sulphur burning and the formation of so3

8/11/2019 Sulphur Burning and the Formation of So3

1/16


2/16

8 8

On to p of this it would be of assistance to know

how moisture in

t he

atmosphere would effect these

values.

Table I an d II Graphs A an d B, supply the

answers here an d all calculations were made

according to

th e

methods outl ined in Lundberg 's

book.

Where moist air was assumed in the theory in

th e calculations, this was taken as air 74 per

cent

saturated with water vapour

at

66. 6F. This

represents roughly

th e

average humidity an d

temperature taken over a number of years

at

Mount

Edgecombe during

th e

months of May to December. '

'Graphs

A

an d B show how the volume of the

burrier

gas:

produced decreases with increasing

SO

2

concentration.

A

comparison of the results

withdry an d

moist air indicates

that

the effect of

moisture is

no t

very marked upon the final gas

coming from the burner.

...

.

Theoretical lame Temperature

When sulphur is burned in air, th e reaction is

exothermic an d therefore the products are at a higher

temperature than that of the reactants.'

For

every

particular concentration of SO2 in a gas, there will

be a corresponding flame temperature to be reached

in

th e

burner.

U ~ f q ~ t u n a t e l y

th e

deduction of these flame

.temperatures ,from basic principles is a fairly long

and

tedious procedure: A method suggested by

Lundberg was usedhere an d it is reasonably rapid

an d

convenient, although comparison with flame

temperatures calculated from basic thermodynamic

principles indicates that this rapid method ma y be

slightly inaccurate an d especially so at the ow r-

concentrations of, SO2' It must be stressed that

the

temperatures listed in Tables

II I an d

IV are

'approximate only and should be taken more as an

indication of the order of magnitude of

temperature

an d

not as accurate flame temperatures

Anyone

wishing to derive theoretical flame tempera tures

;may.do so

b y

consulting th e technical literature

d e ~ 1 i r i g with this. subject 5 8 _9 10

'.. Once again, results have been caleulated for

dr y an d 'moist air an d in order to show how the

temperature

of the flame

ma y

vary due to

heat

losses by radiation, th e two cases:

a

No Heat Losses

b Radiation of 15 per cent. of the total heat input

have been considered. ' /

.

T ~ e : T a b l e s an d Graphs show how dilution Of the

burner .gas with ai r causes

th e

flametemperature to

-drop.' ;;Moi;;tiire'again appears to have a negligible

effect.

u ~ p u r ~ i o x i d e

ormation

Most of the points of theoretical importance have

been listed, an d all that remains to completejhe

picture is a consideration of

th e

factors influencing

the formation of SO

3

is known in practice

that

a small

amount

of this compound is always formed

along with th e SO2 in the burner, an d therefore

a knowledge of 'the conditions which favour SO

3

formation should be useful in that it would then

be possible to arrange an atmosphere an d conditions

in the burner which would, keep th e amount of SO 3 '

formed at an. absolute minimum.

When SO 3 is formed in the presence of excess

air, the following reaction occurs: . .

25 0

2

2S 0

3

This reaction is reversihle

an d

therefore it will

proceed from left to right until equilibrium is

reached. At this point, as much SO 3 as is being

formed will be dissociating once more

into

its

components., The equilibrium can only be dis

turbed by

removing one of the reactants or product

of

the.

reaction, .or changing the steady state of

conditions prevailing, otherwise,

if

temperature,

concentration, etc. are kept steady, then

t he

equilibrium will be stable an d constant for those

sets of conditions. .

For tuna te ly, this reaction has been studied. in

some detail by

many

observers and it is possible

to calculate the amount of SO

3 that

will be formed

under almost

an y

given

se t

of 'conditions: The

subject of catalysis

an d

the effects of catalysts have

no direct bearing on the theory of SO 3 formation

as it is conceded

that

catalysts do

no t

disturb the

equilibrium' point, bu t merely serve to speed up

the rate

Of

reaction, thereby ensuring

that

equili

brium will be reached in a shorter space of time. They

also allow lower temperatures to be used when

S0 3

is being formed. Catalysts are therefore of im

portance in' th e Contact Process for theproductiori

of sulphuric acid: .

The theoretical conversion percentages which

have been calculated here do not

take

into account

the time factor, that is to say; no allowance has

been made

forthe

period

that

is necessary to ensure

that

th e reaction reaches equilibrium before the

gases pass from the burning apparatus. Often

it will be found in plant practice

that

the conversion

of SO

2

to SO

3

as measured, is considerably less

than

that

predicted in theory under th e conditions

specified. Thus, the conversion figures listed ma y

be

taken

as the maximum possible, which might

never be attained in the plant.

.

....' . ,

In

orderto be in a position to predictthe amount

of conversion of SO

2

to SO3 under a given set of


3/16

89

conditions, it is necessary to know

the

equilibrium

constant, K, for

the

reaction:

S02

+

i

0

2

S03

which occurs

at

various temperatures. This re

action, and not the previous one having double

the

quantities, has been chosen purely for convenience,

The

equilibrium

constant,

K, differs for

the

two

reactions, the former being

the

square of the lat ter,

but the

calculated conversions

would'

be

the

same

in each case.

For the

purpose of this paper,

the

equilibrium

constant has

been calculated from

the

average of

three

authorities, viz.

