Optimization of internal coupling process of NaA zeolite membrane in distillation column to separate ethanol/water mixture

Ethanol dehydration is a challenging topic in industries due to the presence of azeotropic mixtures. To solve this issue, a novel internal coupling process was proposed by placing membrane units inside distillation column to remove water in vapor flow before becoming azeotropic. The internal coupling was experimentally verified by a home-made distillation-vapor permeation (VP) setup using a hollow fiber NaA zeolite membrane module to reveal the separation improvement. The experimental characterization demonstrated that NaA zeolite membrane significantly enhanced the distillation separation capacity in spite of a low impact on vapor flow rate in the column. Compared with middle coupling of the column, placing the membrane in the upper section yielded higher synergistic effects, resulting in a top ethanol content of 97.05 wt%, which was about an increase of 0.38 wt%. This suggested the importance of coupling position in determining the separation performance. To further optimize the integration between the two separation techniques, a steady-state process simulation for internal coupling was conducted, where the membrane area, coupling position, and feed position were extensively optimized. The simulation results indicated that the membrane unit could significantly promote the forward shift of the vapor-liquid composition curve to broaden the distillation boundaries. Compared with external coupling mode, the internal coupling process required a higher membrane area and provided an energy saving percentage of 21.6 % and 25.7 % for reboiler and condenser, respectively. In addition, a novel segmental configuration was designed for internal coupling, where the membrane area was evenly distributed at different stages. The simulation results indicated that such a design could improve the balance between the two units based on different separation principles. It was suggested that when four segmental design were used, the membrane area could be effectively saved by more than 30 %, further developing the application potential of the internal coupling technique.