(393a) Process Synthesis and Optimization of a Combined Simulated Moving Bed and Distillation Separation Scheme for the Recovery of 2,3-Butanediol from a Dilute Fermentation Broth

The biochemical conversion of lignocellulosic biomass to hydrocarbon fuels via 2,3-butanediol (BDO) fermentation offers a promising pathway for the production of second-generation biofuels and added-value chemicals. The National Renewable Energy Laboratory (NREL) performed a detailed techno-economic analysis (TEA) in 2018 of a pathway that produces a blend of diesel and naphtha from corn stover [1]. This process is considered as the baseline state-of-technology (SOT) for our study, where 2,3 butanediol (BDO) is the key intermediate obtained via fermentation and adipic acid is the main co-product from lignin. While the SOT showcased the potential of the BDO pathway, there is still room for improvement and optimization in various stages. More specifically process synthesis and integration within the areas of fermentation, BDO recovery and upgrading steps can be used to enhance the design of the overall biorefinery.

In this study we focused on the purification of BDO from a dilute fermentation broth, which is a critical step in successfully demonstrating the biorefinery’s feasibility. Not only is BDO enrichment needed for the dehydration reactor to produce alkenes (the first stage of the downstream catalytic train) [2], but also there is a high energy cost associated with elevating the dilute stream to the reactor temperature and pressure conditions (250^oC and 60 bar). Thus, we present the synthesis and optimization of a novel hybrid separation scheme to enrich BDO and deliver a dehydration ready feed. The integrated process combines simulated moving bed (SMB) with two distillation columns, one to recover the BDO from the extract stream and the other to regenerate the desorbent (in this case ethanol) from the raffinate stream. These units are modelled through first principles in Pyomo using non-linear programming (NLP) and the resulting optimization problem solved using IPOPT. The proposed scheme presents an improved alternative to distillation since BDO has a high boiling point (~180^oC) and this requires vaporizing the large water content of the stream. Moreover, despite being mature technologies, to our knowledge, there are no studies in literature where SMB is simultaneously optimized with distillation, giving an opportunity for further exploration [3].

Ongoing research at Georgia Tech has demonstrated that adsorption-based options, like the SMB technology, offer an efficient solution for BDO recovery from dilute water mixtures. Using a hierarchical all-silica MFI zeolite adsorbent, SMB runs have shown that real fermentation broth (~10 wt% BDO) can yield over 99.9% recovery in the extract with purities from 20-80 wt%. The results were verified by the proposed model and were used to refine key system parameters. The accuracy in predictions guided the scale-up from the lab bench and successful experimental pilot-plant runs were performed to produce over 0.35 kg_BDO/day, the largest reported production of BDO to date from fermentation broths using SMB.

The hybrid separation scheme adds the two rectifying distillation columns to the SMB. The complex interactions between the SMB and distillation make identifying the optimal operating conditions and the level of separation performed in each unit difficult. The optimization problem was set up to minimize the total annual cost (TAC) subject to an overall 99.5% BDO recovery. The optimal process led to a 21% lower TAC value compared to our previously reported results [4] and a 45% improvement compared to the straight distillation alternative. This represents a reduction to $2.45/GGE in the minimum fuel selling price (MFSP) and a reduction of 15% in CO2e/GGE, global warming potential (GWP), compared to the SOT biorefinery. In addition, a sensitivity analysis was performed to assess the effect of adsorbent cost on the TAC optimization. Lastly, the optimization procedure was repeated with GWP as the objective function. We fixed the optimal amount of adsorbent from the economics optimization and when the GWP is minimized it leads to an increase of approximately 14% in the TAC. This underscores a potential trade-off between techno-economic (TEA) and lifecycle analysis (LCA), a subject that will further be explored.

Finally, a sensitivity analysis was performed to assess the effects of feed concentration and post-distillation purity. These values dictate the fermentation titer and dehydration feed composition, which are crucial for understanding the biorefinery from a systems level. As expected, the TAC and GWP were increased with decreasing BDO feed concentration and with increasing BDO post-distillation purity. However, a notable observation is that the titer influence on the optimal TAC is less pronounced as the post-distillation purity increases. The fermentation tank and dehydration reactor modelling and costing are still a subject of research being done in collaboration with NREL, and Georgia Tech and ExxonMobil, respectively. The presented optimization procedure revealed the optimal purity in the extract (level of SMB separation), amount of required adsorbent, and desorbent recovery and concentration, among other operating conditions. These process variables have seldom been considered in SMB studies and the gained insights can be applied to the recovery of other platform chemicals, not only BDO. This can help inform decisions related to scale-up and early-stage biorefinery commercialization.

References

[1] Davis, R., Grundl, N., Tao, L., Biddy, M.J., Tan, E.C.D., Beckham G.T., Humbird, D., Thompson D.N., Roni, M.S. Biochemical Deconstruction and Conversion of Biomass to Fuels and Products via Integrated Biorefinery Pathways. National Renewable Energy Lab, 2018.

[2] Adhikari, S.P., Zhang, J., Guo, Q., Unocic, K.A. Tao L., Li. Z. A Hybrid Pathway to Biojetfuel via 2,3-butanediol. Sustainable Energy & Fuels,. 4(8), 2020.

[3] Rodrigues A., Pereira, C. Minceva, M., Pais, L.S., Ribeiro, A.M., Ribeiro, A., Silva, M., Graca, N., Santos, J.C. Simulated Moving Bed Technology: Principles, Design, and Process Applications. Waltham/MA: Butterworth-Heinemann, 2015.

[4] Avendano, M., Fu, Q., Lao, J., Gupta, J., Lu, M., Nair, S., Realff, M. (2022, November 13-18). TEA and LCA analysis of the Production of Renewable Hydrocarbon Fuels and Co-products from Lignocellulosic Biomass via 2,3 BDO fermentation, separation, dehydration, oligomerization and dehydrogenation [Oral Presentation]. 2022 AIChE Annual Meeting. Phoenix, AZ, United States.