Analytical solutions are presented that confirm the existence of a combined condensing and vaporizing displacement mechanism in enriched gas drives. The solutions are derived for dispersion free, one-dimensional displacements in four-component hydrocarbon systems. A simple geometric construction is used to find a key tie line in the ″crossover″ tie line. This tie line is shown to control the development of miscibility in condensing/vaporizing systems, and it connects the condensing and vaporizing portions of the displacement.
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