Cellulose hydrolysis step

Processes for Triacetate. There are both batch and continuous process for triacetate. Many of the considerations and support faciUties for producing acetate apply to triacetate however, no acetyl hydrolysis is required. In the batch triacetate sulfuric acid process, however, a sulfate hydrolysis step (or desulfonation) is necessary. This is carried out by slow addition of a dilute aqueous acetic acid solution containing sodium or magnesium acetate (44,45) or triethanolamine (46) to neutrali2e the Hberated sulfuric acid. The cellulose triacetate product has a combined acetic acid content of 61.5%. 

A hydrolysis step is involved in the pulp industry in order to concentrate the cellulose from wood. This uses large-scale processes whereby a liquid fraction, the lignocellulose, is formed as a by-product in the process, and contains high levels of phenolic components and their derivatives. These compounds also constitute an environmental problem due to their possible introduction into rivers, lakes, and/or seas. Chlorophenols from the cellulose bleaching process have traditionally attracted most of the interest in the analysis of industrial waste because of their high toxicity. 

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. ... 

The first step in cellulose hydrolysis is the adsorption of the enzyme onto the cellulose. The rate of adsorption depends on the viscosity and agitation of the system. In a dilute, well mixed system, adsorption equilibrium is established in about 5 min. The adsorption equilibrium is described empirically by a Langmuir Adsorption isotherm [31] ... 

Fig. 9.5 Overlay presentation for the conformational change of the Glc3 sugar ring. Panel A is for glycosylation step, and panel B is for deglycosylation step. Red color is for ES or EI2, green color is for transition states, while purple color is for Ell and EP [69]. Reprinted with permission from (Liu J, Wang X, Xu D (2010) QM/MM study on the catalytic mechanism of cellulose hydrolysis catalyzed by cellulase Cel5A from addothermus cellulolyticus, J Phys Cham B 114 1462-1470). Copyright (2010) American Chemical Society...

Economically feasible processes for biomass conversion to ethanol requires the fermentation of the sugars generated in the pretreatment and hydrolysis steps. In agricultural residues and hardwoods, xylose constitutes about 45% of the total sugars, and therefore xylose conversion to ethanol is important for high yields. Dilute acid hydrolysis of cellulosic biomass generates inhibitory compounds such as furfural, hydroxymethyl furfural, and acetic acid [1], These inhibitors affect the ability of yeasts to ferment the hydrolyzates, and therefore a detoxification step is usually included in fermenting acid... 

The surrounding hemicellulose and/or lignin are removed, and the cellulose microfibre structure is modified. The hemicellulose polymer is hydrolyzed to a large extent in the pre-treatment step whereas cellulose hydrolysis needs an additional step (acid or preferably enzymatic). 

Corn fiber is composed of approximately 20% starch, 35% hemicellulose, 18% cellulose, 11% protein, 3% oil, and 6% ash. The hemicellulose is primarily composed of xylose (-55%) and arabinose (-36%). Corn fiber can be fractionated using combinations of enzymatic and thermochemical (heat plus acid or base) methods. Many different hydrolysis schemes have been carried out on the corn fiber to obtain various industrially useful components. The hydrolysis step creates mainly oligosaccharides, with lower concentrations of monosaccharides and degradation products. These oligosaccharides are further hydrolyzed to monosaccharides by a secondary hydrolysis. In this experiment, the corn fiber was hydrolyzed at 30% solids at 140°C for 30 minutes followed, optionally, by an acid hydrolysis or enzyme hydrolysis. The most optimal hydrolysis method was the initial thermochemical hydrolysis method without the further acid or enzyme hydrolysis. The glucose and xylose monosaccharides present in the hydrolysates were fermented to ethanol by a recombinant strain of Saccharomyces cerevisiae 424A to produce up to 58 g/L of ethanol in the fermentation broth. 

The most commercially feasible hydrolysis method is the initial hydrolysis at 140°C for 30 minutes. This method hydrolyzes most of the starch and 72% of the hemicellulose. This leaves the cellulose and a portion of the hemicellulose to act as a carrier for the corn steep liquor and stillage to make corn gluten feed. The additional acid hydrolysis step creates a large amount of degradation products, which would be inhibitory to the ethanol fermentation, without giving a greater monosaccharide concentration. The enzyme hydrolysis step is not feasible without enzymes that contain activities specifically for the corn fiber hemicellulose matrix. These types of enzymes are not commercially available, or would be prohibitively expensive, therefore, enzyme hydrolysis of corn fiber is not currently commercially feasible. 

Fig. 5 Cascade reaction from cellulose to LA in water. Required catalysts are indicated. Bottlenecks are usually in step 1 (hydrolysis of solid cellulose) or step 3 (retro-aldol)...

Concentrated sulfuric acid has been used to dissolve and hydrolyse native cellulose (see Figure 7.6). The concentrated acid can disrupt hydrogen bonding between the cellulose chains and thus decrystallize the eellulose. Then, water is added to rapidly hydrolyse cellulose into glucose. The diluted sulfuric acid is re-concentrated for the next eyele of decrystallization and hydrolysis steps. The final produets inelude a mixture of C5 and C6 sugars. The hydrolysis proeess is generally more complex than pyrolysis or liquefaction. However, hydrolysis enables selective decomposition of the biomass polymers and thus provides access to useful platform chemicals that are unavailable from pyrolysis or liquefaction techniques. 

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