Liquid hot water pretreatment process Essay

Liquid hot water pretreatment (LHW) process is similar to steam explosion but uses water in the liquid state at high temperatures instead of steam. The terms solvolysis, hydrothermolysis, aqueous fractionation, and aquasolv have been used to describe LHW for pretreatment of biomass. This process results in lignin removal and hemicellulose hydrolysis rendering cellulose in the biomass more accessible. It avoids the formation of fermentation inhibitors that occur at higher temperatures (Yang and Wyman, 2004). In this process, lower temperatures  optimum between 180190 C for corn stover and low dry matter about 18% content are used resulting in the production of more poly and oligosaccharides. The temperature of 160190 C for pH controlled LHW pretreatment and 170230 C have been reported depending on the severity of the pretreatment (Bobleter 1994 Wyman et al. 2005). LHW pretreatment has been conducted in three different types of reactor configurations  cocurrent, counter current and flow through reactor configuration depending on the direction of the flow of the water and biomass into the reactor. The water and biomass are brought in contact for up to 15 minutes at temperatures of 200230 C. The hot water cleaves hemiacetal linkages thus liberating acids during biomass hydrolysis, which facilitates the breakage of ether linkages in biomass (Antal 1996). Mosier et al. (2005b) stated that the cleavage of Oacetly groups and uronic acid substitutions on the hemicellulose could help or hinder LHW pretreatment, because the release of these acids helps to catalyse the formation and removal of oligosaccharides, or further hydrolyze hemicellulose to monomeric sugars, which can be subsequently degraded to aldehydes i.e. furfural from pentoses and 5hydroxymethyl furfural from hexoses that inhibit microbial fermentation (Palmqvist and HahnHgerdal 2000a). The formation of monosaccharides and the subsequent degradation products that further catalyze hydrolysis of cellulosic material during LHW pretreatment can be reduced by maintaining the pH between 4 and 7 (Kohlmann et al. 1995). The flow through reactor configuration in which hot water is passed over a stationary bed of lignocellulose was reported by Yang and Wang (2004) to be the more effective configuration for removing hemicellulose and lignin at same severity. High lignin solubilization hinders recovery of hemicelluloses (Mok and AntalJr 1992, 1994). A catalyst such as an acid can be added making the process similar to dilute acid pretreatment. But catalytic degradation of sugars results in undesirable side products. During pretreatment the pH and pKa of water is affected by temperature, so potassium hydroxide is used to maintain the pH above 5 and below 7 to reduce the formation of monosaccharides that are degraded to fermentation inhibitors (Mosier et al. 2005b Weil et al. 1998).

LHW pretreatment has been studied at lab scale, and pH controlled LHW pretreatment is considered for largescale pretreatment of corn fibre. Mosier et al. (2005a) have reported successful pretreatment of corn fibre slurry in a 163 L/min reactor with a 20 minutes residence time, showing that possibility of scalingup LHW pretreatment and its practicability in pretreating large quantities of corn fibre.