After several days of saccharification and

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enzyme production (A400) and other nutrients, is added to the first fermentor. After several days of saccharification and fermentation, most of the cellulose and xylose will have been converted to ethanol. The resulting beer with 4-5% by weight ethanol is sent to product recovery. Product recovery (A500) consists of a beer column to distill the ethanol from the majority of the water and residual solids. The vapor exiting the beer column is 35% by weight ethanol and feeds the rectification column. A mixture of nearly azeotropic (92.5%) ethanol and water from the rectification column is purified to pure (99.5%) ethanol using vapor-phase molecular sieves. The beer column bottoms are sent to the first effect of a three-effect evaporator. The rectification column reflux condenser provides heat for this first effect. After the first effect, solids are separated using a centrifuge and dried in a rotary dryer. A portion (25%) of the centrifuge effluent is recycled to fermentation and the rest is sent to the second and third evaporator effects. Most of the evaporator condensate is returned to the process as fairly clean condensate (a small portion, 10%, is split off to waste water treatment to prevent build-up of low-boiling compounds) and the concentrated syrup contains 15%-20% by weight total solids. Biogas (containing 50% methane, and with a heating value of approximately 12,000 British thermal units, or Btu, per pound) is produced by anaerobic digestion of organic compounds in wastewater treatment. The treated water is considered suitable for recycling and is returned to the process, so there is no water discharge from the process. The solids from distillation, the concentrated syrup from the evaporator, and biogas from anaerobic digestion are combusted in a fluidized bed combustor, or FBC, (A800) to produce steam for process heat. Soluble components in the wet boiler feed are combusted and some water vapor exits through the stack. The majority of the steam demand is for the pretreatment and distillation areas. Generally, the process produces excess steam that is converted to electricity for use in the plant; any excess electricity is sold to the local power grid. III.3 Primary Process Differences There are some major differences in the processing of corn starch versus stover. Stover requires more feed handling; it is envisioned that stover will be delivered in bales that must be washed, shredded, and then milled to achieve a particle size that can be conveyed to the process. Corn requires milling to a fine meal. The steps to reduce the carbohydrate polymers in stover to simple sugar monomers take considerably longer and are more energy intensive than for the starch in corn. The cellulose requires pretreatment
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10 approaching 180´-200´C (356´-392´F) with dilute acid to make the cellulose digestible by cellulase enzyme versus 80´-90´C (176´-194´F) for cooking the corn starch. After pretreatment, the cellulase enzyme and fermentation organism require about 7 days for conversion to ethanol, compared to 2 days for starch. The longer residence time increases the chance for contamination during SSCF. The resultant beer is more dilute, and the mixing power requirements are higher due to a higher solids content.
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  • Spring '10
  • BLANCHE
  • The Land, ........., corn ethanol industry

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