174864

ABSORPTION OF NITROGEN OXIDES INTO DILUTED AND CONCENTRATED NITRIC ACID

J. B. Lefers

Delft University Press


T3

m o o o U1 U l

BIBLIOTHEEK TU Delft P 1608 4302

C 456614


PROEFSCHRIFT ter verkri jging van de graad van doctor in de technische wetenschappen

aan deTechnische Hogeschool Delft,

op gezag van de rector magnif icus

Prof. dr. ir. F. J.Kievits,

vooreen commissie aangewezen

door het college van dekanen te verdedigen op woensdag 12 maart 1980 te 16.00 uurdoor Jan Bernard Lef ers scheikundig ingenieur geboren te Enschede

Delft University Press/1980

£0 02250 % D0ELEHSIR.101 £~

Dit proefschrift is goedgekeurd door de promotor PROF. DRS. R J. VAN DEN BERG

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Aan mijn ouders

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I

V O O R W O O R D

Dit p r o e f s c h r i f t i s tot stand gekomen met de medewerking van een groot aantal personen. Langs deze weg w i l i k hen voor hun enthousiaste inzet h a r t e l i j k en oprecht danken. In het bijzonder gaat mijn dank u i t naar

de afstudeerders: Okke de Boks, Arend Bos en Jan Laverman, die aan d i t onder

zoek hebben meegewerkt;

mijn c o l l e g a Cock van den Bleek voor de p r e t t i g e samenwerking binnen de NO^-onderzoekgroep en voor z i j n stimulerende bijdrage i n de d i s c u s s i e s ;

a l l e medewerkers van v e r s c h i l l e n d e servicegroepen voor de vervaardiging, hers t e l l i n g en verbetering van de apparatuur en de u i t v o e r i n g van de t a l r i j k e analy s e s , zonder wier werkzaamheden d i t p r o e f s c h r i f t zeker n i e t tot stand gekomen was;

Wim Jongeleen en Koos Kamps voor het tekenen en verkleinen van de figuren;

Saul Lemkowitz voor het corrigeren van de engelse tekst;

Marian Wijnen voor het zorgvuldig uitgevoerde typewerk en de lay-out van het

manuscript.

C O N T E N T S

SUMMARY 1

SAMENVATTING 3

1. INTRODUCTION 5

1.1 General remarks 5

1.2 Aim of t h i s work 9 1.3 Outline of the th e s i s 9

References 10

2. THE ABSORPTION APPARATUS 11

2.1 Introduction 11

2.2 S e l e c t i o n of laboratory absorber 11 2.3 Description of the absorption apparatus 14 2.4 Mass t r a n s f e r i n a laminar f a l l i n g l i q u i d f i l m 16

2.4.1 Theory 16 2.4.2 Experimental 18

2.4.3 Results 18 2.5 Gas phase mass t r a n s f e r i n laminar plug flow gas streams 19

2.5.1 Introduction 19 2.5.2 Theory 20 2.5.3 Experimental 22

2.5.4 Results 23 2.6 Conclusions 27

References 28

3. SPECTR0PH0T0METRIC DETERMINATION OF NITROGEN OXIDES AND NITRIC ACID

VAPOUR 30

3.1 Introduction 30 3.2 Experimental 30

3.3 Results 33

3.4 Conclusions 39 References 41

4. THE ABSORPTION OF NO„/N„0_ INTO DILUTED AND CONCENTRATED 2 2 4

NITRIC ACID 42

4.1 Introduction 42

4.2 Review of l i t e r a t u r e 42 4.2.1 Absorption of NO^/I^O^ i n t o aqueous so l u t i o n s 42 4.2.2 NO^/N^O^ absorption i n t o concentrated n i t r i c a c i d s o l u t i o n s 48

4.3 Experimental 53

4.4 Results 55

4.4.1 The absorption of NgO^ into d i l u t e d n i t r i c a c i d s o l u t i o n s 55

4.4.2 The absorption of ̂ 2^4 i n t o concentrated n i t r i c a c i d s o l u t i o n s 59

4.5 Conclusions 65

References 65

5. THE OXIDATION AND ABSORPTION OF NO BY NITRIC ACID 67

5.1 Introduction 67

5.2 Proposed mechanism 67

5.3 Experimental 70 5.4 Mathematical model and r e s u l t s 70 5.5 Discussion 86 5.6 Conclusions 87

References

6. AN ABSORPTION MODEL FOR THE DESIGN OF A DILUTED NITRIC ACID ABSORBER AND METHODS TO DECREASE THE NO -CONTENT IN TAIL GASES 89

x

6.1 Introduction 89 6.2 Absorption model for the production of d i l u t e d n i t r i c acid 89 6.3 Methods to decrease the NO^-content i n t a i l gases of n i t r i c

a cid plants 94

6.3.1 Wet processes 95

6.3.1.1 Extended absorption 95

6.3.1.2 H2°2 s c r u b b i n S process 95 6.3.1.3 N i t r i c acid scrubbing 97

6.3.2 Dry processes 100 6.3.2.1 Adsorption 100 6.3.2.2 Non-selective reduction processes 100

A

6.3.2.3 S e l e c t i v e reduction processes

References 101 102

APPENDIX I. THE ADDITIVITY OF RESISTANCES FOR MASS TRANSFER IN A WETTED WALL COLUMN 105

1. Introduction and general theory 105 2. Results 107 3. Conclusions 113

References 113

NOMENCLATURE 114

S U M M A R Y

The subject of t h i s t h e s i s i s dealing with the absorption of nitrogen oxides i n t o n i t r i c a c i d s o l u t i o n s to obtain data f o r the design of i n d u s t r i a l absorbers f o r the production of d i l u t e d and concentrated n i t r i c a c i d . From the l i t e r a t u r e concerning the absorption of nitrogen oxides into aqueous soluti o n s i t i s known that NO, NO^, NÔ^ and NÔ^ a l l play an important r o l e during the absorption. Moreover, n i t r i c acid and nitrous a c i d can be formed i n the gas phase and i n the l i q u i d phase. This complex absorption mechanism was investigated i n a s p e c i a l l y designed wetted wall column with a known i n t e r -f a c i a l area between the gas phase and the l i q u i d phase. A laminar f a l l i n g l i q u i d f i l m and a laminar plug flow of the gas phase without a v e l o c i t y gradient perpendicular to the g a s - l i q u i d i n t e r f a c e could be r e a l i z e d i n the wetted wall column. The mass t r a n s f e r i n the laminar f a l l i n g l i q u i d f i l m was

investigated by absorbing pure carbon dioxide into water at a temperature of o

20 C and at a pressure of 1 bar. It was found that within the measured conditions the l i q u i d phase mass t r a n s f e r could be described by the penetration theory. The gas phase mass t r a n s f e r was investigated by absorbing ammonia from a nitrogen gas stream into 2 N s u l f u r i c a c i d s o l u t i o n s . The experimental r e s u l t s showed a good agreement with the t h e o r e t i c a l l y predicted values derived from the s o l u t i o n of the Graetz problem.

The absorption of NOg/NÔ^ gas mixtures from a nitrogen gas stream into o o

n i t r i c a c i d was c a r r i e d out at a temperature of 20 C and 30 C and at a pressure of about 1 bar. The experimental r e s u l t s could be in t e r p r e t e d with the following model: a) NOg and NÔ^, which are i n continuous e q u i l i b r i u m with each other, d i f f u s e

from the gas phase to the g a s - l i q u i d i n t e r f a c e .

b) NÔ^ i s the only species which d i f f u s e s into the l i q u i d phase. c) In the experiments with d i l u t e d n i t r i c a c i d (25% and 40%) the d i f f u s i o n of

NÔ^ i s accompanied by a rapid pseudo f i r s t order r e a c t i o n in the l i q u i d phase between N 20^ and water. It was found that the absorption rate of NÔ into d i l u t e d n i t r i c a c i d decreases with increasing a c i d strength. In the experiments with concentrated n i t r i c a c i d (63%-80%) the rea c t i o n of NO with water can be neglected and N O d i s s o l v e s p h y s i c a l l y i n the a 4 ^ 4

l i q u i d phase.

1

The s o l u b i l i t y of N^O^ i n concentrated n i t r i c a c i d s o l u t i o n s was ca l c u l a t e d from the t o t a l vapour pressure data of the system NgO^-HgO-HNOg. It was found that within the conditions studied Henry's law i s v a l i d . Furthermore, the s o l u b i l i t y of NgO^ i n concentrated n i t r i c a c i d increases strongly with i n creasing acid strength and decreasing temperature.

The oxidation of NO i n a nitrogen gas stream by 40%-80% n i t r i c a c i d o

solut i o n s was investigated i n the wetted wall column at a temperature of 20 C and 30°C. In the experiments with 63% and 80% n i t r i c a c i d s o l u t i o n s the experimental r e s u l t s were in t e r p r e t e d with the following model:

a) The oxidation reaction takes place i n the gas phase between NO and n i t r i c a c i d vapour and can be considered to be i n f i n i t e l y f a s t . NO and n i t r i c a c i d vapour are t r a n s f e r r e d by molecular d i f f u s i o n from, r e s p e c t i v e l y , the gas bulk and the g a s - l i q u i d i n t e r f a c e to the rea c t i o n zone or plane. It was found that Danckwerts' s o l u t i o n for instantaneous i r r e v e r s i b l e reactions i n the l i q u i d phase can also be applied to gas phase reactions.

b) The NO^ and N^O^ produced, which are i n continuous, equi l i b r i u m with each other, d i f f u s e from the rea c t i o n plane or zone to the gas bulk and to the g a s - l i q u i d i n t e r f a c e . At the g a s - l i q u i d i n t e r f a c e only Ng0 4 d i s s o l v e s p h y s i c a l l y into the concentrated n i t r i c a c i d .

Experiments with 57% n i t r i c a c i d showed that the oxidation of NO also proceeds p a r t i a l l y i n the l i q u i d phase. Under these circumstances the gas phase reaction i s too slow to be considered to be i n f i n i t e l y f a s t , a f a c t which may be caused by the rather low n i t r i c a c i d vapour pressure. In the experiments with 40% n i t r i c a c i d i t was found that the oxidation takes place completely i n the l i q u i d phase. Under these circumstances the f i n a l oxidation product i s mainly n i t r o u s acid.

Based on general chemical r e a c t i o n engineering considerations a mathematical model was developed to describe the absorption mechanism which occurs i n the absorber f o r the production of d i l u t e d n i t r i c acid. F i n a l l y various methods to decrease the amounts of nitrogen oxides content i n t a i l gases of n i t r i c a c i d plants were b r i e f l y discussed.

2

S A M E N V A T T I N G

Dit p r o e f s c h r i f t heeft a l s onderwerp de absorptie van s t i k s t o f o x i d e n i n salpeterzuur, hetgeen van belang i s b i j het ontwerp van industriële absorbers voor de produktie van verdund en geconcentreerd salpeterzuur. U i t een l i t e r a t u u r o n derzoek i s gebleken dat de absorptie van s t i k s t o f o x i d e n i n waterige oplossingen zeer gecompliceerd i s , waarbij NO, NÔ^, NO2 and NÔ^ een bel a n g r i j k e r o l spelen . Bovendien kunnen salpeterzuur en salpeterigzuur i n de v l o e i s t o f f a s e en i n de gasfase worden gevormd. Om een d e r g e l i j k gecompliceerd absorptieproces te bestuderen i s een natte wand kolom ontwikkeld waarin het contactoppervlak tussen gas- en v l o e i s t o f f a s e goed bekend i s . In deze kolom kon een laminaire s t r o ming van de vallende v l o e i s t o f f i l m en een laminaire propstroming van de gasfase zonder snelheidsgradiënt loodrecht op het g a s - v l o e i s t o f contactoppervlak worden verkregen. Het sto f t r a n s p o r t i n de laminair vallende v l o e i s t o f f i l m werd onder-

o zocht door zuiver CO^ te absorberen i n water b i j een temperatuur van 20 C en een druk van 1 bar. U i t de experimenten kon worden geconcludeerd dat het s t o f -transport i n de v l o e i s t o f f a s e beschreven kon worden door de pe n e t r a t i e t h e o r i e . Het s t o f t r a n s p o r t i n de gasfase werd onderzocht door NH^ te absorberen van een s t i k s t o f gasstroom i n 2 N zwavelzuur. De experimentele r e s u l t a t e n bleken goed overeen te komen met de theor e t i s c h voorspelde waarden u i t het Graetz-model.

De absorptie van NOg/NgO^ gasmengsels van een s t i k s t o f gasstroom i n salpe-o o

terzuur werd uitgevoerd b i j een temperatuur van 20 en 30 C en een druk van

ongeveer 1 bar. De experimentele r e s u l t a t e n konden worden beschreven met het

volgende model. a) N0 2 en NgO^, w e ^ k e voortdurend i n evenwicht z i j n , diffunderen van de gas

fase naar het fasegrensvlak. b) N 20 4 i s de actieve component die i n de v l o e i s t o f f a s e d i f f u n d e e r t .

c) In de experimenten met verdund salpeterzuur (25% en 40%) gaat de d i f f u s i e van NÔ^ gepaard met een s n e l l e pseudo Ie orde r e a c t i e i n de v l o e i s t o f f a s e tussen NÔ^ en water. U i t de experimenten bleek dat de absorptiesnelheid van NgO^ i n verdund zuur afnam b i j toenemende zuursterkte.

In de experimenten met geconcentreerd salpeterzuur (63%-80%) bleek dat de r e a c t i e tussen water en ^ 0 ^ verwaarloosd kon worden. In dat geval l o s t N O s l e c h t s f y s i s c h op i n de v l o e i s t o f f a s e .

3

De oplosbaarheid van NÔ^ i n geconcentreerd salpeterzuur werd berekend u i t literatuurgegevens betreffende de t o t a l e dampdruk van het systeem NÔ^-HgO-HNOg. Binnen de beschouwde c o n d i t i e s b l i j k t dat de wet van Henry g e l d i g i s . Verder b l i j k t de oplosbaarheid van N 20 4 i n geconcentreerd salpeterzuur sterk toe te nemen met toenemende zuursterkte en dalende temperatuur.

De oxydatie van NO i n een s t i k s t o f gasstroom door 40%-80% salpeterzuur werd

onderzocht i n de natte wand kolom b i j een temperatuur van 20°C en 30°C. De ex

perimentele r e s u l t a t e n met 63% en 80% salpeterzuur konden worden beschreven met

het volgende model.

a) De oxydatie r e a c t i e vindt p l a a t s i n de gasfase tussen NO en salpeterzuur-damp en kan a l s oneindig snel worden beschouwd. Salpeterzuurdamp en NO d i f funderen r e s p e c t i e v e l i j k van het g a s - v l o e i s t o f contactoppervlak en de gas-bulk naar het r e a c t i e v l a k of de reactiezöne. Experimenteel bleek dat Danckwerts' oplossingen voor instantane i r r e v e r s i b e l e r e a c t i e s i n de v l o e i s t o f f a s e tevens kunnen worden toegepast op instantane gasfase r e a c t i e s .

b) Het gevormde NOg en NÔ^, welke voortdurend i n evenwicht z i j n , diffunderen

van het r e a c t i e v l a k of de reactiezöne naar de bulk van de gasfase en naar

het fasegrensvlak. Op het fasegrensvlak l o s t a l l e e n N2 ° 4 f y s i s c h op i n de

v l o e i s t o f f a s e .

Experimenten met 57% salpeterzuur toonden aan dat de oxydatie van NO ook ged e e l t e l i j k verloopt i n de v l o e i s t o f f a s e . In d i t geval kan de gasfase r e a c t i e n i e t a l s oneindig snel worden beschouwd, hetgeen wordt veroorzaakt door de lage salpeterzuurdampdruk. B i j 40% salpeterzuur vindt de oxydatie van NO v o l l e d i g i n de v l o e i s t o f f a s e p l a a t s , waarbij s a l p e t e r i g zuur het u i t e i n d e l i j k gevormde produkt i s .

Toepassing van de algemene beginselen van de chemische reactorkunde op de absorptie van s t i k s t o f o x i d e n i n salpeterzuur resulteerde i n een wiskundig model voor het ontwerpen van industriële absorbers b i j de produktie van verdund salpeterzuur. Tenslotte werden de v e r s c h i l l e n d e mogelijkheden om het NO x~gehal-te i n afgassen van salpeterzuurplants te verlagen met elkaar vergeleken.

4

1. INTRODUCTION

1.1 GENERAL REMARKS

N i t r i c a c i d i s one of the most important inorganic acids and i t i s used i n the production of f e r t i l i z e r s , dyestuffs, r e s i n s and explosives. Further a p p l i c a t i o n s are s t a i n l e s s s t e e l p i c k l i n g and metal etching. About three-fourths of the n i t r i c a c i d produced i s used i n the f e r t i l i z e r industry, mainly f o r the production of ammonium n i t r a t e , ammonium phosphates and compound f e r t i l i z e r s . The n i t r i c a c i d needed i n the f e r t i l i z e r industry i s us u a l l y d i l u t e d n i t r i c a cid with a concentration of 50-70%. For most other a p p l i c a t i o n s , such as n i t r a t i o n reactions, 90-100% n i t r i c acid i s used.

Since the development of the Haber-Bosch ammonia synthesis i n 1913 nearly a l l n i t r i c acid plants are based on the oxidation of ammonia and the subsequent absorption of nitrogen oxides i n t o water.

Diluted nitric acid production

An example of a flow sheet for the production of d i l u t e d n i t r i c acid based on the D.S.M. n i t r i c a c i d process i s given i n F i g . 1 (mono pressure system). Ammonia mixed with a i r enters a converter (B) i n which the gas mixture i s passed over a platinum gauze c a t a l y s t at a temperature of 850-920°C. The ammonia i s oxidized to NO according to

4NH3 + 50 2 -»• 4N0 + 6H 20 (1)

The converter can be operated at atmospheric, medium (3-5 bar) or high pressure (7-10 bar). The hot gases leaving the converter are cooled i n a waste heat b o i l e r to generate steam. The temperature i s fu r t h e r reduced to 20°-40°C i n a cooler-condenser (D), and at the same time the NO formed i s oxid i z e d to N0 2

2N0 + 0 2 X 2N0 2 (2)

5

7

NH3-

0- B

T

12 -H 20

11

10 8

6 0 ° / . HNO3

Fig. 1 The D.S.M. n i t r i c acid process (mono-pressure system).

A: air compressor, B: converter, C: tail gas heater, D: cooler-

condenser, E: absorption column, F: bleaching column, G: expansion

turbine.

1: air, 2: NH , 3: 10% NH in air, 4: NO, 5: N02, 6: weak acid, 7: tail

gas containing 200-2000 ppm 8: unbleached 60% n i t r i c acid, 9: NO^,

10: bleached 60% n i t r i c acid, 11: air, 12: water.

The water formed condenses i n the cooler-condensor and some weak ac i d i s

produced. The gas mixture, containing about 10 volume % NOg, enters the a c i d

absorber (E) where i t reacts with water i n the l i q u i d phase at ambient

temperature and at the operating pressure i n the converter:

3N0 2 + H 20 •* 2HN03 + NO (3)

The n i t r i c a c i d formation i s accompanied by NO evolution which i s re-oxidized i n the gas phase by molecular oxygen according to r e a c t i o n (2). The N0 2

produced i s subsequently absorbed i n t o the l i q u i d phase. In the top of the acid absorber the re-oxidation rate of NO i s very slow, and as a f i r s t approach t h i s may be assumed to be the rate determining step i n the absorption process. The t a i l gas, containing about 200-2000 ppm N0 x, leaves the absorber and i s heated. Energy i s subsequently recovered by expansion i n a turbine a f t e r which the gas stream i s vented to the atmosphere.

The 60% n i t r i c a c i d containing some d i s s o l v e d nitrogen oxides leaves the

absorber and i s st r i p p e d with a i r i n a bleaching column (F) .

6

A variant of the mono-pressure process i s the dual pressure process, at which the conversion takes place at a lower pressure than the absorption. In a dual pressure system a nitrous gas compressor i s needed. The main advantage o f f e r e d by a low pressure i n the converter i s the decrease of the ammonia and platinum c a t a l y s t l o sses. On the other hand the investments required are higher.

S i m i l a r processes such as the D.S.M. n i t r i c a c i d process can be found i n the l i t e r a t u r e [1].

Concentrated nitric acid production

There i s a s u b s t a n t i a l need for stronger ac i d , p a r t i c u l a r l y f o r a c i d with a concentration i n the range of 90-100%. Such a c i d i s , for example, used i n n i t r a t i o n r eactions. However, concentrated n i t r i c acid can not be d i r e c t l y prepared by d i s t i l l a t i o n of d i l u t e d a c i d of 60%, because n i t r i c a c i d and water form a constant b o i l i n g mixture (azeotrope) between an acid strength of 68-69%. E x t r a c t i v e d i s t i l l a t i o n with s u l f u r i c a c i d or m o d i f i c a t i o n of the azeotropic composition with magnesium n i t r a t e and subsequent d i s t i l l a t i o n of the r e s u l t a n t mixture are rather expensive methods.

Recently some new processes have been developed to produce concentrated n i t r i c a c i d of 80%, which can be d i r e c t l y d i s t i l l e d to produce 100% n i t r i c a c i d . F i g . 2 gives a s i m p l i f i e d flow sheet of the Du Pont de Nemours concentrated n i t r i c a c i d process [2]. The NO produced by oxidation of ammonia i n a converter (A) i s , a f t e r recovery of energy i n a waste heat b o i l e r , further cooled i n a cooler-condenser (B). In the cooler-condenser the NO i s o x i d i z e d to NO at a temperature of 15°-70°C according to r e a c t i o n (2). Moreover, the water produced by reaction (1) i s condensed and some weak acid i s produced. The weak acid enters the weak acid d i s t i l l a t i o n column (D) where the excess of water i s removed. N i t r i c a c i d of 68% leaves the bottom of the d i s t i l l a t i o n column.

The gas mixture from the cooler condenser i s fed to a physical absorber (C) where N0 2 i s disso l v e d i n 80%-85% n i t r i c a c i d at 0°C-10°C and at a pressure of 10-12 bar. The gas mixture leaving the p h y s i c a l absorber contains some n i t r i c a cid vapour and about 2000 ppm NOg. This can be decreased to 200 ppm by scrubbing with water. The 80%-85% n i t r i c acid leaving the bottom of the absorber contains about 15-30% dissolv e d N

2 ° 4 • This s o l u t i o n enters reactor (E) in which the NgO^ i s converted to n i t r i c acid at 40°-100°C with a i r .

2N 20 4 + 2H 20 + 0 2 - 4HN03 (4)

7

NH 3

S

2 ~ A B

6 10

<68 °/o HNO3

c <90"/.HN03

14

12 c:

Fig. 2 The Du Pont de Nemours concentrated n i t r i c acid process.

A: converter, B: cooler-condenser, C: physical absorber, D: weak acid

d i s t i l l a t i o n column, E: reactor, F: strong acid bleaching column, G:

strong acid d i s t i l l a t i o n column.

1: 10% NH^ in air, 2: NO, 3: NO2 to physical absorber, 4: weak acid to

d i s t i l l a t i o n column, 5: <_ 68% n i t r i c acid, 6: 80% n i t r i c acid with

dissolved N^O^, 7: 80% bleached n i t r i c acid, 8: tail gas to water or

diluted acid scrubber, 9: 85% n i t r i c acid to bleacher, 10: N0^ in air,

11: NOg in air, 12: 85% n i t r i c acid solution to d i s t i l l a t i o n column,

13: air, 14: > 90% n i t r i c acid, 15: water.

N i t r i c a c i d of about 68% i s fed to the reactor to supply the water required f o r the r e a c t i o n . Note that during the ac i d formation no NO i s formed [3,4]. The l i q u i d e f f l u e n t from the reactor (85% n i t r i c a c i d ) containing about 5-10% diss o l v e d N 2 0 4 i s stri p p e d with a i r i n a bleaching column (F), A f t e r s t r i p p i n g the concentrated n i t r i c acid i s d i s t i l l e d to produce 90-100% n i t r i c a c i d . The bottom product (80-85% n i t r i c acid) i s recycled to the phy s i c a l absorber. In the l i t e r a t u r e other processes are reported to produce concentrated n i t r i c acid such as the SOLNOX-process of Ugine Kuhlmann [5,6], the SABAR-process of Davy Powergas GmbH [7,8], the HYCON-process of Chemico Construction Corp. [9,10], the concentrated n i t r i c acid processes of Sumitomo Chemical Engineering Co. Ltd. [1,12] and F r i e d r i c h Uhde GmbH [13].

3

Regulations of NO^ emission

T a i l gases of n i t r i c a c i d plants contain nitrogen oxides which are e s s e n t i a l reactants i n the formation of photochemical smog, i n addition to being p o l l u t a n t s themselves. In the United States the present emission l e v e l i s 1.5 kg NO ( c a l c u l a t e d as N0„) per ton a c i d f o r new plants. This i s equivalent to

x * j

about 200 ppm. For e x i s t i n g plants a l e v e l of 400 ppm w i l l be required. In

Europe the l i m i t v a r i e s from country to country. Presently for new plants a

value of 400 ppm may be assumed, depending on the l o c a l s i t u a t i o n .

