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Carbon dioxide, in its supercritical state, is being used to replace conventional organic solvents in chemical processes
Many reactions, extractions, separations and other operations in the chemical process industries (CPI) involve the use of organic solvents. In addition to handling and disposal issues, organic solvents can pose a number of environmental concerns, such as atmospheric and land toxicity. In many cases, conventional organic solvents are regulated as volatile organic compounds (VOCs). In addition, certain organic solvents are under restriction due to their ozonelayer-depletion potential.
Supercritical carbon dioxide is an attractive alternative in place of traditional organic solvents. CO2 is not considered a VOC. Although CO2 is a greenhouse gas, if it is withdrawn from the environment, used in a process, and then returned to the environment, it does not contribute to the greenhouse effect. There have been an increasing number of commercialized and potential applications for supercritical fluids. This article summarizes the fundamentals of supercritical CO2 properties and processing, and presents a number of current and potential applications.
Supercritical fluids
Above its critical values, a compound's liquid-vapor phase boundary no longer exists and its fluid properties can be tuned by adjusting the pressure or temperature. Although supercritical fluid has hquid-like density, it exhibits gas-like diffusivity, surface tension and viscosity. Its gas-like viscosity results in high mass transfer. Its low surface tension and viscosity lead to greater penetration into porous solids. Because of its liquid-like density, a supercritical fluid's solvent strength is comparable to that of a liquid.
The critical temperatures and pressures of materials vary quite significantly (Table 1). Generally, substances that are very polar at room temperature will have high critical temperatures since a large amount of energy is needed to overcome the polar attractive energy.
At critical conditions, the molecular attraction in a supercritical fluid is counterbalanced by the kinetic energy. In this region, the fluid density and density-dependent properties are very sensitive to pressure and temperature changes. The solvent power of a supercritical fluid is approximately proportional to its density. Thus, solvent power can be modified by varying the temperature and pressure. Because their properties are a strong function of temperature and pressure, supercritical fluids are considered tunable solvents. In contrast, conventional liquid solvents require relatively large pressure changes to affect...