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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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In addition to the process benefits, there are cost and environmental benefits associated with the supercritical process. The supercritical fluid process has low operating energy costs when compared to other alternative solvent processes and the cost of the carbon dioxide used to supply the system is orders of magnitude less than the purchase costs of chlorofluorocarbons, especially with the added taxes imposed by the federal government. In addition, carbon dioxide is a more environmentally friendly material and does not have the disposal costs associated with other alternatives.
Supercritical fluids were identified in early 1989 as a possible CFC replacement for certain niche applications at Honeywell Space Systems. At that time there was very limited data on either the application of supercritical fluids for precision cleaning or compatibility studies with materials common to the aerospace industry.
Carbon dioxide was selected as the gas of choice for cleaning of precision guidance instruments. It was the gas most widely discussed in literature, and the solubility of supercritical carbon dioxide was often compared to heptane or cyclohexane which are excellent organic solvents. It was felt that if supercritical carbon dioxide did behave like these hydrocarbons, it could potentially dissolve most contaminants removed by CFC-113.
Several vendors of supercritical extraction systems were contacted and/or visited by Honeywell to determine if this might be a viable process. Samples of common contaminants and material compatibility coupons were taken to the vendors to determine if supercritical carbon dioxide would be compatible with common
materials and be successful in removing the contaminants. Based on the positive outcome of these initial tests, the original supercritical cleaning system was ordered in late 1989 and delivered in March of 1990. Table 1 contains a summary of the subsystems and general specifications which were used when ordering this system.
Table 1. System Specification
Subsystem or Description
(Cleaning Vessel 4" ID X 12 depth
(2.5 liter) with threadless pin
Compressor Electrically driven diaphragm,
10,000 psi maximum pressure
Flow Rate Variable, 0.1 - 6.5 ACFM
How Control Manual micrometering valve
Flow Monitoring Digital meter, turbine type with
Pressure Control Manually variable pressure
control valves on compressor,
liquid cosolvent pump, extractor
and inlet line.
Heating System Heated SCF inlet line and
extractor vessel
Temperature Control Control of extractor and inlet
temperature with over
temperature protection. Monitors
for extractor, separator and SCF
inlet temperature.
Cosolvent Reservoir and high pressure
Separator 1.5 L with easy-open closure
Safety 6,600 psi rupture disk and
variable pressure control on
extractor vessel.
Pressure System rated for 6000 psi
operating pressure
Designing and specifying this first cleaning system was a somewhat difficult process. Many equipment vendors were just starting up and had limited resources. Other vendors considered equipment design a proprietary technology and were unwilling to fully assist in the development of equipment specifications and design. These problems, and a lack of experience on our part, led to construction of a cleaning system with many deficiencies. Supercritical extraction systems had been used for many years in the laboratory and the food
industries for extraction and concentration of selected components, but this system was one of the first to be designed specifically for the purpose of precision cleaning. These problems were both major and minor, but all had to be corrected for the system to operate as a precision cleaning system. Table 2 is a summary of some of the more important problems encountered in getting the system to an operational state. Figure lisa diagram of the current supercritical cleaning system.
Table 2. Problems Encountered with As-Received SCF System
Rupture disk rated at Wrong rupture disk Replaced with 6000 psi disk
5000 psi installed
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