Application of Near Critical/Supercritical Solvent Cleaning Processes
Replacement for Cleaning Solvents
The use of NC/SCCO2 for parts cleaning is a natural outgrowth of the analytical and production scale extraction processes. Precision cleaning of surfaces is a crucial step in many manufacturing activities. Removal of contaminants, including oils, greases, inorganic residues and particulates, is needed to prepare surfaces for subsequent operations such as coating, plating, or final assembly . In addition, removal of residues from piping and fluid handling equipment is essential when these contaminants are incompatible with process fluids. An important example is the cleaning of parts used in equipment which exposes the parts to high pressure oxygen gas. Even minor traces of grease or oils can react violently with high pressure O2 gas or liquid oxygen. As a result of these considerations, cleaning operations can be a very significant component of the manufacturing and remanufacturing costs .
Contrastingly, NC/SCCO2 is an environmentally benign cleaning technology that is very effective at removing contaminants from parts, including those with complex geometries. Supercritical carbon dioxide is an excellent solvent for precision cleaning, particularly for porous, intricate parts .
Current сleaning practice and NC/SCCO2
NC/SCCO2 cleaning is indicated for parts that are currently cleaned with solvents such as Freon-113 that are in limited supply, or with hazardous solvents, or for which no acceptable cleaning processes exist. Because the process occurs at elevated pressures (1100 psi or greater), parts that can fit within a moderately sized pressure vessel are best candidates for NC/SCCO2 cleaning. Parts geometry is very important. Components that incorporate blind holes, or other surface features that can prevent aqueous cleaning solutions from effective contact with contaminants, may be strong candidates for NC/SCCO2.
NC/SCCO2 cleaning is well suited to parts that are sensitive to corrosion, since carbon dioxide is compatible with all metals and evaporates completely, leaving no residue. As with many traditional solvents, NC/SCCO2 can swell certain elastomers and damage some polymeric materials. Although NC/SCCO2 is most effective at removing nonpolar contaminants such as greases or oils. Introducing a small quantity of a "co-solvent" such as methanol can effectively dissolve polar soils such as fingerprints or soldering flux. In some cases, NC/SCCO2 is effective as one component of a multistage cleaning process.
Although NC/SCCO2 can clean components to very exacting standards it may be a cost effective solution even for routine cleaning problems. In some cases, lower pressure and temperature processes using subcritical liquid carbon dioxide (NC), rather than SCCO2 may be more effective.
There is a considerable amount of interest throughout the research community in NC/SCCO2 cleaning processes for solvent replacement and treatment. One potential end-user of this technology is the Department of Defense of the USA, because it operates many weapons systems and platforms that require frequent maintenance and re-work operations. It uses large quantities of chemical solvents such as Freon-113 and Trichloroethane. In addition, many its installations produce hazardous waste streams that must be treated before discharge.
The Applied Research Laboratories (ARL) of the Pennsylvania State University has acquired a NC/SCCO2 system in order to evaluate the effectiveness of NC/SCCO2 technologies for problems of interest to the Department of Defense. The system used for most ARL NC/SCCO2 studies is an SFT-1000 SFE/SFR Processing Unit manufactured by Supercritical Fluid Technologies, Inc. fig. 1 shows a schematic view of the apparatus. Supercritical Fluid Technologies, Inc. acted as the system integrator as well as brought several fundamental patented technologies, such as specialized equipment designs and controls for pumping carbon dioxide in a tightly controlled and efficient manner [6, 7].
The system had the following features:
Operation at pressures up to 6000 psi (41.4.MPa) at temperatures of 392ºF (200ºC);
The extraction vessel itself is ASME Code-Stamped for 5500 psi (37.9 MPa) at 650ºF (343ºC) operation. It has an interior volume of one gallon (3.785 L) and is fabricated from 316SS and features a bolted closure. In operation at relatively low pressures (3300 psi), the solid metal seal can be replaced with an elastomer o-ring;
The extraction vessel contains a magnetically driven impeller to provide agitation of the fluid bath;
Mechanical safety components including rupture disc assemblies and relief valves are included to protest the equipment and operators from potential over pressurization conditions;
The CO2 pump is capable of sustaining a continuous flow of 350 cm3 of liquid CO2 per minute at the maximum operating pressure;
Carbon dioxide is delivered to the pump under its own vapor pressure (nominally 850 psi at Room Temperature). In order to prevent cavitation, a refrigerated chiller is used to circulate a water/ethylene glycol mixture through a heat exchanger attached to the pumping head;
All valves and lines downstream of the extraction vessel are heated to prevent formation of solid CO2 deposits. A heated back pressure regulator is used to control the rate of carbon dioxide flow. A solid state mass flow meter can be installed downstream of the "decompression vent" in order to measure CO2 flowrate;
A co-solvent (alcohols etc.) delivery system is included;
All process parameters (pressure, temperature, impeller speed, co-solvent delivery rate) are governed by a Programmable Logic Controller (PLC) with software developed at Supercritical Fluid Technologies, Inc. This device is capable of storing a number of process methods. The PLC is programmable by a keypad located on the front panel of the unit;
The system can be monitored by the Data Acquisition Package on a PC attached by an RS-232 link. The software displays a graphical process monitor showing the status of all the valves and other mechanical components, while real-time data traces track parameters such as temperature, pressures, and impeller speed.
