Proposal for the introduction of the concept of multiplicity of solutions in steady-state distillation problems in the syllabus of separation processes

Abstract

The existence of multiplicity in steady-state separation problems based on the use of equilibrium stage cascades has been known since the second half of the 20th century. Nevertheless, there are few textbooks -among those usually accessible to Chemical Engineering students- explaining this behavior and even when it is considered it is not deeply treated. In this work, we propose a distillation problem whose resolution allows the instructor the opportunity to introduce some specific characteristics and behavior that may otherwise pass unnoticed. Thus, the simulation of a column for the heterogeneous azeotropic distillation of ethanol using benzene as an entrainer is suggested, and the results obtained do not seem to be compatible with the distillation boundaries of the ternary system. The apparent contradictions can be explained by means of the representation of mass balances on the residue curve map, and allow the students to actually understand the requirements for the feasibility of the separation. The obtainment of two different solutions depending on the initial composition profiles used for starting the calculations allows the instructor to introduce the steady-state multiplicity concept. Errors associated with bad checking of the thermodynamic model are also pointed out.