Solids removal, hemicellulose

Corn stover, a well-known example of lignocellulosic biomass, is a potential renewable feed for bioethanol production. Dilute sulfuric acid pretreatment removes hemicellulose and makes the cellulose more susceptible to bacterial digestion. The rheologic properties of corn stover pretreated in such a manner were studied. The Power Law parameters were sensitive to corn stover suspension concentration becoming more non-Newtonian with slope n, ranging from 0.92 to 0.05 between 5 and 30% solids. The Casson and the Power Law models described the experimental data with correlation coefficients ranging from 0.90 to 0.99 and 0.85 to 0.99, respectively. The yield stress predicted by direct data extrapolation and by the Herschel-Bulkley model was similar for each concentration of corn stover tested. 

These results could suggest that what has been traditionally been described as "biphasic" behavior may reflect a combination of chemical reaction and mass transfer effects, with each limiting xylan reaction and removal at different stages or modes of operation. This effect might be better described by a model that incorporates reaction of solids to form soluble species as a function of temperature and acid concentration coupled with a second mass transfer step that is affected by flow. On this basis, we plan to investigate whether the pore leaching model could be simplified and adapted in this way to better describe hemicellulose hydrolysis. 

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. 

Alkali treatment of cellulosic materials can remove hgnin as well as part of the hemicellulose and expose cellulose for fungi to digest. In addition, the alkah treatment method can swell and disrupt the structure of cellulose. Commonly used alkalis includes NaOH, Ca(OH)2 or CaO and aqueous as well as gaseous NH3. For solid-state fermentation, after alkah treatment, the cellulosic materials do not need to be washed, because all the solubilized hgnin and hemicellulose can be retained in the raw material [15]. After the addition of other nutrients, the pH has to be adjusted to a value of approx. 6.5 with H2SO4. 

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,... 

When hardwoods are to be used, it is necessary to remove a part of the hemicellulose first by prehydrolysis. This has been done on a large scale with straw, a substance chemically similar to hardwood, by heating it in an 8-to-l liquid-to-solid ratio, with 0.5 percent sulfuric acid for 2 to 3 hours at 130°C (266°F). Without prehydrolysis, hardwoods and straws form slimy materials, probably because of their high hemicellulose content, which prevent the flow of the hydrolyzing acid. 

Solid wt% to second pretreatment Wash water feed ratio Removal of sol. hemicellulose... 

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