ARL has performed a series of experimental studies to evaluate the effectiveness of NC/SCCO2 solvent processes for a variety of cleaning tasks.
Cleaning of Oxygen System Instrumentation
Components of oxygen systems must be cleaned to very stringent standards to ensure safe operation. Even traces of oils, greases, or particulates can react with high-pressure oxygen gas or liquid and lead to catastrophic fires or explosions . Aqueous cleaners such as Tribasic Sodium Phosphate have been used as alternatives to Freon-113 in oxygen piping systems. Unfortunately, these water-based solutions are not acceptable for cleaning instrumentation such as pressure transducers or gauges [9, 10]. A typical pressure gauge incorporates a curved bourdon tube. This tube is flattened and sealed at one end, it is difficult for an aqueous cleaner, with its relatively high viscosity and surface tension, to penetrate into the recesses of the tube to remove contaminants. In addition, it is impossible to ensure that all of the water and cleaner residues are removed from the tube. For these reasons, oxygen gauge cleaning operations still use Freon-113, which is in increasingly short supply due to a production ban.
SCCO2 offers a viable solution to these problems. Because of the exceptional penetrating power, SCCO2 should be able to reach contaminants even in "direct drive" gauges, which have tubes as small in diameter. Since residual CO2 evaporates completely to form an inert gas that can easily be flushed from the gauge, there is no concern about cleaner residue remaining behind within the part to react with oxygen (gas or liquid) while the Bourdon tube pressure gauge is in service.
The program used a test protocol for oxygen gauge cleaner qualification developed by the NASA White Sands Test Facility, and accepted by NAVSEA . Simulated gauges were constructed from 16-inch lengths of 1/8" OD tubing and bent into 4" diameter circles. These gauges were then doped with a test mixture containing five contaminants; including synthetic and petroleum based hydraulic fluids, lubricating oil, a gauge calibration fluid, and perfluoralkylether grease. The gauges were pressurized with SCCO2 to approximately 200 bar (3000 psi), and then vented to 100 bar (1500 psi) over a 30 second time period. The fluid temperature ranged from 100ºC (212ºF) to 130ºC (266ºF). The pressure was raised and lower five times over the course of a five-minute cleaning cycle. Cleaning efficiencies as high as 97% were achieved with this procedure that is equivalent to that obtained with Freon-113 cleaning techniques.
Although the results of this test showed that pulsed SCCO2 technique can be very effective for cleaning gauges that are rated for operation above 3000 psi, this process was not suitable for gauges with pressure ratings below 3000 psi. Since approximately 40% of the oxygen gauges processed at Puget sound Naval Shipyard are rated for the lower pressures, it was necessary to identify an alternative approach that will work for all gauges. An apparatus was developed at ARL that allowed the pressure pulse technique to be applied to any gauge, regardless of pressure rating. Fig. 2 is a schematic of this device. The gauge to be cleaned is placed in the main pressure vessel. The vessel headspace is pressurized with an inert gas (nitrogen or argon). At the same time, liquid carbon dioxide is delivered to the gauge via a dome loaded regulator, which tracks the vessel pressure, automatically admitting CO2 as needed to equalize the pressures inside and outside of the bourdon tube. The system is held at the desired pressure for a period of time to allow contaminants to dissolve in the carbon dioxide, which is either near critical or supercritical state, depending on the temperature of the pressure vessel. After a suitable interval, the 3-way inert gas valve is switched to the "vent" position. Carbon dioxide flows out of the gauge through the check valve, then the CO2 and inert gas are vented from the pressure vessel.
In this manner, large pressure pulses can be applied but the differential pressure across the gauge remains small, allowing even low pressure gauges to be cleaned. A 92% cleaning effectiveness with a standard deviation of 5% was achieved with this method. It appears likely that the NC/SCCO2 pressure pulse technique can be further improved and optimized, replacing Freon-113 in this gauge cleaning application.
