(455e) Techno-Economic Analysis of Ultrasound Technology for Ethanol-Water Mixture Separation Process

The main source of CO₂ emission is thermal energy-based industrial facilities such as distillation operations which accounts for about 95% of the total separation energy consumption, but the thermodynamic efficiency of the distillation column is typically under 10%.¹ Therefore, replacing the conventional thermal separation processes while reducing CO₂ footprint using electrochemical energy is highly in interest. Since the separation of ethanol from an ethanol-water mixture by classic distillation is challenging because of its azeotrope formation,² other technologies are required to break the azeotrope and produce anhydrous ethanol. Conventional methods for obtaining anhydrous ethanol from 5-12 wt% ethanol-water mixture reached through biomass fermentation³ include azeotropic distillation,^4,5 extractive distillation,⁶ pressure swing adsorption,⁷ and a new method which is ultrasonic separation. The distillation process uses thermal energy for phase change and requires fossil-fuel-derived entrainers such as cyclohexane. Pressure swing adsorption is operated under high pressure to break the azeotrope, but ultrasonic separation does not need additional driving force or is accompanied by a phase change and is known to have better separation efficiency.⁸ In this regard, we have focused on developing nonthermal, nonequilibrium ultrasonic ethanol-water mixture separation process in pilot-scale to substitute conventional ethanol-water separation technologies.

The physical mechanism of the power ultrasonic separation is the acoustic vibration which travels through a bulk liquid ethanol-water solution and releases mist consisting of liquid droplets. Ultrasound-generated liquid droplets are enriched in ethanol with higher concentrations than in the bulk solution. Ultrasonic separation can avoid the azeotropic bottleneck of distillation and low energy efficiency due to phase change.⁹ Although ultrasonic separation has shown economic and environmental potential as a new ethanol-water separation method, economic assessments of ultrasonic separation on a commercial scale remain questionable due to undefined process design (e.g. pre-distillation + ultrasound, multi-stage ultrasound) and unestablished capital cost evaluation method. Therefore, the economic feasibility evaluation of ultrasonic separation has to be performed for futural commercialization. Also, the environmental impact must be evaluated for precise comparative study with conventional processes.

Here, we develop a data-driven ultrasonic separation model and report a techno-economic analysis of the ultrasound azeotropic separation process for the ethanol-water solution to produce 99.9 wt% anhydrous ethanol. We first design the conceptual design of the conventional ethanol-water separation process including azeotropic distillation, extractive distillation, pressures swing adsorption, and the new separation method, ultrasonic separation. Then, the conceptual designs developed by the process simulator are employed to predict the total annualized cost (TAC), including capital cost and operating cost of each process. In the case of ultrasonic separation, the optimal experimental condition is determined, and an additional experiment is performed based on the optimal condition. Further analysis is performed to identify the ultrasound column diameter. Then the effect of mist collection rate and transducer cost, which are likely to change with the advancement of ultrasonic separation technology, is examined. Also, we identify the global sensitivity of mist collection rate, transducer cost, utility cost, and ultrasound column diameter to understand optimal configuration compared with conventional processes. As steam used in the reboiler of the pre-distillation column is the major cost driver, we propose an optimal process design of ultrasonic separation to reduce steam use by finding the optimal pre-distillation column specification. Finally, the environmental impact of each process is evaluated and compared. Since ultrasonic separation technology is still in the development stage, the capital cost of ultrasonic separation is about twice of that of traditional technologies, but this can be supplemeted sufficiently with technology development. The operating cost of ultrasonic separation is 35-80% of conventional methods, resulting in unit production cost of $5.79/kmol ethanol which is feasible based on the prior study.¹⁰ This study revealed that ultrasonic separation is sufficiently competitive compared to other technologies economically and environmentally on a commercial scale. The analysis emphasizes the promise that the ultrasonic separation process significantly reduces the energy usage and environmental footprint and shows the potential of the bioethanol process based on renewable energy.

¹ U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Industrial Technology Program. (2005). Hybrid separations/distillation technology: research opportunities for energy and emissions reduction

² Li, G. and P. Bai (2012). New Operation Strategy for Separation of Ethanol–Water by Extractive Distillation. Industrial & Engineering Chemistry Research 51(6): 2723-2729.

³ Al-Asheh, S., et al. (2004). Separation of Ethanol–Water Mixtures Using Molecular Sieves and Biobased Adsorbents. Chemical Engineering Research and Design 82(7): 855-864.

⁴ Castillo, F. J. L. and G. P. Towler (1998). Influence of multicomponent mass transfer on homogeneous azeotropic distillation. Chemical Engineering Science 53(5): 963-976.

⁵ Bastidas, Paola & Gil, Ivan & Rodríguez, Gerardo. (2010). Comparison of the Main Ethanol Dehydration Technologies through Process Simulation. 20th European Symposium on Computer Aided Process Engineering: ESCAPE 20.

⁶ Jaimes Figueroa, J. E., et al. (2012). Improvements on Anhydrous Ethanol Production by Extractive Distillation using Ionic Liquid as Solvent. Procedia Engineering 42: 1026.

⁷ Mulia-Soto, J. F. and A. Flores-Tlacuahuac (2011). Modeling, simulation and control of an internally heat integrated pressure-swing distillation process for bioethanol separation. Computers & Chemical Engineering 35(8): 1532-1546.

⁸ Sato, M., et al. (2001). Ethanol separation from ethanol-water solution by ultrasonic atomization and its proposed mechanism based on parametric decay instability of capillary wave. The Journal of Chemical Physics 114(5): 2382-2386.

⁹ Kirpalani, D. M., & Toll, F. (2002). Revealing the physicochemical mechanism for ultrasonic separation of alcohol–water mixtures. The Journal of chemical physics, 117(8), 3874-3877.

¹⁰ Ebrahimiaqda, E. and K. L. Ogden (2017). Simulation and Cost Analysis of Distillation and Purification Step in Production of Anhydrous Ethanol from Sweet Sorghum. ACS Sustainable Chemistry & Engineering 5(8): 6854-6862.