Optimal operation of hybrid distillation/pervaporation systems

DAVIOU, MARIA C.; HOCH, PATRICIA M.; ELICECHE, ANA M.; ORTIZ, INMACULADA

Separations Technology - Vol I - AIChE Pub. N° 150

American Institution of Chemical Engineers

Lugar: New York; Año: 2001; p. 144 - 149

The main objective of this work is the optimization of the azeotropic distillation column in a hybrid distillation/pervaporation system using a rigorous simulation of the pervaporation membrane and the distillation column. Optimization of such processes is a new alternative, which has not been explored previously. A methodology is proposed for the simultaneous optimization of the hybrid system that includes an algebraic and differential equation system, where the optimization variables are the operating conditions and the location of feed, recycle and side draw. Numerical results of the optimization are shown for the separation of a mixture of i-butene, methanol and MTBE. Debottlenecking and azeotrope breaking are fruitful fields for hybrid distillation/membrane systems. Some studies propose a combination between distillation/pervaporation as an interesting alternative for solving these problems in industrial processes. The objective was to replace a sequence of azeotropic columns with a hybrid distillation/pervaporation process, as these processes are generally less energy consuming than distillation, and they are not influenced by the equilibrium between components. Recent patents propose hybrid distillation/pervaporation technologies for azeotrope breaking processes involving the separation of alcohols and ethers (Chen et al., 1988, Chen et al., 1989) applied to the MTBE process, replacing the Hüls process. The pervaporation membrane used shows high flux and high selectivity to the permeation of Methanol, effectively breaking the azeotrope Methanol-MTBE. The process called "Total Recovery Improvement for MTBE" (TRIM™) is a combination of an organophilic pervaporation membrane and distillation, using two different layouts. The integration of the TRIM™ process to an existing one would be attractive if the production could be increased by 5%. Lipnizki et al (1999) present an extensive review of pervaporation-based hybrid processes, focusing on industrial applications and pointing out the need of optimization of some of the analyzed processes. One of the fields where there is need of optimization is the distillation/pervaporation hybrid process. Hömmerich and Rautenbach (1998) studied the integration of pervaporation and vapor permeation into the Hüls process. They analyzed the influence of the operating conditions in a hybrid distillation-pervaporation-vapor permeation system for the Methyl tert-Butyl Ether (MTBE) production. González and Ortiz (2001a and b) carried out experimental work to find a semi-empirical model for the pervaporation membrane to separate methanol and MTBE, and an approach to define a hybrid process distillation/pervaporation based on the Hüls process, performing a cost analysis. The whole process is modeled using gPROMS (2000). The formal optimization of the debutanizer column with a pervaporation membrane to treat the side stream has not been attempted previously. Previous work on the optimization of non-conventional distillation columns (Hoch and Eliceche, 1991) does not contemplate the azeotropic distillation case.