Xylan with lignin

The pentosan polysaccharides, xylan and arabinan, commonly known as hemiceUulose, are the principal precursors of furfural and are always found together with lignin and cellulose in plant materials. 

Although ASPEN-Plus is widely used to simulate petrochemical processes, its uses for modeling biomass processes are limited owing to the limited availability of physical properties that best describe biomass components such as cellulose, xylan, and lignin. For example, Lynd et al. (1) used conventional methods to calculate the economic viability of a biom-ass-to-ethanol process. However, with the development by the National Renewable Energy Laboratory (NREL) of an ASPEN-Plus physical property database for biofuels components, modified versions of ASPEN-Plus software can now be used to model biomass processes (2). Wooley et al. (3) used ASPEN-Plus simulation software to calculate equipment and energy costs for an entire biomass-to-ethanol process that made use of dilute-H2S04 acid pretreatment. 

The composition of poplar wood was usedasamodel for the feedstock composition however, as used in this simulation, the poplar is modeled as consisting of only cellulose, xylan, and lignin, with compositions of 49.47, 27.26, and 23.27%, respectively. Laboratory results for carbonic acid pretreatment are relatively scarce, so for the purpose of this comparative study, stoichiometry of pretreatment reactions was assumed to be equal to those used in the comparison model (3) cellulose conversion to glucose 6.5% xylan conversion to xylose 75 and lignins solubilized 5%. Thus, economic comparisons made with this model assess different equipment and operating costs but not product yields. For the successful convergence of the carbonic acid model, the simulation required initial specification of several variables. These variables included initial estimates for stream variables and inputs for the unit operation blocks. 

Cellulose pyrolysis in the presence of salts, acids, or bases may be used as a model of cellulose pyrolysis in a wood matrix. A series of differences are usually seen during pyrolysis of microcrystalline cellulose and wood, even considering only those components of wood that are generated by cellulose pyrolysis. The differences can be explained by the influence of inorganic components in wood, but also by the presence of chemical bonding of cellulose with lignin and hemicelluloses (xylans, etc.). 

Figure 2.8. (a) Hemicelluloses with long persistence lengths are relatively inflexible and can only snake between microfibrils, (b) Schematic assembly of polydiverse xylans and lignins during secondary wall formation of a hardwood, (c) Microfibrils cluster in arrays surrounded by wider regions of matrix material (schematic from spruce wood). 

Enzymes that can be harnessed for the breakdown of hemicellulose in cereal crops, and crop fiber biomass are becoming increasingly important because of their pivotal role in the utilization of these renewable energy sources. Hemicelluloses (xylans, aiabinoxylans) are widely found as stmctmal components in plant cell walls, where they cross-link with lignin and are extensively hydrogen-bonded to cellulose [1]. Structurally, xylans are heteropolysaccharides consisting of a linear P-d-(1— 4)-linked xylopyranoside backbone that. 

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