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[DANA-IF]

Hi friends

 

As I know         c_p = (δh / δT)_p and c_v = ( δh / δT)_{v     are two specific heat defined for vapors and gases} 

 

_{when I look into hysys for a liquid stream , they give}  c_p  and c_{v    values .}

 

 

What does mean that please ?

Edited by N.BH, 13 March 2014 - 01:18 PM.

[aqassam]

The Hysys documentation (on support.aspentech.com) provides a very good description of what these properties are:

 

For Cp:

EOS Enthalpy, Entropy & Cp Method Specification

 

With most of the Equations of States, you may have two or three alternative calculation methods for enthalpy, entropy, and Cp. The property calculation methods that are available include: the EOS selected, and the Lee-Kesler method.

 

EOS Selected:

With this selection, the enthalpy, entropy, and Cp calculation methods contained within the Equation of State are used.

 

Lee-Kesler

The Lee-Kesler method may be used for the calculation of enthalpies, entropies and Cp values. This option results in a combined Property Model, employing the appropriate equation of state for vapor-liquid equilibrium calculations and the Lee-Kesler equation for the calculation of enthalpies and entropies. This method yields comparable results to Aspen HYSYS standard equations of state and has identical ranges of applicability.

Lee-Kesler enthalpies may be slightly more accurate for heavy hydrocarbon systems, but require more computer resources because a separate model must be solved.

Once the vapor phase is selected, the liquid phase needs to defined.

 

For Cv

"Heat Capacity is calculated using a rigorous Cv value whenever HYSYS can. The method used is given by the following equations:

 

Cp - Cv  = -T (dV/dT)^2 / (dV/dT)                (A.62)

 

However, when ever this equation fails to provide an answer, HYSYS falls back to the semi-ideal Cp/Cv method by computing Cp/Cv as Cp/(Cp-R), which is only approximate and valid for ideal gases."