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ENHANCED PERVAPORATION SEPARATION EFFICIENCY VIA STAGED FRACTIONAL CONDENSATION (DEPHLEGMATION) OF PERMEATE VAPOR
Citation:
Vane*, L M., F R. Alvarez*, A. Mairal, AND A. Ng. ENHANCED PERVAPORATION SEPARATION EFFICIENCY VIA STAGED FRACTIONAL CONDENSATION (DEPHLEGMATION) OF PERMEATE VAPOR. Presented at 13th National Meeting of No. American Membrane Society, Long Beach, NC, May 11 - 15, 2002.
Impact/Purpose:
To inform the public.
Description:
In traditional pervaporation systems, the permeate vapor is completely condensed to obtain a liquid permeate stream. For example, in the recovery of ethanol from a 5-wt% aqueous stream (such as a biomass fermentation broth), the permeate from a silicone rubber pervaporation membrane may contain 35-wt% ethanol. While this concentration factor is attractive, the 35-wt% ethanol permeate requires further processing to make it useable as a solvent or fuel additive (>99-wt% ethanol is generally required). Research has been reported in the literature in which multiple permeate vapor condensers operated in series and at different temperatures were used to achieve additional separation of the permeate species. However, the literature contains no reports of pervaporation modules interfaced with a fractional condensation column, generally referred to as a "dephlegmator", to provide a high degree of permeate separation. Such a separation would increase the overall selectivity of the pervaporation system, possibly altering the downstream processing requirements.
A dephlegmator column is analogous to a distillation column in which comounds are separated by providing multiple vapor-liquid equilibrium (VLE) stages. However, in distillation, the feed is a liquid and, in simplistic terms, heat is provided at the base of the column to create the upflowing vapor. In dephlegmation, the feed is a warm vapor and cooling is provided to create the downflowing liquid. Compounds in the vapor feed mixture with a low vapor pressure and high boiling point will preferentially condense in the dephlegmator, whereas those with a high vapor pressure and low boiling point will remain in the vapor phase. Thus, for the ethanol/water system, water is expected to be removed as a condensate at the bottom of the dephlegmator while ethanol will remain mostly in the overhead vapor phase.
Chemical process simulations were performed to determine the potential effectiveness of employing dephlegmation for permeate vapor separation of the ethanol/water system. In the base-case scenario, a feed vapor containing 34.6-wt% ethanol (balance water) at 60 oC and 30 torr absolute pressure was introduced into a simulated four-stage dephlegmator. Establishing an overhead temperature of 14.5oC and a condensate temperature of 24.5oC resulted in an overhead ethanol concentration of 85-wt% and a condensate ethanol concentration close to 5-wt%. These simulations indicated that substantial additional separation of a ethanol/water permeate vapor is at least theoretically possible.
Subsequently, two pilot-scale dephlegmator systems were constructed at USEPA T&E Facility in Cincinnati, OH, Experiments were performed with these units using a surrogate pervaporation ethanol/water permeate stream to assess the accuracy of the simulation results and to determine the height of theoretical equilibrium stages in the two dephlegmator designs. These experiments confirmed the simulation results. The final phase of this proof-of-concept project will involve integrating a dephlegmator pilot unit with a Membrane Technology & Research, Inc. pervaporation pilot unit. In this presentation, results of the dephlegmator computer simulations and surrogate vapor dephlegmator experiments will be detailed. In addition, results from the simulation of an integrated pervaporation/dephlegmator system will be presented.