Eco-efficient downstream processing of fermentation products by intensified distillation processes

Abstract

The rapid rise in global energy demands has accelerated the depletion of fossil fuel reserves, raising significant concerns about long-term energy security. Beyond resource exhaustion, this growing dependency on fossil fuels has contributed to severe environmental pollution and climate change, leading to strict environmental regulations. Consequently, extensive research efforts have been devoted to developing methods for producing a wide range of platform chemicals from renewable sources. In that respect, industrial biotechnology holds significant potential to replace fossil fuel-dependent production of various chemicals. Nonetheless, improvements in the fermentation and downstream processing are often necessary to enhance the competitiveness of large-scale bioprocesses. Significant research efforts have been put into genetic engineering to improve microorganisms’ performance and optimization of the fermentation process. However, downstream processing after fermentation has not been nearly as promptly addressed. A common limitation of the fermentation processes is the inhibition of microbial production by the products themselves which limits achievable product concentrations in the broth (often <10 wt%). Additionally, the formation of by-products and the presence of microorganisms complicate recovery process after fermentation. Consequently, downstream processing costs may significantly contribute to the total production costs (20-40% for the bulk biochemicals).
Process systems engineering is essential in developing competitive (bio)chemical processes, as it integrates process design and simulation in biotechnology. Hence, this thesis aims to advance industrial biotechnology by developing advanced, large-scale downstream processes for the recovery of fermentation products such as alcohols, diols, esters, carboxylic acids, and aromatic compounds. A systematic approach to downstream process development was undertaken through multiple case studies that were organized into chapters based on the characteristics of the primary product being recovered. Aspen Plus software was used as a computer aided process engineering tool for process design. Various process intensification techniques (e.g. dividing-wall column and reactive distillation), heat integration and heat pumping were studied to reduce energy requirements and decrease total process costs. The performance of the developed processes was assessed through economic analysis and the evaluation of key sustainability metrics, including energy requirements, greenhouse gas emissions, water consumption, water losses, and toxic materials and pollutants intensity.
Chapter 2 focuses on designing downstream processing strategies tailored for low-boiling alcohols produced through fermentation, including ethanol, isopropanol, isobutanol, and hexanol. Since these products are sufficiently volatile, the end-product inhibition can be mitigated by continuously removing the product from the broth using a distillation-based loop around the bioreactor. Heat pump-assisted vacuum distillation was demonstrated as an effective method for initial product recovery from the broth. Additionally, a novel pass-through distillation process was designed specifically for ethanol recovery. This innovative technique separates the evaporation and condensation parts of distillation by introducing an absorption-desorption loop between them. As a result, these operations can be conducted at different pressures, allowing the use of inexpensive cooling utilities while maintaining mild evaporation temperatures. Furthermore, a hybrid approach combining gas stripping with heat pump-assisted vacuum evaporation was developed for the recovery of isobutanol from fermentation broth. In all cases, the initial separation of the product from most of the broth under reduced pressure lowers the operational temperature, making it possible to reuse the remaining broth along with the microorganisms. This may improve fermentation performance by avoiding biomass loss, increasing substrate-to-product yield and reducing water requirements. Following this initial step, a preconcentration step using heat pump-assisted atmospheric distillation was carried out to remove most of the remaining water. The final purification method depended on the properties of the target product. Ethanol and isopropanol, which form homogeneous azeotropes with water, were purified using extractive distillation with ethylene glycol. In contrast, isobutanol and hexanol, which form heterogeneous azeotropes, were purified through a combination of simple distillation and liquid-liquid phase separation. Furthermore, a general methodology for recovering volatile alcohols from fermentation broths was developed and the performance of downstream processing was compared for the recovery of ethanol, isopropanol, isobutanol, and hexanol. Due to the higher hydrophobicity of longer-chain alcohols, their concentrations in the fermentation broth tend to be lower (e.g. 6.14 wt% for ethanol, 5.00 wt% for isopropanol, 1.61 wt% for isobutanol, and 0.24 wt% for hexanol). More dilute fermentation broths result in higher processing costs (0.08, 0.11, 0.16, and 0.53 $/kg, respectively) and increased energy requirements (0.96, 1.35, 2.02 and 3.07 kWthh/kg, respectively) of the downstream processing. Finally, it was concluded that the primary factor influencing downstream processing efficiency is the achievable product concentration in the fermentation broth, rather than the alcohol chain length.
