Project Summary
This project addresses the environmental and economic challenges of fossil fuel dependency by designing a commercial-scale chemical plant at Florida Tech that converts corn stover, an agricultural waste, into 300,000 metric tons of absolute ethanol per year. Utilizing a hybrid thermochemical-biochemical pathway, the process begins with mechanical pretreatment via a hammer mill and high-temperature fluidized bed gasification at 850 °C to produce syngas. To protect the biological catalysts used downstream, the raw syngas is purified using a water scrubber and a zinc oxide (ZnO) desulfurization reactor to remove contaminants such as H2S. The cleaned gas is then fermented by Clostridium autoethanogenum in a bubble column bioreactor, followed by a rigorous separation sequence involving flash separation, beer stripping, rectification, and a 20-unit 3A molecular sieve system to overcome the water-ethanol azeotrope. This integrated design provides a sustainable, high-purity renewable energy solution while effectively managing agricultural residues.
Manufacturing Design Methods
The manufacturing process was designed using a comprehensive simulation framework in Aspen Plus, which utilized RYield and RGibbs reactors to model the complex decomposition and chemical equilibrium during biomass gasification. Mechanical pretreatment was meticulously modeled using Vogel selection and breakage functions within a hammer mill simulation to achieve specific particle size targets below 6 mm. For the biochemical section, fermentation was modeled using yield-based assumptions, specifically a productivity of 2 g/Lh, to bypass the limitations of simulating microbial kinetics and mass transfer in Aspen. Finally, the separation units employed RadFrac distillation models with 25 and 30 stages, respectively, to concentrate the ethanol before it entered a specialized molecular sieve block for final dehydration.
Specification
The plant is designed to handle a massive feedstock throughput of 187,667 kg/hr of corn stover, which is processed in a gasifier operating at 850 °C and 1 bar. The fermentation stage is specified to operate under anaerobic conditions at 37 °C, 3–5 bar, and a pH of 5, utilizing Clostridium autoethanogenum to target an 80/20 carbon split between ethanol and acetate. To ensure continuous operation, the dehydration system specifies a total of 20 molecular sieve units, with one bed operational while the remaining 19 undergo a 460-minute regeneration phase.
Analysis: Technical analysis of the simulation confirms that the gasifier residence time must be maintained at 2 seconds to prevent ash melting and slagging while ensuring complete biomass decomposition. The purification analysis indicates that the zinc oxide reactor is highly effective, achieving a 99% conversion rate of H2S and requiring a steady feed of 263.65 kg/hr of ZnO to prevent downstream catalyst poisoning. Furthermore, the separation analysis demonstrates that while standard distillation can only achieve 95 wt% ethanol, the inclusion of the 3A molecular sieve breaks the azeotrope, achieving the "absolute" purity required by industrial fuel standards.
Other Information
Beyond the core process design, this project included a HAZOP analysis and a review of environmental and societal impacts to ensure the facility meets modern safety and sustainability standards. The design incorporates efficient resource management, such as a 1:1 liquid-to-gas ratio in the water scrubbing system and the use of heat exchangers to recover energy from the 850 °C raw syngas stream. These considerations ensure that the plant is not only technically feasible but also economically and environmentally responsible.
Acknowledgement
This comprehensive checkpoint report and the associated process design were prepared by the student engineering team consisting of Bre Venditti, Jacob Dymock, Eva Shealy, and Muath Alharbi. The work was conducted under the auspices of the Department of Chemical Engineering at Florida Tech's College of Engineering.
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