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Assembling of matrix takes very long

Posted Sep 20, 2018, 5:23 a.m. EDT General, Chemical Reaction Engineering Version 5.3a 4 Replies

Till Henrich
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Hello!

I'm trying to solve a (quite simple) 2D-axissym. model of a fixed bed reactor. I have four species where only three a actually taking part in the exothermic reaction. The chemistry is modeled in re (reaction engineering module) and exported to the 2D-model where I use the following modules: tds (transport diluted species), ht (heat transfer in fluids) and br (Brinkmann-equations).

As far as I can see, I set up the model correctly. But when I want to solve it, it takes hours to assemble the matrix!!!

Is this a normal behaviour? Or where could I start finding the mistake?

Thanks in advance! Till

4 Replies Last Post Sep 24, 2018, 5:14 a.m. EDT
Till Henrich
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Posted: 6 years ago Sep 21, 2018, 7:47 a.m. EDT

Found an interesting behaviour:

I simplified the calculation of the heat capacity of the mixture. I left out the temperature-pendency of the species and took averaged values.

Now it solves much much faster!

Found an interesting behaviour: I simplified the calculation of the heat capacity of the mixture. I left out the temperature-pendency of the species and took averaged values. Now it solves much much faster!
Edgar J. Kaiser Certified Consultant
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Posted: 6 years ago Sep 21, 2018, 9:25 a.m. EDT

Till,

that's not too surprising. A temperature dependent heat capacity makes the heat transfer physics nonlinear.

Cheers Edgar

-------------------
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Till, that's not too surprising. A temperature dependent heat capacity makes the heat transfer physics nonlinear. Cheers Edgar
Till Henrich
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Posted: 6 years ago Sep 24, 2018, 4:04 a.m. EDT

Hi Edgar,

thanks for replying. But is this the effect that strong? Computing a solution with fixed Cp takes about 1 minute. With Cp(T) it takes several hours! BTW: I have modeled the heat conductivity also as a function of T in both cases. This doesn't affect the solution time at all...?

Cheers Till

Hi Edgar, thanks for replying. But is this the effect that strong? Computing a solution with fixed Cp takes about 1 minute. With Cp(T) it takes several hours! BTW: I have modeled the heat conductivity also as a function of T in both cases. This doesn't affect the solution time at all...? Cheers Till
Edgar J. Kaiser Certified Consultant
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Posted: 6 years ago Sep 24, 2018, 5:14 a.m. EDT

Till,

yes that is at least a little surprising. Is the derivative Cp(T)/dT nicely smooth?

Cheers Edgar

-------------------
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Till, yes that is at least a little surprising. Is the derivative Cp(T)/dT nicely smooth? Cheers Edgar

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