1.2 AIM OF THIS WORK

In the absorber f o r the production of d i l u t e d as well as concentrated n i t r i c a c i d several reactions and e q u i l i b r i a occur. Many i n v e s t i g a t i o n s can be found in the l i t e r a t u r e concerning the absorption of N0 2 into water and aqueous sol u t i o n s , but the absorption mechanism i s s t i l l not well understood. This hiatus i s caused mainly by the fact that NO, N

2 ° 3 ' N 0 2 3 1 1 1 1 N2°4 a 1 1 P l a y a n

important r o l e i n the absorption process. Moreover, n i t r i c a c i d and n i t r o u s a c i d are produced i n the gas phase as well as i n the l i q u i d phase. Investigations concerning the absorption of nitrogen oxides into n i t r i c a c i d s o l u t i o n s are p a r t i c u l a r l y poor. In t h i s work the absorption mechanism of nitrogen oxides into n i t r i c a c i d s o l u t i o n s w i l l be investigated to obtain data f i r s t l y , f o r design of i n d u s t r i a l absorbers for the production of n i t r i c a c i d and secondly f o r the design of scrubbers f o r the removal of nitrogen oxides from t a i l gases of n i t r i c acid and n i t r a t i o n plants.

1.3 OUTLINE OF THE THESIS

A laboratory absorber with a well defined i n t e r f a c i a l area between the gas phase and the l i q u i d phase i s developed to i n v e s t i g a t e the complex absorption mechanism (Chapter 2). The q u a n t i t a t i v e analysis of nitrogen oxides i n the gas phase i s given i n Chapter 3. Absorption measurements of NOg/NgO^ gas mixtures into d i l u t e d and concentrated n i t r i c a c i d are c a r r i e d out i n the laboratory absorber. The r e s u l t s are described i n Chapter 4. In Chapter 5 an o x i d a t i o n -absorption mechanism of NO by n i t r i c acid s o l u t i o n s i s proposed. Experimental r e s u l t s are compared with the t h e o r e t i c a l l y predicted values. Chapter 6 gives a mathematical model f o r the design of a d i l u t e d n i t r i c a c i d absorber based on general chemical r e a c t i o n engineering considerations. At the end of t h i s

chapter various methods to decrease the N0 x content i n t a i l gases of n i t r i c a cid plants are b r i e f l y discussed. F i n a l l y i n Appendix 1 the a d d i t i v i t y of i n d i v i d u a l phase resistances f o r mass t r a n s f e r i n the laboratory absorber i s discussed.

REFERENCES

1. Honti, G.D., The Nitrogen Industry, Akademia Kia D6, Budapest, 1976. 2. Du Pont de Nemours, B r i t . 1419645, 1975, December 31.

3. Franck, H.H. and Schirmer, W., Z. Elektrochem. ,1950, 54, 254.

4. Shneerson, A.L., Mlnovich, M.A. F i l i p p o v a , Zh.M. and Platonov, P.A., J. Appl. Chem. USSR (Engl. Transl. ) , 1965, 38, 1627.

5. Ugine Kuhlmann, Ger. Offen. 2128382, 1971, December 23.

6. Anon., Nitrogen No. 106, , 1977, March/April, 35.

7. Anon., Hydrocarbon Process. , 1975, November, 164.

8. Hellmer, L., Chem. Eng., 1975, December, 98. 9. Newman, D.J. and K l e i n , L.A., Chem. Eng. Progr., 1972, 68, 62. 10. Chemico Construction Corp., U.S. Patent 3542510, 1970, November 24. 11. Komiyama, D., Ohrui, T. and Sakakibara, Y., Hydrocarbon Process., 1972,

A p r i l , 145.

12. Sumitomo Chemical Co. Ltd., Ger. Offen. 2125677, 1971, December 2. 13. F r i e d r i c h Uhde GmbH, Ger. Offen. 2148329, 1973, A p r i l 5.

10

2. THE ABSORPTION APPARATUS

2.1 INTRODUCTION

Absorption phenomena are commonly studied i n absorbers with a well defined i n t e r f a c i a l area between gas and l i q u i d i n order to obtain data for the design of i n d u s t r i a l equipment (packed beds and pl a t e columns). From l i t e r a t u r e data concerning the absorption of nitrogen oxides into n i t r i c acid solutions i t i s known that gas phase reactions as well as l i q u i d phase reactions may occur [1, 2,3,34,35,36,37], To i n v e s t i g a t e such a complex absorption mechanism on laboratory scale a s p e c i a l design of the absorption apparatus i s necessary. In t h i s Chapter a laboratory absorber with a well defined i n t e r f a c i a l area between gas and l i q u i d i s developed i n which gas absorption with simultaneously occurring gas phase reactions can be studied. L i q u i d phase and gas phase mass tr a n s f e r behaviour i n t h i s absorption apparatus i s investigated separately by absorbing pure CO^ into water and NH^ from a nitrogen gas stream i n t o s u l f u r i c a c i d s o l u t i o n s and i n t o water. The r e s u l t s are compared with the t h e o r e t i c a l l y predicted values.

2.2 SELECTION OF LABORATORY ABSORBER

Several types of laboratory absorbers can be found i n the l i t e r a t u r e [4]. Table 1 gives a survey of the normal operating conditions of various absorbers.

To i n v e s t i g a t e gas absorption with simultaneously occurring f a s t gas phase reactions the behaviour of the absorber has to be " i d e a l " i n r e l a t i o n to the l i q u i d phase and the gas phase. In a continuous flowing system favoured cases may be:

a) complete mixing i n the l i q u i d phase and i n the gas phase, so that the bulk concentrations i n each phase are uniform (and equal to the o u t l e t concentration).

b) plug flow of the gas phase and the l i q u i d phase without a v e l o c i t y gradient perpendicular to the g a s - l i q u i d i n t e r f a c e .

11

LAMINAR JET

WETTED -WALL COLUMN

WETTED SPHERE

STRING OF WETTED SPHERES

ROTATING DRUM

MOVING BAND

STIRRED C E L L

LIQUID

g a s

It

GAS

À 1 I

LIQUID LIQUID

gas

GAS

FILM

LIQUID

g ° 5 ^ i

4M-BAND

gas

l iquid

t i m e o f e x p o s u r e o f f r e s h l i q u i d e l e m e n t s t o g a s i n s e c o n d s

10 - 0 . 2 5

> 100 a l s o a s b a t c h

r e a c t o r

i n t e r f a c i a l a r e a , 2

d i a m e t e r ; 11,4 cm

l e n r j h t 12.4 cm

f l o w c o n d i t i o n s o f t h e

l i q u i d p h a s e

l a m i n a r u n i f orrr, v e l o c i t y

l a m i n a r h a l f

p a r a b o l i c v e l o c i t y

l i s t r i b u t i o n

l a m i n a r u n i f o r m v e l o c i t y

t u r b u l e n t

'flow c o n d i t i o n s o f t h e gas p h a s e

t u r b u l e n t l a m i n a r

t u r b u l e n t l a m i n a r t u r b u l e n t t u r b u l e n t t u r b u l e n t

l a m i n a r

a b s o r p t i o n m o d e l

g e n e r a l r e f e r e n c e

p e n e t r a t i o n m o d e l

[4,5,6]

p e n e t r a t i o n model

[4,7,8,9,33]


[4,10,lî]

p e n e t r a t i o n o r

f i l m model

m o d e r a t e

[ 4 , 1 2 , l J ]


p e n e t r a t i o model f i l m model

[4,16,17,1a]

Table 1 A comparison of various laboratory absorbers

Ad a). A continuously s t i r r e d c e l l with independent c o n t r o l of the a g i t a t i o n rates i n the gas phase and i n the l i q u i d phase may be s u i t a b l e f o r r e a l i z i n g complete mixing. To insure a smooth i n t e r f a c e excessively high s t i r r e r speeds have to be avoided e s p e c i a l l y i n the l i q u i d phase. This f a c t o r may be c r i t i c a l for the maintenance of uniformity of the bulk concentration i n the l i q u i d phase. Godfrey [16] studied mixing phenomena of the l i q u i d phase i n a s t i r r e d c e l l with and without b a f f l e s by using t r a c e r techniques. The mixing time was found to be 2-5 seconds. Godfrey [16] assumed that uniformity i n the l i q u i d bulk was r e a l i z e d i f the mixing time i s l e s s than 3% of the average residence time of the l i q u i d phase i n the s t i r r e d c e l l . This means a rather long residence time for the l i q u i d phase.

12

Mixing i n the gas phase i s much l e s s c r i t i c a l due to the higher a g i t a t i o n rates which can be applied without d i s t u r b i n g the smooth g a s - l i q u i d i n t e r f a c e . Very l i t t l e information can be found i n the l i t e r a t u r e concerning mixing times i n the gas phase i n s t i r r e d c e l l s . Sada et a l [17] found that at a residence time of about 10 seconds and with a s t i r r e r speed of 100 rev/min the bulk of the gas phase was completely mixed. The most important advantage of the s t i r r e d c e l l i s that the d e s c r i p t i o n of absorption phenomena can be based on the two-f i l m theory, which means that even f o r complex mechanisms rather simple mathematical expressions are obtained. On the other hand the accuracy of the experimental r e s u l t s i s rather low. Further i t should be noted that the gas phase rea c t i o n of NO with n i t r i c acid vapour i n the presence of l i q u i d n i t r i c acid produces water, which condenses on the walls of the gas compartment. This i s an u n r e a l i s t i c s i t u a t i o n f o r i n d u s t r i a l absorbers such as packed beds and p l a t e columns.

Ad b)• Plug flow of the gas phase and the l i q u i d phase can be r e a l i z e d i n a wetted wall column [7,19,20,21] i n which an i d e a l f a l l i n g l i q u i d f i l m and a gas stream are flowing laminar and cocurrently i n a v e r t i c a l tube. A f l a t v e l o c i t y p r o f i l e of the gas stream i n the wetted tube can be created by choosing the surface v e l o c i t y of the l i q u i d f i l m equal to the gas v e l o c i t y . The flow model i s presented i n F i g . 1. Although the v e l o c i t y d i s t r i b u t i o n i n an i d e a l laminar f a l l i n g l i q u i d f i l m i s h a l f parabolic,plug flow of the l i q u i d f i l m can be assumed i f the penetration depth of the absorbing gas i s small compared to the f i l m thickness. In t h i s way the following advantages can be obtained:

1. Known hydrodynamic behaviour of the laminar f a l l i n g l i q u i d f i l m . 2. F l a t v e l o c i t y p r o f i l e of the laminar flowing gas stream without a

v e l o c i t y d i s t r i b u t i o n perpendicular to the g a s - l i q u i d i n t e r f a c e .

3. Equal contact time values i n the absorption apparatus of the gas phase and the l i q u i d phase.

4. The mass t r a n s f e r i n the gas phase and l i q u i d phase can be described by a molecular d i f f u s i o n process i n r a d i a l d i r e c t i o n . Because of the equal v e l o c i t i e s of the gas phase and the l i q u i d phase, there i s zero drag at the i n t e r f a c e and no influence of the moving i n t e r f a c e on the mass tr a n s f e r can be expected.

Absorption phenomena i n wetted wall columns are commonly described with the penetration theory, and with complex absorption mechanisms t h i s may lead to d i f f i c u l t mathematical expressions. On the other hand, the r e s u l t s with wetted wall columns are accurate (Table 1). The water produced by the gas phase

13

reaction of NO and n i t r i c a c i d vapour condenses on the l i q u i d i n t e r f a c e and t h i s s i t u a t i o n i s r e a l i s t i c f o r the absorption process i n i n d u s t r i a l absorbers. The mean advantages of a wetted wall column compared to a s t i r r e d c e l l f o r studying the absorption of nitrogen oxides i n t o n i t r i c a c i d are the higher accuracy and the fact that a better correspondence with the absorption process i n i n d u s t r i a l absorbers may be r e a l i z e d .

On the contrary the mathematical d e s c r i p t i o n of the absorption process i n wetted wall columns i s more d i f f i c u l t than i n s t i r r e d c e l l s . Although the choice between both laboratory absorbers i s rather a r b i t r a r y i t was decided to use a wetted wall column f o r studying the absorption mechanism of nitrogen oxides into n i t r i c a c i d s o l u t i o n s .

l iqu id gas f i lm e (

y

R fc

-r

r

Fig. I Flow model and coordinate system.

2.3 DESCRIPTION OF THE ABSORPTION APPARATUS

The wetted wall column i s schematically presented i n F i g . 2. It c o n s i s t s of an upper and lower s t a i n l e s s s t e e l calming s e c t i o n each with a length of 25.0 cm. The upper calming section contains a porous s t a i n l e s s s t e e l f i l t e r to suppress gas eddies and to create a f l a t v e l o c i t y p r o f i l e of the gas. The lower end of the upper calming s e c t i o n f i t s i n t o the top of a c y l i n d r i c a l absorption column (glass) with an inner diameter of 3.45 cm. The length of the glass c y l i n d r i c a l absorption column i s 13.0 cm, 33.0 cm and 50.0 cm. The l i q u i d f i l m was i n t r o duced to the wetted wall column through an adjustable annular s l i t , which i s normally of the same width (^0.4 mm) as the thickness of the l i q u i d f i l m . The f i l m covering the inner surface of the tube flowed down and was i n c l i n e d to an

14

\

annular pool with a small surface area. In t h i s way the gas was separated from the l i q u i d . The l i q u i d l e v e l i n the r e s e r v o i r was kept on a constant height by means of a l e v e l c o n t r o l l e r . Pressure taps were located 11 cm above the wetted wall s e c t i o n and 11 cm under the wetted wall s e c t i o n . Temperatures of the i n -and outgoing gas and l i q u i d were measured by means of thermocouples.

2.4 MASS TRANSFER IN A LAMINAR FALLING LIQUID FILM

The p h y s i c a l absorption rate of a pure gas into an i d e a l f a l l i n g l i q u i d f i l m may under c e r t a i n conditions be described by the theory of penetration. In order to check the hydrodynamic behaviour of the l i q u i d f i l m i n the previously described wetted wall column,experiments were c a r r i e d out by absorbing COg i n t o water. The measured absorption rates were compared with the values predicted by the penetration theory.

2.4.1 Theory

The p h y s i c a l absorption of a pure gas into a laminar f a l l i n g l i q u i d f i l m may under c e r t a i n conditions be considered as a non-stationary d i f f u s i o n process i n a s e m i - i n f i n i t e medium. The absorption process i s then described by the following equation:

dt dx

The i n i t i a l and boundary conditions are

t = 0 X > 0 C l = ° l , o

t > 0 X = 0 C£,i t > 0 X = 0 0

C £ = °SL,o

(2)

(3) (4)

The s o l u t i o n of t h i s equation i s :

The absorption rate can be found be d i f f e r e n t a t i o n of equation (5):

3C i ' " it*

16

The amount of gas absorbed per un i t of surface area during contact time T i s

given by:

1 * TT m(T) = 2 ( C £ . - C l o ) V — (7)

For an i d e a l laminar f a l l i n g l i q u i d f i l m without r i p p l e s flowing down a v e r t i c a l tube we have:

_ _h_ v s

, s = 2 (g/3V) [ - J

3V<j> B \ 1/3

(8)

(9)

6 f = l - i g T J ( 1 0 )

A laminar f a l l i n g l i q u i d f i l m has a h a l f parabolic v e l o c i t y d i s t r i b u t i o n and

equation (7) can only be applied i f the gas penetrates into a l i q u i d l a y e r

whose v e l o c i t y does not d i f f e r too much from the surface v e l o c i t y . This implies

that penetration depth of the absorbing gas i s small compared to the f i l m

thickness. According to N i j s i n g [5] the l i q u i d f i l m can be considered to be 2 2 i n f i n i t e l y deep i f D^T/Sf < 0.04. In our experiments D^T/Sf i s smaller than

0.065, and the deviation from equation (7) i s about 0.15%. The formation of a

f a l l i n g f i l m through a s l i t introduces a deviation from the steady s t a t e r e l a t i o n s h i p f o r the v e l o c i t y given i n equation (9). Its e f f e c t on the t o t a l contact time i s n e g l i g i b l e i f the f i l m height i s more than about 20 times the f i l m thickness [5,7]. The c o r r e c t i o n needed f o r the end e f f e c t i s generally an

order of magnitude more.

In p r a c t i c e some r i p p l e s may appear on a f a l l i n g f i l m . P o r t a l s k i and Clegg [23] found that these r i p p l e s give no important increase of the i n t e r f a c e . Ripples cause some mixing and give enhanced mass t r a n s f e r rates compared to the

penetration theory [24]. These r i p p l e s are eliminated by adding small amounts

of surface a c t i v e agents ("Teepol") to the absorbing l i q u i d without introducing an a d d i t i o n a l resistance to mass t r a n s f e r [5,7,8].

A f a l l i n g f i l m has a stagnant surface with a height of a few cm above the l e v e l of the r e c e i v i n g l i q u i d . In t h i s stagnant surface the absorption can be

neglected and the t o t a l height h has to be corrected by substracting the height

of t h i s end e f f e c t [5].

17

2.4.2 Experimental

Ordinary d i s t i l l e d water contained i n an overhead r e s e r v o i r was fed by g r a v i t y to the absorber shown i n F i g . 2. In t h i s way a constant l i q u i d flow was obtained which was necessary f o r a smooth f i l m . The column length of the absorber was 15.9, 34.9 and 51.9 cm. The water was completely degassed by high vacuum, and 0.05% by weight of "Teepol" was added to the water i n order to eliminate r i p p l e s of the l i q u i d f i l m . CO^ from a c y l i n d e r was saturated with water vapour at the experimental temperatures and then supplied to the wetted wall column. The CX>2 absorption rate was measured from the decrease of the C0 2

volume at constant pressure with a soap f i l m i n a c a l i b r a t e d tube according to N i j s i n g [5].

2.4.3 Results

In order to check the hydrodynamic behaviour of the l i q u i d f i l m the experimental absorption rates were compared with the rates predicted by the penetration theory. In our experiments the height of the stagnant l i q u i d f i l m formed above the c o l l e c t i n g l i q u i d r e s e r v o i r was found to be 1.6 cm which agrees with the data found by Lynn et a l [8-10] and N i j s i n g [5]. In t h i s part of the f i l m the absorption rate can be neglected. A p l o t of m (T) versus ^ ( h - AlO/v^ should

give a s t r a i g h t l i n e through the o r i g i n with a slope of 2C^ i y ^ j j / ^ " 3 ) • Regression analysis shows that the deviation of the experimental points i s always less than 2% and the upper bound and the lower bound of the 95%

-5 2 confidence i n t e r v a l for the intercept was r e s p e c t i v e l y 0.155 x 10 kg/m and

-5 2 -0.102 x 10 kg/m . The upper bound of the 95% confidence i n t e r v a l of the

"™™ —5 2 4 slope 2C„ WD./l was found to be, r e s p e c t i v e l y , 7.97 x 10 kg/m .sec and 2 4 -5 2 4 7.68 x 10~5 kg/m .sec which agrees with the value of 8.04 x 10 kg/m .sec found by N i j s i n g [5].

Hence i t was concluded that the absorption rate i n the l i q u i d phase i s well predicted by the penetration theory and that the hydrodynamic behaviour of the l i q u i d f i l m agrees well with the t h e o r e t i c a l model.

18

12

Fig. 3 Absorption rate of CO^ into water as a function of the contact time

(P = 1.013 bar, t = 20°C).

Symbol h (cm)

A 15.9

0 34.9

V 51.9

2.5 GAS PHASE MASS TRANSFER IN LAMINAR PLUG FLOW GAS STREAMS

2.5.1 Introduction

The e f f e c t of gas and l i q u i d flow rates on the gas phase mass t r a n s f e r i n laminar gas streams was t h e o r e t i c a l l y and experimentally studied i n wetted wall columns and rectuangular g a s - l i q u i d flow [7,20,22,25,26].

I f however the gas v e l o c i t y i s uniform and i s equal to the surface v e l o c i t y of the l i q u i d , no influence of the moving i n t e r f a c e on the gas phase mass t r a n s f e r can be expected. S t r i c t l y speaking t h i s i s true when there i s zero

19

drag at the i n t e r f a c e . H i k i t a and Ishimi [22] s u p e r f i c i a l l y studied t h i s s i t u a t i o n i n wetted wall columns, but no information i s given at high Graetz-numbers. Dekker [7] investigated the absorption of ammonia into water at Graetz-numbers from 135 up to 245 i n a wetted wall column which was constructed i n a s p e c i a l way to create a f l a t v e l o c i t y p r o f i l e of the gas. Comparison of his measured absorption rates with the t h e o r e t i c a l l y predicted values was, however, based upon the assumption of a small l i q u i d phase resistance for mass tr a n s f e r which was not experimentally confirmed. i . <

In t h i s part these gaps of information are in v e s t i g a t e d i n the previously described wetted wall column ( F i g . 2) by absorbing ammonia from a nitrogen gas stream into water and s u l f u r i c acid s o l u t i o n s . The measured mass t r a n s f e r rates are compared with the t h e o r e t i c a l l y predicted rates.

2.5.2 Theory

Gas and l i q u i d are assumed to flow co-currently and i n a laminar way i n a v e r t i c a l wetted wall column. The gas v e l o c i t y i s assumed to be uniform throughout the column and equal to the surface v e l o c i t y of the l i q u i d f i l m . The flow model and the co-ordinate system are given i n F i g . 1. Assuming that a l l the relevant p h y s i c a l properties remain constant and that mass t r a n s f e r from the gas phase to the l i q u i d phase takes place by molecular d i f f u s i o n only i n r a d i a l d i r e c t i o n , the d i f f u s i o n equation can be written as:

3C . 3 2C 1 3C v 6 = D ( ^ + - E ) (11)

8 3h g l 3 r 2 r 3r '

with the boundary and i n i t i a l conditions:

h = 0 0 < r < R - 6, C = C (12) f g g>o

h > 0 r = 0 3C g/3r = 0 (13) h > 0 r = R - <$„ C = C (14)

f g g. i

Attention should be paid to the fact that the i n t e r f a c e concentration should

have a constant value C . along the f i l m . According to Carslaw and Jaeger g. i

[32] the s o l u t i o n of these equations i s :

2 C (h,r) - C . o o 2 r . -D h a . c -c 6 , 1 = E , I T j T Ü T V \ i r r ' - p p - 7 g,o g , i n=l n 1 n f v^R-ö,) '

in which J and J are Bessel functions of the f i r s t kind and, r e s p e c t i v e l y , of

20

the zero and f i r s t order, while a are the roots of the equation

W = 0 ( 1 6 )

The f i r s t eight roots are given i n Table 2.

a 1 = 2.4048 a = 5.5201 a 3 = 8.6537 a 4 = 11.7915 a g = 14.9309 a. = 18.0711 o a ? = 21.2116 a = 24.3525 o

Table 2 First eight eigenvalues

The bulk average value of C g(h,r) on a given height h i s :

H-Sf

C (h) = » r.C (h,r) dr (17) ( R - 6 f ) 2 J

From equations (15) and (17) the value of cg ( n ) i s obtained.

C g ( h ) _ C g i » 1 I a n v \ -S § ^ = 4Z — exp (--5—J (18) C - C n=l a V Gz ' S,o g , i n

in which Gz i s the Graetz number defined as

G z = ^ D T E ( 1 9 )

g

The average mass t r a n s f e r c o e f f i c i e n t k^ between the i n l e t and ou t l e t of the mass t r a n s f e r section may be defined i n terms of a logarithmic mean d r i v i n g force as follows:

_ _ (C -C .)-(C (h)-C .) -ir(R-Ô.) v (C -C (h)) = k 2Tf(R-6" ) h x — g ' ° 8 , 1 g (20) i s g.o g ft i r— *" -.(h) - C

- c o g , i

21

A f t e r s u b s t i t u t i o n of equation (18) i n equation (20) the average Sherwood

number can be written as:

2 G z o o 4 , a TT .

Sh = - - In Z=1 - exp ( - — - (2D IT a Gz n

r If the Graetz number i s la r g e r than about 150 the gas phase may be considered to be i n f i n i t e l y deep, and t h i s s i t u a t i o n corresponds with the penetration theory. Under t h i s condition the logarithmic average Sherwood number may be represented as:

Gz / — In I Sh g = e ) " * J

The deviation of equation (21) with equation (22) at Graetz numbers l a r g e r than

150 i s smaller than 6%.

2.5.3 Experimental

The absorbing l i q u i d ( d i s t i l l e d water, IN s u l f u r i c acid and 2N s u l f u r i c acid) containing 0.05% by weight "Teepol" was fed to the wetted wall column i n the same way as described f o r the CO^ absorption experiments. The experimental

equipment i s shown i n F i g . 4. Ammonia and nitrogen were supplied from c y l i n d e r s

and meted with flow c o n t r o l l e r s . The gas flow rates of ammonia and nitrogen were determined from the pressure drop across a c a l i b r a t e d s t a i n l e s s s t e e l c a p i l l a r y tubing immersed i n a t h e r m o s t a t i c a l l y c o n t r o l l e d water bath maintained at 20°C. A f t e r mixing the ammonia and nitrogen gas streams the gas

mixture was led i n co-current flow through the wetted wall column. The flow

rate of the gas mixture was chosen i n such a way that the average gas v e l o c i t y was equal to the surface v e l o c i t y of the l i q u i d f i l m . In our experiments the concentration of ammonia i n the ingoing gas stream was v a r i e d from 2% to 10% by volume. The concentration of ammonia i n the i n - and outgoing gas streams was

determined by adding 0.1N s u l f u r i c a c i d to a gas sample and then analysing on

NH + content by means of a c o l o r i m e t r i c method based on the Berthelot reaction 4 i n an Auto-Analyzer. The NH^ content of the i n - and outgoing l i q u i d was analysed i n the same way. In the experiments with water a known amount of 0.1 N s u l f u r i c a c i d was added to the l i q u i d sample to prevent desorption of ammonia. A mass balance around the ammonia absorption could be established, and the deviation was found to be l e s s than 1.5%.

