Pergamon J. Aerosol Sci. Vol. 28. Suppl. l, pp. $397--$398, 1997 01997 Elsevier Science Ltd. All rights reserved

Printed in Great Britain PI1 :S0021-8502(97)00245-0 0021-8502m s17.~0.00

A DEPOSITION MODEL FOR FIBROUS PARTICLES IN GRANULAR BED FILTERS

Yue Zhou, Jan Marijnissen, Saul Lemkowitz, Albert Podgorski" and Henk Bibo

Sections Risk Management & Particle Technology, Delft University of Technology Julianalaan 136, 2628BL, Delft, The Netherlands

"Department of Chemical and Process Engineering, Warsaw University of Technology Warynskiego 1, 00-645 Warsaw, Poland

KEYWORDS: Fibrous particles, deposition model, granular bed filter

INTRODUCTION The paper presents experimental studies on the deposition of fibrous particles in a granular bed filter. In a previous study (Zhou et al., 1994), a granular bed filter was used to model the deposition of fibers in the lower parts of the human lung. In the bed filter each layer, which is composed of glass spheres of a given size, represents an airway generation of the human lung. Experimental work was done by Zhou et al. (1996) to develop a deposition model for spherical particles in a granular bed. On the basis of this initial research, the model for spherical particles was modified to describe the deposition of fibers. This abstract describes the development of a semi-empirical model for fiber deposition in granular bed filters based on the authors' experiments and a comparison between this model and a theoretical model developed by Podgorski et al. (1996).

EXPERIMENTAL STUDY FOR FIBER DEPOSITION IN GRANULAR BED FILTERS The experimental study includes the production, dispersion and measurement of fibers with various sizes. For large fibers (length > 3 /am), glass fibers with a uniform diameter were dispersed by a fluidized bed aerosol generator and their length distribution and concentration were measured by a laser fiber monitor. For the sub-micron size, a nebulizer was used to disperse uniform akaganeite fibers: their concentration was measured by a differential mobility particle sizer. Deposition was determined by measuring particle concentration at the beginning and end of the granular bed filter. The experimental results showed that the model modified from the spherical particle deposition model could not be used to predict fiber deposition in a granular bed filter. Therefore a semi-empirical model

Table. Fiber deposition model in granular bed filters

P~. exp( -3(1 -E)LT1T): = fir = rls+T1D+rlI+rlR

2d,l~

Stk~'~ 41 1.75Ree 1] I = : S tk tt = (I.0+ )S t k

2.05 +Stk~ 41 150( I -E)

11 s = 0.022(Grv)°S+0.1 I(Grv) ~75

2 Re -~ lqt~ = B ( R e ) S c t'lR~Re/:(R,,, .[ i(Re ) = - _ +

3 6(Re3+2xl05) 2

B ( R e ) = 8; .f~(Re) = - 7 (Re<30):

B ( R e ) = 40 : .f~(Re) ~ - I. 15 (30<Re<100)

- ~ 4 ~ ~ L/ qe = 6 . , ~ - R,-; R, d

$397

$398 Abstracts of the 1997 European Aerosol Conference

was developed as shown in the table, Based on the experimental data in various deposition mechanism regimes, this model includes the fiber equivalent diameter, the aspect ratio of a fiber, and the fiber rotation. Predictions of fiber deposition made using this semi-empirical model agree quite well with our experimental results.

COMPARISON OF THE THEORETICAL MODEL AND THE SEMI-EMPIRICAL MODEL A theoretical model based on individual fiber trajectories in a granular bed filter was developed by Podgorski et al. (1996). This fundamental approach is based on the constricted tube model for the geometrical representation on the meso-scale internal structure of a granular bed. The aerodynamic behaviour of a fiber in an incompressible Newtonian fluid, including fiber orientation, was considered in the theoretical model. This model was calculated with the same conditions as the experiments.

It is very difficult to compare all the experimental data with the theoretical model, especially for high velocities, because the theoretical model requires long calculation times. For both the theoretical and the semi-empirical models, comparisons were obtained using experimental data for small fibers at very low superficial velocities (no experimental data exists for large fibers) as shown in the figure. Agreement between the theoretical model and the experimental data was poor. It is therefore concluded that the present theoretical model cannot be used for calculating fiber deposition in granular beds. The semi-empirical model, however, is in very good agreement with the experimental data.

1 ............. t h . o ~ , = ~ r . o n . i ] ~ - - ~ - - semi.empirical model ~ i

0 8 I • expefimen~ data _ I / perpendicular /

~ ' 1 - - mndo m ~ - - ~ ~ / /

// $ ) L - 1~.0~ m I parall~. .............. ~ ' ~ / '

0 2 - 1 " " • 1 ......... ~." .................. | ~ " 0.92 ;am J ....... :::::: .......... perpendicular / ~'° '3s I

0 - ~ J r ' 1 10 1 0 0

Flber aspect redo (-)

Figure. Comparison of the theoretical model, the semi-empirical model, and experimental data for fiber deposition in a granular bed filter with various orientations,

SYMBOL LIST

d:,, bead diameter [cml St/( Grv gravity number [-] L the height of the bed [cml ~l, rb,, L t fiber length [cm] rls.'qR Pr total penetration for fibers 1-] Re Reynolds number [-1 qr SC Schmidt number l-I

REFERENCES Podgorski, A. el al. (1996), J. Aerosol Sci., 27. $479-480

Stokes number [-] porosity of the granular bed filter [-] single sphere collection efficiencies for impaction, diffusion, sedimentation and interception [-] single sphere collection efficiency for fibrous particles [-]

Zhou, Y. et al, (1996), in Aerosol Inhalation: Recent Research Frontiers, (Ed. J. Marijnissen and L.Gradon) Kluwer Academic Publisher, p127-142 Zhou, Y. et al. (1994), Proceedings of Fourth International Aerosol Conference, Los Angeles, p882-883