calculate hydrocarbon dewpoint from Natural Gas composition

[Clifford Bradford]

Suppose I know the composition of a sample of natural gas in mol%/vol% how do I go about calculating the hydrocarbon dewpoint at a given pressure?

https://en.wikipedia.org/wiki/Hydrocarbon_dew_point

 I thought I would calculate the saturation temperature (dewpoint) of each component at the partial pressure of the component but the results I got from that seemed unreasonably low. Can anyone provide suggestions/pointers to details on how this is calculated?

Thanks

[Andrés Riedemann]

if we suppose ideal gases, you need to find the temperature such as:
sum(yi/P0i(T)) = 1/P_dew
where P0i is the saturation pressure of each component.

Now, calculating the saturation temperatures for all components allows to determine an starting point, after all, the temperature T is between T0i(P)[min] and T0i(P)[max] .

solving that non-linear problem should give you the temperature at which a mixture of ideal gases presents a dew point at the specified composition y.

[Ian Bell]

How is the hydrocarbon dew point different from the conventional dewpoint of the mixture? They sound the same, as you can't get a single component dewpoint once you have a mixture.

[Lynn McGuire]

Hi Ian,

The dewpoint of a mixture can be either hydrocarbon or aqueous, depending on how much water is in the mixture.  We use the "Three-Phase Flash and Distillation" by K. K. Bool & J. A. Schuil of Badger B.V. in The Netherlands to determine a preliminary dewpoint by iterating an isothermal four phase flash using a merged kvalue system.  From Computers and Chemical Engineering Vol. 9 No. 3, pp 295 - 300, 1985. 

Then we use the "Separation Process Principals", Second Edition, J. D. Seader and Ernest J. Henley, 2006, John Wiley and Sons, Inc., pages 150-151, "Rigorous Method for a Vapor-Liquid-Liquid System" four phase flash method which solves two k-value systems (aqueous and hydrocarbon) simultaneously to get a rigorous answer.  The four phases are vapor, hydrocarbon liquid, aqueous liquid, and solid.

Sincerely,
Michael Lynn McGuire
President
WinSim Inc.

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