(66b) Incorporating Unsteady-State Fermentation Models in Aspen Plus:  Fed-Batch and in Situ Gas Stripping Simulation and Analyses

Authors: 
Darkwah, K., University of Kentucky
Seay, J., University of Kentucky
Knutson, B. L., University of Kentucky
Process simulation can predict the most informative experiments to conduct for process design, control and optimization. Traditional refinery processes have been extensively simulated and analyzed using the thermodynamic models and separation unit operations in process simulation tools, such as Aspen Plus. However, these essential tools for optimizing refinery processes have not been fully extended to biorefineries as a result of the inherent unsteady-state nature of fermentation processes. One of the approaches used to reduce substrate and product inhibition to fermentation microorganisms and increase fermentation productivity is fed-batch fermentation and in situ product recovery techniques, such as gas stripping. In the fed-batch fermentation, a bioreactor is charged with a relatively low concentration of substrate and as the substrate is consumed, fresh substrate is supplied at a rate that keeps the substrate concentration in the reactor below the inhibitory levels to the microorganisms. Gas stripping as a chemical separation method allows for the selective removal of the volatile components from the aqueous fermentation solution using a gas bubbled through the fermentation broth.

In our previous study, the batch acetone-butanol-ethanol (ABE) fermentation was used as a model system to simulate an unsteady-state batch ABE fermentation using a Fortran user kinetics subroutine linked to the batch reactor (RBatch) in Aspen Plus as a starting point for a biorefinery. This study extends the Aspen Plus batch model to fed-batch fermentation and in situ gas stripping with the aim of comparing the trends in the simulation to their corresponding laboratory fed-batch and in situ gas stripping experiments. An existing ABE fermentation model was reformulated to describe cell growth and inhibition using Monod kinetics, making it sensitive to changes in the fermentation environment as a result of fresh substrate addition (fed-batch) and product accumulation. This study provides a platform that can be used to guide integrated fed-batch fermentation and in situ gas stripping experiments and assess the accuracy of literature data. Additionally, a universally accepted process simulator, Aspen Plus, can be used in the simulation of fermentation processes linked with separation unit operations to provide strategies for the design and optimization of biorefineries.