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Transport of diluted species (Problem with catalyst pellet radius)

Posted 6 apr 2014, 03:24 GMT-4 Chemical Reaction Engineering Version 4.3b 4 Replies

Arvind Nanduri
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Hi,

I am modeling a catalyst pellet for fischer tropsch synthesis (43 species in the reaction). I have the model set up but it is converging only for small particle diameters (0.01mm) when ever I increase the particle radius, the concentration of one of the reactants become negative.

I tried increasing the surface concentration of the reactants but still I keep getting negative numbers at high particles radius values ( I have good results for 0.01 mm radius).

I know that there is a section in knowledge base dedicated to negative concentration profiles but I am working on an expired class kit license and I do not have access to the knowledge base.

I would really appreciate any help. ( I really need some help soon)
4 Replies Last Post 7 apr 2014, 08:02 GMT-4
Lasse Murtomäki
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Posted: 1 decade ago 7 apr 2014, 05:21 GMT-4
Without seeing the model I can ask only:
1. Is it a steady-state or a time dependent problem?
2. Is there a diffusion boundary layer around the particle?

br
Lasse
Without seeing the model I can ask only: 1. Is it a steady-state or a time dependent problem? 2. Is there a diffusion boundary layer around the particle? br Lasse
Arvind Nanduri
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Posted: 1 decade ago 7 apr 2014, 06:23 GMT-4
Hello Dr. Lasse,
My model is a stationary one. The is no diffusion boundary layer around the particle. In the model CO gets completely consumed towards the center of the particle and eventually becomes zero. As I increase the radius the concentrations become either zero or it throws "Undefined value found in the equation residual vector" error. I am attaching the model for 0.017 mm pellet which is working just fine with adaptive mesh refinement. I really appreciate your help.
Hello Dr. Lasse, My model is a stationary one. The is no diffusion boundary layer around the particle. In the model CO gets completely consumed towards the center of the particle and eventually becomes zero. As I increase the radius the concentrations become either zero or it throws "Undefined value found in the equation residual vector" error. I am attaching the model for 0.017 mm pellet which is working just fine with adaptive mesh refinement. I really appreciate your help.
Lasse Murtomäki
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Posted: 1 decade ago 7 apr 2014, 07:48 GMT-4
I looked at your model. It is terribly complicated, so it is difficult to see where the problem resides. Reaction-diffusion systems do not always have a steady solution, but may oscillate when the diffusion and reaction time constants couple in a suitable way. Naturally, an existing industrial chemical process cannot oscillate.

This is a lousy reply, I am sorry.

BR
Lasse
I looked at your model. It is terribly complicated, so it is difficult to see where the problem resides. Reaction-diffusion systems do not always have a steady solution, but may oscillate when the diffusion and reaction time constants couple in a suitable way. Naturally, an existing industrial chemical process cannot oscillate. This is a lousy reply, I am sorry. BR Lasse
Arvind Nanduri
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Posted: 1 decade ago 7 apr 2014, 08:02 GMT-4
Thank you for your reply. But is there a way to limit the concentration to positive values?? I tried if(CO<0,esp,CO) as an initial condition for concentration of CO but still comsol gave me negative concentrations for CO.
Thank you for your reply. But is there a way to limit the concentration to positive values?? I tried if(CO

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