Batch fermentation is the most commonly studied process for butanol production. In the batch process the substrate (feed) and nutrients are charged into the reactor that can be used by the culture. In a batch process, a usual substrate concentration of 60-80 gL-1 is used as higher concentration results in residual substrate being in the reactor. The reaction mixture is then autoclaved at 121°C for 15 minutes followed by cooling to 35-37°C and inoculation with the seed culture. During cooling, nitrogen, or carbon dioxide is swept across the surface to keep the medium anaerobic. After inoculation, the medium is sparged with these gases to mix the inoculum. Details of seed development and inoculation have been pub­lished elsewhere (Formanek et al., 1997; Qureshi and Blaschek, 1999a). Depend­ing on the size of the final fermentor, the seed may have to be transferred several times before it is ready for the production fermentor.

Various substrates can be used to produce butanol including corn, molasses, whey permeate, or glucose derived from corn (Qureshi and Blaschek, 2005). However, some substrates may require processing prior to fermentation, known as “upstream processing,” such as dilution, concentration, centrifugation, filtra­tion, hydrolysis, etc. The usual batch fermentation time lasts from 48 to 72 h after which butanol is recovered, usually by distillation. During this fermentation period, ABE up to 33 gL-1 is produced using hyperbutanol producing C. beijer­inckii BA101 (Chen and Blaschek, 1999; Formanek et al., 1997). This culture results in a solvent yield of 0.40-0.42 (Formanek et al., 1997). The ABE con­centration in the fermentation broth is limited due to butanol inhibition to the cell. At a butanol concentration of approximately 20 gL-1, strong cell growth inhibition occurs that kills the cells and stops the fermentation. Butanol produc­tion is a biphasic fermentation where acetic and butyric acids are produced during acidogenic phase followed by their conversion into acetone and butanol (solven — togenic phase). During the acidogenic phase, the pH drops due to acid production and subsequently rises during solvent production. At the end of fermentation, cell mass and other suspended solids (if any) are removed by centrifugation and sold as cattle feed. Figure 6.2 shows fermentation profile of butanol production in a typical batch fermentation process from cornstarch using C. beijerinckii BA101.

Butanol can be produced both by using corn coproduct from i) corn dry-grind and ii) wet-milling processes. During the dry-grind process corn fiber and germ are not removed prior to fermentation. At the end of fermentation (after starch utilization during fermentation), corn fiber and other insoluble solids are removed by centrifugation, dried, and sold as cattle feed. The dried solids are known as “Distillers Dry Grain Solids” or DDGS. On the contrary, during the wet-milling

process, corn fiber and germ are removed prior to fermentation. In this process, cornstarch can be converted to any of the three products (liquefied cornstarch, glucose syrup, or glucose) each of which is fermentable by C. beijerinckii to produce butanol. It should be noted that often corn refineries add sodium met­abisulfite during the wet-milling operation as a corn kernel softening agent and preservative to the liquefied cornstarch. The presence of sodium metabisulfite may interfere with the direct fermentation of the liquefied cornstarch. However, glucose syrup or glucose does not contain any such fermentation inhibitors. The unit operations that are applicable to the corn dry-grind and wet-milling fermen­tation of butanol are given in Table 6.2.

During the 1940s and 1950s, production of butanol on an industrial scale (Terre Haute, IN, and Peoria, IL) was carried out using large fermenters ranging in capacity from 200,000 to 800,000 L. The industrial process used 8-10% corn mash, which was cooked for 90 min at 130-133°C. Corn contains approximately 70% (dry weight basis) starch. The use of molasses offers many advantages over using corn, including the presence of essential vitamins and micronutrients (Paturau, 1989). In industrial processes, beet and invert and blackstrap molasses were diluted to give a fermentation sugar concentration of 50 to 75 gL-1, most commonly 60 gL-1. The molasses solution was sterilized at 107 to 120°C for 15 to 60 min followed by adding organic and inorganic nitrogen, phosphorus, and buffering chemicals. The yield of solvent using C. acetobutylicum was usually low at 0.29-0.33. Distillation has been the method of choice to recover butanol; however, during the last two decades a number of alternative techniques have been investigated for the economical recovery of butanol, which will be discussed in the recovery section.