22

r f n . T .

- g , " " V o - c

Fig'. 4 Expérimental set-up for ammonia absorption experiments in a wetted wall column.

2.5.4 Results

The absorption of ammonia into s u l f u r i c a c i d s o l u t i o n s i s accompanied with a fast chemical reaction i n the l i q u i d phase. The i n t e r f a c e concentration of ammonia i s equal to zero i f an increase of the a c i d i t y of the absorbing l i q u i d does not further increase the absorption rate. Experiments with water, IN s u l f u r i c a c i d and 2N s u l f u r i c a c i d showed no influence of the a c i d i t y of the absorbing l i q u i d on the absorption rate (see Table 3).

Hence i t was concluded that during the absorption of ammonia into 2N s u l f u r i c acid s o l u t i o n s the i n t e r f a c e concentration of ammonia i s equal to zero, and a further s i m p l i f i c a t i o n of equation (18) i s p o s s i b l e . The absorption rates i n 2N s u l f u r i c a c i d s o l u t i o n s were investigated as a function of the contact time values between gas and l i q u i d , and the r e s u l t s were compared with the t h e o r e t i c a l l y derived expressions. The contact time values between gas and l i q u i d were v a r i e d from 0.25 up to 1.2 seconds and the concentration of ammonia in the ingoing gas stream from 2.3% up to 10.5% by volume.

23

C g,o

% v o l

t o t a l pressure bar

V s

m/sec

C (h) g

C S,o

water 2 72 1 184 0 543 0.516

1NH 2S0 4 2 71 1 191 0 539 0.479

2NH oS0„ 2 4 2 76 1 201 0 537 0.505

water 4 85 1 095 0 399 0.424 1NH 2S0 4 4 84 1 096 0 400 0.430

2NH-S0. 2 4 4 93 1 111 0 401 0.415

Table 3 Influence of the acidity of the liquid phase on the absorption rate of

ammonia into water and sulfuric acid solutions at 20°C in a wetted

wall column with a height of 51.9 cm and with an inner diameter of

3.45 cm

Due to the absorption of ammonia from the gas phase into the l i q u i d phase the gas v e l o c i t y decreases. Therefore at Graetz numbers smaller than 100 excessively high ammonia concentrations i n the gas phase should be avoided, and the maximum ammonia concentration i n these experiments was therefore about 5% by volume.

In the experiments with 2N s u l f u r i c a c i d the stagnant l i q u i d f i l m above the re c e i v i n g l i q u i d was observed to be about 2.0 cm. For a highly soluble gas such as ammonia i n s u l f u r i c a c i d s o l u t i o n s , i t i s doubtful i f the absorption rate i n the stagnant l i q u i d f i l m may be neglected as was done i n the carbon dioxide absorption experiments.

From l i t e r a t u r e data [27,28] i t i s known that the evaporation rate of a stagnant water surface into d r i e d a i r flowing across t h i s surface i s strongly reduced by the addition of small amounts of surface a c t i v e agents. Long s t r a i g h t - c h a i n alcohols, for example, may reduce the evaporation rate to only 12% of the rate at a clean water surface. D.W. Thompson [29] found that small amounts of surface a c t i v e agents decrease the absorption rate of ammonia into water i n an u n s t i r r e d container. Experiments with 1-octadecanol and 1-hexa-decanol showed a reduction of r e s p e c t i v e l y 41% and 51% i n the absorption rate.

24

c g.o

% by volume

P bar

TT Gz

red

C g (h) C g.o

3 67 1 296 0 0131 0 .782 3.77 1 295 0 0134 0 .772 2 31 1 276 0 0136 0 .780

10 4 1 193 0 0167 0 739 5 35 1 189 0 0167 0 .731 2 45 1 208 0 0161 0 744

10 7 1 113 0 0194 0 700 5 83 1 143 0 0196 0 694

10 5 1 113 0 0234 0 683 4 72 1 107 0 0237 0 688 5 34 1 117 0 0225 0 690 5. 20 1 092 0 0262 0 653 5. 49 1 092 0 0256 0 658 4. 05 1 215 0 0391 0 593 5. 30 1 117 0 0567 0 541 4. 53 1 120 0 0571 0 527 2. 74 1 200 0 0621 0 503 2. 78 1 201 0 0626 0 507 4. 98 1. 111 0 0891 0 419 4. 88 1. 111 0 0908 0 411

Table 4 Absorption experiments of NH„ into 2N sulfuric acid solution at 20 C

Reasoning from the above-mentioned we assumed 75% of the stagnant l i q u i d f i l m i n the wetted wall column to be i n a c t i v e f o r the absorption of ammonia into s u l f u r i c a c i d (Ah = 1.5 cm), and the Graetz numbers were corrected f o r t h i s en e f f e c t . For the c a l c u l a t i o n of the Sherwood and Graetz numbers the gas phase d i f f u s i o n c o e f f i c i e n t of ammonia i n nitrogen was taken from the data given by Mason and Monchick [30].

-5 2 o (D = 2.3 x 10 m /sec at 25 C and 1.01325 bar). 3 2

25

Corrections f o r temperature and pressure deviations were c a r r i e d out using the r e l a t i o n of Wilke-Lee [31]. The experimental r e s u l t s are given i n Table 4.

The r e l a t i v e concentration of ammonia i n the gas phase as a function of TT/Gz r e d i s p l o t t e d i n F i g . 5.

The logarithmic mean value of the Sherwood number as a function of the Graetz number (Gz .) i s p l o t t e d i n F i g . 6. These fi g u r e s show that the red deviation of the measured points from the t h e o r e t i c a l l y predicted values i s small. At Graetz numbers of more than 150 the deviation from the penetration theory i s smaller than 6%.

Fig. 5 NH3 absorption experiments in a wetted wall column into 2N F.^S0^ at

20°C; comparison between experimental results and theory (

theoretical lines).

Symbol h (cm)

0 14.9

V 34.9

A 51.9

26

Fig. 6 Mean Sherwood number as a function of the Graetz number ( equation

(21), assymptotic solution).

Symbol h (cm)

0 14.9

A 34.9

V 51.9

2.6 CONCLUSIONS

A wetted wall column was developed i n which gas absorption with simultaneously occurring gas phase reactions such as the absorption of nitrogen oxides i n n i t r i c a c i d may be investigated. In t h i s wetted wall column a laminar f a l l i n g l i q u i d f i l m and a laminar plug flow of the gas phase without a v e l o c i t y gradient perpendicular to the g a s - l i q u i d i n t e r f a c e could be r e a l i z e d . The hydrodynamic behaviour of the l i q u i d f i l m was checked by absorbing COg i n t o water. It was found that the absorption rate was well predicted by the penetration theory. Gas phase mass tra n s f e r was inv e s t i g a t e d by absorbing

27

ammonia from a nitrogen gas stream into s u l f u r i c acid s o l u t i o n . The experimental

r e s u l t s show a good agreement with the Graetz model.

REFERENCES

1. Andrew, S.P.S. and Hanson, D., Chem. Eng. Sci., 1961, 14, 105.

2. H o f t i j z e r , P.J. and Kwanten, F.J.G., Absorption of n i t r o u s gases, from Nonhebel, G., Gas P u r i f i c a t i o n Processes f o r A i r P o l l u t i o n Control, Newnes-Butterworths, London, 1972.

3. Detournay, J.P. and Jadot, R.H. , Chem. Eng. Sci., 1973, 2_8, 2099. 4. Danckwerts, P.V., Gas-Liquid Reactions, McGraw-Hill, London, 1970. 5. N i j s i n g , R.A.T.O., PhD Thesis, D e l f t U n i v e r s i t y of Technology, D e l f t , The

Netherlands, 1957. 6. Kramers, H., B l i n d , M.P.P. and Snoeck, E., Chem. Eng. Sci., 1961, 14, 115.

7. Dekker, W.A., PhD Thesis, D e l f t U n i v e r s i t y of Technology, D e l f t , The

Netherlands, 1958. 8. Lynn, S., Straatemeier, J.R. and Kramers, H. , Chem. Eng. Sci., 1955, 4_, 49. 9. Lynn, S., Straatemeier, J.R. and Kramers, H. , Chem. Eng. Sci., 1955, 4_, 58.

10. Lynn, S., Straatemeier, J.R. and Kramers, H., Chem. Eng. Sci., 1955, 4, 63. 11. Wild, J.D. and Potter, O.E., J . Chem. E. Symposium Series, 1968, 28, 30. 12. Alper, E. and Danckwerts, P.V., Chem. Eng. Sci., 1976, 31, 599. 13. Kameoka, Y. and Pigf o r d , R.L., Ind. Eng. Chem. Fundam., 1977, 16, 163. 14. Govindan, T.S. and Quinn, I.A., AIChE J., 1964, 10, 35. 15. Danckwerts, P.V. and Kennedy, A.M., Chem. Eng. Sci., 1958, 8, 201.

16. Godfrey, J.H., PhD Thesis, Oregon State U n i v e r s i t y , U.S.A., 1973.

17. Sada, E., Kumazawa, H., Yamanaka, Y., Kudo, I. and Kondo, T., J. Chem. Eng.

Japan, 1978, 11, 276. 18. H i k i t a , H., Asai, S., Ishikawa, H. and Saito, Y., Chem. Eng. Sci., 1975, 30,

607. 19. Dekker, W.A. Snoeck, E. and Kramers, H. , Chem. Eng. Sci., 1959, 11̂ , 61.

20. Aihara, K., Ukawa, N., Hozawa, M. and Tadaki, T., Int. Chem. Eng., 1976, 16,

494.

21. H i k i t a , H. and Ishimi, K., J. Chem. Eng. Japan, 1976, 9_, 357. 22. ' H i k i t a , H. and Ishimi, K. , J. Chem. Eng. Japan, 1976, 9̂, 362. 23. P o r t a l s k i , S. and Clegg, A.J., Chem. Eng. Sci., 1971, 26, 773. 24. Banerjee, S., Rhodes, E. and Scott, D.S., Chem. Eng. Sci., 1967, 22, 43. 25. Byers, H.C. and King, J.C., AIChE J., 1967, l j ! , 628. 26. Byers, H.C. and King, J.C., AIChE J., 1967, 13̂ , 637.

28

27. Sherwood, T.K., Pigf o r d , R.L. and Wilke, C.R., Mass Transfer, McGraw-Hill, 1975.

28. Davies, J.T. and Rideal, T.K., I n t e r f a c i a l Phenomena, Academic Press, New York, 1963.

29. Thompson, D.W., Ind. Eng. Chem. Fundam. , 1970, j}, 243.

30. Mason, E.A. and Monchick, L. , J. Chem. Phys. , 1962, 36, 2746. 31. Reid, R.C., Prausnitz, J.M. and Sherwood, T.K., The Properties of Gases and

Liqu i d s , McGraw-Hill, 1977. 32. Carslaw, H.S. and Jaeger, J.C. Conduction of Heat i n So l i d s , Oxford

U n i v e r s i t y Press, 1959.

33. Lefers, J.B., Van den Bleek, CM. , Bos, A.S. and Van den Berg, P.J., paper presented at the 6th International Congress of Chemical Engineering, Chemical Equipment Design and Automation, Prague, August 1978.

34. Kaiser, E.W. and Wu, C.H., J. Phys. Chem., 1977, 81, 1701. 35. Kaiser, E.W. and Wu, C.H., J. Phys. Chem., 1977, 81, 187.

36. S t r e i t , G.E. and Wells, J.S., Fehsenfeld, F.C., Howard, C.J., J. Chem. Phys., 1979, 70, 3439.

37. McKinnon, I.R., Mathieson, J.G. and Wilson, I.R., J. Phys. Chem., 1979, 83, 779.

29

3. SPECTROPHOTOMETRY DETERMINATION OF NITROGEN OXIDES AND NITRIC ACID VAPOUR

3.1 INTRODUCTION

For p o l l u t i o n c o n t r o l purposes much at t e n t i o n has been paid to the determination of nitrogen oxides and several a n a l y s i s methods have been developed [1-5]. The disadvantage of most methods i s that they are not a p p l i c a b l e i n the higher concentration range which occurs i n the manufacture of n i t r i c a c i d . Infrared spectroscopy, however, can also be used f o r the determination of nitrogen oxides at higher concentrations. Infrared absorption c o e f f i c i e n t s of NOg and NO for p o l l u t i o n c o n t r o l have been measured as a function of the o p t i c a l path length and the temperature [6]. Guttman [7] i n v e s t i g a t e d integrated absorption i n t e n s i t i e s of pure N0„ and NO. at temperatures of 50°C up to 100°C and at pressures up to 2 MPa. The r e s u l t s of Guttman i n d i c a t e that Beer's law i s v a l i d . Often water vapour i s also present i n gas mixtures containing nitrogen oxides. Due to the reaction of nitrogen oxides with water vapour n i t r i c a c i d vapour and/or n i t r o u s acid vapour can be formed [8,9], e s p e c i a l l y at higher concentrations of nitrogen oxides. Very l i t t l e information, however, can be found i n the l i t e r a t u r e concerning the q u a n t i t a t i v e a n a l y s i s of NO, N0 2 > NgO^ and n i t r i c a c i d vapour i n such gas mixtures. Such information may be of importance f o r the manufacture of n i t r i c a c i d and p o l l u t i o n c o n t r o l purposes. Using i n f r a r e d absorption Fontanella [10] studied the concentration of NO at a wave number of 1915 cm 1 , of N0 2 at 1606 cm * and n i t r i c a c i d vapour at 1326 cm 1 i n the stratosphere using the sun as source. This method i s not a p p l i c a b l e at higher concentrations of N0 2 due to the strong overlap of n i t r i c a c i d vapour and the N0 2 absorption band.

In t h i s Chapter a method i s developed f o r the determination of NO, NOg, NgO^ and n i t r i c a c i d vapour i n gas mixtures at concentrations which occur in the manufacture of n i t r i c acid.

3.2 EXPERIMENTAL

A l l s p e c t r a l measurements were c a r r i e d out on a Perkin-Elmer Model 117 i n f r a r e d

30

spectrophotometer. The i n f r a r e d absorption gas c e l l was constructed of glass

with an inner diameter of 3.5 cm and a path length of 10.0 cm. S i l v e r c h l o r i d e

windows were cemented on the gas c e l l which was kept i n a l l experiments at a o

constant temperature of 25.0 C by a thermostat. The experimental apparatus f o r the sample preparation of n i t r i c oxide gas mixtures and nitrogen dioxide gas mixtures i s given i n F i g . 1. In the f i r s t step of sample preparation the experimental apparatus, i n c l u d i n g the gas c e l l , was flushed with dry nitrogen to remove the oxygen and then evacuated. In every experiment the evacuation was always checked by means of a mercury vacuum gauge. A f t e r evacuation valves 1, 2 3 and 5 were closed and n i t r i c oxide was l e d i n t o the system. No attempts were made to p u r i f y the n i t r i c oxide. (Matheson Gas Products, p u r i t y : 99,2%.) The small amounts of n i t r o u s oxide and nitrogen dioxide which are present i n commercial n i t r i c oxide were small enough to be neglected. A f t e r f i l l i n g the system with n i t r i c oxide the p a r t i a l pressure of n i t r i c oxide was measured with a d i f f e r e n t i a l manometer f i l l e d with bromo-naphthalene, i n which the s o l u b i l i t y of nitrogen oxides i s very low. The vapour pressure of bromo-naphthalene i s s u f f i c i e n t l y low that no o p t i c a l i n t e r f e r e n c e was found from i t s vapour. The gas c e l l was closed by valve 4, and a f t e r removing the n i t r i c oxide i n the sample container the gas sample c e l l was pressurised with dry nitrogen t i l l an absolute pressure of 0.1067 MPa was obtained.

The preparation of samples of N0 2 which i s i n e q u i l i b r i u m with NgO^ i n the same way as described for NO was found to be very inaccurate. Due to the strong temperature dependence of the above mentioned e q u i l i b r i u m [11] a l l temperatures in the experimental apparatus such as i n the gas-sample c e l l , the sample container and i n the d i f f e r e n t i a l manometer should be kept very constant. Small deviations i n the temperature cause large e r r o r s i n the c a l c u l a t i o n of the p a r t i a l pressures of NO^ and NgO^ with the e q u i l i b r i u m constant. Therefore NO^ and N 2 0 4 was prepared by o x i d i z i n g n i t r i c oxide supplied as described above with dry oxygen i n the gas sample c e l l at an absolute pressure of 0.1067 MPa. The oxidation of NO i s complete within a few minutes and the p a r t i a l pressures of N0 2 and NgO^ were then c a l c u l a t e d by means of the equi l i b r i u m constant given in Table 1. In t h i s way only the temperature of the gas sample c e l l should be kept accurately constant. The use of oxygen i n place of nitrogen for p r e s s u r i s i n g the gas sample c e l l had no influence on the i n f r a r e d absorption measurements.

N i t r i c a c i d vapours were prepared by bubbling d r i e d nitrogen gas through concentrated n i t r i c a c i d s o l u t i o n s (Merck a n a l y t i c a l grade) which were kept at a constant temperature of 20°C. To prevent condensation of the n i t r i c a c i d vapour and the water vapour the gas stream was then heated to 25.0°C and

31

continuously l e d through the gas c e l l . The presence of some water vapour d i d not influence the i n f r a r e d absorption measurements. The n i t r i c a c i d concentrat i o n i n the nitrogen gas stream was changed by varying the concentration of the n i t r i c a c i d s o l u t i o n . The concentration of n i t r i c a c i d vapour i n the gas stream was determined by adding a known amount of 0.1 N a l k a l i to a gas sample and then analysing on the n i t r a t e and n i t r i t e content with a c o l o r i m e t r i c method. In t h i s method the n i t r a t e i s reduced to n i t r i t e by a copper-cadmium reductor column. The n i t r i t e ion then reacts with sulfanilamide under a c i d i c conditions to form a diazo compound. This compound was coupled with N-l-naphthylethylene-diamine dihydrochloride to form a reddish-purple azo dye [12]. In the samples the n i t r i t e concentrations were always very small compared to the n i t r a t e concentration ( l e s s than 0.1% of the n i t r a t e content).

NO-

t o t h e r m o s t a t

O , Np

t o v a c u u m p u m p

U J

Fig. 1 Experimental set-up for sample preparation.

A: sample container; B: molecular sieves; C: differential manometer

f i l l e d with bromo^naphthalene; D: infrared sample gas cell; E: mercury

manometer; F: vacuum gauge; 1, 2, 3, 4, 5 valves.

32

3.3 RESULTS

Infrared absorption c o e f f i c i e n t s can be determined by using Lambert-Beer's law. For our purposes at constant temperature the absorbance (A) can be written as:

A = log — = a.b.P (1) o

A = Absorbance 1,1 = the re s u l t a n t and incident i n t e n s i t i e s o a = a b s o r p t i v i t y MPa .cm b = path length cm P = p a r t i a l pressure MPa bP = o p t i c a l path length MPa.cm

Of each component (NO, N0 2, Ng0 4 and n i t r i c a c i d vapour) c a l i b r a t i o n curves were prepared at wave numbers at which no absorbance of other components were found. The absorbance of each component was determined with the base-line method. C a l i b r a t i o n curves for NO at a wave number of 1908 cm 1 f o r high o p t i c a l path lengths and low o p t i c a l path lengths (< 0.015 MPa.cm) are presented i n F i g . 2 and F i g . 3. Least square f i t s to the experimental points i n d i c a t e that Beer's law i s only v a l i d at low o p t i c a l path lengths (< 0.015 MPa.cm). At higher o p t i c a l path lengths the i n f r a r e d absorbance f a l l s o f f as the square root of the o p t i c a l path length. Campani et a l [6] found a much lower absorbance f o r NO at 1908 cm * compared with the r e s u l t s presented here. This deviation may be due to the higher r e s o l u t i o n at our i n f r a r e d absorption measurements.

C a l i b r a t i o n curves of N0 2 at 2908 cm" and N2 0 4 at 2960 cm - 1 and at 3120

cm are given i n F i g s . 4, 5 and 6. Regression a n a l y s i s shows that Beer's law i s v a l i d and that the deviations from the o r i g i n of the 2908 cm * NOg peak and

the 3120 cm ^2^4 p e a k a r e s m a H - The d e v i a t i o n from the o r i g i n of the 2960 cm ^ ^2^4 P e a k m a y be due to the small overlap of the N0g band and the Ng0 4

band. A d i r e c t comparison of these r e s u l t s with the measurements of Guttman [7] o

i s d i f f i c u l t since his r e s u l t s were measured at temperatures of 50 C up to 100°C. Neglecting the influence of the temperature on the absorbance at 50°C the r e s u l t s presented here agree well with those of Guttman [7].

It should be noted that at high concentrations of n i t r i c a c i d vapour ( o p t i c a l path length > 0.003 MPa.cm) i n gas mixtures containing N0„ and NO a

^ ^ £ 4 small overlap of the 2960 cm N2°4 a°sorption band with a weak n i t r i c a c i d

33

band was observed. Therefore the determination of i n t h e P r e s e n c e ot high n i t r i c a c i d vapour concentrations should be c a r r i e d out with the 3160 cm ^ " 2 ^ 4

absorption peak or c a l c u l a t e d from the e q u i l i b r i u m constant at known NOg concentrations (see Table 1).

The c a l i b r a t i o n curve of n i t r i c a c i d vapour of the 895 cm * peak i s presented i n F i g . 7. From l e a s t square f i t s i t can be concluded that Beer's law i s v a l i d and that the deviation from the o r i g i n i s small.

Fig. 2 Calibration curve of NO at 1908 cm 1 (25.0° C, 0.1067 MPa).

(A = 1.3918 PNQ.b - 0.065.)

34

Fig. 3 Calibration curve of NO at 1908 am~ for low concentration (25.0°C,

0.1067 MPa).

(A = 7.0186 PNQ.b + 0.00210.)

In order to study the accuracy of t h i s method gas mixtures containing NO, NOg, NgO^ and n i t r i c a c i d vapour were analysed. These gas mixtures were prepared by p a r t i a l oxidation of a known amount of NO, dosed as described above, with a i r i n the presence of water vapour i n the gas sample c e l l . A f t e r the p a r t i a l oxidation the gas sample c e l l was pressurised with nitrogen to 0.1067 MPa. The following reactions and e q u i l i b r i a may occur i n the gas sample c e l l :

2N0 + 0 2 •+ N0 2 (2)

NO + N0 2 t N 2 0 3 (3)

2N0 2 $ N 2 0 4 (4)

3N0 2(N 20 4) + H 20 X 2HN0 3(g) + NO (5)

NO + N0 2 + H 20 X 2HN0 2(g) (6)

o The e q u i l i b r i a constants at 25 C are given i n Table 1. The concentrations of NO, NOg, N 2 0 4 and n i t r i c a c i d vapour i n the gas sample c e l l were determined a f t e r the oxidation with i n f r a r e d absorption using the c a l i b r a t i o n curves. The small amounts N 2 0 3 which are present i n these gas mixtures were c a l c u l a t e d using the equi l i b r i u m constant. A representative record of the i n f r a r e d absorption spectrum of such a gas mixture i s given i n F i g . 8.

35

0 - 2 0

0 - 1 5

0 1 0

< 0 0 5

0 0 1 0 0 2 0 0 3

O p i c a l p a t h l e n g t h M P o . c m

0 0 4

Fig. 5 Calibration curve of N 0 at 2960 cm 1 (25.0°C, 0.1067 MPa). 2 4 (A = 5.794 7 b- 0.01245.)

2 4

jQ <

0 - 15 -

0 - 10 -

0 0 5 -

0 0 1 0 0 2 0 0 3

O p t i c a l p a t h l e n g t h M P a . c m

0 0 4

Fig. 6 Calibration curve of N90. at 3120 cm 1 (25.0°C, 0.1067 MPa).

(A - 3.902 P N2°4

2^4 b - 0.00194.)

37

• O p t i c a l p a t h l e n g t h M P a c m

Fig. 7 Calibration curve of n i t r i c acid vapour at 895 crn1 (25.0°C, 0.1067 MPa).

(A - 71.897 PUMn .b - 0.0153.)

E q u i l i b r i a E q u i l i b r i u m constant Value at 25°C Reference

3N0 2+H 20 X 2HN03+N0 K ^ l

P 2 .P HNO NO

P 3 .P N 0 2 »2°

0.00130 MPa"1 [18]

2N0 2 X N2°4 \ P N 2 ° 4 2

p N0 2

0.654 MPa _ 1 [11,13]

NO + N0 2 X N2°3 \ P n 2 ° 3

P .P NO N0 2

0.0517 MPa"1 [13]

N02+N0+H20 X 2HN02 \ P 2

HN02

P .P .P NO N0 2 H 20 0.140 MPa"1 [14-17]

Table 1 Equilibrium constants

3 8

wavenumber (cm* 1) 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1S00 1600 1400 1200 1000 8 0 0 6 0 0

~i r

N 2 0 . .

NoO,

N 2 0

u Fig. 8 Infrared absorption spectrum of nitrogen oxides and nitric acid vapour.

Small amounts of water vapour d i d not produce serious o p t i c a l i n t e r f e r e n c e . The i n i t i a l amount of NO supplied before the r e a c t i o n was compared to the amount of nitrogen oxides and n i t r i c a c i d vapour a f t e r r e a c t i o n had occurred i n the gas sample c e l l and e q u i l i b r i u m had been attained. From Table 2 i t can be concluded that the deviation i n the mass balance i s small. In the spectra an absorption band of n i t r o u s a c i d vapour at 850 cm ^ [19] was sometimes found. The occurrence of the absorption band of nitrous a c i d can be explained by equi l i b r i u m (6) and i t s presence i s a function of the r a t i o of the NO concentration and the N0 2 concentration i n the gas sample. N i t r i c a c i d vapour was only detected i n the gas sample at rather low NO concentrations (see Table 2).

