aerosol filtration

Commission of the European Communities

Aerosol filtration

Report EUR 8951 EN

Commission of the European Communities

nuclear science and technology

Aerosol filtration

M. Klein, W.R.A. Goossens

Technical assistance : M. De Smet, J. Trine, M. Hertschap

Studiecentrum voor Kernenergie/Centre d'étude de l'énergie nucléaire SCK/CEN Chemistry Division CIT Boeretang 200 B-2400 Mol

Contract No WAS 228-81-8B(RS)

Final report

Work performed within the framework of the indirect programme (1980-84) of the European Atomic Energy Community

'Management and storage of radioactive waste'

Directorate-General ilei enee, Research and Development

1984 EUR 8951 EN

Published by the COMMISSION OF THE EUROPEAN COMMUNIT IES

Directorate-General Information Market and Innovation

Bâtiment Jean Monnet LUXEMBOURG

LEGAL NOTICE

Neither the Commission of the European Communities nor any person acting on behalf of the Commission is responsible for the use which might be made of the

following information

Cataloguing data can be found at the end of this publication

Luxembourg, Office for Official Publications of the European Communities, 1984

ISBN 92-825-4269-6 Catalogue number:

© ECSC-EEC-EAEC, Brussels · Luxembourg, 1984

Printed in Luxembourg

CONTENTS

Page

LIST OF SYMBOLS V

1. INTRODUCTION 1

2. DESCRIPTION OF BEKINOX FILTRATION MATERIALS 1

3. PARAMETRIC STUDY OF THE FILTRATION CHARACTERISTICS 2

3.1. Description of the experimental procedure 2 3.2. The parametric study of the Bekipor WB porous mats 3 3.3. The parametric study of the Bekipor sintered webs 4

4. FILTRATION AND REGENERATION BY WASHING FOR TOO FILTER CONFIGURATIONS 5

5. AFHT AEROSOL FILTRATION AT HIGH TEMPERATURE 7

5.1. Description of the test unit and of the test procedure 8 5.2. The data obtained in AFHT 9 5.3. Discussion of the results 15

6. GENERAL DISCUSSION 18

7. CONCLUSION 18

FIGURES 19

III

LIST OF S Y M B O L S

C dust inlet concentration (mg/m3N dry gas basis)

dp aerosol particle diameter (ym)

d fibre diameter (pm)

D dilution air flowrate (N m3/h d g b)

D total air flow rate (m3/h)

DF decontamination factor

DP total pressure drop (Pa)

DP pressure drop through the filter medium (Pa)

DP,. pressure drop through the dust layer (Pa)

DP initial pressure drop during filter drying (KPa)

(H2O) water vapour concentration (%)

I insoluble fraction of cesium (%)

I D insoluble fraction of dust (%)

Ps s

Ko characteristic constant of the filtration medium ( " -—) cm

K. characteristic constant of the accumulated dust layer

r Pa

n l s2 ,

min" mg " cñF"

η number of filter layers

MMAD mass median aerodynamic diameter (Um)

Q /S total quantity of dust recovered by washing divided by the filtration

surface (mg/cm2)

S filtration surface (cm2)

S soluble fraction of cesium (%) c

S soluble fraction of dust (%)

sg standard deviation for a log normal distribution (D 84 %/D 50 %)

T„ calcination temperature (°C)

T filter temperature (°C)

t drying time (min)

U superficial velocity (cm/s)

V_ ..„.water volume necessary to recover 90 (95 %) of the loaded dust (I)

ε bed porosity (%)

1. INTRODUCTION

In many nuclear process plants, the filtration of aerosols in process streams is generally performed by various purification systems put in line. The final system always consists of high efficiency HEPA filters arranged in series and in parallel ; a characteristic of these HEPA filters is their high DF and their low dust loading capacities in relation to the low pressure drop they can withstand. The aim of this study is the development of prefilters with high dust loading capacities and which could be in-situ regenerated in order to extend their operational life time. They should retain most of the aerosol charge of the gaseous effluent, should withstand the nature of the process stream (N0X in the case of reprocessing streams or HLLW calcination streams] and should support the stream temperature in the case of high temperature processes (Incineration or vitrifications processes).

Various types of prefilters can meet these requirements, among them the sintered metal filters, the glass fiber filters and the metallic fiber filters. The latter type has been chosen in this study. The experimental study has been concentrated on two type of filters (high porosity mats and sintered webs) purchased by a Belgian company BEKAERT N.V.

2. DESCRIPTION OF BEKINOX FILTRATION MATERIALS

BEKINOX is the trade mark of minute fibres made of various alloys and metals such as : - Inconel 601

- Titanium, Nickel.

The fibres are available in different forms such as staple and bulk fibres or yarns and in different diameters ranging from 4 to 22 ym. The high porosity mat type with its trade name BEKIPOR WB is a stainless steel porous medium with high porosity. The main characteristics of the different types of mats tested are given in Table I.

