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How to fix the problem of non-integer exponent and temperature dependent variables
Posted 07.01.2019, 16:47 GMT-5 Chemical, Chemical Reaction Engineering, Parameters, Variables, & Functions Version 5.3a 1 Reply
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Hello everybody, I'm simulating a porous reactive catalitic media for secondary reforming in an ammonia plant in order to optimize the yield and the selectivity of the process as a function of the considered shape of the catalyst at a prefixed composition.
I have to add a user defined reaction rate, which depends on temperature and some other parameters (kinetic constants, adsorption constant, etc). I have to study three different reaction rates, two of them depends on k1 and k3, which have the following unit:
(kmolbar^0.5/(kgh))
When I fill the parameters section out, the writing turns yellow with a "unit syntax error". How can i fix this problem? I read about a non-integer exponents problem with comsol, but i don't know how to deal with a non-integer exponent in the unit of a variable. I'm using the Comsol 5.3a version.
Then, I want to ask you also how can I define a variable which depends on the temperature of the system, because I considered a tubular reactor and the temperature is not constant with the length, i don't know how to consider a variable which changes progressively with the temperature along the length.
Thank you in advance, I hope someone of you could help me because I've gone crazy about these problems for several days!
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For a non-isothermal problem you need to couple heat transfer physics with chemical reaction physics. See e.g. thermal_decomposition tutorial model.
What comes to the non-integer reaction rates, use plain number as the reaction rate constant and write the reaction using (p/p0) instead of p, where p0 = 1bar. Usually the yellow color does not harm calculation, it is only a warning that Comsol does not fully understand.