The continuous culture technique is often used to improve reactor productivity and to study the physiology of the culture in steady state. A number of studies exist for continuous fermentation of butanol, and they all give some insight into butanol fermentation and the behavior of the culture under these conditions. Because of the production of fluctuating levels of solvents and the complexity of butanol fermentation, the use of a single-stage continuous reactor does not seem to be practical at the industrial scale. In continuous culture, a serious problem exists in that solvent production may not be stable for long time periods and ultimately declines over time, with a concomitant increase in acid production. In a single-stage continuous system, high reactor productivity may be obtained, however, at the expense of low product concentration compared to that achieved in a batch process. In a single-stage continuous reactor using C. acetobutylicum, Leung and Wang (1981) produced 15.9 gL-1 total solvents (ABE) at a dilution rate of 0.1 h-1 resulting in a productivity of 1.6 gL-1h-1. The productivity was improved further to 2.55 gL-1h-1 by increasing the dilution rate to 0.22 h-1. It should be noted that the product concentration decreased to 12.0 gL-1. In a related continuous fermentation process using a hyperbutanol producing strain of C. beijerinckii BA101, Formanek et al. (1997) was able to produce 15.6 gL-1 ABE at a dilution rate of 0.05 h-1 resulting in a productivity of 0.78 gL-1h-1. However, solvent concentration decreased to 8.7 gL-1 as dilution rate was increased to 0.2 h-1. This resulted in an increase in productivity to 1.74 gL-1h-1.

As a means of increasing product concentration in the effluent and reducing fluctuations in butanol concentration, two or more multistage continuous fermen­tation systems have been investigated (Bahl et al., 1982; Yarovenko, 1964). Often, this is done by allowing cell growth, acid production, and ABE production to occur in separate bioreactors. In a two-stage system, Bahl et al. (1982) reported a solvent concentration of 18.2 gL-1 using C. acetobutylicum DSM 1731, which is comparable to the solvent concentration in a batch reactor. This type of mul­tistage bioreactor system (7-11 fermenters in series) was successfully tested at the pilot scale and full plant scale level in the Soviet Union (now Russia) (Yarovenko, 1964). However, 7-11 fermenters in series add to the complexity of the system for a relatively low-value product such as butanol. It is viewed that such a multistage system would not be economical.

Immobilized and Cell Recycle Reactors

Increased reactor productivity results in the reduction of process vessel size and capital cost thus improving process economics. In a butanol batch process, reactor productivity is limited to less than 0.50 gL-1h-1 due to a number of reasons including low cell concentration, down time and product inhibition (Maddox, 1989). Increasing cell concentration in the reactor is one of the methods to improve reactor productivity. Cell concentration can be increased by one of two techniques namely, “immobilization” and “cell recycle.” In a batch reactor a cell concentration of <4 gL-1 is normally achieved. In an attempt to improve the reactor productivity, Ennis et al. (1986a) were among the early investigators to use the cell immobilization technique for the butanol fermentation. These authors used cell entrapment technique and continuous fermentation with limited success in productivity improvement. The same group investigated another technique involving cell immobilization by adsorption onto bonechar and improved reactor productivity to approximately 4.5 gL-1h-1 (Qureshi and Maddox, 1987) followed by further improvement to 6.5 gL-1h-1 (Qureshi and Maddox, 1988). The culture that was used in these studies was C. acetobutylicum P262. In an attempt to explore clay bricks as an adsorption support for cells of C. beijerinckii, Qureshi et al. (2000) were able to improve reactor productivity to 15.8 gL-1h-1. In another approach, Huang et al. (2004) immobilized cells of C. acetobutylicum in a fibrous support, which was used in a continuous reactor to produce ABE. In this reactor a productivity of 4.6 gL-1h-1 was obtained.

Cell recycle technique is another approach to increase cell concentration in the reactor and improve reactor productivity (Cheryan, 1986). Using this approach, reactor productivities up to 6.5 gL-1h-1 (as compared to <0.5 gL-1h-1 in batch fermentation) have been achieved in the butanol fermentation (Afschar et al., 1985; Pierrot et al., 1986). In a similar approach, Mulchandani and Volesky (1994) used a single-stage spin filter perfusion bioreactor in which a maximum productivity of 1.14 g L-1 h-1 was obtained; however, the ABE concentration fluctuated over time.