Table I : Characteristics of BEKIPOR WB mats

Type

04/300 08/300 12/300

Diameter of the fibre

ym

4 8 12

Superficial weight g/m2

300 300 300

Porosity of the web

%

99 99 99

Thickness of the web

cm

0.35 0.35 0.35

1 -

The second type tested is a sintered web with trade name BEKIPOR ST. !;It is a depth type stainless steel filter medium composed of very fine stainless fibres randomly laid into a 3 dimensional labyrinth structure. This mat.is further compacted and sintered to produce a filter of high mechanical strength but of lower porosity. The AL series have high dust holding capacities and can be used either for liquid filtration of high viscosity fluids, either for gas filtration. The main characteristics, of the 3 type BEKIPOR ST/AL tested, are given in Table II,

Table II : Characteristics of BEKIPOR ST sintered webs

BEKIPOR ST

Absolute filter rating for liquid filtration ym Superficial weight g/m2

Porosity % Thickness H cm

10AL2

10 530 80 0.033

25AL2

25 1100 75 0.055

. 40AL2

40 1100 80 0.065

3. PARAMETRIC STUDY OF THE FILTRATION CHARACTERISTICS

3.1. Description of the experimental procedure

The flow sheet of the test unit i,s given in Fig. 1.. The main parts.of the. unit are ; a Collison atomizer as aerosol generator, a dilution system, a filter holder and an aerosol measuring system. The flow ranges in the different parts are given in Fig. 2;' It illustrates that the dilution system allows to perform tests in a superficial velocity range of 3.5 to 19 cm/s with flat filters of an useful diameter of 15 cm. The filter holder accepts up to 15 layers of flat filters in series. The characteristics of the spectrometers ASAS-X and CSAS are given in Table III. These apparatus which operate on the principle of light scattering by a particle illuminated in a laser beam, produce a signal which depends on the size, shape and refraction index of the particle.

The same experimental procédure was' used for all tests' ·; a diluted water solution of polystyrene monodisperse particles is sprayed using the Collison atomizer. The water spray, containing the latex aerosols is diluted and dried with clean air. The aerosol particle flow (part/s) is alternatively measured before and after the filter with the two lasers·spectrometers using the proper size range channels.

- 2

Table III : Characteristics of lasers spectrometer

Size range

Working temperature

Sample gas flow

Sheat gas flow

Sample volume

Collecting solid angle

Laser

Energy density

ASASX

0.60 3.00 ym

0.24 0.85 ym

0.150.30 ym

0.09 0.195 ym

in 15 channels

25 °C

0.5 1.5 cc/s

20 cc/s

Hydrodynamically

focused jet 70 ym

35 120 °C (< 2Π sterad)

HeNe 2 mw 6328 Ä

TEM°° mode

500 W cm2

CSAS100 HTS

0.32 0.755 ym

0.5 2.7 5 ym

1.0 12.25 ym

2.0 20.0 ym

in 15 channels

max 370 °C

15 20 m3/h

Optically defined

volume 150 ym

HeNe 5 mw 6328 Ä

High order random mode

30 W cm2

3.2. The parametric study of the BEKPOR WB porous mats

During the experimental investigation of the porous mats, the different parameters

were varied as follows :

: 04, 08, 12

1, 3, 5

3.5, 10, 13, 19 cm/s

0.091, 0.109, 0.173, 0.220

0.330, 0.481, 0.620, 0.720

0.945, 2.20 ym.

nature of the web

number of layers n

superficial velocity u

particle diameter dp

During all the runs, the pressure drop over the filter remained equal to the

value measured before the run with clean air. It means that the loading with

latex aerosols at concentrations lower than 0.5 mg/m3 had no influence on the

bed porosity during the short time of a run. Also the DF remained constant

during a run.

Qualitative observations on the influence of the tested parameters on the values

of the DF are given hereafter.

3

ύψ Ψ U 4- η f dp t

DF i * f * * DP f Ψ f ->

The most important results obtained are shown in Fig. 3, 4 and 5. Vertical lines around a data point indicate the uncertainty range of the particular data point.

As illustrated in Fig. 3 and 5, a minimum of DF is observed in the size range 0.11 to 0.173 ym due to the well known decrease of the diffusion mechanism and to the increase of the interception mechanism as a function of aerosol size.

The main practical informations drawn from these tests are :

- the webs 08 and 12 show very low DF and DP unless the number of layers is high ;

- the web 04 is the most effective when the superficial velocity is lower than 5 cm/s and when the number of layers is high i.e. for η = 5. In these conditions, the DF is higher than 100 for a 0.1 ym aerosol and higher than 1000 for a 1 ym aerosol ; the pressure drop remaining at a reasonable value of 70 Pa ¡

an alternative to this optimum situation, is to use the three webs in series

with the coarser layer WB12 first followed by the WB08 and finally the WB04

as last layer. With such a set of layers it is also possible to reach satis

factorily results (DF of 40 for the 0.1 ym and 1000 for the 1 ym) with low

pressure drop (DP = 80 Pa).

3.3. The parametric study of the BEKIPOR sintered webs

The parameters were varied as follows :

nature of the mat

superficial velocity u

particle diameter dp

10AL2, 25AL2, 40AL2

3.5, 6, 19 cm/s

0.091 to 2.2 ym.

The DF and DP of the different runs are given in Fig. 6. Qualitative observations

on the influence of the tested parameters on the values of the DF and the DP are

given hereafter.

d. f u f dp f tf

DF Ψ * f f

DP Ψ f * f

4

For the runs at low superficial velocity, no significant increase of pressure

drop is measured for the three kinds of mats. At high superficial velocity

(19 cm/s), and for the finest mat 10AL2, a significant increase is measured

during bed loading with PSL aerosols. In Fig. 7, the relative increase of the

DF for an increase of the pressure drop by a factor of 2 is given as a function

of the particle size.

