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rate of reaction units do not match

Posted 7 nov. 2017, 14:15 UTC−5 Chemical Reaction Engineering Version 5.3 3 Replies

Antonios Arvanitis
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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

3 Replies Last Post 10 nov. 2017, 01:48 UTC−5
Lasse Murtomäki
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Posted: 7 years ago 8 nov. 2017, 02:00 UTC−5

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

Hi You wrote the expression of the equlibrium constant, not the rate law that should be r = kf*p_CO^a*p_H2O^b - kb*p_CO2^c*p_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
Antonios Arvanitis
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Posted: 7 years ago 9 nov. 2017, 12:52 UTC−5

Hi Lasse, it worked! Thank you

Hi Lasse, it worked! Thank you
Lasse Murtomäki
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Posted: 7 years ago 10 nov. 2017, 01:48 UTC−5

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

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

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