3.4 CONCLUSIONS

A method has been developed f o r the determination of NO, NOg, ^2

04 a n d n i t r i c

a c i d vapour i n gas mixtures by means of i n f r a r e d spectroscopy which may be

important for the manufacture of n i t r i c a c i d and f o r p o l l u t i o n c o n t r o l . The following i n f r a r e d absorption peaks can be used: f o r NO: 1908 cm , f o r N0„:

2908 cm ; f o r N O : 2980 cm 1 or 3160 cm 1 and f o r n i t r i c a c i d vapour: 895 -1 2 4

cm . At rather high concentrations of n i t r i c a c i d vapour i n gas mixtures containing nitrogen oxides the 2980 cm 1 N

2 ° 4 absorption band shows a small overlap with a weak n i t r i c acid band. Therefore the determination of N 20 4 i n the presence of high concentrations of n i t r i c acid vapour should be c a r r i e d out

39

O No. PN0' s u p p l i e d After p a r t i a l oxidation % Deviation

kPa P NO

kPa kPa PN0 kPa

p HNC

kPa 3

P N 2 ° 3 kPa

i n mass balance

(2908 cm"1) (3160 cm - 1) (1908 cm - 1) (89E cm

1 12.67 4 80 1.51 5.27 0 131 5 4 1 12.67 4 80 1.51 5.27 0 131

2 11.85 5 95 2.48 0.771 0. 091 0 024 -0 3

3 12.21 6 12 2.60 0.817 0. 091 0 026 0 6

4 13.37 5 45 1.93 3.80 *) o 107 -0 3 4 13.37 5 45 1.93 3.80 107 -0

5 11.48 6 12 2.53 0.423 0. 111 0 013 2 3

10.91 3 00 0.56 6.27 *) 0 097 _3 o 6 10.91 3 00 0.56 6.27 *)

0 097

7 12.57 4 07 1.01 6.40 *) 0 135 1 5 7 12.57 4 07 1.01 6.40 0 135

8 10.87 5 60 2.47 0.147 0. 156 0 004 -0 2

g 12.07 3 68 0.85 6.64 *) 0 126 1 7 g 12.07 3 68 0.85 6.64 0 126

nitrous a c i d vapour found i n the gas sample.

Table 2 The amount of nitrogen oxides and nitric acid vapour found after partial oxidation of NO in the

presence of water vapour

with the 3160 cm absorption peak or c a l c u l a t e d from the equilibrium constant

at known N0 2 concentration.

In gas mixtures containing nitrogen oxides and water vapour also nitrous a c i d vapour could be detected, e s p e c i a l l y at high NO-concentrations compared to the NOg concentration.

REFERENCES

1. Saltzman, B.E. and Cuddeback, J.E., Anal. Chem. , 1975, 47, 1. 2. Saltzman, B.E. and Burg, W.R., Anal. Chem., 1977, 49, 1. 3. A l l e n , J.D. and P h i l , M., J. Inst. Fuel, 1973, 46, 123.

4. Lievens, F., Rapp. Cent. Étude Energ. Nucl. B.L.G., 1973, 480.

5. Forweg, W., V.D.I.Ber. (Ver. Dtsch. Ing.), 1974, 24, 247. 6. Campani, P., Fang, C.S. and Prengle, H.W., Appl. Spectroscopy, 1972, 26,

372. 7. Guttman, A., J. Quant. Spectrosc. Radiât. Transfer, 1961, 2, 1.

8. England, C. and Corcoran, W.H., Ind. Eng. Chem. Fundam. , 1974, 13, 373.

9. England, C. and Corcoran, W.H., Ind. Eng. Chem. Fundam., 1975, 14, 55. 10. Fontanella, J.C., O f f i c e s National d'Etudes et de Recherches Aérospatiales

1974, Note technique no. 235. 11. Bodenstein, M. and Boës, F., Z. Physik. Chem., 1922, 100, 68. 12. Technicon Auto-Analyzer II, I n d u s t r i a l Method No. 230-72A/Tentative 1974.

13. Hisatsune, I.C., J. Phys. Chem., 1961, 65, 2249. 14. Ashmore, P.G. and Ty l e r , B.J., J. Chem. Soc, 1961, 1017. 15. Wayne, L.G. and Yost, D.M., J. Chem. Phys., 1951, 19, 41.

16. Karavaev, M.M, and Skvortsov, G.A., Russian Journal of Physical Chemistry,

1962, 36, 566. 17. Waldorf, D.M. and Babb, A.L., J. Chem. Phys., 1963, 39, 432. 18. Nonhebel, G., Gas P u r i f i c a t i o n Processes f o r A i r P o l l u t i o n Control, Newnes-

Butterworths, London, 1972. H o f t i j z e r , P.J. and Kwanten, F.J.G., Absorption of n i t r o u s gases.

19. Jones, L.H., Badger, R.M. and Moore, G.E., J. Chem. Phys., 1951, 19, 1599.

41

4. THE ABSORPTION OF N0 2/N 20 4 INTO DILUTED AND CONCENTRATED NITRIC ACID

4.1 INTRODUCTION

The absorption of nitrogen dioxide into n i t r i c a c i d s o l u t i o n s i s very important fo r the production of n i t r i c acid and the subsequent stack gas problems. In the l i t e r a t u r e many i n v e s t i g a t i o n s can be found concerning the absorption of NO^/ NgO^ gas mixtures into water. K i n e t i c data concerning the absorption of nitrogen dioxide into n i t r i c a c i d are, however, of much greater importance f o r the design of i n d u s t r i a l absorbers. No r e l i a b l e data were found i n the l i t e r a t u r e .

In t h i s Chapter the absorption mechanism of HO^/H^O^ gas mixtures into d i l u t e d and concentrated n i t r i c a c i d i s investigated i n the previously described wetted wall column. Moreover the e q u i l i b r i a data of the system NgO^-HNOg-HgO w i l l be c r i t i c a l l y discussed.

4.2 REVIEW OF LITERATURE

4.2.1 Absorption of NOg/NgO^ into aqueous s o l u t i o n s

The major re a c t i o n by which n i t r i c a c i d i s formed i n i n d u s t r i a l n i t r i c a c i d absorbers to produce d i l u t e d n i t r i c a c i d can be represented by the rea c t i o n of NO„, which i s i n equili b r i u m with NO., with water.

2N0 2 (N 20 4) + H 20 •* HN0 3 + HNOg (1)

In a c i d s o l u t i o n s the n i t r o u s a c i d may decompose:

3HN0„ X HNO„ + 2N0 + H o0 (2)

The o v e r a l l r e a c t i o n can then be written as:

3N0 2 (N 20 4) + H 20 •> 2HN03 + NO (3)

42

Reaction (3) may also proceed i n the gas phase and n i t r i c a c i d vapour or mist can be produced [1,2,3]. L i t t l e i s known concerning the n i t r i c a c i d mist formation i n the gas phase.

Detournay and Jadot [4] found that under normal conditions the n i t r i c a c i d formation i n the gas phase may be neglected compared to the n i t r i c a c i d formation i n the l i q u i d phase.

Equi libria

The most important e q u i l i b r i u m f o r the n i t r i c a c i d formation i s given by the o v e r a l l r e a c t i o n (3). This equation determines the maximum n i t r i c a c i d concentration that can be obtained at a given composition of the ni t r o u s gases:

P 2 P HNO NO K p 3 (4)

1 P P H 2 ° N ° 2

The value of the equili b r i u m constant i s given i n Table 1. The same d e f i n i t i o n may be applied to the heterogeneous equilibrium and

then P„„_ and P are the vapour pressures of HN0_ and H„0 over the l i q u i d HNU3 "20 o £

phase. The vapour pressure of HNOg and HgO over n i t r i c a c i d s o l u t i o n s may be taken from the data of the binary system HNOg/HgO. Therefore i t i s p r a c t i c a l to define the following equilibrium constants:

\-¥ 5 p

N0 2

P 2

HNO K p = " p - 1 (6)

6 H 2 ° Measurements of the equili b r i u m constant K p as a function of the n i t r i c a c i d strength and the temperature can be found i n the l i t e r a t u r e [ 5 , 6 , 7 ] .

According to Carberry [5] K and K should be based on N-0. which i s P i P5

generally accepted to be the active species during the absorption i n t o d i l u t e d n i t r i c a c i d .

\ p3/2 ( ? )

N2°4

Carberry [5] c o r r e l a t e d several l i t e r a t u r e data and found that K p was i n

dependent of the temperature.

4 3

E q u i l i b r i a E q u i l i b r i u m constant (bar 1) Reference

H N 0 3 N ° , „ 1 n - 9 ,4644, ' r , Q l

3N0 Q + H o0(g) X 2HN0„(g) + NO K = = 1.75 x 10 exp <-=—) [39] 2 2 3 P l P d .P

N 0 2 H 2 °

P 2N0 2 $ N 2 0 4 K p = = exp ( ^ S L _ 2 1 . 2 4 4 ) [37,38]

2 P NO 2

P NgOg q 4869

NO, + NO $N,0, K = = p = 41.82 x l o " ' exp (-^-) [37] 2 Z d 3 NO N0 2

P 2

H N 0 2 -6 4723 ,., NO„ + NO + H.O X 2HN0„ K = p p = 0.185 X 10 exp ( — — ) [1J

2 2 * P4 NO' NOg' HO

Table 1 Equilibrium constants of the reactions of nitrogen oxides

log K = 7.412 - 20.28921 x w + 32.47322 x w2 - 30.87 x w3 (8) P 7

i n which w i s the weight f r a c t i o n of HNO^. The vapour pressures of HNOg and H^O were measured by Vandoni and Laudy [8]

and can be described by the Margules-Duhem equation [9].

Mechanism of NO^/Ncfi^ absorption into aqueous solutions

The absorption rate and mechanism of N0 2 which i s i n equilib r i u m with N^O^ into water have been studied by several authors [1,10-18,23]. It has been generally accepted that i f a NO^/NgO^ gas mixture i s absorbed i n t o aqueous solutio n s NgO^ i s the ac t i v e species and p h y s i c a l l y d i s s o l v e s i n the water. A f t e r d i s s o l u t i o n the NgO^ reacts with water to form n i t r i c acid. The following reaction scheme represents t h i s :

2N0 2(g) J N 20 4(g) (9)

N 2 0 4 t N 2 0 4 ( £ ) (10)

N2°4 + H 2 ° * H N 0 3 + H N 0 2

The equi l i b r i u m represented by equation ( 9 ) i s so r a p i d l y established that i t may be assumed that NgO^ and N0 2 are continuously i n equili b r i u m with each other [19]. Applying the theory of mass t r a n s f e r with a rap i d pseudo f i r s t order r e a c t i o n i n the l i q u i d phase the absorption rate can be written as:

If NOg/NgO^ i s absorbed i n t o aqueous solutio n s from an i n e r t gas stream the gas phase mass t r a n s f e r i s also important. N0 2 and N 2 0 4 are tr a n s f e r r e d , i n continuous e q u i l i b r i u m with each other, from the gas bulk to the g a s - l i q u i d i n t e r f a c e .

The value of k as defined i n equation (12) does incorporate the molar

concentration of water. Values of ^ )/"^J?, ^ o r w a t e r have been measured with laboratory absorbers. Table 2 gives a review of these i n v e s t i g a t i o n s . From t h i s table i t can be concluded that the values of H JkD„ agree rather well with

2 4' one another.

The r e a c t i o n rate constant k was c a l c u l a t e d from these values with the

45

10 3 X H „ \/kD„ N 20 4 V I k sec Method of measurement

2 kmol/m .sec.bar.

20°C 25°C 30°C 20°C 25°C 30°C

Caudle and Denbigh [11] 1.09 506 absorption Wendel and Pigf o r d [13] 0.57 138 absorption Dekker [14] 1.09 506 absorption Kramers et a l [15] 0.76 0. 88 250 330 absorption Corriveau [18] 0.56 136 absorption Kameoka and P i g f o r d [16] 0.68 195 absorption H o f t i j z e r and Kwanten [1] 0.92 361 absorption Gerstacker 1.0-1.1 >490 absorption Discussion at r e f . [15] Moll [20] 267 l i q u i d N0 2

i n j e c t i o n into water T r e i n i n and Hayon [21] 300+100 f l a s h photolysis Komiyama and Inoue [16] 194 desorption

—9 2 D = 1.41 X 10 m /s at

2°4,1 3

H =1.29 kmol/m .bar at 2 4

25°C "» —9 2 D = 1.41 X 10 m /s at

2°4,1 3

H =1.29 kmol/m .bar at 2 4

25°C J Kramers et a l [15]

Table 2 Comparison of literature data concerning the absorption of Np0. into water

d i f f u s i o n c o e f f i c i e n t and Henry c o e f f i c i e n t found by Kramers et a l [15]. The

agreement between the reaction rate constant derived from the absorption

measurements i s rather poor. Moll [20] i n j e c t e d l i q u i d NÔ^ i n t o water and the rea c t i o n rate constant k

agreed well with the value measured by Kramers et a l [15] with laminar j e t

experiments. T r e i n i n and Hanson [21] found roughly the same values by means of

f l a s h p h o t o l y s i s . In i n d u s t r i a l absorber design the value of H„ „ i/kD. i s much N 2 0 4 V I

more important than the reaction rate constant. H o f t i j z e r and Kwanten [1] found the following equation f o r water:

l 0 g V o . l / 5 * = " 0.53 -760 (3° - 75°C) (13)

2 4

It i s known that the absorption rate of NÔ^ decreases with increasing a c i d strength. This may be a t t r i b u t e d to the decrease of the Henry c o e f f i c i e n t with increasing i o n i c strength and a decrease of k as the molar concentration of " f r e e " water becomes r e l a t i v e l y small i n more concentrated n i t r i c a c i d [1].

No r e l i a b l e data can be found i n the l i t e r a t u r e concerning the influence of the n i t r i c a c i d strength on the H i / kD. values.

N2O4V Jo Longstaff and Singer [24] found that i f N0 2 gas i s i n contact with 60%

n i t r i c a c i d unreacted NÔ^ may be present i n the l i q u i d phase and that the nit r o u s a c i d concentration may be neglected. The r a t i o C /(C +C )

N2°4,J> H N 0 2 , J l N20 4,Jo in the l i q u i d phase as a function of the n i t r i c a c i d strength given i n F i g .

1 0

0 5

50 100

1 N 0 3

Fig. 1 The distribution of ^2^4 m ^ ^n ^ t r i e acid solutions as a function

of the nitric aoid strength.

47

From t h i s f i g u r e i t can be concluded that the ph y s i c a l absorption of N0 2/

"2^4 ^ a s m l x * u r e s into n i t r i c a c i d s o l u t i o n s becomes important f o r n i t r i c a c i d s o l u t i o n s above 55% and that the r e a c t i o n of N„0. with water may be neglected.

z 4

4.2.2 NCvj/NgO^ absorption i n t o concentrated n i t r i c acid s o l u t i o n s

The absorption of NO„/N 0 into n i t r i c a c i d s o l u t i o n s of more than 55% i s £ £ 4 important f o r the production of d i l u t e d n i t r i c acid as well as the production of concentrated n i t r i c acid. In the l i t e r a t u r e very l i t t l e information can be found concerning the absorption of N0 2/N 20 4 gas mixtures i n t o concentrated n i t r i c a c i d . Atroshchenko and Kaut [26] inv e s t i g a t e d the absorption of NOg/NgO into 70 - 98% HNOg and found that the absorption proceeds purely p h y s i c a l l y . This f i n d i n g was also confirmed by Karavaev and Visloguzova [25]. In the l i t e r a t u r e no information can be found concerning the absorption mechanism.

Equilibrium

The s o l u b i l i t y of NgO^ into concentrated n i t r i c a c i d s o l u t i o n s i s very important for the design of i n d u s t r i a l absorbers. In the l i t e r a t u r e only values of the t o t a l vapour pressure of the system N^-HNOg-HgO were found [27,28,29,30]. In order to study the s o l u b i l i t y of NgO^ into n i t r i c a c i d s o l u t i o n s i t was assumed that t h i s t o t a l vapour pressure can be described as:

P = P + P + P + P + P „ + P (14) tot HNO3 H 20 N0 2 N 2 0 4 N 2 0 3 NO

P„„ and P„ _ are very low above concentrated n i t r i c acid s o l u t i o n s and there-NO N 2 0 3 * fore they may be neglected. The vapour pressures of HNO^ and HgO were taken from the data of the binary system HNO^-HgO measured by Vandoni and Laudy [8].

2 With a i d of the eq u i l i b r i u m constant (K = P / P H n ' o f t h e e q u i l i b r i u m

2 N2 4 2

2N0 2 X N 2 0 4 (15)

the vapour pressures of N0 2 and N2 0 4 were c a l c u l a t e d .

If i t i s assumed that mainly N2 0 4 i s present i n the l i q u i d phase [27,29,30,

31,32,33] the Henry c o e f f i c i e n t H„ „ should be defined as: N2°4

° N2°4 I

48

Henry c o e f f i c i e n t s (H„ _ ) were c a l c u l a t e d from the t o t a l vapour pressure data N 2 ° 4

of Klemenc and Rupp [28] and the r e s u l t s are given i n F i g . 2 and F i g . 3. From these f i g u r e s i t can be concluded that the Henry c o e f f i c i e n t i s independent of the amount of NgO^ i n the l i q u i d phase. At small NgO^ contents some deviations

2 0

15

10

o 6

S CM

V V — v — V

o

o o o ri

o u o

~~ A. A A A A. A A a. û

5 10 15

°/o N 2 0 4 by w e i g h t in H N O 3

0 ° C

12 5 °C

2 5 ° C

20

Fig. 2 Hjy Q as a function of the FS^O^ content in 16 N HNO^.

(V: 0°C; 0: 12.5°C; A: 25°C) [28].

From the t o t a l vapour pressure data measured by Weinreich [27] and Karavaev and Yarkovaya [29] Henry c o e f f i c i e n t s were c a l c u l a t e d as a function of the temperature and the ac i d strength (see F i g . 4). Some own experiments were c a r r i e d out to i n v e s t i g a t e the Henry c o e f f i c i e n t more d i r e c t l y . N i t r i c a c i d of 75% was saturated with nitrogen gas containing 5-20 volume % of N0 2 by means of a saturator at a pressure of 1.04 bar. The gas phase was analysed f o r i t s N

20 4 _

content with i n f r a r e d spectroscopy (Chapter 3). The NgO^-content i n the l i q u i d phase was determined by i n j e c t i n g a l i q u i d sample of 50 y l in t o a 10 ml 0.8 N NaOH s o l u t i o n and then analysing f o r n i t r i t e content with a c o l o r i m e t r i c method [34]. It was found that the measured Henry c o e f f i c i e n t was independent

49

of the p a r t i a l pressure of N^O^ i n the gas phase. The experimental r e s u l t s are

summarized i n Table 3.

30

25

20 -

15

! io o

E

3

o °c

-o o o o o-

A A C ft-

12 • 5 °C

o o

25 ° C

5 10 15 %> N 2 0 4 b y w e i g h t i n HN0 3

20 25

Fig. 3 H n as a function of the N„0. content in 19 N nitric acid. "24 (V; 0°C; 0: 12.5°C; A: 25°C) [28].

5 0

0-30

Fig. 4 H - as a function of the temperature and n i t r i c acid concentration. a2 4

a Weinreich [27] 25% by weight of Ng04 in 75% HN03

+ Weinreich [27] 20% by weight of ffg0^ in 75% HN03

0 Weinreich [27] 10% by weight of N204 in 75% HNO^

A Klemenc and Rupp [28]

• Karavaev and Yarkovaya [29]

0 This work

From F i g . 4 i t can be seen that there i s a rather good agreement with the re s u l t s derived from the vapour pressure data.

It should be noted that at a rather high N^O^ content i n the l i q u i d phase and low temperatures l i q u i d i m m i s c i b i l i t y may occur [31,32,33].

The heat of s o l u t i o n of N O into n i t r i c a c i d s o l u t i o n s can be c a l c u l a t e d

51

from:

d i n (H ) AH g

_ _ * J L = ( 1 7 )

d (^) R

in which T i s the absolute temperature, R the gas constant and AH g the heat of so l u t i o n at the temperature considered (taken as negative). In a f i r s t approximation AHg may be assumed to be constant over a small range of temperature. The r e s u l t s for d i f f e r e n t n i t r i c a c i d strengths are given i n Table 4.

From t h i s table i t can be seen that the heat of s o l u t i o n of NgO^ in t o n i t r i c a c i d s o l u t i o n s i s rather constant f o r d i f f e r e n t a c i d strengths.

Temperature (°C) H Measured range ™ 2 4 kmol/m 3.bar bar P

20.3 7 0 0 0106 - 0 0611

25.2 6 3 0 008 - 0 05

35.2 4 9 0 006 - 0 04

45.3 3 4 0 003 - 0 007

Table 3 Henry coefficient of N204 in 75% n i t r i c acid as a function

of the temperature

% HN03 AH g (kJ/kmol N 20 4)

65 - 23 2 X i o 3

70 - 25 3 X i o 3

75 - 27 2 X i o 3

19 N - 25 3 X i o 3

Table 4 Heats of solution of Np0. into concentrated n i t r i c acid solutions

52

4.3 EXPERIMENTAL

The experimental apparatus i s schematically presented i n F i g . 5. N i t r i c a c i d s o l u t i o n s were pumped to an overhead r e s e r v o i r and then fed by g r a v i t y to the wetted wall column described i n Chapter 2. About 0.05% by weight of an a l k y l sulphonate was added to the l i q u i d i n order to eliminate r i p p l e s of the l i q u i c f i l m . The f i l m height h was corrected for the end e f f e c t caused by the presence of the surface act i v e agent and the e f f e c t i v e f i l m height h' was i n these experiments 13.7 cm and 34.6 cm. Nitrogen dioxide ( b o i l i n g point: 21.2°C) was supplied from a c y l i n d e r immersed i n a water bath at 47.5°C. To avoid condensation the pipes containing pure nitrogen dioxide were heated with resistance heating wire (Pyrotenax Ltd., Hebburn-on-Tyne, England) and ins u l a t e d with glass wool. The N0 2 gas stream was meted with a needle valve immersed i n a th e r m o s t a t i c a l l y c o n t r o l l e d water bath of 50°C and then mixed with a nitrogen gas stream. The gas mixture was l e d i n co-current flow through the wetted wall column i n the same way as was described i n Chapter 2. The gas mixture leaving the wetted wall column was scrubbed with an a l k a l i n e hydrogen peroxide s o l u t i o n the remove the nitrogen oxides before venting i t to the atmosphere. The n i t r i c a c i d leaving the wetted wall column was stored i n a s t a i n l e s s s t e e l v e s s e l . In the experiments with 63% and 78% the n i t r i c a c i d s o l u t i o n was stripped with nitrogen to remove the di s s o l v e d N„0^ from the n i t r i c a c i d . The st r i p p e d n i t r i c a c i d was then recycled. In the experiments with 25% and 40% n i t r i c acid, the n i t r i c a c i d l e a v i n g the wetted wall column was drained o f f .

The i n - and outgoing l i q u i d were analysed for t h e i r NgO^ and/or HNOg content by i n j e c t i n g a l i q u i d sample (50 - 250 y l ) into a 10 mol 0.8 N NaOH so l u t i o n . A f t e r reaction the n i t r i t e content was determined with a c o l o r i m e t r i c method [34]. Note that with t h i s method HN02 and N 20^ i n the n i t r i c a c i d samples can not be distinguished. The outgoing gas streams were analysed f o r t h e i r NOg, N

2 0 4 and NO content with i n f r a r e d spectroscopy (Chapter 3). The concentration of N0 2 and N

2 ° 4 i n t n e ingoing gas stream was c a l c u l a t e d by e s t a b l i s h i n g a mass balance around the wetted wall column.

53

to air

Fig. 5 The experimental set-up for the NO^ and NO absorption experiments into

nitric acid solutions.

1: wetted wall column; 2: stripper; 3: scrubber; 4: stainless steel

vessel; 5: stainless steel vessel for stripped nitric acid solution;

6: vessel for alkaline hydrogen peroxide; 7: calibrated glass pipe;

8: rotameter; 9: overhead reservoir; 10: flow controllers; 11: stain

less steel filter; 12: cyclone; 13: thermostatically controlled water

bath; 14: needle valve; 15: infrared gas sample cell.

54

4.4 RESULTS

4.4.1 The absorption of N 20 4 into d i l u t e d n i t r i c a c i d s o l u t i o n s

According to the model NO^ and NgO^ are transferred, i n continuous equi l i b r i u m

with each other, from the gas phase to the g a s - l i q u i d i n t e r f a c e . The d i f f u s i o n

of a N0 2/N 20 4 mixture i n the gas phase was regarded as the d i f f u s i o n of one

f i c t i t i o u s component Q defined as:

CQ = X + 2 C N n 4 ( 1 8 )

2,g 2 4,g

The gas phase d i f f u s i o n can under our measured conditions be described by the

Graetz model (Chapter 2) and the d i f f u s i o n rate of N0 2 from the gas phase to

the g a s - l i q u i d i n t e r f a c e per unit of surface area can be written as:

n Q

The d i f f u s i o n c o e f f i c i e n t of the f i c t i t i o u s component Q as a function of D 2

and D N ^ was derived by Dekker [35].

