1 Perspectives on Preparation of Cellulose for Hydrolysis E D W A R D S. LIPINSKY
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Resource Management and Economic Analysis, Battelle Columbus Division, Columbus, O H 43201
Destruction of fiber and achievement of large surface areas are goals in the preparation of cellulose for hydrolysis to fermentable sugars. Pretreatments and cellulosic raw materials that are appropriate differ from those employed in fiber-conserving pulping systems. Alternative pretreatments to disrupt the lignin-hemicellulose-cellulose complex, remove extractives, and generate large surface areas are discussed. The Purdue processes yield amorphous cellulose by treatment of the raw material with Cadoxen, ferric tartrate, or sulfuric acid as solvents. The Iotech process employs explosive depressurization to remove lignin and to increase surface area while maintaining cellulose crystallinity. Plants with low lignin and extractives content and cell morphology that facilitates downstream processing are preferred in systems designed to optimize the production of fermentable sugars.
"economical hydrolysis of cellulose to obtain fermentable sugars for the manufacture of chemicals and fuels is a major goal of research and development on renewable resources. Enzymatic hydrolysis of lignocellulosic materials proceeds slowly because of steric problems. Acid hydrolysis of lignocellulosic materials leads to loss in yield through degradation. Pretreatment of lignocellulose to remove interfering constituents is a strategy that has been explored repeatedly as a means to alleviate cellulose hydrolysis problems. Several definitive reviews on pretreatment of lignocellulosic materials for improving cellulose hydrolysis (1,2,3) appeared a few years ago. More recently, two pretreatment methods (the Purdue process and the Iotech process) have been announced that claim superior performL /
0-8412-0460-8/79/33-181-001$05.75/0 © 1979 American Chemical Society
In Hydrolysis of Cellulose: Mechanisms of Enzymatic and Acid Catalysis; Brown, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
HYDROLYSIS OF CELLULOSE
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ance in preparing cellulosic materials for hydrolysis. These processes are somewhat controversial because they resemble well-established analytical techniques and well-known alternatives to xanthation for solubilizing cellulose (4,5,6). This chapter provides a framework for generating and evaluating pretreatment concepts. The Purdue and Iotech processes can be put into perspective by using this framework. This monograph is focused primarily on hydrolysis of cellulose. However, the choice of technology for cellulose hydrolysis depends both on the state of the cellulose when it reaches the hydrolysis process and on the fermentation (or other) technology to be applied to the output of the cellulose hydrolysis process. Dr. Humphrey's chapter treats the downstream fermentation technology; this chapter is concerned primarily with preparation of cellulose for hydrolysis. Chemicals and Fuels System An overview of cellulose-based chemicals and fuels systems is presented in Figure 1. In this cyclic system, demand for fuels and chemicals leads to the organization of factors of production (land, labor, capital, etc. ) for the generation of biomass for use as a renewable resource. The "plant culture" activity is called "silviculture" if trees are the desired biomass or "agriculture" if herbaceous plants are grown Numerous materials-handling activities are required to move the biomass in the field to the factory. These activities include harvesting, storage, transportation, and, possibly, preservation activities. Typically, the raw material for cellulose processing is a ligninhemicellulose-cellulose ( L H C ) complex that is not very amenable to hydrolysis. Various pretreatments involving size reduction, separation of constituents of the complex, and processes to increase the accessibility of cellulose to hydrolytic agents may be required. These activities convert a relatively intractable raw material into a cellulosic substrate. The cellulosic substrate is depolymerized by water (or possibly other agents ) so that simple sugars can be obtained. If the pretreatments totally separate cellulose from hemicellulose, the simple sugar would be glucose. Otherwise, the glucose will be mixed with other carbohydrates that may interfere with the fermentation. Application of microbiological technology to the simple sugars converts them into chemicals and fuels, which can be further transformed and formulated into such products as liquid motor fuels, plastic containers, and tires. These end-use products, when distributed, constitute the supply that consumers use to satisfy demands. Thus the cycle is complete. The overall system has an impact on cellulose hydrolysis primarily through determining which cellulosic raw materials are available and the
In Hydrolysis of Cellulose: Mechanisms of Enzymatic and Acid Catalysis; Brown, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
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Cellulose Preparation
LiPiNSKY
prices associated with them. That is, the system determines availability, quality, and cost. For example (Figure 1 ) , part of the consumed cellulose becomes a biomass resource through recycling. To the extent that hemicellulose is removed in pulping processes, the system simplifies the biomass resource for hydrolytic processing. Conversely, when paper is coated with olefin polymers, the biomass resource is rendered less suitable for hydrolysis. The cost of the biomass resource depends on its alternative uses, scale of production, and location. CONSUMPTION 1
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DEMAND ^ —
SUPPLY
PLANNING
RECYCLING
FACTORS OF PRODUCTION PLANT CULTURE BIOMASS RESOURCES
t
MATERIALS HANDLING
RAW
DISTRIBUTION
MATERIALS PRETREATMENT CELLULOSIC SUBSTRATES DEPOLYMERIZATION SIMPLE SUGARS CONVERSION CHEMICALS A N D FUELS
Figure 1.
FORMULATION
_ END USE "PRODUCTS
Overview of cellulose-based chemicals and fuels systems
In Hydrolysis of Cellulose: Mechanisms of Enzymatic and Acid Catalysis; Brown, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
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HYDROLYSIS O F C E L L U L O S E
Composition The raw material that reaches the factory is a far cry from Avicel or Solka Floe that are used by many experimenters to simulate cellulosic products (Table I). In addition to the fact that the cellulose is tied up in a lignin-hemicellulose-cellulose ( L H C ) complex, it should be recognized that the terms "lignin and "hemicellulose" are catchall terms covering a range of compositions and molecular weights.
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Table I.
Raw Materials Birch Wood Pine Wood Bagasse Wheat Straw Avicel Solka Floe a
Comparison of Selected Biomass Species with Avicel and Solka-Floc (Percentage)* HemiCellulose cellulose 40 41 40 30 100 99.5
36 27 32 50
— —
Lignin
Extractives
21 27 20 13
3 4 6 4
— —
— —
Ash