During filtration, a layer of aerosol deposits on the filter surface and produces

an additional resistance to flow which depends on the properties of the aerosol

layer. A visual examination of the filter after the runs revealed indeed the

presence of a dust layer on the surface of the bed. No significant penetration

into the mass of the filter was determined by 10 χ microscope magnification. The

deposit of aerosols becomes the filtering medium for succeding particles which

explains the increase of efficiency observed during the two successive runs.

The main practical informations drawn from these tests are :

- only the 10AL2 gives a good DF. The webs 25 and 40 are not acceptable as gas

filtration materials for submicronic aerosols ;

- for the 10AL2, it is advisable to work at a low superficial velocity in order

to have acceptable DP. The effect of the superficial velocity on the DF

obtainable with a non-loaded mat is négligeable within the experimental

detection limits.

The low dependence of the DF on the superficial velocity and the small residence

time in the filter indicate that interception is the main filtration mechanism at

the superficial velocity range studied. In this case, diffusion plays a minor

role in the filtration mechanism. Filtration by the accumulated dust becomes

preponderant after a certain operating period depending on the particle size and

on the aerosol concentration.

4. FILTRATION AND REGENERATION BY WASHING FOR TWO FILTER CONFIGURATIONS

An aerosol of methylene blue has been used as challenge aerosol for the study,

at room temperature, of two filter configurations : a candle type filter and a

flat type filter configuration.

The characteristics of the test performed and of the challenge aerosol are given

in Table IV.

Descriptions of the candle filter holder and of the flat type filter holder are

given in Fig. 8 and 9.

The Fig. 10, 11 and 12 give the DF evolution with bed loading measured by methylene

blue sampling and the corresponding pressure drop increase.

- 5 -

Table IV : Characteristics of the filter configurations and of the corresponding tests

ε SF

°T U

C

MMAD

%

cm

m3/h

cm/s

mg/m3

ym Sg

Type A 5 layers Bekipor WBD4

3 candles * diam. inlet 90 mm

outlet 52 mm * height 250 mm

99

2100

15.6

2.9

1.2

0.5 0.62

Type B 1 layer Bekipor ST10AL2

3 candles * diameter 52 mm

* height 250 mm

80

1230

15.6

3.8

3.9

0.6 0.67

Type C 3 layers WB12 + 3 layers WB08 + 3 layers WB04

flat type filter * diameter 280 mm

* height 17 mm

98

616

15.6

7

2.3

0.6 0.72

The Fig. 13, 14, 15 give the "latex efficiency curves" (DF in function of particle diameter for aerosols from 0.09 to 1.09 ym) before and after bed loading with methylene blue and after filter regeneration by washing.

The main conclusions drawn from these tests were :

- For the type A and C, equipped with porous filtration materials, the pressure drop increase by a factor of 1.2 only during bed loading with methylene blue aerosols. The DF's measured by methylene blue sampling increase by a factor of 3 to 10, while the DF's measured with latex aerosols increase by a factor of 7 to 30. During loading, the particle size distribution of the methylene blue aerosol is shifted towards smaller values so that the penetrating aerosol lies around 0.1 to 0.2 ym.

The quantity of methylene blue inside the filter material decreases exponentially with the bed depth so that the major fraction is accumulated in the first layer of the bed. The regeneration of the filter by water washing has been tested with low pressure jet and spraying nozzles. Good water penetration of the filter and wetting of the fibres is only obtained by spray washing.

The washing technique used is not suited to the initially designed candle type

- 6 -

filter of small diameter when the deep BEKIPOR WB mats are used.

Indeed, the washing of candles with small diameter and great length is only possible from outside to inside when the deep BEKIPOR mats need also a washing from inside to outside. Furthermore, a great number of nozzles per candle (9 in the case of a candle of diameter 92 mm and length 250 mm) is necessary for a good washing of a candle. The drying of a candle with BEKIPOR mats WB is not easy since all the water tends to accumulate at the bottom of the candles.

The spray washing technique, with spray nozzles above and under the filter surface, is well suited for flat filters. Washing from both sides is necessary in order to have an efficient regeneration of the filter. The drying of the filter requires a heated air flow in order to vaporize the water accumulated into the porous mats.

- For the type B, equipped with a low porosity sintered web, the pressure drop increases by a factor of 5 during bed loading. The methylene blue aerosols form a thin layer of deposit on the filter surface which becomes the filtering medium for succeeding particles so that the DF measured by methylene blue is increased by a factor of 20, while the DF measured with latex aerosols greater than 0.3 ym is increased by a factor of 104.

The regeneration of the filter by spray washing of the dust layer is very efficient and only relatively small quantities of spraying solutions are necessary.

Conclusively, only the flat type filter configuration will be tested at high temperature with spraying nozzles above and under the filter surface and with gas flowing upwards so that the outlet side of the filter cannot be polluted by the washing solution.

5. AFHT AEROSOL FILTRATION AT HIGH TEMPERATURE

The aim of the AFHT unit, is to test the filter materials at high temperature (400 °C) with a representative aerosol obtained by calcination of a simulated waste solution and to test the regeneration of the filter by spray washing techniques.