D = D + _ - (20) Q N 2 ° 4 Jl + 8K P„ . + ./ 1 + 8K P / l + 8K P„ . + ,/" P 2 Q,o

-5 2 o D„„ = 1.36 x 10 m /s at 20 C and 1.0132 bar N0 2

D = 0.96 x 10~ 5 m2/s at 20°C and 1.0132 bar 2 4

i n which P_ . i s the p a r t i a l pressure of N0„ + 2N„0„ at the i n t e r f a c e and P„ Q,i t- t- 2 2 4 Q,o

i s the p a r t i a l pressure i n the middle of the wetted wall column. The N2 ° 4

reacts with water:

N2°4 + H 2 ° ~* H N 0 3 + H N 0 2 ( 2 1 )

Decomposition of the ni t r o u s a c i d produced according to

4HN02 •* 2N0 + N 20 4 + H 20 (22)

55

does not take place r a p i d l y i f i t s concentration i s low. NO i s very poorly soluble i n aqueous soluti o n s and i t can e a s i l y d i f f u s e into the gas phase. Under our experimental conditions no NO could be detected i n the gas phase with i n f r a r e d spectroscopy. Therefore i t may be assumed that the decomposition of HNOg can be neglected [1,13,14,15,16,35]. Applying the theory of mass t r a n s f e r with a r a p i d pseudo f i r s t order r e a c t i o n i n the l i q u i d phase the absorption rate per uni t of surface area based on the penetration theory can be written as:

This equation holds only i f kx » 1 [36]. In accordance with t h i s equation i t was assumed that N^O^ i s the ac t i v e species during the absorption.

The temperature r i s e near the i n t e r f a c e as a r e s u l t of the absorption was

ca l c u l a t e d with [36]:

( - A H - A H )

A, T = _ 2 S_ H P V — — (24) 2P C p N2°4 N2°4,i

The heat of s o l u t i o n of NgO^ i n t o aqueous s o l u t i o n s and the heat of reaction

were taken from the data of Moll [20]. Within the measured conditions i t can

e a s i l y be shown that the temperature r i s e was small enough to be neglected (< 0.2°C). The p a r t i a l pressures of N 2 0 4 on the g a s - l i q u i d i n t e r f a c e were c a l c u l a t e d from the measured absorption rates, equation (19) and equation (20) by means of an i t e r a t i o n procedure. The measured absorption rate as a function

of P was p l o t t e d f o r 25% n i t r i c a c i d i n F i g . 6 and f o r 40% n i t r i c a c i d i n N2°4,i

F i g . 7. This should give a s t r a i g h t l i n e through the o r i g i n with a slope of

H„ „\/kD N2O4V . From these f i g u r e s i t can be concluded that N 2 0 4 i s the ac t i v e species during the absorption. A least-square method gave the slope of the st r a i g h t l i n e and these values are given i n Table 5.

From t h i s table i t can be seen that hN 2 q decreases with increasing

n i t r i c acid strength. For low n i t r i c a c i d concentration t h i s i s mainly caused

by the decrease of H, „ with increasing i o n i c strength. According to H o f t i j z e r N2O4 and Kwanten [ l ] the influence of the i o n i c strength on H can be described

2°4 with:

(H ) = (H N Q ) exp (- 0.075 I) (25) 2 4 n i t r i c a c i d 2 4 water

where I i s the i o n i c strength defined by

56

n 2 I = i I (Z. C.)

. - 1 1 (26)

From F i g . 8 i t can be seen that the approximation proposed by H o f t i j z e r and Kwanten [1] i s v a l i d f o r n i t r i c a c i d concentrations up to about 25%. Above 25% n i t r i c a c i d large deviations occur. This may be caused by the fact that the pseudo f i r s t order r e a c t i o n rate constant k decreases with increasing n i t r i c a c i d strength since the molar concentration of the " f r e e " water tends to become r e l a t i v e l y small i n more concentrated n i t r i c a c i d . Moreover the d i f f u s i o n c o e f f i c i e n t of NgO^ into n i t r i c a c i d decreases with increasing n i t r i c a c i d strength.

5 7

• b a r

P N 2 0 4 , i * 1 , 2 P N 0 2 , i ( a )

Pig. 7 The absorption rate of N204 into 40% HNC>3 at 20° C as a function of the

driving force.

(h> = 0.346 m; x = 0. 722 sec; 0: P Q ; A: P Q + P m f t ) . lhU4,i lV2u4,i auZ,%

H m n V k D o Reference N 2 4

x 10^ kmol/m .bar

water 0 76 Kramers et a l [15] H o f t i j z e r and Kwanten [ l ]

25% HN03 0 49 + 0 03 t h i s work

40% HNOg 0 16 + 0 02 t h i s work

Table 5 Q ykD^ values as a function of the nitric acid strength at 20 C

58

1 0

Hn n V kDo values as a function of the n i t r i c acid strength. 2 4

(A ; Kramers et al [ 1 5 ] j Hoftijzer and Kwanten [1]; 0: this worl

approximation proposeu by Hoftijzer and Kwanten [1]).

Some authors [10,13,14,35] observed n i t r i c a c i d mist during t h e i r experiments concerning the absorption of NgO^ i n t o water e s p e c i a l l y at r e l a t i v e l y high molar concentrations of N^O^. During the experiments presented here no n i t r i c a c i d mist was observed. The n i t r i c a c i d formation i n the gas phase does not seem to be very important as was found by Detournay and Jadot [4]-

4.4.2 The absorption of N„0„ into concentrated n i t r i c a c i d s o l u t i o n s 2 4

Previously i t was derived that t h i s absorption process proceeds purely p h y s i c a l l y and that NgO^ may be assumed to be the a c t i v e species. The gas phase d i f f u s i o n of N0„ and NO can be described by the Graetz-model according to:

59

The p h y s i c a l absorption process of N 2 0 4 i n t o the l i q u i d phase can under our experimental conditions be written as:

J = 2(H P - C ) V — (28) N2°4 N2°4 N2°4,i N2°4,*,o T T

The temperature r i s e near the i n t e r f a c e as a r e s u l t of the physical absorption was c a l c u l a t e d with [36]:

-AH AT = H P V — < 2 9> P C N o0„ N.O^ a p 2 4 2 4 , i v

It was found that within the measured conditions t h i s temperature r i s e may be o

neglected (< 0.2 C). Table 6 and Table 7 give the experimental r e s u l t s . P a r t i a l pressures of NgO^ on the i n t e r f a c e were c a l c u l a t e d from equation (27) and equation (20) with an i t e r a t i o n procedure. The measured absorption rate as a function of the d r i v i n g force (H„ „ P„ _ - C„ _ ) should be given a

N 204 N 2 0 4 i N 2 0 4 ) £ ) 0

s t r a i g h t l i n e through the o r i g i n . From Fig s . 9, 10, 11 and 12 i t can be concluded that N 2 0 4 i s the ac t i v e

species during the absorption. With equation (28) the t h e o r e t i c a l absorption rates were c a l c u l a t e d and compared with the measured absorption rates. The d i f f u s i o n c o e f f i c i e n t of N_0„ into n i t r i c a c i d s o l u t i o n s was c a l c u l a t e d with

2 4 the r e l a t i o n of Wilke and Chang [36].

D„ „ = 0.88 x 10~ 9 m2/s f o r 78% HN0„ at 20°C N2°4,£ 3

D „ = 0.77 x 1 0 - 9 m2/s f o r 63% HNO, at 20°C N2°4,il 3

From Table 6 and Table 7 i t can be concluded that the proposed absorption model

describes the experiments f a i r l y w e l l .

6 0

Exp. P T c

bar

N2°4,£,o 3 3 xlO kmol/m

NO 2,o

bar

2 4,o

bar

2 4,i

bar

N 2 ° 4 N2°4 6 2 6 2 xlO kmol/m .sec xlO kmol/m .sec

measured penetration theory

35, 1 1. 093 20 0. 346 1 .58 85 3. 05 0 ,0458 0 ,0214 0, 0148 1.11 1, 21

35 2 1 ,093 20 0. 346 1. 58 85 3 05 0. 0673 0, 0463 0. 0297 2.39 2 50

37. 1 1 040 20 0. 346 2 , 40 85 2. 61 0 ,0165 0 ,00279 0. 00231 0.11 0 11

37. 2 1 040 20 0, 346 2 40 85 2 61 0 ,0379 0 ,0147 0 0107 0.58 0 70

37 3 1, 040 20 0. 346 2 40 85 2, 61 0 ,0556 0 ,0316 0 0212 1.31 1 44

37 4 1 040 20 0, 346 2 40 85 2 ,61 0 .0664 0 ,0451 0 0296 1.80 2 03

37 5 1 040 20 0. 346 2 40 85 2 61 0 ,0759 0 ,0590 0 0376 2.34 2 60

31 1 1 070 30 0, 137 0 .513 65 3 33 0 ,0712 0 ,0241 0 0180 2.13 2 12

31 .2 1 ,070 30 0. 137 0 .513 65 3 33 0 ,0931 0 ,0411 0 ,0281 3.92 3 .41

34 1 1 .110 30 0 346 0 .986 65 3 ,10 0 .0538 0 .0137 0 0105 0.88 0 .85

34 .2 1 .110 30 0 346 0 .986 65 3 , 10 0 .0975 0 .0450 0 ,0323 2.64 2 .85

36 .1 1 .064 30 0 .346 1 .77 65 2 , 82 0 .0190 0 .00172 0 ,00164 0.03 0 .04

36 ,2 1 .064 30 0 ,346 1 .77 65 2 .82 0 .0436 0 .0090 0 .00745 0.35 0 ,44

36 . 3 1 .064 30 0 .346 1 .77 65 2 ,82 0 .0723 0 .0248 0 ,0191 0.98 1 .23

36 .4 1 ,064 30 0 ,346 1 . 77 65 2 .82 0 .0915 0 .0397 0 .0289 1.69 1 .90

36 .5 1 .064 30 0 346 1 . 77 65 2 ,82 0 .111 0 .0582 0 ,0413 2.49 2 .75

Table 6 N0„ absorption experiments into 63% HNO^

Exp. P c T h' T

bar C m sec

m C P K~0. . N0„ 2 4,10,0 2,o

3 3 xlO kmol/m bar

P n 2 ° 4 , o P n 2 ° 4 , 1

bar bar

N2°4

6 2 xlO kmol/m .sec measured

N2°4 6 2 xlO kmol/m .sec

penetration theory

12 1 1 075 20 0 346 1 71 210 3 9 0 0676 0 0468 0 0187 4 10 4 05

12 2 1 075 20 0 346 1 71 210 3 9 0 1006 0 1034 0 0377 8 47 8 24

13 1 1 051 20 0 346 2 15 210 3 6 0 0644 0 0424 0 0194 3 02 3 74

13 2 1 051 20 0 346 2 15 210 3 6 0 0740 0 0561 0 0236 4 02 4 58

13 3 1 051 20 0 346 2 15 210 3 6 0 0847 0 0734 0 0269 5 52 5 24

14 1 1 051 20 0 346 2 15 210 2 6 0 0323 0 0107 0 00570 0 84 1 07

14 2 1 079 20 0 346 1 71 210 2 6 0 0348 0 0124 0 00648 1 11 1 37

15 1 1 067 20 0 346 2 15 210 2 75 0 0122 0 00151 0 00107 0 13 0 15

15 2 1 087 20 0 346 1 71 210 2 75 0 0126 0 00163 0 00110 0 17 0 17

19 1 1 063 20 0 137 0 43 210 3 35 0 0541 0 0299 0 0136 5 47 5 85

19 2 1 063 20 0 137 0 43 210 3 35 0 0785 0 0631 0 0223 12 0 9 67

18 1 1 039 30 0 137 0 61 146 2 55 0 0746 0 0264 0 0138 4 28 3 76

18 2 1 039 30 0 137 0 61 146 2 55 0 0981 0 0456 0 0217 7 06 5 99

17 1 1 014 30 0 137 0 91 146 2 55 0 0638 0 0193 0 01127 2 35 2 49

17 2 1 014 30 0 137 0 91 146 2 55 0 0994 0 0469 0 0242 5 41 5 48

16 1 1 053 30 0 346 2 15 146 3 64 0 0106 0 000533 0 00040 0 08 -— 16 2 1 053 30 0 346 2 15 146 3 64 0 0310 0 00456 0 00335 0 31 0 41

16 3 1 053 30 0 346 2 15 146 3 64 0 0786 0 0293 0 0170 1 93 2 45

16 4 1 053 30 0 346 2 15 146 3 64 0 0582 0 0161 0 0104 1 02 1 47

16 5 1 053 30 0 346 2 15 146 3 64 0 0999 0 0473 0 .0246 3 17 3 59

Table 7 NO^-absorption experiments into 78% nitric acid

N 2 0 4 , i " N 2 0 4 . l,o / n N 2 0 4

(o) bar

N 2 0 4 , i * ' 2 K N O z ,i ~ L N 2 0 4 , l.o ' H N 2 0 4

Fig. 9 The absorption rate of N^O^ into 63% HNO^ at 20 C as a function of the

driving force.

(h' = 0.346 m; T = 2.40 sec; 0: P FN0 ^ ~ CN 0 ^HN 0 ' NU2,i N2U 4,l,o 2 4

N„0„ . °N„0 /HN„0j A : PN„0, +

2 4,i 2 4,l,o 2^4 2 4,i

0 0 5

P N 2 0 4 ,i " C N 2 0 4 , l , o / H N 2 0 4

(o)

P N 2 0 4 , i * 1 ' 2 P N 0 2 , i - C N 2 0 4 . l , o / H N 2 0 4

( i )

Fig. 10 The absorption rate of N^O^ into 63% HN03 at 30 C as a function of the

driving force.

(h> = 0.346 m; T = 1.77 sec; 0: P -C /H ; A : P +

p m _ r /n 2 4 > l 2 4 > * > ° 2 4 2 4 > l

2,1 2 4,%,o 2 4

63

0 0 2 0 0 4

P N 2 0 4 , i " C N 2 0 4 , l.o / H N 2 0 4

(o )

P N 2 0 4 . i * 1 / 2 P N O s . I " C N 2 Q 4 , l , o / H N 2 0 4

< A )

Fig. 11 The absorption rate of N^4 into 78% HN03 at 20° C as a function of the

driving force.

Ch' - 0.346 m; T - 2.15 sec; 0: P

-NO 2,i /2 - C

N2°4,lJHW

N2°4,i ' X°4,l,o/HW \ 0 4 J

64

4.5 CONCLUSION

The absorption of NOg/NgO^ into d i l u t e d n i t r i c a c i d i s accompanied by a rapid pseudo f i r s t order reaction between NÔ^ and water. From the absorption measurements i t can be concluded that with increasing a c i d strength the values

decrease of the s o l u b i l i t y of N„0, i n n i t r i c a c i d with increasing i o n i c stength. Above 25% n i t r i c acid s o l u t i o n s t h i s approximation i s not v a l i d .

The absorption of NO^/NgO^ gas mixtures into concentrated n i t r i c a c i d (> 63%) can be considered as pure p h y s i c a l process. It can be concluded that NÔ^ i s the ac t i v e species during the absorption process. The s o l u b i l i t y of NÔ^ i n concentrated n i t r i c a c i d s o l u t i o n s was c a l c u l a t e d from the t o t a l vapour pressure data of the system NgO^HNOg-HgO, and i t can be concluded that Henry's law i s v a l i d .

REFERENCES

1. H o f t i j z e r , P.J. and Kwanten, F.J.G., Absorption of ni t r o u s gases i n : G. Nonhebel, Gas p u r i f i c a t i o n processes f o r a i r p o l l u t i o n c o n t r o l , Newnes-Butterworths, London, 1972, p. 164.

2. Goyer, G.G., J. Coll. Sci. , 1963, 18, 616. 3. England, C. and Corcoran, W.H., Ind. Eng. Chem. Fundam. , 1974, K3, 373. 4. Detournay, J.P. and Jadot, R.H., Chem. Eng. Soi. , 1973, 28, 2099. 5. Carberry, J . J . , Chem. Eng. Soi., 1959, ¡3, 189. 6. Theobald, H. , Chemie-Ing. -Teohn. , 1968, 1_5, 763. 7. Tereshchenko, L.Ya., Panov, V.P., Pozin, M.E. and Zubov, V.V., J. Appl.

Chem. USSR (Engl. Transl.), 1968, 41, 1995. 8. Vandoni, R. and Laudy, M., J. Chim. Phys., 1952, 49, 99. 9. Aunis, G., J. Chim. Phys., 1952, 49, 103.

10. Chambers, F.S. and Sherwood, T.K., Ind. Eng. Chem., 1937, 2J3, 1415. 11. Caudle, P.G. and Denbigh, K.G., Trans. Far. Soo., 1953, 39, 39. 12. Peters, M.S. and Holman, J.L., Ind. Eng. Chem., 1955, 47, 2536. 13. Wendel, M.M. and Pigf o r d , R.L., A.I.Ch.E. Journal, 1958, 4, 249. 14. Dekker, W.A., Snoeck, E. and Kramers, H., Chem. Eng. Soi., 1959, IX, 61. 15. Kramers, H., B l i n d , M.P.P. and Snoeck, E., Chem. Eng. Sci., 1959, 14, 115. 16. Kameoka, Y. and Pigf o r d , R.L., Ind. Eng. Chem. Fundam., 1977, 16, 163. 17. Denbigh, K.G. and Prince, A.J., J. Chem. Soo., 1947, 790.

approximation t h i s i s mainly caused by the

65

18. Sherwood, T.K., Pigf o r d , R.L. and Wilke, C.R., Mass Transfer, McGraw-Hill,

1975.

19. Carrington, T. and Davidson, N., J. Phys. Chem. , 1953, 57, 418. 20. Moll, A.J., PhD Thesis, Washington, 1966. 21. T r e i n i n , A. and Hayon, E., J. Am. Chem. Soa., 1970, 92, 5821. 22. Komiyama, H. and Inoue, H., J. Chem. Eng. of Japan, 1978, 11, 25. 23. Counce, R.M., Master Thesis, U n i v e r s i t y of Tennessee, Kn o x v i l l e , 1978. 24. Longstaff, J.V.L. and Singer, K., J. Chem. Soa. , 1954, 2610. 25. Karavaev, M.M. and Visloguzova, V.G., J. Appl. Chem. USSR (Engl. Transl.),

1974, 47, 1001. 26. Atroshchenko, V . l . and Kaut, V.M., J. Appl. Chem. USSR (Engl. Transl.),

1958, 31, 340. 27. Weinreich, G.H., PhD Thesis, U n i v e r s i t y of Toulouse, France, 1955. 28. Klemenc, A. and Rupp, J . , Z. Anorg. Allg. Chem., 1930, 194, 51. 29. Karavaev, M.M. and Yarkovaya, V.A., J. Appl. Chem. USSR (Engl. Transl.),

1967, 40, 2340. 30. Karavaev, M.M. and Bessmertnaya, A.I., The Soviet Chemical Industry, 1969,

7, 30. 31. Audinois, R., J. Chim. Phys., 1965, 62, 439. 32. Audinois, R., J. Chim. Phys. Physicochim. Biol., 1969, 66, 489. 33. Audinois, R., CR. Acad. Sei. Paris Sec. C. , 1968, 266, 117. 34. Technicon Auto-Analyzer II, I n d u s t r i a l Method No. 230-72A/Tentative 1974. 35. Dekker, W.A., PhD Thesis, D e l f t U n i v e r s i t y of Technology, D e l f t , 1958. 36. Danckwerts, P.V., Gas-Liquid reactions, McGraw-Hill, London, 1970. 37. Hisatsune, I.e., J. Phys. Chem., 1961, 65, 2249. 38. Bodenstein, M. and Bogs, F., Z. Physik. Chem., 1922, 100, 68.

39. Forsythe, W.R. and Giauque, W.F., J. Am. Chem. Soa., 1942, 64, 48.

5. THE OXIDATION AND ABSORPTION OF NO BY NITRIC ACID

5.1 INTRODUCTION

The oxidation of NO to N0 2 by concentrated n i t r i c a c i d solutions may be of importance f o r the production of concentrated n i t r i c a c i d . In add i t i o n concentrated n i t r i c a c i d may be used as a scrubbing l i q u i d f o r the removal of nitrogen oxides from the t a i l gas of n i t r i c acid plants, since i t has two a t t r a c t i v e properties. F i r s t l y , i t i s a very strong o x i d i z i n g agent and i t can e a s i l y o x i d i z e NO to N0 2. Secondly, N 2 0 4 which i s i n equili b r i u m with N0 2, dis s o l v e s p h y s i c a l l y very well i n t o concentrated n i t r i c a c i d .

In t h i s Chapter the mechanism and the k i n e t i c s of t h i s oxidation by 63%-78% n i t r i c a c i d are investigated to obtain data f o r the design of i n d u s t r i a l absorbers. Furthermore some preliminary experiments concerning the absorption of NO into 40% n i t r i c a c i d s o l u t i o n s are c a r r i e d out to check i f the r e s u l t s gathered i n the concentrated region apply also to the d i l u t e d system.

5.2 PROPOSED MECHANISM

The reaction of NO with n i t r i c a c i d i s presented with the following o v e r a l l reaction:

NO + 2HN03 t 3N0 2 + H 20 (1)

This r e a c t i o n i s the reversed r e a c t i o n of a c i d formation. Concentrated n i t r i c a c i d solutions of 55%-80% have a considerable n i t r i c a c i d vapour pressure. Therefore, t h i s r e a c t i o n may take place i n the gas phase [1]. In order to inve s t i g a t e t h i s phenomenon experiments were c a r r i e d out by passing a nitrogen gas stream containing 1% NO over a 65% n i t r i c acid s o l u t i o n . It was observed that the produced water vapour condensed i n the gas phase on the glass reactor walls close to the g a s - l i q u i d i n t e r f a c e . Furthermore, large amounts of the brown coloured N0 2 were found in the gas phase. Tereshchenko et a l [2]

67

investigated the oxidation of NO i n a nitrogen gas stream by n i t r i c acid solutions of 60% to 80% i n a bubbling apparatus and found that the oxidation rate was c o n t r o l l e d by gas phase d i f f u s i o n of NO from the gas bulk to the gas-l i q u i d i n t e r f a c e . These observations can be explained by regarding the gas phase re a c t i o n between NO and n i t r i c a c i d vapour as i n f i n i t e l y f a s t . The reac t i o n may take place i n a small r e a c t i o n zone or i n an asymptotic case on a reaction plane very close to the g a s - l i q u i d i n t e r f a c e (see F i g . 1). Reactions which proceed i n a reaction zone may be treated as though they are instantaneous i f the rea c t i o n zone i s not too large [11,12],

Fig. 1 Absorption-oxydation model

The r e a c t i o n i s a c t u a l l y much more complicated than equation (1) suggests, and i t may proceed v i a a mechanism composed of the following steps:

k l NO + HNOg HN02 + N0 2 (2)

k = 0.2 - 9 m 3/kmol.sec at 298°K [4,5,6].

The HN02 produced reacts very r a p i d l y with n i t r i c a c i d vapour [4,5,6].

k2

HN02 + HN03 + 2N0 2 + H 20 (3)

k g = 6 x 10 3 - 9 x 10 3 m 3/kmol.sec at 300°K [4,5,9], 68

The r e a c t i o n rate constant k 2 i s about 10 times higher than the r e a c t i o n rate constant of the primary r e a c t i o n . With i n f r a r e d a n a l y s i s i t was found that the amount of nitrous acid vapour i n the gas phase could be neglected. It i s therefore assumed that the nitrous a c i d reacts instantaneously as i t i s formed. For t r a n s i e n t evaporation of n i t r i c a c i d into a gas, the condition f o r instantaneous gas phase reaction i s [11,12]:

/ Z „ / I k . JHN0 3 ,/ J°mO, CN0,o ./ % 2C D D 2C D HNO . HNO, NO HNO, . HN0„

J , 1 O O , 1 O

(4)

i n which J„„„ i s the amount of n i t r i c acid which would be evaporated per u n i t HNUg

area i f the concentration of NO i n the neighbourhood of the surface retained i t s bulk value C instead of becoming depleted by r e a c t i o n . Using the reaction rate constant k^ i t can be shown that the condition f o r instantaneous reaction i s not f u l f i l l e d , which implies that r e a c t i o n (2) i s too slow. Near the i n t e r f a c e , however, large amounts of N0 2 and water vapour are present, and therefore the following reactions are important i n the r e a c t i o n zone:

N0 2 + NO + H 20 X 2HN02 (5)

NO + N0 2 X N 2 0 3 (6)

From the l i t e r a t u r e [3,7,8,10,13] i t i s known that reactions (5) and (6) are much f a s t e r than r e a c t i o n (2) . The HN02 and ^ 0 ^ produced by these reactions react with n i t r i c a c i d vapour according to, r e s p e c t i v e l y , equation (3) and

N 20 g + HN03 2N0 2 + HN02 (7)

From the above i t can be concluded that the gas phase oxidation of NO by n i t r i c a c i d vapour i s a very complex reac t i o n , and under these circumstances i t seems to be an a u t o c a t a l y t i c r e a c t i o n .

It should be noted that the l i q u i d phase oxidation of NO by d i l u t e d n i t r i c a c i d s o l u t i o n s (< 25%) which only nitrous a c i d produces, i s also an autoc a t a l y t i c reaction [14,15,16,30]. Presently, however, too l i t t l e i s known concerning k i n e t i c s and mechanism to confirm the conditions f o r instantaneous gas phase reacti o n . The NOg produced i s i n e q u i l i b r i u m with NgO^.

