GCAP Phase Diagram? 1

[jhamil1]

All,

I have been using GCAP8.2 equation of state software to develop phase diagrams to determine if liquids are dropping out of a natural gas stream. The gas composition has been entered into the software and a phase diagram is produced. It also produces two additional curves for hydrate dew point and water dew point with respect to temperature and pressure. Here is a link of what I am talking about

http://files.engineering.com/getfil...8459-17a312b06b0e&file=GCAP_Phase_Diagram.pdf

I don't know if any one has used this program or if it is a good program. I do not do this on a regular basis and I need some clarification. I know the circondentherm temperature is the point at which liquids will begin to drop out. I have this information from the chromatograph data. What I don't understand about the GCAP software is the hydrate dew point curve is above this point.

+ Will hydrates of heavier hydrocarbons drop out before the circondentherm temperature is reached?

 

[dcasto]

it is not a hydrate dew poit curve, it is a hydrate solids curve. It says that if you have sufficient water in the gas, at 600 psia, you will make solid "ice" at 49 degrees F. If you have free Liquid" water at those conditions, instead of getting a nice liquid phase, you will get a solid phase!

You told the program that you have saturated gas at 874 psia and 96 F, so you will make hydrates at those temperatures listed. Furthermore, at 874 psia and a temperature below 96, water will drop out because of your inputs, when water drops out, you make hydrates.

If you tell the program you only have 7lbs/mmscfd, the water dew point at 874 psia will drop to 30 F and therfore the hydrate point becomes the water dew point curve.

Finally, The program is a EOS and is valid, but I must warn you, the controvery is in the definition of C6. Your chromatgraph does a backflush and sweeps out all the molecules that are bigger than C6, it's called C6+. The GPA has been studing this and as a default you should select a pysdo compound. Many people use a brakedown such as this C6+ is psydo compound is: 60% C6, 30% C7 and 10% C8.

 

[jhamil1]

dcasto,

Thank you for your explanation, very very helpful. The reason I was questioning this, is because the chromatograph also tells me that the cricondentherm temperature is at 5 degF at 319 psia. Using the GCAP software, the cricondentherm temperature is not what the software computes. Could you offer any words of advice as to why the software is not computing something close to the chromatograph output? Do you possibly know where I can input the water content into the software (i.e., 7 lbs/MMscf) which is standard for transmission lines. Typically the water content is very low, but I could use this number as a safety comparison. Thank you for your help.

 

[dcasto]

I can't imagine that the chromatagraph can have a true equation of state subroutine in it. It's like liquid chromatographs calculate a vapor pressure of a stream by simple mole percent mixing rules x1*VP1 + x2*VP2 + xi*VPi = VP.

I dowloaded the trial version. You can trial and error on the saturation temperature to get the 7lb/mmscf (you'll have to take the CO2 to 0.0). The program uses SRK Soave redlich-kwon.

I'll make sure I tell John that I helped you when I see him in 2 weeks.

 

[jhamil1]

Who are you referring too?

 

[dcasto]

John Campbell, he comes to the convention normally.