![]()
The phase behavior of H2O 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 H2O 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. Because hydrogen bonds are significantly stronger than those between hydrocarbon molecules, hydrocarbon solubility in water (and that of water in hydrocarbons) is very small. Hydrogen bonds are responsible for most of the unusual properties water displays. One example is water's very high heat of vaporization, which absorbs large amounts of heat and buffers many hydrocarbon reservoir temperatures. Another example is the very high normal boiling point water has relative to its molecular weight. This chapter discusses H2O 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 H2O hydrocarbon phase behavior is beyond the current state of the art. 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 also explains qualitative trends, to help the engineer to understand the implications of temperature, pressure, and composition changes. Such a qualitative understanding and a few hand calculation methods serve as an initial check on the quantitative predictions of computer programs. This chapter is divided into three main sections.
Site News