2N0 2 t N 2 0 4 (8)

69

This e q u i l i b r i u m i s established very r a p i d l y [17]. Therefore, i t i s assumed that N0„ and N„0 d i f f u s e i n continuous equi l i b r i u m with each other from the A ¿1 4 r e a c t i o n zone or r e a c t i o n plane to the gas bulk and to the g a s - l i q u i d i n t e r f a c e .

At the g a s - l i q u i d i n t e r f a c e only N 2 0 4 d i s s o l v e s p h y s i c a l l y i n t o the

concentrated n i t r i c acid, i n which i t i s highly soluble (see Chapter 4). The

proposed model i s presented i n F i g . 1.

5.3 EXPERIMENTAL

The experiments we c a r r i e d out i n the equipment which was previously described (Chapter 2 and Chapter 4). The in-going and out-going gas and l i q u i d were analysed f o r nitrogen oxides content and a mass balance around the N0-oxidation was established. It was found that the deviation was l e s s than 5%.

5.4 MATHEMATICAL MODEL AND RESULTS

NO oxidation

According to the proposed model the oxidation rate of NO i s c o n t r o l l e d by gas phase d i f f u s i o n from the gas bulk to the r e a c t i o n zone or reaction plane. Gas phase mass t r a n s f e r i n the wetted wall column takes place by molecular d i f f u s i o n only i n the r a d i a l d i r e c t i o n and therefore the f r a c t i o n a l concentration change of NO i n the gas phase can be written as:

- 2 C oo 1 -a it = 4 I — exp ( ) (9)

C„„ n=l a Gz„„ NO,o n NO

The gas phase d i f f u s i o n c o e f f i c i e n t of NO i n nitrogen was c a l c u l a t e d using the -5 2 o

r e l a t i o n of Chapman-Enskog (I> N 0 = 1.98 x 10 m /s at 20 C and 1.0132 bar) [24]. The experimental r e s u l t s f o r 78%, 63% and 57% n i t r i c a c i d s o l u t i o n s are

given i n F i g s . 2, 3 and 4. From these f i g u r e s i t can be concluded that the oxidation rate for 78% and 63% n i t r i c a c i d i s completely gas phase d i f f u s i o n c o n t r o l l e d and agrees with the proposed model. According to equation (9) the f r a c t i o n a l concentration change of NO should be independent of the i n l e t concentration of NO. From Tables 1 and 2 i t can be concluded that the f r a c t i o n a l concentration change of NO tends to increase with i n c r e a s i n g P„_ . This

NO, o suggests, that the reaction i s very complicated.

70

Fig. 3 Fractional concentration change of NO as a function of the Graetz-

number for 63% nitric acid (0: 20°C; A : 30°C; — equation (9)).

72

' O r

Fig. 4 Fractional concentration change of NO as a function of the Graetz-

number for 67% nitric acid (0: 20°C; equation (9)).

73

Exp.. h > 6

h T T p c o P NO, o

P N0 2 P N 2 ° 4 P N 2 ° 3 \ °4 °N0

xlC > 6 m sec °C bar kmol bar bar bar bar x l O 6 CN0,o 3. m /sec N 2 V

m3

kmol N 2 ° 4 / m2. s

1.1 5 44 0 346 1 26 20 1 093 6 5 0 0419 0 0284 0 00825 0 000268 5 . 25 0.327

1.2 5 44 0 346 1 26 20 1 093 6 5 0 0866 0 0388 0 01541 0 000845 11. 50 0.365

1.3 5 44 0 346 1 26 20 1 093 3 8 0 1274 0 0509 0 02648 0 00177 18. 92 0.396

1.4 5 44 0 346 1 26 20 1 093 3. 8 0 1683 0 0599 0 0367 0 00252 24. 71 0.363

1.5 5 44 0 346 1 26 20 1 093 3 8 0 2099 0 0689 0 0486 0 00395 31. 73 0.396

1.6 5 44 0 346 1 26 20 1 093 3 8 0 0433 0 0248 0 00628 0 000258 5 28 0.349

2.1 7 38 0 346 1 03 20 1 115 2 2 0 0410 0 0224 0 00515 0 000265 5 64 0.420

2.2 7 38 0 346 1 03 20 1 115 2 2 0 0782 0 0304 0 00947 0 000750 11 34 0.458

2.3 7 38 0 346 1 03 20 1 115 2 2 0 1075 0 0373 0 0143 0 00125 15 73 0.452

2.4 7 38 0 346 1 03 20 1 115 2 2 0 1596 0 0479 0 0235 0 00245 23 08 0.466

2.5 7 38 0 346 1 03 20 1 115 2 2 0 193 0 0560 0 0321 0 00362 27 66 0.486

3.1 11 05 0 346 0 79 20 1 148 10 6 0 0626 0 0264 0 00749 0 000578 9 93 0.508

3.2 11 05 0 346 0 79 20 1 148 10 6 0 0989 0 0340 0 01184 0 00117 15 28 0.509

3.3 11 05 0 346 0 79 20 1 148 20 7 0 1247 0 0390 0 01553 0 00180 17 85 0.538

3.4 11 05 0 346 0 79 20 1 148 20 7 0 1553 0 0454 0 0211 0 00256 23 95 0.527

3.5 11 05 0 346 0 79 20 1 148 37 0 0 1764 0 0487 0 0243 0 00314 28 88 0.531

5.1 3 45 0 346 1 71 20 1 087 2 5 0 0811 0 0469 0 0223 0 000708 8 35 0.270

5.2 7 38 0 346 1 71 20 1 087 2 5 0 1362 0 0636 0 0413 0 00148 14 47 0.247

9.1 7 38 0 137 0 43 20 1 069 3 3 0 1631 0 0435 0 0194 0 00317 36 41 0.649

9.2 7 38 0 137 0 43 20 1 069 3 3 0 0788 0 0288 0 00851 0 00102 17 60 0.651

Table 1 Experimental r e s u l t s of N0-oxidation by 78% HNO

E X P - *l h ' T T P c PN0,o PN0, P N O PN n JN n _^N0_

xlO m sec °C bar xlO bar bar bar bar x l O 6 NO, o 3 kmol kmol

m /SeC W W m 3 m 2 0

20 1 3 17 0 137 0 75 20 1 060 1 11 0 0593 0 0318 0 0104 0 000635 5 25 0 489

20 2 3 17 0 137 0 75 20 1 060 1 11 0 1208 0 0498 0 0253 0 00224 12 43 0 541

22 1 7 69 0 137 0 42 20 1 087 5 06 0 0263 0 0150 0 00231 0 000182 2 69 0 669 22 2 7 69 0 137 0 42 20 1 087 5 06 0 0483 0 0232 0 00549 0 000523 5 87 0 677

22 3 7 69 0 137 0 42 20 1 087 5 06 0 0752 0 0285 0 00832 0 00102 9 98 0 689 22 4 7 69 0 137 0 42 20 1 087 5 06 0 1124 0 0363 0 0135 0 00197 14 07 0 701 22 5 7 69 0 137 0 42 20 1 087 5 06 0 1410 0 0387 0 0153 0 00269 17 9 0 714 24 1 7 69 0 346 1 05 20 1 103 1 93 0 0259 0 0233 0 00555 0 000194 2 23 0 467 24 2 7 69 0 346 1 05 20 1 103 1 93 0 1085 0 0479 0 0235 0 00169 8 19 0 471 26 1 4 15 0 346 1 58 20 1 091 2 30 0 0309 0 0311 0 00992 0 000244 1 53 0 369

26 2 4 15 0 346 1 58 20 1 091 2 30 0 1277 0 0648 0 0429 0 00247 7 16 0 433

Table 2 Experimental results of the NO-oxidation by 63% HNO

Ul

In some experiments with 63% n i t r i c a c i d the influence of N0 2 on the oxidation rate of NO fo r equimolar i n l e t q u a n t i t i e s of NO and N0 2 was investigated. The experimental r e s u l t s are given i n Table 3 and from t h i s table i t can be concluded that the influence of N0 2 on the f r a c t i o n a l concentration change of NO i n the gas phase i s of minor importance.

The oxidation r a t e of NO by 57% n i t r i c a c i d s o l u t i o n s i s not completely gas phase d i f f u s i o n c o n t r o l l e d , and the oxidation also takes place p a r t i a l l y i n the l i q u i d phase (see F i g . 4). Under these circumstances the gas phase re a c t i o n rate i s too slow to be considered as instantaneous, a fact which may be caused by the very low n i t r i c a c i d vapour pressure. At more d i l u t e d n i t r i c a c i d (< 40%) no N0 2 was found i n the gas phase, and under these conditions the rea c t i o n takes place only i n the l i q u i d phase. It should be noted that N0 2 and/or N 20 4

cannot e x i s t i n d i l u t e d n i t r i c a c i d (< 40%), and i n t h i s case n i t r o u s acid i s the f i n a l product [14,15,16].

The l i q u i d phase oxidation of NO by d i l u t e d n i t r i c a c i d can be presented with the following o v e r a l l r e a c t i o n :

2N0 + HN03 + H 20 •* 3HN02 (10)

Some experimental r e s u l t s with 40% n i t r i c a c i d are given i n Table 4.

If the theory of mass t r a n s f e r with a rap i d pseudo f i r s t order r e a c t i o n i n

the l i q u i d phase may be applied the absorption rate per uni t surface area can

be written as:

1 provided that kx » 1.

The absorption r a t e (J„„) p l o t t e d as a function of P . should give a NO NO, 1 s t r a i g h t l i n e through the o r i g i n with a slope of H J J Q ^ ^ D ^ (see F i g . 5). With regression a n a l y s i s the slope was found to be:

HJJQ /̂~ki>£ = 2.81 + 0.15 x 10~ 5 kmol/m 2.bar.sec at 20°C (12)

The oxidation of NO in t o 5-25% n i t r i c a c i d s o l u t i o n s seems to be a u t o c a t a l y t i c [14,15,16,25,26,27,28,29,30]. Abel et a l [14,25,26,27,28] proposed the following r e a c t i o n scheme:

HN0 3 + HN02 "* N 2 0 4 + H 20 (13)

76

measured model

Exp. p NO,o

P N0„ 2,o P N„0„ 2 4,o

p N0 2 P N 2 ° 4

CN0 V C Q , i CN0 V C Q , i J n 2 ° 4 P N0„ 2,o

P N„0„ 2 4,o p N0 2 P N 2 ° 4

°N0,o CQ,o" CQ,i °N0,o Q,o Q,i J n 2 ° 4

bar bar bar bar bar X 1 0 6

2 kmol/m .s

X 1 0 6

2 kmol/ra . s

40.1 0.0199 0.0171 0.00296 0.0293 0.00871 0.482 0.53 1.90 0.490 0.610 1.76

40.2 0.0704 0.O4O3 0.0164 0.0574 0.0334 0.546 0.67 7.03 0.490 0.610 7.68

41.1 0.0484 0.0285 0.00819 0.0458 0.0213 0.523 0.62 4.76 0.490 0.610 4 . 85

41.2 0.0941 0.0486 0.0239 0.0671 0.0455 0.544 0.69 9.78 0.490 0.610 10.62

Table 3 Influenae of N0g on the oxidation of NO by 63% HNO y (T - 20°C, T = 1.05 sea, 10 bar).

N 20 4 + 2N0 + 2H 20 * 4HN02 (14)

In t h i s r e a c t i o n mechanism reaction (13) i s rather slow, while equi l i b r i u m (14) i s e s t a b l i s h e d very r a p i d l y . The i n i t i a l formation rate of nitrous a c i d of the o v e r a l l r e a c t i o n (10) was found to be f i r s t order with respect to the ni t r o u s a c i d concentration and f i r s t order with respect to the n i t r i c acid concentration [15,16]. Furthermore the o v e r a l l r e a c t i o n rate was found to be independent of the p a r t i a l pressure of NO [15,16]. From the above i t can be concluded that within the measured conditions t h i s a u t o c a t a l y t i c behaviour i s not v a l i d f o r the oxidation of NO by 40% n i t r i c a c i d s o l u t i o n s .

<J>£ x 10 5 h' T CHN0 2 X l ° 3

2, o p N0

g N0

3, m /s m sec. 3 kmol/m bar 2

kmol/m .sec.

1.21 0 136 0.284 2.65 0 0402 1.40

1.21 0 136 0.284 2.65 0 1038 2.72

1.21 0 346 0.722 1.92 0 0302 0.97

1.21 0 346 0.722 2.62 0 0578 1.91

1.21 0 346 0.722 2.62 0 0870 2.58

1.21 0 346 0,722 2.62 0 109 3.14

1.21 0 346 0.722 2.62 0 129 3.59

1.21 0 346 0.722 2.62 0 0274 0.88

Table 4 Experimental results of the absorption rate of NO into 40% HN0

(T = 20°C; P -1.16 bar).

78

Fig. 5 The absorption rate of NO into 40% nitric acid (0: x =0.722 sec; A;

x = 0.284 sec).

UOp/IlpO^ diffusion from the reaction plane

According to the proposed model N0 2 and N^O^, which are i n continuous equil i b r i u m with each other, d i f f u s e from the r e a c t i o n plane to the gas bulk and to the g a s - l i q u i d i n t e r f a c e . At the g a s - l i q u i d i n t e r f a c e only N„0 di s s o l v e s

& 4 p h y s i c a l l y i n the concentrated n i t r i c a c i d . For a q u a n t i t a t i v e d e s c r i p t i o n of

these d i f f u s i o n processes the concentration of N0 2 and N^O^ on the r e a c t i o n

plane should be known. It should be noted that the distance from the rea c t i o n plane to the g a s - l i q u i d i n t e r f a c e (6 ) va r i e d i n our experiments from 0 -

-4 3 x 10 m (moving boundary). From these r e s u l t s i t can be derived that the concentration decrease of N0 2 and NgO from the rea c t i o n plane to the gas-l i q u i d i n t e r f a c e i s small (< 5%), and therefore the concentration of NOg and N 2 0 4 on the r e a c t i o n plane was assumed to be equal to that at the i n t e r f a c e . The concentration of N0 2 and N 2 0 4 on the r e a c t i o n plane i s c a l c u l a t e d from a mass balance around the rea c t i o n plane by assuming a quasi-stationary s i t u a t i o n [18,19].

- 3D, NO 8r

3C, NO D Q 3r

Q - 2D (15)

Q = N0 2 + 2N 20 4

i n which the s o l u b i l i t y m i s defined as:

79

C N2°4,£,i H N 2 0 4 m = ~ = 2T~ <16)

2 4,g,i

For small values of the contact time the gas phase and the l i q u i d phase may both be considered to be i n f i n i t e l y deep, and therefore the l o c a l mass f l u x can be c a l c u l a t e d from the penetration theory.

The l o c a l mass fl u x of NO from the gas bulk to the r e a c t i o n plane i s :

N NO " " D NO T I = cm,o^k (17)

R-6 f

For zero i n i t i a l N0 2 concentration i n the gas phase the l o c a l mass f l u x of N0 2

and N^O^ from the re a c t i o n plane to gas bulk becomes:

3C / D N = - D -—• I = C V — (18) Q Q 3r ^ Q, i TTt K '

If there i s N0„ present i n the in-going gas stream, the l o c a l mass fl u x N can 4 0,

be written as:

NQ " ( C Q , i - C Q , o ) V i ? <19>

The l o c a l mass fl u x of N 2 0 4 i n the l i q u i d phase can be described by:

3C

V . = • V , « " L a , = V , , , " V m . . * ^ * * (20)

A f t e r s u b s t i t u t i o n i n t o the mass balance the following equation i s obtained:

3C V — — = (C - C )¥ — t 2(1 C„ „ - C „ ) x NO.o' TTt Q,i Q V TTt N2°4,g,i N2°4,£,o

D n 2 ° 4 I * 4 > X (21)

These equations are only v a l i d i f the concentration of N0 2 on the r e a c t i o n

plane i s higher than the concentration of N0 2 i n the gas phase. The d i f f u s i o n

c o e f f i c i e n t D was c a l c u l a t e d as was described i n Chapter 4. With the known Q 2 equilib r i u m constant K„ = P /P„„ , the concentrations of N0„ and N„0„ on "2 N2O4 NO2 2 2 4

the r e a c t i o n plane can be c a l c u l a t e d from equation (21) by means of an i t e r a t i o n

80

procedure. Note that these concentrations are independent of the contact time

value. If the gas phase may not be considered to be i n f i n i t e l y deep, an average

concentration of N0„ and N„0. on the reaction plane over the e f f e c t i v e f i l m 2 2 4 height h' was c a l c u l a t e d from an o v e r a l l mass balance:

2 2 o o l a i T °° i a i r

3<j) (1 - 41 — exp (- - 5 — ) ) = <j) (C _ C )(1 - 4E - r exp (- )) + g n=l a 2 Gz g Q > 1 Q ' ° n=l a 2 Gz„ n NO . n Q

* T7t 8TT(R - 6 ) h'(m C - C )Y Zl ' <22>

W 2 U 4 , g , i W2 U4,)l,o

The concentrations of NOg and NgO^ on the r e a c t i o n plane obtained with

equation (22) deviate under our experimental conditions les s than 8% from those

obtained with equation (21). The d i f f u s i o n of N0 2 and NgO^ from the re a c t i o n plane to the gas bulk can

be described with: — 2 G„ - C_ . oo , a TT

- * x \ - <- <*-> Q,o Q,i n=l a Q n

The values of C„ - C„ ,/C - C„ . were c a l c u l a t e d from the C„ . of the Q Q,.t Q,o Q,i Q > 1 _ t h e o r e t i c a l model according to equation (22) and the measured value C from the

experiments. In F i g . 6 and F i g . 7 the t h e o r e t i c a l and the measured values are

compared f o r i n i t i a l zero concentration of NO^ i n the gas phase. The influence

of NOg i n the in-going gas stream on the d i f f u s i o n rate of NOg from the

reaction plane to the gas bulk i s given i n Table 3. From the above i t can be

concluded that the experiments agree rather well with the model.

The amounts of N 2 0 4 which were p h y s i c a l l y absorbed i n t o the n i t r i c a c i d

s o l u t i o n s can under our experimental conditions be described with the

penetration theory.

' * ITT J N o = 2 < m V o 4 . " V o 4 , > V - i f T ^ (24) 2 4 2 4,g, i 2 4,J!,,o

The measured absorption rates were compared with the t h e o r e t i c a l l y predicted

values according to equation (24). In F i g . 8 and F i g . 9 the measured absorption rates and the t h e o r e t i c a l predicted absorption rates are p l o t t e d f o r i n i t i a l zero N O concentrations i n the gas phase. The influence of i n i t i a l N0 o ^ 2 concentration i n the gas phase on the N 20 4 absorption rate i s given i n Table 3. From the above i t can be concluded that the measured N 2 0 4 absorption rate i s rather well predicted by the model.

81

Fig. 6 The diffusion of iVOg and N 04 from the reaction plane to the gas bulk

for 78% nitric acid (0: 20°C; A : Z0°C; equation (22)).

82

1 0

0 - 5 h

O U

o IU

o u

0 0 5 0 - 1 0

G z Q , r e d

Fig. 7 The diffusion of N02 and Nfl^ from the reaction plane to the gas bulk

for 63% nitric acid (0: 20°C; A: 30° C; equation (23)).

83

5.5 DISCUSSION

In the proposed mechanism a few assumptions were made which w i l l be discussed

i n more d e t a i l .

a) According to the proposed model water vapour i s produced on the reaction plane very close to the g a s - l i q u i d i n t e r f a c e . In some experiments the gas phase leaving the wetted wall column was analysed f o r i t s water vapour content. It was found that the amount of water vapour i n the gas phase could be neglected.

This implies that a l l the water vapour produced condenses on the n i t r i c a c i d

l i q u i d f i l m . N i t r i c a c i d d i f f u s e s from the l i q u i d f i l m into t h i s t h i n water layer and water d i f f u s e s from the i n t e r f a c e i n the n i t r i c a c i d f i l m . These concentration gradients may have some influence on the s o l u b i l i t y of NgO^. It can be shown that the average thickness of t h i s layer i s very small

_7 (6 < 3 x 10 m). The concentration gradient of water i n t h i s l a y e r was layer roughly c a l c u l a t e d under statio n a r y conditions with:

D

H 2 °

H 20

layer AC

H 2 ° (25)

i n which J i s the condensation rate of water vapour. From t h i s value i t was 2 -2 3 c a l c u l a t e d that AC < 10 kmol/m under our experimental conditions. This H2O

implies that the a c i d strength i n the t h i n layer may also be assumed to be equal to the strength i n the n i t r i c a c i d l i q u i d f i l m .

b) In the c a l c u l a t i o n s the influence of a temperature change near the i n t e r f a c e as a r e s u l t of heat of reac t i o n , heat of condensation of the water vapour, heat of mixing of the condensed water and n i t r i c a c i d , heat of evaporation of the n i t r i c acid and heat of s o l u t i o n of NgO^ into n i t r i c a c i d was neglected (see Table 5).

A H 2 9 8 1 Reference

NO + 2HN0g <--> 3N0 2 * H 20 AH 1 = 38.6 X 10 3 J [23]

2HN03 (£) -> 2HN03 (g) AH 2 = 78.6 X i o 3 J [23]

H 20 (g) H 20 (A) AH 3 = -44.2 x i o 3 J [23]

3N0 2 (g) -> f N2°4 <«> AH 4 = -85.9 x i o 3 J [23]

N 2 ° 4 <*> N2°4 <*> AH S = -25.3 X 19 3 J t h i s work

Table 5 Heat effects near the interface

86

The temperature change near the i n t e r f a c e was c a l c u l a t e d from a heat balance by

assuming that the heat of mixing may be neglected and that a l l heat e f f e c t s

only contribute to a temperature change i n the l i q u i d phase near the i n t e r f a c e .

P V (26)

< A H1 * A H 2 + A H 3 + ^ A H 4 ) CN0,o yf%0,

+ pc P v

i n which y represents the f r a c t i o n of the NO^ produced which i s converted to N^O^. Within the experimental conditions the temperature change near the i n t e r

face va r i e i

neglected.

o o face v a r i e d from - 0.6 C to 0 C. This was found to be small enough to be

5.6 CONCLUSIONS

The oxidation of NO by concentrated n i t r i c a c i d (63-78%) can be considered to be an instantaneous gas phase re a c t i o n i n a reaction zone or on a rea c t i o n plane very close to the g a s - l i q u i d i n t e r f a c e . Presently too l i t t l e i s known concerning the mechanism and k i n e t i c s to prove t h i s hypothesis using the c r i t e r i a f o r instantaneous reactions.

It was found that Danckwerts' soluti o n s f o r instantaneous i r r e v e r s i b l e reactions i n the l i q u i d phase can also be applied to gas phase reactions. The NOg and ̂ 0 ^ produced, which are i n continuous equi l i b r i u m with each other d i f f u s e from the r e a c t i o n zone or reaction plane to the gas bulk and to the g a s - l i q u i d i n t e r f a c e . At the i n t e r f a c e only NgO^ di s s o l v e s p h y s i c a l l y into the concentrated n i t r i c a c i d . The mathematical model presented to describe these d i f f u s i o n processes was found to be i n good agreement with the experiments.

The absorption of NO by 40% n i t r i c acid s o l u t i o n s takes place i n the l i q u i d phase and under these circumstances nitrous a c i d i s the f i n a l product. The absorption rate can be described by the theory of mass t r a n s f e r with a rapid pseudo f i r s t order r e a c t i o n i n the l i q u i d phase.

REFERENCES

1. Dohnalek, R. and Vesely, S., Neth. Appl. 6401801, 1965. 2. Tereshchenko, L.Ya., Panov, V.N. and Pozin, M.E., J. Appl. Chem. USSR (Engl.

87

Tränst.), 1972, 45, 241. 3. Kaiser, E.W. and Wu, C.H., J. Phys. Chem., 1977, 81, 1701. 4. Kaiser, E.W. and Wu, C.H., J. Phys. Chem., 1977, 81, 187. 5. S t r e i t , G.E., Wells, J.S., Fehsenfeid, F.C. and Howard, C.J., J. Chem.

Phys., 1979, 70, 3439. 6. McKinnon, I.R., Mathieson, J.G. and Wilson, I.R., J. Phys. Chem., 1979, 83,

1979. 7. Wayne, L.G. and Yost, D.M., J. Chem. Phys., 1951, 19, 41. 8. Chan, W.H., Nordstrom, R.J., Ca l v e r t , J.G. and Shaw, J.H., Paper presented

before the Division of Environmental Chemistry American Chemical Society,

p. 251-253, A p r i l 4-9, 1975, New York. 9. England, C. and Corcoran, W.H., Ind. Eng. Chem. Fundam., 1974, 13, 373.

