Fundamentals of particle separation and air filters

J. Aerosol Sci., Vol. 22, Suppl. I, pp. $727-$730, 1991. Printed in Great Britain.

FUNDAMENTALS

0021-8502/91 $3.00 + 0.00 Pergamon Press pie

OF PARTICLE SEPARATION AND AIR FILTERS H. Emi

Department

of Chemistry and Chemical Engineering, Kanazawa 2-40-20 Kodatsuno Kanazawa 930, Japan

University,

ABSTRACT All techniques developed so far to separate particles from their liquid or gas dispersion systems are classified into three categories; (1) techniques which utilizes force field only, (2) those which use both force field and separation media (collection bodies) , and (3) those which have separation media without any force field. The basic principles of these techniques are discussed in conjunction with the development of new types of particle separator. Fibrous air filters are classified into the second category. The optimal filter structure is discussed by studying the influence of fiber diameter, fiber orientation, binder content on a Filter Figure of Merit. Membrane filters are now widely used as an inline filter for pressurized gases. In order to develop prediction method of these membrane filters, the membrane filters are classified into five groups according to the internal structure, and the collection performance are evaluated in terms of the Filter Figure o f Merit. Then, the prediction methods of collection efficiencies of membrane filters in each group are discussed. KEYWORDS Particle separator, Membrane filters

Classification,

Basic

principle,

Air

filtration,

Fibrous

filters,

BASIC PRINCIPLES OF PARTICLE SEPARATION All particle separators for liquid and air dispersion systems are classified into three basic separation forms as shown in Table 1. The first form is that which uses only the force field. The collection efficiency is generally low, because particles must travel a long distance before reaching the collection wall. Since the pressure drop is low, the critical factor to determine the collection performance is -the deposition velocity of particles. If obstacles are placed in a fluid stream, suspended particles readily reach the obstacle surface with the aid of small force between particle and obstacle. Because the insertion of obstacles into the stream raises the pressure drop, pressure drop as well as the collection efficiency should be taken into account to evaluate the collection performance. The third form is the separators that utilizes only obstacles without any force field. Geometrical size of a channel in an obstacle should be smaller than the particle size. The third form guarantees the perfect removal of particles with the diameter larger than the channel, but the p r e s s u r e drop is evidently high compare to the other forms. Therefore, the most crucial factor for this form is the pressure drop. Table 2 gives the examples of force fields, obstacles and the separators. In each column, the elements are listed in random order. By choosing any element in the S727

S728

H. EM] Table 1. Basic forms of panicle separator from dis)ersed systems Elementary

Force f t e l d

Force f i e l d

cause

Obstacles

and o b s t a c l e s

,J' t,

j

//F

Form

I

Efficiency Pressure drop Critical factor for performance

Example

e

I

,1, t

/

If

tacle

/

low low • Deposition veloctty

ie

~/Obstacle

• Thickener

medium medium • Collision efficiency • Pressure drop • Venturl scrubber

•Ftlter

• ESP

• Fibrous

• Bag f i l t e r

• Cyclone

• G r a n u l a r bed

filter

htgh htgh • Pressure drop

press

• Membrane f i l t e r

Table 2. Examples of force fields, obstacles and separators Force Field Gravity Centrifugal Electrostatic Magnetic Thermophoretlc Diffusiophoretlc Lift Inertia Diffusion

Table

Obstacle Fiber l a y e r Granular bed F l u l d i z e d bed Membrane Droplet Porous media Woven c l o t h Felt Screen

3. Prediction

Dust C o l l e c t o r S e t t l i n g chamber ( h o r i z o n t a l flow)

Separator

equations

I n e r t i a l dust c o l l e c t o r Fabric f i l t e r Fibrous f i l t e r Electrostatic filter Centrifuge Filter press Scrubber

for fractional

Er

collection

efficiencies

Remarks

voS/Q : voS/Qll 1 : vaS/Q>l l-exp(-voS/e)

no p a r t i c l e mixing

Centrifugal separator

1-exp(-vcS/Q)

complete mixing

Electrostatic precipitator

I-exp(-vsS/Q)

D e u t s c h ' s Eq.

Scrubber

l-exp(-kvc

Packed bed

3 ~ l-exp( 2 I-~ 4 ~ l-exp(1-a

complete mixing

LLW/dc) L dc ~ c) L v c) dc

atomized d r o p l e t s g r a n u l a r bed f i b r o u s bed

L ' : water to gas r a t i o , L : e f f e c t i v e length f o r p a r t i c l e c o l l e c t i o n (m) dc : r e p r e s e n t a t i v e dimension of a s i n g l e o b s t a c l e (m) v : d e p o s i t i o n v e l o c i t y (m/s)

Fundamentals of particle separation

$729

force field and combining it with an element in the obstacles, we may create a separator. This kind of classification is helpful for students in understanding the principle of various separators as well as for engineers in creating new types of particle separator. Expressions for predicting fractional collection efficiency, Ef, are listed in for various dust collectors with different principles. An interesting feature figure is that every predicting equation in the table is given by a similar of particle deposition velocity, vd, in the force field or of the single collection efficiency, tic, due to a combination of several forces.

