Abstract:
An ammonia oxidation catalyst basket design has two support grids. A first grid supports the primary catalyst and a separate, second grid supports the secondary catalyst. This dual grid design separates the two catalysts, and enables the catalysts to be independent of each other. Any interruption in the primary or the secondary catalyst does not impede or adversely impact on the structure or function of the other catalyst.

Description:
BACKGROUND 
       [0001]    This application claims the priority benefit of U.S. Provisional Application Ser. No. 62/039,278, filed Aug. 19, 2014, the entire disclosure of which is incorporated herein by reference. 
         [0002]    The present disclosure relates to a catalyst basket and associated support method, and more particularly to an improved ammonia oxidation catalyst basket design that has two support grids and a method of independently supporting primary and secondary catalysts. 
         [0003]    During the manufacture of nitric acid, significant quantities of pollutants are emitted to the atmosphere. One of the principal pollutants is the greenhouse gas nitrous oxide (N 2 O). Nitric acid, HNO 3 , is produced by oxidizing ammonia in the presence of a precious metal catalyst, “primary catalyst”. This process produces nitrogen oxides NOX. The main form of the NOx is nitrogen monoxide, NO, that is further processed with water to manufacture nitric acid, HNO 3 . There is N 2 O present in the production of the NOx and takes no further part in the chemistry of the nitric acid process and consequently is emitted to the atmosphere. The N 2 O can be reduced greatly by using a “secondary catalyst”. The secondary catalyst is typically installed underneath the primary catalyst. As the NOx is produced by the primary catalyst, the NOx passes over the secondary catalyst that selectively destroys the N 2 O. 
         [0004]    The initial process of converting ammonia to NO for the production of HNO 3  is facilitated in a piece of equipment called a converter or burner. Inside the converter there is a catalyst containment device generally referred to in the industry as an ammonia oxidation catalyst basket, along with some heat exchange equipment. Historically over the last approximately 60 years, ammonia oxidation catalyst baskets have been designed with a single support grid. The support grid is designed to hold and seal the primary catalyst in position for the ammonia oxidation process. The single support grid design is used for either a direct support of the primary catalyst or as a dual support of a fill material and the primary catalyst. For example, it is known to provide a fill material with the primary catalyst installed on top of the fill material. 
         [0005]    With the onset of greenhouse gas reduction requirements for HNO 3  production plants, several ways have been proposed to selectively eliminate the N 2 O. One of those ways has been to replace the fill material in the ammonia oxidation catalyst basket with a secondary abatement catalyst. This method has been proven to work very well in many plants. Unfortunately, these revisions of adding a secondary abatement catalyst to an existing ammonia oxidation catalyst basket design have resulted in process issues. 
         [0006]    One of these process issues is known as gapping, i.e., with normal operations of a converter there will be a “gapping” of the secondary catalyst. This gapping forms at the secondary catalyst and at the ammonia oxidation catalyst basket sidewall interface. When this happens there is a preferential flow path that can cause additional issues. 
         [0007]    Another process issue is that the abatement catalyst material has been known to compress or become compacted and in doing so reduces the height of the fill. When the height of the fill is reduced, the primary catalyst seal can be interrupted/impacted and causes still other issues. 
         [0008]    Still another process issue results from interruptions in the process (“shutting off”) called a trip. If there are an unusual number of trips, the secondary catalyst can dome and lift the primary catalyst, and thereby interrupt a seal around the perimeter of the primary catalyst. This consequence can likewise cause additional issues. 
         [0009]    As is evident, a need exists to address these issues in an efficient and effective manner, and at a reasonable cost that is easily incorporated into existing design parameters, and that solves these problems and others. 
       SUMMARY 
       [0010]    The present disclosure is directed to an improved ammonia oxidation catalyst basket design that has two support grids. 
         [0011]    A first grid supports the primary catalyst and a separate, second grid supports the secondary catalyst. 
         [0012]    A main feature of this dual grid design is the separation of the two catalysts. This dual grid design enables the catalysts to be independent of each other. Any interruption in the primary or the secondary catalyst does not impede or adversely impact on the structure or function of the other catalyst. 
         [0013]    A primary benefit is that the dual grid design separates the catalysts from each other and allows for these catalysts to perform independently of each other. 
         [0014]    A benefit resides in that any movement of the secondary catalyst does not impact the primary catalyst. 
         [0015]    Another advantage of the design of the present disclosure is that the primary catalyst remains flat during a campaign cycle and this is important to maintain the seal. 
         [0016]    Still other benefits and advantages of the present disclosure will become more apparent from reading and understanding the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a perspective view of a catalyst basket assembly employing the dual grid features of the present disclosure. 
           [0018]      FIG. 2  is an enlarged, partial sectional view through the catalyst basket assembly. 
