Automatic development of global phase diagrams for binary systems in pressure-temperature space

Abstract

Global phase diagrams of binary systems in pressure-temperature (PT) space are very useful. In this project the techniques to automatically develop global phase diagrams in PT space were created. The codes to compute different components of a global phase diagram in PT space were developed. These codes were then successfully incorporated into a single functional program. To generate the binary PT phase diagram, the overall composition was varied from pure component 2, the least volatile component (LVC) to pure component 1, the most volatile component (MVC). The step size for changing mole fraction was varied in the calculation of different parts of a global phase diagram. When the points near the joining points between different parts were computed, the step size was set to a rather small value. The step size was then increased to twice of the last value for each subsequent point computed. When the MVC mole fraction was approaching one, the step size was set to a small value to obtain enough points needed to minimize the chances of missing important phenomena. The techniques to set initial guesses for evaluation of different components of a global phase diagram were discussed. The code performance, including the number of iterations for different convergence criteria and the sensitivity of the algorithm were presented. Using the code developed, phase diagrams of type I, type II, type III and type V were generated using representative binary systems from the petroleum processing field. The boundary states between different types of phase behaviour were also explored. It was observed that with the increase of the binary interaction parameters, the phase behaviour of the ethane + ethanol binary system changes from type I to type II to type III while the methane + n-hexane binary system changes from type V to type III. These conclusions matched the results of van Konynenburg and Scott (1980). It was also concluded that with the increase of the binary interaction parameter for a binary system, the system showed a trend to exhibit more liquid-liquid immiscibility.