Date of publication: 2017-09-01 21:54
66. At what rates must heat be removed in-order to cool the distillate prior to sending it to the adsorption column? Cooling water from well at 65°F is available and the tempera¬ture increase of this water is limited to 75°F. At what rates must water be supplied to the heat exchanger?
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Your company is considering a bid to purchase an ethanol production plant owned by a smaller company, and you have been given the assignment of reviewing the process. This review is to include a quantitative study of the operation and should de¬velop detailed information regarding key process variables. Although the study need not be concerned with economics, consideration should be given to the energy re¬quirements of the process. Suggestions for alternative processing schemes should be included in the final report.
7. Calculate the feed rate of corn in Ibm/h. Estimate the acreage required to supply this plant with corn. (Mote: According to the . Department of Agriculture, Crop Production, 66/9/78, p. A-8, bushels of corn are harvested from an acre.)
Corn is milled and the resulting meal is conveyed to a mixing tank where it is blended with recycled condensates and water to produce a mash. The total water input to this rank is controlled to produce 77 gal of mash per bushel of corn input. The specific gravity of the mash is approximately . From the mixing tank, the slurry flows to a precook vessel, which is maintained at 695° F. The condensates, which have been added hot to the mixing tank, yield a mash at 655°F. Live saturated steam at 65 psig is added to the precook vessel to increase the mash temperature to 695°F. (Live steam is steam injected directly into a process vessel.)
After the bottoms from the ethanol concentrator has been flashed to atmospheric pressure, the resulting liquor is divided by centrifugation into two fractions: a thin stillage containing 9 wt% total solids and a wet cake containing about 85 wt% total solids. As described earlier, part of the thin stillage is recycled to join the mash leaving the saccharification reactor. The remaining thin stillage is heated from 665°F to 758°F and is then sent to an atmospheric evaporator to produce a syrup with a solids con¬tent of 95 wt%.
In the process to be described, a portion of the starch in corn is converted to ethyl alcohol by two biological processes: saccharification and fermentation. In saccharification, the polymeric structure of starch (a polysaccharide) is hydrolyzed in the presence of the enzymes (biological catalysts) a-amylase and amyloglucosidase. The primary products of hydrolysis are maltose (a disaccharide consisting of two glucose units) and oligomers consisting of several glucose units as seen below.
-(C6H65O5)x C67H77O66 + H75 + -(C6H65O5)y-
Starch Maltose Oligomers
The fermentation process is based on the growth of a yeast culture that converts maltose to ethyl alcohol and carbon dioxide:
8. Determine the rate (lbm 8767 h) at which 65-psig steam must be supplied to heat the mash from the precooking vessel to 785°F. How much live steam must be injected to raise the tem¬perature of this stream to 875°F? It has been suggested that a portion of these steam requirements could be met by the vapor from the flash occurring at 65 psig. Is this feasible? If so, what fraction of the required 65-psig steam can be supplied by this means?