Prospective and development of butanol as an advanced biofuel

Butanol has been acknowledged as an advanced biofuel, but its production through acetone–butanol–ethanol (ABE) fermentation by clostridia is still not economically competitive, due to low butanol yield and titer. In this article, update progress in butanol production is reviewed. Low price and sustainable feedstocks such as lignocellulosic residues and dedicated energy crops are needed for butanol production at large scale to save feedstock cost, but processes are more complicated, compared to those established for ABE fermentation from sugar- and starch-based feedstocks. While rational designs targeting individual genes, enzymes or pathways are effective for improving butanol yield, global and systems strategies are more reasonable for engineering strains with stress tolerance controlled by multigenes. Compared to solvent-producing clostridia, engineering heterologous species such as Escherichia coli and Saccharomyces cerevisiae with butanol pathway might be a solution for eliminating the formation of major byproducts acetone and ethanol so that butanol yield can be improved significantly. Although batch fermentation has been practiced for butanol production in industry, continuous operation is more productive for large scale production of butanol as a biofuel, but a single chemostat bioreactor cannot achieve this goal for the biphasic ABE fermentation, and tanks-in-series systems should be optimized for alternative feedstocks and new strains. Moreover, energy saving is limited for the distillation system, even total solvents in the fermentation broth are increased significantly, since solvents are distilled to ~ 40% by the beer stripper, and more than 95% water is removed with the stillage without phase change, even with conventional distillation systems, needless to say that advanced chemical engineering technologies can distil solvents up to ~ 90% with the beer stripper, and the multistage pressure columns can well balance energy consumption for solvent fraction. Indeed, an increase in butanol titer with ABE fermentation can significantly save energy consumption for medium sterilization and stillage treatment, since concentrated medium can be used, and consequently total mass flow with production systems can be reduced. As for various in situ butanol removal technologies, their energy efficiency, capital investment and contamination risk to the fermentation process need to be evaluated carefully.     

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

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.     

Evaluation of solvents for extractive butanol fermentation with Clostridium acetobutylicum and the use of poly(propylene glycol) 1200

Summary In an effort to improve the viability of acetone-butanol-ethanol fermentation by extractive fermentation, 63 organic solvents, including alkanes, alcohols, aldehydes, acids, and esters, were experimentally evaluated for biocompatibility with Clostridium acetobutylicum by observing gas evolution from cultures in contact with candidate solvents. Thirty-one of these solvents were further tested to determine their partition coefficient for butanol in fermentation medium. The biocompatible solvent with the highest partition coefficient for butanol (4.8), was poly(propylene glycol) 1200, which was selected for fermentation experiments. This is the highest partition coefficient reported to date for a biocompatible solvent. Extractive fermentations using concentrated feeds were observed to produce up to 58.6 g·l^–1 acetone and butanol in 202 h, the equivalent of three control fermentations in a single run. Product yields (based on total solvent products and glucose consumed) of 0.234 g·g^–1 to 0.311 g·g^–1 and within run solvent productivities of 0.174 g·l^–1·h^–1 to 0.290 g·l^–1·h^–1 were consistentwith conventional fermentations reported in the literature. The extended run-time of the fermentation resulted in an overall improvement in productivity by reducing the fraction of between-run down-time for fermentor cleaning and sterilization.     

萃取耦合技术对玉米秸秆水解液发酵产丁醇的影响

本研究以玉米秸秆水解液为原料,通过萃取发酵技术生产燃料丁醇,以提高丁醇产量,降低生产成本。通过对萃取剂的筛选与条件优化,确定纤维丁醇发酵的萃取剂为油醇,添加时间为发酵0 h,添加比例为1:1 (V/V)。该条件下发酵32 g/L糖浓度的玉米秸秆水解液,丁醇和总溶剂产量分别为3.28 g/L和4.72 g/L,比对照分别提高958.1%和742.9%。以D301树脂脱毒后5%总糖浓度的玉米秸秆水解液进行丁醇萃取发酵,丁醇和总溶剂产量分别达到10.34 g/L和14.72 g/L,发酵得率为0.31 g/g,与混合糖发酵结果相当。研究结果表明萃取发酵技术能够显著提高原料的利用率和丁醇产量,为纤维丁醇工业化生产提供了技术支撑。     

Acetone–butanol–ethanol competitive sorption simulation from single,binary, and ternary systems in a fixed‐bed of KA‐I resin

