Sesquiterpenes are a versatile group of 15-carbon molecules, traditionally extracted from plants for diverse applications ranging from fuels to fine chemicals and pharmaceuticals. Scarcity of natural resources and emergence of new applications have encouraged the development of sustainable solutions to produce sesquiterpenes. The recent development of engineered microbial strains able to produce and secrete sesquiterpenes reaching fermentation titres in the order of g per L, is a promising alternative to produce diesel-like biofuels from renewable biomass sources, like sugar cane bagasse. The most attractive aspect of sesquiterpene fermentations is that the extracellular product readily forms an oil phase separated from the aqueous fermentation broth in the reactor. The difference of densities between the aqueous broth and the light product phase opens the opportunity of integrating cost-efficient separation techniques (e.g. gravity separation, hydro-cyclones) with the reactor. This scenario could contribute to significantly lowering equipment and utility costs as well as reducing cost of raw materials by allowing for cell recycling. The scale-up of sesquiterpene fermentations has unveiled processing challenges that were not prominently present at laboratory scale. @en

Sesquiterpenes are a group of versatile, 15-carbon molecules with applications ranging from fuels to fine chemicals and pharmaceuticals. When produced by microbial fermentation at laboratory scale, solvents are often employed for reducing product evaporation and enhancing recovery. However, it is not clear whether this approach constitutes a favorable techno-economic alternative at production scale. In this study empirical correlations, mass transfer and process flow sheeting models were used to perform a techno-economic assessment of solvent-based processes at scales typical for flavors and fragrances (25 MT year⁻¹) and the fuel market (25 000 MT year⁻¹). Different solvent-based process options were compared to the current state of the art, which employs surfactants for product recovery. The use of solvents did reduce the sesquiterpene evaporation rate during fermentation and improved product recovery but it resulted in costs that were higher than, or similar to, the base case due to the additional equipment cost for solvent-product separation. However, when selecting solvents compatible with the final product formulation (e.g. in a kerosene enrichment process), unit costs as low as $0.7 kg⁻¹ can be achieved while decreasing environmental impact.

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In multiphase fermentations where the product forms a second liquid phase or where solvents are added for product extraction, turbulent conditions disperse the oil phase as droplets. Surface-active components (SACs) present in the fermentation broth can stabilize the product droplets thus forming an emulsion. Breaking this emulsion increases process complexity and consequently the production cost. In previous works, it has been proposed to promote demulsification of oil/supernatant emulsions in an off-line batch bubble column operating at low gas flow rate. The aim of this study is to test the performance of this recovery method integrated to a fermentation, allowing for continuous removal of the oil phase. A 500 mL bubble column is successfully integrated with a 2 L reactor during 24 h without affecting cell growth or cell viability. However, higher levels of surfactants and emulsion stability are measured in the integrated system compared to a base case, reducing its capacity for oil recovery. This is related to release of SACs due to cellular stress when circulating through the recovery column. Therefore, it is concluded that the gas bubble-induced oil recovery method allows for oil separation and cell recycling without compromising fermentation performance; however, tuning of the column parameters considering increased levels of SACs due to cellular stress is required for improving oil recovery.

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