Acid hydrolysis of cellulosic materials has long been practiced and is rela­tively well understood. Among the important specific factors in chemical hydrolysis are surface-to-volume ratio, acid concentration, temperature, and time. The surface-to-volume ratio is especially important in that it also deter­mines the magnitude of the yield of glucose. Therefore, smaller particle size results in better hydrolysis, in terms of the extent and rate of reaction [41]. With respect to the liquid-to-solids ratio, a higher ratio leads to a faster reac­tion. A trade-off must be made between the optimum ratio and economic feasibility because the increase in the cost of equipment parallels the increase in the ratio of liquid to solids. For chemical hydrolysis, a liquid/solids ratio of 10:1 seems to be most suitable [41].

In a typical system for chemically hydrolyzing cellulosic waste, the waste is milled to fine particle sizes. The milled material is immersed in a weak acid (0.2 to 10%), the temperature of the suspension is elevated to 180 to 230°C, and a moderate pressure is applied. Eventually, the hydrolyzable cel­lulose is transformed into sugar. However, this reaction has no effect on the lignin which is also present. The yield of glucose varies, depending upon the nature of the raw waste. For example, 84-86 wt% of kraft paper or 38-53 wt% of the ground refuse may be recovered as sugar. The sugar yield increases with the acid concentration as well as the elevation of temperature. A suit­able concentration of acid (H2SO4) is about 0.5% of the charge.

A two-stage, low-temperature, and ambient-pressure acid hydrolysis process that utilizes separate unit operations to convert the hemicellulose and cellulose to fermentable sugars was developed [42] and tested by the Tennessee Valley Authority (TVA) and the U. S. Department of Energy (DOE). Laboratory and bench-scale evaluations showed more than 90% recovery and conversion efficiencies of sugar from corn stover. Sugar product concen­trations of more than 10% glucose and 10% xylose were achieved. The inhibi­tor levels in the sugar solutions never exceeded 0.02 g/100 ml, which is far below the level shown to inhibit fermentation. An experimental pilot plant was designed and built in 1984. The acid hydrolysis pilot plant provided fer­mentable sugars to a 38 L/h fermentation and distillation facility built in 1980. The results of their studies are summarized as follows.

• Corn stover ground to 2.5 cm was adequate for the hydrolysis of hemicellulose.

• The time required for optimum hydrolysis in 10% acid at 100°C was 2 hours.

• Overall xylose yields of 86 and 93% were obtained in a bench-scale study at 1- and 3-hr reaction times, respectively.

• Recycled leachate, dilute acid, and prehydrolysis acid solutions were stable during storage for several days.

• Vacuum drying was adequate in the acid concentration step.

• Cellulose hydrolysis was successfully accomplished by cooking stover containing 66 to 78% acid for six hours at 100°C. Yields of 75 to 99% cel­lulose conversion to glucose were obtained in the laboratory studies.

• Fiberglass-reinforced plastics of vinyl ester resin were used for con­struction of process vessels and piping.