Note 84: Vacuum Pump Exhaust Filters - Charcoal Exhaust Traps

Part III - Charcoal Exhaust Traps

By John J. Manura, Scientific Instrument Services, Ringoes, NJ
4/28/00

INTRODUCTION

Vacuum pumps are widely used in scientific laboratories on a variety of scientific instruments including mass spectrometers, electron microscopes and many other instruments. These pumps can be a major source of laboratory air contamination. Indoor air contamination by vacuum pumps originates from both the lubricating oils used in the pumps, and also from chemicals that have contaminated the pump oil. When the vacuum pumps are used in instruments such as mass spectrometers, all the residual organics analyzed by the mass spectrometer are trapped in the vacuum pump oil. Eventually these organics are purged out of the oil and into the laboratory air via the vacuum pump exhaust port. Often the organic chemicals analyzed by these vacuum system instruments can be quite toxic or carcinogenic. This can present a serious environmental health problem in the laboratory. These pumps can be a major source of indoor contamination and therefore it is normally recommended that these pumps be vented outside the room or to a laboratory exhaust hood. However this is not always practical when the instrumentation is located within the interior of a building.

This is the third part of three articles reporting on the effectiveness of filters designed for use on the exhaust ports of vacuum pumps (1, 2 & 3). The previous article in this series (2) demonstrated that oil mist eliminators were effective at trapping the heavy oils from the vacuum pumps, but they were ineffective at trapping the more volatile organics from the vacuum pumps. This article discusses the effectiveness of the charcoal trap as a vacuum pump exhaust trap to solve this problem of trapping the more volatile organics in vacuum pump exhaust. The charcoal trap provides minimal backpressure to the vacuum pump exhaust and its large volume of trapping efficiency can solve most laboratory vacuum pump emission problems. When the charcoal trap is used in series with the oil mist eliminator, a very effective system is achieved which can trap the emissions from vacuum pumps and improve the quality of air in the instrument laboratory (1).

EXPERIMENTAL

Figure # 1 Vacuum Pump Exhaust Filtering System

The two stage vacuum pump exhaust filtering system is shown above (Figure # 1). The two filters attach in series to the exhaust port of a vacuum pump. For these studies, an Alcatel oil mist eliminator was used (Scientific Instrument Services part # 66827) as the stage 1 filter. An NW25 vacuum pump exhaust fitting was first attached to the exhaust port of the vacuum pump, and the Oil Mist Eliminator was clamped to this fitting using a centering ring and clamp. The Stage 2 charcoal trap used for this study is the Koby charcoal exhaust filter (Scientific Instrument Services Part # KA-1), which is fitted with an adaptor to mate it to the NW fitting on top of the oil mist eliminator. The two filters are clamped together using a centering ring and clamp.

All studies were performed on a BOC Edwards model E2M1.5 rotary vacuum pump, which is the standard model used on the Agilent mass spectrometers. The old oil in the pump was drained and the pump was charged with a volume of Inland 45 pump oil (Scientific Instrument Services). Inland 45 pump oil is the vacuum pump oil of choice because it is a highly refined oil comprised of aliphatic hydrocarbons with chain lengths of between 20 and 40 carbon units. The pump was run for two hours and the oil drained. The pump was again filled with a full charge of Inland 45 pump oil and run for 2 hours with the gas ballast valve open and then for an additional 2 hours with the gas ballast valve closed before any testing was done. This cleaning and flushing procedure assured that the pump was free of contaminants from the previous use of the vacuum pump. For this study a micro-needle valve was attached to the intake port of the vacuum pump to permit a calibrated flow of air into the vacuum pump. The flow through the needle valve was adjusted to 100 ml/min (or 300 ml/min for some studies) and calibrated with a Gilibrator Air Flow Calibration System (Scientific Instrument Services Part # 800844-2).

Figure # 2 Analysis of Vacuum Pump Exhaust in the Two Stage Exhaust Filtering System

For these studies, air samples were collected at three levels in the exhaust stream from the vacuum pump (Figure # 2). The first level (Stage A) is directly from the pump exhaust with no filtering. Another air sample was collected after the oil mist eliminator (Stage 1) and the final air samples were collected after the charcoal trap (Stage 2) and right before the exhaust air would enter the laboratory air.

