Hydrocarbon Dew Point

[Strawb]

Does anyone know how to calculate (simply) the dew point of a natural gas mixture given its composition?

There was a very informative thread798-142881 now closed about cricondentherm etc.

I seem to have a basic misunderstanding which is:
Our client states a cricondentherm temp of 4-10degC for a gas mixture (at 65Barg). However the gas is composed of about 5% Propane and C3+. I have checked vapour pressure charts and see that Propane is liquid at 12Bar (30degC) and Butane at about 3 Bar (30 degC).

I don't understand why these fractions don't 'drop out' of the gas as these pressures are reached. Our client insists that temp needd to drop to below the Cricondentherm temp before any liquifaction. Do the components behave differently when in a mixture? Or am I oversimplifying my understanding of vapour pressure charts?

Help please.

 

[25362]

Non-polar hydrocarbons in mixtures show these critical properties.

You could see some P-T diagrams at the end of

http://www.transcanada.com/Alberta/presentations/gas_quality_INGAA_Presentation_2004.pdf

and some explanation in:

http://www.ingaa.org/Documents/Final Hydrocarbon Liquid Dropout Technical Paper.pdf

 

[MortenA]

In a simplified way you can say that the light fractions (methane/ethane) acts as a "solvent" that can contain a certian amouth of heavier frations at a given pressure/temperature.

A lot of thoughs have then gone into trying to determine exactly how much

Best regards

Morten

 

[dcasto]

you are going to need the Peng-Robison equation of state and solve for the dewpoint temperature at 65 bar. The gas will more likely have its lowest dew point temperature at between 30 to 40 bar.

The biggest problem is not with the equation and solving it (a similator program like WinSim, Hysys, and Prosim will do this in 100 nano seconds), its defining exactly what all the componets are down to the C10's.

Search around

http://www.gasprocessors.com

for papers written over the last three years on this topic.