Phase equilibrium calculations for continuous and semicontinuous mixtures

Abstract

Significant amounts of computation time are required for phase equilibrium calculations involving mixtures of very many components, such as polymer solutions and petroleum reservoir fluids. In this paper we consider the question of describing such mixtures as a collection of discrete components and a continuum distribution of species represented by a distribution function. We show that (1) the distribution function for the feed in a vapor-liquid calculation will not, in general, represent the coexisting vapor and liquid with equal accuracy, and (2) that to achieve the benefits of a continuous or semicontinuous description of a mixture, one need not rewrite phase equilibrium programs now in use, but instead choose component lumpings based upon Gauss numerical quadrature formulas. The specific Gauss formula to be used depends upon the distribution function which describes the mixture. Further, we have generalized the Gauss quadrature method by considering the case of finite limits of integration, which we show to be important when considering heavy oils. Finally, we show that relatively few pseudocomponents chosen in this way lead to phase equilibrium calculations of an accuracy comparable to that with a very much larger number of pseudocomponents chosen in the conventional way.