Figure # 3 SIS AutoDesorb Short Path Thermal Desorption System Theory of Operation

Thermal Desorption tubes were packed with 100 mg of Tenax® TA and flow conditioned with pure nitrogen at 50 ml/min at 300 degrees C for 3 hours. Air samples were collected from the three pump stages onto the Tenax adsorbent traps at a collection rate of 25 ml/min for 20 minutes (500 ml total) using a Gilian LFS-113 Air Sampling Pump (Scientific Instrument Services). The collected samples were analyzed using the SIS Automated Short Path Thermal Desorption System (AutoDesorb Model 2000, Scientific Instrument Services) which is described in previous articles (4) and shown below (Figure # 3). The desorption tubes were attached to the AutoDesorb system and first purged with 30 ml of helium. The desorption tubes were then desorbed from 100 to 250 degrees C at a rate of 60 degrees per minute and held at 250 degrees for a total desorption time of 6 minutes. The desorbed analytes were trapped on a GC Cryo-Trap (Scientific Instrument Services), which was pre-cooled to -65 degrees C using liquid CO2.

The AutoDesorb system was attached to an Agilent Technologies 6890 GC for the separation of the analytes and the Agilent 5973 MSD to detect and identify the analytes. The GC column for these studies was an Agilent DB5-MS, 0.25 u film thickness, x 0.25 mm I.D. x 30 meters long. A 6" long DB-5, 1.5 u film thickness x 0.53 mm I.D. guard column was used at the front of the GC column and inside the Cryo-Trap. When the desorption was complete, the Cryo-Trap was rapidly heated to 250 degrees C. The GC column was held at 40 degrees C for 5 minutes and then temperature programmed to 280 degrees C at 10 degrees per minute. The total GC run time was 29 minutes. The Agilent MSD was operated in the EI mode and scanned from 35 to 450 daltons.

For the studies of the ability of the filters to trap volatiles, 10 ul of gasoline was injected directly into the vacuum pump. Gasoline was selected because it contains a wide range of volatile and semi-volatile organics and would accurately simulate the wide range of chemicals that might be injected into a mass spectrometer.

DISCUSSION

Charcoal traps are ideally suited for the trapping of volatile and semi-volatile organics in air. Similar charcoal traps are used to purify air in GC carrier gas purifiers and clean room air purification systems.

The Koby charcoal trap used for this study has been designed for the trapping of volatile and semi-volatile organics from the vacuum pump exhausts. Each trap contains 50 grams of carbon with a total surface area of 807,290 sq. ft.. This high surface area provides for very efficient air purification. The charcoal filters have a high adsorption capacity and affinity for adsorbing organic chemicals. In addition to trapping volatile and semi-volatile organics, the charcoal traps will adsorb water, oil mist dirt (to 0.5 microns) and bacteria (to 0.5 microns). The Koby charcoal traps produce minimal backpressure to the vacuum pump, and can therefore operate safely on the exhaust port of the vacuum pump. The filters are low in cost and are designed to be disposable.

The Koby charcoal traps are encased in a nickel plated metal case with brass 1/4" NPT male threads on both ends of the trap. Each filter contains 50 grams of activated carbon adsorbent resin. The filter also contains an internal coated metal baffle and multi-fiber adsorption filtration discs inside the housing to contain the charcoal adsorbent. Various fittings can be attached to the ends of the Koby filter to adapt them for mating with the NW flanges on all vacuum pumps and oil mist eliminators. It is normally recommended that these disposable filters be replaced monthly, or when the outward air reaches an unacceptable condition or exceeds permissible exposure levels. They should be replaced more frequently if extremely hazardous materials are being trapped on the filters. The Koby filters are disposable according to your local regulations and the types of organics that are trapped on the traps.

Figure # 4 Trapping Efficiency of Three Filter Stages (1.0 ul of gasoline injected into pump)

Figure # 4a 100 X Scale Expansion of Koby Filter Exhaust

In this first study, 1.0 ul of gasoline was injected directly into the vacuum pump and the exhaust air from all three levels or stages of the pump filters was collected and analyzed as described above (Figure # 4). At the exhaust at the top of the vacuum pump (top chromatogram - stage A) high levels of the volatile organics from the gasoline as well as the heavy hydrocarbons from the pump oil were detected. At the exhaust after the oil mist eliminator (middle chromatogram - Stage 1), high concentrations of the aromatics and hydrocarbons from the gasoline were detected but the heavy hydrocarbons from the pump oil were removed. At the final stage of filtering (Stage 2), after the Koby charcoal filter, all the volatile organics from the vacuum pump exhaust were removed. Figure # 4a is a 100 times expansion of the Koby filter exhaust in Figure # 4 (bottom chromatograph). The background is almost negligible and only consists of trace levels of siloxanes from the GC septum or capillary column liquid phase.

