Chapter 9 - Phase Diagrams

Abstract

This chapter focuses on fluid phase equilibria and their relation to hydrate formation. Phase diagrams are useful for both the theoretical discussion of gas hydrates and for engineering design. Some general observations about phases are useful for developing phase diagrams. The first of these is that gases are always miscible. Therefore, a system, regardless of the number of components, cannot contain two vapor phases. A pure component has only one liquid phase. Thus, a single component cannot exhibit liquid phase immiscibility. Binary and multicomponent systems can and do exhibit liquid phase immiscibility. A critical point is a point where the properties of the coexisting phases become the same. Critical points exist when a vapor and a liquid are in equilibrium and when two liquids are in equilibrium. A binary system existing in two-phase equilibrium occupies a region in the pressure-temperature plane, as opposed to a curve for a pure component. On the other hand, for three-phase equilibrium, which is triple points for a one-component system, there are curves in the pressure-temperature plane for a binary system. With fluid phase equilibria, in multicomponent systems the design engineer usually constructs a phase envelope, a map that shows the regions where the stream exists as a liquid, a vapor, or as two phases. The construction of a phase envelope is virtually impossible without computer software.