Butanol Recovery by Extraction, Perstraction, Supercritical CO ₂ Extraction, and Ionic Liquid Extraction

Extraction processes offer the potential of a significant reduction of the energy requirement needed for separation and concentration when used within a hybrid process in combination with distillation. For a classical liquid-liquid extraction, the implemented solvent needs to fulfill several requirements, such as a high distribution coefficient and separation factor. The extraction step can be realized in situ directly within the fermenter or in an external separation unit, with circulation of the fermentation broth. In both versions the organic solvent is in direct contact with the fermentation broth and will be dissolved accordingly. Many solvents are toxic to the microorganisms, but oleyl alcohol has often been used, achieving a high fermentation productivity of around 3 g/L · h. To avoid the development of emulsions during the extraction process, a membrane can be applied between the fermentation broth and the organic solvent. With this perstraction process a fermentation productivity of around 2.3 g/L · h could be achieved. Using supercritical CO 2 extraction, a proper pressure system is needed. With cell recycle and immobilized cell reactors, high productivities in the order of 6.5–16 g/L · h can be achieved. Ionic liquids can be applied directly as solvent or implemented in pervaporation membranes (e.g., PDMS membranes) with good separation performance and high selectivity reached, but the proof with long-duration fermentation applications is still needed. Due to high separation selectivity reached in extraction processes, the hybrid process with distillation offers a significant reduction of energy demand for separation and concentration of the fermentation products.