10. England, C. and Corcoran, W.H. , Ind. Eng. Chem. Fundam., 1975, 14^, 55. 11. Danckwerts, P.V., Gas-Liquid Reactions, McGraw-Hill, London, 1970. 12. A s t a r i t a , G., Mass Transfer with Chemical Reaction, E l s e v i e r P u b l i s h i n g

Company, Amsterdam, 1967. 13. Vla s t a r a s , A.S. and Winkler, C.A., Can. J. Chem., 1967, 45, 2837. 14. Abel, E. and Schmid, H., Z. Physik. Chem., 1928, 132, 55. 15. Schmid, G. and Bahr, G., Z. Physik. Chem., 1964, 41, 8. 16. Usubillaga, A.N., PhD Thesis, U n i v e r s i t y of I l l i n o i s , U.S.A., 1962. 17. Carrington, T. and Davidson, N., J. Phys. Chem., 1953, 57, 418. 18. Hisatsune, I.C., J. Phys. Chem., 1961, 65, 2249. 19. Technicon Auto-Analyzer I I , I n d u s t r i a l method No. 230-72A/Tentative 1974. 20. H i k i t a , H. Asai, S. and Takatsuka, T., Chem. Eng. J., 1972, 4, 31. 21. Van de Vusse, J.G., Chem. Eng. Sei., 1966, 21, 631. 22. Dekker, W.A., PhD Thesis, D e l f t , 1958. 23. Forsythe, W.R. and Giauque, W.F., J. Am. Chem. Soc. , 1942, 64, 48. 24. Reid, R.C., Prausnitz, J.M. and Sherwood, T.K., The Properties of Gases and

Liqu i d s , McGraw-Hill, 1977. 25. Abel, E. and Schmid, H., Z. Physik. Chem., 1928, 134, 279. 26. Abel, E., Schmid, H. and Babad, S., Z. Physik. Chem., 1928, 136, 135. 27. Abel, E., Schmid, H. and Babad, S., Z. Physik. Chem., 1928, 136, 419. 28. Abel, E. , Schmid, H. and Babad, S., Z. Physik. Chem., 1928, 136, 430. 29. Abel, E., Schmid, H. and Römer, E., Z. Physik. Chem., 1930, 148, 337. 30. Axente, D., Lacoste, G. and Mahenc, J . , J. Inorg. Nucl. Chem., 1974, 36,

2057.

88

6. AN ABSORPTION MODEL FOR THE DESIGN OF A DILUTED NITRIC ACID ABSORBER AND METHODS TO DECREASE THE NO x CONTENT IN TAIL GASES

6.1 INTRODUCTION

Although many i n v e s t i g a t i o n s can be found i n the l i t e r a t u r e concerning the absorption of nitrogen oxides into n i t r i c a c i d s o l u t i o n s , the mechanism i s s t i l l not well understood i n absorbers f o r the production of d i l u t e d n i t r i c a c i d . This i s mainly due to the fac t that various nitrogen oxides NO, NgO^, NO^ and NgO^ a l l play an important r o l e i n the absorption process i n both the l i q u i d and the gas phase. Furthermore n i t r i c a c i d as well as n i t r o u s a c i d can be formed i n both phases. Oxygen i s normally present i n the gas phase o x i d i z i n g NO to NOg. This o x i d a t i o n i s an unusual r e a c t i o n with an apparent negative temperature c o e f f i c i e n t as was shown by Bodenstein [1].

In t h i s Chapter an absorption model, based on general chemical r e a c t i o n engineering considerations, i s derived f o r the design of i n d u s t r i a l absorbers fo r the production of d i l u t e d n i t r i c a c i d . In addition, various methods of decreasing the concentrations of nitrogen oxides i n t a i l gases of n i t r i c a c i d plants w i l l be b r i e f l y discussed.

6.2 ABSORPTION MODEL FOR THE PRODUCTION OF DILUTED NITRIC ACID

The o v e r a l l reaction f o r the a c i d formation i n the absorption column can be presented with:

This equ i l i b r i u m determines the maximum ac i d concentration that can be obtained at a given composition of the n i t r o u s gases (Chapter 4). The NO produced i s very poorly soluble i n aqueous so l u t i o n s , and i s t r a n s f e r r e d to the gas phase, where i t reacts with molecular oxygen.

3N0 2 (N 20 4) + H 20 % 2HN03 + NO (1)

2N0 + 0 2 •* 2N0 2 (2)

89

The r e a c t i o n rate of t h i s oxidation can be expressed by [1-5]:

- d t - • k PN0 \ ( 3 )

The reaction rate constant k increases with decreasing temperature. The reverse reaction may be neglected under the conditions p r e v a i l i n g i n the absorption column. E s p e c i a l l y at the top of the absorption column where the p a r t i a l pressure of NO i s low, the reoxidation rate of NO i s small. As a f i r s t approximation the oxidation of NO can be considered to be the rate determining step i n the absorption process.

The NO produced has a considerable influence on the absorption rate of NgO^ i n t o water and d i l u t e d a c i d . This e f f e c t may be due to the formation of

HNOg and N2^3 i n t n e *=as P h a s e :

N0 2 + NO + H 20 % 2HN02 (g) (4) NO + N0 2 X N 2 0 3 (g) (5)

The n i t r i c a c i d formation i n the gas phase seems to be of minor importance under the conditions p r e v a i l i n g i n the absorption column [6,7]. The equili b r i u m constants of reactions (4) and (5) were given i n Chapter 4 (Table 1). The concentrations of N 2 0 3 and HN02 are small under e q u i l i b r i u m conditions but due to the high s o l u b i l i t i e s and the r a p i d establishment of these e q u i l i b r i a , the t r a n s f e r of NgO^ and HN02 from the gas phase to the l i q u i d phase can not be neglected. A f i r s t attempt to describe such a complex absorption process was done by Andrew and Hanson [8]. More r e c e n t l y H o f t i j z e r and Kwanten [7] proposed an absorption model which i s schematically presented i n F i g . 1. In the mathematical d e s c r i p t i o n they neglect the t r a n s f e r of N„0 from the gas phase to the l i q u i d phase. In t h i s work a model i s set up i n which the NgOg t r a n s f e r and the HNOg t r a n s f e r are both taken i n t o account. The model i s based on:

a) D i f f u s i o n of N0 2 and NgO^ from the gas bulk to the g a s - l i q u i d i n t e r f a c e k 2k g,NO _ e> N2°4

J =23 = (P - P ) + — (P - P ) (6) N0 2 N 2 0 4 RT N0 2 N0 2 >.> RT 2 4, N2°4,i

b) Transfer of N2

0 4 ' N 2 ° 3 a n d H N 0 2 f r o m t n e g a s - l i q u i d i n t e r f a c e to the l i q u i d phase.

The N2 0 4 reacts with water to produce n i t r i c a c i d and n i t r o u s acid, as was

90

3HN02 - H N 0 3 > 2 N O » H 20

LIQUID - BULK

Fig. 1 Absorption model according to Hoftijzer and Kwanten [?].

discussed i n Chapter 4. According to Corriveau [9] the N^O^ reacts r a p i d l y with water i n the l i q u i d phase to produce n i t r o u s a c i d .

N2°3 «2° 2HN0„ (7)

This r e a c t i o n may be considered to be a rap i d pseudo f i r s t order r e a c t i o n . The

nit r o u s a c i d formed i n the gas phase d i s s o l v e s p h y s i c a l l y i n t o the s o l u t i o n .

The absorption rate can then be written as:

N„0. = 2 P N + 4 P H N O „ /«HNO + V o „ / V o A JN0 = 2 J N ° 2 "2"4 "2"4,i "2"4 2, i 2 2 3,i 2 3

( 8 )

In absorption columns f o r the production of d i l u t e d a c i d the l i q u i d phase may be assumed to be nearly saturated with NO and under these conditions the reverse reaction becomes important. The absorption rate i s then represented by:

H = 2 v 4 " 2 \ ' p » o 2 A v « i f o ( 1 • ( ^ ) 2 / 3 )

i H H N 0 2 ^ P 4 • PN0, i • PN0 2 ^ . • PH 20, 3. 1/6

k „ ( l - ( - ^ ) )

8. 1/3 kD„ (1 - ( ~ ) ) K (9)

91

Values of the equili b r i u m constants K p , Kp , K p and Kp were given i n Chapter 4 (Table 1). The value of p\ i s defined as:

PNO,i P 3

N 0 2 , i

(10)

Note that equation (9) may only be applied to d i l u t e d a c i d . The absorption of N0 2/N 20 4 into concentrated a c i d should be considered to be purely p h y s i c a l .

c) Transfer of NO from the g a s - l i q u i d i n t e r f a c e to the gas-bulk.

JN0 = 3 JN0 2 = ~wT^ ( P N 0 , i " PN0 }

In the gas bulk the reoxidation of NO with oxygen takes place [1-5]. It may be assumed that the gas phase i s saturated with water. The water vapour pressure as a function of the ac i d strength can be taken from the binary system HN0,-H 0 measured by Vandoni and Laudy [10] . The values of H •ƒ kD„ as a function of the ac i d strength were given i n Chapter 4. L i t t l e information can

HHN0 2 3 n d HN 20 3V be found i n the l i t e r a t u r e concerning the values of H m i n_ and H H n %ƒ kD„. Abel

and Neusser [12] determined the HN02 vapour pressure above n i t r o u s a c i d

s o l u t i o n s . Values of H_„t„ were c a l c u l a t e d from the eq u i l i b r i u m measurements of Hi\L)2

Theobald [13] concerning the heterogeneous system n i t r i c a c i d / n i t r o u s gases. The p a r t i a l pressures of HN0_ i n the gas phase were c a l c u l a t e d from P , P„„ ,

£ NO NO2 PtI _ and the equili b r i u m constant K_ . The values of H T I„. as a function of the n 20 P4 HNO2 acid strength thus found are about twice the value measured by Abel and Neusser

[12] (see F i g . 2). This discrepancy requires further i n v e s t i g a t i o n . Values of H„ „ U kD. were c a l c u l a t e d from the absorption measurements of Hofmeister and

N 2 ° 3 V l

Kohlhaas [14]. The r e s u l t s are given i n Table 1. Corriveau [9] used a laboratory absorber containing f i v e wetted spheres to i n v e s t i g a t e the absorption rate of N„0„ i n t o water. From Table 1 i t can be seen that the value of H„ _ \/ kD„ *s 3 "2O3* "

reported by Corriveau [9] i s much lower than that of Hofmeister and Kohlhaas [14]. No information was found i n the l i t e r a t u r e concerning the influence of the n i t r i c a c i d strength on the values of H \l kD.. As a f i r s t approximation

N 2 O 3 » * N 2 0 3

ecre

i o n i c strength. I t i s c l e a r that more work i s needed to obtain r e l i a b l e data

t h i s influence may be c a l c u l a t e d from the decrease of H„ _ with increasing N 2 0 3

92

l i q u i d method of

2 kmol/m .s.bar measurement

Hofmeister, Kohlhaas [14] 5 x 10~ water laminar j e t Corriveau [9] 1.58 x 10~ water wetted spheres

Table 1 Comparison of literature data concerning the absorption of N^O^ into

water at 25.0°C

From the proposed model i t can be concluded that the absorption rate w i l l be increased by (see also H o f t i j z e r and Kwanten [7]):

a) high p a r t i a l pressures of the nitrogen oxides;

b) low temperatures i n both phases; c) high degree of oxidation of the nitrogen oxides; d) large g a s - l i q u i d i n t e r f a c i a l area.

93

For a s i m p l i f i e d mathematical model of the absorption column i n the d i l u t e d n i t r i c a c i d production the reader i s r e f e r r e d to the l i t e r a t u r e [16,17],

6.3 METHODS TO DECREASE THE NO CONTENT IN TAIL GASES OF NITRIC ACID PLANTS x

T a i l gases of n i t r i c a c i d plants contain between 100 and 3000 ppm of nitrogen

oxides and t a i l gases of some very o l d plants even more. Because of i t s harmful

e f f e c t on the ecosystem an e f f e c t i v e removal of N0^ i s necessary. At present

the emission l e v e l i s 1.5 kg NO ( c a l c u l a t e d as N0 o) per ton a c i d f o r new X £t

plants i n the United States. This i s equivalent to about 200 ppm. For e x i s t i n g plants a l e v e l of 400 ppm w i l l be required. In Europe the l i m i t v a r i e s from country to country. For new plants a l i m i t of 400-500 ppm may be assumed, depending on the l o c a l s i t u a t i o n . Presently several methods to decrease the nitrogen oxides content i n these t a i l gases are known i n the l i t e r a t u r e [15,18]. Table 2 gives a review of the most important methods.

•— Wet process

NO abatement

Dry process

Extended absorption (water scrubbing) [19,20,21]

.H 20 2 scrubbing [22]

I—HN0, scrubbing [23-37]

— Adsorption [41-43]

.Non-selective reduction [44-49]

S e l e c t i v e reduction [15,44,45,46, 50,51]

Table 2 Methods to decrease the NO^ content in tail gases of nitric acid

plants 94

6.3.1 Wet Processes

6.3.1.1 Extended absorption [19,20,21]

Increasing of the absorption volume decrease the nitrogen oxides content i n

t a i l gases of n i t r i c acid plants. The degree of oxidation of the nitrogen

oxides i n these t a i l gases i s about 0.5. In the l i q u i d phase mainly HN02 i s

produced.

NO + N0 o + H O 2HN0 (£) (12) ¿ 2 ^

The HNOg may be decomposed p a r t i a l l y :

3HN02 •> HN03 + 2N0 + H 20 (13)

The NO produced i s very poorly soluble i n aqueous s o l u t i o n s , hence i t i s tr a n s f e r r e d to the gas phase where i t reacts with oxygen. At the top of the absorber the reoxidation rate of NO w i l l be very slow as the p a r t i a l pressure of NO i s small. This implies that a r e l a t i v e l y large absorption volume i s required for a high degree of oxidation. The extended absorption method i s now rather often applied i n new plants. By working at a pressure of 12 bar i n the absorber and by coo l i n g the absorption system with water the N0 x content i n the t a i l gas may be reduced to 200 ppm. Even i n e x i s t i n g plants extended absorption can be applied, provided that the pressure i n the main absorber i s not too low (Fig. 3). The extended absorber i s positioned down stream r e l a t i v e l y to an e x i s t i n g absorber. Condensate i s cooled and enters the extended absorber. The r e s u l t i n g weak acid becomes the feed f o r the main absorber ( F i g . 3). If the degree of the oxidation of NO i s low such a process i s not economical due to the large absorption volume required.

6.3.1.2 H O scrubbing process [22] A 2

The t a i l gas of the ac i d absorber (A) i s scrubbed with H 20 2 (see F i g . 4). The following o v e r a l l reactions occur:

NO + N0 2 + 2H 20 2 2HN03 + HgO (14)

2N0 + 3H 0 •* 2HN0 + 2H 0 (15)

95

o 2

NH 3

1

A

2 .r

H p O

0°/. H N O ,

Fig. 3 Simplified flow sheet for the production of diluted nitric acid with

extended absorption.

A: converter; B: cooler/condenser; C: absorber; D: extended absorber.

1: feed to converter; 2: 10% NO; 3: NO oxidized to NO^; 4: 2000-600

ppm N0x; 5: 200-400 ppm N0x; 6: water; 7: weak nitric acid; 8: 60%

nitric acid to bleacher.

H 2 0 -

N 0 2

H o O 2 u 2

60 % HNO3

Fig. 4 Simplified flow sheet of the NÔ^ scrubbing process [22],

A: acid absorber; B; UÔ^ scrubber.

1: feed to acid absorber; 2: 2000-4000 ppm N0x; 3: 200-400 ppm N0x;

4: weak nitric acid with unreaated HÔ^; 5: recycling EÔ^; 6: fresh

H000; 7: water to acid absorber; 8: 60% nitric acid to bleacher.

96

The t a i l gas treatment takes place at ambient temperatures and at a pressure

equal to that i n the a c i d absorber. The weak ac i d leaving the scrubber and

containing some unreacted H O enters the top of the a c i d absorber. From the ¿1 2

reaction equations (14) and (15) i t can be seen that t h i s process overcomes the chemical and p h y s i c a l l i m i t a t i o n s which e x i s t i n the extended absorption method. Own experiments were c a r r i e d out to i n v e s t i g a t e the absorption rate of NO i n t o H„0 s o l u t i o n s . It was found that H O decomposed rather r a p i d l y as soon as i t was a c t i v a t e d by r e a c t i o n . The molecular oxygen produced d i f f u s e d from the l i q u i d phase to the gas phase. In the gas phase the oxygen i s rather i n e f f e c t i v e . The l o s s of H 20 2 by decomposition i s a serious disadvantage of t h i s process.

6.3.1.3 N i t r i c acid scrubbing [23-40]

a) D i l u t e d n i t r i c a c i d scrubbing [23-29,40] The Humphreys/Glasgow and Bolme process uses a 30% n i t r i c a c i d s o l u t i o n as scrubbing l i q u i d [23,24]. A s i m p l i f i e d flow sheet i s given i n F i g . 5. The gas stream from the a c i d absorber containing 2600 ppm N0^ enters scrubber (A) i n which N0„ and NO can be absorbed at ambient temperature according to

NO + N0 2 + H 20 + 2HN02 (16)

2N0 + HNOg + H 20 * 3HN02 (17)

The pressure i n the scrubber i s about the same as i n the acid absorber CW bar). A f t e r scrubbing the gas contains 150-250 ppm NO . The n i t r i c a c i d l e a v i n g the

o scrubber i s regenerated by heating to 70 C and s t r i p p i n g with a i r or steam. Under these conditions the nitrous a c i d i s decomposed. The N0^ produced i s cooled and recycled to the acid absorber. The main advantage of t h i s process i s that NO can also be absorbed and recovered. Several v a r i a n t s of t h i s process are known i n l i t e r a t u r e [25-29].

b) Concentrated n i t r i c acid scrubbing [30-37] The S0LN0X n i t r i c a c i d process of Ugine Kuhlmann produces weak acid of 60-63% and concentrated n i t r i c acid of 80% [30,31,32], An important step i n the SOLNOX-process i s the d i s s o l u t i o n of NgO^ into concentrated n i t r i c acid; t h i s d i s s o l u t i o n i s also the method f o r cleaning the t a i l gas. A s i m p l i f i e d flow sheet i s presented i n F i g . 6. The combustion gas leaving the converter (1) f i r s t enters a precondenser (A). In the precondenser the gas i s cooled with

9 7

3

1%

Fig. 5 Simplified flow sheet of the Humphreys/Glasgow and Bolme process [23,

24] .

A: n i t r i c acid scrubber; B: regeneration column.

1: tail gas of acid absorber; 2: 30% n i t r i c acid solution; 3: treated

tail gas (200 ppm NO^); 4: 30% n i t r i c acid containing nitrous acid; 5:

recovered NO^ to main acid absober; 6: stream or air for regeneration

of scrubbing liquid.

c i r c u l a t i n g cooled weak acid. The cooled gas and a portion of the cooled weak acid from the precondenser c i r c u i t pass to the co-current condenser (B) where the gas and the l i q u i d are both cooled to 0°C with c o l d brine. In t h i s way the water vapour i s removed from the gas phase. The n i t r i c a c i d s o l u t i o n leaving

the co-current condenser has a concentration of about 62-63%. The cold, dry and

f u l l y o x i d i z e d gas then enters a p h y s i c a l absorber (C) i n which N0 2/N 20 4 i s o

d i s s o l v e d into a 80% n i t r i c a c i d s o l u t i o n at a temperature of -10 to 0 C and a pressure of about 8 bar. The gas leaving the top of the absorber (5) contains about 600 ppm NOg. This can be e a s i l y lowered to 200 ppm by scrubbing i t with water or d i l u t e d n i t r i c a c i d . The concentrated n i t r i c a c i d containing 10-30 wt % of NgO^ enters reactor (D). In the reactor the d i s s o l v e d NgO^ i s converted to

o o n i t r i c acid with water and a i r at 60 -80 C and a pressure of 8 bar.

98

N2°4 + H 2 ° + i02 "* 2 H N 0 3 (18)

Under these conditions no NO i s produced [38,39]. The strong acid i n the lower part of the reactor i s bleached and p a r t i a l l y r e c y cled to the p h y s i c a l absorber. Processes s i m i l a r to the SOLNOX process can be found i n the l i t e r a t u r e [33-37] (see also Chapter 1).

Fig. 6 Simplified flow sheet of the SOLNOX process [30],

A: precondenser; B: co-current condenser; C: physical absorption

column; D: reactor.

1: gas stream from converter to precondenser; 2: gas stream to co-

current condenser; 3: weak nitric acid; 4: cooled weak acid; 5: NO^ gas

stream to physical absorber; 6: 60% nitric acid; 7: bleached and cooled

80% nitric acid; 8: gas containing NO^ to physical absorber; 9: 80%

nitric acid containing 10-30% by weight N^Q^S 10: 60% nitric acid to

bleacher; 11: unbleached 60% nitric acid to reactor; 12: air to reactor;

13: bleached 80% nitric acid.

99

6.3.2 Dry processes

6.3.2.1 Adsorption [41-43]

NO can be removed and recovered from n i t r i c a c i d t a i l gas streams by fixed-bed x adsorption on molecular sieves. Molecular sieves show a high adsorption capacity for N0 2 at ambient temperatures, but the adsorption capacity f o r NO i s very low. In the presence of oxygen the molecular sieves can catalyze the

oxidation of NO to N0 2 which i s adsorbed on the molecular sieves. T a i l gases of

n i t r i c a c i d plants contain water vapour and water vapour w i l l f i r s t be adsorbed.

This decreases the adsorption capacity f o r NOg. An emission l e v e l of 50 ppm N0 x

can be obtained. The adsorbed N0 2 i s p e r i o d i c a l l y desorbed and recycled to the aci d adsorber. The desorption process takes place at a temperature of about

o 150-250 C. A s i m p l i f i e d flow sheet i s presented i n F i g . 7.

1 3

4

2

Fig. 7 Adsorption process for the removal of NO^ from tail gases of n i t r i c

acid plants.

A: fixed bed adsorption column; B: regeneration of the adsorption

column.

I: tail gas from n i t r i c acid absorber (2000 ppm NO^); 2: treated tail

gas containing 50 ppm N0^; 3: recovered N0^ to n i t r i c acid absorber;

4: gas for regeneration of the adsorption column.

6.3.2.2 Non-selective reduction processes [44-49]

Non-selective reduction of nitrogen oxides i s characterized by the reaction of a reducing agent (CH 4, CO, Hg, naphta, etc.) with N0 x and oxygen i n the

100

presence of a c a t a l y s t . Noble metal c a t a l y s t s are used based on Pt, Pd and Rh, deposited on a s u i t a b l e i n e r t c a r r i e r . In t h i s process N0 x can only be reduced i f a l l the oxygen i s removed. T a i l gases of n i t r i c acid plants contain about 3% oxygen which i s an orde of magnitude higher than the N0 x content. This implies that a large amount of reducing agent i s needed f o r the reduction of NO^. The heat evolved by r e a c t i o n can be recovered i n a waste heat b o i l e r . Furthermore, due to the temperature r i s e of the t a i l gas more energy can be recovered at the expansion turbine. Van den Bleek and Van den Berg [46] put forward a hypothesis explaining why these processes are non-selective r e l a t i v e to oxygen. This w i l l be b r i e f l y reviewed. The reactions occurred can be presented as follows:

cat

N0 2 + Red -* NO + Red 0 (19)

cat N 2

NO + Red •* N 20 + Red 0 (20)

i n which Red and Red 0 represent a reducing agent (CH^, H 2, CO, naphtha) and

i t s oxidation product, r e s p e c t i v e l y . According to Van den Bleek and Van den

Berg [46] , N0 2 i s e a s i l y reduced to NO while reaction (20) i s very slow. The

main reason why NO i s not reduced s e l e c t i v e l y i s due to the reaction

cat NO + 0 2 •+ N0 2 (21)

The NO produced by r e a c t i o n (19) w i l l be e a s i l y and qui c k l y oxidized with oxygen. In t h i s way the reduction c y c l e has to s t a r t again consuming another quantity of reducing agent. Only when a l l of the oxygen i s consumed r e a c t i o n (20) becomes important.

6.3.2.3 S e l e c t i v e reduction processes [15,44,45,46,50,51]

A s e l e c t i v e reduction of nitrogen oxides i n t a i l gases of n i t r i c a c i d plants i s p o s s i b l e with NH^ as a reducing agent. This saves a large amount of reducing

agent. The reduction can be c a r r i e d out i n a f i x e d bed at a temperature of o o

200 -500 C using a c a t a l y s t based on Pt, Pd, Rh or metal oxides such as V„0_, z 5

Fe 0 , Cr 0 and CuO [51]. In t h i s way the NO content i n these t a i l gases can Z o Z o X

be e a s i l y decreased to 200 ppm. The temperature r i s e i n the t a i l gas as a r e s u l t of the heat evolved from the reduction reactions i s small (about 20°C). Van den Bleek and Van den Berg [46] postulated why a reduction of N0 x with NH^ i s s e l e c t i v e r e l a t i v e to oxygen. The following reactions are considered i n t h e i r hypothesis:

101

cat cat N 20 N0 2 + NH 3 - "NH 4N0 2 / 3" -> ^ + ^ 0 (21)

cat cat NgO NO + NH 3 + "NH 4N0 2 / 3" + R

+ H2 ° ( 2 2 )

2

On the c a t a l y s t surface they assume the formation of n i t r a t e or n i t r a t e - l i k e complexes as intermediate products. These n i t r a t e or n i t r a t e - l i k e complexes decompose to N and NO. In t h i s way no NO i s produced, and which of course implies that the reoxidation of NO according to re a c t i o n (20) can not take place.

REFERENCES

1. Bodenstein, M., Z. Physik. Chem. , 1922, 100, 87.

2. Hisatsune, I.e. and Zafonte, L. , J. Phys. Chem., 1969, 7̂3, 2980. 3. Greig, J.D. and H a l l , P.G., Trans. Faraday. Boo., 1967, 63, 655. 4. Mahenc, J . , C l o t , G. and Bes, R., Bull, de la Société Chimique de France,

1971, 5̂ , 1578.

5. England, C. and Corcoran, W.H., Ind. Eng. Chem. Fundam., 1975, iA, 55.

6. Detournay, J.P. and Jadot, R.H., Chem. Eng. Sei., 1973, 28, 2099. 7. H o f t i j z e r , P.J. and Kwanten, F.J.G., "Absorption of ni t r o u s gases", i n

G. Nonhebel, Gas P u r i f i c a t i o n Processes f o r A i r P o l l u t i o n Control, Newnes-Butterworths, London, 1972, p. 164.