The main components of this unit are : (Fig. 16) - a high temperature stainless steel calciner ; - a filter holder (Fig. 17) - a washing system comprising two spraying nozzles placed above and under the flat filter ;

- 7 -

- a condensor with water cooling in a special tube and shell configuration j - two sampling systems (Fig. 18) placed before and after the filter and comprising a flat filter holder and a 6 stages cascade impactor.

5.1. Description of the test unit and of the test procedure

The nitric acid solution, containing various nitrate salts representative of a specific liquid waste (Table V), is fed at a flowrate of 0.2 to 0.3 1/h into the 3 kW calciner operating at 600 or 700 °C. Dilution air is also fed at the calciner inlet at a maximum flow rate of 10 Nm3/h. The off gases from the calciner contain besides air, water vapour and N0X gases formed by calcination of nitric acid and nitrate salts. The off gases contain also a certain amount of dust and aerosols comprising, nitrates not completely calcined, oxides formed by nitrates decomposition and oxides volatilized at the calcination temperature. During cooling of the off-gases, the volatilized oxides condense to sub-micronic aerosols.

Table V : Composition of the simulated liquid waste

Element

Na Fe Cr Al Mn Cs Sr Ba Ce Zr Mo Rb Y La

g/i 4.65 1 .57 0.17 0.8 0.4 0.15 0.05 0.01 0.18 0.02 0.13 0.025 0.03 0.16

Chemical form

NaN03

Fe(N03')3 Cr(N03)3

A1((\I03)3 rln(N03)2 CsN03

Sr(N03)2

Ba(N03)2

Ce(N03)3

Zr(N03U NaaMoOit RbN03

Y(N03)3

La(N03)3

The same procedure is used for all the filters tested. Each test cycle comprises the following steps :

7. Vete.ninination ofi the "latex e^lciency cu&ve" o{ the new mate&ial

The "latex efficiency curve" is determined by testing the filter at room tempe

rature with air loaded with various monodisperse latex aerosols.

2. Loading tut

The filter is loaded with an aerosol generated by high temperature calcination

of the reference solution traced with 131

*Cs. The filter efficiency (rip) is

determined by activity measurements of absolute sampling filters placed before

and after the filter tested and the particle size distribution at the filter

inlet is determined with a cascade impactor.

The following values are determined :

C : dust inlet concentration (mg/m Ν dry gas basis)

HMAD : mass median aerodynamic diameter (ym)

sg : standard deviation of the distribution D84/D50

°i < 1 ym : fraction smaller than 1 ym.

3. VeteAmination o& the. latex e^ldenay cwive. a^ten. loading

4. VUXeh. washing

The filter is washed at room temperature with the spraying nozzles localised up

and downwards the filter. The washing is performed by intermittent short

spraying pulses followed by washing water drain.

The following values are determined :

Qj/Sp : total quantity of dust recovered by washing

divided by the filtration surface (mg/cm )

IQ : insoluble fraction of dust

SQ : soluble fraction of dust

IQ : insoluble fraction of Cesium

SQ . : soluble fraction of Cesium

Vgg(95) : water volume necessary to recover 90 (95 %) of the loaded dust (1).

5. TÁJitoA dn.ylng

The filter is dried with heated air at a constant flow rate of 10 m /h. The

initial pressure drop (DPw = kPa) and the drying time (tn, = min) are determined.

6. Latex. e^ldency cuAve a^ten. ωαοking

This test procedure is repeated 4 times in order to test the filter regeneration

technique and its effect on the filter performance.

5.2. The data obtained in AFHT

Three filter types in the flat filter configuration have been submitted to the

above mentioned procedure.

9

The characteristics of the filter type tested are given in Table VI. A description of the filter holder is given in Fig. 17 and Fig. 18 shows the flowsheet of the gas sampling unit. In the Tables VII, VIII, IX are summarized the values of the main parameters and the main results of all the tests. The gas characteristics are defined by : Dr

F (H 2 o:

υ τ

dilution air (Nm /h dry gas basis) calcination temperature (°C) filter temperature (°C) concentration of water vapour (%) superficial filtration velocity (cm/si

In these tables, are also given the aerosol characteristics (C, MMAD, Sg, % < 1 ym), the dust deposit characteristics (Qy/Sp, Ipj, Srj, Irj, Sfj), the washing volumes (Vgrj, Vg5) and the drying characteristics (DP^, trj) .

In the figures 19 to 26 are given : - the pressure drop evolution with loading time ; - the filter DF evolution with loading time ; - the "latex efficiency curve" before and after loading

• for the type I, the latex efficiency curve has been determined for an aerosol of 0.220 ym at superficial velocities of 10, 19, 28 cm/s,

• for the types II and III, the latex efficiency curve has been determined at a given superficial velocity for four monodisperse aerosols.

