In this case the hydrolysis of cellulose is carried out separately from the fermentation but if the products of hemicellulose are to be included a second bioreactor is used to

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ferment pentose sugars such as xylose. The use of a separate bioreactor gives a higher yield of ethanol and less energy is required. Considerable efforts have been made to improve the yield of ethanol using the normal yeast fermentation. A number of process changes have been investigated in order to improve the economics of etha­nol production. The traditional method of fermentation has been batch culture in a non-stirred cylindro-conical vessel where the sugar and salts are inoculated with yeast and the fermentation allowed to proceed until the sugar is exhausted. Other forms of bioreactor operations and designs have been investigated in order to improve ethanol productivity, as this affects the cost of the final product. The fer­menter can be operated in a batch-fed mode where batches of fresh medium are added at times during the fermentation. This avoids substrate inhibition where a high substrate concentration at the beginning of the fermentation may inhibit growth. In the continuous mode medium is added continuously throughout the fer­mentation and cells and medium removed at the same rate to keep the volume in the fermenter the same. This allows the operator to run the fermenter for a long period without having to waste time cleaning, refilling and sterilizing the fermenter. Fermenters of various designs where cell recycling has been used have considerably higher productivity which is due to maintaining a high cell density throughout the process.

The traditional fermentation vessel is not stirred but stirred tanks can be used to give good mixing and a more rapid growth rate. Alternative fermenter designs have been tested in order to improve the rate of growth and ethanol production. The tower fermenter is just an elongated tank with a high aspect ratio. The fluidized bed fer­menter operates by mixing the cells by pumping the medium up through the base of the fermenter, thus fluidizing the cell mass at the bottom of the tank. A membrane fermenter keeps the cells separate from the medium with a semi-permeable membrane. This allows the fermenter to retain a high cell density and thus a higher rate of etha­nol production. Examples of the productivity of these various systems are given in Table 6.9.

Another method for maintaining a high cell density is to immobilize the cells on or in some form of support. This retains the cells within the bioreactor at a high density and allows for a continuous process. Examples of immobilized cells are given in Table 6.9. It is difficult to compare results as the glucose used differs in concentra­tion but it is clear that cell recycling results in increased productivity.

Simultaneous saccharification and fermentation (SSF)

One of the most important advances in ethanol production was the development of simultaneous saccharification and fermentation (SSF). In this system, yeast ferments the glucose produced by the cellulase enzymes in the same vessel and at the same time. The cellulase enzymes therefore do not suffer from feedback inhibition from their products glucose and cellobiose as the fermentation removes these inhibitors. This increases hydrolysis rates, reduces enzyme levels, shortens process time and requires smaller bio­reactor volumes. The drawbacks to the system are the differences in the optimal condi­tions for the enzymes and yeasts which reduces the process rate. The cellulase normally operates at 40-50°C, whereas yeast fermentation is carried out at 30°C. One way of avoiding this is to use thermotolerant yeasts like Kluyveromyces marxianus. The use of yeasts that can assimilate pentoses, for example Candida acidothermophilum, C. brassicae and Hansenula polymorpha, would also improve the process.

The hydrolysis can be carried out by an enzyme mixture or enzyme-producing microorganisms. The organisms used in SSF are often T. reesei which provides the enzymes and S. cerevisiae for the fermentation, run at a temperature of 38°C. The temperature is a compromise between yeast optimum of 30°C and the hydrolysis optimum of 45-50°C. The major advantages of SSF have been found to be increase in hydrolysis due to the reduction in feedback inhibition, lower enzyme or organism requirement, higher product yield, less contamination as sugar levels are kept low, shorter process time and smaller bioreactor volumes.