Separation of alcohol-water mixtures: Understanding fundamentals of pervaporation and gel stripping with polymeric materials

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2016-08

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Abstract

Cost-efficient recovery of bioalcohols from dilute aqueous fermentations poses a challenge due to the low concentration of alcohols obtained. Therefore, design of alternative, energy-efficient processes for recovery of bioalcohols from dilute fermentations (80 to 120 g/L for bioethanol and 10 to 20 g/L for biobutanol) is vital because of the high energy requirements for traditional multi-stage distillation. Development of novel, energy-efficient separation and purification technologies will play a very important role in the successful development and commercial implementation of biofuels.

The emphasis of this dissertation is on design of a membrane-based separation technique called as pervaporation for recovering ethanol from dilute ethanol-water mixtures. To permit efficient recovery of ethanol, pervaporation membranes must exhibit both high equilibrium selectivity for ethanol and a high rate of ethanol permeation. However, design of highly selective, highly permeable membranes is a non-trivial problem due to the complex interplay between diffusivity selectivity and equilibrium solubility selectivity. Given that thousands of potential membrane materials exist, screening methodology for rapid identification of pervaporation membrane materials with superior properties is a pressing need. While most studies of polymeric pervaporation membranes have focused on silicones due to their hydrophobic nature and very low glass transition temperature (Tg), this dissertation examines polyacrylates, which offer greater tunability in chemical composition. Random copolymer networks are screened in a high-throughput, combinatorial fashion by systematically varying the materials' hydrophilicity and crosslinker concentration. The screening methodology described is a powerful means to accelerate the search for new membrane materials, as it is adaptable to a broad range of polymer chemistries.
Any copolymer network in a mixture of solvents is inherently a quaternary system (two monomer units, two solvents) at its simplest, and no established thermodynamic framework exists for modeling the equilibrium swelling behavior. Therefore, a generalized extension of classical Flory-Rehner (FR) theory has been derived to describe swelling of homopolymer or copolymer networks in any number of solvents. The applicability of the multi-component FR theory is validated by examining the swelling data of random copolymer networks in a mixture of two solvents using a specific four-component (two monomer units, two solvents) extension of the generalized FR model. To understand the fundamentals of pervaporation, a novel sorption-diffusion-desorption (SDD) model is developed based on the multi-component FR theory. The predicted permeability selectivity and fluxes of ethanol and water show quantitative agreement with the experimental results, validating the SDD model. Because diffusivity selectivity always favors permeation of water molecules, which are smaller than alcohols, the permeability selectivity of pervaporation membranes is usually lower than solubility selectivity. The situation is exacerbated in butanol-water mixtures, owing to much larger difference in the molecular sizes of butanol and water. The final part of this work therefore examines an alternative process called "gel stripping" based upon equilibrium sorption. Without requiring sophisticated techniques or processing equipment, gel stripping potentially provides high separation efficiency with low energy cost.

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Keywords

Alcohol-Water Separation, Pervaporation, Gel Stripping, Multi-Component Flory-Rehner Theory

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