Fairlie

4

T 476

10glOI< =

T - 4.474

Lewis and Randall

4936

10glOK

=

T -

4.665

5186.5

10glOK

=

O.611log

1

- 6.7497

where K is the equilibrium constant

and

T is the

absolute temperature

in degrees Kelvin.

The above equations showthat K is dependent

upon temperatu re only. Natu ra lly, the equili

brium constant can be calculated from thermo

dynamic

first principles along these lines:

K [

S03J

h . th tivit

. [

S02J

[ OJt'

were a IS re ac IVI y co-

efficient for each respect ive reactant or produc t,

and this equation

may

be re-written as

K

()

PSO

3 h . h . I

PSG 2 PO

2)f

were

p

IS

t

e

partia pressure

of each substance as. indicated. However, the

calculat ion of K from first principles is somewhat

lengthy,

but

should this expedient be necessary

or desired, reference should be made to

textbooks

on thermodynamics.

B 9 10

Knowing K, it is then possible to deduce the

expected conversion of SO

2

to

SO 3

tinder various

conditions. However, the conversions calculated

in this paper

were carried

out

on

the

lines suggested

by

Browning andKress.? . The equilibrium constant,

.K, was calculated

from

an equahonwhere the

variables were in terms of concentrat ions of the

various products as follows:

K=_x_

l Xjb 1..

2

ax

, 100 - -. lax

where a = initial concentration of SO2 in

where b = initial concentration of

in

where x = Fraction

( cOnverSion)

of S03

100

K is calculated from assumed values of

conversion

for a gas of specific SO2 content, and then the

equilibrium temperature for

that

value of K cal

culated from

the

Lewis and Randall equation given

previously. . ,.

Table

V

and Graph

Edetai l the

var iation of

equilibriumconstant, K, with temperature, Tables VI

VII

and Graphs

F and

G, record

the

equilibrium

temperature for

any

assumed degree of conversion

with a gas of known composition. '

With

a gas of known strength, t he amount of

SO3likely to be formed can be reduced

by

increasing

the

temperature. Generally, in order to reduce

the

possibility of trioxide format ion,

the

SO2

content of the gas should be increased,

.i.e. the

amount of excess air

used,

in burning should be

kept

at

a minimum. On

top

of this, high

corn

bustion chamber tempera tu res help to reduce

the

incidence of SO:I formation. However,

theory

indicates that it is literally impossible to prevent

SO2 undergoing a certain amount.of oxidation,

albeit a very small amount at temperatures over

1,000C. .

The sugar chemist is attempting to produce a

burner

gas containing S02 only,

and

none of

the

higher oxide,

and

in

the

light of

the

foregoing .theory

it would appear

that

this could best be accomplished

by the following:-

a

Maintaining the

quantity

of excess

air

used

at an absolute minimum for the plant in

question. .

b

Ensuring that the flame temperature. in

the

burner

is

kept

as high as possible

and heat

losses due to radiation are minimised.

c Arranging for

the

hot burner gases to be

cooled as rapidly as possible in order that they

need not be held at those temperatures for

any

length of time, where maximum conversion

of SO2 to SO:i is likely to occur.

d On top of this it would be helpful to

draw

the

gases through

the

burner as

rapidly.

as

possible, with the apparatus in use, to

deavour to reach a

state

'of affairs where the

reaction

250

2

0 2 2'50

3

, does riot have

time to reach equilibrium.' However,' this

rate of flow of

the

gases in

the

burner will

naturally be dictated largely by the rate at

which

sulphur can be burned without sublim

ation taking place.


4/16

This completes

the

theoretical study of sulphur

burning, and all that remains is to observe what

occurs in practice. The theory is in fair agreement

with what

is experienced in

actual

pant operation

andit is hoped that

the

connections between theory

and practice will be obvious.

P RT

PR CTICE

The most convenient raw material for

the

pro

duction of SO2 is sulphur, and although there are

many

alternative sources available, sulphur still

proves

the

popular choice for a

variety

of obvious

reasons. The sulphur used in

the

sugar

industry

in this country is obtained almost solely from

the

U.S.A., this source being chosen with consideration

to price, quality

and

availability.

In

1949

the

production of crude sulphur in

America amounted to approximately 41 million

long

tons.v

mined by

the

Frasch-process

and

it is

interesting to note that

the

Texas Gulf

Sulphur

Company was the major producer. During the

same period

the

Union of South Africa is credited

with

the

importation of

just

over 65,000 long tons

of crude

sulphur.P

most of which was used in

the

manufacture of sulphuric acid.

can be seen,

therefore, that

the

sugar industry's requirements of

between 4,500

and

5,500 long tons per annum, are

quite small when compared with

the

total imports

into the country.

Sulphur is

abundantly

distributed in nature,

and

on the Gulf Coast it is usually associated with

salt dome intrusions. The element is found to

occur in

the

limestone, gypsum

and

anhydrite cap

rock

and

a few of these domes contain commercial

quantit ies of sulphur .i The occurrence of these

domes appears to be limited mainly to

the

Gulf

Coast region of

the States

of Texas

and

Louisiana.

Detai ls of the mining and production of sulphur

are contained in a highly informative circular en

titled Sulphur-General Information issued

by

the United

States Bureau of Mines-

and

anyone

interested would be well-advised to procure a copy

of this publication.

The element sulphur is non-metall ic

and

exists

in a var ie ty of allotropic forms

and

therefore

may

be said to have a series of properties depending on

the

form in which it

found.