Separation of butanol based on sorption methodology from acetone–butanol–ethanol (ABE) fermentation broth has advantages in terms of biocompatibility and stability, as well as economy, and therefore gains much attention. In this work a chromatographic column model based on the solid film linear driving force approach and the competitive Langmuir isotherm equations was used to predict the competitive sorption behaviors of ABE single, binary, and ternary mixture. It was observed that the outlet concentration of weaker retained components exceeded the inlet concentration, which is an evidence of competitive adsorption. Butanol, the strongest retained component, could replace ethanol almost completely and also most of acetone. In the end of this work, the proposed model was validated by comparison of the experimental and predicted ABE ternary breakthrough curves using the real ABE fermentation broth as a feed solution. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 31:124–134, 2015     

Butanol recovery from aqueous solution into ionic liquids by liquid–liquid extraction

Biobutanol has currently attracted considerable attention as an alternative biofuel to the petroleum-derived fuel due to several advantages including high energy content, low water absorption and easy application to the existing gasoline infrastructure. However, its production has still faced many obstacles to overcome including lack of energy-efficient butanol separation process from fermentation broth. To solve this issue, the extraction behavior of butanol from aqueous media into a variety of imidazolium-based ionic liquids (ILs) was investigated by liquid–liquid extraction. To understand the effect of ILs properties, the solvent characteristics of ILs such as mutual solubility of feed solvent (water) and extraction solvent (IL), distribution coefficient of butanol between water and IL, selectivity, and extraction efficiency were correlated with hydrophobicity and polarity of ILs. The butanol distribution between ILs and water strongly depends on the hydrophobicity of anions of ILs followed by the hydrophobicity of cations of ILs. On the other hand, butanol extraction efficiency and selectivity depend on the polarity of ILs. Considering extraction efficiency and selectivity, [Tf₂N]-based ILs among the tested ILs showed to be the best extract solvent for the recovery of butanol from aqueous media. Among the studied ILs, [Omim][Tf₂N] showed the highest butanol distribution coefficient (1.939), selectivity (132) and extraction efficiency (74%) at 323.15 K, respectively.     

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

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.     

Ex situ product recovery for enhanced butanol production by <Emphasis Type="Italic">Clostridium beijerinckii</Emphasis>

In situ butanol recovery fermentation has been intensively studied as an effective alternative to conventional butanol production, which is limited due to the cellular toxicity of butanol. However, the low biocompatibility of adsorbents often leads to failure of in situ recovery fermentations. In this study, Clostridium beijerinckii NCIMB 8052 was cultured in flasks without shaking and in situ recovery fermentation was performed by using an adsorbent L493. The amounts of acetone, butanol, and ethanol (ABE) increased by 34.4 % in the presence of the adsorbent. In contrast, cell growth and production of organic acids and ABE were retarded in the 7-L batch fermentations with in situ butanol recovery. Cell damage occurred in the fermentor upon agitation in the presence of the adsorbent, unlike in static flask cultures with in situ recovery. Ex situ recovery fermentation using circulation of fermentation broth after mid-exponential phase of cell growth was developed to avoid adsorbent-cell incompatibility. No apparent cell damage was observed and 25.7 g/L of ABE was produced from 86.2 g/L glucose in the fed-batch mode using 7 L fermentors. Thus, ex situ recovery fermentation with C. beijerinckii is effective for enhancing butanol fermentation.     

Acetone butanol ethanol (ABE) production from concentrated substrate: reduction in substrate inhibition by fed-batch technique and product inhibition by gas stripping

Acetone butanol ethanol (ABE) was produced in an integrated fed-batch fermentation-gas stripping product-recovery system using Clostridium beijerinckii BA101, with H₂ and CO₂ as the carrier gases. This technique was applied in order to eliminate the substrate and product inhibition that normally restricts ABE production and sugar utilization to less than 20 g l^–1 and 60 g l^–1, respectively. 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 the integrated fermentation-gas stripping product-recovery system with CO₂ and H₂ as carrier gases, we carried out fed-batch fermentation experiments and measured various characteristics of the fermentation, including ABE production, selectivity, yield and productivity. The fed-batch reactor was operated for 201 h. At the end of the 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.     

Enhancement of butanol tolerance and butanol yield in Clostridium acetobutylicum mutant NT642 obtained by nitrogen ion beam implantation

As a promising alternative biofuel, biobutanol can be produced through acetone/butanol/ethanol (ABE) fermentation. Currently, ABE fermentation is still a small-scale industry due to its low production and high input cost. Moreover, butanol toxicity to the Clostridium fermentation host limits the accumulation of butanol in the fermentation broth. The wild-type Clostridium acetobutylicum D64 can only produce about 13 g butanol/L and tolerates less than 2% (v/v) butanol. To improve the tolerance of C. acetobutylicum D64 for enhancing the production of butanol, nitrogen ion beam implantation was employed and finally five mutants with enhanced butanol tolerance were obtained. Among these, the most butanol tolerant mutant C. acetobutylicum NT642 can tolerate above 3% (v/v) butanol while the wide-type strain can only withstand 2% (v/v). In batch fermentation, the production of butanol and ABE yield of C. acetobutylicum NT642 was 15.4 g/L and 22.3 g/L, respectively, which were both higher than those of its parental strain and the other mutants using corn or cassava as substrate. Enhancing butanol tolerance is a great precondition for obtaining a hyper-yield producer. Nitrogen ion beam implantation could be a promising biotechnology to improve butanol tolerance and production of the host strain C. acetobutylicum.     

