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Liquid biofuels produced from biomass have the potential to partly replace gasoline. One of the most promising biofuels is butanol which is produced in acetone-butanol-ethanol (ABE) fermentation. The ABE fermentation is characterized by its low butanol concentration in the final fermentation broth. In this research, the simulation of three in situ recovery methods, namely, vacuum fermentation, gas stripping and pervaporation, were performed in order to increase the efficiency of the continuous ABE fermentation by decreasing the effect of butanol toxicity. The non-integrated and integrated butanol production systems were simulated and optimized based on a number of objectives such as maximizing the butanol productivity, butanol concentration, and butanol yield. In the optimization of complex industrial processes, where objectives are often conflicting, there exist numerous potentially-optimal solutions which are best obtained using multi-objective optimization (MOO). In this investigation, MOO was used to generate a set of alternative solutions, known as the Pareto domain. The Pareto domain allows to view very clearly the trade-offs existing between the various objective functions. In general, an increase in the butanol productivity resulted in a decrease of butanol yield and sugar conversion. To find the best solution within the Pareto domain, a ranking algorithm (Net Flow Method) was used to rank the solutions based on a set of relative weights and three preference thresholds. Comparing the best optimal solutions in each case study, it was clearly shown that integrating a recovery method with the ABE fermentation significantly increases the overall butanol concentration, butanol productivity, and sugar conversion, whereas butanol yield being microorganism-dependent, remains relatively constant.

Due to environmental challenges, depleting oil resources, rising cost of oil and instability in oil-producing countries, biofuel production has attracted a lot of attention in recent decades. Biobutanol is one of the biofuels showing the most potential as an alternative for partly replacing petroleum-based fuels. Both researchers and industrialists are currently working at developing an energy-effective process to produce biobutanol at a large scale. Acetone-butanol-ethanol (ABE) fermentation is the biological process of biobutanol production and Clostridia are the most common bacteria used to produce biobutanol. However, there are several challenges in the butanol bioproduction process that should be addressed to make this process economically viable. The main challenge in the biobutanol production process is the low concentration of butanol in the ABE fermentation broth. It is therefore important to develop an efficient separation method. Several separation methods such as distillation, liquid-liquid extraction (LLE), pervaporation, gas stripping and adsorption have been considered to recover butanol from dilute solutions and ABE fermentation broths. Adsorption is considered as one of the most promising methods due to its high performance and energy efficiency for butanol separation. In this study, the focus was on developing an efficient separation method for butanol recovery from dilute model solution and fermentation broth using adsorption. A comprehensive adsorbent screening was first carried out to identify the best commercially available adsorbent among a series of potentially promising adsorbents. Activated carbon (AC) F-400 was selected for further experimentation since it showed high adsorption capacity and adsorption rate in addition to high selectivity toward butanol. AC F-400 was then tested extensively in packed adsorption column experiments for binary and ABE model solutions and fermentation broths to investigate the competitive adsorption between butanol and other components present in ABE broths. The results showed that the butanol adsorption capacity was not affected by the presence of ethanol, glucose and xylose while the presence of acetone led to a slight decrease in adsorption capacity at low butanol concentrations. On the other hand, the presence of acids (acetic acid and butyric acid) in the ABE broth showed a significant effect on the butanol adsorption capacity over a wide ii range of butanol concentration and this effect was more pronounced for butyric acid. At the end, different competitive adsorption isotherm models were also studied to appropriately represent the behaviour of the competitive adsorption. Desorption of butanol was subsequently investigated to evaluate both the desorption capacity of butanol and the capability of the adsorbent particles to be used for multiple adsorption-desorption cycles. The results of this set of experiments showed that AC F-400 can retain its initial adsorption capacity after 6 adsorption/desorption cycles. The recovery of butanol from butanol-water (1.5 wt%) binary and ABE model solutions was 84 and 80% with butanol adsorption capacity of 302 and 171 mg/g, respectively. The combination of adsorption and gas stripping techniques was also studied to investigate the performance of CO2 gas stripping of solvents from the model solutions and fermentation broths followed by adsorption. The results showed that the butanol adsorption capacity of the overall system for binary solutions (260 mg/g for a binary butanol-water solution of 15 g/L with vapour phase concentration of 5.8 mg/L), ABE model solutions (192 mg/g for a corresponding vapour concentration of 5.2 mg/L) and ABE fermentation broths (247 mg/g for a corresponding vapour phase concentration of 2.5 mg/L) was higher than what has been published in the literature. Finally, a model was developed and adequately validated the experimental data to predict the behaviour of the ABE compounds in a packed bed adsorption column for butanol separation from dilute solutions.

