The effect of droplet size distribution on liquid dispersion in a venturi scrubber

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Pergamon

J. Aerosol Sci. Vol. 28, Suppl. 1, pp. $291-$292, 1997 01997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

PII:S0021-8502(97)00193-6

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T H E E F F E C T OF D R O P L E T SIZE D I S T R I B U T I O N O N L I Q U I D D I S P E R S I O N IN A V E N T U R I S C R U B B E R

J. Fathikalajahi and M. R. Talaie Department of Chemical Engineering, Shiraz University Shiraz, Iran Abstract

A three-dimensional dispersion model of our previous work was used to study liquid dispersion in a venturi type scrubber. The model in this recent work takes into account the effect of droplet size distribution on droplet dispersion in the scrubber. The results from the model are in good agreement with the experimental data. Introduction

In Venturi scrubbers liquid is atomized into high velocity gas stream. The formed droplets with different sizes are dispersed throuthout the scrubber due to eddy difusivity and gas velocity. Several attempts have been made on mathematical modelling of liquid dispersion in a venturi scrubber {Taheri and Shieh (1975); Placek and peters (1982); Viswanathan et. al (1984); Fathskalajahi et. al. (1995)}. In this study the model developed by Fathikalajahi et. al (1995) was modified and used to specify the effect of droplet size distribution on liquid dispersion. M a t h ~ m a tical m o d e l

Droplet concentration distribution for a specific drop size was obtained by solving the following three-dimensional dispersion equation :

a(VdxCd(m) ) @(Va~Cd(D) ) a(Vd~Cd(D) ) a2Cd(m--------~) ÷ a2Cd(m--) ) +S N(D) @X + @y ~ @Z :EM( @y2 @Z 2

(i)

Where, N(D) is the frequency distribution of drop size (number fraction of droplets having the size btween D and D+dD) at the begining of atomization and Cd(D) is the frequency distribution of drop size at any section of scrubber depending on x,y,z. This equation can be obtained by writing differrential mass balance for droplets over a differential control volume. The details of solving the above equation and evaluating the needed parameters are given by Fathikalajahi et. al (1995). concentration at any section of scrubber was found by following integration:

(2)

Ca= f Ca(D) dm Gin

Results

Figures

1 and 2 show the comparison between experimental

S291

data of

Abstracts of the 1997European Aerosol Conference

$292

V i s w a n a t h a n et. al (1984) and the results of m a t h e m a t i c a l model b a s e d on b o t h d r o p size d i s t r i b u t i o n and m e a n drop size for two d i f f e r e n t sets of o p e r a t i n g conditions. As it can be seen in b o t h figures, by c o n s i d e r i n g droplet size distribution the results will be in b e t t e r agreement with e x p e r i m e n t a l data.

Acknowledgement: Research

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This research was financially supported by of Shiraz U n i v e r s i t y u n d e r grant No 73-EN-854-499.

Nomenclature Cd = Total d r o p l e t s c o n c e n t r a t i o n Cd(D)= F r e q u e n c y d i s t r i b u t i o n of drop size D = M e a n d i a m e t e r of d r o p l e t s Ed = E d d y d i f f u s i v i t y of d r o p l e t s f(d) = F r e q u e n c y d i s t r i b u t i o n of drop size N(d) = F r e q u e n c y d i s t r i b u t i o n of drop size Re = R e y n o l d s n u m b e r ( @VD/# ) S = Source s t r e n g t h Vd = Droplets velocity x = Length y = Height z = Width

( No./m 3 ) ( No.lmOlm )

(m) ( m21s ) (m-z) (NO./m) (dimensionless) ( N o . / m 3. s ) ( mls ) (m) (m) (m)

References l-Fathikalajahi,J., M. Taheri and M.R. Talaie, " T h e o r e t i c a l study of l i q u i d d r o p l e t s d i s p e r s i o n in a venturi scrubber", Journal of the A I R and W A S T E M a n a g e m e n t Association, 45 ,181-185 , (1995). 2-Placek,T. D. and L.K. peters, "Analysis of p a r t i c u l a t e removal in venturi s c r u b b e r s - Role of heat and mass transfer", A. I. Ch. E. J., 28,31-39, (1982). 3-Taheri, M. and Ch. Sheih, "Mathematical m o d e l l i n g of a t o m i z i n g scrubber", A. I. C h . E . J., 21,153, (1975). 4-Viswanathan, S, A. W. Gnyp, C. C St. Pierre, " E x a m i n a t i o n of gas liquid flow in a v e n t u r i scrubber",Ind. Eng. Chem. Fundm., 23, 303308, (1984).

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Figure 1 The c o m p a r i s o n b e t w e e n the r e s u l t s and e x p e r i m e n t a l data.

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Figure 2 The c o m p a r i s o n b e t w e e n the results and e x p e r i m e n t a l data