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Materials 2019, 12, 350 7 of 22 The treatment of barley straw hydrolysate with lime also improved butanol production, from around 4.5 g/L with a yield of 0.21 g/g total sugar to 18.0 g/L with a yield of 0.29 g/g total sugar [127]. Similar results have been reported with corn stover and switch grass hydrolysates [107]. The high process costs associated with some feedstocks, product toxicity and low product concentrations are a few of the challenges involved in the production of biobutanol. Identifying the challenges involved in converting lignocellulosic biomass to biobutanol and assessing key process improvements can contribute towards making biobutanol more attractive commercially [120]. The efficiency of batch and fed-batch fermentation processes may be affected not only by the presence of numerous fermentation inhibitors, but also by the need for additional and time-consuming steps: sterilization of the bioreactors or re-inoculation. These limitations can be avoided by using continuous fermentation processes. The most common strategies for continuous fermentation are free cell systems, immobilized cell systems and cell recycling in the free cell fermentation process, the cells in the fermentation broth move freely, due to mechanical or air-lift agitation. This keeps the microbial cells and nutrients in suspension and helps promote mass transfer. However, fermentation processes using immobilized cells have important advantages over free-cell continuous fermentation processes. These include enhanced fermentation productivity, the possibility of continuous processing on stable fillings and lower recovery and downstream processing costs. Immobilization eliminates the lag phase and enables efficient continuous operation without repeated inoculation. It also improves cells stability and the catalytic effects of biocatalysts. Immobilization may improve genetic stability and protect cells against shear forces [128]. Immobilization has been applied in different bioreactors. Using a fibrous bed bioreactor, Huang et al. [129] achieved a significantly higher butanol yield from corn with a strain of C. acetobutylicum than could have been expected using conventional continuous fermentation techniques. Using butyric acid as co-substrate shortened the acid-producing phase (acidogenesis) in the feed stream and increased the duration of the solvent-producing phase (solventogenesis). Napoli et al. used a continuous fermentation process in a packed bed reactor for butanol production [130]. According to Qureshi et al. [131], nutrient limitation should be used to avoid excessive cell growth in packed beds. A modified and improved version of free-cell continuous fermentation is the cell recycling and bleeding process. This process has been investigated in a continuous butanol production system with high-density C. saccharoperbutylacetonicum [132]. To recycle the cells, a membrane module (for filtration) was implanted into the bioreactor. The removal of excess bacterial cells optimized the dilution rate and facilitated cell bleeding, leading to up to six-fold higher yields of butanol in comparison to conventional continuous fermentation without cell recycling. Further research efforts are necessary in order to scale up continuous systems for the industrial production of butanol [117]. Current technologies for the production of biobutanol by fermentation include the purification of biobutanol. Sánchez-Ramírez et al. compared steam stripping distillation using distillation columns to distillation columns with a liquid–liquid extraction column. The results showed the second process to be the more efficient design [133]. An interesting flash fermentation technology was used by Mariano et al. [117] in order to obtain higher productivity from butanol synthesis. This method could also reduce distillation costs. The technology consists of three interconnected units: a fermenter, a cell retention system and a vacuum flash vessel. Using this method, final butanol concentrations of more than 20 g/L were obtained. Extractive fermentation is another promising development, which could reduce the butanol inhibition effect. In this method, butanol is simultaneously produced and selectively removed to keep the concentration of butanol in the fermentation medium low [134]. Other common techniques for the removal of butanol include: liquid–liquid extraction, gas stripping, adsorption, perstraction, reverse osmosis and pervaporation [135]. Liquid–liquid extraction, using extractant containing 20% decanol in oleyl alcohol and C. acetobutylicum ATCC 4259 enhances butanol formation under conditions of controlled pH [136]. Alone, decanol is toxic to microorganisms; however, used in a mixture with oleylPDF Image | Butanol Synthesis Routes for Biofuel Production
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