Abstract:
A process is provided for removing hydrogen peroxide and oil out the exhaust gas emitted from a sterilization chamber, such as from a STERRAD® medical instrument sterilizer. The exhaust gas is passed through either alumina or a mass of metal wool, which sorbs the lubricating oil out of the exhaust gas, and then through alumina coated with a precious metal catalyst, such as Pd, Pt, Rh and alloys and compounds thereof, which decomposes the hydrogen peroxide into water and oxygen. The precious metal coating on the alumina may be sufficiently discontinuous to allow residual lubricating oil in the exhaust gas to be sorbed into the alumina.

Description:
This application claims priority from provisional application No. 60/064,048 filed Nov. 3, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and device for reducing the percentage of hydrogen peroxide in gas exhausting from a sterilization chamber containing hydrogen peroxide vapor. 
     BACKGROUND OF THE INVENTION 
     A very common method for sterilizing medical instruments and devices comprises contacting the instruments and devices with vapor phase hydrogen peroxide. Several systems are available commercially to effect such sterilization, such as the STERRAD® Sterilization Systems available from Advanced Sterilization Products division of Johnson &amp; Johnson Medical of Irvine, Calif. In general, such sterilization systems comprise a chamber into which the instruments to be sterilized are placed. Air within the chamber is evacuated and hydrogen peroxide in the vapor phase is admitted to the chamber. Some systems, such as the STERRAD®, additionally induce a plasma upon the hydrogen peroxide sterilization gases. This enhances the sterilization and has the added benefit of disassociating the hydrogen peroxide molecules such that when the hydrogen peroxide plasma returns to its gaseous state, the molecules recombine to form simple oxygen and water vapor thereby lessening the need to deactivate the hydrogen peroxide before exhausting it from the sterilization chamber into the-environment. 
     Some systems forego the plasma and merely rely upon the hydrogen peroxide vapor to effect sterilization. A common practice for such systems, is to vent their exhaust to a vent exterior of the room housing the sterilizer, typically to a vent pipe on the roof. Little or no attempt is made to chemically alter the hydrogen peroxide and it leaves the vent in active form. A drain may be provided to collect any liquid hydrogen peroxide. Such a venting system inhibits movement of the sterilizer making it impractical to move between laboratories etc. as the vent requires the sterilizer to remain at a fixed location. 
     Even with most or all of the hydrogen peroxide being broken down by induction of a plasma, it is desirable to provide some means to deactivate hydrogen peroxide. For instance, the admission of hydrogen peroxide raises the pressure within the chamber and it is often desirable to further evacuate the chamber to lower the pressure. The evacuated gasses will thus contain a concentration of hydrogen peroxide. The STERRAD® sterilization system currently available employs a filter comprising copper wool over which gases being exhausted from the chamber are passed. Copper is a well-known catalyst for hydrogen peroxide, although a rather inefficient one. The contact between the hydrogen peroxide and the copper wool tends to break down at least a portion of any hydrogen peroxide exhausting from the chamber into water and oxygen. 
     SUMMARY OF THE INVENTION 
     Applicants have developed an improved method and devices for reducing hydrogen peroxide emissions from such a sterilization chamber. 
     A method, according to the present invention, is provided for cleaning gas exhausting from a chamber which contains a chamber gas comprising hydrogen peroxide. Gas exhausting from the chamber passes along a flow path from the chamber to an exit port. A bed of members coated with a precious metal catalyst for hydrogen peroxide is disposed within the flow path and the exhaust gas passes over the members and catalytically reacts therewith to convert the hydrogen peroxide to water and oxygen. Thus, the exhaust gas reaching the exit has a substantially reduced amount of hydrogen peroxide compared to the chamber gas. Preferably, the precious metal catalyst is selected from the group consisting of palladium, platinum, rubidium and compounds and alloys thereof. 
