(449d) Continuous-Flow, Packed-Bed Fermentations: New Scaffold Materials and Reactor Design Issues

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 renewable, non-food feedstocks, namely cellulosic biomass, has received much attention. If cellulosic EtOH is to become an economically sustainable source of fuel in the future, drastic improvements to process economics must be realized, however. Part of the cost reduction can be realized by lowering the cost of raw materials and increasing the efficiency of conversion to EtOH, while other reductions must be made in the areas of capital equipment and operating costs by design of more efficient fermentation processes. Our work focuses on design of an efficient continuous-flow bioconversion process for production of EtOH, with emphasis on cellulose-derived feedstocks, which may contain particulate material and inhibiting compounds. Immobilized cell reactor (ICR) fermentations via ethanologens attached to a solid support or scaffold offer several advantages over batch fermentations. Continuous flow flushes away the EtOH as it is produced, keeping its concentration low near the reactor inlet, reducing product inhibition. The conversion of sugars to EtOH is enhanced near the inlet due to the ability to maintain conditions of moderate sugar concentration (as high as 12 to 15 %) and low EtOH concentration. Cell density in immobilized systems can significantly exceed the maximum value attainable in free cell suspensions. Thus, volumetric EtOH productivities 5 to 10 times higher than those achieved in comparable batch fermentations have been reported. The expense of separating cells from the product mixture is also greatly reduced or eliminated. However, perhaps the most important advantage of the ICR is that high dilution rates (and thus volumetric productivities) can be achieved. A recent advance has been made in the design of synthetic, porous polymer scaffold materials (SPPS) for use in packed-bed ICR. The SPPS are polymer gels of relatively low water content that contain pores and channels of typical diameter 1 to 30 micrometers, allowing entry and exit of micoorganisms of typical size 1 to 5 micrometers. SPPS are fabricated in the form of irregular particles of typical mesh size 1 to 2 mm. SPPS materials are a great improvement over traditional polymer gel materials such as calcium alginate, carageenan, and polyacrylamide because the pores greatly reduce mass transfer resistance and allow facile venting of CO₂ bubbles during fermentation. In addition, packed beds of SPPS particles may allow particulate solids to pass through the bed, lowering the risk of clogging compared to traditional gel-based ICR. An important distinction between these SPPS and traditional gels (e.g. calcium alginate) is the mode of loading with cells. In traditional ICR systems, cells are usually encapsulated in the gel prior to fermentation. In contrast, for the SPPS, the material is processed, loaded into the reactor with growth medium, and sterilized before inoculation with cells, which are able to enter through surface-accessible pores. After cells reach the exponential growth phase, continuous flow is initiated. Because cells are free to enter and exit the SPPS, immobilization is understood to be partial, and some minimal loss of cells occurs. However, the loss of cells is not enough to cause washout even at dilution rates above 3.0/h, and is it not necessary to employ cell-recycle schemes. The advantages of facile gas ventilation and substrate transport through the pores outweigh any lost productivity due to consumption of sugars to produce new cells. Our work has so far examined conversion of glucose at an inlet feed concentration of 1.0 % w/w by E. coli strain LY01. A continuous-flow column ICR packed with an SPPS material achieved volumetric productivity 14 times higher than that of a comparable batch fermentation, while the porous structure of the SPPS bed mediated problems with CO₂ holdup. (We anticipate reporting results for more concentrated feedstocks and mixed sugar fermentations at the AIChE Annual Meeting). The concentrations of glucose and EtOH exiting the column reactor (determined by HPLC) at a dilution rate 1.0/h, are plotted in Fig. 1. Due to the drastic improvements in volumetric productivity possible with an SPPS packed bed ICR, design and optimization of efficient ICR systems for cellulosic EtOH is promising. We are presently evaluating vertical column reactors with different packing arrangements that optimize ventilation of CO₂, while minimizing the possibility of clogging of the bed by particulate matter and/or cells. Our presentation will discuss the advantages and limitations of the SPPS column reactors, design issues, and the conversion of different feedstocks to EtOH.