Butanol recovery from fermentations by liquid-liquid extraction and membrane solvent extraction

Extraction can successfully be used for in-situ alcohol recovery in butanol fermentations to increase the substrate conversion. An advantage of extraction over other recovery methods may be the high capacity of the solvent and the high selectivity of the alcohol/water separation. Extraction, however, is a comprehensive operation, and the design of an extraction apparatus can be complex. The aim of this study is to assess the practical applicability of liquid-liquid extraction and membrane solvent extraction in butanol fermentations. In this view various aspects of extraction processes were investigated.Thirty-six chemicals were tested for the distribution coefficient for butanol, the selectivity of alcohol/water separation and the toxicity towards Clostridia. Convenient extractants were found in the group of esters with high molar mass.Liquid-liquid extraction was carried out in a stirred fermenter and a spray column. The formation of emulsions and the fouling of the solvent in a fermentation broth causes problems with the operation of this type of equipment. With membrane solvent extraction, in which the solvent is separated from the broth by a membrane, a dispersion-free extraction is possible, leading to an easy operation of the equipment. In this case the mass transfer in the membrane becomes important.With membrane solvent extraction the development of a process is emphasized in which the extraction characteristics of the solvent are combined with the property of silicone rubber membranes to separate butanol from water. In the case of apolar solvents with a high molar mass, the characteristics of the membrane process are determined completely by the solvent. In the case of polar solvents (e.g. ethylene glycol), the permselectivity of the membrane can profitably be used. This concept leads to a novel type of extraction process in which alcohol is extracted with a water-soluble solvent via a hydrophobic semipermeable membrane. This extraction process has been investigated for the recovery of butanol and ethanol from water. A major drawback in all processes with membrane solvent extraction was the permeation of part of the solvent to the aqueous phase.The extraction processes were coupled to batch, fed batch and continuous butanol fermentations to affirm the applicability of the recovery techniques in the actual process. In the batch and fed batch fermentations a three-fold increase in the substrate consumption could be achieved, in the continuous fermentation about 30% increase.     

Acetone-butanol-ethanol fermentation and isoflavone extraction using kudzu roots

The economics of Acetone-butanol-ethanol (ABE) fermentation is greatly affected by raw materials, and the use of readily available starchy materials from marginal farming lands could be a viable option for reducing costs. Kudzu, a rapidly growing perennial leguminous vine, has been planted on marginal farming land and widely distributed in Asia and America. This study investigated ABE fermentation by C. acetobutylicum ATCC 824 using kudzu roots and isoflavone extraction from kudzu fermentation residue (KFR). The kudzu roots could be used as a sole substrate for ABE fermentation without nutritional supplements. Batch culture containing 140 g kudzu/L produced 17.99 ± 1.08 g/L solvent (ABE), including 11.20 ± 0.79 g/L butanol, 5.54 ± 0.20 g/L acetone, and 1.15 ± 0.09 g/L ethanol, with a productivity of 0.19 g/(L/h) and a yield of 0.33 g solvent/g sugar after 96 h of fermentation. Isoflavone yield extracted from KFR was 1.90/100 g KFR, approximately 48% higher compared with that extracted from raw kudzu. A kinetic analysis of the extraction process showed that both the isoflavone yield and the extraction rate obtained from KFR were higher than the corresponding values obtained from raw kudzu. These results indicate that kudzu may provide a new potential raw material for ABE production and the process of ABE fermentation integrated with isoflavone extraction may provide a new way to reduce fermentable substrate costs.     

Development of a supported emulsion liquid membrane system for propionic acid separation in a microgravity environment

Perstractive fermentation is a good way to increase the productivity of bioreactors. UsingPropionibacteria as the model system, the feasibility of using supported emulsion liquid membrane (SELM) for perstractive fermentation is assessed in this study. Five industrial solvents were considered as the solvent for preparing the SELM. The more polar a solvent is, the higher the partition coefficient. However, toxicity of a solvent also increases with its polarity. CO-1055 (industrial decanol/octanol blend) has the highest partition coefficient toward propionic acid among the solvents that has no molecular toxicity towardPropionibacteria. A preliminary extraction study was conducted using tetradecane as solvent in a hydrophobic hollow fiber contactor. The result confirmed that SELM eliminates the equilibrium limitation of conventional liquid-liquid extraction, and allows the use of a non-toxic solvent with low partition coefficient.     

