Nanocellulosic materials, either as cellulose nanofibrils (CNF) or cellulose nanocrystals (CNC), have a wide range of potential applications in different industrial sectors, due to their renewable nature and remarkable properties. Here, a sustainable and environmentally friendly method to obtain nanocellulose was evaluated using hydrolysis with citric acid, an organic acid that can be obtained as a biorefinery product. This approach resulted in a single-step extraction of nanocellulose, with carboxyl functionalization of the surface varying according to hydrolysis reaction times from 1.5 to 6 h, at 120 °C, as evidenced using NMR to measure the degree of substitution. The charged surface groups of CNC and CNF resulted in improved colloidal stability, with ζ-potential values from −36 to −48 mV. Both CNC and CNF extracted using different reaction times were thermally stable, but the increase of carboxyl groups reduced the degradation temperature. Techno-economic analysis (TEA) showed that the cost of citric acid had the greatest influence on the minimum product selling price (MPSP) of the nanocellulose, indicating that the production of citric acid within the biorefinery could be an interesting way to make this approach feasible.

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.

BackgroundSugarcane is the most efficient crop for production of (1G) ethanol. Additionally, sugarcane bagasse can be used to produce (2G) ethanol. However, the manufacture of 2G ethanol in large scale is not a consolidated process yet. Thus, a detailed economic analysis, based on consistent simulations of the process, is worthwhile. Moreover, both ethanol and electric energy markets have been extremely volatile in Brazil, which suggests that a flexible biorefinery, able to switch between 2G ethanol and electric energy production, could be an option to absorb fluctuations in relative prices. Simulations of three cases were run using the software EMSO: production of 1G ethanol + electric energy, of 1G + 2G ethanol and a flexible biorefinery. Bagasse for 2G ethanol was pretreated with a weak acid solution, followed by enzymatic hydrolysis, while 50% of sugarcane trash (mostly leaves) was used as surplus fuel.ResultsWith maximum diversion of bagasse to 2G ethanol (74% of the total), an increase of 25.8% in ethanol production (reaching 115.2 L/tonne of sugarcane) was achieved. An increase of 21.1% in the current ethanol price would be enough to make all three biorefineries economically viable (11.5% for the 1G + 2G dedicated biorefinery). For 2012 prices, the flexible biorefinery presented a lower Internal Rate of Return (IRR) than the 1G + 2G dedicated biorefinery. The impact of electric energy prices (auction and spot market) and of enzyme costs on the IRR was not as significant as it would be expected.ConclusionsFor current market prices in Brazil, not even production of 1G bioethanol is economically feasible. However, the 1G + 2G dedicated biorefinery is closer to feasibility than the conventional 1G + electric energy industrial plant. Besides, the IRR of the 1G + 2G biorefinery is more sensitive with respect to the price of ethanol, and an increase of 11.5% in this value would be enough to achieve feasibility. The ability of the flexible biorefinery to take advantage of seasonal fluctuations does not make up for its higher investment cost, in the present scenario.

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