     Preferably, a vacuum pump is used to exhaust gas from the chamber. The bed may be located between the vacuum pump and the exit. If so, the members are preferably comprised of alumina, and more preferably the coating of precious metal catalyst on the members is sufficiently discontinuous to allow lubricating oil entrained within the exhaust gases to be absorbed into the alumina members whereby the amount of any such entrained lubricating oil available to deposit upon the catalyst and reduce an available surface area thereof is reduced. These members are preferably substantially spherical, although ovoid or other smooth shapes are desirable. When located downstream of the vacuum pump undue turbulence of the flow may enhance distribution of any entrained lubricating oil from the pump and hasten poisoning of the catalyst. A mass of metal wool disposed within the flow path upstream and adjacent the bed entraps within the wool oil entrained in the exhaust gas and diffuses the exhaust gas to enhance its distribution through the bed. Further, a portion of the bed adjacent the wool can be provided with alumina or other oil absorbent members without the precious metal coating to absorb entrained oil before it reaches the catalyst. 
     The bed can also be disposed between the chamber and the vacuum pump. The members may be provided with bores therethrough so that the exhaust gas flows through the bores to induce and enhance turbulence thereby enhancing the catalytic reaction. Preferably, such members are tubular. These members may be formed of a polymer such as polyethylene with the catalyst thereon. 
     A converter, according to the present invention, for cleaning gas exhausting from a chamber comprises a casing having a gas flow path therethrough, the flow path extending to an exit port on the casing. A connector on the casing connects to the chamber whereby gas exhausting from the chamber flows through the flow path. A bed of members coated with a precious metal catalyst for hydrogen peroxide is disposed within the flow path whereby hydrogen peroxide in gas exhausting from the chamber catalytically reacts with the catalyst to convert the hydrogen peroxide to water and oxygen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a sterilization system employing the method and device for cleaning sterilizer gases according to the present invention; 
     FIG. 2 is a perspective view of an embodiment of the device of FIG. 1 located downstream of the pump; 
     FIG. 3 is a further perspective view of the device of FIG. 2; 
     FIG.  4 . is a sectional view taken along lines  4 — 4  of FIG. 2; 
     FIG. 5 is an exploded perspective view of the device of FIG. 2; and 
     FIG. 6 is a front elevational view of an alternative embodiment of the device of FIG. 1 located upstream of the pump. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 discloses in diagram form a sterilization system  10  comprising a sterilization chamber  12  having a flow path  14  from the sterilization chamber  12  to an exit  16 . Typically, the exit  16  exhausts to atmosphere within a room containing the sterilization system  10 . It is thus desired to reduce or eliminate hydrogen peroxide in the gases exhausting from the exit  16 . A vacuum pump  18  draws gases from the sterilization chamber  12  and passes them out through the exit  16 . 
     During a typical sterilization cycle, instruments to be sterilized (not shown) are placed within the sterilization chamber  12  and the vacuum pump  18  exhausts gases within the sterilization chamber  12  to reduce the pressure therein down to 0.5 Torr or below. Hydrogen peroxide is then admitted to the chamber  12 . The admission of hydrogen peroxide will somewhat increase the pressure within the sterilization chamber  12 , typically back to around 10 Torr. To enhance vaporization of the hydrogen peroxide, to enhance penetration of the vapor into crevices in any items to be sterilized, and if a plasma is desired, to ensure its proper ignition upon inducement of an electromagnetic field, it is desirable to reduce the pressure in the chamber  12  back to the level prior to admission of hydrogen peroxide by exhausting additional gases with the vacuum pump  18 . Such exhausted gases, although small in quantity, will contain highly concentrated hydrogen peroxide. Typically on the order of 59% hydrogen peroxide or higher. 
     An upstream catalytic converter  20 , in the flow path  14  upstream of the vacuum pump  18 , and a downstream catalytic converter  22 , in the flow path  14  downstream of the vacuum pump  18 , catalytically act upon any hydrogen peroxide in gases passing along the flow path  14  to reduce or eliminate the hydrogen peroxide by the time such gases reach the exit  16 . Although both the upstream and downstream catalytic converters  20  and  22  may be employed together, suitable hydrogen peroxide reduction can be effected with either one thereby eliminating the need and expense to employ two catalytic converters. 
     FIGS. 2 and 3 depict a downstream catalytic converter cartridge  24  corresponding to the catalytic converter  22 . The cartridge  24  comprises a cylindrical body  26  having a male threaded connection  28  at a first end  30  thereof to allow removable connection to the sterilization system  10  (see FIG. 1) thereby facilitating removal and replacement of the cartridge  24 . Preferably, the body  30  is formed of an inexpensive easy to form polymeric material which is compatible with hydrogen peroxide, such as polyvinyl chloride (PVC), although other suitable materials such as aluminum will be apparent to those of skill in the art. 