Extraction of rifamycin B from native and aqueous solutions]

Optimal conditions for extraction of rifamycin B from aqueous solutions and fermentation broth filtrates at pH values within 2.0-7.0 were determined. When the antibiotic was extracted from the aqueous solutions, the highest yield was obtained at pH 2.0. When the antibiotic was extracted from the fermentation broth filtrates, it was found that chloroform was the most selective solvent with respect to rifamycin B, the chloroform selectivity being increased at pH 3.5-4.0. It was shown that rifamicin B passed from the buffer solutions with a concentration of 3-20 mg/ml to chloroform in amounts of 6-7 mg/ml and to ethylacetate and butanol in amounts of 20 mg/ml. Such conditions of chloroform and butanol (9 : 1) increased the rifamycin B contents in the extract up to 40 mg/ml.     

High-titer n-butanol production by clostridium acetobutylicum JB200 in fed-batch fermentation with intermittent gas stripping

Acetone–butanol–ethanol (ABE) fermentation with a hyper‐butanol producing Clostridium acetobutylicum JB200 was studied for its potential to produce a high titer of butanol that can be readily recovered with gas stripping. In batch fermentation without gas stripping, a final butanol concentration of 19.1 g/L was produced from 86.4 g/L glucose consumed in 78 h, and butanol productivity and yield were 0.24 g/L h and 0.21 g/g, respectively. In contrast, when gas stripping was applied intermittently in fed‐batch fermentation, 172 g/L ABE (113.3 g/L butanol, 49.2 g/L acetone, 9.7 g/L ethanol) were produced from 474.9 g/L glucose in six feeding cycles over 326 h. The overall productivity and yield were 0.53 g/L h and 0.36 g/g for ABE and 0.35 g/L h and 0.24 g/g for butanol, respectively. The higher productivity was attributed to the reduced butanol concentration in the fermentation broth by gas stripping that alleviated butanol inhibition, whereas the increased butanol yield could be attributed to the reduced acids accumulation as most acids produced in acidogenesis were reassimilated by cells for ABE production. The intermittent gas stripping produced a highly concentrated condensate containing 195.9 g/L ABE or 150.5 g/L butanol that far exceeded butanol solubility in water. After liquid–liquid demixing or phase separation, a final product containing ～610 g/L butanol, ～40 g/L acetone, ～10 g/L ethanol, and no acids was obtained. Compared to conventional ABE fermentation, the fed‐batch fermentation with intermittent gas stripping has the potential to reduce at least 90% of energy consumption and water usage in n‐butanol production from glucose. Biotechnol. Bioeng. 2012; 109: 2746–2756. © 2012 Wiley Periodicals, Inc.     

Selection of organic solvents for in situ extraction of fermentation products fromClostridium thermohydrosulfuricum cultures

Summary Fifteen organic solvents were examined to determine their biocompatibility for in situ extraction of fermentation products from cultures of the thermophilic anaerobeClostridium thermohydrosul furicum. Five solvents (hexadecane, isooctane, kerosene, oleyl alcohol, Shellsol TD) were found to be non-toxic toClostridium thermohydrosul furicum. Interfacial tensions, phase separation and partition coefficients for ethanol of the biocompatible solvents were compared. With the exception of kerosene, these solvents showed good separation from the aqueous phase. Oleyl alcohol had the highest partition coefficient for ethanol (K_D=0.34 at 65°C) and appears to be suitable for extractive ethanol fermentation.     

Salting-out extraction and crystallization of succinic acid from fermentation broths

In this study, salting-out extraction (SOE) and crystallization were combined to recover succinic acid from fermentation broths. Of the different SOE systems investigated, the system consisting of organic solvents and acidic salts appeared to be more favorable. A system using acetone and ammonium sulfate was investigated to determine the effect of phase composition and pH. The highest partition coefficient (8.64) and yield of succinic acid (90.05%) were obtained by a system composed of 30% (w/w) acetone and 20% (w/w) ammonium sulfate at a pH of 3.0. Additionally, 99.03% of cells, 90.82% of soluble proteins, and 94.89% of glucose could be simultaneously removed from the fermentation broths. Interestingly, nearly 40% of the pigment was removed using the single-step salting-out extraction process. The analysis of the effect of pH on salting-out extraction indicates that a pH lower than the pK of succinic acid is beneficial for the recovery of succinic acid in an SOE system. Crystallization was performed for the purification of succinic acid at 4 °C and pH 2.0. By combining salting-out extraction with crystallization, an identical total yield (65%) and a higher purity (97%) of succinic acid were obtained using a synthetic fermentation broth compared with the actual fermentation broth (65% and 91%, respectively).     

