In alcoholic fermentation processes, ethanol is the main component that is toxic to yeast because it acts as a noncompetitive inhibitor of metabolism. One way of overcoming the inhibition effect on yeast is to extract the ethanol from the broth during the fermentation. The present work evaluates ethanol production by extractive batch fermentation using CO 2 as a stripping gas. Investigation was first made of the influence of specific CO 2 flow rate (ϕ) and solution temperature on ethanol stripping. The best results, in terms of ethanol removal, were obtained at 2.0 vvm and 34.0 °C. Modeling of conventional and extractive ethanol fermentation was then performed considering cell growth, substrate consumption, ethanol production, and the entrainment of ethanol and water using first-order equations. The hybrid Andrews−Levenspiel model was able to describe the kinetics of the conventional fermentation process, and a model proposed here could accurately predict the behavior of the extractive fermentation. In all the extractive fermentations, there was faster substrate uptake and earlier substrate exhaustion, compared to the conventional fermentation. Extractive fermentation, with stripping initiated after 3 h at an ethanol concentration of 43.3 g•L −1 , resulted in an ethanol productivity (in g•L −1 •h −1 ) that was around 25% higher, and finished about 2 h earlier, compared to the control fermentation.

The ethanol accumulated in the broth during fermentation is the main component toxic to yeast, causing slower yeast growth and decreased ethanol production. One way of overcoming this inhibition effect is to use extractive fermentation, where the ethanol is removed from the broth during the fermentation process. The present work evaluates ethanol production by extractive fed-batch fermentation with CO 2 stripping, under different conditions of substrate concentration in the must feed (Cs F ), vat filling time (F t ), and start time of ethanol stripping with CO 2 . First, the process kinetic parameters were estimated by modeling of conventional fed-batch fermentations (without stripping) in a 5 L bubble column bioreactor, with fitting of the model to experimental data. This procedure used a sucrose concentration of 180 g•L −1 in the must feed, temperature of 34.0 °C, and vat filling times of 3 and 5 h. Subsequently, extractive fed-batch ethanol fermentations were performed at 34.0 °C with a sucrose concentration of 180 g•L −1 in the feed, specific CO 2 flow rate (ϕ) of 2.5 vol•vol −1 •min −1 (vvm), and F t of 3 or 5 h, starting ethanol stripping with CO 2 after 3 or 5 h of fermentation. The hybrid Andrews−Levenspiel model was able to provide accurate descriptions of the behaviors of the conventional and extractive fed-batch ethanol fermentations, considering the removal of ethanol and water from the broth. Use of F t of 5 h and start of ethanol stripping at 3 h of fermentation substantially reduced the inhibitory effects of the substrate and ethanol on the yeast cells. This condition enabled the extractive fed-batch ethanol fermentation to be performed using substrate concentrations of up to 240 g•L −1 in the feed, with substrate exhaustion occurring after approximately 12 h. The total ethanol concentration reached 110.3 g•L −1 (14 °GL (degrees Gay-Lussac)), around 33% higher than that obtained using conventional fed-batch fermentation without ethanol removal.

One way of overcoming the substrate and ethanol inhibition effects in the industrial ethanol production process is to use fed-batch fermentation coupled with an ethanol removal technique. This work describes the optimization and experimental validation of sugar cane ethanol production by fed-batch fermentation with in situ ethanol removal by CO2 stripping. The optimization employing a genetic algorithm (GA) was used to find the optimum feed flow rate (F) and the ethanol concentration (C E0) in the medium at which to initiate stripping, in order to obtain maximum ethanol productivity. Conventional ethanol fermentation employing the optimum feed flow rate was performed with must containing 257.1 g L–1 of sucrose (180 g L–1 of total sucrose concentration), resulting in achievement of an ethanol concentration of 82.2 g L–1. The stripping fed-batch fermentation with high total sucrose concentration (260–300 g L–1) or 371.4–428.6 g L–1 in the must feeding was performed with optimal values of the feed flow rate and the ethanol concentration (C E0) in the medium at which to initiate stripping. At the highest sucrose feed (total concentration of 300 g L–1), the total ethanol concentration reached 136.9 g L–1 (17.2 °GL), which was about 65% higher than the value obtained in fed-batch fermentation without ethanol removal by CO2 stripping. This strategy proved to be a promising way to minimize inhibition by both the substrate and ethanol, leading to increased sugar cane ethanol production, reduced vinasse generation, and lower process costs.

