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Modelling reversible chemical reaction.

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Old   August 20, 2010, 09:19
Default Modelling reversible chemical reaction.
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Marek
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Hello,

Iím trying to model a liquid-liquid reversible reactions. What is more that is axissymetric problem with swirl. I have some problems, because Iím relatively new in Ansys Fluent.
To start, my reactions are as follows:

1A + 1B <-> 1C + 1D
1C + 1B <-> 1E + 1D

I have all the data for reaction, k constant and activation energy. Iíve calculated pre-exponential factor based on Arrhenius equation. Also I got all required data in materials panel. I know that constant k for reversible reaction is calculated from standard state entropy and enthalpy.
First question is what is rate exponent and how to calculate it? Chemical kinetics is not my best side, so I donít know, if I calculated it properly.

Still it wonít work properly. I use unsteady solver, laminar flow (there is no need to use turbulence model - it is based on known Reynolds number), 2ddp. In species transport, there is used volumetric type of reaction (laminar finite rate).

Another question is about species transport. At the outflow boundary, there is backflow of last (E) compound (I know that the last one is complementary compund). The same situation was, when I was using pressure-outlet type of boundary. Any ideas?

Last but not least question, is about mesh and time step size. Do these problems can result from too small amounts of mesh elements or too large time step, whether to few iteration per time step?

Thanks in advance for all kinds of ideas or suggestions.
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Old   August 20, 2010, 12:08
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Allan Walsh
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Often the rate exponents for the reactants will be the same as the stoichiometric coefficient in the reaction. For example, in the reaction
Ca + 0.5 O2 -> CaO, then
rate = k [Ca] [O2]^0.5.
If you follow the crude computer notation for the equation, then the rate exponent for oxygen is 0.5.
In practice it can be much more difficult. You should research your specific equations to see if there are suggested literature values for a mechanism with rate exponents. Also, the Fluent (Ansys) materials database has quite a few built-in reaction mechanisms and you may get lucky with yours.
In my experience, it is easy to make mistakes on the units for the pre-exponential factors and I recommend doing a hand calculation as a check of the rate coefficients calculated by Fluent.
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Old   August 20, 2010, 13:49
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The reaction is transeterification and accurate data is hard to find, because it happens in spinning disk reactor (we have only data from batch reactor) and that is a new problem for us.

I've done hand calculation for pre-exponential factor. Based on Fluent help, I found that reaction k constant units are m^3/(kmol*s) so pre-exponential factor should have consitnt units. Am I thinking right?

Also may I ask if you could tell me how to change built-in reaction mechanism, because I think I'm using a default one?
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