(132a) Supercritical Fluid Extraction for Process Intensification of Biological Processes
The energy efficiency of many biological processes is limited by recovery of dilute fermentation products. Ethanol is an exception to this rule, as it can be tolerated by microbes at concentrations greater than 10 wt%. Other products are much more toxic, which limits their endpoint concentrations and makes energy intensive their recovery. While distillation remains the separation technology of choice when feasible, energy efficient alternatives are required for instances where endpoint concentrations are insufficient for distillation, when products are insufficiently volatile, or when products are thermally sensitive. One promising technology, which has been known since the 1980s but incompletely developed, is supercritical fluid extraction. Supercritical fluid extraction combines elements of distillation with liquid-liquid extraction. The extraction solvent, typically carbon dioxide or a light hydrocarbon, is present in its supercritical state, meaning that the thermodynamic driving force is based on solute partitioning â similar to liquid-liquid extraction. Subsequent to extraction, the target molecule can be recovered in high purity simply by decreasing the pressure, meaning that supercritical fluid extraction does not require downstream separation steps â a feature that is held in common with distillation. Ideal targets should be amenable to biomanufacturing, but with titers limited by end product inhibition. They should be water soluble at concentrations less than their inhibition limit and partition favorably into one or more supercritical solvents and out of aqueous solution. Pressure and temperature can be adjusted to fine tune the separation. Butanol is used as a test case to show dramatic, >10-fold decreases in energy requirements for separation. New advances that combine microbes that are tolerant of supercritical fluids and capable of bioproduction of desirable target molecules hold promise for even greater improvements in energy efficiency, as demonstrated by a recent study using the newly identified strain Bacillus megaterium SR7. Integrated fermentation and extraction benefits from optimized process design including solvent recycling and co-production of carbon dioxide at high pressure. Future studies can investigate mass transfer in the supercritical fluid-water environment. Future development work can focus on reducing costs.