(587i) Membrane Pervaporation in a Fast Pyrolysis Biorefinery

Fast pyrolysis of biomass produces a complex mixture of sugars, organic oxygenates including carboxylic acids, alcohols, aldehydes, esters, ethers, furans, and ketones, and phenolic monomers, dimers, and oligomers with a variety of functional groups. Water is an additional product, of which the sugars and carboxylic acids are highly soluble and many of the other organic species have limited or very little solubility. The mixture complexity and presence of multiple phases necessitates several levels of separation to obtain individual compounds of interest. Fractional condensation of the fast pyrolysis vapor product stream directly from the reactor is the first level of separation that partitions the exiting vapor and aerosol products into several distinct fractions according to the respective component vapor pressures.

This work explores the use of several types of membranes and membrane processes and their application to the different fractions of bio-oil produced by the unique fractional condensation system under development at Iowa State University. The primary applications investigated include hydrophobic membranes for the pervaporative removal of low concentration phenolic species from the aqueous fraction and hydrophilic membranes for vapor and liquid phase dehydration of the aqueous fraction before and after the primary condensation from the pyrolysis reactor. Also explored is the use of these membranes in tandem with other separation processes applied to the bio-oil product fractions such as liquid-liquid extraction and adsorption. In this manner the membranes function to recover the organic solvents and clean water used in the separations with the potential of reducing the energy and operating costs as compared to distillation and other more traditional separation methods in a biorefinery.

Early tests with commercially available PDMS membranes have demonstrated effectiveness for removing a large amount of phenolic species from the aqueous product stream of the pyrolysis fractional condensation process. Approximately 10 wt% of the overall permeate is phenolic species, which phase separate from the permeate water due to their low solubilities, for example 0.5 and 2.7 wt% for 4-ethylphenol and o-cresol, respectively, at 25 °C. This allows for easy decantation of the permeate organic phase and recycle of the phenolic saturated aqueous phase. Preliminary experiments with commercially available silica based membranes have shown to be effective for dehydration of the aqueous pyrolysis product fraction and tests continue to examine the long-term performance of these membranes and their ability to achieve full dehydration of the aqueous product fraction.