1. CFD MODELLING OF PRESSURE DROP AND FLOW DISTRIBUTION IN PACKED BED FILTERS K Taylor & AG Smith - S&C Thermofluids Ltd S Ross & MW Smith - DERA Porton Down

2. OVERVIEW
Packed bed filters remove toxic agents from contaminated airstreams
CFD potential design tool for predicting the flow and pressure drop
Mathematical model for predicting radial voidage distribution in bed
Non-uniform voidage distribution included in CFD model
Validated against measurements of pressure drop and velocity distribution
Potential for CFD modelling of adsorption process also investigated

3. GEOMETRY OF FILTER BED

4. VOIDAGE DISTRIBUTION IN CYLINDRICAL FILTER BEDS
Radial voidage distribution in ‘snowstorm’ packed filter beds is a function of the ratio: particle size/bed diameter
Affects the velocity distribution within the filter bed
Measurements made of voidage distribution for range of particle sizes
Fitted to modified ‘Mueller’ model

5. e = eb + (1- eb)e-brJo(ar*)

6. GEOMETRY OF FILTER BED

7. CFD MODEL
2-d axi-symmetric BFC model
Grid distribution determined from voidage distribution to ensure adequate grid resolution near walls
Local voidage distribution coupled to Ergun-Orning equation for pressure loss through bed: Dp/L = 5 So2(1-e)2mU/e So(1-e)rU2/e3 | | viscous loss turbulent loss
Substantial improvement in predictions compared to model using average voidage

8. PRESSURE DROP - 3mm PARTICLES

9. PRESSURE DROP VS GRID DENSITY

10. VELOCITY DISTRIBUTIONS

11. ADSORPTION MODEL
Transient model to predict ‘breakthrough’
Steady state flowfield used as initial conditions
Adsorption rate source term: ¶C/¶t = 1/e So k (C - Ci)
Rate of uptake in adsorbent: ¶m/¶t = e/(1-e) (-¶C/¶t)/rz
Maximum uptake from isotherm equation: mmax = a.b.RH/(1 - RH)

12. VAPOUR UPTAKE IN FILTER BED

13. VAPOUR PENETRATION

14. IMPLEMENTATION WITHIN PHOENICS
Pre-processor - interprets voidage distribution and basic input parameters - outputs Q1 file
Additional Q1 commands for adsorption model
GROUND coding for porosity from voidage distribution inlet boundary conditions source terms for pressure loss and adsorption rate

15. CONCLUDING REMARKS
Method for prediction of pressure and flow distribution validated for range of parameters
Implemented within PHOENICS user routines
Potential for adsorption model demonstrated
Areas for further work: improvement and validation of asdorption model improved user interface turbulence modelling within the filter bed