Hydrolysis

FIGURE 3.4

Hydrolysis of cellulose.

sugars can be subsequently converted to ethanol using appropriately selected micro-organisms via a process called fermentation. The fermenta­tion of ethanol from 6-carbon sugars (such as D-glucose) follows the stoichio­metric equation.

Fermentation

2CH3CH2OH + 2CO2

According to the stoichiometric equation, one mole of D-glucose produces 2 moles of ethanol and 2 moles of carbon dioxide. Considering the molecu­lar weights of glucose, ethanol, and carbon dioxide being 180, 46, and 44, respectively, the maximum theoretical yield of ethanol by weight % from the process would be 92/180 = 51%. Nearly half the weight of the glucose 88/180 (49%) is converted to carbon dioxide at its theoretical maximum. As such, a significant amount of carbon dioxide is generated by the fermentation step, which needs to be captured or utilized for economically beneficial purposes.

Hemicellulose is made up of the 5-carbon sugar, xylose, arranged in chains with other minor 5-carbon sugars interspersed as side chains. Similarly to the cellulose case, the hemicellulose can also be extracted from the plant material and treated to liberate xylose that, in turn, can be fermented to produce ethanol. However, xylose fermentation is not as straightforward or efficient as glucose fermentation based on currently available technology. Depending on the micro-organism and conditions employed, a number of different fermentation paths are possible or conceivable. The array of prod­ucts can include ethanol, carbon dioxide, and water as

Fermentation

2CH3CH2OH + CO2 + H2O

Actually, three different reactions have been documented with yields of ethanol ranging from 30 to 50% of the weight of xylose as the starting mate­rial (i. e., weight ethanol produced/weight xylose). They are:

3 Xylose ^ 5 Ethanol + 5 Carbon Dioxide Xylose ^ 4 Ethanol + 7 Carbon Dioxide Xylose ^ 2 Ethanol + Carbon Dioxide + Water

The first reaction yields a maximum of 51% (= 5 * 46/(3 * 150)), the second 41% (= 4 * 46/(3 * 150)), and the third 61% (= 2 * 46/150), respectively. Although the maximum theoretical ethanol yields from these fermentation reactions range between 41 and 61%, the practical yields of ethanol from xylose as starting material are in the range of 30 to 50%.

In the discussion of potential yields of ethanol from various starting mate­rials, two different ranges of efficiencies of hemicellulose-to-xylose conver­sion and xylose-to-ethanol conversion have been combined to provide an overall conversion efficiency of hemicellulose to ethanol of about 50%. Just as with the glucose fermentation, the conversion of carbon dioxide to value — added products would vastly improve the overall process economics of etha­nol production, because the yield of carbon dioxide is not only significant in amounts but also inevitable. It must be noted that even though xylose fermentation to ethanol is also mentioned in this chapter, the main focus of this chapter is on glucose fermentation, more particularly corn sugars into ethanol. Ethanol-from-corn technology involves glucose fermentation, not xylose fermentation, as required in cellulosic ethanol technology. Xylose fer­mentation or hemicellulose fermentation is treated in depth in Chapter 4.