10 -

Table VI : Characteristics of the filter types used in AFHT

TYPE I Bekipor WB04

High porosity mat

5 layers WB04

Bed porosity 99 %

Filtration surface 154 cm2

Useful filter diameter 14 cm

Filter height 3 cm

TYPE II Bekipor 12/8/4

High porosity mat

3 layers WB12 + 3 layers WB08 + 3 layers WB04

Bed porosity 98 %



Filter height 3 cm

TYPE III Bekipor ST10AL2

Low porosity web

1 layer sintered web ST10AL2

Bed porosity 80 %



Filter height 0,033 cm

Table VII : Type I BEKIPOR WB04

GAS CHARACTERISTICS

AEROSOL CHARACTERISTICS

DUST DEPOSIT CHARACTERISTICS

WASHING

DRYING

SYMBOLS

°G TC TF (H20) U

C MMAD Sg < 1 y

QT/Sp ID sD

ic sc

v90 % v95 %

DPW tD

UNITS

Nm3/hd.g.b. °C °C %

cm/s

mg/Nm3 ym

mg/cm2 % % % %

1 1

kPa min

CYCLE 1

10 600 270 17.4 44

940 2.2 0.45 15

60

12 20

CYCLE 2

2.6 600 400 10 13

280 4.8 0.21 16

31

4.5 35

CYCLE 3

2.6 600 400 10 13

360 6.5 0.31 6

42

0.65 0.86

5.6 15

CYCLE 4

3 700 400 16 16

580 1.6 0.47 23

66 14 86 0.5 99.5

C.77 1.2

7.2 30

- 12

Table VIII : Type II BEKIPOR 12/8/4

GAS CHARACTERISTICS



WASHING

DRYING

SYMBOLS

°G TC TF

(H20) U

C MMAD Sg

< 1 y

QT/SF

ID SD ic sc

V90 % v95 %

DPW tD

UNITS


cm/s

mg/Nm3 ym

mg/cm2 q, Ό

% % %

1 1

kPa min

CYCLE 1

3 700 400 10 15

188 2 0.45 20

50 2 98 0.7 99.3

0.9 1.4

5.4 15

CYCLE 2

3 700 400 10 15

320 1.9 0.40 25

44 11 89 1 .3 98.7

0.7 0.9

5.4 20

CYCLE 3

3 700 400 10 15

360 2.4 0.40 16

53 11 89 1.2 98.8

0.7 0.9

5.5 20

I

CYCLE 4

3 700 400 10 15

230 1 .5 0.5 28

-

- 13 -

Table IX : Type III BEKIPOR ST10AL2

GAS CHARACTERISTICS



WASHING

DRYING

SYMBOLS

°G TC

TF

(H20) U

C MMAD Sg < 1 y

QT/SF

ID SD

ic sc V90 % V95 %

DPW tD

Filter t starting

UNITS


cm/s

mg/Nm ym

, 2 mg/cm

% % % %

1 1

kPa min

emperature mu drying flow

CYCLE 1

3 700 400 10 9

470 3 0.43 12

21 19 81 1.5 98.5

1

st be hig

CYCLE 2

3 700 400 10 9

240 2.8 0.36 16

18 17 83 1.7 98.3

0.5

1 .6 10

her than

CYCLE 3

3 700 400 10 9

146 2 0.33 26

10 19 Θ1 2.4 97.6

0.86

2.0 10

CYCLE 4

3 700 400 10 9

130 1.7 0.47 25

100 °C before

- 14

5.3. Discussion of the results

7. decontamination {¡actoA

The minimum and maximum values of the measured DF are given in Table XII for

each type in function of the cycle number.

The minimum values are always observed at the start of the loading cycle and

the maximum values at the end of the cycle when the pressure drop is the

highest (Fig. 19, 20, 22, 23, 25).

The "latex efficiency curves" measured between each cycle when the filter has

been washed are not significantly modified. This means that the filter regene

ration does not modify the filtration characteristic of the filter.

The "latex efficiency curves" determined after a loading cycle show always an

increase of the DF of the filter ; as can be observed from the figures 21, 24,

and 26, the higher the pressure drop increase, the higher the relative DF

increase and the bigger the particle size, the higher the relative DF increase.

The type I filter, with layer of 4 ym fibres, is the most efficient filter type

particularly at the start of the run when the filter is not loaded with dust.

1. VfieÁòuAe. cbxop

The filter resistance comprises the resistance to air flow presented by the

filtering medium plus the layer of dust particles trapped by the filter

material.

Total resistance through the filter can be expressed as follows :

DP = DPo + DP!

Because of the low filtration velocities, streamline flow takes place so that

the resistance through the filter is a linear function of velocity

DPo = Ko . U

(Pa) C^l) (cm./s). cm

The resistance through the accumulated layer of dust DP1 : increases

with the dust concentration (C), the superficial velocity (U ) and the loading

time (t)

DPi = Ki · C · U 2 · t ·

rD ι fmg, rcm. . . (Pa) (—) · (—) · (min).

m;

15

ρ 3 2

The value of the dust resistance coefficient (Ki : —:— · — · — T ) depends on

min mg cm

the density, porosity, particle size and bulk density of the dust layer.

The values of Ko and Ki are given in Tables X and XI for the different filter

types in function of the cycle number.

The filter which has the lower Ko and Ki values, is the filter which will have

the lower operating pressure drop and the longer loading cycle for specific

values of the superficial velocity and of the aerosol concentration.

The type II filter with layers of decreasing fibre diameters has the lowest

Ko and Ki values. This filter type is also the least efficient type of the

different types tested.

The type I filter with layers of 4 ym fibre diameters has values of Ko and Ki

a bit higher than for the type II but also is the most efficient filter type

at the start of a cycle when no dust layer is present.

