Mr.Mishra,

Can you please be more specific, any further references, I wouldn't mind.

For Pseueo: Pc = Pc1*mole_fraction1+Pc2*mole_fraction2+Pc3*mol_fraction3... Etc

TC = Tc1*mole_fraction1+Tc2*mole_fraction2+Tc3*mol_fraction3... Etc

Where mole_fraction is the fraction of each component (by knowing the composition) 

Pc1.. Etc and Tc1 .. Etc are critical pressures and temperatures of each component (like methane, ethane, propane, .. Etc)

You will get stuck on the plus fraction and then you have two options: measure it or estmate it from given tables.

For the true Pc abd Tc, they are thermodynamacillay calculated. There is a discussion here: 

http://www.cheresources.com/invision/topic/19355-true-or-pseudo-critical-pressure-for-sizing-control-valves/

I hope this helps and good luck. 

Mr.Khaldoon,

What you mentioned are the pseudo critical pressure and temperature for the mixture,

Ppc=summation(pc1*Mw1+...+pcn*Mwn)

Tpc=summation(Tc1*Mw1+...+Tcn*Mwn)

Under high pressure they deviate largely from the true mixture critical pressure and temperature.

Pseudo Critical pressure and temperature are properties of a substance that do not change due to conditions (same as molecular weight). We usually do correction due to existence of inorganic impurities (ie. CO2, N2, H2S).

For the true ones: use hem, hne and nhne method. I am sure HEM exists in hysis. 

More in link attached. 

https://www.chem.purdue.edu/gchelp/liquids/critical.html

The critical pressure and temperature of a substance differ from that in mixture, in which complex equations are used to describe multi-phase behavior, the Pseudo critical pressure and temperature can be used as a replacement equation for mixture critical pressure and temperature at low to moderate pressures, however, under high pressures these values tend to give wrong results, perhaps the formula you just mentioned used at low to moderate pressures "10-1500 psi" and it can be found in software, but I assure you it is not the formula used in Petroleum Engineering software. I'm writing a Thesis about this subject and I had tried your formula along with others.

I have one question Mr.Khaldoon how do you know by heart, that software HYSIS uses the Pseudo formula to obtain critical properties. Is it one of the options?. As for reference material I will include them as soon as I finish the Thesis "Don't worry I just want to be sure".

Not by heart. If you choose thermodynamic modelling then you get the true critical pressure and temperature by second dervitation of Gibbs free energy equal to zero. I have gone through my cases as I told you plus I am interested in this topic and have done some fluid modelling before as you can see in my profile/publications. 

Please refer to those as true critical pressure and true critical temperature since they might be confused with pseudo ones. The pseudo ones are used for z estimation which has a great importance especially in gas reservoir engineering. 

I will wait for your publication/thesis and best of luck.