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Über dieses Buch

Recent economic trends, especially the worldwide decline in oil prices, and an altered political climate in the United States have combined to bring about major reductions in research on renewable energy resources. Yet there is no escaping the "facts of life" with regard to these resources. The days of inexpensive fossil energy are clearly numbered, the credibility of nuclear energy has fallen to a new low, and fusion energy stands decades or more from practical realization. Sooner than we may wish ,we will have to turn to renewable raw materials - plant "biomass" and, especially, wood - as significant suppliers of energy for both industry and everyday needs. It is therefore especially important to have a single, comprehensive and current source of information on a key step in any process for the technological exploitation of woody materials, cellulose hydrolysis. Further­ more, it is essential that any such treatment be unbiased with respect to the two methods - chemical and biochemical - for the breakdown of cellulose to sugars. Researchers on cellulose hydrolysis have frequently been chided by persons from industry, especially those individuals concerned with determining the economic feasibility of various technological alternatives. They tell us that schemes for the utilization of wood and other such resources fly in the face of economic realities.

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract
Cellulose is an abundantly available carbohydrate polymer in nature. This polymer is continually replenished by photosynthetic reduction of carbon dioxide catalyzed by sunlight. The estimated volume of existing cellulosic resources is 324 billion cubic meters. Furthermore, the annual net yield of photosynthesis is 1.8 trillion tons of biodegradable substances, about 40% of which is cellulose. The abundance coupled with the renewability renders cellulose to be the most promising feedstock for production of energy, food, and chemicals. In addition, the nature’s balance and aesthetic value are not diminished by its use as feedstock [4]. Only a small fraction of cellulosic resources are currently utilized to manufacture products such as lumber, fuel, textiles, paper, plastics, films, foils, explosives, varnishes, thickeners, and glues. Moreover, easily harvestable trees, which are rich in cellulose, tend to be exploited for this purpose [2].
Liang-tseng Fan, Mahendra Moreshwar Gharpuray, Yong-Hyun Lee

2. Nature of Cellulosic Material

Abstract
The hydroysis of native lignocellulosics, especially that catalyzed by enzyme, is a slow process. The heterogeneous degradation of lignocellulosics is governed primarily by their structural features since (1) cellulose present in biomass possesses a highly resistant crystalline structure, (2) lignin surrounding the cellulose forms a physical barrier, and (3) the reactive sites available are limited. The cellulose present in lignoeellulosics is composed of crystalline and amorphous components [7]. The amorphous component is usually more reactive than the crystalline component, and thus any means that will increase the amorphous content will enhance the hydrolysis rate [12,16,17, 50]. The presence of lignin forms a physical barrier for attack by either enzyme or acid molecules; therefore, treatments causing disruption of the lignin seal will increase the accessibility of cellulose to enzyme or acid molecules and eventually its hydrolysis rate. The limitation of available reactive sites stems from the fact that the average size of the capillaries in biomass is too small to allow the entry of reactive molecules, especially the large enzyme molecules.
Liang-tseng Fan, Mahendra Moreshwar Gharpuray, Yong-Hyun Lee

3. Enzymatic Hydrolysis

Abstract
Enzymatic hydrolysis of cellulose accomplishes degradation of cellulose to glucose. This heterogeneous catalytic reaction is typically characterized by an insoluble reactant (cellulose) and a soluble catalyst (enzymes) [150]. The rate of this reaction is influenced by both structural features of cellulose and mode of enzyme action. Unfortunately, the enzymatic hydrolysis of native cellulose proceeds at an extremely low rate, and therefore, its pretreatment prior to hydrolysis is essential to enhance the rate of hydrolysis. This chapter elaborates on the nature of lignocellulosic structural resistance, properties and mode of enzyme action, different pretreatment methods, and a variety of kinetic models for enzymatic degradation.
Liang-tseng Fan, Mahendra Moreshwar Gharpuray, Yong-Hyun Lee

4. Acid Hydrolysis of Cellulose

Abstract
Cellulosic materials consist of three major components, namely, cellulose, hemicellulose, and lignin. The two modes of converting the carbohydrate components into their constituent sugars are enzymatic hydrolysis and acid hydrolysis. The former has been reviewed in the preceding chapter [38,39, 81 ]. The present chapter covers the latter with the focus on mechanism and kinetics of acid hydrolysis.
Liang-tseng Fan, Mahendra Moreshwar Gharpuray, Yong-Hyun Lee

5. Design and Economic Evaluation of Cellulose Hydrolysis Processes

Abstract
In this chapter, commercial hydrolysis processes for a variety of cellulosic materials are described; both enzymatically and acidically catalyzed processes are included. It appears, however, that the former remains largely at the conceptual or developmental stage; most of the commercial processes in operation, while rather limited in number, are said to belong to the latter. Nevertheless, the procedure for process economic evaluation and optimization of hydrolysis of cellulosic materials is illustrated with two examples from the former one utilizing relatively pure cellulose and the other lignocellulosic material as substrates, because of the future potential of the enzymatically catalyzed processes.
Liang-tseng Fan, Mahendra Moreshwar Gharpuray, Yong-Hyun Lee

6. Epilogue

Abstract
Numerous and complex factors need be taken into account in economically assessing processes for hydrolytic utilization of cellulose. Cellulose hydrolysis must compete with hydrolysis of starch from grain crops. While lignocellulosic residues are abundant, their distributions are diffused; the availability of commercially viable collection systems for them is limited. Of all crops, corn is a primary source of starch because its supply is ample, its cost is relatively low, and commerically viable systems are on hand for storing and transporting it over long distances. Cellulosic biomass, approximately at US $ 30 per dry ton currently, is far cheaper than corn approximately at US $ 100 per dry ton. Nevertheless, it is very difficult for most naturally occurring organisms to hydrolyze cellulose due to the inaccessible nature of cellulose crystallites. Consequently, a tradeoff exists between the low raw material cost and high investment cost for cellulose hydrolysis. To enhance hydrolyzability, a cellulosic biomass need be subjected to a variety of pretreat-ments; however, this increases the cost of processing.
Liang-tseng Fan, Mahendra Moreshwar Gharpuray, Yong-Hyun Lee

Backmatter

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