Analysis and multiple steady states of an industrial heterogeneous azeotropic distillation

A study of an industrial multicomponent heterogeneous azeotropic distillation is presented. The process concerns an organic acid dehydration using an immiscible entrainer. First, a validation of the MESH and thermodynamic models through a comparison between pilot-plant experimental reconciled data and simulation results is conducted. A four-component mixture is considered for the simulation. Case studies of the boiler heat duty are automatically generated by an operating path tool. An infinity/infinity analysis is performed for the heterogeneous azeotropic pilot column and an industrial column with a decanter. Because of practical constraints, the pilot and the industrial columns do not have the same reflux policies. This leads the infinity/infinity analysis to predict multiple steady states for the industrial unit but not for the pilot column. However, multiple steady states are found by simulation both for the pilot and for the industrial unit. Multiple steady states are confirmed by simulation and experimental data for the industrial unit. Because of the positive infinity/infinity analysis, they are attributed to the phase equilibrium properties of the quaternary system. For the pilot column, multiple steady states are found by the simulation and linked to experimental observations. The multiplicity is not caused by the phase equilibrium properties; rather, it is attributed to interactions between the material and energy balances. An analysis of the simulation results helps explain the behavior of the industrial unit: the temperature of the sensitive tray gives rise to a peak in heat. This peak is located very close to the industrial temperature set point and is correlated with an impurity content minimum in the main product stream. An impurity minimum is also evidenced by the simulation for the pilot column. This complex behavior can explain observed difficulties in controlling the process at the industrial set point.