The type III filter with one layer of sintered web has values of Ki one order

of magnitude higher than for the types I and II which means that frequent

regenerations are needed.

3. T-ilteA Ae.ge.neh.ation by ¿pAay ujoòhing

The filter regeneration by spray washing allows for all the filter types to

remove most of the dust trapped.

For the filter types I and II, spray washing must be done first downwards to

remove the dust layer from the filter surface and then upwards to remove the

dust accumulated into the filter mass. For the filter type III, only downwards

spray is necessary to remove the dust layer from the filter surface.

Generally only a slight increase of the Ko values is observed, which means that

the regeneration is efficient and does not damage the filter.

Visual examination of the filter materials after 4 cycles revealed for type I

and type II filter equipped with high porosity layers, a slight compression and

hardening of the layers ; for type III filter equipped with sintered web no

modifications of the filter materials were observed.

4. TFÁJüteA drying

After washing and draining of the filter, a certain amount of water remains

trapped into the filter material.

This amount is important for the types I and II porous filters, since a water

volume of about 50 % of the filter volume is held up into the filter.

For the thin web filter of the type III, this amount is négligeable since the

filter thickness is only 0.3 mm.

In order to have a rapid filter drying, it is necessary to heat first the

16

Table Χ : Permeability constant Ko of the filter Ko (Pa.s/cm)

Cycle 1 Cycle 2 Cycle 3 Cycle 4

TYPE I WB/04

17 18.5 21 .5 21 .5

TYPE II WB/12/8/4

15 13 13 15

TYPE III ST10AL2

28 28 28 33

Pa m3 s ^ Table XI : Dust resistance coefficient Ki (—τ—)(—)(—) min' ̂ mg' cnr


TYPE I

7.5 10~5

7.4 10~5

4.6 10 - 5

5.5 10~5

TYPE II

2.6 10~5

3.5 10"5

6.5 10 - 5

7.4 10 - 5

TYPE III

1 .4 10"1* 3.1 10-1* 9.3 10"4

7.4 IO-1*

Table XII : Minimum and maximum DF


TYPE I

min

32 24 30 100

max

-80 240 550

TYPE

min

7 19 26 15

II

max

70 180 400 160

TYPE

min

11 60 18 22

III

max

650 700 70 70

- 17 -

filter holder above 100 °C before flowing heated air through the filter. The pressure drop, initially high at the start of the drying, rapidly drops to its "clean filter" value when the water has been vaporized.

6. GENERAL DISCUSSION

The three filter types tested can be used as prefilters at high temperature and in corrosive environment. They can also be in-situ regenerated by spray washing techniques and withstand to several regeneration cycles.

Nevertheless, differences are observed between the different filter types tested. The sintered web (type III) has a dust loading capacity lower than the porous mats (type I and II) which means that the regeneration frequency would be higher. For example, at the same operating conditions (U = 1 0 cm/s, C = 100 mg/m3, DP max = 3000 Pa) a cycle of 6 hours must be followed for the type III whereas for the types I and II a cycle of 60 to 80 hours must be observed. For the sintered web, an alternative to the wet regeneration technique could be a dry regeneration technique such as countercurrent blow back technique. An advantage of the dry regeneration technique is the absence of temperature cycling bound to a wet regeneration technique. For the porous mats (type I and II), the dry regeneration technique is not applicable because the filter strength resistance is too low and moreover the dust is not only retained at the surface of the filter but has also penetrated deeply into the filter mass. The types I and II have roughly the same dust holding capacities so that cycle times of the same order of magnitude could be used. The type I filter, has nevertheless a better efficiency than the type II when the filter is not loaded with aerosol. This type is more advisable for applications when the aerosol concentrations are low (C ̂ 10 mg/m3).

7. CONCLUSION

The parametric study of the aerosol filtration performance of various filter materials allowed to select three filter types which were further tested at ambient temperature in two geometrical configurations using artificial aerosols.

The performance of these three filtertypes in a flat filter configuration was also investigated at a temperature of 400 °C using an aerosol generated by calcining a simulated nitric acid waste solution traced with 13l4Cs. Regeneration of these filters by spray washing appeared feasible without observable deterioration effects. The regeneration frequency of the sintered web filter being ten times higher than that of the porous mats filters, dry regeneration techniques are rather preferable for the former filter. The results obtained in this study allows an optimal design for prefilters as a function of specific off-gas process conditions.

- 18 -

F I G U R E S

- 19

OUTLET AEROSOL

Fig . 1 - Test u n i t for aeroso l f i l t r a t i o n a t low temperature

- 21 -

INLET AEROSOL

/

'

1.6-9.2m3/h 1 '

F I L Τ E R

y

fi L tn U mJ/h

ι r-

HEPA

OUTLET ^EROSOL

ι a 111 /π

pure Ν2

BY PASS i i ■>

HEPA

0.63 rrr/h

Aerosol laden N2

15.63 m3/h

▼ 0.5-1.5 10"

CSAS

6m

3/h

ASAS

Fig2.FLOW SHEET OF A.F.L.T.

F i g . 2 - Flow s h e e t of A . F . L . T .

