The free energies of the compounds were obtained in the last post. In this post, the contributions of pressure and mixtures are developed. I will start with the general case for non-ideal gases or liquids and then proceed to simplify to more restrictive cases.
The first term involves the compound free energies from the last post. The over-arrow term creates a vector of relative fugacities. Thus, both terms involve the dot product of two vectors. The overall result is a scalar.
The first simplification is to consider only gases, the case needed for the methane reforming system.
Next, assume ideal solutions of non-ideal gases.
The formula looks like the previous one, but the fugacity coefficient is now for pure compounds, unaffected by mixture.
At this point, the Gibbs Free Energy formula is...
Finally, let's assume that the gases are ideal, which is often the case at low pressures.
The ln0 function is a Mathcad function that returns an infinitesimal value when the argument is zero. This allows computations to continue without error when a compound is not present.
In the next post, the last formula will be used to determine the equilibrium curve for the methane reforming example.
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