Natural

sulphur

usually occurs as the

more

stable rhombic or

a-sulphur form. However, it can exist in

many

forms,

bu t

they all tend to revert to the rhombic

form. A study of the various properties

and

allotropic forms of sulphur is outside the scope

of this

paper

and a brief account of those properties

of relative importance to sulphur burning only,

will be given. Sulphur, S 8 ' is a bri tt le, yellow ele

ment which is solid at room temperature, is in

soluble in water, bu t

will dissolve in carbon bisul-

90

phide. The rhombic form melts at 112. 8C. and

the

melt forms a mobile liquid which becomes more

viscous as the temperature is increased, unti l at

2 C it becomes so thick it will not flow. However,

at

350C., the melt again becomes mobile. It ignites

at

about 248C., finally boiling

at

444.6C.2 The

above temperatures should be

taken

as approximate

only as Mellor gives specific figures for

the

known

allotropic forms

and any

particular allotropic

mixture would probably have its own characteristic

physical

and

chemical constants.

A typical analysis of the sulphur supplied to the

sugar industry is roughly as follows:-

16

Moisture

at

105C.

0.10

Total Residue on Burning 0.13

Organic residue 0.03

Mineral residue

0.10

Arsenic as As

20

3

2 p.p.m.

Selenium as Se Negligible

Acidity as H 2S0

4


5/16

of

the

burner, bu t it is possible that in

many

cases

they

may

reach a proportion where they cause

trouble.

Types of urners

a

Rotary urner

This burner is proably

the

most commonly used

and

is known as

the

Glens Falls

Rotary

Burner.

The makers, Glens Falls Machine Works of America,

claim that maximum efficiency can be

attained

with

unskilled labour

and

that installation

and

operating

costs are low. Furthermore, they emphasise that

losses due to sulphur sublimation or formation of

SO

3

can be eliminated almost entirely.' With

this type of burner it is possible to operate con

tinuously at SO 2 concentrations ranging between

5 per cent. and about 18 per cent. In order to

prevent fluctuations in gas strength it is desirable

to

feed

the

sulphur by mechnical means and not

by hand.

The sulphur in the lower part of the horizontal

rotat ing drum forms a molten pool as well as a thin

film around the circumference of

the

drum. This

film, in effect, increases

the

surface area of the

sulphur exposed to the incoming air

and

a further

increase in surface area is brought about by the

sulphur that drips down from the

top

of the drum.

These factors help to increase the capacity of

the

burner and ensure that combustion is complete.

On

top

of this, it is claimed

that the

sulphur film

protects the drum metal from deteriorat ion due to

hot

SO 2 gases as

the

heat conductivity of sulphur

is less

than that

of cork.

A combustion chamber is fitted at

the

back of

the horizontal drum and this serves to complete

the

combustion of any sublimed sulphur as well as

to mix the gases and dilute them to

the

desired

strength. This chamber is therefore constructed

so as to have air ports and one or more baffles.

Impurities in the sulphur do not readily affect

the

operation of these burners because

the

rotation

of

the

drum agitates the molten sulphur sufficiently

to prevent any blanketing films forming on the

surface. A further refinement that can be added

is a sulphur melter and the heat of radiation from

the

burner

may

be utilised to keep the sulphur

molten. Steam heating coils in

the

Melting Tank

are only required for starting up the burner after

a long shut-down. In order to further improve

the

efficiency of this machine, the molten sulphur

should be strained prior to passing it to the burner.

The Teeding of molten sulphur ensures that the

burner is handling moisture-free material.

Ideal results are obtained with this type of burner

by using sulphur of a minimum

purity

of 99.6

per cent. .The molten sulphur in the horizontal

drum should be at the highest possible level without

allowing any overflow to take place.

91

The makers suggest

the

following capacities for

a burner 30 in. in diameter

and

8 ft. long.

Capacity with l in. water draught is 1351bs. Sulphur

per hour.

Capacity with in. water draught is 200 lbs.

Sulphur per hour.

Capacity with 2 in. water draught is 270 lbs. Sulphur

per hour.

A burner of identical diameter,

but

only 6 ft. long

would have corresponding capacities of about 75

per cent. of those given above. As a rough guide,

it may be taken tha t : .

For a

1

in. draught,

the

burner consumes approxi

mately 2 lbs. Sulphur/sq. ft yhr

For

a

in. draught, the burner consumes approxi

mately 3 lbs. Sulphur/sq.

ft /hr

For

a 2 in. draught, the burner consumes approxi

mately 4:.2 lbs. Sulphur/sq.

ft.yhr.

Another authoritv considers that a burner 36 in.

in diameter

and

ft. long is capable of handling

nearly 8 lbs. of Sulphur/sq. ft./hour. Fairlie

finds it practical, without resorting to high draught,

to run a burner

ft. in diameter at a rate of 1 ton

of Sulphur per

day

per foot of length.

The size of the combustion chamber is variable

between fairly wide limits and it should be borne

in mind that a large chamber permits a higher

concentration of SO 2 in

the

burner gas without

danger of sulphur sublimation. is suggested that

a chamber spa e of 60 cubic feet per ton of sulphur

per 24 hours- is adequate, while the makers of this

type of burner indicate that a minimum space of

it cubic feet per pound of sulphur per hour per

square foot of burner area should suffice for burners

up to 30 in. in diameter. These figures should be

taken as approximate indications only

and

no

hard

and fast rules seem to apply. The size of

the

combustion chamber should be dictated by practical

considerations

and

previous experience.

