Non ideal fluids
When I wrote the reactor book, I did not have access to a physical property program. Thus, it would have involved considerable effort to compute non ideal fugacities. Fortunately, for the methane reforming example, the assumption of ideal gases and ideal solution is valid, as I will show.
Since writing the book, I have acquired the Prode Physical Properties (PPP) program. Now I can easily examine non ideal systems.
The fugacities via PPP
The mc_StrFv function produces a vector of fugacities for each component in a stream. These fugacities are the component partial fugacity coefficients multiplied by the total pressure. Dividing by the pressure results in the fugacity coefficients.
The coefficients are all close to 1 indicating an ideal system. Let's redo the example without making the ideal assumption.
Gibbs free energy for non-ideal system
I wrote the program below to define the Gibbs free function for a non-ideal gas system.
I set the standard state fugacities of each compound to 1 atm. This was necessary because the F(T) free energies were based on that standard state in two ways. First, the free energies at the standard state of ideal gas at 298 K and 1 atm were used. Second, the heat capacities were for ideal gases.
Compare non-ideal vs. ideal
The GFreal function can be minimized as before to obtain the equilibrium composition as a function of T and P.
Since the system and conditions originally chosen are almost ideal, I have increased the pressure for the comparison below.
The non-ideal results were obtained using the cubic plus association/Peng Robinson equation of state with interaction parameters.
The plot demonstrates two effects. First, the ideal assumption slightly overpredicts the methane conversion at a given temperature. Second, the high pressure reduced the conversion for both cases.
A later post will show an example using GFreal for a much more non ideal system.
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