Liquid-liquid extraction for recovery of paclitaxel from plant cell culture: Solvent evaluation and use of extractants for partitioning and selectivity

A major challenge in the production of metabolites by plant cells is the separation and purification of a desired product from a number of impurities. An important application of plant cell culture is the biosynthesis of the anticancer agent paclitaxel. Liquid–liquid extraction plays a critical role in the recovery of paclitaxel and other valuable plant‐derived products from culture broth. In this study, the extraction of paclitaxel and a major unwanted by‐product, cephalomannine, from plant cell culture broth into organic solvents is quantified. Potential solvent mixtures show varying affinity and selectivity for paclitaxel over cephalomannine. The partition coefficient of paclitaxel is highest in ethyl acetate and dichloromethane, with measured values of 28 and 25, respectively; however, selectivity coefficients are less than 1 for paclitaxel over cephalomannine for both solvents. Selectivity coefficient increases to 1.7 with extraction in n‐hexane, but the partition coefficient decreases to 1.9. Altering the pH of the aqueous phase results in an increase in both recovery and selectivity using n‐hexane but does not change the results for other solvents significantly. The addition of extractants trioctylamine (TOA) or tributylphosphate (TBP) to n‐hexane gives significantly higher partition coefficients for paclitaxel (8.6 and 23.7, respectively) but no selectivity. Interestingly, when 20% hexafluorobenzene (HFB) is added to n‐hexane, the partition coefficient remains approximately constant, but the selectivity coefficient for paclitaxel over cephalomannine improves to 4.5. This significant increase in selectivity early in the purification process has the potential to simplify downstream processing steps and significantly reduce overall purification costs. © 2012 American Institute of Chemical Engineers Biotechnol. Prog., 28: 990–997, 2012     

Screening and characteristics of a butanol-tolerant strain and butanol production from enzymatic hydrolysate of NaOH-pretreated corn stover

As a gasoline substitute, butanol has advantages over traditional fuel ethanol in terms of energy density and hydroscopicity. However, solvent production appeared limited by butanol toxicity. The strain of Clostridium acetobutylicum was subjected to mutation by mutagen of N-methyl-N'-nitro-N-nitrosoguanidine for 0.5?h. Screening of mutants was done according to the individual resistance to butanol. A selected butanol-resistant mutant, strain 206, produced 50?% higher solvent concentrations than the wild-type strain when 60?g glucose/l was employed as substrate. The strain was also able to produce solvents of 23.47?g/l in 80?g/l glucose P2 medium after 70?h fermentation, including 5.41?g acetone/l, 15.05?g butanol/l and 3.02?g ethanol/l, resulting in an ABE yield and productivity of 0.32?g/g and 0.34?g/(l?h). Subsequently, Acetone-butanol-ethanol (ABE) production from enzymatic hydrolysate of NaOH-pretreated corn stover was investigated in this study. An ABE yield of 0.41 and a productivity of 0.21?g/(l?h) was obtained, compared to the yield of 0.33 and the productivity of 0.20?g/(l?h) in the control medium containing 52.47 mixed sugars. However, it is important to note that although strain 206 was able to utilize all the glucose rapidly in the hydrolysate, only 32.9?% xylose in the hydrolysate was used after fermentation stopped compared to 91.4?% xylose in the control medium. Strain 206 was shown to be a robust strain for ABE production from lignocellulosic materials and has a great potential for industrial application.     

Improving performance of a gas stripping-based recovery system to remove butanol from Clostridium beijerinckii fermentation

The effect of factors such as gas recycle rate, bubble size, presence of acetone, and ethanol in the solution/broth were investigated in order to remove butanol from model solution or fermentation broth (also called acetone butanol ethanol or ABE or solvents). Butanol (8 g L^–1, model solution, Fig. 2) 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 (Fig. 3, model solution). In Clostridium 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 was used to remove ABE (Fig. 4, results reported as butanol/ABE concentration). The productivity was reduced as a result of addition of an excessive amount of antifoam used to inhibit the production of foam caused by the smaller bubbles. This suggested that the fermentation was negatively affected by antifoam.Mention of trade names of commercial products in this article is solely for the purpose of providing scientific information and does not imply recommendation or endorsement by the United States Department of Agriculture.