Sutermeister' states that

rotary

sulphur burners

produce a gas of varying composition owing to

sudden rushes of cold air through the apparatus,

but

Fairlie claims that with a continuous feed, it is

possible to maintain

the

gas at a uniform con

centration of SO 2 with only minor fluctuations.

He stipulates, however, that this is dependent on

the

depth of the molten sulphur in

the

rotating

cylinder being kept uniform. Any change in this

depth will alter

the

area of

the

unsubmerged film

covered surface and this in tum will alter

the

rate

of combustion of sulphur.


6/16

92

b The cme Burner

This

type

is deigned to give a constant rate of

burning for any given concentration of SO 2 in the

gas. The makers, Acme Coppersmithing and

Machine Company, point out that the rate of com

bustion of sulphur bears a definite relation to the

amount

of air supplied. The amount of air re

quired for a given concentrat ion of SO 2 is given by

the

expression:-

o/SO . C b ti G 90.65

2 In om us On ases

A 0.026

where A

lbs. of air

at

70F. per lb. of sulphur

burned when the burner is operating a ta

rate

of

t

lbs. of sulphur per square foot of burning surface.

Should the sulphur burning rate be changed, then

the above formula has to be modified slightly.

is claimed

that

a

tray

or rotary burner does

not

satisfy the requirements of a definite constant

burning

rate

because of the following

factors:-

1

Ordinary sulphur contains organic matter

and this accumulates as a carbonaceous film

on the surface of the burning sulphur which

may eventually extinguish the flame.

(2) Additions of fresh charges of sulphur to the

burning surface disrupts the burning rate and

normal conditions are only returned to afte r

s v r ~

hours have elapsed.

With this burner, the makers have overcome the

first drawback by operating the burner at a rate

greater

than

2

lbs.of

Sulphur per Square foot of

burning surface per hour, finding that this prevents

the formation of carbonaceous scum by burning it

off with the rest _of the gases. The drawbacks of

factor (2) were prevented

by

arranging a special

feeding device

that fed molten sulphur in a manner

that did not dis turb the burning surface, but at

the same time, ensured that a constant level of

molten sulphur was maintained in the burner.

This

burner is opera ted on compressed air

and

a special sulphur melter is supplied with the

apparatus. Sulphur is fed to the

bottom

of the

burner

via a main feed pipe, and a side feeder arm

on this main feed allows for positive control of

sulphur feed, thereby guaranteeing a constant

level of molten sulphur in the burner.

All the data. given here and the brief description

of the principles behind this burner were taken from

a pamphlet issued

by.

the manufacturers, viz.

Acme Coppersmithing and Machine Company of

Oreland, Pennsylvania. A description of this burner

may be found in the technical literature.19

c

pray ype

Burner

The Research Department of the Texas Gulf

Sulphur Company is accredited with

the

invention

of this burner. The development of apparatus

was described in a technical paper issued in 1934

20

and it was therein explained how the burner was

operated. Briefly, molten sulphur is fed to a spray

nozzle which injects a fine spray of this material

into the burner. At the same time the desired

amount of air is forced into the burner chamber

and ignition of

the

sulphur takes place. The

burner chamber is fitted with baffies to mix the

resultant gases and prevent any unburned sulphur

passing through to the absorption apparatus.

The burning

and

combustion chambers are in one

uni t. Normally,

the

burner chamber is lined with

fire-brick to reduce heat losses and minimise the

formation of SO 3

Certain advantages are claimed for this burner

and among those listed are:-

(1) Operation is simple and the burner requires

little

attention

once

the rate

of combustion

has been set. The combustion rate can be

changed simply by altering the stroke of the

sulphur

pump

and adjusting the compressed

air supply accordingly.

(2) Starting up and shutting down operations are

comparatively easy, and if the burner is started

up when cold, maximum gas concentration can

be reached within } hours. In order to shut

down it is merely necessary to stop the sulphur

metering pump and shu t off

the

compressed

air supply.

(3) The burner is extremely flexible insofar

as

rate

of combustion is concerned

and

there

fore it is possible to vary the capacity of the

burner within fairly wide limits.

(4) will produce a gas with an SO

2

content of

anything up to 20 per cent. when run con

tinuously.

(5) Sublimation of sulphur should not occur

provided proper care is taken, and since there

is no burning-down period, aswith conventional

equipment, the hazards of possible sublim

ation are reduced to an absolute minimum.

(6) Provided the gas concentration is kept high

and

the burner is lined with refractory brick,

l itt le SO 3 should be formed

at

the elevated

temperatures (2,400 to 2,700F.) of operation.

During tests it was found that the SO 3

content of the gas passing to the cooler was

only 0.14 per cent. of the sulphur burned.

(7) The maintenance costs are not likely to be

high and the initial cost of installation should

compare favourably with that of any of the

more usual types of burner.

(8)

Kress

found

that

power costs could be re

duced by 75 per cent. over that of conven

tional burners.


7/16

93

Anyone wishing to study in detail the design of

a spray type burner is well-advised, to read the

paper by Conroy

and

johnstone on this subject.

The authors indicate that the internal volume of a

rotary burner should be 13/14 cubic feet per ton of

Sulphur per day and on top of this a combustion

chamber space of 60 cubic feet per ton of Sulphur

per day

is required, whereas the

total

volume of a

spray type burner is 24 cubic feet per ton of Sulphur

per day.

