ULTRASONIC CLEANING OF EFFLUENT GAS FILTERS
Arthur E. Desrosiers
Bartlett Services, Inc.
P.O. Box 1800
Plymouth, MA 02360
800-225-0385, Ext. 300
C.S. Yam and Robert Kaiser
Entropic Systems, Inc.
P.O. Box 397
Winchester, MA 01890-0597
(617) 938-7588
ABSTRACT
A commercial scale ultrasonic cleaning system based on the Sonatol process has been applied by Entropic Systems, Inc. (ESI) and Bartlett Services, Inc. to decontaminate large effluent gas filters (24" x 24" x 6") that are used to clean up effluent gas from an incineration facility at a DOE site. The medium used in the ultrasonic cleaning process is a solution of high molecular weight fluorocarbon surfactant in a perfluorinated carrier liquid, which is recycled in the process, is inert, nonflammable, generally safe to use, and does not present a hazard to the atmospheric ozone layer. Particles removed from the effluent gas filters in the Sonatol process are captured by a series of disposable incinerable polypropylene filters (11" Diameter. X 15" Ht., typical size). The pore sizes of these disposable filters range from 30 m m (Pre-filter) to 0.3 m m (Final Filter). Previous experiments, carried out at the MIT Nuclear Reactor Laboratory, demonstrated that the Sonatol process can decontaminate highly radioactive electronic test components in less than one hour and that 99.5% of the removed radioactivity is captured by a single 0.22 m m filter. Twenty-seven effluent gas filters were processed during the initial cleaning campaign in April 1997. Particles were effectively removed from the processed filters, which significantly reduced the pressure drop across the filters, thus allowing them to be returned to service. This is the first known successful demonstration of cleaning this type of filter for reuse.
INTRODUCTION
The Bartlett Sonatol process, one of the applicants of ESI’s enhanced particle removal process, is a means of nondestructively decontaminating nuclear equipment. None of the decontamination methods now available can be used to decontaminate sensitive parts, such as electronic and electrical equipment, without irreversible damage. They also cannot treat geometrically complex parts such as effluent gas filters effectively. For these reasons, ESI received support from the Nuclear Regulatory Commission to further develop this process. A laboratory scale ultrasonic decontamination system has been developed to demonstrate the application of ESI’s enhanced particle removal process1 to the decontamination of radioactive electronic circuit boards2. The process uses inert perfluorinated liquids as the working media; the liquids have zero ozone depletion potential, are inert, nonflammable, and are generally recognized as non-hazardous materials. The parts to be cleaned are first sonicated with a dilute solution of a high molecular weight fluorocarbon surfactant in an inert perfluorinated liquid. The combination of ultrasonic agitation and liquid flow promotes the detachment of the particles from the surface of the part being cleaned, their transfer from the boundary layer into the bulk liquid, and their removal from the cleaning environment, thereby reducing the probability of particle redepostion, After the cleaning steps, the parts are rinsed with the pure perfluorinated liquid to remove residual surfactant, and dried. The process is operated in a closed flow loop, thereby minimizing the consumption of the process liquids.
Bartlett Services, Inc. has received a contract to decontaminate large effluent gas filters (24" x 24" x 6") that are used to clean up effluent gases from a government incineration facility. These filters contain multiple layers of a woven Hastalloy and fiberglass filtration medium. A mobile decontamination system dedicated to this project, based on the Sonatol process, was developed and constructed by Bartlett and ESI. Twenty-seven, non-radioactive, effluent gas filters were received for initial cleaning tests. The purpose of this initial cleaning test was to determine the decontamination ability and compatibility of the Sonatol process with the material and structure of the effluent gas filters.
DECONTAMINATION SYSTEM
The Sonatol Filter Decontamination System was constructed in a 44 ft. long Bartlett mobile trailer. The process layout is outlined in Fig. 1. The system consists of three major components: (i) an ultrasonic cleaning workstation, (ii) a filter tray and (iii) a liquid storage tank.
Figure 1. Sonatol cleaning system.
The dimensions of the ultrasonic cleaning tank are 26" x 26" x 26" with an inclination of 0.5" at the bottom for draining purposes. Two immersible ultrasonic transducers were installed at the bottom of the cleaning tank with a total power of 2000 watts (W). The ultrasonic energy was supplied by four 500 W ultrasonic power generators located outside the cleaning tank. The ultrasonic frequency used in this system was 40 + 2kHz square wave. A sparger with 49 spray nozzles was also installed near the bottom of the tank to remove superficial bulk materials collected at the surface of the effluent gas filters.
The dimensions of the filter tray are 48" x 48" with a 24" wall on one side for access. The filter tray was designed so that all the liquid transfers between the storage tank and the ultrasonic cleaning system were performed inside this self contained barrier. Therefore, any liquid leakage during liquid transfer would be captured by the tray itself. The major components inside the filter tray were (i) four disposable filters, (ii) a 1/3 hp magnetic drive pump and (iii) double acting air actuated valves. Two filter streams, one for wash and one for rinse, were used for liquid filtration. There were two disposable polypropylene profile filters, pre-filter and 10 gallons for the final filter. The filter pore size was 15 m m for the pre-filter and 0.3 m m for the final filter.
