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Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
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This indicated the necessity to completely clean the system when changing between different parts to be cleaned. A typical cleaning process involves performing a 1 hour SCF run with an empty chamber. Cross-contamination can be a problem when using one piece of equipment to clean multiple contaminants, especially when one of those materials is being collected for reuse.
6.5 Hardware Compatibility
In addition to material compatibility testing, there were a significant number of hardware compatibility tests performed. Unlike the materials testing, this testing detected problems with certain pieces of hardware. End housings, as described earlier, were put through a variety of tests including electrical, magnetic, dimensional and microscopic examination to determine if any changes were detected. No impact of the SCF process was detected by these tests other than small weight increases and dimensional changes which were reversible by vacuum baking.
Hermetically sealed assemblies were not as compatible and, as expected, did not tolerate the pressure of the cleaning process. A motor stater, composed of wire windings potted in place with an unfilled epoxy was also found to be incompatible with the cleaning process. In this case, the carbon dioxide would fill voids in the epoxy and then during depressurization cause fracturing at these voids. Later this problem was alleviated by cleaning at a lower pressure and decreasing the decompression rate.
The issues raised during testing of these different assemblies illustrate the need for effective and complete compatibility analysis prior to implementation of a cleaning process. Materials analysis alone is insufficient when determining if supercritical fluid cleaning can be used in a proposed application.
As development of the SCF cleaning process continued, it became apparent that certain changes were required to the cleaning
system. These were needed to improve cleaning efficiency and enhance the capabilities of the system.
The first modification was the acquisition of an additional cleaning vessel. This vessel had interior dimensions of 3 inches diameter and 8 inches deep (0.9 liter) and was equipped with a magnetically driven stirring propeller i The smaller chamber would allow a lower cleaning volume for experimentation purposes and the lower thermal mass would allow more precise control of the extractor temperature. The system was connected to the pump station through a solenoid valve so that either chamber could be selected for use. The stirring capability was added so that the effects of stirring on cleaning effectiveness could be evaluated. It was believed that stirring would improve the cleaning efficiency for bulk removal of damping fluids, but would not greatly improve efficiency during removal of small amounts of contaminants trapped in deep cracks and crevices.
During execution of the large number of test plans, it became apparent that monitoring and adjusting the temperature, pressure and flow rate were difficult to perform in a completely manual mode. A PC-based monitoring system was added to the SCF system to provide monitoring of the various temperatures and pressures within the system. The PC was set up to control and monitor the SCF system, but due to time and budget constraints only the monitoring hardware was installed. With this system, monitoring was performed of chamber inlet and outlet temperatures, separator temperatures, pump and chamber pressures, water bath temperatures and flow rates. To make this system more readable and meaningful to the operator, an animated display system was used to display real time data with updates every second.
Another change was made when the electrically powered high pressure pump that was supplied with the original system was replaced with an air-operated high pressure pump with higher capacity. The original pump took approximately 20-30 minutes to reach operating pressures with the existing cleaning chambers and would have taken an excessively long time to obtain operating conditions in the larger chamber which was planned for later implementation. The air-driven pump had been used in supercritical cleaning systems for similar applications and had shown excellent performance. The pump
operates using an in-plant air supply and would reach operating pressure within a few minutes with the existing extraction chambers. The pump was installed and has worked extremely well. By ducting the exhaust from the pump into the plant vent system, the noise level from this pump was similar to that of the electric pump. The airoperated pump also added another degree of safety to the system. The maximum liquid delivery pressure is directly related to the incoming air pressure. Tightly controlling the inlet air pressure lowers the probability of system overpressure.
A third cleaning chamber was purchased and installed. This chamber was much larger (8.5 inches diam. * 16 inches deep, 14.9 liter capacity) than either of the other two chambers. It was equipped with an pneumatically operated hydraulic closure and locking mechanism and was equipped with a variable speed magnetic drive. The larger size was necessary so that all components of a system to fill guidance instruments could be cleaned in one SCF cleaning run.
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