Qureshi reviewed the ABE fermentation in various types of reactor systems and recovery by gas stripping with 13 references. Gas stripping is a simple technique which does not require expensive apparatus, does not harm the culture, does not remove nutrients and reaction intermediates and reduces butanol toxicity (inhibition). As a result of butanol removal by gas stripping, concentrated sugar solutions can be used to produce ABE solvents. Compared to sugar utilization of 30 g L-1 in a control batch reactor, sugar utilization of 199 g L-1 has been reported with 69.7 g L-1 solvent production. In fed-batch reactors concentrated sugar solutions (350 g L-1) have been used. Additionally, the process of ABE production results in concentrated product streams containing 9.1-120 g L-1 ABE solvent. In the integrated ABE production and recovery systems, selectivity figures of 4-30.5 have been reported [118]. The effect of factors such as gas recycle rate, bubble size, presence of acetone, and ethanol in the solutions or broth were investigated in order to remove butanol from model solution or fermentation broth. Butanol stripping rate was found to be proportional to the gas recycle rate. In the bubble size range attempted (< 0.5 and 0.5-5.0 mm), the bubble size did not have any effect on butanol removal rate. In C. beijerinckii fermentation, ABE productivity was reduced from 0.47 g L-1 h-1 to 0.25 g L-1 h-1 when smaller (< 0.5 mm) bubble size and an excessive amount of antifoam (to inhibit production of foam caused by smaller bubbles) were used. This suggested that fermentation was negatively affected by antifoam [119].

Gas stripping can be performed by using fermentation gases (H2 and CO2) formed during fermentation. Concentrated sugar solutions (250-500 g/L) were used in continuous fermenta­tion of Clostridium beijerinckii BA101, which operated for 21 d (505 h), producing 460 g acetone-BuOH/L [120]. In the integrated fed-batch fermentation and product recovery system, solvent productivities were improved to 400% of the control batch fermentation productivities. In a control batch reactor, the culture used 45.4 g glucose L-1 and produced 17.6 g total solvents L-1 (yield 0.39 g g-1, productivity 0.29 g L-1 h-1). Using integrated fermentation-gas stripping product recovery system with CO2 and H2 as carrier gases, the fed-batch reactor was operated for 201 h. At the end of fermentation, an unusually high concentration of total acids (8.5 g L-1) was observed. A total of 500 g glucose was used to produce 232.8 g solvents (77.7 g acetone, 151.7 g butanol, 3.4 g ethanol) in 1 L culture broth. The average solvent yield and productivity were 0.47 g g-1 and 1.16 g L-1 h-1, respectively [121]. Using a potential industrial substrate (liquefied corn starch, 60 g L-1) in a batch process integrated with gas stripping resulted in the production of 18.4 g L-1 ABE solvents, with 92% utilization of sugars present in the feed. In a fed-batch reactor fed with saccharified liquefied corn starch, 81.3 g L-1 ABE was produced as compared to 18.6 g L-1 in the control. In this integrated system, 225.8 g L-1 corn starch sugar (487% of control) was consumed. In absence of product removal, it is not possible for C. beijerinckii BA101 to utilize more than 46 g L-1 glucose [122].