Figure # 5 Trapping Efficiency of Three Filter Stages (10 ul of gasoline injected into pump)

In this second study, 10 ul of gasoline was injected directly into the vacuum pump and the exhaust air from all three levels or stages of the pump filters was collected and analyzed as described above (Figure # 5). At the exhaust at the top of the vacuum pump (top chromatogram - stage A) as well as the exhaust after the oil mist eliminator (middle chromatogram - Stage 1), high concentrations of the aromatics and hydrocarbons from the gasoline were detected. However the charcoal trap was extremely efficient at removing all the volatile organics from the vacuum pump exhaust. The exhaust from the top of the charcoal trap (bottom chromatogram - Stage 2) contained no peaks of any volatile or semi-volatile organics. As shown above the light hydrocarbons including butane and pentane were not detected in the charcoal trap exhaust, nor was acetone or any of the aromatics common in gasoline present in the charcoal trap exhaust.

Figure # 6 Vacuum Pump Exhaust After the Koby Air Filter

An additional study (Figure # 6) demonstrates that the air exhausted by the vacuum pump through the Koby charcoal traps was cleaner than the normal indoor air in our testing facilities. In this study the exhaust after the Koby air filter was compared with the same volume of air sampled in the laboratory. Note that the results shown in this graph are shown at 50X scale expansion from the data shown in Figure # 5 previously. The small peaks shown in the graph are not from the gasoline, but are siloxanes from the GC septum.

Determination of the Capacity of the Koby Filter

Figure # 7 Setup for Determining the Capacity of the Koby Filter

A study was next conducted to determine the capacity of the Koby charcoal trap for a series of volatile organics. Gasoline was injected into the Koby filter using the apparatus shown above (Figure # 7). The Koby trap was attached to an injection port adaptor device which permits liquid gasoline to be injected via a syringe directly onto the Koby filter through a septum with no sample loss to the environment. It was continually purged with clean nitrogen at a flow rate of 100 ml/min. Any volatile organics from the gasoline which would break through the Koby filter would be trapped on the desorption tube. Gasoline samples were injected in 1 to 5 ml (up to a total of 75 ml) slugs and the desorption tubes were sampled for 5 minutes after the injection (total 500 ml of gas sample). The desorption tubes were then analyzed to detect the presence of any volatiles from the gasoline. Gasoline was selected for this study due to the wide range of organics present in this sample (hydrocarbons from butane on up, aromatics, etc). Gasoline was injected into the Koby trap until at a final volume of 75 ml was injected on a single trap, the odor of gas was detected at the exhaust of the Koby trap. This final sample was not analyzed, because of the potential that it would seriously overload the mass spectrometer.

Figure # 8 Determining the Capacity of the Koby Charcoal Trap.

For all the Koby exhaust samples analyze, none of the gasoline volatile organics were detected. The two charts above (Figure # 8) compare a Tenax blank tube (in which no material was injected) to the final sample analyzed in which 70 ml of gasoline liquid was injected into the Koby trap. Note that the scale expansion in this chart is 50X that of the other studies to show the low background of volatiles passed through the Koby Filter. The only peaks present in the two chromatograms above are a few siloxanes from either the GC septum or the liquid phase on the capillary column. This study confirms that the Koby charcoal trap is very efficient at trapping a wide range of volatile organics and has an exceptionally high capacity for these organic compounds.

Figure # 9 Koby Filter Mounted Directly on a Vacuum Pump.

This study justifies that in some cases, the Koby filter can be attached to the vacuum pump without the use of an oil mist eliminator. If the gas ballast valve is not used on the vacuum pump or the pump is not pumping large quantities of air, the Koby trap would probably last for at least a month without being saturated. However for optimum life, an oil mist eliminator is recommended. The oil mist eliminator will trap the heavy oils from the vacuum pump so that these oils do not saturate the charcoal trap. This will provide a greater degree of capacity for trapping other lighter volatiles and organics by the charcoal trap.