Chapter 3 focuses on developing recovery processes for aliphatic diols that can be produced through fermentation, including 1,3-propanediol, 2,3-butanediol, 1,4-butanediol, and 1,3-butanediol. Due to their high boiling points, simple distillation is unsuitable for separating these compounds from the fermentation broth. Instead, initial separation steps are required to remove biomass, as well as large organic and inorganic molecules. These initial steps result in a clarified, desalted solution, which then undergoes further treatment through preconcentration and final purification. Since these final two steps contribute significantly to overall purification costs due to their high energy requirements, this research focuses on enhancing them to improve the efficiency of downstream processing for aliphatic diols. The preconcentration step was designed to remove most of water along with some light impurities using a heat pump-assisted vacuum distillation. The final purification step, which separates high-purity diol from residual light and heavy impurities, was effectively carried out in a dividing wall column. Furthermore, a general methodology for recovering high boiling components from dilute aqueous solutions was developed. Recovery processes for 1,3-propanediol, 2,3-butanediol, 1,4-butanediol, and 1,3-butanediol, designed according to this methodology, were compared. The performance of downstream processing was primarily determined by the product concentration in the fermentation broth, though the presence of impurities also played a significant role. Nonetheless, in all cases, the proposed methodology enabled the cost effective (total recovery costs of 0.29-0.70 $/kg) and energy-efficient (primary energy requirements of 1.35-2.01 kWthh/kg) recovery of valuable propanediol and butanediol from dilute aqueous solutions (7.0-10.0 wt%). Lastly, adaptable process design offers the possibility of using a single process configuration to produce various aliphatic diols depending on the market demand.
Chapter 4 discusses the development of recovery processes for various organic chemicals produced through different biotechnology-based processes, such as ethyl acetate, propionic acid, 2-phenylethanol, acetic acid, furfural, etc. While these processes differ due to the unique properties of each compound, they all rely on distillation as a primary recovery method. It was employed as the main separation technique to efficiently recover propionic, acetic, and succinic acids from dilute aqueous solutions. An adaptable dividing-wall column design was demonstrated to effectively recover propionic and acetic acids or perform esterification with methanol to recover methyl-propionate and methyl-acetate (total recovery costs of 0.40-0.47 $/kg). Distillation was also utilized as the primary technique in the recovery of ethyl acetate. Due to its high volatility, most of the ethyl acetate is separated in the fermenter off-gas. Consequently, product recovery was carried out from both the vapor and liquid phases, with an azeotropic dividing-wall column used effectively for final purification (total recovery costs of 0.61-1.09 $/kg). Furthermore, distillation-based downstream processing was effectively used for the solvent recovery and final purification of 2-phenylethanol after in-situ product recovery by liquid-liquid extraction. An adaptable azeotropic dividing-wall column was designed to recover high-purity 2-phenylethanol or perform esterification and recover 2-phenylethyl acetate (total recovery costs of 0.64-0.73 $/kg). Lastly, a sequence of distillation steps was developed to recover by-products (acetic acid, formic acid, furfural, 5-hydroxymethyl furfural) from the lignocellulosic biomass pretreatment process using liquid hot water (total recovery costs of 0.78 $/kg). This process could enhance the competitiveness of lignocellulosic biorefineries by enabling the valorization of additional products.
Furthermore, this thesis emphasizes the importance of advancing both upstream and downstream processes simultaneously to ensure the competitiveness and viability of entire bioprocesses. While the primary focus of this research is on recovery process development, it serves as an initial step toward achieving that broader goal. Chapter 5 highlights the significance of integrating upstream and downstream processes in bioprocess development. Isopropanol, along with acetone, and 2-phenylethanol, were selected as representative examples of low boiling and high-boiling biochemicals. Large-scale bioprocesses for their production were designed to showcase the practical application of integrated process design. Isopropanol and acetone were produced through gas fermentation, with different combinations of products (isopropanol and acetone together or separately) considered. Previously developed advanced recovery processes, including heat pump-assisted vacuum distillation, followed by heat pump-assisted atmospheric distillation and extractive distillation, were employed. Detailed techno-economic analysis and life cycle assessments demonstrated competitive economic performance (total production costs of 0.57-0.74 $/kg) and reduced environmental impacts compared to fossil fuel-based production (reduced global warming potential by 138-160%). 2-phenylethanol was produced de-novo through fermentation from glucose. Two different in-situ product recovery processes were compared: liquid-liquid extraction using oleyl alcohol and product adsorption. In both cases, distillation-based separations were used to effectively recover the solvent and obtain high-purity 2-phenylethanol. Additionally, a sensitivity analysis was conducted to examine the influence of 2-phenylethanol concentration on total production costs. This analysis revealed that minimal costs were achieved at a concentration of 1.5 g/L, with concentrations above 1 g/L not resulting in a significant reduction in costs due to increased fermentation costs. Regardless of the used in-situ recovery technique, the developed processes for producing 2-phenylethanol were found to be highly cost-effective (total production costs of 9.03-9.40 $/kg).
Summarizing, this thesis illustrates the significant impact that advancements in downstream processing can have on overall process performance. Specifically, it supports the development of novel advanced separation processes, based on process intensification principles, which have received relatively limited attention in industrial biotechnology. These innovations can play a decisive role in determining the feasibility of bioprocesses for industrial scale applications. Furthermore, the importance of incorporating downstream processing at the early stages of developing biotechnology-based processes is highlighted. The findings presented in this thesis have the potential to drive progress in industrial biotechnology and contribute to the successful commercialization of biotechnology-based products.