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Series parallel reaction Eddy Dissipation vs Finite Rate Chemistry

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Old   October 10, 2019, 13:53
Default Series parallel reaction Eddy Dissipation vs Finite Rate Chemistry
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Rajat Sehgal
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Hi, I am a beginner with CFD and CFX. I have a problem to simulate multiple reactions for thermal chlorination of methane in a plug flow reactor. The reactions are:
CH4 + Cl2 --> CH3Cl + HCl
CH3Cl + Cl2 --> CH2Cl2 + HCl
CH2Cl2 + Cl2 --> CHCl3 + HCl
CHCl3 + Cl2 --> CCl4 + HCl

I have three coaxial inlets for each of Cl2, HCl (inert/carrier gas) and CH4. The flow rates for Cl2 and CH4 are equimolar and for HCl is fixed to constant value higher than either of the other two. Mesh used is coarse (default element size). I modeled these as 4 single step, irreversible, 2nd order reactions and chose the finite rate/eddy dissipation model(combined). The pre exponent factors are of the order of 10^6 m3/mol.s and activation energies around 3 kcal/mol. The solution converges well for EDM and combined model, but there is too much of CCl4 produced. For an equimolar CH4 and Cl2 mixture there should be very little CCl4 produced. If CCl4 is ignored, the other products are in right proportions to each other. It is probably due to CCl4 not having consumption term. I also tried Finite rate chemistry, hoping it may do the trick , but it does not converge unless I reduce the pre exp factor by order of 10^5 to 10^6.

I am unable to understand why it reacts to such an extent and how I can avoid it. Also is there a way to have a kinetically controlled model for such fast reactions? (By this I mean for example here the reactions are all very fast but some faster than others. How could we model that? Also could eddy dissipation model reversible reactions if they are fast as well?)

Analysis: Steady state
Fluid model --> Turbulence model: k-epsilon, Heat transfer: Thermal energy
I am attaching some pictures with more info about reaction rates and concentrations. Top inlet is Cl2, middle Hcl and bottom CH4. Axissymmetric about x axis.
Attached Images
File Type: jpg Reaction_concs.jpg (84.9 KB, 8 views)
File Type: png TurbKE_dissip.png (68.7 KB, 7 views)
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cfx pre, eddy dissipation, reacting flow

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