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# Critical Pressure of mixture

## Critical Pressure of mixture

(OP)
Hi,

What could be the critical pressure of a mixture (suppose N2 and H2O)?
Would it be the critical pressure of N2 and H2O (Both)?
Or, it would be a different one other than both of these?

Many thanks.
Shiplu48

### RE: Critical Pressure of mixture

It all depends on the temperatures and pressures...

### RE: Critical Pressure of mixture

From Elementary Principals of Chemical Processes by Felder and Rousseau, the critical pressure of 100% N2 is 33.5 atm, and the critical pressure of 100% H2O is 218.3 atm. For mixtures, one should use a "mixing rule" that fits to the ideal, non-ideal, polar, non-polar, etc. nature of the components in the mixture. That can be done by consulting a reference like The Properties of Gases and Liquids by Reid, Prausnitz, Sherwood, Poling, and possible others depending on which Edition you look at. The simplest mixing rule is "Kay's rule", which is a mole fraction average as follows:

Psuedocritical Pressure = P'c = yN2 x PcN2 + yH2O x PcH2O

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Critical Pressure of mixture

Of course that is when both components are in the gas phase. My impression was a more general case was being considered.

### RE: Critical Pressure of mixture

(OP)
Thanks Latexman for your valuable feedback.

Is Kay's Rule is similarly applicable for liquid mixtures?

Is there any dependency of Critical Temperature and pressure on operating T and P?

Thanks.

### RE: Critical Pressure of mixture

You are welcome. By the way, there's a button (Great post!) for that.

Critical temperature (Tc) is a pure component physical property / constant. Tc is the highest temperature at which a pure component can coexist in two phases (liquid and vapor), and the corresponding pressure is the critical pressure (Pc). Being pure component constants, they are not affected by the operating temperature and pressure of your process. Does temperature and pressure affect the molecular weight of nitrogen? No, of course not. It's the same with Tc and Pc; they are experimentally determined constants.

Yes, Kay's Rule is similarly applicable for liquid mixtures. If you had a liquid mixture of 50% N2 and 50% H2O and a vapor mixture of 50% N2 and 50% H2O, you would calculate T'c and P'c using Kay's Rule exactly the same way for both mixtures.

Your process temperature and pressure will affect the psuedoreduced temperature and pressure of the mixture:

T'r = T/T'c

P'r = P/P'c

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Critical Pressure of mixture

(OP)
Again Thanks Latexman.

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