4 replies to this topic

[smuk]

I am trying to read the chart for fugacity of hydrogen given in "Data Book of Hydrocarbons"by J.B.Maxwell. I am not sure what is the correct way to read the chart. Can anyone help?

 

Thanks

[MrShorty]

I am optimistic that we can help. Is there something specific you are having trouble with?

 

It looks to me like the H2 chart is mostly read just like the other fugacity charts:

1) Get the "effective pressure" -- total pressure * y(HC)^0.5

2) From effective pressure and temperature, read the value of the fugacity function off of the chart.

3) From the other 2 charts, get the A (based on molecular weight of solvent) and B (based on characterization factor of solvent) correction factors and apply them to the value read in step 2.

4) Recognize that the fugacity function obtained this way represents the product of the effective pressure and the volatility (F=Pe*K=Pe*y/x) where Pe is the effective pressure from step 1.

 

Comparing this to some data I have for hydrogen in heptane, the procedure appears to underpredict the volatility of H2, but it is in the right ball park -- especially for an old graphical based method.

[smuk]

Thanks Mr. Shorty,

 

I am trying to do VLE calculations for natural gas (containing hydrogen, CO2, N2, Argon - inert gases) condensation due to isenthalpic expansion. In this case what should be taken as the solvent - is it the condensed liquid, whose composition is not known at the beginning, and hence will need to be assumed, say liquid containing no hydrogen, methane and inert gases. Also how do I calculate the characterization factor of the condensed liquid, meaning what correlation should  I use. After completing the VLE calculations, I can check back if the assumed condensed liquid composition was reasonable. However, I feel that this would be a rather tedious approach. Is there a better way to do this. Thanks for your patience.

[MrShorty]

In this case what should be taken as the solvent - is it the condensed liquid, whose composition is not known at the beginning, and hence will need to be assumed, say liquid containing no hydrogen, methane and inert gases.

I believe the expectation for "solvent" is the condensed liquid considering only the hydrocarbon components, so ignoring the H2, CO2, Ar, etc. dissolved in the liquid). Yes, the procedure is a tedious "iterative" procedure where you would "guess" a solvent, perform the calculation, see how close your guess was, use the result to improve your guess, and tediously repeat until you are confident you have an adequate guess at what the liquid/solvent composition is.

 

Also how do I calculate the characterization factor of the condensed liquid, meaning what correlation should  I use.

Maxwell has a couple of charts in section 2 for determining characterization factor (based on normal boiling point and density).

 

Is there a better way? Newer methods are not always "better" (and maybe it depends on how you define "better"). Most VLE calculations require some kind of tedious iteration, so most modern methods for VLE calculations will employ suitable mathematical models programmed into computers (process simulators or whatever you have access to). For most natural gas type scenarios, a phi-phi model based on your favorite equation of state (Peng-Robinson seems to be a common favorite) is the usual mathematical model chosen. It depends on how you define "better" and what you have access to.

[smuk]

Thanks Mr. Shorty for your help.