Design and control of alternative downstream processes of IBE fermentation

Abstract

In this thesis, three alternative downstream process configurations are designed to obtain high-purity alcohol products from isopropanol-butanol-ethanol (IBE) fermentation broth. In order to make a fair comparison between process configurations, the same IBE fermentation broth, which is recovered by adsorption and gas stripping as in-situ recovery methods, is utilized as a feed stream. The fermentation broth, containing two homogeneous and one heterogeneous azeotropes, is highly dilute. Therefore, the downstream separation process of IBE fermentation broth is very energy intensive. The purpose of this thesis is to propose an energy-efficient process configuration and develop a heat-integrated version of the proposed configuration to further reduce the energy requirement and capital cost. In addition, the examination of controllability of the proposed process configuration and its heat-integrated version by dynamic simulations is also aimed. The steady-state designs of alternative configurations are compared in terms of economic and gas emissions. Optimization of the design parameters for alternative process configurations are done based on economic analysis. Total annual cost (TAC) is used as the objective function of economic optimization. The minimum value of the objective function is searched by using sequential iterative optimization method that can be defined as a grid search method. In addition to the economic evaluation, gas emissions of the configuration are calculated as an important metric for environmental evaluation. Pure distillation configuration has eight distillation columns and one decanter. Firstly, a significant amount of water present in the fermentation broth is taken away from the mixture by a preconcentration column. Then, water in homogeneous azeotropes and water in heterogeneous azeotrope are removed by extractive distillation and decanter-distillation systems, respectively. In this configuration, butanol, isopropanol (IPA) and ethanol are obtained in high purity. On the other hand, hybrid extraction-distillation configuration includes extraction, extractive distillation and conventional distillation units to separate butanol from IBE fermentation broth. In this configuration, a consecutive extractor and extractive distillation system is utilized to remove excess amount of water. In order to take away IBE mixture from water, too much solvent is required in this configuration. Therefore, capital and operating costs of this configuration are high due to the liquid circulating in the downstream units. In reactive distillation configuration, a significant part of water in the fermentation broth is removed by a preconcentration column, and the rest of water is consumed in the reactive distillation by an ethylene oxide-water reaction which forms ethylene glycol. This configuration reduces the energy consumption, operating and capital cost significantly compared to other two configurations. Based on the results, the reactive distillation configuration is selected as the proposed configuration since it is more economical and has less gas emissions compared to alternative configurations. Once the final steady-state designs are completed and the results are examined, then the dynamic controllability of the proposed configuration for downstream separation of IBE fermentation is investigated. Dynamic simulation of the process is created and three plantwide control structures are designed for the proposed process. The results of plantwide control structures are examined in terms of robustness. The robustness of control structures is tested against disturbances in feed flowrate and feed composition. Keeping biobutanol purity at its set-point against disturbances is the main aim of the designed plantwide control structures. The responses against the disturbances show that the control structure including dual temperature control for reactive distillation and biobutanol purification columns provides a robust control. Lastly, a heat-integrated version of the most energy efficient and economical alternative, reactive distillation configuration, is developed. Heat integration between condenser and reboiler of the columns is applied to reduce the energy consumption of the reactive distillation configuration in addition to process-to-process heat transfer between product and feed streams. The results show that the heat-integrated reactive distillation configuration provides remarkable reduction in costs and energy consumption.