The Recycle Of Sulfuric Acid And Xylose In The Prehydrolysis Of Corn Stover


Ethanol may be produced from agricultural residues by using a two-stage acid hydrolysis followed by acid recovery, fermentation, and distillation. With sulfuric acid as a catalyst, xylose-rich and glucose-rich streams can be obtained from corn stover in the prehydrolysis and hydrolysis steps, respectively. After acid separation, the sugar solutions are fermented to ethanol and concentrated by distillation. The acid recovery and distillation steps are the most expensive portions of the process; therefore, any reduction in the requirements for these steps will significantly improve the economics of the overall process. One means by which this may be accomplished is the addition of a recycle stream in the prehydrolysis step. The result will be an increased xylose concentration in the the prehydrolyzate fluid, which will reduce the acid recovery and distillation costs for each unit of ethanol produced. Thus, both capital and operating costs can be lowered while the net energy production is increased. In this investigation, the prehydrolysis step was carried out in a batch reactor at a temperature of 100 °C and a reaction time of 80 min. A 2-1. reaction vessel was used for the first two batches, each of which resulted in approx. 600 milliliters (ml) of prehydrolyzate fluid. This fluid, consisting primarily of xylose dissolved in the dilute sulfuric acid solution, was then mixed in varying ratios with fresh acid and used as the acid catalyst in later experiments to determine the effects of recycle on sulfuric acid activity and xylose concentration. In one set of recycle experiments, the activity of the recycled acid was as much as 90% of the activity of fresh acid on a volume per volume basis. Because the recycled fluid contained approx. 3% xylose, the xylose concentrations obtained in these experiments depended on the ratio of recycled acid to fresh acid. Concentrations averaged 27.5 grams per liter (g/l) for an acid residence time of 80min and 51.4 g/l for an acid residence time of 160 min. The effect of the increased concentration of xylose on the economics of a 4.5 x 106 gallon per year ethanol plant were estimated to be a 16% reduction in capital costs and a 17% reduction in operating costs. For corn stalks at $25 per ton and ethanol at $1.185 per gallon (ga), the estimated rate of return on investment would increase from 3.4% without recycle to 18.0% with recycle. © 1982.


Chemical and Biochemical Engineering

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Article - Journal

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Publication Date

01 Jan 1982