Downstream Processing of Butanol Produced by Fermentation: Thermodynamic Measurements and Modelling

Several factors, including environmental concerns and the concept of sustainability, are leading to an increased interest in processes able to produce fuels and chemicals from alternative, renewable resources. In this context, the interest in the Acetone-Butanol-Ethanol (ABE) fermentation of lignocellulosic biomasses has been intensified in the last years as a route alternative to the petrochemical one for producing butanol both for the chemical industry and as a biofuel. This work focuses on the downstream separation process of the ABE fermentation, which represents a key challenge due to its high energy demand. The attention is devoted to the thermodynamic framework in order to properly describe the phase equilibrium conditions involved in the process. To this purpose, new experimental data, collected at the Process Thermodynamics laboratory (PT lab) of Politecnico di Milano, are presented as for Vapour-Liquid Equilibria (VLE) for the binary system ethanol+water and for Liquid-Liquid Equilibria (LLE) for the pair 1-butanol+water. The collected data show good agreement with the experimental data available in the literature. Then, the performances of the Non-Random Two-Liquid (NRTL) thermodynamic model, coupled with the Redlich-Kwong (RK) Equation of State for the calculation, respectively, of the activity coefficient in the liquid phase and of the fugacity coefficient in the vapor phase, are discussed to assess its reliability in the thermodynamic characterization of the systems of interest. The model can predict the vapour-liquid equilibrium temperature with a percent Average Absolute Deviation (AAD%) in the range 0.17 – 0.26 %, the mole fraction of ethanol and water in the vapor phase with an AAD%, respectively, of 23.3 – 31.6 % and 4.3 – 12.1 %. Concerning the LLE measurements for the system 1-butanol+water, the model predicts the mole fraction of water with an AAD% of 2.4 % and 0.17 %, respectively, in the organic and in the aqueous phases.