Dynamic Control of Liquid Biomass Digestate Distillation Combined with an Integrated Solar Concentrator Cycle for Sustainable Nitrogen Fertilizer Production

Global food supply and security challenges necessitate the development of sustainable technologies that combine nutrient recovery from waste sources with energy inputs originating from the sun. Solar integration into energy-intensive conventional distillation technology has been slow because of the intrinsic operational reasons of the chemical and gas processing industry, such as the lack of accessible land for solar installations and the availability of energy-dense byproducts as energy carriers. New opportunities, however, arise in integrating solar energy with the chemical industry to produce concentrated nitrogen-containing fertilizers from agricultural waste. This would constitute the decentralized production of key fertilizers sustainably because of the availability of land for solar installations in rural areas. In this work, a process design study was performed to model the dynamic compatibility and day/night controllability of the distillation column that uses biomass digestion liquid as a feed and concentrates it into NH4+-rich distillate solution to be later crystallized as a green NH4HCO3 fertilizer. The energy needed for the reboiler was supplied during the day from 68 parabolic trough collectors utilizing liquid ethylene glycol and an auxiliary reboiler with no need for an external thermal energy storage. The key controlled process variable was the concentration of NH3 in the bottom stream that is typically discharged into the environment and can contribute to significant ecotoxicity if not maintained under 1 ppm. A proposed successful control structure that ensured good dynamic controllability entailed a feed-forward type of control with a total column heat temperature controller. Additionally, to effectively handle excess solar energy availability during the day at reduced feed flow, a steam generator was used and controlled using an ethylene glycol bypass flow override controller. While 3 ppm swings were observed in a basic control structure for the extended periods of 2 h, the optimal control structure resulted in 1.5 ppm swings for up to 15 min. The override control allowed steam generation during the day under the decreased feed flow disturbance of −20% for the design where solar collector duty matched the peak duty of the reboiler generating and returning to the steam header about 25% of the peak steam consumption in the reboiler. Overall, the work reports one of the very few dynamic controllability studies that combine solar energy harvesting and its use in sustainable process engineering.