8. Andrew, S.P.S. and Hanson, D., Chem. Eng. Sei., 1961, 1£, 105.

9. Sherwood, T.K., Pigf o r d , R.L. and Wilke, C.R., Mass Transfer, McGraw-Hill, 1975. •

10. Vandoni, R. and Laudy, M., J. Chim. Phys., 1952, 49, 99. 11. Aunis, G., J. Chim. Phys., 1952, 49, 103. 12. Abel, E. and Neusser, E., Monatsh. Chem., 1929, 54, 855.

13. Theobald, H. , Chenrie-Ing.-Techn. , 1968, 1J5, 763. 14. Hofmeister, H.K. and Kohlhaas, R., Ber. Bunsenges. Physik. Chem., 1965, 69,

232. 15. Yamaguchi, M., Matsushita, K. and Takami, K., Hydrocarbon Process. , 1976,

August, 101.

16. Emig, G., Wohlfahrt, K. and Hoffmann, U., Paper presented at the 12th Symposium on Computer App l i c a t i o n s i n Chemical Engineering, Montreux Casino, 1979, 242.

102

17. Holma, H. and Sohlo, J . , Paper presented at the 12th Symposium on Computer Appl i c a t i o n s i n Chemical Engineering, Montreux Casino, 1979, 228.

18. S i d d i q i , A.A., T e n i n i , J.W. and K i l l i o n , L.D., Hydrocarbon Process., 1976, October, 94.

19. Swanson, C.G., Prusa, J.V., Hellman, T.M. and E l l i o t t , D.E., Pollut. Eng.,

1978, 10, 52. 20. Brown, M.L., Environ. Symp. Proceedings, Washington, 1976, 137. 21. Newman, D.J., Chem. Eng. Progr., 1971, 67, 79.

22. Adrian, J.C. and Ve r i l h a c , J . , Paper presented at the 2nd Inte r n a t i o n a l Conference on the Control of Gaseous Sulphur and Nitrogen Compound Emission, U n i v e r s i t y of Sa l f o r d , England, 1976.

23. Bolme, D.W. and Horton, A., Chem. Eng. Progr., 1979, March, 95. 24. Bolme, D.W., U.S. Patent 4053555, 1977, October 11.

25. American Hydrocarbon Company, U.S. Patent 4081518, 1978, March 28.

26. Chenoweth Development Laboratories, Inc., U.S. Patent 4081517, 1978, March 28.

27. Mayland, B.J. and Heinze, R.C., Environ. Symp. Proceedings, Washington, 1976, 143.

28. Mayland, B.J. and Heinze, R.C., Chem. Eng. Progr., 1973, 69, 75. 29. F r i e d r i c h Uhde, U.S. Patent 3809744, 1974, May 7. 30. Ugine Kuhlmann, Ger. Offen 2128382, 1971, December 23. 31. Anon., Nitrogen No. 106, 1977, March/April, 35. 32. Adrian, J.C. and Vidon, B., Paper presented at the 2nd Inte r n a t i o n a l

Conference on the Control of Gaseous Sulphur and Nitrogen Compound Emission, U n i v e r s i t y of Sa l f o r d , England, 1976.

33. Sumitomo Chemical, Ger. Offen 2125677, 1971, December 2. 34. Du Pont de Nemours, B r i t . 1419645, 1975, December 31. 35. Ohrui, T., Okubo, M. and Imai, 0., Hydrocarbon Process., 1978, November,

163. 36. Hellmer, L., Chem. Eng., 1975, December, 98. 37. Anon., Hydrocarbon Process. , 1975, November, 164.

38. Franck, H.H. and Schirmer, W., Z. Elektrochem., 1950, 54, 254.

39. Shneerson, A.L., Minovich, M.A., F i l i p p o v a , Zh.M. and Platonov, P.A.,

J. Appl. Chem. USSR (Engl. Transl.), 1965, 38, 1627. 40. Osa, T., Fujieda, S. and Abe, Y. , Chem. Lett., 1976, 9, 1029. 41. Kiovsky, J.R., Koradia, P.B. and Hook, D.S., Chem. Eng. Progr. , 1976, August,

98.

42. Buck, B.J. and Matthews, W.G., Environ. Symp. Proceedings, Washington, 1976, 157.

103

43. J o i t h e , W., B e l l , A.T. and Lynn, S., Ind. Eng. Chem. Process. Pes. Develop.,

1972, 11, 434. 44. Klimisch, R.L. and Larson, J.G., The C a t a l y t i c Chemistry of Nitrogen

Oxides, Plenum Press, New York, 1975. 45. Shelef, M. , Catal. Rev. Soi. Eng., 1975, 11, 1.

46. Van den Bleek, CM. and Van den Berg, P.J. submitted f o r p u b l i c a t i o n i n

J. Chem. Tech. Biotechnol.

47. Fischhof, H., Chem. Eng., 1977, December, 863. 48. F r e i t a g , W. and Packbier, M.W., Ammonia Plant Saf., 1978, 20, 11. 49. Searles, R.A., Paper presented at the 2nd International Conference on the

Control of Gaseous Sulphur and Nitrogen Compound Emission, U n i v e r s i t y of Salf o r d , England, 1976.

50. Youn, K.C., Hydrocarbon Process., 1979, February, 117. 51. Kiovsky, J.R., Koradia, P.B. and Lim, C.T., Paper presented at the 3rd

International Symposium on The Control of Sulphur and other Gaseous

Compounds, U n i v e r s i t y of Sa l f o r d , England, 1979.

104

A p p e n d i x 1. THE ADDITIVITY OF RESISTANCES FOR MASS TRANSFER IN A WETTED WALL COLUMN

(1)

1. INTRODUCTION AND GENERAL THEORY

In chemical engineering design the a d d i t i v i t y of i n d i v i d u a l mass t r a n s f e r

resistances f o r g a s - l i q u i d systems i s often applied, which was o r i g i n a l l y

derived from the two-film theory:

1 1 K k mk. og g I

It should be noted that the use of t h i s r u l e i s based on the assumption of

steady state t r a n s f e r at a l l times i n both phases and equation (1) w i l l hold

true i f the following two conditions are met:

1. The d i s t r i b u t i o n c o e f f i c i e n t m must be a constant or known as a function of the t r a n s f e r r e d component i n the l i q u i d phase.

2. No other resistance may be present other than those expressed by

1/mk^ and V k g -

The gas phase mass t r a n s f e r c o e f f i c i e n t and the l i q u i d phase mass t r a n s f e r

c o e f f i c i e n t may vary with the contact time of renewable surfaces or may vary

over a f i n i t e surface. Equation (1) may then be applied f o r the l o c a l values of

the mass t r a n s f e r c o e f f i c i e n t s .

(2) K k mk„ , og , l o c a l g , l o c a l I,local

In p r a c t i c e i t i s customary to define average s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s . I f , f o r instance, the s i n g l e mass t r a n s f e r c o e f f i c i e n t s vary with the contact time of renewable surfaces we get f o r the time average mass tr a n s f e r c o e f f i c i e n t s :

T - o k & , l o c a l d t (3) K £ T

105

and T ƒ"* , dt g , l o c a l

k = - ( 4 ) g

The true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t (K ) can be c a l c u l a t e d & og,true with: T

— i — ~ r ~ « ƒ K , dt > + — (5) og,local o k , mk„ , — o _ g , l o c a l t,local

og,true T T

In chemical engineering design the average o v e r a l l mass t r a n s f e r c o e f f i c i e n t i s , however, often derived from the a d d i t i v i t y of i n d i v i d u a l average mass t r a n s f e r r e s i s t a n c e s :

Kog, addition 1 I T 1 t 1 — + r • dt r — • dt — — 1 k i mk„ , , k mk„ o g . l o c a l o I,local g £ — + 1

T T

In general equation (5) i s not equal to equation (6). King [1] pointed out that equation (5) i s equal to equation (6) only i f the following a d d i t i o n a l conditions are f u l f i l l e d :

3. The mass t r a n s f e r c o e f f i c i e n t s of the gas phase and the l i q u i d phase must not i n t e r a c t .

4. The l o c a l value of mk„/k must be constant at a l l points of the A< g

g a s - l i q u i d i n t e r f a c e .

King [1] found that i f high l o c a l values of k^ tend to coincide with low l o c a l values of k^ and reversed the true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t may deviate under c e r t a i n conditions by about 20% from the value obtained with equation (6). I f , however, high l o c a l values of k^ coincide with high l o c a l

values of k the d e v i a t i o n w i l l be rather small. The d e v i a t i o n i s a function of g

the r a t i o mk„/k , and a maximum appears at mk„/k = 1. Both s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s can vary inherently with the

same power of age or distance along the i n t e r f a c e , and under these conditions a l l four conditions f o r the a d d i t i v i t y of phase resistances are f u l f i l l e d .

In Chapter 2 s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s were investigated i n a wetted wall column, i n which a co-current laminar flow of a f a l l i n g l i q u i d f i l m and a gas core with a f l a t v e l o c i t y p r o f i l e could be established. It was found

106

that the l i q u i d phase mass t r a n s f e r can be described under our experimental conditions with the penetration theory. The gas phase mass t r a n s f e r can be described with the s o l u t i o n of the Graetz-problem. The t r a n s f e r c o e f f i c i e n t s vary l o c a l l y and not with the same power of age along the g a s - l i q u i d i n t e r f a c e , and t h i s phenomenon implies that not a l l four conditions are f u l f i l l e d . At large Graetz-numbers i t was found that the gas phase as well as the l i q u i d phase may be considered to be i n f i n i t e l y deep. Both mass t r a n s f e r c o e f f i c i e n t s can be described with the penetration theory and therefore the a d d i t i v i t y of phase resistances holds [1,2].

In t h i s section the a d d i t i v i t y of i n d i v i d u a l phase resistances i n the previously described wetted wall column (Chapter 2) i s studied. The true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t i s c a l c u l a t e d from a numerical s o l u t i o n and i s compared with the value obtained from equation (6).

2. RESULTS

In Chapter 2 a wetted wall column was developed i n which a co-current laminar flow of a f a l l i n g l i q u i d f i l m and a gas core with a f l a t v e l o c i t y p r o f i l e could be established. The flow model and the coordinate system are given i n F i g . 1. The following a d d i t i o n a l assumptions have been made f o r the absorption model:

1. The absorption i s purely physical and heat e f f e c t s as a r e s u l t of the absorption may be neglected.

2. A l l the relevant p h y s i c a l properties remain constant during the

absorption.

3. D i f f u s i o n i n the gas and l i q u i d phase takes place only i n r a d i a l d i r e c t i o n .

4. At the i n t e r f a c e e q u i l i b r i u m e x i s t s between the gas and the

l i q u i d .

The d i f f u s i o n process can be described by the following equations:

Gas phase

L i q u i d phase v

v s

s z

+ r

1 3C (8)

(7)

with the following i n i t i a l and boundary conditions:

107

h = 0 r > R - S C £ = C„ (9) , o

h = 0 r < R - 6 C = C (10) t g g.o

h > 0 r = 0 3C g/3r = 0 (11)

h > 0 r - + c o c = C „ (12) — ic Je, o

The i n t e r f a c e conditions are:

3CÇ 3C h > 0 r = R _ 6 D = D - t t- 6- (13)

h > 0 r = R - 6. m C . = C . . (14) f g , i fc.,i

The double penetration model (asymptotic solution)

An asymptotic s o l u t i o n of these equations can be found at large Graetz-numbers i f the gas phase may also be considered to be i n f i n i t e l y deep. The mass tr a n s f e r i n the gas phase and i n the l i q u i d phase can then be described with the penetration theory.

A l l four conditions f o r the a d d i t i v i t y of i n d i v i d u a l phase resistances are

f u l f i l l e d and then the true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t can be

c a l c u l a t e d with:

Kog,true i i ( 1 5 )

2 ^ D /TTT + 2m^ D^/TTT

Van den Berg and Hoornstra [5] described the p h y s i c a l absorption of c h l o r i n e into benzene by means of t h i s double penetration model.

Numerical solution

Equations (7-14) have been approximated by f i n i t e d i f f e r e n c e s according to

Crank-Nicolson [3,4] and then solved by a Gaus-Seidel i t e r a t i o n procedure.

A problem a r i s e s at the i n t e r f a c e f o r h = 0 where no e q u i l i b r i u m e x i s t s

between the gas and the l i q u i d . Therefore the concentration p r o f i l e s i n the gas

phase and i n the l i q u i d phase were approximated i n the f i r s t step by means of

the a n a l y t i c a l s o l u t i o n of the penetration theory [4]. The f r a c t i o n a l

concentration change of the t r a n s f e r r e d component i n the gas phase was

c a l c u l a t e d f o r i n i t i a l zero gas concentration i n the l i q u i d phase as a function

of the Graetz-number and the s o l u b i l i t y (see F i g . 2). From t h i s f i g u r e i t can

108

liquid gas jfiim] 5 f

/

Fig, 1 Flow model and coordinate system.

Fig. 2 The fractional concentration change of a transferred component in the

gas phase as a function of T T / G S for D./D = 0.0001. x, g

( Numerical solution, — Double penetration model).

109

be seen that the asymptotic s o l u t i o n (double penetration model) i s only v a l i d at Graetz-number la r g e r than 100. The true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t was c a l c u l a t e d from the numerical s o l u t i o n by e s t a b l i s h i n g a simple mass balance around the wetted wall column. Based on the logarithmic mean d r i v i n g force between the i n l e t and o u t l e t the mass balance can be written as:

1> (C g g,o C (h)) g

K A og

V g,o m / v g h)

I n

'l,o B,o

C (h) L e

(16)

A comparison of the true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t with the average o v e r a l l mass t r a n s f e r c o e f f i c i e n t c a l c u l a t e d from the a d d i t i v i t y of i n d i v i d u a l phase resistances i s only possible i f a l l mass t r a n s f e r c o e f f i c i e n t s are based on the same d r i v i n g force. For the l i q u i d phase i t was found that under c e r t a i n conditions the mass t r a n s f e r can be described by the penetration theory. The average Sherwood number based on the logarithmic mean d r i v i n g force between the i n l e t and the o u t l e t can be written as [6]:

2

Fo„ I n

O CO

,2 Z , V n e X p (" «- n=l

2 11 Fo ) n t

(17)

i n which

(18)

The eigenvalues m , the c o e f f i c i e n t s A and the functions F are given i n 0 n n n Table 1.

In Chapter 2 i t was found that the average gas phase Sherwood-number can be described by the s o l u t i o n of the Graetz-problem.

Sh = - — In I ^ g V n=l a 2

n

2 ( a IT \

Gz exp (19)

The values of a are given i n Table 1 of Chapter 2. n The deviation of the true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t

compared to the average o v e r a l l mass t r a n s f e r c o e f f i c i e n t obtained from the a d d i t i v i t y of i n d i v i d u a l phase re s i s t a n c e with equations (19), (17) and (6) was ca l c u l a t e d f o r mk„/k a 1 at which the de v i a t i o n i s pronounced.

y» g 110

For i n i t i a l zero gas concentration i n the l i q u i d phase the dev i a t i o n i s

p l o t t e d as a function of the TT/Gz-number (see F i g . 3). Because the inaccurancy

i n the c a l c u l a t i o n of the true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t i s

about 5 °/oo t h i s deviation i s not relevant. The disadvantage i n the

c a l c u l a t i o n of the l i q u i d phase mass t r a n s f e r c o e f f i c i e n t , based on the

logarithmic mean d r i v i n g force, i s that the thickness of the l i q u i d f i l m must

be known. Therefore i t i s use f u l to define an average l i q u i d phase mass

tr a n s f e r c o e f f i c i e n t , as follows:

k„ = 2V — (20)

This equation can be d i r e c t l y derived from the penetration theory and i t has

been based on an a r i t h m e t i c a l d r i v i n g force.

The average o v e r a l l mass t r a n s f e r c o e f f i c i e n t following from the a p p l i c a t i o n

of the a d d i t i v i t y of the i n d i v i d u a l phase resistances was c a l c u l a t e d from

equation (19) and equation (20). It should be noted that i n t h i s case the

si n g l e phase mass t r a n s f e r c o e f f i c i e n t s have not been based on the same d r i v i n g

force. The l i q u i d phase mass t r a n s f e r c o e f f i c i e n t i s based on an a r i t h m e t i c a l

d r i v i n g force, while the gas phase mass t r a n s f e r c o e f f i c i e n t i s based on a

logarithmic-mean d r i v i n g force.

The true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t , c a l c u l a t e d with the

numerical s o l u t i o n , was therefore based on a combination of an a r i t h m e t i c a l and

a logarithmic-mean d r i v i n g force defined as:

* « .î«»,.ï >- " V ° - V M >

g g,o g og

In (C - ^ ) g,o m

g m _

The deviation of t h i s true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t with the

average o v e r a l l mass t r a n s f e r c o e f f i c i e n t obtained from the a d d i t i v i t y of

i n d i v i d u a l phase resistances was p l o t t e d as a function of the ir/Gz-number f o r

mk./k 2 1 and for zero i n l e t gas concentration i n the l i q u i d phase ( F i g . 4).

From t h i s f i g u r e i t can be seen that there i s a p o s i t i v e deviation, which i n

creases at increasing ir/Gz-numbers. This d e v i a t i o n i s caused by the fact that

unequal d r i v i n g forces are added, leading to a systematic error. Within the

measured conditions, however, t h i s deviation i s small enough to be neglected.

I l l

1 - 0 5

1 0 0

0 9 5 -

0 0 5 0-1

TI G z

Fig. 3 The deviation of the additivity of individual phase resistances for

mass transfer in a wetted wall column as a function of tt/Gz (D^/V^ =

0.0001; Foi = 0.01).

(Average overall mass transfer coefficient based on a logarithmic-mean

driving force.)

o

1 0 5

1 0 0

0 9 5

Tt G z

_L_ 0 0 5 0 - 1

Fig. 4 The deviation of the additivity of individual phase resistances for

mass transfer in a wetted wall column as a function of TT/GZ (D„/D -J6 g

0.0001).

(Average overall mass transfer coefficient based on a combination of an

arithmetical and a logarithmic-mean driving force.)

112

n F n

1 2

3 4 5

6 7 8 9 10

2.26313 6.29782

10.30802 14.31325 18.31657 22.31892 26.32070 30.32213 34.32432 38.32519

Table 1 Eigenvalues

1.33823 -0.54556

0.35893 -0.27211 0.22113 -0.18732

0.16313 -0.14488 0.13060 -0.11908

0.393429 -0.118857

0.067046 -0.045787 0.034377

-0.027320

0.022551 -0.019128 0.016559 0.014565

3. CONCLUSIONS

The s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s i n a previously developed wetted wall column do not vary with the same power of the contact time of renewable surfaces. Under these conditions the a d d i t i v i t y of i n d i v i d u a l phase resistances for mass t r a n s f e r does not hold and deviations may occur.

In order to study t h i s deviation, a true average o v e r a l l mass t r a n s f e r c o e f f i c i e n t was c a l c u l a t e d from a numerical s o l u t i o n , which was compared with the value obtained from the a d d i t i v i t y of i n d i v i d u a l phase resistances. It can be concluded that the deviation i s small enough to be neglected.

REFERENCES

1. King, J.C., A. I. Ch. E. Journal, 1964, 10, 671.

2. Szehely, J . , Chem. Eng. Sai. , 1965, 20, 141.

3. C r o f t , D.R. and L i l l e y , D.G., Heat Transfer C a l c u l a t i o n s Using F i n i t e D ifference Equations, Applied Science Publishers Ltd., London, 1977.

4. Crank, J . , The Mathematics of D i f f u s i o n , Clarendon Press, Oxford, 1975. 5. Van den Berg, H. and Hoornstra, R. , Chem. Eng. J., 1977, 12!, 191. 6. Brauer, H., Stoffaustausch, Sauerlander A.G., Aarau, 1971.

113

NOMENCLATURE

d

D

F

n

F o

g Gz

Gz red

h' Ah H

AH r

AH s

I

J

J„

roots of the equation J (a ) = 0 o n

thermal d i f f u s i v i t y

area of i n t e r f a c e

c o e f f i c i e n t

concent rat ion

s p e c i f i c heat

diameter

d i f f u s i o n c o e f f i c i e n t funct ion F o u r i e r number g r a v i t a t i o n a l a c c e l e r a t i o n Graetz number

Graetz number corrected f o r the i n a c t i v e f i l m height f i l m length or co-ordinate of length i n flow d i r e c t ion e f f e c t i v e f i l m length i n a c t i v e f i l m length Henry's law constant heat of re a c t i o n heat of s o l u t i o n i o n i c strength absorption rate per un i t of surface area

Bessel function of the f i r s t kind and zero order

Bessel function of the f i r s t kind and f i r s t order

2, m /sec

kmol/m 3

kg/m Joule/kg. K

2, ra /sec

m/sec

m m m

3 kmol/m .bar Joule/kmol Joule/kmol

3 k-ion/m

2 kmol/m .sec

_og og,addi-

t ion

K og,true K

sec m/sec m/sec m/sec

rea c t i o n rate constant gas phase mass t r a n s f e r c o e f f i c i e n t

l i q u i d phase mass t r a n s f e r c o e f f i c i e n t

o v e r a l l mass t r a n s f e r c o e f f i c i e n t based on gas side o v e r a l l gas phase mass t r a n s f e r c o e f f i c i e n t derived from the a d d i t i v i t y of i n d i v i d u a l average mass t r a n s f e r resistances m/sec true o v e r a l l gas phase mass t r a n s f e r c o e f f i c i e n t

e q u i l i b r i u m constant

s o l u b i l i t y (= C. ,/C .) A,l g , l

m/sec

bar

114

m(T)

n N P

Sh t T

eigenvalues quantity of gas absorbed per unit of surface area a f t e r contact time I

number

l o c a l mass fl u x pressure N0„ 2N 20 4

distance i n r a d i a l d i r e c t i o n radius of wetted wall column gas law constant Sherwood number time

temperature

surface v e l o c i t y of the l i q u i d f i l m mass flow rate of the gas

co-ordinate of length across flow d i r e c t i o n valencies of ions

kmol/m 2

kg/m

kmol/m .sec

bar

m m

o Joule/kmol. K

sec °K

m/sec

kg/sec m

GREEK SYMBOLS

lay e r

the f r a c t i o n of NO^ converted to ^^0^ thickness of the laminar l i q u i d f i l m thickness of f i c t i t i o u s water layer

distance from r e a c t i o n plane to g a s - l i q u i d i n t e r f a c e at instantaneous reactions kinematic v i s c o s i t y density contact time gas flow rate l i q u i d flow rate

2, m /sec 3

kg/m sec

3 . m /sec 3 , m /sec

SUBSCRIPTS

c wetted wall column f l i q u i d f i l m g gas phase i g a s - l i q u i d i n t e r f a c e & l i q u i d phase

l o c a l l o c a l values 115

o i n l e t

Q N0 2 + 2N 20 4

r r e a c t i o n plane

s l i q u i d surface

SUPERSCRIPTS

bulk average or mixing up value

116

; — • — - - - -!

S T E L L I N G E N

1. B i j gas-vloeistofcontact bestaat de mogelijkheid dat r e a c t i e optreedt i n de

gasfase tussen een vluchtige component u i t de v l o e i s t o f f a s e en een reactant

i n de gasfase.

Er kan sprake z i j n van een r e a c t i e v l a k i n de gasfase indien de r e a c t i e als oneindig snel mag worden beschouwd. (Dit Proefschrift)

2. De algemene r e l a t i e s betreffende gasabsorptie gepaard gaande met een oneind i g s n e l l e r e a c t i e i n de v l o e i s t o f f a s e tussen een opgelost gas en een reactant i n de v l o e i s t o f f a s e z i j n i n p r i n c i p e tevens g e l d i g voor systemen waarb i j een vluchtige component u i t de v l o e i s t o f f a s e oneindig snel reageert i n de gasfase met een daarin aanwezige reactant.

(Dit Proefschrift)

3. De sterkte van verdund salpeterzuur a l s wasvloeistof voor de verwijdering

van nitreuzen u i t afgassen wordt voor een groot deel bepaald door de oxida

ti e g r a a d van NO en de oplosbaarheid van s a l p e t e r i g z u u r i n salpeterzuur.

" V .

4. Het ondergronds vergassen van kool i s ongunstig vanuit reactorkundig oog

punt .

5. Het succes van innovatie-gerichte onderzoekprogramma's aan u n i v e r s i t e i t e n

en hogescholen z a l sterk afhangen van de bereidheid van het b e d r i j f s l e v e n

om de nodige informatie en gegevens te verstrekken.

6. De k w a l i t e i t van i n groepsverband uitgevoerde researchwerkzaamheden i s boven een bepaald minimum niveau onafhankelijk van de grootte van de beschikbare financiële middelen.

(New Scientist, 1979, 84_ (1176), 91)

7. De samenleving zou een beter i n z i c h t hebben i n de wetenschap en techniek indien met name j o u r n a l i s t e n en T.V.-presentatoren een grotere deskundigheid en bekwaamheid op d i t t e r r e i n bezaten.

8. De stellingname dat i n het " v r i j e " Westen v r i j h e i d van meningsuiting zou heersen berust op een vooroordeel.

9. De regering i s met name door de Wet op de Investeringsrekening (WIR) medeverantwoordelijk voor de huidige zuiveloverschotten i n ons land.

10. " M a c r o b i o t i c i " kunnen i n bepaalde opzichten beschouwd worden als "luxe

wilde beesten".

11. Hoogbouw i s laag-bij-de-gronds.

D e l f t , 12 maart 1980 J.B. Lefers

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