It is felt

that

the three burners listed cover the

types of particular and possible interest to the sugar

industry. In order to complete our practical con

siderations of sulphur burning, it is only necessary

to study how SO3 formation. occurs and what can

be done to minimise its formation as this information

should then make it possible to successfully produce

a gas that is rich in SO2and, at the same time, free

of all but traces of SO3

The Formation and Dissociation

SO

3

The conditions favouring the formation of SOa

are:-

(i) Decomposition of any sulphuric acid

that

may be present in the commercial sulphur.

(ii) Formation of trioxide during the actual

burning of the sulphur.

(iii) Oxidation of the SO 2 present in the burner

gas.

The first source accounts for only a relatively

small amount of the SO3normally found in burner

gas. Naturally, sulphur will oxidise. to a certain

extent in the presence of moist air, but the amount

of sulphuric acid formed is bound to be extremely

small.

Undoubtedly, a certain amount of trioxide will

be formed during the burning of the sulphur, but

this is difficult to determine accurately. t will

depend, to a large extent, upon the proportion of

excess air present and upon the temperature reached

in the burner.

t

can be seen therefore that the

SO content of the gas should be as high as possible

for the type of burner in use.

The third source, i.e. the oxidation of SO

accounts for the major portion of trioxide found in

a burner gas

and

consequently this aspect of the

problem will have to be examined in fairly minute

detail. In practice it has been found that oxidation

will take place only unti l a certain ratio of dioxide

to trioxide is reached At this point equilibrium

is established between the reactants and products

oof the reaction and as fast as SO3 is being formed,

it is again decomposed into SO2 and oxygen.. This

equilibrium may be distrubed or changed by altering

the concentration of the reactants on either side of

the equation:

O

2

2S0

2

2S0

3

A rotary steel or cast-iron burner operating

continuously has been found to convert between

1 per cent. and 2 per cent. of the S02 into S032,

while a similar burner lined with refractory brick

will produce a gas of much lower trioxide content.

Obviously, then, the materials of construction have

an effect upon the oxidation of SO2and an investig

ation led to the discovery

that

certain metals or

their oxides could increase the rate at which trioxide

was formed. These metallic substances are known

as catalysts and while they do not take part in the

reaction in a quantitative way, that is to say,

they

are not used up or depleted during the course of

oxidation, they most certainly speed up the rate of

reaction. These substances, therefore, warrant

study as well. .

t will probably be advisable to study the various

effects of physical and chemical conditions tlpon

trioxide formation separately to ensure that the

picture be complete.

Effects

Excess

ir

Comog and co-workers found that when sulphur

vapour was burned in the presence of 250 per cent.

excess air at 4:60C approximately 3.4 per cent.

to 3.8 per cent of the sulphur appeared as the

trioxide. However, Browning and Kress observed

less than 0.02 per cent. conversion under similar

conditions in the absence of a flame. The obvious

inference is that atomic oxygen in a flame has a

considerable influence upon the reaction. The

results of their experiinents indicated

that

trioxide

formation could be reduced by decreasing the

equilibrium conversion of SO2 to SO 3 by keeping

the quantity of excess air at a minimum in con

junction with high combustion chamber temper

atures. A perusal of the theory will show agreement

with this statement.

Obviously, then, the amount of air supplied to

the burner should be carefully metered, or, failing

that, regular analyses of the gases from the com

bustion chamber should be carried out to ensure

that the SO

2

content is being maintained at a

maximum.

A rough guide to the correct amount of air re

quired is based on observation of the flame in the

burner. the sulphur burns with a blue flame,

conditions are just right. A flame with a brown

tinge indicates insufficient air and sublimation of

sulphur. However, these observations do not

prevent one from operating the burner with an

excess quantity of air and one is well-advised to

rely more on chemical tests and mechanical aids

than on rule-of-thumb methods such as those men

tioned above.

A study of Table

VII

will show how the quantity

of excess air can influence the formation of SO3


8/16

94

Should the burner be operated with insufficient

air to complete the combustion of sulphur, then

sublimation takes place and slight mists of unburned

sulphur may form.

nfluence Temperature

Theory has already indicated to us that temper

ature exerts a considerable influence upon the

amount of trioxide formed. In practice it has been

found that the optimum temperature range favour

able to the formation of SO 3 lies between

MOC

and 980C22 Frohberg- found that maximum

oxidation occurred at 400

0 500e.

while at an

approximate temperature of 900/1,000C., dissoci

ation took place. t has been shown that at

1,000C. nearly 50 per cent. of the SO

3

formed is

decomposed, this decomposition starting at 700e.

at which temperature the conversion of SO2 to

S03

is about 60 per

cent.

Above 1,000e. the

dissociation of the trioxide is more rapid than its

formation. t may be generally stated that below

400e. the rate of formation of SO3 is too slow to

be regarded as a nuisance, while above 1,000e.,

even though SO 3 is formed rapidly, the rate of

decomposition predominates. However, even at

305e., oxidation of S02 has been found to occur,

albeit at a very slow rate. Thus we have two con

flicting states, viz. where the rate of conversion of

SO2 to SO

3

increases rapidly with increasing

temperature,

but

where the dissociation of SO3

overhauls the rate of conversion, and at the higher

temperature (above 1,000e.), therefore, the per

centage conversion, as measured, tends to decrease.