     Turning also to FIGS. 4 and 5, an inlet filter  32  is positioned within the cylindrical body  26  at the first end  30 . The inlet filter  32  comprises first and second perforated discs  34  and  36  held in parallel spaced relations by tabs  38  extending from the first disc  34  and received within slots  40  in the second disc  36 . 
     Preferably, the discs  34  and  36  are formed of aluminum. Aluminum wool filter material  42  resides between the first and second discs  34  and  36 . A lip  44  at the cylindrical  401  body first end  30  holds the inlet filter  32  in the proper position. 
     An exit filter  46  is positioned at a second end  48  of the cylindrical body  26 . It comprises a discoidal cap  50  preferably formed of PVC, having a plurality of parallel, elongated slots  52  therethrough. A disc of particulate filtering foam  54  sits within a recess  56  in the cap  50  inwardly of and adjacent the slots  52 . If desired, activated charcoal may also be employed to help reduce odor emissions from the sterilizer. A perforated aluminum disc  58  covers the recess  56  to hold the foam  54  within the cap  50 . The exit filter  46  abuts a lip  60  within the body  26  and is adhered therein with an appropriate adhesive. 
     A filter bed  62 , comprising a plurality of alumina spheres  64  coated with palladium, fills the body  26  between the inlet and exit filters  32  and  46 . Preferably, the spheres are comprised of 0.5% Pd/alumina, type SOB having a diameter of between 2 and 4.75 millimeters, a metal area of 10 m 2 /g, an apparent bulk density of 0.75 g/cm 2  and a pore volume of 0.45 cm 2 /g. 
     In operation, exhaust gases enter the downstream catalytic converter  22  at the first end  30  passing through the inlet filter  32 , where the aluminum wool filter  42  traps lubricating oil which may be contained in the exit stream from the vacuum pump  18 . Vacuum pumps are typically provided with efficient epoxy glass microfiber filter elements to trap and recycle entrained lubricating oil in their exit flow, and it is recommended that to avoid poisoning of the palladium catalyst that the oil removing capacity of the standard filter be increased by a factor of 2 or more until no visually apparent oil mist is deposited on parts downstream of the vacuum pump. 
     The aluminum wool filter  42  provides a measure of added protection for the filter bed  62 . It is not thought that oil chemically interacts with the palladium coating of the spheres  64 , but rather may occlude portions thereof to reduce the effective surface area which may catalytically react with gases passing through the filter bed  62 . The filter  42  also acts as a diffuser to more efficiently distribute the exhaust gases through the filter bed  62 . 
     As the gases leave the inlet filter  32 , they pass over the filter bed  62  to come into intimate contact with the palladium coated spheres  64 . Any hydrogen peroxide in the gases tends to catalytically react with the palladium, breaking down into simple oxygen and water vapor. No heating of the catalyst is required. The flow then passes through the exit filter  46  and out to the environment. 
     FIG. 6 depicts the upstream converter  20  (see also FIG.  1 ). It comprises a body  66  having connections  68  at its first and second ends  70  and  72  and a bed  74  of tubular members  76  coated with platinum. The members  76  are shown loosely packed for clarity, but in practice it is preferable to pack the members  76  closely within the body  66 . A pair of screens  78  and  80  hold the members  76  within the body  66 . The screens function similarly to their counterparts  32  and  46  in the downstream converter  22  (see also FIG.  5 ), but the inlet screen  78  need not filter entrained oil as the converter  20  is positioned upstream of the pump  18 . 
     The members preferably are formed of polyethylene, are 6 mm long, have in internal diameter of 6 mm, and an external diameter of 10 mm. The platinum can be coated onto the members  76  using platinum dissolved in a salt and acid solution. Such a service is provided by Johnson Matthey of 2001 Nolte Drive, West Deptford, N.J. 
     The tubular shape of the members enhances turbulence of the flow through the bed  74 . Due to the turbulence, performance of this type of bed  74  per unit surface area of catalyst exceeds that of the bed  62  of spherical members  64 . 
     While the invention has been particularly described in connection with specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and that the scope of the appended claims should be construed as broadly as the prior art will permit