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This project evaluates the feasibility of using cyanobacteria to produce ethylene from CO2. A recent paper published by the National Renewable Energy Lab (NREL) showed that it was possible to produce ethylene continuously in lab scale experiments. The cyanobacteria uses CO2, water, and light to photosynthesize ethylene.
We were tasked to design a plan to produce 100MM lb/year of ethylene. It was quickly determined that at the current published production rate the process would not be economically feasible. The rate would need to be significantly increased before the process becomes economically viable. Also, at the current state of technology, no commercially available or patented photobioreactor can support this process. The presence of both a gas feed and effluent pose significant obstacles for reactor design. It was also determined that due to the endothermic nature of the reaction and the inefficiency of photosynthesis, the process must rely predominantly on sunlight.
This project includes specifications and pricing for water purification, cell growth, and two separation systems. The present value of the process without the reactor section was calculated to determine the maximum reactor investment and annual operating cost to yield a return on investment of 15%. Location of the plant was also determined. Due to carbon dioxide and seawater needs, this plant will be located along the coast in Santa Rosa, CA, close to an ethanol plant. The plant will operate 340 days per year to allot for any downtime incurred in daily operation.
Cells will be initially grown in seed and growth tanks in batch-type process. Warm seawater supplemented with nitric acid, phosphorous acid, and sodium hydroxide will be used as the media for cell growth. Two separation sections were designed for purifying reactor effluent. The two separation systems investigated were pressure swing adsorption using zeolite adsorbent and cryogenic distillation with a custom nitrogen refrigeration system. These two were compared economically and it was shown that the PSA system yielded favorable economics.
Without the reactor section, the process using pressure swing adsorption yields an IRR of 67.62% with a net present value of $70MM at 15% ROI. The proposed reactor section investment and annual operating cost can have at most a net present value of -$70MM to meet project requirements.
Date Posted: 25 July 2014