The increased demand of fossil fuel along with the depletion of economical crude oil resources, environmental challenges such as the accumulation of CO2 and other greenhouse gases in the atmosphere and the reduction of the dependence on imported oil are some of the motivations for the huge interest in biofuels. Biobutanol produced from ABE fermentation has been considered to be a good partial replacement for fossil fuels. However, challenges such as the need for inexpensive feed-stocks, improved fermentation performance to achieve higher final butanol concentration and higher yield, an efficient method for solvent recovery, and water recycle are the main obstacles to make the production of this alcohol economically viable. Pervaporation, a membrane-based process, is considered to be an attractive separation method to remove butanol from ABE fermentation broth. Among the membranes used for butanol separation, PDMS membranes showed reasonable performance such as good permeability, and appropriate selectivity for butanol separation by pervaporation. However, PDMS membranes need to be improved in terms of performance to be applicable in large scale butanol production plants. In this study, activated carbon nanoparticles have been embedded into the matrix of the PDMS membrane to improve its separation performance and, in particular, the permeation flux and butanol selectivity. Result showed that the presence of nanoparticles improves the PDMS membrane performance up to a certain particle loading. Moreover, it was shown that the operating conditions have a major impact on the pervaporation membrane separation process. The best membrane for pervaporation separation of butanol from binary aqueous solutions was obtained for a 6 wt% particle concentration where the total permeation flux and butanol selectivity increased by 42.6% and 51.9%, respectively, compared to neat PDMS membranes. Moreover, the best performance for the separation of butanol from ABE model solutions was achieved for an 8 wt% nanoparticle loading. Both the selectivity for butanol and the total permeation flux more than doubled in comparison to neat PDMS membranes prepared in this study. Moreover, in order to compare the PDMS/AC mixed matrix membrane performance for pervaporation separation of butanol from binary and ABE model solutions with PDMS membranes available on the market, experiments were also performed with a commercial PDMS membrane. Result of butanol separation from ABE model solutions showed that mixed matrix membranes with 8 wt% nanoparticles loading had a higher permeation flux than that of the commercial membranes. It was clearly shown that the presence of activated carbon nanoparticles in the matrix of the PDMS would be beneficial for the pervaporation separation of butanol from ABE fermentation broths. To better comprehend how the presence of activated carbon nanoparticles in the polymeric membranes enhance the performance of the membranes, a series of numerical simulations were performed. A finite difference model was developed to simulate the mass transfer of permeating components through mixed matrix membranes by pervaporation for a wide range of relative permeability, nanoparticle loading, particle shape, particle size and different filler adsorption isotherms. Finally, an investigation has been performed to optimize the butanol pervaporation separation process from ABE fermentation broth at an industrial scale.

Liquid biofuels produced from biomass have the potential to partly replace gasoline. One of the most promising biofuels is butanol which is produced in acetone-butanol-ethanol (ABE) fermentation. The ABE fermentation is characterized by its low butanol concentration in the final fermentation broth. In this research, the simulation of three in situ recovery methods, namely, vacuum fermentation, gas stripping and pervaporation, were performed in order to increase the efficiency of the continuous ABE fermentation by decreasing the effect of butanol toxicity. The non-integrated and integrated butanol production systems were simulated and optimized based on a number of objectives such as maximizing the butanol productivity, butanol concentration, and butanol yield. In the optimization of complex industrial processes, where objectives are often conflicting, there exist numerous potentially-optimal solutions which are best obtained using multi-objective optimization (MOO). In this investigation, MOO was used to generate a set of alternative solutions, known as the Pareto domain. The Pareto domain allows to view very clearly the trade-offs existing between the various objective functions. In general, an increase in the butanol productivity resulted in a decrease of butanol yield and sugar conversion. To find the best solution within the Pareto domain, a ranking algorithm (Net Flow Method) was used to rank the solutions based on a set of relative weights and three preference thresholds. Comparing the best optimal solutions in each case study, it was clearly shown that integrating a recovery method with the ABE fermentation significantly increases the overall butanol concentration, butanol productivity, and sugar conversion, whereas butanol yield being microorganism-dependent, remains relatively constant.

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Butanol production by four different Clostridium sp. strains was investigated using glucoseP2-medium supplemented with increasing concentrations of butyric acid, added as cosubstrate.Batch fermentations were carried out in serum bottles (freely-suspended cellcultures) and fibrous-bed bioreactor (FBB) with medium recirculation (immobilized cells).Butyric acid clearly revealed to inhibit cellular growth with all specific growth rates decliningupon the increase of butyrate concentrations. However, the presence of low and moderatelevels in the medium can readily enhance the ABE-fermentation and increase butanolproduction through a shift induction towards the solventogenic phase controlled by themedium pH. In all cases it was found that 4.0 g⋅l-1 is the optimal concentration of butyratethat maximizes the yields for all ABE-solvents and butanol productivities. The non-mutant C.acetobutylicum ATCC 824 was singled out as the most efficient butanol productive strainamong all bacteria tested (10.3 g⋅l-1 butanol versus 0.72 g⋅l-1 with and without 4.0 g⋅l-1butyrate, respectively) showing a productivity augment in the order of 0.078 g⋅l-1⋅h-1 (78.5%)and yields of 0.3 g⋅g-1 from substrate and 7.6 g⋅g-1 from biomass versus 0.072 g⋅g-1 and 0.41g⋅g-1 with and without the optimal butyrate concentration, respectively. This strain alsorevealed the best overall tolerance over increasing butyrate concentrations up to ∼6.0 g⋅l-1 andthe highest glucose uptake (65.5%) among all bacteria. Furthermore, the beneficial effects ofbutyric acid were also observed through the use of a fibrous bed-bioreactor when the mutatedstrains of C. beijerinckii ATCC 55025 and BA 101 were tested. The use of this immobilizedcell system effectively improved butanol production over the free system with butanol titersin the fermentation broth around 11.5 g⋅l-1 and 9.4 g⋅l-1 for the two bacteria, respectively,roughly doubling the values attained with the corresponding suspended cell cultures when themedia were supplemented with 4.0 g⋅l-1 of butyrate. All these results confirm theenhancement of butanol formation using either free or immobilized cell culturessupplemented with butyric acid concentrations up to 4.0 g⋅l-1 in the media.