Separate Hydrolysis and Fermentation (SHF)
In the separate hydrolysis and fermentation method aqueous sugar solution from the enzymatic or acid hydrolysis is taken for the fermentation. The main advantages of this method are that each step can be processed at its optimal operating conditions, and separate steps minimize interaction between the steps. The disadvantages of the SHF method are that the end product inhibition minimizes the yield of ethanol, and there is a chance of contamination due to the long period processes [18-20].
The supernatant from enzymatic hydrolysis of lignocelluloses can contain both 6-carbon (hexoses) and 5-carbon (pentoses) sugars when both cellulose and hemicellulose are hydrolyzed. Depending on the lignocellulose source, the hydrolyzate typically consists of glucose, xylose, arabinose, galactose, mannose, fucose, and rham — nose. According to theoretical calculations one ton of 6-carbon polysaccharides, glucan, galactan, or mannan yields 1.11 tons of six-carbon sugars and could be fermented into 172.0 gallons of bioethanol [21], and fermentation of the representative 6-carbon sugars like glucose can be represented by the equation:
C6H12O6 ^ 2 CH3CH2OH + 2 CO2 (8.1)
Similarly, one ton of 5-carbon polysaccharides, xylan or arabi — nan, yields 1.14 tons of 5-carbon sugars and could be fermented theoretically into 176.0 gallons of bioethanol [21]. Fermentation of 5-carbon sugars like xylose can be represented by the equation:
3 C5H10O5 ^ 5 CH3CH2OH + 5 CO2 (8.2)