The reaction:

2S0

2

O

2

2S0

3

takes place nearly to completion at 450C in the

presence of a catalyst , and. it has been shown

that

while the

rate of the forward reaction is only

moderate at 400e., it increases to 40 times this

value at 500e. the decomposition reaction only

becoming perceptible at 550e. and upwards.

t may therefore be said that up to a temperature

of 450C., reaction of formation prevails, and only

well above this temperature does dissociation come

into play.25

Sulphur trioxide is very stable in the absence

of contact substances.

and

once formed is difficult

to dissociate. Generally, decomposition will not

take place completely at temperatures as high as

1,100/1,200e. Decomposition of trioxide already

formed cannot be relied upon to keep the loss from

this source at a reasonable value at temperatures

below 1,00e., the amonnt of dissociation at this

temperature only reducing the loss by an amount

of less than 0.5 per cent.

It is well worth .remembering that the final

equilibrium condition of the reaction depends only

upon the temperature

and

the composition of the

gas mixture, and oxidation of SO2 will take place

until a certain ratio of trioxide to dioxide is reached.

High temperatures favour a low ratio and low

temperatures a high ratio of SO

3

to SO

2

7

Presence Moisture

The air drawn into a burner is not normally

dried,

but

to prevent the formation of corrosive

sulphuric acid mists, the water content of the air

used should not be more than 5 mgm. per cubic

foot of air at S.T.P.24 Under conditions prevailing

in Natal, this would involve drying the air with

either concentrated sulphuric acid or P 205 or :a

similar efficient drying agent.

the air is thoroughly dried with P 205 there

is little oxidation of SO2 up to a temperature of

450e. and in general it may be stated that dry

air helps to retard oxidation. In the presence of

water vapour, oxygen does not combine with SO2

at 100e.

but oxidation does occur, even at this

low temperature, if particles of liquid water are

present.6 Mellor found that moisture does not

appear to affect the oxidation of SO 2

but

the

presence of CO2 and nitrogen causes more SO3

to be formed.

Moisture has a poisoning effect on some catalysts

and Tolley has shown this to be true for iron

oxide. Upon continued exposure, however, cata

lysis slowly increases until after 40 hours the

catalysis with wet gases is half that of dry gases.

He found

that

water vapour has its greatest in

hibiting action at a temperature of 475e., put

even at 635e., the catalytic act ivity of iron oxide

is reduced to of

that

with dry gases.

Browning and Kress? investigated the dew-point

of burner-gas mixtures and showed that the cor

rosion to be expected would be greatest at the dew

point of the particular gas because of the combined

action of scale and condensed liquid upon the metal

base. Above the dew-point, only gases are present

and corrosion is much reduced, while at temper

atures below the dew-point, the rate of corrosion

is considerably less on account of the slowing up

of the chemical reactions at these lower temperatures.

Their experiments revealed that the dew-point

increased as the SO 2 content of the gas was raised

and for safe operating practice, using moist air,

the temperature of the gas in iron should be kept

above

200C

to prevent undue corrosion of the

metal. When the gas is required to be cooled

below this temperature, it should be transferred to

a lead pipe.


9/16

As previously stated,. moisture in the air can

cause mists to form in the burner gas

and

these are

most difficult to, remove. A method which helps

in the removal of mists consists of cooling the mixed

gas to below 46C., when the S03 will condense.

The gas should now be passed through a filter-box

packed with charcoal or iron borings

and

then

through a water-spray .scrubbing tower. This

method, however, is not always very successful

and it is easier to prevent mists than, having once

formed, to remove them.

ction Catalysts

Catalysts do not affect the equilibrium point of

a

gas

mixture,

bu t

merely alter the rate at which

equilibrium is approached. The ratio of the original

and

final substances present in the gas will not be

changed upon contact with a catalyst . The contact

substance will only ensure that this equilibrium

ratio, is reached in less time.

A list of various contact substances normally

encountered follows, and these, with their effects

upon SO3 formation, have been listed separately

for the sake of clarity. Anyone wishing to know

more about the effects of various catalysts on SO3

formation, should read the monumental work of

Browning and Kress, as this covers the subject

most comprehensively.

Iron Compounds

Iron or its oxides have long been known to exhibit

cata lytic properties as far as the oxidation of SO2

is concerned and the Mannheim Process for the

manufacture of suphuric acid utilised iron oxide

as a catalyst.

For

reasons of efficiency, this oxide

has been replaced by either plat inum or vanadium

in

the

Contact Process.

Tolley predicted tha t the first reaction to occur

when steel was in contact with sulphur dioxide

and

oxygen at reasonably high temperatures, would

be a combination of oxide and sulphide formation,

represented as

2Fe

S02-

2FeO

S

Fe

S- FeS

or

alternatively:-

5Fe

2S0

2

- 2FeS'

Fe 30 4

He pointed out that as FeO cannot exist below

570C., the first reaction shown above could only

occur above this temperature. The experiments

showed that the catalytic activitiy of mild steel

increased rapidly during the first few hours of

exposure to S02 but after about 10 hours the

activity became reasonably constant. was felt

that this initial rapid increase in the rate of oxid

ation of SO2was probably due to the formation of

iron oxide.

95

The catalytic effect of iron compounds depends to

a large extent upon their physical

and

chemical

state . Experiments have shown

that

iron oxides,

as such, are not good catalysts, bu t their activity

may be increased after a certain period by the

formation of sulphates and other compounds.

