Partly as another explored route for process cost reduction but also as a means to avoid the accumulation of ethanol concentrations inhibitory to cell growth or toxic to cellular biochemistry, technologies to remove ethanol in situ, that is, during the course of the fermentation, have proved intermittently popular.254 Seven different modes of separation have been demonstrated in small-scale fermentors:

• A volatile product such as ethanol can be separated from a fermentation broth under vacuum even at a normal operating temperature; a system with partial medium removal and cell recycling was devised to minimize the accumula­tion of nonvolatile products inhibitory to yeast growth and productivity.152

• If the fermentor is operated normally but the culture liquid is circulated through a vacuum chamber, the ethanol formed can be removed on a con­tinuous basis; this arrangement avoids the need to supply O2 to vessels maintained under vacuum.255

• Solvent extraction with a long chain alcohol (я-decanol) with immobilized cells of S. cerevisiae; up to 409 g/l of glucose (from glucose syrup) could be metabolized at 35°C.256

• As with water removal from concentrated ethanol, ethanol can be selectiv­ity adsorbed by different types of resins with hydrophobic surfaces, includ­ing cross-linked divinylbenzene polystyrene resins widely used in modern chromatographic separations of alcohols, sugars, and carboxylic acids; such resins work efficiently with ethanol at low ethanol concentrations, and the ethanol can be desorbed with warm dry N2 gas at 60-80°C.257258

• Hollow-fiber microfiltration is effective for ethanol and other small-mol­ecule products (such as lactic acid) but is slow and difficult to sterilize.259

• In membrane pervaporation, the cells are retained by a semipermeable membrane while a partial vacuum is applied to the permeate side; etha­nol concentrations could be maintained below 25 g/l for five days while a concentrated ethanol efflux stream of 17% w/v was achieved.260 Polyvinyl alcohol membranes operate better at elevated temperature, and this sug­gests that thermophilic ethanologens would be very suitable in a membrane pervaporative process.261

• Gas stripping of ethanol can be effected in an air-lift fermentor, a type of vessel originally developed for viscous microbial fermentation broths but also used for some of the more fragile and shear-sensitive mammalian cells in culture; this is another example of a technology that would inevitably work better with a thermophilic ethanologen and an elevated fermentation temperature.262 Alternatively (and more economically, with reduced power consumption for gas volume flow), the fermentation broth is circulated through an inert packed column and continuously sparged with a strip­ping gas (see figure 4.10) — such arrangements can result in highly stable continuous fermentations (for >100 days), with near-theoretical yields of ethanol from concentrated glucose solutions (560 g/l) in corn steep water to provide nutrients.263 264

How many (if any) of these advanced downstream technologies become adopted for industrial use will depend heavily on their economics — ethanol stripping is, for example, assessed at providing a significant cost savings for fuel ethanol production from cornstarch.265 With lignocellulosic substrates being used more widely, espe­cially in developing economies, a much simplified technology can provide surpris­ingly elegant solutions. Solid-state fermentations[42] have long been used for fermented foods and sake but can easily be adapted to manufacture (under more stringent con­ditions and with a reduced labor intensity) many fine chemicals and enzymes.266 A continuous process has been engineered to process and ferment feedstocks such as fodder beet and sweet sorghum in a horizontal tubular bioreactor, the ferment­ing material (with a low moisture content) moved along with the aid of a spiral screw.267,268 Some ethanol volatilization will occur at any temperature above ambient (caused by the fermentation process), but the bulk of the product could be recovered by a gas or air flowing through the container before the ethanol is condensed and transferred to a final dehydration step (as in the gas stripping technology). Although originally devised for farm-scale facilities (by the Alcohol Fuel Research Labora­tory, South Dakota State University), this solid-phase bioprocess yielded 87 l of ethanol/tonne of feedstock and was sufficiently productive to allow distillation from 8% v/v outputs. Echoing some of the discussion in chapter 1, the net energy balance (see section 1.6.1) was calculated to be unambiguously positive for fodder beet (2.11 for pasteurized pulped beet fodder, 3.0 for unpasteurized substrate), although much less persuasive for sweet sorghum (1.04 and 1.30, respectively) but was — even in 1984, when world oil prices were unpredictable and high after the price inflation of the 1970s — uncompetitive with then current gasoline prices (figure 1.3).