Summary We examined the effect of gas-stripping on the in situ removal of acetone, butanol, and ethanol (ABE) from batch reactor fermentation broth. The mutant strain (Clostridium beijerinckii BA101) was not affected adversely by gas stripping. The presence of cells in the fermentation broth affected the selectivities of ABE. A considerable improvement in the productivity and yield was recorded in this work in comparison with the non-integrated process. In an integrated process of ABE fermentation-recovery using C. beijerinckii BA101, ABE productivities and yield were improved up to 200 and 118%, respectively, as compared to control batch fermentation data. In a batch reactor C. beijerinckii BA101 utilized 45.4 g glucose l )1 and produced 17.7 g total ABE l )1 , while in the integrated process it utilized 161.7 g glucose l )1 and produced total ABE of 75.9 g l )1 . In the integrated process, acids were completely converted to solvents when compared to the non-integrated process (batch fermentation) which contained residual acids at the end of fermentation. In situ removal of ABE by gas stripping has been reported to be one of the most important techniques of solvent removal. During these studies we were able to maintain the ABE concentration in the fermentation broth below toxic levels.

Production of acetone, butanol and ethanol by Clostridium beijerinckii BA101 and in situ recovery by gas stripping

Anaerobic bacteria such as the solventogenic clostridia can ferment a wide range of carbon sources (e.g., glucose, galactose, cellobiose, mannose, xylose, and arabinose) to produce carboxylic acids (acetic and butyric) and solvents such as acetone, butanol, and ethanol (ABE). The fermentation process typically proceeds in two phases (acidogenic and solventogenic) in a batch mode. Poor solvent resistance by the solventogenic clostridia and other fermenting microorganisms is a major limiting factor in the profitability of ABE production by fermentation. The toxic effect of solvents, especially butanol, limits the concentration of these solvents in the fermentation broth, limiting solvent yields and adding to the cost of solvent recovery from dilute solutions. The accepted dogma is that toxicity in the ABE fermentation is due to chaotropic effects of butanol on the cell membranes of the fermenting microorganisms, which poses a challenge for the biotechnological whole-cell bio-production of butanol. This mini-review is focused on (1) the effects of solvents on inhibition of cell metabolism (nutrient transport, ion transport, and energy metabolism); (2) cell membrane fluidity, death, and solvent tolerance associated with the ability of cells to tolerate high concentrations of solvents without significant loss of cell function; and (3) strategies for overcoming poor solvent resistance in acetone and butanol-producing microorganisms.

Achievements and perspectives to overcome the poor solvent resistance in acetone and butanol-producing microorganisms

With the incessant fluctuations in oil prices and increasing stress from environmental pollution, renewed attention is being paid to the microbial production of biofuels from renewable sources. As a gasoline substitute, butanol has advantages over traditional fuel ethanol in terms of energy density and hygroscopicity. A variety of cheap substrates have been successfully applied in the production of biobutanol, highlighting the commercial potential of biobutanol development. In this review, in order to better understand the process of acetone-butanol-ethanol production, traditional clostridia fermentation is discussed. Sporulation is probably induced by solvent formation, and the molecular mechanism leading to the initiation of sporulation and solventogenesis is also investigated. Different strategies are employed in the metabolic engineering of clostridia that aim to enhancing solvent production, improve selectivity for butanol production, and increase the tolerance of clostridia to solvents. However, it will be hard to make breakthroughs in the metabolic engineering of clostridia for butanol production without gaining a deeper understanding of the genetic background of clostridia and developing more efficient genetic tools for clostridia. Therefore, increasing attention has been paid to the metabolic engineering of E. coli for butanol production. The importation and expression of a non-clostridial butanol-producing pathway in E. coli is probably the most promising strategy for butanol biosynthesis. Due to the lower butanol titers in the fermentation broth, simultaneous fermentation and product removal techniques have been developed to reduce the cost of butanol recovery. Gas stripping is the best technique for butanol recovery found so far.

Problems with the microbial production of butanol.

The rising cost of crude oil combined with the depletion of the resources, political instability in oil producing countries, and the desire to reduce the current dependence on imported oil are some of the reasons that have motivated the different waves of interest for renewable and sustainable fuels. In addition to bioethanol and biodiesel, biobutanol is attracting significant interest as a biofuel mainly due to the recent advances in biotechnology for its production. Biobutanol has lower water miscibility, flammability, and corrosiveness than ethanol, and has the enviable advantage to be able to directly replace gasoline in car engines without requiring modifications. Butanol can be produced from a wide variety of waste biomass feedstock which does not compete with food. In butanol bioproduction, the most widely used microorganisms for acetonebutanol–ethanol (ABE) fermentation are anaerobic bacteria such as the solventogenic Clostridia including Clostridium acetobutylicum and Clostridium beijerinckii. However, the production of biobutanol via fermentation is facing significant engineering challenges due to the very low final concentration and low yield due to the severe butanol toxicity to microorganisms. It is therefore important to find an efficient separation technique to recover butanol at the end of fermentation or during the fermentation to reduce the level of toxicity and prolong the fermentation. In this article, the main butanol separation techniques such as adsorption, gas stripping, liquid–liquid extraction (LLE), perstraction, reverse osmosis (RO) and pervaporation were discussed. It was concluded that adsorption and pervaporation are the separation techniques that offer the most potential for butanol separation from dilute solutions.

Separation techniques in butanol production: Challenges and developments

Butanol has a great potential as a biofuel and to date a lot of research has been done both in terms of more efficient butanol production as well as in developing product recovery methods. Many of them deal with separation techniques which can be used for selective recovery of acetone, n-butanol and ethanol from model solutions and fermentation broths. This work is a review of techniques used for ABE recovery, such as distillation, adsorption, gas stripping, liquid–liquid extraction, pertraction, membrane distillation, sweeping gas pervaporation, thermopervaporation and vacuum pervaporation. Advantages and disadvantages of using particular methods, examples of applications and integrated processes are also described.

ABE fermentation products recovery methods—A review

The problem of dilute solvent concentration in butanol-acetone fermentations can be solved by using reverse osmosis to dewater the fermentation liquor. Polyamide membranes have a potential application in a butanol-acetone fermentation and exhibited rejection rates as high as 98 percent. Optimum rejection of butanol in a fermentation liquor occurred at recoveries of 20-45 percent. Flux ranged from .05 to .6 L m/sup -2/ min/sup -1/.

Butanol fermentation liquor production and separation by reverse osmosis

The problem of dilute solvent concentration in butanol-acetone fermentations can be solved by using reverse osmosis to dewater the fermentation liquor. Polyamide membranes have a potential application in a butanol-acetone fermentation and exhibited rejection rates as high as 98%. Optimum rejection of butanol in the fermentation liquor occurred at recoveries of 20-45%. Flux ranged from 0.05 to 0.6 L m(-2) min(-1).

James L. Fischer

Papers

Butanol fermentation liquor production and separation by reverse osmosis

Butanol fermentation liquor production and separation by reverse osmosis