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PERVAPORATION SEPARATION IMPROVEMENTS VIA FRACTIONAL CONDENSATION (DEPHLEGMATION): IMPACT OF DEPHLEGMATOR DESIGN ON PERFORMANCE
Citation:
Vane*, L M., F R. Alvarez*, A. Mairal, AND A. Ng. PERVAPORATION SEPARATION IMPROVEMENTS VIA FRACTIONAL CONDENSATION (DEPHLEGMATION): IMPACT OF DEPHLEGMATOR DESIGN ON PERFORMANCE. Presented at 2003 North American Membrane Society National Meeting, Jackson Hole, WY, 05/13/2003.
Description:
Traditionally, pervaporation systems have been operated using a total condenser to deliver the final permeate liquid product. Over the past two years, we have investigated the use of a condensation process called "dephlegmation" to enhance the separation performance of pervaporation systems. Such a dephlegmator would be inserted in the permeate path between the membrane and the final complete condenser/vacuum pump. In this method, permeate vapor is introduced into a high surface area dephlegmator column in which cooling is provided to partially condense the vapor. Establishing a temperature profile in the column creates multiple vapor-liquid equilibrium (VLE) stages which results in the production of an overhead vapor product enriched in one species and a bottoms condensate that is depleted in that species. For example, an ethanol-water permeate would be fractionated into an ethanol-rich overhead vapor product and an ethanol-depleted bottoms condensate. In dephlegmator systems, sufficient heat and mass transfer are both critical. A number of dephlegmator configurations are possible which will provide sufficient heat and mass transfer. In one configuration, a high surface area heat exchanger is used alone as a dephlegmator. In this configuration, heat removal and mass transfer occur on the same surfaces. At the other extreme, heat removal is achieved in a heat exchanger separate from the mass transfer device, although heat and mass exchange between the vapor and liquid phases still occur within the contactor column. In this second mode, the heat exchanger provides the downflowing reflux condensate into a high surface area packed mass transfer column. In pervaporation systems, the dephlegmator reflux liquid could obtained from the condensate product from the final total condenser, thus avoiding the need for a separate reflux condenser on top of the dephlegmator.
We have previously reported results of chemical process simulations of dephlegmators (2001 AIChE National Meeting), pilot-scale dephlegmation experiments with surrogate permeate vapors (2002 NAMS meeting), and experiments with an integrated pervaporation-dephlegmation pilot-scale system (2002 AIChE National Meeting). In this presentation, characteristics of the dephlegmator configurations mentioned above will be described. Specifically, experimental results obtained with a plate-fin heat exchanger dephlegmator will be compared to recent results obtained with a shell-and-tube dephlegmator containing a structured packing. The latter dephlegmator was operated both in a heat exchanger configuration and in a simple packed column configuration with an overhead reflux condenser. An MTR pervaporation pilot unit equipped with a silicone rubber membrane was used as the permeate source. For most experiments, the feed solution to the pervaporation unit contained approximately 5 wt% ethanol yielding a permeate containing 30 wt% ethanol. Both dephlegmator units could be operated to produce at least 90 wt% ethanol overhead product with at least 89% of the ethanol recovered in that product. Thus, the overall separation factor for the combined pervaporation-dephlegmation system exceeded 170 as compared to a separation factor of 8 for the membrane alone. The packed shell-and-tube dephlegmator exhibited a greater number of VLE stages, but was more sensitive to the vapor loading rate than the plate-fin dephlegmator. The shell-and-tube model could be operated effectively as a simple packed column with heat removed in a separate reflux condenser, reducing the complexity and cost of implementing dephlegmation. The application of dephlegmation to pervaporation separations other than the ethanol-water system will also be discussed.
*This is an abstract of a proposed presentation and does not necessarily reflect U.S. EPA policy.