Critical reflux, parametric sensitivity, and hysteresis in azeotropic distillation of isopropyl alcohol + water + cyclohexane

In this work, the characteristics of heterogeneous azeotropic distillation of isopropyl alcohol (IPA)+water (H₂O) with cyclohexane (CyH) as an entrainer were investigated. Critical reflux, parametric sensitivity, and hysteresis were found by theoretical analysis and supported by experiment using a laboratory-scale sieve plate distillation column. At each given reflux rate, there are two branches of stable steady states as reboiler duty varies, one with desirable purity and another with almost no separation. The highest attainable purity is achieved at a specific reboiler duty. Beyond this duty there is a catastrophic loss in IPA purity. At higher refluxes, the temperature profiles of the column with highest attainable purity pass around the IPA+CyH azeotrope. A plateau at 69.3 °C characterizes this type of profile. There is little change of highest attainable IPA purity as reflux varies. When reflux is reduced below a critical value, the temperature profile of the column with highest attainable purity passes around the IPA+H₂O azeotrope. The highest attainable IPA purity decreases substantially as reflux is reduced. The critical reflux represents an operating state that achieves high IPA purity with minimum energy consumption. Furthermore, at a given pair of reflux and reboiler duty, the steady-state temperature profile that achieves good separation is not unique. The exact position of the temperature front can be fixed at any position inside the column using a cyclic series of step changes in reboiler duty. This hysteresis phenomenon is attributable to changes in entrainer inventory inside the column.