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| Overview: | Supercritical fluid is a high pressure cleaning process that takes advantage of the fact that the fluid chosen becomes an extremely effective solvent for many organic materials, once in its supercritical state. It is a cleaning process that penetrates small openings and is especially useful for precision or intricate components such as gyroscopes, accelerometers, nuclear valve seals, laser optic components, special camera lenses, electromechanical assemblies, and porous ceramics. The process works well removing liquid contaminants, including silicone, petroleum and dielectric oils, flux residues, lubricants, adhesive residues, and fats and waxes. However, it is not very effective on heavy soils, nor for removal of particles or salts, except in circumstances where it is used in conjunction with agitation or ultrasonic cleaning.
The supercritical point is the pressure and temperature condition above which a chemical can no longer be vaporized, but, at the same time, the fluid does not retain its liquid-phase characteristics. Supercritical fluids have qualities unique to their fluid state; that is, unlike the characteristics and properties of either the vapor or the liquid phases. Small changes in temperature and pressure produce significant changes in density and solvent power. This combination of characteristics allows for greater mass transfer rates, effectively decreasing the time required to move the contaminants into the bulk supercritical fluid stream, thus providing rapid cleaning. Carbon dioxide (CO2) is probably the most widely used fluid in supercritical cleaning applications. CO2 is especially useful, since it is non-toxic, non-flammable, and non-ozone depleting; has a supercritical temperature near ambient temperatures (good for temperature sensitive parts); and exhibits excellent solvent properties in its supercritical state. Carbon dioxide supercritical cleaning requires high operating pressures in the range of 1,500 to 2,000 psig, but operating temperatures of only 35 to 65oC. As a result, most supercritical cleaning equipment has been designed for high pressure operation and is relatively small. High-pressure cylindrical chambers of supercritical cleaning equipment are intended to hold primarily small, intricate parts or parts with deep crevices, tiny holes, or very tight tolerances that normal alternative precision cleaning processes, specifically aqueous or semi-aqueous processes, have difficulty cleaning. A basic CO2 supercritical cleaning system has two primary cleaning vessels: the extraction vessel, in which the component to be cleaned is placed and flooded with supercritical carbon dioxide and, as the CO2 dissolves the contaminants, it flows to a separator vessel where the fluid is subjected to a pressure and temperature change (pressure is reduced and the carbon dioxide vaporizes). As that occurs, the solubility of the contaminant in the carbon dioxide decreases, causing the contaminant to separate from the bulk fluid. Once all the CO2 is evacuated from the separator, the concentrated contaminant is usually in residue form, often as an oily or tar-like liquid that is simply drained from the separator. The residue can then be recovered, recycled, or reused, if suitable; otherwise, the residue is disposed as the sole component; no solvents, wastewater, or other contaminants are present to increase the volume of waste disposed. The greatest concern when using supercritical cleaning processes is the safety risk of high operating pressures. Equipment must be properly maintained to prevent over pressure or failure of high-pressure components. Although carbon dioxide is non-toxic and non-flammable, it can displace oxygen and cause asphyxiation if leakage occurs in closed, occupied spaces. A CO2 monitor may be useful for closed areas, despite the fact that there are early warning symptoms, primarily difficulty in breathing (unlike nitrogen, which can quickly cause asphyxiation without warning). |
| Compliance Benefit: | Supercritical fluid cleaning eliminates
the production of waste solvents at a facility. Hazardous waste reduction is required
under RCRA (40 CFR 262) and Executive Order 13148. In addition, the
reduction of hazardous waste may help facilities reduce their generator status and
lessen the amount of regulations (i.e., recordkeeping, reporting, inspections,
transportation, accumulation time, emergency prevention and preparedness, and emergency
response) they are required to comply with under RCRA (40 CFR 262). Using a
non-ozone-depleting substance (ODS) will also help facilities meet the requirements under 40 CFR 82 and
EO 13148 requiring federal agencies to maximize the use of safe alternatives
to Class I and Class II ozone-depleting substances. Moreover, the decrease in solvents
may reduce the possibility that a facility will meet any of the reporting thresholds
for solvents under SARA Title III (40 CFR 355, 370, and 372) and EO 13148.
However, increased on-site use of carbon dioxide may increase the possibility of
meeting reporting thresholds for that chemical under SARA Title III.
The compliance benefits listed here are only meant to be used as general guidelines and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved |
| Materials Compatibility: | Carbon dioxide, in its supercritical state, is compatible with virtually all metals; however, non-metallic components, such as plastics, gaskets, and o-rings must be checked for compatibility. In general, cross-linked polymers and high-density polyethylene are not affected by CO2 supercritical cleaning. Cellulose acetate butyrate is one plastic that is not compatible with supercritical carbon dioxide. Other plastics that are susceptible to damage from supercritical cleaning are generally affected because the carbon dioxide solvates the plasticizers within the plastic and once removed, the absence of plasticizer tends to make the cleaned plastics more brittle. This is usually an undesirable result for plastic components. Compatibility should always be checked and tested, if necessary.
The extremely high pressures at which supercritical cleaning takes place make it unsuitable for components containing gas or evacuated spaces because they could implode or deform during the cleaning cycle. |
| Safety and Health: | The primary safety concern when using supercritical fluids is the high pressure and/or temperature operating range of the equipment. Proper design, operation, and maintenance are critical to safe use of the equipment. In addition, the hydrocarbon gases are flammable; thus, their use requires excellent maintenance measures to safeguard against leaks.
Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing any of these technologies. |
| Benefits: |
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| Disadvantages: |
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| Economic Analysis: | Supercritical cleaning systems are expensive, but operating and waste disposal costs are usually low.
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| NSN/MSDS: | None identified.
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| Approving Authority: | Appropriate authority for making process changes should always be sought and obtained prior to procuring or implementing any of the technology identified herein.
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| Points of Contact: | For more information |
| Vendors: |
This is not meant to be a complete list, as there may be other manufacturers of this type of equipment. CF TECHnologies, Inc. One Westinghouse Plaza, Suite 200 Hyde Park, MA 02136-2059 Phone: (617) 364-2500 FAX: (617) 364-2550 Contact: Mr. John Moses E-mail: JohnMoses@cftechnologies.com URL: http://www.cftechnologies.com/
EnviroPro Technologies |
| Related Links: |
None |
| Sources: | EPA Solvent Alternative Guide, SAGE 2.0, EPA and ICOLP guides for "Eliminating CFC-113 and Methyl Chloroform in Aircraft Maintenance Procedures," Oct 93, and "Eliminating CFC-113 and Methyl Chloroform in Precision Cleaning Operations," revised Oct 94. Pirrotta, R. and T. Pava, "Replacement of CFCs with Supercritical Carbon Dioxide for Precision Parts Cleaning," Proceedings of the International Conference on CFC and Halon Alternatives ‘94, p. 532-539, October 94. Mr. Bill McGovern, CF TECHnologies, Inc., Hyde Park, Massachusetts. |