Study and optimization of distillation process schemes for the production of bio-products from enzymatic fermentation

Butanol and isopropanol are currently produced by the petrochemical route. The production of bio-butanol and bio-isopropanol can also be carried out by fermentation (IBEA production: Isopropanol – Butanol – Ethanol – Acetone). However, the alcohol concentration in the fermentation broth obtained is less than 24 g/L, mainly due to the inhibition of butanol. The most used system to purify the IBE leaving the fermenter is distillation but, given the very low concentrations of the distillation feed, the separation costs can be very high. The main objective of this work is the optimization of the process to minimize the separation cost. The optimization process begins with the optimization of the number of plates, the feeding plate, the feeding temperature, and the heat exchanger technology. In this way, the cost of separation of the classic scheme has decreased by about 28.8%. Optimization of the scheme in terms of energy integration was then performed. Two energy integration schemes are proposed: one involves the use of a compressor to compress the overhead vapors of the column and uses them to provide heat to the reboiler and a second scheme in which the heat exchanger network has been optimized. While the case with the compressor was not advantageous, because of high investment costs, the heat exchangers optimization has reduced the separation costs by 27.9% compared to the non-optimized classic case. Finally, given the possibility of increasing the concentration of IBEA in the feed, using ISPR (In Situ Product Recovery) systems, or by engineering the strain of bacteria, it is shown how the IBEA concentration affects the cost of separating the scheme. The same procedure was used to estimate the separation cost of an IFPEN patented distillation scheme. With the energy optimization of this scheme, it was possible to reduce the cost for the separation of IBEA by 34.3%. Moreover, given the high energy consumption of the beer column and its impact on the separation cost, two other schemes for the concentration of IBEA were simulated. The first alternative scheme is a hybrid liquid-liquid extraction/distillation scheme. This scheme involves an extraction column to remove water from the feed using a solvent. Then the solvent is eliminated in a distillation column whose energy consumption is significantly reduced. The second scheme is a combination of an extractive and azeotropic distillation in which the butanol contained in the feed is used as a separation agent. Regardless of the scheme used, starting from very diluted IBEA, the obtained streams are water (vinasses) to be recycled to the fermenter or to be sent to water treatment, pure butanol, acetone, and isopropanol with an azeotropic composition (containing a small percentage of ethanol). For the production of pure isopropanol, it is necessary to dehydrate this stream. The isopropanol-water dehydration methods are the Pressure Swing Distillation (PSD), the Same Pressure Distillation, azeotropic distillation with cyclohexane (or benzene), pervaporation, and adsorption. The dehydration cost must therefore be added to the cost of IBEA separation.