There are two sources of cellulosic feedstocks for corn to ethanol, dry grind facilities: corn pericarp (fiber) and corn stover. Both materials qualify for D3 RINs (renewable identification numbers for cellulosic ethanol), and could be an attractive resource for re-purposing corn to ethanol facilities for producing cellulosic ethanol. This approach would employ mixtures of endo- and exo-cellulases, pectinases, xylanases, protease, beta-glucosidase, as well as other possible auxiliary enzymes to liquefy and/or hydrolyze the cellulosic substrates. Tests, both in our laboratory and other research centers have demonstrated successful conversion of pretreated corn stover and other lignocellulosic materials. Processes for conversion of pretreated cellulose-containing distiller’s solids have been demonstrated and are being marketed. Reports on this technology, available in the literature, show that a $10 to $12 million capital investment enables a 6% increase in ethanol production in existing corn to ethanol plants. This process requires pretreatment prior to hydrolysis and fermentation of the cellulosic portion of the corn kernel.
Reports by Scott, Wyman, Schell, Elander, Kumar, Saville, Lawson and others have discussed the impact of mixing and reactor configuration on increasing both rate and yield of enzyme hydrolysis of cellulose, and/or liquefaction of pretreated lignocellulosic biomass (corn stover and hardwood). One significant factor that impacts economic viability of cellulose ethanol is being able to process biomass at high solids concentration in order to reduce energy for heating and other processing steps and increase the concentration of the ethanol produced. We recently reported the liquefaction of steam-exploded (pretreated) sugar cane bagasse at concentrations of up to 300 g/L.
We present here the impact of reactor operation on the liquefaction of untreated corn stover and pericarp (corn fiber), and compares this to our recently reported liquefaction of sugar cane bagasse. The objective is to prepare these materials in pumpable slurries, thereby simplifying subsequent hydrolysis procedures, and reducing the cost of equipment that would otherwise require introduction of solid materials into a pressure vessel (i.e, a pulping digester).
We have found that bioreactor design is a key consideration for the time-effective and economic processing of untreated lignocellulosic material, into a fermentable slurry. If the rate of addition and the rate of mixing are respectively too fast or too slow, a solid mass of wet material results. We report fed-batch reactor that enables liquefaction of untreated, 1 to 3 cm pieces of corn stover and pericarp into pumpable slurries. The liquefaction occurs over a 72 hour period with aliquots of cellulosic biomass materials being added to formulated cellulase/hemicellulase liquid mixtures. These materials, when inoculated with 1 g/L yeast in YPD media were readily fermented to ethanol.
The fed-batch reactor design for liquefying cellulosic biomass required the directed addition of pre-determined quantities of lignocellulosic material at time intervals of 1, 2, 3, 6, 9 and 12 hours until a concentration of either 200 (corn stover) or 300 (pericarp) g/L was attained. This enables the enzyme to change the biomass structure into a combination of particulate material and oligosaccharides at viscosities estimated to be in the range of 500 cp at 50°C and shear stress of 100s-1. The resulting mixing enables mass transfer of enzyme to the surface of the untreated pericarp.
The effect of agitation speed, the transition from stagnant to turbulent regimes, power input required to achieve initial mixing of enzyme with solids at 50°C in a batch reactor, and changes in biomass characteristics that occur during liquefaction is also described. Acceleration of liquefaction by a proper agitation profile reduces power input and allows for proper mixing by promoting contact between enzymes and substrate, which improves hydrolysis yields and process efficiency. The role of enzymes in changing particle characteristics and viscosity in a stirred tank reactor, thus enabling liquefaction, will be discussed in the context of mechanisms proposed in literature.
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