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Reaction rate in chemical equilibrium reactions

Posted Jul 20, 2022, 7:09 a.m. EDT Chemical Reaction Engineering Version 5.6 3 Replies

Hidayat Ullah
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Hello everyone. I am working on transport of diluted species in porous media (tds). I have 4 equilibrium reactions with specified equilibrium constant values (Keq).

In the formula of diluted species (d(εci)/dt+ ∇ ⋅ Ji + u ⋅ ∇ci = Ri), the chemical reactions are introduced as source terms in the form of Ri (reaction rate expression [mol/m^3-s]).

I do not understand how COMSOL calculates this Ri term. By looking at its unit it seems like rate of change of concentration of a specie (dc/dt). But this is not true because there is a difference between the Ri and dc/dt values.

Can someone explain the process of Ri calculation in COMSOL?

Thanks in advance. HK

3 Replies Last Post Sep 15, 2022, 2:25 a.m. EDT
Lasse Murtomäki
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Posted: 2 years ago Aug 1, 2022, 6:33 a.m. EDT

When you define the chemical reaction, you must give the rate constants in a proper dimension so that Ri's becomes mol/(m^3·s). If they are no properly defined the reaction term appears in yellow color.

Lasse

When you define the chemical reaction, you must give the rate constants in a proper dimension so that Ri's becomes mol/(m^3·s). If they are no properly defined the reaction term appears in yellow color. Lasse
Hidayat Ullah
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Posted: 2 years ago Sep 14, 2022, 4:40 a.m. EDT

Thank you for your response dear Lasse.

I have defind it well. In the equilibrium reaction node I define the equilibrium constant i.e., Keq which has no units. My question is how COMSOL calculates the Ri term from Keq?

Thank you for your response dear Lasse. I have defind it well. In the equilibrium reaction node I define the equilibrium constant i.e., Keq which has no units. My question is how COMSOL calculates the Ri term from Keq?
Lasse Murtomäki
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Posted: 2 years ago Sep 15, 2022, 2:25 a.m. EDT

I have never suceeded in using an equilibrium reaction, it just does not converge. Instead, I define an arbitrarily high forward rate constant and calculate the backward rate constant with the equilibrium constant. For example like this:

Reaction: A --> B

Ri = kf[A] - kb[B]

Keq = kf/kb ==> kb = kf/Keq.

Because the reaction rate constants are high, the reaction appears an an equilibrium reaction. You may check it by calculating [B]/[A] if it equals to Keq.

I have never suceeded in using an equilibrium reaction, it just does not converge. Instead, I define an arbitrarily high forward rate constant and calculate the backward rate constant with the equilibrium constant. For example like this: Reaction: A --> B Ri = kf*[A] - kb*[B] Keq = kf/kb ==> kb = kf/Keq. Because the reaction rate constants are high, the reaction appears an an equilibrium reaction. You may check it by calculating [B]/[A] if it equals to Keq.

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