Cp/Cv correction in Hysys 3

In Hysys simulator application, the Specific Heat Ratio shown for streams is incorrect. There is a formuala for calculating the correct Cp/Cv values. Would anybody out there know what this correct formula be. Thanks very much.

 

[KernOily]

How do you know it's incorrect? Do you have a table of values to compare the HYSYS calculation against?

Is it incorrect every time, for every gas and every gas mixture?

Have you called Hyprotech to discuss this with them? Thanks!
Pete

 

[kbander]

I'm not sure if this is the case here...but the problem may be confusing real gas Cp/Cv with ideal gas Cp/Cv.

Worth checking...


Regards,

Bob

 

[will48]

The Cp/Cv value in Hysys is the 'actual' value, based on the equations of state in thermodynamic package you are using.
You can calculate the 'ideal' value by the following formula:

Cp/Cv (ideal) = Cp / (Cp - R)

where Cp = molar heat capacity of gas stream (kJ/kmolK)
R = universal gas constant = 8.314 kJ/kmolK

I suggest (although cannot confirm) that you use the ideal value in most formulae (e.g. PSV sizing) since these formulae are often derived with some 'ideal' assumptions.
If in doubt, use the value which gives the most conservative result.

 

[jcaiken]

Hysys has been calculating the 'real' Cv value and the associated Cp/Cv since v2.1.3. Prior to that they used the (Cp/Cp-R)- 'semi-ideal' as the Cp value was calculated from the EoS. The result is a Cp/Cv value that appears to be closer to experimental data but can differ considerably from the Cp/(Cp-R) value. Particularly in the 60-100 barg range at normal temperatures (ambient ±100°C) where the ideal value approaches 1.0 as pressure increases but the new value approaches goes in the opposite direction to more than 2.

We are finding that less experienced engineers are using the 'real' Cp/Cv data in datasheets sent to vendors for purchase. For RV's this is less conservative but not necessarily unsafe - it depends on the vendor sizing calculations, margins, whether they use HYSYS and are aware of this change.

Quoting both values to the vendor could cause confusion so I'd support Will48's suggestion to use the Cp/(Cp-R) on datasheets.