Ethanol Production by Acid Hydrolysis and Fermentation

Although hardwoods have a lower lignin content and so give a higher yield of sugars, softwoods are preferred where ethanol is the desired end product. This is because hardwoods have more pentose sugars, which are not readily fermented by common yeasts. In a typical batch process sawdust and wood chips are loaded into the reactor vessel and treated with dilute sulphuric acid (0.5% concentration) at temperatures between 130 and 200°C for about three hours. Ideally, the sugars should be removed from the reaction zone before they, in turn, have time to break down. 

New developments aim to increase the yield further, by relying on fast ( 1 min), high temperature ( 240°C) and continuous hydrolysis. At higher temperatures the rate of hydrolysis of cellulose increases more rapidly than does the rate of degradation of the newly formed sugars so it should be possible to obtain a slightly better yield. 

The hexose sugars, glucose and mannose, are subsequently converted to ethanol by fermentation with a yeast such as Saccharomyces cerevisiae within the first 12 hours at 35°C  

The process described by Burton et al. (1984) uses proven technology and demonstrates the point that the production of ethanol from wood is likely to be viable only when integrated as a multiproduct operation. The obvious areas for improvement are in increasing the ethanol yield and in encouraging fermentation to continue as the concentration of ethanol builds up in the solution, whieh would significantly reduce the cost of distillation. 

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