Abstract:
A drum vent filter useful for venting hydrogen gas from transuranic wastes in order to remove particles from the gas while controlling the release of VOCs has a carbon-to-carbon filter media of increased density with a relatively large filter face and reduced thickness. In addition, a protective lid is provided over the filter element which has been substantially increased so as to enhance the diffusion of hydrogen gas through the filter element.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to an improved drum vent filter. More particularly, the present invention is directed to an improved vent filter which has improved hydrogen gas permeability. 
     BACKGROUND OF THE INVENTION 
     Disposing of toxic waste, especially transuranic waste, presents modern societies with a daunting challenge. The waste is produced at one site, such as an industrial site or power generation site and must be eventually transported to a waste disposal or a waste storage site. Transporting these materials is hazardous because the materials are transported by trucks over existing highway systems. Frequently these materials generate gases which are also hazardous. The gases may result in combustion which can disperse the waste before the waste reaches the waste disposal or waste storage site. It has been determined that hydrogen gas is generated by radioactive waste. Accordingly, the Department of Transportation has set limits on the amount of hydrogen gas in a container of radioactive waste which may be safely shipped even if the container is vented. The rate of venting is therefore an important consideration because it is desirable to vent as much gas from the container as possible before the container is shipped and while the container is awaiting permanent storage. After the drum has been filled with waste such as transuranic waste, it is important that the gas being vented is filtered so as to remove particulate matter which may be radioactive and also to impede the diffusion of volatile organic compounds (VOCs) out of the drums. 
     In view of these considerations, there is a need to improve the drum vent filters currently used. 
     SUMMARY OF THE INVENTION 
     In view of the aforementioned considerations, the present invention is directed to an improved drum vent filter for filtering hydrogen gas to remove particles therefrom. The drum vent filter comprises an annular housing having an axially extending portion with a threaded exterior surface and a cylindrical interior surface defining a cylindrical interior space with first and second axial openings. The cylindrical interior surface has an annular lip therein having an axially facing support surface and a free surface. The free surface is spaced a selected distance from the first axial opening and the support surface is spaced a selected difference from the second axial opening. A radial flange extends outwardly from the outwardly extending portion of the housing and has a top surface which is flush with the second opening of the cylindrical interior surface and a bottom surface adjacent the threaded exterior surface of the cylindrical portion. A cylindrical filter element comprised of a carbon-to-carbon filter media is disposed within the cylindrical space and has a bottom surface abutting the annular lip and a top surface flush with the top surface of the radial flange. The filter element is retained within the cylindrical space with a peripheral layer of adhesive sealant. Disposed in spaced relation to the top surface of the radial flange is a lid which defines an axial gap between the top surfaces of the filter element and radial flange and a radial gap between the top surface of the radial flange and the bottom surface of the lid. An annular seal is disposed about the cylindrical portion of the housing and abuts the bottom surface of the radial flange wherein the axial gap between the bottom surface of the lid has a height in the range of about 0.125 inch to about 0.175 inch, and preferably about 0.150 inch; wherein the cylindrical space has a diameter of at least 0.8 inch and wherein the housing has an axial length no greater than about 0.375 inch, whereby diffusion of hydrogen gas through the vent is enhanced. 
     In a further aspect of the invention, the carbon-to-carbon filter media has a thickness in the range of one quarter to three-tenths inch, a density of 0.2 to 0.3 g/cm 3  and a diameter in the range of 0.60 inch to 1 inch and preferably about 0.70 inch. 
     In a further aspect of the invention, the housing has overall height in the range of 0.65 inch to 0.85 inch, and preferably about 0.75 inch and is made of type 304 stainless steel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of a prior art drum vent filter over which the drum vent filter of the present invention is an improvement; 
     FIG. 2 is a top view of the prior art drum vent filter of FIG. 1; 
     FIG. 3 is a side perspective view of a drum vent filter configured in accordance with the principles of the present invention; 
     FIG. 4 is a bottom perspective view of the drum vent filter of FIG. 3; 
     FIG. 5 is a side elevation of the drum vent filter of FIGS. 3 and 4, and 
     FIG. 6 is a top view of the drum vent filter of FIGS.  3 - 5 . 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIGS. 1 and 2, there is shown a prior art drum vent filter  10  which includes a housing  12  having a cylindrical portion  14  and a radially extending flange  16 . The threaded cylindrical portion  14  is externally threaded for threading into a vent opening in a drum (not shown) containing a toxic waste material such as transuranic waste material. The housing  12  has a gasket  20  which seals against the lid (not shown) of the drum so that hydrogen gas within the drum can only vent through the housing. Within the housing  12  there is a filter element  21  made of a carbon-to-carbon composite filter media  22 . The filter media  22  traps particulate matter while absorbing volatile organic compounds. The filter media has a diameter of about 0.73 inch, a thickness of about 0.6 inch, and a carbon density of about 0.15 g/cc. The diameter of the exposed portion of the inner face  23  of the filter media is about 0.375 inch because of coverage by annular flange  24  on housing  12 . The filter media  22  is retained within the housing with a sealant  25 . 
