388396 Continuous-Flow Production of Bioethanol: Reactor Design Considerations and Energy-Efficient Recovery
Due to limited global supplies of fossil fuel resources, rising prices, and environmental problems, future energy needs must be met in part or entirely by fuels derived from renewable resources. Production of ethanol (EtOH) from cellulosic biomass is one of the more attractive alternatives for producing a portable, liquid fuel. Although EtOH is already used as an additive in gasoline, drastic improvements to process economics must be realized if cellulosic EtOH is to become an economically sustainable source of fuel in the future. While many processes and economic factors are involved in the conversion of lignocellulosic biomass to anhydrous ethanol, a significant cost reduction can be achieved by designing more energy-efficient fermentations and separation processes for alcohol recovery.
One emphasis of our work is design of an efficient continuous-flow reactor system for bio-ethanol production. While continuous-flow reactor systems potentially offer great advantages in terms of increased volumetric productivity and continuous operation, the biofuels industry continues to prefer batch fermentation processes due to instabilities encountered in continuous-flow systems. We have developed a continuous-flow fermentation process based upon E. coli strain LY01, which is immobilized on a low volume fraction (<0.01) of poly(ethylene terephtalate) fibers of 12-25 μm diameter. Stirred-tank and vertical column reactor designs have been compared with simple glucose feedstocks. Stable fermentation with glucose conversion greater than 90 % can be obtained with the tank reactor using 70 g/L glucose in the feed, while the immobilization of cells allows dilution rate and volumetric productivity to exceed the values obtained in chemostat mode of operation. The column reactor provides exceedingly high cell densities and productivities when glucose concentration in the feed is low (< 20 g/L), but suffers from instabilities and poor conversion when the feedstock concentration approaches commercially viable levels. Thus, the stirred tank with an immobilized cell section appears to be the more attractive design. Current experiments involve assessing the performance of the continuous-flow system with mixed sugars.
The second emphasis of our work is on development of an energy-efficient separation process ("gel stripping") that is readily adaptable to continuous-flow fermentation schemes. Another key challenge limiting the success of cellulosic ethanol is the high energy consumption of traditional multi-stage distillation processes. Gel stripping is a column-based absorption process for recovery of bioethanol (or biobutanol) from dilute ferementation effluents that may significantly reduce the energy cost of separation. The alcohol is stripped from the fermentation effluent as the fermentation effluent passes through a packed bed containing particles of a selectively absorbent polymeric gel material. Proteins, salts, and cells are unable to enter the gel due to its small mesh size and drain from the column. The absorbed alcohol is recovered from the gel by re-extraction into a low-boiling solvent with low latent heat of vaporization. Using high-performing gel materials identified by a combinatorial screening approach, the gel stripping process has been applied to dilute alcohol-water mixtures with high recovery rates. 95%+ removal of alcohols from dilute aqueous solutions has been demonstrated, and a mathematical model of the unsteady-state gel stripping process has been developed.
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