During fermentation of raw substrates containing carbohydrates, yeasts or bacteria are capable of converting the sugar monomers consisting of five (“C5 sugars”, pentoses) and/or six carbon atoms (“C6 sugars”, hexoses) into ethanol (Huber et al., Chem. Rev. 2006, Vol. 106, pp. 4044 to 4098). Ethanol is called “bio-ethanol” when it is prepared from biogenic raw materials. Bio-ethanol is suitable as a bio-fuel, as an admixture to petrol for spark-ignition engines or for further chemical processing. In the past, it has primarily been recovered from sugar and grain containing starch, but so far not from ligno-cellulose biological (LCB) matter in amounts worth speaking of (Huber et al., Chem. Rev. 2006, Vol. 106, pp. 4044 to 4098; Kamm and Kamm, Chem. Ing. Tech., 2007, Vol. 79, pp. 592 to 603).
Separation of the ethanol from the fermentation solution is necessary for preparing ethanol by fermentation. These techniques include pervaporation, extraction, adsorption, reverse osmosis and gas stripping (Windsperger et al., Verfahrenstechnik 1989, Vol. 23, pp. 16 to 21; Qureshi et al., Bioprocess Biosyt. Eng. 2005, Vol. 27, pp. 215 to 222). In particular, the ethanol may be converted to the gas phase. Gas stripping is a selective removal of volatile substances from the fermentation solution (Ezeji et al., J. Ind. Microbiol. Biotechnol. 2007, Vol. 34, 771 to 777).
After conversion to the gas phase, the ethanol needs to be separated from the ethanol-gas mixture. In particular, the ethanol must be separated from the ethanol-carrier gas mixture after stripping. Different techniques are available for this purpose, such as condensation or selective adsorption of the ethanol to an adsorber. CA 1 195 258, for example, describes a process where the fermentation solution is subjected to gas stripping after fermentation has been completed and the ethanol-carrier gas mixture is then adsorbed to a molecular lattice under conditions avoiding the capillary condensation of water. However, this process does not allow controlling the ethanol concentration during fermentation.
However, such control of the ethanol concentration in the fermentation solution is important for preparing bio-ethanol at an industrial scale. One problem in the production of bio-ethanol is the increasingly inhibiting effect and the toxic influence of the ethanol formed on the micro-organisms during fermentation. As a result of the inhibiting effect and the toxic influence of products formed during fermentation, various techniques have been developed to separate these in situ during fermentation.
For example, Walsh et al. (Biotechnology and Bioengineering Symp., No. 13, 1983, pp. 629 to 647) describe a method where C6 sugar is fermented to obtain ethanol and the ethanol is separated from the fermenter in situ by gas stripping and adsorbed to activated carbon. This method permits adjusting the ethanol concentration during fermentation in the range around 6% (w/v). Given the low selectivity of activated carbon for ethanol, however, activated carbon is not suitable for an efficient process.
However, such a control at 6% (w/v) is not sufficient for preparing ethanol from ligno-cellulose bio-matter which requires the fermentation of C5 sugars. For example, Dominguez et al. (Biotech. Bioeng., 2000, Vol. 67, pp. 336-343) have been able to show that the reaction of C5 sugars to form ethanol with the yeast Pichia stipitis is inhibited at only 2% (w/v) of ethanol. Therefore, Dominguez et al. have developed a process where the ethanol concentration may be kept below 2% (w/v) during the fermentation of xylose, with ethanol being condensed on an ice-cooled condenser after stripping in situ in an especially designed fermenter with a side arm.