Polymers for Biomass Energy Conversion: Porous Scaffold Materials for a Continuous-Flow, Immobilized-Cell Fermentation Process

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 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 therefore 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.  Sponge-like polymeric materials having micrometer-scale porosity are a key element of a new immobilized-cell, packed-bed fermentation process that already exhibits volumetric productivity more than an order of magnitude higher than comparable batch fermentations.  Synthetic porous polymer scaffolds (SPPS) are fabricated in the form of irregular particles of typical mesh size 1 to 2 mm, which are used to immobilize fermenting microorganisms as a packed bed within a columnar reactor.  Design of optimal (SPPS) for the immobilized cell reactor process is an emerging materials challenge within the broad field of biomass energy conversion.

Optimal SPPS are inexpensive polymers or gels of relatively low water content that contain pores and channels of typical diameter 1 to 30 micrometers and have a high pore volume fraction (0.4 to 0.8).  Pores should have extensive surface connectivity, facilitating entry and exit of both reactants and fermenting micoorganisms.   Ethanologens of typical size 1 to 10 micrometers are targeted, such as Escherichia coli, Zymomonas mobilis, or Saccharomyces cerevisiae, and maximizing the cell density within the SPPS material is critical.  Two general approaches to SPPS design are being pursued:

1) Preparation of a bicontinuous blend with one crosslinked component and one extractable component, which is removed to generate randomly structured, interconnected pores. 

2)  Crosslinking of a matrix polymer around extractable polymer microfibers, which are removed to leave well-defined cylindrical pores.  

In both cases, the extractable phase is removed prior to fermentations by leaching in a solvent, preferably water, leaving pores or channels in the SPPS with a high degree of connectivity to the surface.  SPPS materials are colonized naturally by cells during the exponential growth phase in batch operation, after which continuous reactor operation is initiated.   

Our work has so far examined conversion of glucose at an inlet feed concentration of 1.0 to 5.0 % 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.  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 allow small particulate solids to pass through the bed, lowering the risk of clogging.  Our presentation will discuss the advantages and limitations of the SPPS column reactors, materials design issues, and progress in the conversion of sugars to EtOH.