Conversion processes, cellulose

Foust, T.D., Aden, A., Dutta, A., Phillip, S. (2009). An economic and environmental comparison of a biochemical and a thermochemical lignocellulosic ethanol conversion processes. Cellulose, 16, 547-565. 

Milling breaks the outer cellulose protective wall around the kernel and exposes the starch to the cooking and conversion processes. Distillers require an even grind with as small a particle size as can be physically handled by the facility. 

Conversion to cellulose II and cellulose III via caustic mercerization and Hquid ammonia treatment are commercial textile processes that are discussed later. Figure 7 shows the characteristic diffractograms (CuKa radiation) of native cellulose, cellulose mercerized with sodium hydroxide, and cellulose treated with Hquid ammonia. 

Over the past two decades, considerable interest has been directed toward the conversion of cellulosic biomass (such materials as wood wastes, bagasse, and straw) into useful products, notably fuels. Several procedures, including fermentation, gasification, liquefaction, and pyrolysis, have been commercially applied to carbohydrates with various degrees of success. In order to use the polysaccharides present in lignocel-lulosic materials as a substrate in fermentation processes, pretreatments are necessary, such as with steam (under slightly acid conditions) or... 

Cost sensitivity studies have shown that the successful commercialization of cellulase-based processes, such as the conversion of cellulose to fermentable sugars, is highly dependent on the cost of enzyme production (i). Because fungal -D-glucosidase (EC 3.2.1.21) is the most labile enzyme in this system under process conditions (2), and k to efficient saccharification of cellulose, this enzyme was targeted for application of stabilization technology, both through chemical modification and immobilization to solid supports. 

This paper is concerned with the potential for production of liquid fuels from biomass in Canada. To this end, the availability and cost of wood wastes, surplus roundwood, bush residues, energy plantation trees, and municipal solid wastes (mostly cellulosic) are assessed and promising thermal, chemical and biochemical conversion processes reviewed. 

Abstract Polyfunctionality of carbohydrates and their low solubility in conventional organic solvents make rather complex their conversion to higher value added chemicals. Therefore, innovative processes are now strongly needed in order to increase the selectivity of these reactions. Here, we report an overview of the different heterogeneously-catalyzed processes described in the literature. In particular, hydrolysis, dehydration, oxidation, esterification, and etherification of carbohydrates are presented. We shall discuss the main structural parameters that need to be controlled and that permit the conversion of carbohydrates to bioproducts with good selectivity. The conversion of monosaccharides and disaccharides over solid catalysts, as well as recent advances in the heterogeneously-catalyzed conversion of cellulose, will be presented. 

Cellulosic Materials. Over 900 x 106 metric tons of carbohydrate-containing cellulosic wastes are generated annually. The technology for converting this material into ethanol is available, but the stoichiometry of the process is disadvantageous. Even if each step in the process of the conversion of cellulose to ethanol proceeded with 100% yields, almost two-thirds of the mass would disappear during the sequence, most of it as carbon dioxide in the fermentation of glucose to ethanol. This amount of carbon dioxide leads to a disposal problem rather than to a raw material credit (209). 

Cellulose is the most abundant renewable resource available for con- version to fuel, food, and chemical feedstocks. It has been estimated by Ghose (11) that the annual worldwide production of cellulose through photosynthesis may approach 100 X 109 metric tons. As much as 25% of this could be made readily available for the conversion processes. A significant fraction of the available cellulose, i.e., 4-5 X 109 t/year, occurs as waste, principally as agricultural and municipal wastes. Cellulose must be viewed, therefore, as an important future source of fuel, food and chemicals (see Table I). 

Figure 1. The centrality of cellulose hydrolysis in the conversion processes...

A great amount of time, money and effort is being devoted to the use of cellulose as a feedstock for the production of ethanol. The studies incorporate chemical or enzymatic conversion of the cellulose to glucose and the conversion of this to ethanol with yeast (Saccharomyces) or bacteria (Zymomonas). However, a third process is presently under development at Massachusetts Institute of Technology whereby the direct conversion of cellulose to ethanol is being attempted without a separate hydrolysis step. ... 

At high concentrations, corrosion-resistant reactors and an effective acid recovery process are needed, raising the cost of the intermediate glucose. Dilute acid treatments minimize these problems, but a number of kinetic models indicate that the maximum conversion of cellulose to glucose under these conditions is 65 to 70 percent because subsequent degradation reactions of the glucose to HMF and lev-ulinic acid take place. The modem biorefinery is learning to exploit this reaction manifold, because these decomposition products can be manufactured as the primary product of polysaccharide hydrolysis (see below). 

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