U = 3.5 cm/s

Τ WBO¿ n =

I DP=70Pa

?>*S

WB

f 04 n = 3

08 π =3

12 n-3

DP=80 Pa

0VVBO8 n=5

DP=50Pa

+ WB 12 n= 5

DP=3 OPa

dpüjm)

0.1

F i g . 3 - Inf luence of f i l t e r m a t e r i a l and con f igu ra t i on

- 22

DF

BEKl POR WB04

n = 1

» 0.173 jjm

• 0.480 i|m

li , 1 U(cm/5)

DF 50

n = 3

1Û.

I 1 I I I I II 3 5 10 20 3 5 10 20

F i g . 4 - Inf luence of s u p e r f i c i a l v e l o c i t y

BEKIPOR WB04

U=3.5cm/s n DP(Pa)

A 5 70 • 3 40 + 1 20

il ui dp(Mm)

0.1

Fig. 5 - Influence of number of layers

- 23

BEKIPOR ST

DF

ie* Δ +

tl Δ

10

10AL2 U DP cm/s Pa

+ 3.5 40 Δ 6 120 ol9 280-450

25 AL2 • Ur 3.5 DPr30

A 4 0 A L 2 Ur 3.5 D P= 20

j j L J ι l I dp(i|m)

0.1

Fig. 6 - Parametric study of BEKIPOR ST filter

Fig. 7 - Relative increase of the DF for a pressure drop increase of a factor 2

- 24 -

Candle φ Sl/si. mm

J eng 14 3 σο

sectie -</g

Fig. 8 - Candle type filter

Fig. 9 - Flat type filter holder

F La. t type filter holder

À spraying

nonle

filter layers-

spraying

non le s***

JL

M outlet washing > U u/ater

Scale 4/5

- 25 -

DF

10¿

-

10

=

-

_

B E K I P O R WB04

3 cand les w i t h 5 l aye rs

^

^ ^

k^^

▲ mass ef f ic iency

• pressure drop

2 f i n a l bed loading r0 .87mg/cm Q D D P o = 1 4 0 P a ( p a )

200

_ . _ »— · time(hoursJnn:

Ι ι ' ι ι ι ι ι , 1 IUU-3 5 10 100

Fig. 10 Performance of the type A filter for a methylene blue aerosol

DF

B E K I P O R ST 10AL2

m e t h y l e n e blue l oad ing Qp

(Pa]

10 —

500 I ι I L J I I

9 0 0qf

( m g ) ) 0 2

Fig. 11 Performance of the type Β filter for a methylene blue aerosol

26

flat type filter :3xdl*3*08**>*o'i

methylene blue test.

mass Oasis ·*

- 7 -

Fina I bed loading ï. 1 mg/cm3-

DP0- Ì50fa

U = 9-cm/s.

~*C time (hrs)

_3oo.

_2oo.

loo.

u Ï ; ι I^J_

Fig. 12 Performance of the type C

filter for methylene blue

Fig. 13 "Latex efficiency curve"

for the type A filter

before and after bed

loading

DF

103

10'

10

BEK1 POR WB04

u r 2.9 cm/s

P S L t e s t s

Δ af ter methy l ene blu e

A before

0.1 1

J L_I I I I I I I I '

dplqm)

- 27

DF PSL t e s t s

• be fo re , , met hy lene blue

I a f t e r ' o af t er w a s h i ng

105

BEKIPOR ST 1 0 A . 2

I

(dp=(L)m)

DF

10

0.1

• DP=490 R>. o DP-loo Pk

dp (i|m)

0.1

Fig. 14 "Latex efficiency curve"

for the type B filter

before and after loading

and after washing

Fig. 15 "Latex efficiency curves"

for the type C filter

PSL tes

DF fta.t type filter

t

3. · Before Loa-ding

n Oaf ter loading

-j. a f ter washing

10£( U = 7 cm/s

Sf= 6/6 cm*

5_

3_

2

Γο3

5 ι

3

2.

2. 10

• S J

3__

10

/ ■y— r 1

— J * ■y-

. « r'

CU.

/ 1

/

·>

A

J

/ f

y

y

m^

. i

/

y

f f

I

_>

fï-r , ,_ ■ C "

.5 .7

/ y

■

~j_

I / I ι I

I j

/

rl· M

cipCfm)

d

28

wss

r®-n (Ά

Λ Η

*pLJ

r—@-+$h—a l r

γ 8t ' U@_T 1 ^ ^S5 Τ

Q) Pump

WSS T i wso Condensor Fitter

SL Sampling Une WSS Washing »praying tysttrn. TC temperature controller wso washing solution oatltt DP pressure drop H Mating resistance

Calciner solution

Fig. 16 - Flow sheet of the Aerosol Filtration at High Temperature unit (AFHT)

Γν1-"1

spacing seals,

π 1 ■

Β Ά \ J [

^ ~

tr η J

/ f

spraying nozzle.

=1— ^. upper grid ^filter Layers "bottom grid - spraying nozzle.

^ —

solution

tu ^\ Sca/e */5

flat type filter holder. AFHT.

Fig. 17 - Flat type filter holder used in AFHT

29

6 stages catead.t impacto?

heated hon

isoKiNetic _ Sampling

batí va.lve g tats ft her filter.

' ' air ejector

h ^ * rΊο* meter

Condensar oLrjino bed.

