(522c) Reducing Process Complexity for Cellulosic Ethanol
Much of the process complexity and high capital costs associate with cellulosic ethanol (dilute acid hydrolysis as a pretreatment and enzymatic saccharification of cellulose) results from side products such as furfural, acetate, and soluble lignin compounds that inhibit the biocatalysts. Inhibitor production has been reduced substantially by replacing sulfuric acid with phosphoric acid, also eliminating the need for exotic metals such as zirconium in the reactor. More robust Escherichia coli biocatalysts have recently been developed with increased resistance to furans and other toxic compounds in hemicellulose hydrolysates. These biocatalysts require only ammonium phosphate, magnesium sulfate, and trace metals for growth. With the phosphoric acid pretreatment, the improved biocatalysts can co-ferment hexose and pentose sugars in a single vessel eliminating liquid-solid separation of fiber (and fiber washing) and the need for sugar cleanup (overliming) and separate fermentation vessels. To facilitate mixing and stirring, a continuous stirred tank reactor for liquefaction is proposed (6 h) followed by a smaller CSTR for pH and temperature adjustment immediately prior to fermentation (termed an L+SScF Process). Ammonia used for pH adjustment and phosphate serves as nutrients for the biocatalysts. All chemicals used are components of crop fertilizer and should retain full value at the end of the process, borrowed for biofuel production together with process water. The proposed process is similar in many respects to the corn dry mill ethanol process. Unit operations at pilot scale (140-L hydrolyzer, 100-L liquefaction, various fermentors up to 140 L) have been used to simulate this process with 10% solids and with 15% solids. Ethanol yields of up to 80 gal/ton can be achieved in 72-96 h with 5 FPU of cellulose/gram of bagasse dry weight.