(561f) System-Level Analysis of Isobutanol Production in Lignocellulosic Biorefineries

Lignocellulosic biomass has received a lot of attention as a promising feedstock for biofuel production. The use of lignocellulosic biomass avoids the food versus fuel competition and is largely available at lower costs. Isobutanol has been considered as an interesting biofuel alternative to bioethanol. Isobutanol has higher energy content and lower vapor pressures compared to ethanol, and it can be blended with gasoline in higher proportions, resulting in blends less corrosive compared to gasoline-ethanol blends. However, the fermentation step to convert sugars to isobutanol is more challenging. Most microorganisms used in isobutanol production have relatively low tolerance to isobutanol, limiting the concentration of isobutanol achievable in the fermentation broth. Several works in the literature have suggested in situ product removal strategies to increase sugar conversion and isobutanol yields without achieving toxic levels of isobutanol during fermentation.

Given the advantages of isobutanol and technological feasibility of sugar-to-isobutanol fermentation, the goal of this work is to perform a system-level analysis of an isobutanol biorefinery. Our goal is to provide useful insights into improvements required to make isobutanol production more economically attractive. We use superstructure-based optimization to study different biorefinery configurations, and we evaluate the impact of critical parameters on the energy efficiency and economics of the system.

The biorefinery is represented via blocks described in terms of conversion, energy demand, and cost parameters utilized to calculate outlet component flows, heat and electricity demands, and production cost. Base values for the block parameters are calculated from the literature and offline simulations. Given that sugar conversion to isobutanol is an active area of research, parameter values for blocks associated with the sugar fermentation and isobutanol recovery are calculated over a wide range of sugar conversion using detailed process simulations. The obtained parameter values, expressed as functions of sugar conversion, are utilized in the biorefinery superstructure optimization model.

Due to the low tolerance of microorganisms to isobutanol, two configurations are considered. The first includes direct fermentation and separation of isobutanol from the fermentation broth and is applicable when sugar conversion is low. The second configuration includes a modified fermentation block to simultaneously remove isobutanol from the broth. The broth is recycled, after an isobutanol-rich vapor stream is removed, to the bioreactor diluting the fermentation broth and avoiding toxic concentrations of isobutanol.

The need for additional unit operations in the second configuration result in higher production cost and energy (i.e., heat and electricity) demand in the fermentation block. Furthermore, these values increase as a function of sugar conversion due to higher isobutanol-rich vapor flows removed from the broth. The production cost and energy demand in the separation block remain approximately constant as a function of sugar conversion.

The results of the sugar-to-isobutanol biorefinery optimization model indicate that sugar conversion is the most important driver in terms of economics. The isobutanol production cost is reduced to $1.2/kg if sugar-to-isobutanol conversion increases to 100% of the theoretical yield. Furthermore, an additional 4–8% reduction in the production cost of isobutanol is attainable if a 50% reduction of the heat demand during isobutanol recovery can be achieved. The optimal biorefinery configuration does not change as a function of sugar conversion. The biorefinery is energetically self-sufficient at most conversion values studied, producing a surplus of electricity to be sold to the grid for extra credit. Finally, we expand our study to analyze a biorefinery co-producing isobutanol and ethanol.