Cp/Cv for gas or for both gas and liquid!!
Cp/Cv for gas or for both gas and liquid!!
(OP)
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
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





RE: Cp/Cv for gas or for both gas and liquid!!
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Cp/Cv for gas or for both gas and liquid!!
zdas04,
If it is applicable in liquid then how could I calculate that?
In the case of gas I had used Cp/Cv=Cp/(Cv-R)
But in liquid, how?
Thanks!
RE: Cp/Cv for gas or for both gas and liquid!!
RE: Cp/Cv for gas or for both gas and liquid!!
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Cp/Cv for gas or for both gas and liquid!!
Please let us know the outcome.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Cp/Cv for gas or for both gas and liquid!!
shiplu48,
if the question is how your software calculates cp and cp/cv then ask technical support,
if the question is general (how software applications calculate cp and cv for both gas and liquid state) you may find answers in many textbooks (see properties of gases and liquids, to mention one),
for example for CV you may utilize a EOS to calculate H and V (plus derivatives),
different software applications export these properties plus derivatives (see for example Nist Refprop or Prode Properties) other applications may calculate derivatives numerically but finally you get your value,
take care that for liquid phase std. EOS (such as Peng-Robinson and Soave-Redlick-Kwong) may show large errors for values of cp, cv, speed of sound, isothermal compressibility etc.
when available you should select methods with specific parameters for each fluid (to calculate H, V etc.)
there are posts (at eng-tips and cheresources) comparing accuracy of std. and modified versions of Peng_Robinson and Soave when calculating liquid properties,
if an accurate model is available (see for example Steam Tables) that should be the preferred option.
RE: Cp/Cv for gas or for both gas and liquid!!
Thank you for your explanation. That things help a lot.
As I have searched the Steam Tables, there are just properties at different T and P.
There is no mention of procedure of cal calculation. Would you please give me the link for calculation or
link of posts (at eng-tips and cheresources) comparing accuracy of std. and modified versions of Peng_Robinson and
Soave when calculating liquid properties?
I would also like to thank georgeverghese,zdas04, Latexman for sharing their knowledge.
Many Thanks.
RE: Cp/Cv for gas or for both gas and liquid!!
http://webbook.nist.gov/cgi/fluid.cgi?P=1&TLow...
RE: Cp/Cv for gas or for both gas and liquid!!
25362, thank you for your link.
I have already visited the site and have found several values for water in liquid phase.
Which equations were used to find that values, if I want to find Cp or Cv?
Could you please mention the name of equations that have been used in case of liquids?
Many thanks.
RE: Cp/Cv for gas or for both gas and liquid!!
https://books.google.co.il/books?id=Xy_km6Z3Mc4C&a...
RE: Cp/Cv for gas or for both gas and liquid!!
RE: Cp/Cv for gas or for both gas and liquid!!
Could you please share some links to eng-tips and/or cheresourses discussions about EoS accuracy?
RE: Cp/Cv for gas or for both gas and liquid!!
however see for example
http://www.cheresources.com/invision/topic/15049-l...
there are some discussions about specific heats, too
RE: Cp/Cv for gas or for both gas and liquid!!
RE: Cp/Cv for gas or for both gas and liquid!!
dH = dU + d(PV)
At const. pressure
Cp.dT = Cv.dT + P.dV
Dividing through by Cv.dT
Cp/Cv = 1 + P.dV/Cv.dT
Here, dV is the incremental change in real fluid molar volume of the liquid at constant pressure over the temp interval dT. In SI units, P= N/m2, Cv = kJ/kgmole/degK, dV = m3/kgmole. Interpolating with this NIST data for example, I get Cp/Cv to be 1.018 at 330degK using this expression (with approx. data from 325degK to 335degK), 1atm while that from NIST would be 1.048. This value from NIST also matches up with data in Perry at 50degC, 1atm(table 2-355 in the 7th edition).
RE: Cp/Cv for gas or for both gas and liquid!!
It was a good one.
What was the range of fluid molar volume change, initial volume and final volume?
Why don't you come up with a full math. It would be clear enough.
RE: Cp/Cv for gas or for both gas and liquid!!
cp = (dH/dT)p
cv = (dU/dT)v
there are different ways to calculate these values, a table of values, a EOS etc.
with a EOS you can calculate cp=(dH/dT)p , V , dV/dP , dV/dT
and finally cv
RE: Cp/Cv for gas or for both gas and liquid!!
RE: Cp/Cv for gas or for both gas and liquid!!
however take care that results in some cases may show large errors (see previous posts)
RE: Cp/Cv for gas or for both gas and liquid!!
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Cp/Cv for gas or for both gas and liquid!!
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.
RE: Cp/Cv for gas or for both gas and liquid!!
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.
RE: Cp/Cv for gas or for both gas and liquid!!
RE: Cp/Cv for gas or for both gas and liquid!!