As ethanol builds up in the fermenter it will begin to inhibit the growth of many microorganisms, and some microorganisms are very sensitive to ethanol (Cardona and Sanchez, 2007). To ameliorate this inhibition four methods for the removal of ethanol during fermentation have been used: vacuum, gas stripping, membranes and liquid extraction (Figs 6.11 and 6.12).

Ethanol has a boiling point of 78°C so that applying a vacuum to the fermenter will remove ethanol from the medium. This technique has not been widely used but in

Concentrated ethanol

►

Vacuum chamber

Feed

Cells

(b) Fermentation with gas stripping

Ethanol + carbon dioxide

Condenser

Fig. 6.11. Ethanol removal from fermentation by (a) vacuum and (b) gas stripping. (Redrawn from Cardona and Sanchez, 2007.)

one case a 12-fold increase in ethanol production was obtained. Gas stripping involves passing a gas, in many cases carbon dioxide, through the culture in a separate column to sweep out the ethanol (Fig. 6.11b). More complex arrangements have been designed and pilot plant units have been operated for some time. The use of ceramic membranes for the separation of ethanol from the growth medium has been investigated using a separate unit from the fermenter known as a pervaporation unit. The use of membranes to immobilize cells can also be combined with ethanol removal. It has been shown that pervaporation can reduce the cost of ethanol by 75% as much of the cost is associated with distillation.

Ethanol can be extracted from the fermentation medium by using a bio — compatable solvent such as aliphatic alcohols я-dodecanol, oleyl alcohol, and dibutyl

126

Ґ

v______

Decanter

To distillation

Aqueous

phase

Fig. 6.12. The removal of ethanol by (a) membrane (pervaporation) and (b) liquid extraction. (From Cardona and Sanchez, 2007.)

phthalate. The solvent can be added to the fermentation and the aqueous and water phases can be separated in a decanter. In a version of this technique a two-phase fermentation can be run using two incompatible polymers and the ethanol can be partitioned into one phase and the biomass in the other.