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rate of reaction units do not match
Posted 7 nov 2017, 14:15 GMT-5 Chemical Reaction Engineering Version 5.3 3 Replies
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Hi I'm trying to type a ''user defined'' rate of reaction in the chemical engineering module. My reaction CO+H2O<=>H2+CO2 has a rate of reaction that is given by: rCO=k((p_CO^ap_H2O^b)/(p_CO2^c*p_H2^d)). With a=1.02, b=0.06, c=...,d=... without using a,b,c,d I'm able to compute but when raising the expressions to the power of a,b,c,d the units do not match and somsol cannot compute. Is there a solution to that problem? Thanks
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Hi
You wrote the expression of the equlibrium constant, not the rate law that should be
r = kfp_CO^ap_H2O^b - kbp_CO2^cp_H2^d
where kf and kb are the rate constants of the forward and backward reaction. What you can do is to define kf and kb in such a way that the dimensions agree. As the reaction rate has the dimension, e.g. bar/s, kf must be of the dimension bar^(1-a-b)/s. Similarly, kb must be of the dimension bar^(1-c-d)/s.
Usually Comsol just nags about the dimensions but still calculates all right.
I wish this helps
Lasse
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Hi Lasse, it worked! Thank you
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Actually, a more pertinent way is to convert concentrations dimensionless by dividing them with the standard concentration c° = 1 mol/L. Then the rate constant must be expressed in the same dimension as the reaction rate, viz bar/s.
BR Lasse