I suspect you want something simple; the following may do the trick for you: The US National Institute of Standards and Technology (NIST) has published extensive tables of very precise thermophysical properties of water. For water vapor pressure, they have a fairly simple equation of state that you can program easily into an algorithm. If the hydrocarbon molecules do not interact strongly with water molecules, then you can calculate the hydrocarbon phase diagram (The Peng-Robinson equation of state would be pretty accurate and simple) independently of the water phase diagram; Calculate the partial pressures of hydrocarbon and water independently and simply sum them together to get the total pressure. CO2 and H2S are not hydrocarbons but often accompany hydrocarbon streams; both are strong gas-hydrate formers, and if they are present, you are going to have problems with almost any simple approach. If the hydrocarbons include molecules with a significant molecular-weight-fraction of functional groups, the interaction of the functional groups with water molecules will give you problems.
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Bob Horton
Chief Technology Officer
Home Run Specialty Chemicals, Inc <www.4bases.net>
Sugar Land, TX
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