(192c) Dynamic Modeling and Simulation of an on-Farm Bioconversion of Lignocellulosic Biomass into Acetone-Butanol-Ethanol (ABE)

Liquid biofuels, such as ethanol and butanol, produced through the biochemical processing of lignocellulosic biomass serve as a sustainable alternative energy source to fossil fuels. The products of Acetone-Butanol-Ethanol (ABE) fermentation are used as solvents, liquid transportation fuels (ethanol and butanol), and for the production of other chemicals. The challenges in using lignocellulosic feedstock in a traditional biorefinery to produce biofuels relate to the seasonality and diversity of the feedstock, and the need to harvest, transport and process large volumes of these feedstock for continuous processing. Farms have the space to store biomass and time to accomplish biomass processing. A proposed on-farm high solids biomass processing, therefore, eliminates the need to transport the feedstock to a central processing facility and uses the existing agricultural framework to convert lignocellulose to ABE fermentation solvents as semi-finished products that can be shipped economically to traditional biorefineries for further processing into products that meet market standards.

ABE fermentation suffers from low products concentrations, yields and productivity.  Various process design schemes such as batch, fed-batch and continuous fermentation are combined with product recovery techniques, such as gas stripping and adsorption, to reduce substrate and product inhibitions and improve productivity and yields. Aspen Plus simulations of the ABE fermentation process using microorganisms have previously relied on using stoichiometric reactions with fixed products yields and distributions, and the autocatalytic production of cells were either ignored or represented with stoichiometric equations in which cell growth was at a fixed yield relative to the formation of other products. This steady-state simulation is inadequate for the time-dependent process, the ABE fermentation process in the proposed on-farm processing system, due to changes in the fermentation environment as a result of the variable glucose availability, time-changing concentrations of cell biomass, substrate and products interactions and inhibition, and the effects of various recycle streams that may contain residual glucose, butyric and acetic acids and ABE.

To this end, this research aims to dynamically model and simulate the integrated ABE fermentation and product recovery, and product concentration to reflect the dynamic product and substrate concentrations over the course of fermentation and changes in the fermentation environment. The process will be simulated by incorporating rate-based kinetic mathematical models in Aspen Plus; Aspen Engineering Suite^TM. The process simulation results in Aspen Plus will be compared with the solution of the kinetic models, a system of ordinary differential equations (ODEs), in MATLAB to highlight the advantages of using the thermodynamic models in Aspen Plus and assess the accuracy of the developed procedure. This research will advance the current steady-state simulation of the ABE fermentation process to dynamic simulations incorporating autocatalytic production of cells, dynamic concentration of metabolites, and substrate and product inhibitions in Aspen Plus for the first time. Furthermore, the simulation will provide a tool that can be used guide ABE fermentation experiments, eliminate infeasible experimental set-ups, and assess the accuracy of literature reported data on the ABE fermentation process.  This simulation will also serve as a basis for assessing the economics of the process as a function of production capacity, product profile and product concentration leaving the on-farm bioprocess.