Fig ±8 flow sheet of sampling unit

Fig. 18 - Flow sheet of sampling unit

Pressure drop DP(Pa)

loading time /·_,, „\ t- „ (mm)

_ l I l _ J

Fig. 19 - Pressure drop of filter type I

- 30

Filter DF 401

- 9 -? cycle! -5

-Ί

-3 -î

dp3-

-6 - 6

- 1

-10

-î -6

-5

-* -3

-a.

-*oo

cycle £

7~ /

■ioo Soo Soo

I 1 I

cyc/e3

/ —

/ /

A

400 l o o jt>o Voo

I I I I

cycle ¿t. /

/

/

/

/

■iOo Soo 300 'iOO I I I I

loading time (min)

Fig . 20 - DF of f i l t e r type I

ι ¿ ι nulls—ï L Γττηπξ ¡. L L

π

a

i o r« Λ cycle * ■ ► · before loaolìng

n3 <3 D C> O OL f ter Ιοα,οίί.

Fig. 21 - "Latex efficiency curve" of filter type I for a 0.22 ym aerosol

31

cycle 4.

5 ° Fil'ter DF JSO

riS

cycle 2 .

/

/ y

C- Ί&& mg /m3

Ak= a.6 ε-s

Pressure drop DPCPa)

I « . te. I«. h . Im b . I,

/ Τ

/ /

■MO ¡at) 3tx>

C-3ÍO

Kt = 3.S40-5

loo Boo Uoo il

Fig. 22 Performance of filter

type II

(cycles 1 and 2)

Ν loading time t(müi)

Fig. 23 Performance of filter

type II (cycles 3 and 4)

cycle 3

Filter DF

-»OO

- 2 O 0

-<ο·

A Ac 'I

ÍO0 SpO 3p0

C =360

Kt- 6.5JO"5 ;

DPCPa)

cycle U-

/ '

/

40O ΙΟΟ 300 \ -

- 230 mg/mi,

- 32 -

LA re χ DF dos

αιψ3 ama osi

__l I I L

Βεκιροη 4îl8lt.

cycle 1 2 3 ί

DP^dlooPa y load. after ο Δ D V

aia 0.313 ι dp (lim) j ι ι ι L Ι Γ '

Fig. 24 - "Latex efficiency curve" for filter type II

Fig. 25 - Performance of filter type III

cycle Ί

Filter DF

■403

1 f :; / 1 ι

40* ¡

:! /

: : /

: /

lio η» -r -ί - S -* -3 -t

-1

ΛΟΟ EOO SOO ι ι ι

C = 4>?0 Ki = 4.1 ■i0~

l>

—3COO

PRESSURE - DROP

Pia. — Sooo

— 40Oo/ \

/ * o o too 3oo

- 1 1 1

cycle Ζ

1

i I

I

1 -h

Aoo tao Soo

ito

3Λ4.0-Ί

/ \ / \

/ \ / \

/ \ / \

' Λοο tOO JOO 1 I 1

cycle3

I 1

1

/

doo Soo 3oo

dV6 9.3 dO-1

/ I / \ 1 \

/ \ / \

Ι ι / t

/ I / \

/ \ I I

• doo too 4bo 1 1 1

cycle H

/

/

τ τ r ι 43 O /mg/m*

7.1d0-1

/ loading

/ t í m e tr ■ \ / ¿(min)

doo loo 3oo

l i l i

33

ΒεκιροΝ STioA.ii

i t i -

LfíT£>i. DF te fore load

cycle d S. 3 Η

before · ▲ ■ ν

load-ing after ο Δ c

.103.tio °·.51? o· 3/j dp(/lm)

_J ι ι ι L!_J ι ι il _

Fig. 26 - "Latex efficiency curve" for filter type III

3A -

European Communities — Commission

EUR 8951 — Aerosol filtration

M. Klein, W.R.A. Goossens

Luxembourg : Office for Official Publications of the European Communities

1984 — V, 34 pp., 26 fig., 12 tab. — 21.0 χ 29.7 cm

Nuclear science and technology series

EN

ISBN 92-825-4269-6

Catalogue number:

Price (excluding VAT) in Luxembourg : ECU 4.34 BFR 200 IRL 3.20 UKL 2.50 USD 3.50

This final report summarizes the work carried out at the SCK/CEN Mol, from 1 July 1981 until 31 December 1982 on the development of fibre metallic prefilters to be placed upstream of HEPA filters for the exhaust gases of nuclear process plants.

1. Investigations of ambient temperature Measurements of the filtration performance of Bekipor porous webs and sintered mats were performed in the AFLT (aerosol filtration at low temperature) unit with a throughput of 15 m3/h. A parametric study on the influence of particle size, fibre diameter, number of layers and superficial velocity led to the optimum choice of the working parameters.

Three selected filter types were then tested with polydisperse aerosols using a candle-type filter configuration or a flat-type filter configuration. The small-diameter candle type is not well suited for a spraying nozzles regeneration system so that only the flat-type filter was retained for high-temperature tests.

2. Investigations at high temperature

A high-temperature test unit (AFHT) with a throughput of 8 to 10 m3/h at 400° C was used to test the three filter types with an aerosol generated by high-temperature calcination of a simulated nitric acid waste solution traced with ,34Cs. The regeneration of the filter by spray washing and the effect of the regeneration on the filter performance was studied for the three filter types. The porous mats (Type I and II) have a higher dust loading capacity than the sintered web (Type III) which means that their regeneration frequency can be kept lower.

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