Specific Heat Ratio Calculation

[ayjay]

Hi all,

I have posted this few days back, but the thread was not published. How can I check whether the thread is available or not?

Again... I posts this topic. Hopefully, it is not redundant with my previous post.

....

I'm currently doing the Specific Heat Ratio calculator for our department internal use. Simple spreadsheet using Excel.
This is for Natural Gas mixtures with methane ~80%, Co2 ~4%, N2 ~0.4%, H2O~0.4 to 4%.

From formulas available online (web/forum), I have found two method so far.. which are:

1. Taking the specific heat capacity, Tc and Pc of each component by multiplying with MW, calculate the sum of Cp, Tc and Pc, find Pr and Tr. Then use the following formula to get the specific heat ratio, K = Cp/(Cp-((R(1+Pr/(Tr^2(0.132+0.712/Tr)^2)/MW)

2. Using Cp = A + B*T + C*T^2 + D*T^3 + E*T^4, Cp for each component was calculated at the specific temperature. Then Cp of mixture is summation of all Cp multiplied with each component MW. K is using K=Cp/(CP-R/MW).

However, the results was not near as expected. From Heat Material Balance (simulated from Hysis), the results should be around 1.4.

From 1st method, I got ~2.2. From 2nd method I got ~1.01.



Appreciate if anyone can advice the correct way of calculating correctly the specific heat ratio. It seems both calculation above is not correct.


Thank you.

[carlo.stenali]

a) you can calculate the ideal cp/cv , if you knew the ideal cp (from temperature dependent polynomial) k = SUM(Wi * Cpi/(Cpi-R))
where Wi is the weight fraction of component i in the mixture

b) you can calculate the departure form ideal conditions with a equation of state such as SRK, PR etc.
this procedure is more complex but gives good accuracy, you'll find detailed descriptions in textbooks as the Properties of Gases and Liquids

c) use a process library directly in Excel, with a library such as PRODE PROPERTIES you obtain directly all these values in Excel,
see
'http://www.prode.com/en/properties.htm'
for additional information