A magnetic drive transfer pump was used to transfer liquids between the storage tank and cleaning tank. The filters were located upstream from the magnetic pump so that any particles in the liquid stream would be removed prior to passing through the pump. At a liquid flow rate of 15 gpm, the system pressure was at about 10 psi, the pressure drop across the pre-filter was 2 psi and across the final filter was 4 psi. Liquid transfers required for different operations were controlled by a series of automated air actuated valves. The on/off positions of theses valves were controlled by a computer during the operation.
SYSTEM OPERATION
The Sonatol Effluent gas Filter Decontamination System is a fully automated cleaning system. The control software runs on a PC 486 computer. The control screen is shown in Fig. 2. There are two modes of operation: (i) Manual and (ii) Automatic. The manual mode is used for system start-up, maintenance, and diagnosis. In the manual mode, operators can control the on/off status of following system components: air actuated valves, pump, wash tank heater, loading lift, cooling water solenoid valves, drying heater, and ultrasonic power. To ensure that the system is running properly, temperature (wash tank, rinse tank 1, rinse tank 2, ultrasonic cleaning tank, and condenser) and pressure (wash tank, ultrasonic cleaning tank, system pressure) are measured on-line and displayed on the screen. The system will shut down automatically in case of high temperature or high pressure.
Fig. 2. CIF Prefilter project.
In the automatic mode, operators can select either "Full Cycle" or "Short Cycle" to meet different cleaning purposes. The Full Cycle or "Surfactant Cleaning Cycle" is the standard Sonatol process which includes four steps: (1) Pre-Rinse, (2) Wash, (3) Final Rinse and (4) Dry. During the Pre-Rinse step, rinse liquid is transferred from rinse tank 1 to the cleaning tank through the magnetic pump and the rinse filter string. When the ultrasonic cleaning tank is filled, the computer stops the liquid transfer and turns on the ultrasonic power for five minutes. The liquid is then drained back to the rinse tank 1 through the rinse stream filters. Any particles removed from the effluent gas filter in this step are collected in the rinse filters. The next step is Wash, which is similar to the Pre-Rinse but a different liquid and filter string are used. In Wash step, wash liquid is drawn from the wash tank to the cleaning tank through the wash filters. The ultrasonic cleaning time is ten minutes. After the sonication, the liquid is drained back to the wash tank through the wash stream filters. Particles removed from the effluent gas filters in this step are collected in the wash filters. The Final Rinse step then follows. The purpose of the Final Rinse step is to remove and recover any surfactant used in the Wash step that remains on the filter materials. The cleaning mechanism of the Final Rinse is exactly like the Pre-Rinse but freshly distilled rinse liquid from rinse tank 2 is used. The used rinse liquid is then purified by filtration, distillation, and phase separation. The surfactant is collected at the bottom of the still.
After the cleaning process, the effluent gas filters are dried for 15 minutes by a stream of hot gas. The vaporized liquid is then recovered by a water cooled condenser located down stream. The Short Cycle or "Non-Surfactant Cleaning Cycle" contains only the Pre-Rinse and Dry steps. The total cycle time for the Full Cycle is about 45 minutes to 60 minutes and the Short Cycle is about 15 minutes to 20 minutes.
RESULTS
Twenty-seven effluent gas filters (Otto York Model 299, Type 3G Demisters) were processed by the Bartlett Mobile Sonatol Decontamination Trailer. The weight and pressure drop (DPs) across each effluent gas filter were measured before an after cleaning. The DPs of the, as received, filters varied widely, ranging from 0.05 in. H2O to 1.05 in. H2O. Nine of the filters measured less than 0.25 in. H2O prior to cleaning. Ten filters measured between 0.25 and 0.50 in. H2O. Eight filters had an initial DP in excess of 0.50 In. H2O, ranging from, 0.60 In. H2O to 1.05 in. H2O. Weight loss was: a) more than 100 grams (g) for four filters, b) between 10 g and 100 g for nine filters, and c) less than 10 g for seven filters. The data were not available for the other seven filters.
DISCUSSION
The Sonatol cleaning process proved to be very effective and removed significant quantities of particles from the processed filters. A reduction in the weight of the filters, as a result of processing, was observed as noted above. This is attributed to the removal of particles. In all cases, there was a significant decrease in DP for the filters that initially had high DP values. All filters, after processing, had a DP of less than 0.25 In. H2O. However, a significant number of filters had DP of less than 0.25 In. H2O even before treatment by the Sonatol process, which indicates that the filters had been compromised. The presence of powder at the bottom of the ultrasonic cleaning tank after cleaning was also observed. The disposable filters, especially the 15 m m filters, rapidly became brown with time.
A sample of powder was examined under a microscope. The powder consisted of material ranging in size from less than 1 m m more than 50 m m.
CONCLUSION
The Sonatol cleaning process effectively removes particulate contamination that plugs effluent gas filters. These filters can be replaced in service, if they are not otherwise compromised. Reusing effluent treatment filters can save replacement costs and avoid disposal costs for radioactive or hazardous materials.
REFERENCES