TABLE OF CONTENTS Phase diagrams are useful for both the theoretical discussion of gas hydrates and for engineering design. In this chapter we concentrate on fluid phase equilibria as they relate to hydrate formation. Thus systems that do not form hydrates are not discussed. This does not mean that they are not interesting or important, just beyond the scope of this book.
In this chapter some rules are presented for constructing pressure-temperature diagrams for single-component systems and binary systems as well as pressure-composition and temperature-composition diagrams for binary systems. These rules can be found throughout this chapter.
Much of this material is based on the Gibbs phase rule. The phase rule is:
| (9-1) |  |
where F is the degrees of freedom, N is the number of components, and p is the number of phases.
An example of its use is as follows: a single component existing in two phases would have one degree of freedom. That means that there is one independent variable to manipulate. Once one variable is fixed, all of the others are fixed as well. For example, a single component existing as a vapor and a liquid has one degree of freedom. If the temperature is specified, then there are zero degrees of freedom the pressure is fixed. This pressure is called the vapor pressure.
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...
