Design and analysis of nonequilibrium separation process problem are discussed. Even though there have been published articles on the design and analysis of equilibrium separation process problems, these issues have not been addressed in great detail as far as nonequilibrium problems are concerned. Strategies are developed for drawing residue curve maps using mass-transfer model equations. In addition the multistage nonequilibrium separation process problem is rigorously analyzed and it is shown that, as in the single-stage case, any multiplicity that occurs is a result of the multiplicity that occurs here in the phase equilibrium calculations at the interface.