Figure # 10 Comparison of Alcatel and Edwards Oil Mist Eliminators & Koby Filter (Gas Ballast Valve Fully Open during this testing)

This final study was done to compare the Alcatel oil mist eliminator, the Edwards oil mist eliminator and the Koby charcoal trap (Figure # 10). This study was conducted with the gas ballast valve fully open with a total exhaust volume of 2400 ml/min. As in previous studies 500 ml of this gas volume was trapped and analyzed. As shown in our previous studies (2), the Alcatel oil mist eliminator does not do an efficient job of trapping the volatile organics purged from the vacuum pump oil (top chromatogram above). The Edwards oil mist eliminator, contains a paper filter element like the Alcatel filter, but it has an additional second element inside the trap (2). Although more effective than the Alcatel oil mist eliminator at trapping volatile and semi-volatile organics, the Edwards oil mist eliminator still failed to trap all of the volatile organics purged out of the pump oil. When the Koby filter was used by itself, virtually all of the volatile and semi-volatile organics are removed from the exhaust and trapped on the charcoal filter. As shown previously (Figure # 6), the vacuum pump air exhausted through the Koby filter, is cleaner than room air. In the Koby charcoal filter exhaust study above (bottom chromatograph), the small peaks present are siloxanes from the GC injection port.

Conclusion

This is Part III of a three part series of articles on the effectiveness of the vacuum pump exhaust traps. Part I describes the two stage vacuum pump exhaust filter system for the complete treatment of vacuum pump exhaust (1). Part II describes the operation and the effectiveness of the oil mist eliminator (2). Part III of this series describes the charcoal trap and its effectiveness at removing volatiles and semi-volatiles from vacuum pump exhaust (3)

The Koby vacuum pump exhaust filter has been shown to be extremely effective in trapping and removing a wide range of volatile and semi-volatile organics from the exhaust of laboratory vacuum pumps. It has been demonstrated that the vacuum pump exhaust which passes through the Koby charcoal trap is actually cleaner than normal room air. In addition it was demonstrated that the Koby charcoal filter has a great capacity for trapping volatile organics. In the studies above, 70 ml of gasoline was trapped on a single Koby cartridge with no breakthrough of even the most volatile components in gasoline.

Due to its outstanding adsorption capacity and affinity for a wide range of volatile and semi-volatile organics, the Koby filter is highly recommended to be used on the exhaust port of vacuum pumps that are not vented. It may be advisable to use them even on pumps that are vented in order to prevent the volatile hazardous organics from being vented outside the laboratory room. The Koby filter is recommended to be used in conjunction with an oil mist eliminator (1& 2). The oil mist eliminator traps the heavy oils from the vacuum pump and prevents these oils from saturating the charcoal trap. This provides a longer life for the charcoal filters. However, due to the high capacity of these traps, the Koby charcoal filters could be attached directly to the exhaust port of vacuum pumps without the use of an oil mist eliminator provided that the vacuum pump gas ballast valve was not used or that high volumes of air were not pumped through the vacuum pump.

In previous articles (1 & 2) the Two Stage Vacuum Pump Exhaust Filter System consisting of an oil mist eliminator and Koby charcoal trap has been shown to be an effective system for removing the volatile and semi-volatile organics from the exhaust of vacuum pumps and can help maintain an environmentally safe atmosphere in the instrumentation laboratory. The oil mist eliminator (Stage 1) has been shown to effectively trap the high molecular weight hydrocarbons from the pump oil and return these oils back into the vacuum pump. The charcoal trap (Stage 2) adsorbs and traps the volatile and semi-volatile organics from the pump exhaust and prevents these organics from entering the laboratory environment. It is recommended that both filters be used for most applications. When both filters are used in series on a vacuum pump exhaust port, the exhaust from the final stage of filtering has been shown to be cleaner than the normal laboratory air.

References

(1) Vacuum Pump Exhaust Filters, Part I Two Stage Vacuum Pump Exhaust Filter System, by John J. Manura, April 2000, SIS Application Note # 82 http://www.sisweb.com/referenc/applnote/app-82.htm

(2) Vacuum Pump Exhaust Filters, Part II Oil Mist Eliminators, by John J. Manura, April 2000, SIS Application Note # 83 http://www.sisweb.com/referenc/applnote/app-83.htm

(3) Vacuum Pump Exhaust Filters, Part III Charcoal Exhaust Traps, by John J. Manura, April 2000, SIS Application Note # 84 http://www.sisweb.com/referenc/applnote/app-84.htm

(4) Design, Development and Testing of a Microprocessor Controlled Automated

(5) AutoDesorb Short Path Thermal Desorption Apparatus, by John J. Manura, Vinod T. Das, Christopher Baker, Daniel Lieske, John C. Miller, John Manos, Roland Roadenbaugh, Thomas G. Hartman, SIS Application Note # 80 http://www.sisweb.com/referenc/applnote/app-80.htm

(6) Two Stage Vacuum Pump Exhaust Filter Systems, Scientific Instrument Services, Ringoes, NJ http://www.sisweb.com/vacuum/sis/exfilter.htm