(503g) Process Intensification of Large-Scale Continuous Biobutanol Production via a Multi-Feed Bioreactor with in situ Gas Stripping

Authors: 
Raftery, J. P., Texas A&M University
Karim, M. N., Texas A&M University
The economical production of alternative liquid fuels is vital to achieving energy independence from fossil fuel sources. While ethanol is often considered the primary candidate, butanol offers the advantages of higher energy density, lower vapor pressure, and lower volatility [1]. However, butanol is more toxic to cell cultures than ethanol, making it difficult to produce biologically at levels necessary for economic viability. Process intensification has been used in bioreactors by combining acetone, butanol, and ethanol (ABE) fermentation with separation phenomena, including pervaporation and gas stripping, to selectively remove the ABE products and overcome this limitation [2, 3]. Additionally, multi-feed inlet configurations have been investigated in continuous bioprocessing systems to limit inhibitory effects in bioreactors by manipulating the substrate concentration and dilution rate independently, limiting substrate concentration in the reactor without necessarily decreasing the reactor productivity [4]. The development of an intensified bioreactor with a multi-feed inlet configuration has the potential to overcome the toxicity limitation, increase process throughput, and produce butanol in a continuous, economically attractive manner that can compete with current bioethanol processes.

In this work, we examine the economic viability of large-scale, continuous production of biobutanol using Clostridium acetobutylicum and in situ gas stripping. First, the optimal control problem for a multi-feed intensified bioreactor is investigated using the flowrate of the stripping gas and inlet feeds as manipulated variable to maximize profitability. Batch reaction kinetics for the system, taken from Votruba et al., are extended to continuous processing in the formulation of the optimal control problem [5]. Solutions for the intensified bioreactor are compared to the economic viability of an optimally-controlled, continuous biobutanol reactor in the absence of gas stripping to emphasize the necessity of intensified process operations.

Once the optimal control policy for the bioreactor is determined, the process synthesis problem for large-scale biobutanol production from lignocellulosic biomass is explored by maximizing profitability. The synthesis problem considers using multiple bioreactors in parallel to process glucose made available from five options of cellulosic biomass, hybrid poplar, corn stover, sorghum, sugarcane bagasse, and switchgrass, in combination with four possible pretreatment methods, ammonia fiber expansion (AFEX), dilute sulfuric acid, sodium hydroxide, and liquid hot water. The use of lignin as a source of steam and salable electricity generation is also considered. The results of this study look to emphasize the importance of process intensification practices in the areas of bioprocess to develop new and otherwise economically impractical processes.

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