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Hi.

 

I have a "problem". I'm using Hysys v8.0 to estimate the True Critical Pressure for a Fuel Gas Stream. Of course, I have all the composition data. According to Peng-Robinson the True Pc is 4189 psia. According to PRO II v9.1 the value is 467.92 psig (for the same thermodynamic model). This is a huge difference. Reviewing some experimental data for Fuel Gas, I found that PRO II is given a "more real value".

 

What's happening here? Could be happening because of the themodynamic calculation?

Not tell you if you placed SRK as a model in Hysys (a very high value of True Pc is obtained).

 

Could be happening because of the gas itself? It's a mixture of different gases, non ideal gases. Attach you will find the composition data for the fuel gas. In that order, Do I have to take the value of Pseudo Critical Pressure (630.5 psia) when I'm working with mixtures???...  I know that the pseudo critical properties are simply the summation of the "apparent" contributions of each pure component. For example, the pseudo Tc of a binary mixture (component 1 and 2) would be calculated as Tc=x(1)*Tc(1)+x(2)*Tc(2).
The true critical properties are the real critical properties of the stream determined thermodynamically by satisfying both the quadratic and cubic forms in the expansion of the Helmholtz free energy... but, the latter applies to gases?

 

Thanks.

 

[PaoloPemi]

I must presume you have selected the same model for fugacity (Peng Robinson and std. alpha function)

BIPs and mixing rules in both simulators,

with these you should find the same critical point(s),

if not it may be a different alpha function, different BIPs or mixing rules or a numerical error,

also I assume you have selected the Heidemann and Khalil procedure

(which is the procedure utilized for calculating

the true critical points in those simulators),

not the estimated values (intersection of

bubble and dew lines in phase envelope) which I have read

it's the default,

you do not provide the mixture composition

and I can't compare results with a different software,

 

http://www.prode.com...aseenvelope.htm

 

this utilizes a different procedure (based on Gibbs) 

and can provide another reference in your tests.

 

by the way I have read that Peng Robinson and Soave Redlick Kwong with standard

alpha parametrs  doesn't calculate accurately critical points so I am not sure how you

have been able to compare the values,

a possibility could be that you are utilizing some specific alpha function,

that could explain the differences.

Edited by PaoloPemi, 31 October 2013 - 05:23 PM.