6 replies to this topic

[adw07]

Hi all

This might be a silly question, but any help appreciated.

I've made a computer algorithm which tests a multicomponent mixture to see if it is in the two phase region or not (Based upon Michelsons Stability Test 1982 using PRSV equation of state).

My question is, if this returns that the mixture is single phase, how do you tell if the phase is a liquid or a gas?

Now, obviously if you looked at the components present and the density, etc. you would be able to assume which phase it is.

However, since this would be difficult for the computer to do by itself I am wondering if there is any test which will tell me which phase it is?

[Technical Bard]

It's been a long time since I did these calculations and wrote such code myself, but if I recall, the way we did it was to identify the fugacities of the saturated vapour and saturated liquid at our process temperature, and then compare the fugacity at the selected condition. This can find a number of things, such as the critical point, or if you are approaching the cricondentherm (fv and fl will start to converge).

Some commercial simulators still just check the density - which is why sometimes programs like HYSYS give you the incorrect vapour fraction, especially near the critical point.

[adw07]

Thanks for this answer - It's been a while but I have now come back to this problem.
I am not sure the method you have outlined is appropriate in my case, since I do not know the saturated vapour and liquid fugacities.
The solution I have at the moment is simply based upon the density - as you have mentioned. I have a used a value of 300 kg/m3 to separate vapour / liquid phases. In practise this works reasonably well, although if anyone has other suggestions any help advice is appreciated. Thanks

[S.AHMAD]

You need to have the k-values (equilibrium constant) at conditions. You can derive this from EOS. Or you need to compute the vapor pressure at the temperature for each components that calculate k-value for each component that is equal to the ratio of vapor pressure to the total pressure.

k = VP/Pt

Calculate the sum of product of mole fraction multiply by k-values (let say Y).

If Y = 1 , the mixture is at bubble point

If Y < 1, it is in liquid state

If Y > 1 it is either vapor or two phases. Then compute the product of mole fraction divided by k-values (let say Z)

If Z = 1 then the mixture is at its dew point.

If Z < 1 then it is in vapor state otherwise it is a mixed state (vapor + Liquid)

Alternatively, do a FLASH calculation to determine the vapor fraction.

Edited by S.AHMAD, 29 September 2011 - 10:23 PM.

[adw07]

Thanks for this answer - I am familiar with this method however I am not sure how to get the necessary K-values.

If the stability test proves the mixture is stable then it means the heavier and lighter trial phases have either converged to a trivial solution or the sum of their mole numbers is < 1, and in either case I am unsure how to extract K-values from the results.

Could you please explain how I would find the K-values from the Equation of State?

I suppose I could run a PT-Flash after the stability test (even though it has already proved single phase), initiate it with Wilson's K-values and see which value the vapour fraction converges to - 0 = liquid phase , 1 = vapour phase. There must be a better method than this however.

Edited by adw07, 05 October 2011 - 05:01 AM.

[S.AHMAD]

1. To find k-values using EOS is tedious and require trial & error.

2. Attached is the extract from API Technical Data Book on the procedure

3. Young engineer like you should be enjoying using the procedure. If you manage to write computer program (using Visual Basic), do not forget to send me a copy.

Attached Files

•  API TDB pg800-810.pdf   1.25MB   26 downloads

[breizh]

http://www.ceb.cam.a...hermo/pure.html


This resource should support your query .

Breizh