Table 3 in the function obstacle

PERFORMANCE EVALUATION OF HIGH EFFICIENCY AIR FILTERS Fibrous air filters belong to the second separation form which utilizes obstacles with the aid of force field. In evaluating the collection performance of various fibrous air filters, we must account for both the collection efficiency and the pressure drop. The evaluation of filter performance could be done by introducing a Filter Figure of Merit (USAEC, 1950), which is defined by l=-lnP/Ap where P is the particle penetration through a filter and Ap the pressure drop. The physical meaning of the I is illustrated in Fig.l, where lnP is plotted against pressure drop at a given particle size and filtration velocity. Suppose we have two filters whose particle penetration and pressure drop are represented by open triangle and open circle. Since a filter given by open triangle has a higher particle penetration than that of open circle, we may say that the filter of open triangle has a lower collection performance from the point of collection efficiency. However, we may reduce the particle penetration through the filter of open triangle simply by increasing the filter thickness or packing fraction of fibers. Increase in filter thickness and packing fraction does not affect the value of I because both InP and Ap vary proportionally with filter thickness. Thus, lnP and Ap of filter shown by open triangle change along the straight line with increasing filter thickness and packing fraction. Consequently, the filter of open triangle may have the same particle penetration as that of open circle at a lower pressure drop. The influence of fiber diameter on the value of I has been theoretically studied by Cooper (1982) for various mechanical collection mechanisms. In the present work, the factors affecting the filter performance (fiber diameter, fiber orientation and binder content) in diffusion and interception control regime are experimentally studied to optimize the filter structure. The experiments revealed that an optimal filter is a filter consisting of the finest fibers aligned parallel to the airflow and containing the least amount of binder.

0

_slope : I~ s l o p e : I2

~

I~>I2

\ \ / shift along this Fig. 1.

Illustration of physical meaning of Filter Figure of Merit C~ {D (D

0

P r e s s u r e drop,

Ap

H. Era~

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"~

10-2

~: '. ,,

t~il, , ~

•

.

d p=0. I,um

.

((..,~

--.,~.

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L\',,¢! ~

",\\ "~':. X

I0-~

~,'\

~

\\

\

/

10-~

=_~',~,x ~_. 10-~

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~-',.X xx

\.

1.0

I

-x 1.5

t

2.0 (( 4.0

4.2

d o ( k Pa )

Fig.2.

Comparison of filter performances of various membrane filters

CLASSIFICATION OF MEMBRANE FILTERS AND THEIR PERFORMANCE EVALUATION AS A GAS FILTER Membrane filters have diverse and complex structures depending on the material and the manufacturing process. Therefore, a single prediction method of collection efficiency cannot be applied for all of these membrane filters. We observed the internal structures of over thirty commercial membrane filters and classified them into five groups according to the structure; (1) fiber-like (one-directional), (2) fiber-like (random-directional), (3) net-like, (4) agglomerate-like and porous, (5) pore-like. The comparison of collection performances of various membrane filters is given in Fig.2. The figure clearly shows the transition of the collection performance as the filter structure changes from pore-like structure to fiber-like structure. It should be noted that Gore-Tex filter has a better collection performance than the commercial HEPA filters because Gore-Tex consists of very fine fibers and contains no binder to keep fibers together. In predicting the collection efficiencies of membrane filters, an external flow model of conventional filtration theory for fibrous filters (Fuchs et al., 1973; Stechkina and Zhulanov, 1978) can be applied for the fiber-like membrane filters in Group (1) and (2), while a pore model (internal flow model) is applicable for pore-like filters of Group (5). For the prediction of net-like and agglomerate-like filters (Group (3) and (4)) with relatively high packing fraction, two distinctive effective pore diameters for diffusion and interception were introduced in the pore model. The model well described the dependency of particle penetration on particle size as well as on the filtration velocity. REFERENCES Cooper, D.W. (1982). Atmos. Environ., 16, 1529-1533 Fuchs, N.A., A.A. Kirsch and I.B. Stechkina (1973). Faraday Symposia of the Chemical Society, No.7 Kirsch, A.A. and Ur.V. Zhulanov (1,978). J. Aerosol Sci., 9, 291-296