This paper analyzes the solutions from equilibrium based simulation models for separation processes with respect to multiplicity of solutions. The causes of multiplicity are investigated and it is shown for distillation columns, when and how multiplicity can be obtained and how it is conncted to design and operation of the separation process. Some of the causes of multiplicity are traced to over-design of staged separation processes and to values of specified variables. It is shown that the number of multiple solutions may be related to the number of stages while the number of acceptable solutions depend on the set of specified variables. Numerical examples covering simple distillation column models and heterogeneous azeotropic distillation operations are presented.