Technology
Waste cellulose possible glucose source Fungus-based process could provide new way to dispose of municipal wastes and also be source of valuable chemical raw material The U.S. Army's Natick, Mass., re search laboratories are about to begin pilot plant studies of a process that converts cellulose wastes to glucose. According to Leo A. Spano, manager of the labs' pollution abatement program, the process is the result of work origi nally aimed at overcoming destructive effects of the fungus Trichoderma vir ide in the South Pacific in World War II. The fungus is such an efficient con verter of cellulose that it is now consid ered a principal source of cellulase en zymes, which are the active agents in cellulose conversion. Success in the pilot studies simultaneously will pro vide both a new way to dispose of mu nicipal and agricultural wastes and a source of a valuable chemical raw ma terial. The annual world production of cel lulose is about 100 billion tons. The photosynthetic energy represented by this renewable resource exceeds even the wildest estimates of world energy needs in the foreseeable future. Utiliz ing the energy stored in cellulose, how ever, has never been successful, at least on a large scale and in a conve nient way. The conversion of cellulose is limited to animal metabolism and inefficient combustion. The utilization of cellulose is greatly simplified if the cellulose is first hydrolyzed to its monomer, glucose. Once the glucose is available, it can be converted to chem ical raw materials, single-cell proteins, fuels, and other products. This is the significance of the Army's process, namely that an efficient way to hydrolyze glucose has been achieved on the lab scale. The enzymic conversion of the cellu lose takes place at 50° C. and atmo spheric pressure. The key development in the process is production of highquality cellulase enzymes. During the past 20 years, workers at the Natick laboratories have developed mutant strains of the fungus that produce two to four times as much cellulase en zymes as the original strain. The evi dence also suggests to Mr. Spano that even higher enzyme yields are possible. 20
C&ENMay27, 1974
Because of the high specificity of the enzyme, it is not chemically affected by impurities in the cellulose reactor. Neither is it affected by the kind of cellulose. After many trials with differ ent kinds of cellulose—for example municipal wastes, straw, and peanut wastes—the chemists at Natick select ed newspapers as typical of the avail able variety of cellulose. The pilot plant will use newspapers as the cellu lose feed, after shredding and ball mill ing to particle sizes of about 50 mi crons. The product of the reaction is a glu cose syrup in concentrations of 2 to 10% by weight. The impurities form a black, sticky residue that dries to a hard, unwettable cake. The cake is
cules are strongly interconnected with hydrogen bonds so that many of the glucosidic linkages are not easily acces sible to the large enzyme molecules. The combination of cellulase enzymes that are used in the process include those that are adept at reducing bonds between cellulose chains and those that convert the active forms of cellu lose to glucose. Finer milling appears to increase rates and yields by me chanically breaking up the cellulose chains. In the case of newspapers, about 70% of the bulk is available cel lulose. In some tests run thus far, bulk reduction of more than 50% has been achieved. Despite the high specificity of the enzymes, there is also some evi dence that the presence of noncellulos-
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chiefly lignin, which can be burned as a fuel or used as a source of chemicals. As technical feasibility has already been demonstrated in the lab, the pilot plant will be used to obtain the neces sary engineering and economic data needed for the design of a demonstra tion plant. The initial capacity of the pilot plant will be 1000 pounds of cel lulose per month. The projected dem onstration plant will be designed to handle 200,000 pounds per month. Among the unknowns in the process are the final yields of glucose and the reaction rates that can be expected in the demonstration plant. In the labora tory tests made thus far, the overall hydrolysis is considered to be a series of reactions that are sensitive to sub strate conditions. The cellulose mole
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ic impurities hinders the reaction through unfavorable mass transfer ef fects of the impurities, particularly in concentrated reaction mixtures. This has also delayed the design of a contin uous process, even though such a pro cess is considered feasible. With President Nixon calling for U.S. energy self-sufficiency in the 1980's, Mr. Spano advised the Ameri can Defense Preparedness Association in March that the new process is prac tically achievable on a very large scale by 1980. Considering cellulose to be one of the few rapidly renewable re sources, it follows that the appeal of an efficient process for the production of fuels and chemicals extends to the long term beyond the depletion of fossil fuels.
