Modeling of a DBD Reactor for the Treatment of VOC "

Transcription

1 Presented at the COMSOL Conference 2009 Milan COMSOL 2009 Conferences october 2009, Milan Italy Modeling of a DBD Reactor for the Treatment of VOC " L. Braci, S. Ognier and S. Cavadias Laboratoire de Génie des Procédés Plasmas et Traitement de Surfaces, 11 rue Pierre et Marie Curie, Paris, France 1

2 Dielectric Barrier Discharge The process Air (N HV Electrode 2, O 2 ) Active species e+o 2 or e+n 2 O(1D), O 3, O 2 (1 g), Dielectric + Pollutant(VOC) S S SH Partial oxidation (low energy) OH O O O S S O O Complete oxidation (high energy) SO 2, CO CO 2 Bioremediation Trapping on fluidised or fixed bed Clean air Molecule model: Butane-thiol-1 SH 2

3 The reactor and simulation domain whole domaine Outlet Flow simulation 3D Diffusion of pollutant Post discharge region Electrode Pollutant inlet Discharge region simulation 2D HV electrode Production of active species (O, O3, ) Air inlet Post discharge simulation 3D Reaction of active species with the pollutant HV electrode 3

4 Outlet: P=0 Pa Models used: 1/ Navier-Stokes incompressible (chns) Flow simulation Post discharge region Electrode Air +Pollutant inlet: 1Sl/mn Velocity and Pressure fields 2/ Convection and Diffusion (chcd) Air inlet: 1Sl/mn Concentration of pollutant in the reactor 4

5 1/ Navier-Stokes incompressible (chns) U m/s 0.58 m/s Flow simulation U max =0.25 m/s in the discharge region Streamlines show a rather good mixing between of active species and the pollutant 0 m/s 5

6 2/ Convection and Diffusion (chcd) C mol/m Diffusion of thiol in the discharge region. Flow simulation 0.5 [Thiol] Experimental results show a film formation in the discharge region. air Change of the design of the reactor is necessary

7 Electron density determination Discharge region simulation 2D The active species reacting with the pollutant are created by colisions oxygen-electrons in the discharge region needed [e - ] e+o 2 O O 2 (1 g) O points Matlab 1D model Pollutant Calculation by resolution of a system of 4 equations: HV Flow HV E l e c t r o d e Equation of Poisson and three continuity equations, for electrons, positive ions and negative ions 7

8 Discharge region simulation 2D Electron density determination Resolution of 4 coupled equations: Equation of Poisson e ϕ = ( n ε 2 p n e n n ) relating V with distribution of local densities of charges. three continuity equations: for electrons, positive ions and negative ions: S e n t The species are created by the reactions j 100 points 1D model +. Γj = S j with Γj = D j n j + sign( q j ) n jµ je Flux of Source term particles for particles j Diffusion Flux due to the drift flux of charged particles J: e for electrons, p for positive ions and n for negative ions = α η) e+a A + +2e e+a + A A - +A + A e+a A - β α β ( neve nen p S p = neve nen p nnnp Sn neve nnnp β HV α β β η Ionization Recombination Attachement =η β Discretization Cranck Nicholson and linearization Raphson Newton [e - ] cm -3 8

9 Reaction Engineering Lab 0D kinetic model of Willy Morscheidt: (LGPPTS) 14 reactions, 12 species Discharge region simulation 2D e+o 2 O O 2 (1 g) O 3 ~ u Average velocity: 0.15m/s O O 2 (1 g) O 3 Residence time:~10ms e The model will be exported in COMSOL Multiphysics module t (s) 9

10 Flow and concentration fields chns and chcd models Concentration mol/m 3 Outlet Velocity (m/s) Discharge region simulation 2D Electrode Inlet Axis of symmetry e O 3 0 e- These concentrations and velocity will be used as Inlet BL in chcd and chns models respectively,, in the post discharge reactor 10

11 Outlet Set-up of 3D reactor and velocity field model used chcd U(m/s) 1.5 Post discharge region 3D simulation Active species inlet Reactions: Residence time: 1 to 2 seconds Pollutant inlet Electrode SH + O O 3 Inlet velocity: m/s Rate Constants (cm3/s) K1=1.2e-12 K2=9.6e-12 Products 0 11

12 model used Concentration fields of reactants chcd Post discharge region 3D simulation Outlet Active species inlet Concentration (mol/m 3 ) 0.16 O O 3 Pollutant inlet Outlet Active species inlet Pollutant inlet Concentrations of active species in the inlet BL from 2D model 12

13 C(mol/m 3 ) model used Concentration fields of pollutant and products chcd Post discharge region 3D simulation BL:22, 23 thiol [thiol]=0.041 mol/m e-8 [mol/s], Expression: ntflux_cthiol_chcd, Boundaries: 22, 23 Conversion of thiol:~90% e-7 [mol/s], Expression: ntflux_cthiol_chcd, Boundary: 12 C(mol/m 3 ) 0.44 Experimental results show a depletion from 40% to 100% depending on the induced specific energy 0 products 13

14 Conclusion - It is possible to simulate a convection- diffusion process in 3D chemical reactor for the treatment of VOC. However due to the lack of CPU memory the reactor must be divided in 3 parts and the whole process must be treated sequentially. So the diffusion of the pollutant in the discharge region is not taken into account. -Calculations show a qualitative agreement with experimental results. The prediction can be sensibly improved for kinetic models with more reactions. Current and future work -Use of more powerfull PC with larger memory for the treatment the whole process in one step. --Implementation of the discharge model directly in COMSOL 14

15 Thank your for your attention Questions? Suggestions? Any idea for the implementation of DBD model in COMSOL is welcome 15