           [0019]      FIG. 3  is a plan view of a first grid that supports a primary catalyst. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Turning to  FIGS. 1-3 , there is shown in one preferred arrangement of a catalyst basket assembly  100 . More specifically, the present disclosure is directed to the dual grid basket  100  that has a housing  110  shown here as a generally circular structure for ease of assembly, although other conformations or configurations of the housing are contemplated without departing from the scope and intent of the present disclosure. Received in a first or upstream portion  112  of the housing  110  is a heat shield  114 . For example, the heat shield  114  may include an enlarged, first end  116  ( FIG. 2 ) that tapers inwardly (reduces in cross-sectional dimension) to a generally constant dimension second end  118 . The heat shield  114  directs gas flow into the upstream portion  112  of the housing  110  and the gas flow continues down through the housing where the gas flow exits at a second or outlet end  120 . 
         [0021]    Between the first and second ends  112 ,  120  of the housing  110 , a support flange  122  extends inwardly from an inner perimeter  124  of the housing  110 . The support flange  122  has a generally stepped configuration in cross-section that forms first and second perimeter support surfaces  126 ,  128 . 
         [0022]    A first support grid  140  is supported along an outer perimeter portion thereof by the first surface  126  of the support flange  122 . The first grid  140  is a rigid, foraminous structure (i.e., has a series of openings therethrough) and is shown here as a honeycomb grid although such a description should not be deemed as a limitation on the present disclosure. The first grid  140  supports a primary catalyst  142 . One example of a primary catalyst is a platinum gauze that reacts with the gas flow, namely ammonia of a gas flow mixture that may comprise about 13% ammonia and 87% air, although other mixtures and percentages can be used without departing from the cope and intent of the present disclosure. The platinum gauze extends over the entire cross-sectional area or opening through the basket in preferred arrangements. The platinum reacts with the ammonia in the oxidation process. 
         [0023]    A mesh screen  144  may also be interposed between the first grid  140  and the primary catalyst  142  to provide further support to the primary catalyst. In addition, a weighted ring  150  is received into the first end  112  of the housing  110 , and the weighted ring provides a downward holding force on the primary catalyst  142  and mesh screen  144 , that retains the primary catalyst in place on the first support grid  140 . More specifically, the weighted ring  150  is dimensioned to engage outer perimeter portions of the primary catalyst  142  and the mesh screen  144 , and hold these components against the second surface  128  of the support flange  122 . In this manner, the potential for “gapping” to form between the outer perimeter of the primary catalyst and the housing is reduced. 
         [0024]    A second support grid  160  is axially spaced (in a direction oriented along the gas flow path through the basket) from the first grid  140  in the housing  110 . The second grid  160  and preferably a mesh screen  162  are also foraminous structures that support a secondary catalyst  170 , namely a N 2 O abatement catalyst. In the illustrated embodiment of the present disclosure, the secondary catalyst  170  is shown as pellets that fill the axial space between the first and second grids. Preferably, the second grid  160  supports the secondary catalyst  170  independently of the primary catalyst  120 . By independently supporting the secondary catalyst  170 , the improved ammonia oxidation catalyst basket of the present disclosure overcomes many of the issues noted in the Background. The first and second grids separate the catalysts from each other and allow for these catalysts to perform independently of each other. Any movement of the secondary catalyst (i.e., the pellets) does not impact the primary catalyst so that there is less risk of gapping. Another advantage of the design of the present disclosure is that the primary catalyst  142  remains flat during a campaign cycle and this is likewise important to maintain the seal at the perimeter of the primary catalyst. 
         [0025]    In summary, the platinum primary catalyst  142  is supported along its outer perimeter and sealed against the housing  110 . It is desirable that all of the incoming gas (ammonia and air mixture) flow through the platinum gauze  142 . The foraminous structure or first grid  140  is typically placed beneath the platinum gauze  142  to allow the gas to flow therethrough and provide a support surface beneath a central portion of the platinum gauze. A perimeter portion of the platinum gauze  142  is supported on an inner diameter shoulder  128  of the housing  110 . 
         [0026]    The secondary catalyst  170  or secondary abatement is received on an upper surface of the second grid  160  in axially spaced relation and beneath the platinum gauze  142 . The present disclosure teaches supporting a primary catalyst independently of the secondary catalyst. 
         [0027]    The present disclosure finds particular application in high pressure plants, where high pressure may range from approximately 135 to approximately 160 psi (e.g. about 150 psi), and may likewise find application in medium pressure plants where the operating pressure ranges from approximately 50 psi to approximately 100 psi. Again, however, these details are provided for sake of completeness and should not be deemed as a limitation to the present disclosure. 
         [0028]    This written description uses examples to describe the disclosure, including the best mode, and also to enable any person skilled in the art to make and use the disclosure. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. Moreover, this disclosure is intended to seek protection for a combination of components and/or steps and a combination of claims as originally presented for examination, as well as seek potential protection for other combinations of components and/or steps and combinations of claims during prosecution.