Commercial production of ethanol in the San Luis Valley, Colorado. Final Report Page: 36 of 222
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The yeast normally used in industrial fermentation is saceharomyces cer-
visia which has been cultivated to operate most efficiently in acidic conditions at 300C.
Wild yeast strains and bacteria do not thrive in acidic conditions, so contamination is
minirhized. As the reaction proceeds, heat is evolved which must be dissipated to
prevent sterilization of the yeast culture. The peak temperature must not exceed 350C
(950F). The normal fermentation cycle requires 40-60 hours retention in the fermenta-
tion vats to convert the glucose to ethanol. Normally, a 92 percent conversion from
glucose to ethanol can be achieved under process conditions. The percentage of alcohol
in the beer leaving the fermentation phase is dependent on the percentage of dry solids
in the mash and usually ranges from 6-11 percent alcohol by weight. The rule of thumb
for determining alcohol content is: 2 percent dry solids produces 1 percent of alcohol
by weight in the beer. The dry solids content is limited, primarily by viscosity of the
mash, at 22 percent by weight.
By-product contaminants include higher weight alcohols, succinic acid,
glycerol and aldehydes. These products are not formed in appreciable quantities and
are removed at later stages in the ethanol process. The remaining steps in the produc-
tion of anhydrous ethanol consist of concentrating the ethanol by removal of water and
other impurities in the process stream.
2.2.3 Feedstock Choice for San Luis Valley Ethanol Plant
In order to provide some insulation from abnormal price fluctuations or a
severe supply shortage for a particular feedstock for the San Luis Valley Plant, a feed-
stock mix was selected, the proportions of which would be determined by market
forces. The agricultural composition of the valley and the similarity of processing for
starchy materials resulted in the choice of wheat, barley, corn, or potatoes as
feedstocks for the ethanol plant. These feedstocks will all require six major process
steps: 1) cleaning and size reduction; 2) cooking or liquefaction; 3) saccharification;
4) fermentation; 5) distillation; and 6) dehydration. The details of the conceptual design
for the site specific ethanol plant are discussed in Section 7.
2.3 DISTILLATION AND DEHYDRATION
Ethanol used as motor fuel must be purified to 99.9 percent alcohol. This
process step is required for ethanol produced from the fermentation of biomass or the
hydration of petrochemicals. The removal of water is accomplished through a series of
successive distillation steps which concentrate the alcohol through reflux. Alcohol
cannot be made stronger than 95.6 percent by rectification because water and alcohol
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Hewlett, E.M.; Erickson, M.V.; Ferguson, C.D.; Sherwood, P.B.; Boswell, B.S.; Walter, K.M. et al. Commercial production of ethanol in the San Luis Valley, Colorado. Final Report, report, July 1, 1983; United States. (digital.library.unt.edu/ark:/67531/metadc874948/m1/36/: accessed May 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.