Cp/Cv for gas or for both gas and liquid!! 4

[shiplu48]

Hello,

As far as I know, Cp/Cv is a parameter for gas. But during the simulation in ASPEN HYSYS if I pick up a liquid stream and enter necessary parameters like P, T and mole or mass fraction then it shows the corresponding parameters of liquid and I also get the value of Cp/Cv.

I am not really getting the point about this parameter in case of liquid.

I need help regarding this.

Many Thanks

 

[Latexman]

Yes, Cp/Cv for a liquid is valid, but it's not very useful and prone to large errors.

Good luck,
Latexman

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

 

[georgeverghese]

Error in my last response

a) The units for pressure should be in kN/m2, not N/m2 to keep things consistent in SI units. With this, the value obtained for Cp/Cv is 1.00033 and does not match up with Cp/Cv calculated from the NIST tabulation.
b) The error is due to the expression for dU, which is not just Cv.dT; see the full expression 4-45 in Perry Chem Engg Handbook 7th edition, for which I'm having trouble computing one of these terms.

 

[georgeverghese]

The expression 4-45 in Perry is

Cp - Cv = T.(∂V/∂T)_P.(∂P/∂T)_V

With additional information on the density variation of subcooled liquid water with pressure and with temp(page 2-311, table 2-355), it is possible to confirm the value for Cv for a given Cp value, from which Cp/Cv can be computed. I have just verified this. For example, in the temp interval 40-50degC, pressure interval 50-200bar, Cp/Cv computed is 1.060, while tabulated value is 1.043.

 

[25362]

To georgeverghese, in that range of pressures and temperatures, the NIST webbook provides a series of values.

 

[georgeverghese]

@25362, Previous response provides the theory and manual calc route for real fluid Cp/Cv for water or any other liquid or vapor stream for that matter.