A comparative study on chemical conversion of cellulose between the batch-type and flow-type systems in supercritical water

Microcrystalline cellulose (avicel) was treated in supercritical water using batch- type and flow- type systems. The flow- type system made it possible to shorten the heating, treating and cooling times, compared with the batchtype system. As a result, the flow- type system was able to liquefy avicel without producing any supercritical water- insoluble residue. Although hydrolyzed products such as glucose and fructose, and pyrolyzed products such as levoglucosan, 5- hydroxymethyl furfural, erythrose, methylglyoxal, glycolaldehyde and dihydroxyacetone were found in common from the water- soluble portion treated by both systems, the flow- type system gave a water-soluble portion with more hydrolyzed and less pyrolyzed products, together with water- soluble oligosaccharides consisting of cellobiose to cellododecaose and their decomposed products at their reducing end of glucose, such as [beta-D-glucopyranosyl](1-11) beta-D-levoglucosan, [beta-D-glucopyranosyl](1-11) beta-D-erythrose and [ beta-D-glucopyranosyl] (1-11) beta-D-glycolaldehyde. In addition, the precipitates of polysaccharides were recovered after 12 h setting of the water- soluble portion. These results indicated that the flow- type system can hydrolyze cellulose with minimizing pyrolyzed products. On the other hand, the batch- type system resulted in a higher yield of the pyrolyzed products due to the longer treatment, but a higher yield of glucose due possibly to the higher pressure and concomitantly higher ionic product of water. Based on these lines of evidence, the process to increase the yield of the sugar is discussed under supercritical water treatment.