Chapter 11: Phase Behavior of H ₂ O + Hydrocarbon Systems

The phase behavior of H ₂ O + hydrocarbon mixtures differs significantly from the vapor/liquid equilibria of normal hydrocarbons in two ways: the aqueous and hydrocarbon components usually separate, with very low mutual solubility; and hydrates often form with water and hydrocarbons smaller than n-pentane. Water generally is avoided because it is incombustible, and hydrate solids usually are avoided because their presence creates flow assurance difficulties.

When hydrocarbon contacts water, the two components separate into two phases in which the mutual component solubility is less than 1.0 mol% at ambient conditions. This splitting of phases affects almost all treatments of H ₂ O + hydrocarbon systems and is caused by the different molecular attractions within water and hydrocarbons. Hydrocarbon molecules have a weak, noncharged attraction for each other, while water attracts other water molecules through a strong, charged hydrogen bond.