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pepa August 18, 2003 09:38

Porous media and chemical reaction
 
Could anybody help me? I would like to make a model of tube reactor with catalytic layer (nonzero thickness). But the reaction is equilibrium and it runs only on the surface of porous catalyst. I try something, but it does not work. Please, what is the best approach.

andrew_garrard August 18, 2003 10:26

Re: Porous media and chemical reaction
 
What is wrong with the method you are trying? What is stopping you creating three subdomains, one for the bulk flow, one for the porus zone and one for the catalyst layer?

pepa August 18, 2003 11:12

Re: Porous media and chemical reaction
 
Yes, I did that: bulk zone, porous zone and nothing else. Porous zone is the catalytic layer. I defined the viscous resistance and inertial resistance and the reaction and the wall rurface reaction type. But how can I make the equilibrium reactions? I have some mechanism of catalytic reaction but only forward reaction, but I know the equilibrium constant. I derived the reaction-rate constant for the backward reaction. Is this approach correct? But it still does not work with my numbers. Please, help!

Greg Perkins August 19, 2003 02:41

Re: Porous media and chemical reaction
 
If you have the forward rate FLUENT should be able to work out the appropriate backward rate based on equilibrium. I would consider this to be correct.

Is your chemical system stiff - ie are the arrhenius parameters very high and perhaps different reactions have very different rate constants????

If so, probably you'll want to use the coupled solver and turb on the stiff chemistry option - see the manual.

Greg

pepa August 19, 2003 10:03

Re: Porous media and chemical reaction
 
How? This is equilibrium reaction on the surface of the catalyst. pEpA

Greg Perkins August 19, 2003 23:01

Re: Porous media and chemical reaction
 
FLUENT doesn't have the option of forcing equilibrium of a particular reaction. However, if you include the backward reaction rate, this is calculated using the forward rate and the equilibrium constants, ie it takes account of equilibrium constraints, but doesn't force it to be at equilibrium at each cell.

To set backward rate just click on the option - if unsure read manual.

Greg

pepa August 20, 2003 03:22

Re: Porous media and chemical reaction
 
--To set backward rate just click on the option - if unsure read manual.-- But it's not possible to click on the option. It possible only for reaction in volume not on the wall surface.

Greg Perkins August 20, 2003 03:29

Re: Porous media and chemical reaction
 
Ok, then you could define a 2nd reaction which is the reverse of the first and define its forward rate using the equilibrium constraint.

So for this 2nd reaction you would define the forward rate as kf2 = kf1/Kc where Kc is the equilibrium constant and kf1 is the forward rate of the first reaction. kf2 would have an arrhenius form.

Hope that makes sense.

Greg

pepa August 20, 2003 04:47

Re: Porous media and chemical reaction
 
Yes , I did this thing. It works really good in volume. I defined one forward and one backward reaction as you wrote. But it still doesn't work on the surface :)) and I really don't know why. The reaction is exothermic, but temperature of mixture decreases, why? Thank you Greg.

Greg Perkins August 20, 2003 05:00

Re: Porous media and chemical reaction
 
Not sure about why it doesn't work - the surface reaction stuff is new in Fluent 6/6.1 - there may be a bug!

I have actually written my own udfs to do the surface reaction and in my models I can have forward and backward in the one reaction. I developed this in Fluent 5 when there wasn't such a feature and have stuck with it, and haven't used Fluent's newer internal models so I am not sure.

Greg

pepa August 20, 2003 05:24

Re: Porous media and chemical reaction
 
Is it possible to watch your UDF? I've never written UDF.


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