At elevated temperatures both the dioxide and

trioxide

react with iron, and this would explain

the short life of iron under these conditions of

service.

In a rotary sulphur burner, there will be surges

of SO3 formed when star ting up or burning down

as more iron will be exposed during these periods.

Furthermore, the temperature range at which

maximum SO3 formation occurs will be passed

through during these stages.

Reverting to the influence of physical state upon

catalytic activity, it is worth noting

that

ferric

oxides may exhibit contact properties,

and the

surface of the material will exert the least influence

when freshly precipitated oxides, which are not yet

dried, are used. The activity increases the oxide

has been moderately heated or kept for a long t ime

so as to become dry. Oxides obtained by heating

ferric- or ferrosulphate give a much lower contact

action than that obtained with an oxide prepared

by igniting a precipitated hydroxide or pyrites

cinders. An oxide containing 21 per cent. Arsenic

as As shows considerably more contac t reaction

than that of a pure oxide at 700C., while

the

addition of copper oxide to an oxide of iron is

favourable to the formation of S03.

26

Observations have shown

that

when a burner

gas is in contact with an iron pipe, maximum

conversion occurs at 750C., there being a 13per cent.

maximum conversion for a gas with a

10.5

per cent.

S02 concentration. This conversion drops to 10

per cent. when the SO2content is raised to 14 per

cent. and a further decrease in conversion to 3 per

cent. for a gas of 19.5 per cent. content is observed.

At 1,000C., the conversion at all concentrations

IS

zero.

Ferric oxide begins to exhibit catalytic act iv ity

at 550C., and this activity increases to a maximum

in the temperature range 600

0 620C

6

Increasing

the

S02

content of the gas from

2-12

per cent.

does not affect the conversion to

any

appreciable

extent, although with higher concentrations,

the

yield of SO3 is lower. As a contact substance,

this oxide only becomes really effective at a temper

ature of

600C

when a percentage conversion of

SO2 to SO3 of 40 to 50 per. cent. is attainable.

In practice, however, the conversion never exceeds

60-66 per cent.

Iron oxide formed from pyrites-cinders shows

lit tle activi ty, and this only begins above 650C., .

increasing with temperature

and

reachirig a con-


10/16

version of 1

percent.

at 1 000C. with a gas con

taining 15 per cent. SO

However it should be

considered worthy of note that if 3 per cent. of SO

3

is formed in a burner gas during normal burning

operations this figure may under favourable

conditions be raised to three or four times this

value if the gases are passed through red-hot

pyrites-cinders.

Pure iron oxide exhibits maximum contact action

at 650C. the respective conversion figures being

15

and 5t

per cent. for gases of 10.5 and 19.5 per

cent. SO 2 con tent . These conversion figures drop

to 4i and

Ii

per cent. respectively when the temper

ature is raised to 1 000C. the activity in this

case being only above

5 C 7

Ferrous sulphate has a m aximum c onta ct action

at

65 C

when t he conversion of a gas containing

15 per cent. S02 is 12t per cent. this conversion

decreasing to t per cent. at 1 000C. Similar con

version figures for ferric sulphate are 8 per cent. at

65 C and

t

per cent. at 1 000C.; indica ting

that

it is less a ctive

than

the ferrous form.

All the above observa tions indica te that iron in

its many forms exhibits varying degrees of catalytic

a ct iv it y and if the formation of. SO

3

is to be mini

mised then obviously gases of high S02 concen

tratio n must be produced at the highest possible

combustion c ha mber tem pe ra ture that can be

a tt ai ne d in practice.

is of further benefit to

note

that

in many instances t he amount of SO

3

that

will be formed according to t heo ry will not be

reached in practice as the gas passes through the

combustion apparatus before equilibrium is reached.

Silica and Silicates

is generally conceded that vitreous fused

silica exerts no c at al yt ic effect on the oxidation of

S02.7 The same applies to Dialite brick and

therefore these materials are considered satisfactory

for the lining of furnaces c ombustion chambers

etc.

The Efficient Operation a urner

In America it is a generally accepted fact

that

the air to the burner should be carefully metered

by mechanical means and furthermore allowance

is made fo r fluctuations in the burner. These

changes in burning rate are catered for by the

installation of an automatic recording device which

continuously analyses the SO 2 content of the

gases from the combustion chamber. A dia phra gm

operated valve then automatically operates the

air inlet and sulphur feed and regulates these

t he re by keeping t he SO 2 co nt en t of the b ur ner gas

constant within fairly narrow limits.

96

Furthermore the sulphur feed should be

mechnical to enable a constant rate of feed to be

maintained.

With.

all these refinements it is

possible to arrange for high combustion temper

atures and a burner gas containing the highest

concentrati on of SO 2 possible with the type of

apparatus in use. Even

better

results will be

obtained if the sulphur. is melted

and

strained

before it is fed to the burner for reasons already

gIven.

The combustion chamber should be fitted with

one or more baffles to ensure thorou gh m ix tu re of

the gases and p reven t

any

possibility of unburned

sulphur passing to the absorption plant. Coolers

are normally fitte d a fter the combustion chamber

and it is imperative that cooling of the gas be as

rapid as possible to minimise SO

3

formation.

In it iall y the gases would be air- o r. water-cooled

and t hen passed to eit her a direct or indirect cooler

to bring the temperature down to 200/300C.

At this stage if further cooling is a ttem pted the

pipes conducting the gases should be lead-lined.