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Hexavalent chromium is frequently found in industrial effluents as a result of the industrial applications of this compound and its anti-corrosive features. However, hexavalent chromium is extremely toxic, and its discharge in water is regulated, with a maximum limit of 0.1 mg/L in accordance with legislation established by CONAMA-Brazil (no. 397, April 3, 2008). To achieve lower discharge values, it is necessary to reduce from Cr(VI) to Cr(III), which is less toxic, and an economic alternative involves biological removal of this compound. Residence time distributions (RTDs) were measured to evaluate the behavior of actual biofilter operation conditions in a biofilter flow. The medium residence time distributions used were 8 and 24 h (recommended by the legislation). To optimize this process, a central composite design was used, considering the initial chromium concentration and pH as the independent variables and the removal of hexavalent chromium as the response. The boundary curves and surface response showed optimal behavior at 3.94 mg/L [Cr(0)] and a pH of 6.2. The removal process of hexavalent chromium is mathematically described by the Michaelis-Menten kinetic model. This model appropriately represents the variation of chromium concentration along the bioreactor.

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BACKGROUND In conventional ethanol fermentation processes, the low ethanol content in the wine is due to inhibition of its production by the yeast cells. Extractive fermentation is an alternative process that can be used to overcome product inhibition by removing the ethanol from the fermentation broth. The present study describes the modeling and experimental validation of ethanol production in extractive batch fermentation, with in situ ethanol extraction by oleic acid, in a non‐conventional drop column bioreactor (DCB) operated under industrial conditions. RESULTS A model was developed using the hybrid Andrews–Levenspiel equation and the ethanol distribution coefficient (KDE), which provided an excellent description of the extractive fermentation process with oleic acid. Furthermore, higher ethanol productivities were obtained in the extractive fermentations, with ethanol productivity of 11.27–12.98 kg m‐3 h‐1, 12.7–29.8% higher when compared with the conventional process without ethanol removal. This was especially evident for the best extractive fermentation, which finished around 2 h earlier than the conventional fermentation. CONCLUSION The DCB showed good performance for use in extractive fermentation with liquid–liquid extraction of the ethanol and this technique presented higher ethanol productivity compared with the conventional process. © 2017 Society of Chemical Industry

In the present study, the bioremoval of Cr(VI) and the removal of total organic carbon (TOC) were achieved with a system composed by an anaerobic filter and a submerged biofilter with intermittent aeration using a mixed culture of microorganisms originating from contaminated sludge. In the aforementioned biofilters, the concentrations of chromium, carbon, and nitrogen were optimized according to response surface methodology. The initial concentration of Cr(VI) was 137.35 mg l(-1), and a bioremoval of 85.23% was attained. The optimal conditions for the removal of TOC were 4 to 8 g l(-1) of sodium acetate, >0.8 g l(-1) of ammonium chloride and 60 to 100 mg l(-1) of Cr(VI). The results revealed that ammonium chloride had the strongest effect on the TOC removal, and 120 mg l(-1) of Cr(VI) could be removed after 156 h of operation. Moreover, 100% of the Cr(VI) and the total chromium content of the aerobic reactor output were removed, and TOC removals of 80 and 87% were attained after operating the anaerobic and aerobic reactors for 130 and 142 h, respectively. The concentrations of cells in both reactors remained nearly constant over time. The residence time distribution was obtained to evaluate the flow through the bioreactors.