In SHF, the hydrolysis is carried out in one vessel and the hydrolyzate is then fermented in a second reactor. The most expensive items in the overall process cost are the cost of feedstock, enzyme production, hydrolysis, and utilities. The feedstock and utility costs are high because only about 73% of the cellulose is converted to ethanol in 48 hr, and the remainder of the cel­lulose, hemicellulose, and lignin are burned or gasified. Enzyme production is a costly step due to the large amount of the enzyme used in an attempt to overcome the end-product inhibition as well as its slow reaction rate. The hydrolysis step is also expensive due to the large capital and operating costs associated with large size tanks and agitators. The most important param­eters are the hydrolysis section yield, the product quality, and the required enzyme loading, all of which are interrelated. Yields are typically higher in more dilute systems where inhibition of enzymes by glucose and cello — biose is minimized. Increasing the amount of enzyme loading can help to overcome inhibition and increase the yield and concentration, although it undoubtedly increases the overall cost. Increased reaction times also make higher yields and concentrations.

Cellulase enzymes from different organisms can result in markedly dif­ferent performances. Figure 4.11 shows the effect of yield at constant solid and enzyme loading and the performance of different enzyme loadings. Increase in enzyme loading beyond a particular point has turned out to be of no use. It would be economical to operate at a minimum enzyme loading level. Or, the enzyme could be recycled by appropriate methods. As the cel­lulose is hydrolyzed, the endo — and exoglucanase components are released back into the solution. Because of their affinity for cellulose, these enzymes can be recovered and reused by contacting the hydrolyzate with fresh feed. The amount of recovery is limited because of в-glucosidase, which does not adsorb on the feed. Some of the enzyme remains attached to the lignin and unreacted cellulose; in addition, enzymes are thermally denatured during hydrolysis. A major difficulty in this type of process is maintaining sterility; otherwise, the process system would be contaminated. The power consumed in agitation is also significant and affects the economics of this process [43]. Even though the effect of yield on the selling price of ethanol in the figure was based on more classical ethanol production processes, it does explain the importance of yield on the final product cost.

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Um and Hanley [69] carried out an experimental study on high-solid enzy­matic hydrolysis and fermentation of Solka Floc into ethanol. To lower the ethanol distillation cost of fermentation broths, a high initial glucose concen­tration is desired. However, an increase in glucose concentration typically reduces the ethanol yield due to the decreased mass and heat transfer rate. To overcome the incompatible temperatures between the enzymatic hydro­lysis (50°C) and fermentation (30°C), saccharification, followed by fermenta­tion (SFF), was employed with relatively high solid concentrations (10% to 20%) using a portion loading method. Glucose and ethanol were produced from Solka Floc, which was first digested by enzymes at 50°C for 48 hours, followed by fermentation. In this process, commercial enzymes were used in combination with a recombinant strain of Zymomonas mobilis. The highest ethanol yields of 83.6%, 73.4%, and 21.8%, based on the theoretical amount of glucose, were obtained with substrate concentrations of 10%, 15%, and 20%, respectively. These values also correspond to 80.5%, 68.6%, and 19.1%, based on the theoretical amount of the cell biomass and soluble glucose present after 48 hours of SFF. In addition to the substrate concentration effects, they also investigated the effects of reactor configurations [69].

As a classic study of the mechanism of the enzymatic hydrolysis of cel­lulose, Fan, Lee, and Beardmore investigated the effects of major structural features of cellulose on enzymatic hydrolysis. They found that the hydrolysis rate is mainly dependent upon the fine structural order of cellulose which can be best represented by the crystallinity rather than the simple surface area [76].