Butanol inhibition is one of the most crucial problems for developing industrial scale production of butanol. Butanol-producing bacteria can rarely tolerate more than 2% butanol in broth [70]. More precisely, 1% exposure of butanol caused a

20-30% increment in the fluidity of cell membrane [71, 72]. C. acetobutylicum was found sensitive to the higher concentration of butanol than 12-13 g l-1 [43, 73]. Various attempts are being made at the organism and process level for reducing the butanol inhibition. One attractive development in process is as an integrated system of fermentation and recovery processes, which allows simulta­neous production and selective removal of solvents illustrated very significant results at laboratory scale studies. The common butanol recovery techniques are adsorption, liquid-liquid extraction, perstraction, reverse osmosis, pervaporation, and gas stripping, which can be integrated with ABE fermentation for online removal of products (Table 7.3) [74].

Gas stripping (Fig. 7.3) comprises the most advantageous characteristics such as simple and economical process (no need of expensive equipments), does not harm the culture, does not remove the nutrients and reaction intermediates, and reduces butanol inhibition effectively [39, 40]. At laboratory scale, gas stripping showed the significant results as integrated with various kinds of fermentation processes, batch [39, 40, 53], fed-batch [53, 75], and continuous [40]. While nitrogen [40] and gases produced in fermentation (hydrogen and carbon dioxide) [39, 53, 75] are used for stripping purpose, whereas utilization of nitrogen gas reflected more effective recovery than other gases.