The subject of cooling is outside the scope of th is

paper but is well-worth : pursuing. by reading

Lundberg- and others.

Summary

The practice of sulphur burning the production

of SO 2 and the formation and dissociation of SO

3

has been outlined and the effects of:

Excess Air

Temperature and

Catalysts

upon SO

3

formation detailed. Where possible a

comparison with t he th eo ry has been made and it

has been shown

that

the following points all help

to increase the efficiency of this type of

plant:

a

is preferable to feed the sulphur con

tinuously by mechnical feeder. The feeding

of strained molten sulphur is preferable as

this eliminates moisture and deleterious hydro

carbons.

b The quantity of excess air should be carefully

controlled and the SO2c ontent of the burne r

gas k ep t at a maximum.

c

The temperature of the burner and com

bustion ch amber m us t be mai nt ai ned above

1 000C. if at all possible.

d Automatic recording

and

operating

apparatus

.to maintain an even feed of sulphur and

constant SO 2 content of the gas is helpful.

e

The gas from the combustion chamber should

be filtered and cooled as rapidly as possible

to pre ve nt formation of SO3


11/16

Conclusions.

The authors have made everv endeavour

and

taken all possible precaution s to ensure that the

information given is accurate. However, mistakes

may

have cropped up

and

no responsibility can be

taken for such errata.

Many of the improvements listed

may

possibly

make the production of SO 2 by these methods rather

costly

and

their inclusion should not be taken as

a recommendation, but rather as a guide or example

of how efficiency may be improved. .These refine

ments are in practice in America in many of the

larger paper mills, so obviously they are economical

for the production of large quantities of SO

is admitted that it is literally impossible to

prevent the formation of SO3 entirely, bu t if pre

cautions, along the lines of those mentioned in this

paper, are taken, the

quantity

of SO3 can be reduced

to such a low level

that

it no longer constitutes

a nuisance.

The types of burners listed were limited to those

generally used in the. Sugar Industry and those

which may be of interest. The spray type burner

is proving very popular in America and it produces

a gas of low SO 3 content and furthermore it is very

flexible in that it may be started or shut down in

a very short space of time. Control is easier than

with the conventional rotary burner and paper

mills in America have found

that

installation costs

are not excessive. Maintenance costs are low

and power consumption compares most favourably

with other types.

For the sake of those who would like to know

more about sulphur burning and its applications,

a reading list has been added.

Acknowledgments.

Our sincere appreciation

and

thanks are duly

made to those firms and Institutions in America

who corresponded with us, supplied technical

articles and generally went out of their way to be

helpful. In particular we would like to mention:-

The Paper Institute, Texas Gulf Sulphur Company,

Acme Coppersmithing

and

Machine Co.,

and

the

Glens Falls Machine Works all

American organis

ations that were extremely helpful

and

co-operative.

REFERENCES.

1Beater:

The Distribution

of

Temperature

in

the

Sugar

Belt

of Natal

and

Zululand S.A.S.T.A. 1949.

2Lundberg: Acid Making in

the

Sulphite

Pulp Industry.

3Chemical Control

Plant Da ta :

Booklet issued

by

Chemical

Construction Company.

4

Fairlie: Sulphuric Acid Manufacture.

5Lewis

and Randall : Thermodynamics a nd th e Free Energy

of Chemical Substances.

6 Mellor: A Comprehens ive Treatise on Inorganic an d

Theoretical

Chemistry Vol.

X

97

7Browning and Kress : A

Study

of Some Factors Influencing

t he Formation and Dissociation of 50

3

in Burner Gases Paper

Trade Journal 100 No. 19, 31-43, 1935.

8Glasstone: Thermodynamics for Chemists.

oDodge: Chemical Engineering Thermodynamics.

Whitney Elias and May: Chemical Reaction Equilibria

TAPPI;

34,

No.9 11 51

.

11 The Glens Galls

Rotary Sulphur

Burner: Pamphlet from

Glens Falls Machine Works.

2 Darrah: The Preparation of S02 Paper

Trade Journal

pp. 132, Nov. 30, 11 50

3

Sutermeister: Chemistry of

Pulp

and Paper Making.

14 Josephson an d Downey: Sulphur and Pyrites U.S. Bureau

of Mines Yearbook, lU49.

15Ridgway:

Sulphur General Information U.S.

Bureau

of Mines I.C. 6329. . .

6 Sulphur: Technical Service Note No. 32 African Explosives

an d

Chemical Industries,

Ltd.

7 Furniss: Rogers Manual of

Industrial

Chemistry.

8

General Description of Acme Burner Pamphlet from

Acme Coppersmithlng

and

Machine Co.

Cain

an d

Chatelain: New Low-Capacity

Sulphur

Burner-

Chemical

and

Metallurgical Engineering, 46, Oct. 1939.

2 Kress

and

Others:

Spray

Type. Sulphur Burner The Paper

Mill, Oct.

11 34

2

Conroy

and

Johnstone: Combustion of

Sulphur

in a

Venturi

Spray

Burner Industrial

an d

Engineering Chemistry 41, pp.

2741, Dec. 194U.

22

Barker:

Sulphite Acid Preparation Paper

Trade Journal

pp. 136, Nov. 30, lU50.

23

Newell. Stephenson: P ulp a nd P ap er Manufacture Vols. I

II and III.

2

sulphur burning and the formation of so3

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