This chapter also explains qualitative trends, to help the engineer to understand the implications of temperature, pressure, and composition changes. Quantitative predictions of macroscopic phase behavior are illustrated by example, along with a few results from hand calculations, though the many excellent commercial phase equilibria computer programs now available largely have eliminated the need for the hand calculations. This chapter discusses H 2O + hydrocarbon phase equilibria in macroscopic terms, such as temperature, pressure, concentration, and phase diagrams-more easily applied by the engineer-because a quantitative molecular prediction of H 2O + hydrocarbon phase behavior is beyond the current state of the art. Another example is the very high normal boiling point water has relative to its molecular weight. One example is water’s very high heat of vaporization, which absorbs large amounts of heat and buffers many hydrocarbon reservoir temperatures.
Hydrogen bonds are responsible for most of the unusual properties water displays. Hydrocarbon molecules have a weak, noncharged attraction for each other, while water attracts other water molecules through a strong, charged hydrogen bond.īecause hydrogen bonds are significantly stronger than those between hydrocarbon molecules, hydrocarbon solubility in water (and that of water in hydrocarbons) is very small. This splitting of phases affects almost all treatments of H 2O + hydrocarbon systems and is caused by the different molecular attractions within water and hydrocarbons. 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. Water generally is avoided because it is incombustible, and hydrate solids usually are avoided because their presence creates flow assurance difficulties. The phase behavior of H 2O + 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.