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Reaction a Quasi-Equilibrium
Objective: To show how Reactions at Equilibrium are modeled in REX
In this simple example, we illustrate how one can specify equilibrium reactions in REX and interpret
the results. You may download the REX file for this example.
Features Illustrated
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Modeling Equilibrated Reactions
Reaction Model
Consider a reversible reaction R1 that involves compounds A and B that will be considered as being
at equilibrium; and a second reaction R2 that produces C from B:
After we enter the compounds and the reactions, R1 should be selected as a Quasi-Equilibrium
reaction in the Reactions node:
In the Kinetics node, both the forward as well as the reverse directions must be included for every
quasi-equilibrium reaction. After entering values for the kinetics parameters, and performing the
Initialize Orders action in the Parameters node, a snapshot of the Parameters node is shown below:
Next, an experimental set is defined where the initial contents are: A = 1 gmol, B= 0 gmol, C=0.001
gmol:
After running the project as Simulate Only in Solution Options node, the profiles are shown below:
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It can be seen that the initial concentration of C is exactly the same as the initial value entered in the
experimental set, but for compounds A and B the initial values are different from what was entered in
the experimental set.
This change in the initial composition is done by REX, using the user provided initial values and then
finding the equilibrium values. The subsequent compositions of A and B will also satisfy the
equilibrium constraint.
You can verify for this simple example that A and B satisfy the equilibrium condition for the forward
and reverse rates:
Replacing the rate r according to the kinetics (activation energies are zero in this example):
Substituting the values for the pre-exponentials, we should have:
Additional Comments
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You should be aware that when dealing with an equilibrated reaction, the values of the
preexponential and activation energy for the forward and reverse directions entered in REX
are not individually meaningful: what has significance is the ratio between the forward and
reverse pre-exponentials, and the difference between their activation energies, as they define
the value of the equilibrium constant. Thus, the same results can be obtained in this REX
example if you multiply the preexponential values of both direction of R1 by a nonzero
number.
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In this simple model, we perform a simulation. When estimating parameters for an equilibrated
reaction, the model will optimize the preexponential and activation energy for the forward
direction only, provided they have open bounds. The parameters for the reverse direction of
an equilibrated reaction will be kept fixed.
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