An Output Multiplicity in Binary Distillation: Experimental Verification

Earlier it was believed that distillation multiplicity only could occur as a result of separating a multicomponent mixture. It has, however, been proven theoretically using very simple distillation models that measuring process flows in units of mass or volume (as is always the case in practical applications) instead of moles may give rise to a bifurcation in the mapping between volumetric reflux flowrate and distillate purity from the case of a single steady state to that of two stable and one unstable steady states being possible for a specific set of actuator settings. This paper verifies the existence of multiple steady states on a semi-industrial scale distil-lation column separating a mixture of methanol and isopropanol. Simulation studies show that the column will exhibit multiplicity in central parts of the operating window. An exper-iment is designed by which the unstable branch is mapped and the singular points identified under stable conditions. The results show that distillation column multiplicity is in fact a phenomenon of practical relevance which may lead to operational problems even for binary distillation columns.