Xylan removal

Prior to xylan removal it is possible to extract from the plant material those hemicelluloses soluble in dilute alkali. Soluble hemicelluloses of this type are the principal impurities extracted with xylan and consist for the most part of low molecular weight polysaccharides and polyuronides. Corn cob holocellulose may be freed of these compounds (called hemicellulose-B fraction, see Figure 2) by extraction with potas-... 

Xylan remaining in solid residues was also analyzed. As seen in Fig.5, increasing velocity significantly increased xylan removal, especially in the first 8 min. For example, xylan removal for operation at 2.8,5.2, and 10.7 cm/ min was 60, 70, and 82% for hot water only pretreatment of corn stover at 200°C after 8 min. However, after 16 min, the differences in xylan removal were less for all velocities run, suggesting that fluid velocity has less impact on the overall degree of hemicellulose hydrolysis. 

Table 1 shows the effect of ARP treatment on the composition and the digestibility of different feedstocks. The optimum conditions described earlier in Materials and Method were applied in these tests. The extent of delignification and xylan removal in com stover is much higher than hybrid poplar. With com stover, more than 50% xylan is lost to liquid. The glucan digestibility of ARP-treated com stover reached 90%. Xylan removal in LL and HL hybrid poplar were 22 and 33%, respectively. The glucan digestibilities of ARP-treated LL and HL hybrid poplar were 66 and 49%, respectively. 

The effect of reaction time on the performance of ARP pretreatment with the HL hybrid poplar is shown in Table 3. Lignin removal of hybrid poplar increases with reaction time. Xylan removal, however, was only slightly affected by ARP reaction time. After a certain point, the xylan content remained constant, indicating that there is a portion of xylan that is resilient and difficult to remove. The 72-h glucan and xylan digestibilities correlate more closely with xylan removal than lignin removal. 

Table 4 shows the effect of ammonia flow rate on the composition and digestibility. It is reaffirmed here that the digestibility correlates better with xylan removal than lignin removal, especially for hybrid poplar. This also indicates that delignification alone is not sufficient to attain high digestibility and that xylan removal is also required. 

In ARP pretreatment of hybrid poplar, deUgnification is increased with the treatment severity, but xylan removal occurs only to a certain extent. Increase of temperature from 175 to 195 °C in ARP does not affect xylan/lignin removal in hybrid poplar, yet the glucan digestibility of treated solid is increased. This is caused by an increase of cellulase accessibility to cellulose created by breakage of certain bonds and reconfiguration of the components in hemicellulose-lignin matrix. Crystallinity index of hybrid poplar increases after the ARP treatment. It is primarily caused by the removal of amorphous components,... 

Xylan removal increased with flow rate and temperature for both water-only and very dilute acid pretreatment, as illustirated by Figure 1. As can be seen from this figure, flowthrough pretreatment of com stover with hot water at a flow rate of 10 mL/min removes more xylan at the same temperature and time than... 

Lignin renK)val was limits to about 30% for a batch reactor without acid addition and dropped to about 20% when dilute sulfiiric acid was used, and the enzymatic digestibility of cellulose in the residual solids increased with xylan removal for both, consistent with observations by others. On the other hand, introducing flow of liquid tiuough the solids enhanced renK)val of lignin... 

Synergy with Xylan Removal and CeUulases Enzymatic Hydrolysis at High Concentrations of Biomass Solids... 

The schematic in Figure 6, illustrates the proposed crystalline and noncrystalline state of xylan in the S2 layer of fibers. The crystalline xylan state develops when deacetylation happens without xylan removal or when epitaxial... 

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