3 replies to this topic

[jprocess]

Question:What is the difference between True Tc and Pc vs. Pseudo Tc and Pc?
Answer made by related COMPANY:
True critical properties are those properties calculated using the mixing rules associated with the property package chosen. Pseudo Critical Properties use simple linear models to estimate the critical properties of a mixture. They are often very different from the true critical points and have no real physical significance.

So what is the application of Pseudo Critical Properties Provided by HYSYS?

[rxnarang]

Two applications come to mind:

a) We report the fluid pseudo Txc and Pc on th eheat exchanger data sheet to help the thermal rating guy. Appaarantely theri prgram works with pseudo rather than real critical properties.

law of corresponding state;- One can compute the Z factor of a gas by computing the pseudo TC and Pc, and then looking up the Z factor graph.

I am sure there are other applications as well.

I have observed in the past that sometimes reporting the pseudo values causes more grief. For instance in a control valve data sheet the vapor pressure can be more than the psedo Pc, which totally freaks out the instrument guy using his standard software.

Regards
Rajiv

[jprocess]

Dear Rajiv,

a) We report the fluid pseudo Txc and Pc on th eheat exchanger data sheet to help the thermal rating guy. Appaarantely theri prgram works with pseudo rather than real critical properties.

You mean softwares like HTFS and B-jac work with pseudo properties?What logical reason caould be behind of it?

[joerd]

I can't speak for HTFS but I know that some correlations and models have been fitted (a long time ago) using pseudo-critical properties. So, if you want to use them, you have to give the proper input, which is a pseudo-property in this case. The same is for Cp/Cv values. Hysys computes them fairly rigorously from the EOS, but oftentimes the equipment vendor has calibrated his design models with Cp/(Cp-R). So you will have to make sure that both parties understand what data you are providing - as always ;-)