Pretreatment processes, cellulose

Pretreatments of the polymeric feedstocks to enhance the hydrolysis of the major components such as cellulose include size reduction, thermal-chemical treatments, and specific enzyme pretreatments (52,57). Although many of the pretreatment processes increase the rate and extent of potymer hydrolysis, the cost for treatment is prohibitive based upon the revenues generated for energy production alone in anaerobic digestion (32). 

These compositional considerations necessitate attention to raw materials evaluation early in process research and development to (1) establish an in-depth knowledge of the principal raw material, including its composition and history of materials handling prior to arrival at the factory, (2) investigate the effects of seasonal variations and storage under various conditions on composition and on hydrolytic performance, and (3) explore alternative pretreatment processes for upgrading the composition and hydrolytic performance of available raw materials. The resource that is for sale is not pure cellulose it is a complex mixture containing cellulose caveat emptor. 

The approach "select favorable raw material has a major impact on the selection of pretreatment processes. For example, the poplar responds splendidly to many pretreatments that fail with Douglas fir or pine-based materials (I). Specific tissues and cells of a given biomass raw material will respond quite differently. For example, the rind fiber of sugarcane bagasse behaves quite differently from the pith fiber (11)- In woody species, the selection of tissues low in bark and extractives is an important factor in the ease or resistance to cellulose hydrolysis. Before embarking on development of processes for hydrolysis of a biomass resource, it is highly desirable to exercise discretion with respect to the choice of raw materials at both the species and tissue levels. This idea is all the more important in an initial choice of species and pretreatment process. 

Polyisoindolenines, which are high-molecular Phthalogen developers and are complexes of heavy metals and indolenine, are developed with reducing agents in a wet treatment. The polyisoindolenine complexes are fiber-affinitive and can be applied by using the exhaustion process. Their substantivity is low but can be increased by pretreating the cellulose with anionic products. Reductive development is performed in an alkaline medium with hydrosulfite. For details, see [82, 83],... 

AFEX [Ammonia Fiber Explosion (or Expansion)] A pretreatment process for ligno-cellulose prior to enzymatic conversion to ethanol. The wood is exposed to liquid ammonia at 60 to 100°C for a short time, and the pressure suddenly released. Invented in 1998 by B.E. Dale and M. Moniruzzaman at Texas A M University and further developed by Dale at Michigan State University. In 2006, AFEX was regarded as the leading nonenzymatic biomass pretreatment process. 

Ionic liquids have only just begun to be investigated for biomass related processes within the last 10 years, yet there are already many exciting examples of how they can be applied in this area. They have been used in cellulose functionalisation, thermochemical depolymerisation, enzymatic depolymerisation, extraction of biomass components, and biomass pretreatment processes. In a growing number of cases, ionic liquid processes have been patented, which suggests future commercial value. 

Because the hemicellulose fraction of biomass materials can be separated from lignin and cellulose by dilute acid treatment, cellulose becomes more reactive towards cellulase. Hemicellulose hydrolysis rates vary with acid concentration, temperature, and solid-to-liquid ratio. With most lignocellulosic materials, complete hemicellulose hydrolysis can be achieved in 5-10 min at 160°C or 30-60 min at 140 °C. Dilute acid hydrolysis forms the basis of many pretreatment processes for example, autohydrolysis and steam explosion are based on high-temperature dilute acid catalyzed hydrolysis of biomass. 

Hardwood and switchgrass residues after a two-stage hydrolysis still contain significant amounts of cellulose (22-47%) and hemicellulose (9-11%). Hydrothermal treating fractions containing cellulose and/or hemicellulose resulted in a high yield of residue and gases. Our experiments reveal that a better pretreatment process needs to be developed. 

A schematic flowsheet for the cellulose acetate process is shown in Figure 11.1. The cellulose sheet is broken up or disintegrated in the shredder. The shredded cellulose is conveyed to the pretreater, where acetic acid is added to pretreat the cellulose. The pretreated cellulose is... 

The purpose of the pretreatment is primarily to open up the structure of the material to facilitate access to the cellulose structure. However, a small amount of sugars from the hemicellulose may be formed already during the pretreatment process. The pretreatment process can be carried out with a variety of methods... 

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