     Disposed above the radially extending flange  16  there is a lid  26  which is welded to the flange by four spaced feet  28  that are welded to the flange using a TIG welding technique. The lid  26  is hexagonal in shape and is separated from the top surface of the filter media  22  and the radial flange  16  by an axial gap  30  which is 0.075 inch in height with the spaced-apart feet  28  providing radial gaps  32 . The gaps  30  and  32  provide openings through which hydrogen gas which has been diffused through the filter  22  enters the atmosphere. 
     Referring now to FIGS. 3-6 where there is shown an improved drum vent filter  40  which replaces the prior art vent filter  10 . The vent filter  40  has several structural differences and dimensional differences which enhance its performance. A first difference is a change in the configuration and mounting of the filter element wherein a filter element  42  is utilized. The filter element  42  has a thickness range of about one quarter to about three tenths inch and a first face  44  which is substantially more open to the interior of the drum because it is not covered with an adhesive sealant material. Rather, the first face  44  rests against the top surface of a radially narrow lip  46  which projects approximately {fraction (1/10)} inch from the cylindrical interior wall  48  of housing  50 . The diameter of the filter element  42  is in the range of 0.60 inch to 1.0 inch and is preferably about 0.70 and in a specific embodiment, about 0.73 inch. The exposed first face  44  of the filter element  42  is about 0.45 inch to about 0.60 inch and preferably about 0.53 inch, which is substantially greater than the 0.375 diameter of the corresponding first face  23  of prior art filter element  22  shown in FIG.  1 . By presenting a larger frontal area, the first face  44  is exposed to a larger amount of gas to be filtered, and by having a shorter diffusion path due to a smaller thickness of ¼-{fraction (3/10)} inch, the filter element  42  is able to diffuse hydrogen gas faster. Due to the relatively small size of hydrogen gas molecules, increase in density from 0.15 grams/cm 3  to a range of 0.2 to 0.3 grams/cm 3  of the carbon-to-carbon filter media  40  does not slow down passage of hydrogen gas through the filter element  42  by a meaningful or substantial amount. 
     The length of the threaded cylindrical portion  54  of the vent filter  40  has been reduced by 0.5 inch as compared to the cylindrical portion  14  of the filtered vent  10  so that the damming effect of the threaded portion within the drum (not shown) is reduced substantially. Consequently, hydrogen gas around the periphery of the threaded portion is more readily able to flow into the space defined by the interior wall  48  and is therefore more likely to allow the diffusion of a greater amount of hydrogen gas through the filter element  42 . An annular gasket  55  made of Neoprene® V is disposed around the threaded cylindrical portion  54  to seal with the top of the container (not shown) which contains the hazardous material. 
     The drum vent filter  40  has a lid  56  in the form of an impervious panel such as the lid  24  of the prior art embodiment; however, the lid  56  is separated by four spaces  57  from the top surface  58  of a radial flange  60  on the housing  50  by a gap  61  in the range of 0.125 to 0.175 inch (and preferably 0.15) rather than a gap of 0.075 inch. This substantial increase in space above the filter element  42  provided by the gap  61  provides for increased flow or diffusion through the filter element  42  while still protecting the filter element  42  from direct mechanical impact. The overall height of the filter element is about 0.75 inch compared to about 0.92 inch for the prior art vent filter  10 . 
     The filter element  42  is sealed with the cavity  63  of the housing  50  only around its periphery with an RTV silicon sealant  62 . The adhesive sealant is protected from direct contact with substances within the drum which might degrade the seal  62  because those substances would have to pass through the filter element  42  before contacting the seal. 
     This is because the lip  46  minimizes the possibility of direct contact with the seal by such substances which would have to flow radially through the carbon-to-carbon filter media  40  before contacting the seal  62 . Since the filter media  40  of the filter element  42  adsorbs such materials, the chances of those materials becoming contacting and degrading the seal  62  is minimized. 
     The preferable material for the filter housing  50  and lid  56  is type 304 stainless steel, although other alloys may be used, for example, HASTELLOY® C-22® alloy available from Corrosion Materials, Inc. of Baker, La. 
     The drum vent filter  40  controls the release of volatile organic compounds (VOCs) while improving the ventilation of hydrogen gas in 55-gallon drums, overpack drums, and standard waste boxes containing transuranic, low-level, hazardous or mixed of waste. The drum vent filter  40  is adapted by its threaded exterior to be threaded into a standard ¾ inch flange of a waste drum lid with about 10 foot-pounds of torque in order to compress the neoprene gasket  55  and accomplish a good seal. 
     The drum vent filter  40  has a resistance to flow of less than 1 inch water column DP at 210 ml per min. and a particle removal efficiency greater than 99.97% of 0.3 to 0.7 μ DOP aerosol. Hydrogen diffusivity is 24 E-06 Mol/sec/Mol fract with hydrogen resistance of 41,666 (category 20,30,40 wastes). It has been found that the hydrogen percentage as a function of time in a standardized test decreases from about an initial hydrogen concentration of 3.5% to a hydrogen concentration of about 2.5% in about 30 minutes, which is a substantial improvement over the prior art vent filter  10  of FIG.  1 . 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The above preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the disclosure in any way whatsoever. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.