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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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Intermittent flow meter Entrained material in C02 Installed particle filter before
operation exiting separator flow meter
Separator freeze-up No heater control on Installed separate heater
separator controls
No pressure gauges for co Gauges not specified Installed gauges
solvent pump
Flow could not be Improper flow valve Changed to micrometering
accurately controlled installed valve
Separator line would Separator line improperly Reconfigured line and added
constantly plug up designed line heaters
Separator would freeze No heater for separator Installed heater and
up temperature controller
C02 entering cleaning Heater control for inlet was Installed controller and
vessel was consistently poorly designed thermocouples
below process
temperature
C02 would cool below No insulation on internal Installed insulation
process temperature tubing
Pump could not maintain C02 was entering pump as Installed tubing insulation,
flow >0.8 ACFM gas instead of liquid heat exchanger and chiller
Pump pressure could not No pump pressure gauges Installed pressure guiges
be monitored
C02 temperature in Insufficient heating of C02 Designed, fabricated and
cleaning vessel was not entering vessel. Poor installed temperature
consistent and was hotter positioning of extractor controlled water bath with
at the top than at the control thermocouple heat exchanger and
bottom repositioned thermocouples
Figure 1. Current SCF system: (1) liquid C02 supply; (2) chiller; (3) plant compressed air supply; (4) high pressure pump; (5) back pressure regulator; (6) cosolvent reservoir; (7) cosolvent pump; (8) tube mixer; (9) hot water bath; (10) rupture disk; (11) cleaning chamber selection valve; (12) cleaning chamber; (13) heater jacket; (14) stirred cleaning chamber; (15) magnetic stirring drive; (16) separator; (17) flow meter; (18) totalizer/indicator; (19) vent to atmosphere; (20) UV detector.
Precision Cleaning: A Case Study 203
The most important problems were associated with the lack of sufficient heat being supplied to the separator where dissolved materials were collected, and the inability of the pump to maintain high flow rates. The carbon dioxide was supplied to the system through a manifold of standard pressurized gas cylinders with siphon tubes to supply liquid carbon dioxide to the pump. At flow rates greater than 0.8 Actual Cubic Feet per Minute (ACFM), the liquid would begin to flash to gas in the transfer tubing and within the pump heads and therefore the high pressure pump could not maintain flow rates above 0.8 ACFM. The solution was to install a heat exchanger and chiller to cool the incoming carbon dioxide to approximately -20F to ensure that it remained liquid all the way to the pump inlet and in the pump head. The flow rate could be maintained at 6.5 ACFM with a constant supply of liquid carbon dioxide.
The addition of the chiller system for the pump added to the problem for the as-supplied heating system for the pressurized carbon dioxide. The system was not capable of maintaining the pressurized carbon dioxide at the required process temperatures of approximately 170-185F. The problem was magnified by the position of the cleaning vessel temperature control thermocouple. The system was originally designed with the inlet on the chamber bottom and outlet on top with the control thermocouple near the inlet port. When the thermocouple sensed low temperature carbon dioxide entering the chamber it would turn on full heat to the vessel. This resulted in the carbon dioxide exiting the vessel well above the set process temperature. The problem was resolved by designing and installing a temperature-controlled water bath that the pressurized carbon dioxide would pass through prior to entry into the extraction vessel. The water bath would raise the temperature of the carbon dioxide to the required process temperatures at both high and low flow rates.
Freezing of the separator during operation at flow rates above 0.5 ACFM was another major problem with the original equipment. Originally, the system was designed as an open loop, with the gaseous carbon dioxide being exhausted to the building vent system after passing through the cleaning chamber and separator. The separator was not designed for the amount of expansional cooling encountered at the desired flow rates and would freeze due to the lack of sufficient
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