Cleaning of Fuel and Oil Filters
NC/SCCO2 cleaning has been evaluated for three aviation fuel and lubrication system components. The hydraulic valve bellows cannot be cleaned with this technique because the smaller bellows are sealed and will not withstand high external pressures. NC/SCCO2 cleaning has, however, been successfully demonstrated for the transmission oil and fuel filters.
The transmission oil filter is currently discarded after a single use. It contains a polymer element supported by a wire mesh structure. As received, the filter was saturated with hydraulic fluid residue, which held a number of metallic particles in place on the outer surface of the elements.
The filter was cleaned in the laboratory scale NC/SCCO2 system shown in fig.1. For this initial test, processing conditions of 4000 psi (27.6 MPa) and 122ºF (50ºC) were used. A total of 93 grams of oily residue was recovered from the filter; this represented approximately one quarter of the gross weight of the filter before cleaning. The metallic particles were also removed, either by action of the carbon dioxide or because the sticky oil residue that originally held them in place was eliminated.
In order to successfully implement NC/SCCO2 cleaning for recycling this filter in an industrial environment, it will be necessary to optimize the process conditions (temperature and pressure) for more efficient and cost effective cleaning. This technique could be applied to remove hazardous materials from other types of oil soaked components, such as automotive oil filters, prior to recycling or landfilling.
The J79 Fuel Filter consists of a folded metal mesh structure and does not have a polymeric element. These units are currently degreased with perchloroethylene (PCE), followed by ultrasonic agitation with MIL-C-25107 "carbon remover," which contains ortho- dichlorobenzene. Implementation of NC/SCCO2 cleaning, therefore, has potential to reduce or eliminate the use of two relatively hazardous chemical solvents. The J79 fuel filter was coated with a black "grime" that was very tenaciously adhered to the surface. Supercritical carbon dioxide treatment recovered a surprisingly large (5 ml) quantity of a viscous oily fluid. The total weight of the contaminant extracted was 3.31 grams. Removal of this sludge loosened the particulate contaminant; the original grimy residue was replaced with a loose, fluffy gray powder, electrostatically attached to the surface of the filter. Attempts to use near-critical liquid carbon dioxide with its higher viscosity, to dislodge the residue were largely unsuccessful. Finally, a soft bristle brush was used to manually remove the powder. Approximately 0.42 grams of powder were easily removed, leaving the surface of the filter quite clean.
As with the transmission oil filter, it will be necessary to optimize the NC/SCCO2 process conditions, improve particulate removal efficiency, and develop QC/QA procedures in order to implement the technique as an alternative to the current practice.
Cleaning of Ball and Roller Bearing Assemblies
Ball and roller bearing assemblies have always presented a particularly difficult cleaning challenge. Bearings inevitably incorporate many small crevices between the curved surfaces of the balls or rollers and the races. Grease and particulate material can become trapped in these interstices. On the surface, it appears that NC/SCCO2 cleaning technology offered a solution to this problem, since the solvent should easily penetrate into these spaces to dissolve contaminants. While it was recognized that greases typically contain inorganic binders and fillers that are not themselves soluble in dense carbon dioxide, it was assumed that removal of the organic fraction would loosen the insoluble residue, allowing these inorganic binders and fillers to be removed manually. This, in fact, was the experience of Farncomb and Nauflett .
There may be some cases where the disadvantages associated with aqueous cleaners and other chemical solvents may make NC/SCCO2 cleaning a viable technology for cleaning bearings. First, some types of greases are adequately removed by NC/SCCO2 cleaning, followed by vacuuming, blowing or brushing to remove particulate material. Second, as discussed in a previous section, new surfactants have recently been introduced that enable carbon dioxide to dissolve otherwise insoluble chemicals.
When used as a replacement for conventional solvents, NC/SCCO2 has significant potential to reduce the environmental impact of certain manufacturing processes. NC/SCCO2 treatment technologies may also be useful in cases where other chemical solvent processes are not applicable or efficient. In particular, when contaminant must be recovered in a highly concentrated form (either because of its intrinsic value or because it is particularly hazardous), NC/SCCO2 extraction should be considered. The use of NC/SCCO2 regeneration of adsorbents is an area that shows promise and should be pursued. New surfactants and chelating agents allow NC/SCCO2 to dissolve chemical species (in particular inorganic compounds and metal ions) that would otherwise be insoluble. This relatively new development promises to greatly expand the scope of solvent replacement and treatment applications of this technology.
In conclusion, while NC/SCCO2 processing is not a universal "drop in" solution to all cleaning and treatment problems, it should be evaluated and investigated along with other technologies. ■