Bioconversion of D-xylose into ethanol by Clostridium thermosaccharolyticum

Bioconversion of lignocellulosic biomass, in particular sawdusts, into ethanol was investigated. Initially, acid hydrolyses at 124°C by dilute sulfuric acids using 0.5, 1.0, and 2.0 weight percent were performed. The optimum hydrolysis condition was found to use 1% acid for 50 minutes hydrolysis time. The total yields of sacchrification, primarily D-xylose, at the optimum condition is 23.7 [g/l]. Secondly, fermentation of two kinds of substrates; (i) a standard CM-4 growth medium, D-xylose as carbon source, and (ii) the hydrolyzate from the sawdusts, primarily C₅-sugars, into ethanol were investigated using Clostridium thermosaccharolyticum (ATGO 31925) in a batch reactor and a continuous up-flow cylindrical reactor. For the continuous up-flow reactor, cells immobilized on polystyrene chips (size 2.79x3.78x2.64 mm) and cells freely suspended in the flowing substrate stream cases were studied with 17.5, 29.5, and 51.2 [ml/hr] flowrates for the immobilized case and with 27.3, 42.2, 66.1, and 109 [ml/hr] flowrates for the freely suspended case. The maximum yield of ethanol obtained from the initial concentration of 6 [g/l] D-xylose was 1.82 [g/l] in a batch reactor. If the dilution rate was less than 0.37 [hr^-1], the yields of the continuous up-flow reactor for both cases approached the maximum of a batch reactor.

Fermentation kinetic parameters for the Monod/Michaelis-Menten model were also obtained for both batch and continuous up-flow reactors. The maximum specific growth rate for the microorganism in CM-4 medium in this investigation was found to be 0.43 [hr^-1].