Supercritical fluids have synergistic properties of both liquids and gases, such as low viscosity, high material diffusivity, and high solvent density, which make them good media for selective extraction [38]. The properties of low viscosity and high molecular diffusivity are gaslike properties, whereas high solvent density is more of a liquidlike property. A fluid is supercritical when both the temperature and pressure are above the critical point values. For example, the critical pressure and temperature of CO2 are 72.8 atm and 31.16°C, respectively [39]. Supercritical carbon dioxide (sc-CO2) has received a great deal of attention in many chemical process applications due to its low critical temperature and inexpensive abundance. As with most other super­critical solvents, the solvent power of sc-CO2 increases as the solvent density of carbon dioxide is increased.

The process for supercritical fluid extraction of oil from algae is similar to that of any vegetable oil. The supercritical CO2 acts as a selective solvent to extract the oil. The oil is soluble in supercritical CO2, in particular very high pressure CO2, but the proteins and other solids are not [39]. To further increase yield, a cosolvent, such as methanol or ethanol, can be used to increase the solubility of more polar components of the oil [38]. Generally speaking, supercritical fluid extraction has the capability of yielding high-quality oil and biomass and sc-CO2 is considered an environmentally benign solvent that gives low environmental impact.

A semi-batch supercritical extraction process of vegetable oil is described in the book by McHugh and Krukonis [39]. In this process, the extraction vessel is filled with crushed algae. The algae must be crushed in order for the oil to be accessible to the sc-CO2 because the extraction rate is limited by the mass transfer rate through the cell wall of a whole cell [38]. Supercritical CO2 is passed through the algae, extracting the oil, and leaving the solid residues in the vessel. The pressure of the supercritical CO2 and oil mixture is reduced so that the oil precipitates. The CO2 is then repressurized and recycled back into the extractor vessel. Several vessels can be used to optimize the system efficiency so that while some are being charged, extraction is happening in others. A schematic of the process flow diagram is shown in Figure 2.5.

The solubility of all vegetable triglycerides is approximately the same and depends largely on the temperature and pressure conditions of a supercriti­cal fluid. At 70°C and 800 atm, CO2 and triglycerides become miscible, and by dropping the pressure by 200 atm, the oil will separate from the CO2 [39]. This fairly low operating temperature allows for extraction of highly unsaturated triglycerides without degradation [38]. Although the process shown in Figure 2.5 was an early attempt at supercritical fluid extraction of algae oil, the pressures of 200 atm and 800 atm are excessively high for most chemical processing operations. In order to reduce the operating pressure, different solvent and cosolvent combinations can be developed for process synergism and implemented accordingly. Supercritical extraction of algae oil is still in the research stage. Research on making biodiesel from algae and

supercritical fluid extraction of algae oil is being done in many labs, includ­ing Sandia National Lab [18].

For the production of goods from algae oil to be feasible, the cost must be competitive with that of petroleum-based or other biomass-based products. The cost of the oil extraction process is a critical component of the overall cost of production [19]. The biggest obstacle facing the economical use of supercritical extraction technology is the possibility to feed and remove the solids continuously [39], as well as the ability to extract oil at a manageably low pressure by using an appropriate solvent combination. One possible option to overcome this obstacle is to have multiple vessels whose stages of operation can be alternated during the process cycles and stages. The other possibility is the use of a synergistically effective cosolvent supercritical fluid system, which has an enhanced solubility toward algae oil without requir­ing an excessively high pressure condition. Another economical obstacle currently preventing production of biofuels based on the supercritical fluid extraction from being competitive is the capital cost associated with use of expensive and energy-intensive equipment. Recent advances achieved in the supercritical fluid technology in other chemical, biological, and petrochemi­cal areas, in particular, pressure vessel designs, advanced reactor materials, solids-handling capability, binary and ternary solvent systems for maximum process synergism, tunability of fluid properties, ingenious energy and pro­cess integration, and extraction and development of high-value by-products could offer substantially enhanced process options for algae oil extraction based on supercritical fluid technology.