Cleaning and liquid contact with solids – Apparatus – With means for collecting escaping material
Reexamination Certificate
2000-09-15
2002-04-09
Coe, Philip R. (Department: 1746)
Cleaning and liquid contact with solids
Apparatus
With means for collecting escaping material
C134S105000
Reexamination Certificate
active
06367491
ABSTRACT:
BACKGROUND
Field of the Invention
The invention relates to methods and apparatus for cleaning articles using supercritical and/or near-supercritical fluids. In particular, the present invention relates to using differences in contaminant solubility and solvent density at various temperatures and/or pressures to effect cleaning action, to influence solvent and/or contaminant movement in cleaning apparatus, and to facilitate concentration of contaminants within cleaning apparatus and their subsequent removal.
Cleaning Using Solvent Action
It has long been known to use solvents in removing organic and inorganic contaminants from art articles. In such processes, the contaminated article to be cleaned is contacted with the solvent to solubilize and remove the contaminant. In a vapor degreaser, subsequent evaporation of the solvent separates the solvent and the contaminant, and the solvent vapors are redirected to the article to further clean it. The contaminant is typically concentrated in the evaporation step, being removed as a precipitate, a separate liquid phase, or as a concentrated solution in the original solvent.
An example of the above process is described in U.S. Pat. No. 1,875,937, issued Sep. 6, 1932 to Savage. Grease is removed from the surface of metal castings and other nonabsorbent bodies by means of solvents, while contaminants collect in the bottom of the apparatus and are drawn off from time to time through a valve.
One of the drawbacks of this type of cleaning process is that the cooling surfaces also have a tendency to condense water out of the atmosphere in addition to cooling and condensing the solvent. This condensed water then becomes associated with the solvent and thus comes into contact with the metal parts of the cleaning apparatus and with the article being cleaned.
U.S. Pat. No. 2,123,439, issued Jul. 12, 1938, to Savage, describes how this problem of condensing water with the solvent may be overcome by first contacting the atmosphere with condensing surfaces at a temperature above the dew point of the atmosphere in which the operation is being carried out, but substantially below the condensing temperature of the solvent. The condensed solvent is drawn off for use in the cleaning process, while the remaining vapors are brought into contact with still cooler surfaces (cooler than the dew point) to condense out the water so it can be removed.
An alternative to the above process of condensing the solvent on a cold surface and then contacting the article to be cleaned with condensed solvent is to cool the article itself. For example, U.S. Pat. No. 3,663,293, issued May 16, 1972, to Surprenant et al., describes how the degreasing of metal parts may be accomplished by generating vapors of a solvent from a liquid sump, establishing a desired level of solvent vapor by adjusting the temperature of condensing means, and introducing a contaminated cold article into the solvent vapors, thereby causing the vapor to condense on the article. Condensate containing the contaminant falls from the article into the sump, and the article is removed from the solvent vapor when its temperature reaches the solvent vapor temperature (thus precluding further solvent condensation on the article).
Cleaning Using Supercritical Fluids
In an effort to improve on vapor degreasing methods, supercritical (and near-supercritical) fluids have been used as solvents to clean contaminants from articles. NASA Tech Brief MFS-29611 (December 1990), describes the use of supercritical CO
2
as an alternative for hydrocarbon solvents conventionally used for washing organic and inorganic contaminants from the surfaces of metal parts.
A typical supercritical fluid cleaning process involves contacting the part to be cleaned with a supercritical fluid. The supercritical fluid, having solubilized contaminants and thus removing them from the part, then flows to a zone of lower pressure through an expansion valve. This depressurization causes the solvent fluid's state to change from supercritical to subcritical, resulting in separation of the solute (that is, the contaminant) from the solvent. Relieved of its burden of contaminant, the cleaned solvent fluid is then compressed back to a supercritical state and again brought into contact with the part if further cleaning is desired.
A different approach to cleaning with supercritical fluids is described in U.S. Pat. No. 4,944,837, issued Jul. 31, 1990 to Nishikawa et al. The method is applied to cleaning a silicon wafer in an atmosphere of supercritical carbon dioxide which contacts the wafer to solubilize the contaminant. After cleaning is complete, carbon dioxide is cooled to below its supercritical temperature (i.e., the system pressure is reduced and the carbon dioxide attains equilibrium between the liquid and gas phases) before removal of the cleaned wafer from the apparatus.
While effective, these processes are relatively inefficient because of the energy consumed in each pressurizaton-depressurization cycle. Further energy losses and increases in equipment complexity are associated with moving the solvent through the apparatus in both supercritical and subcritical states.
SUMMARY OF THE INVENTION
The present invention (an improved cleaner using supercritical and/or near-supercritical fluids) includes an apparatus which avoids or reduces several of the shortcomings noted above by keeping the solvent fluid in a supercritical or near-supercritical state in a pressure vessel throughout the cleaning and contaminant removal process. The pressure vessel comprises sealable access means to the vessel interior such as a door, lid, pressure lock, hatch, valve, etc. Note that a pressure lock may itself comprise a pressure vessel. Sealable access means may also comprise ports to introduce and/or remove articles to be cleaned, to remove (and if desired, recover) concentrated contaminants (including contaminated solvents), and to replenish the solvent as needed. Note that cosolvents and/or adjuvants which may be present as components in a solvent fluid may or may not also be in a supercritical state during normal operation of the cleaner.
Solubilized contaminants are concentrated and recovered through use of heating and/or cooling means within the pressure vessel which cause temperature changes in a solvent fluid which change contaminant solubility in the fluid. Even during contaminant recovery in the above improved cleaner, however, the solvent fluid remains in a supercritical or near-supercritical state. Consequently, the energy consumption is reduced (and efficiency is increased) over existing cleaners in which the solvent must be heated to account for enthalpy losses upon depressurization and compression to recycle the solvent and use it in the supercritical state.
In preferred embodiments of the improved cleaner, mechanical pumps are virtually unnecessary (initial pressurization and replacement of solvent fluid during operation can simply be accomplished by heating liquid carbon dioxide) because bulk-flow and micro-flow convection currents provide the desired fluid circulation. Additionally, because of the large density changes with low temperature differences and the low viscosity, supercritical fluids can move very quickly in response to relatively small temperature differences in different fluid zones. Such rapid solvent fluid movements, however, are detrimental to creating relatively large temperature differences within the solvent fluid necessary to effect large solubility differences within the supercritical fluid thereby diminishing the internal cleaning/recycling functionality of the invention. The rapid movement of the supercritical fluid past a heat exchanger surface reduces the amount of heat transfer; greater temperature differentials between the fluid and heat exchanger surface aggravates the problem. A solution is to increase the effective area for heat transfer by providing more contact time with the supercritical fluid by altering the fluid flow patterns through use of insulated baffle means.
In certain preferred embodiments, heat pu
Franjione John G.
Freitas Christopher J.
Marshall Mary C.
Coe Philip R.
Paula D. Morris & Associates P.C.
Southwest Research Institute
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