The process of cleaning one item invariably involves making something else dirty. Whether that something else is an organic or halogenated solvent, soapy water, or a rag, we seldom address the dirtying that accompanies any cleaning process. If we are to achieve environmentally benign cleaning, we must look at the life cycle of solvents employed for cleaning, including the potential for recycling, reuse, or release into the environment. Truly “green” cleaning processes not only minimize the amount of waste generated; but also they prevent the dispersal of that waste into large amounts of solvent, water, soil, or air. Dense-phase carbon dioxide is a great cleaning solvent from a pollution-prevention viewpoint. By-product CO2 generated by other industrial processes can be captured, so it is not necessary to generate CO2 specifically for cleaning. Spills of CO2 will not contaminate groundwater or create a need for soil remediation. Carbon dioxide even has advantages for the work environment, since no chronic, harmful effects are known from repeated inhalation of low concentrations of CO2. The barriers to using CO2 as a cleaning solvent have centered around two issues: the expense of high-pressure equipment and the poor solubility of many contaminants in CO2. Micell Technologies, Inc., based in Raleigh, NC, has addressed the equipment issue by using liquid CO2 just below ambient temperature (∼18–22 °C) and vapor pressure (∼50 bar). The equipment needed to contain this pressure is considerably less expensive than that needed for supercritical CO2 processes. As for the second barrier, Micell has surfactant packages that enhance the ability of CO2 to dissolve many contaminants commonly found on clothes or on metal parts. Micell is in the process of designing and bringing to market integrated CO2 solutions, including equipment and appropriate chemistries, to replace the organic solvents or water traditionally used in garment dry cleaning, metal degreasing, and textile processing. Dry cleaning is a bit of a misnomer, in that clothes are cleaned in a liquid solvent. “Dry” simply means that exposure of a garment, such as a wool suit or silk blouse, to water is minimized to prevent damage to hydrophilic fibers.
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