The product is condensed cooled stored denatured with

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contains more than 99% by weight ethanol. The product is condensed, cooled, stored, denatured with gasoline (5% by volume), and shipped. Ethanol storage capacity is 12 days. The whole stillage is partially evaporated in the first three stages of a six-effect vacuum evaporator. The partially evaporated whole stillage is separated in a decanter centrifuge (one operating plus one standby). The wet grains leave the centrifuge at 35% by weight total solids. The thin stillage from the centrifuge is partially recycled as backset, and the remainder is concentrated in the final three stages of the evaporator to syrup containing 55% by weight total solids. To conserve steam and cooling water, the condensation of overhead vapors from the rectifier to provide reflux for distillation is accomplished in the evaporator. The syrup and wet grains are mixed and dried in a gas-fired rotary dryer. The DDG leaving the dryer contains 9% moisture by weight. The process is designed to be essentially zero-discharge. Makeup water is added only for the cooling tower and the CO 2 scrubber, and no wastewater is produced.
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8 III.2 Lignocellulose Feedstock-to-Ethanol Process Description The process used in this analysis can be briefly described as using co-current dilute acid prehydrolysis of the lignocellulosic biomass with simultaneous enzymatic saccharification of the remaining cellulose and co-fermentation of the resulting glucose and xylose to ethanol. In addition to these unit operations, the process involves feedstock handling and storage, product purification, wastewater treatment, enzyme production, lignin combustion, product storage, and other utilities. In all, the process is divided into nine areas (see Figure 2). Details of the process can be found in the NREL design report for the dilute acid prehydrolysis and enzymatic hydrolysis process. 16 Figure 2. Lignocellulose-to-ethanol process flow The feedstock, in this case corn stover, is delivered to the feed handling (A100) area for storage and size reduction. From there, the biomass is conveyed to pretreatment and conditioning (A200). In this area, the biomass is treated with dilute sulfuric acid at a high temperature for a very short time, liberating the hemicellulose sugars and other compounds. Ion exchange and overliming is required to remove compounds liberated in the pretreatment that will be toxic to the fermenting organism. Only the liquid portion of the hydrolysis stream is conditioned. After pretreatment, a portion of the hydrolyzate slurry is split off to enzyme production (A400). In enzyme production, seed inoculum is grown in a series of progressively larger aerobic batch fermentors. The inoculum is then combined with additional hydrolyzate
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9 slurry and nutrients in aerobic fermentors to produce the enzyme needed for saccharification. Simultaneous saccharification and co-fermentation, or SSCF, (A300) of the hydrolyzate slurry is carried out in a series of continuous anaerobic fermentation trains. The recombinant fermenting organism Zymomonas mobilis is grown in progressively larger batch anaerobic fermentations. This inoculum, along with cellulase enzyme from enzyme production (A400) and other nutrients, is added to the first fermentor.
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