Abstract:
A vertical, fixed-bed ammonia converter wherein a fixed-bed catalyst zone is configured into two mechanically separated catalyst volumes and two gas streams that operate in parallel. The design maintains the ratio of gas flow to catalyst volume so that there is no catalyst effectiveness penalty. The catalyst beds and gas flow paths are configured so that gas flow is downward through each catalyst volume. Each fixed-bed catalyst zone in the present invention can hold the catalyst in an annular space formed between two concentric shrouds arranged around a shell and tube heat exchanger. The two catalyst beds associated with each zone are situated above one another along the length of an interstage heat exchanger. Pipes or conduits are disposed through the beds to effect the parallel gas flow configuration, or alternatively, annular flows are created via passages through the internal shrouds that contain the catalyst beds.

Description:
BACKGROUND OF INVENTION 
     This invention relates to ammonia converters for catalytically reacting a gaseous feed stream containing nitrogen and hydrogen to produce ammonia. 
     Elaborate and sophisticated reactor designs have been developed for converting nitrogen and hydrogen in the gas phase in a fixed catalyst bed to form ammonia. The designs have attempted to optimize the ratio of gas flow to catalyst volume for maximum catalyst effectiveness. Even so, it is still desirable to reduce the reactor size relative to the ammonia production capacity. The size of the reactor, of course, has an impact on its cost. 
     Ammonia converters are complicated by the fact that ammonia synthesis from nitrogen and hydrogen gas is exothermic and the reactions take place at high temperatures and pressures. Thus, interstage cooling is generally used between a series of catalyst zones to maintain kinetic and equilibrium conditions appropriate for optimum conversion efficiency. There must also be provision made for servicing the catalyst zones, e.g. periodically removing and replacing catalyst when it loses its effectiveness. 
     The use of the radial flow and mixed axial-radial flow arrangements in ammonia converter designs have become the standard for vertical ammonia converters. These designs, however, generally require the use of a freeboard or other catalyst volume that is ineffective. These designs can also complicate catalyst loading and removal, and require care in the design to avoid the potential for catalyst fluidization at the upper end of the radial flow catalyst volume. 
     SUMMARY OF INVENTION 
     The present invention is directed to a vertical, fixed-bed ammonia converter in which a fixed-bed catalyst zone is configured into two mechanically separated catalyst volumes and two gas streams that operate in parallel. The design maintains the ratio of gas flow to catalyst volume throughout the bed so that there is no catalyst effectiveness penalty compared to vertical radial-flow designs. The invention provides a reduction in reactor size since the two volumes can be optimally arranged within the reactor shell. The catalyst beds and gas flow paths are configured so that gas flow is downward through each catalyst volume, thus eliminating both the ineffective catalyst volume and the catalyst fluidization potential. The design facilitates the usual state-of-the-art alignment of heat exchangers and catalyst beds popular in the vertical, radial fixed-bed ammonia converters of the prior art. 
     Each fixed-bed catalyst zone in the present invention preferably holds the catalyst in an annular space formed between two concentric shrouds arranged around a shell and tube heat exchanger. The two catalyst beds associated with each zone are situated above one another along the length of an internal heat exchanger. In one preferred split-flow design, pipes or conduits are disposed through the beds to effect the parallel gas flow configuration. In another preferred embodiment, annular flows are created via passages through the internal shrouds that contain the catalyst beds. 
     In one embodiment, the present invention provides a vertical ammonia converter including a vessel having an upright cylindrical shell and a plurality of fixed bed catalyst zones vertically spaced apart in the vessel, including uppermost and lowermost catalyst zones and at least one intermediate catalyst zone. At least the uppermost and intermediate catalyst zones are concentrically disposed about a respective shell and tube heat exchanger for interstage cooling of effluent gas from the catalyst zones. Magnetite catalyst is disposed in the uppermost catalyst zone, and high activity catalyst in the intermediate and lowermost catalyst zones. At least the intermediate catalyst zones include at least two mechanically separated catalyst beds disposed vertically with respect to each other and configured for parallel downward gas flow split between the at least two catalyst beds. 
     The lowermost catalyst zone preferably has at least two mechanically separated catalyst beds disposed vertically with respect to each other and configured for parallel downward gas flow split between the at least two catalyst beds. The vessel shell preferably has a substantially uniform diameter along the length of the catalyst zones to facilitate fabrication. The vertical ammonia converter preferably includes respective pluralities of conduits passing through each respective catalyst bed to effect the parallel gas flow split, or respective annular flow passages around each catalyst bed to effect the split. 
     In another embodiment, an ammonia converter includes an upright cylindrical shell and at least one fixed bed zone disposed within the shell between an upper gas inlet zone and a lower gas outlet zone. The fixed bed zone has upper and lower catalyst volumes configured for downward gas flow in parallel through each volume. An annular housing for the catalyst volumes is formed by inner and outer concentric shrouds around a shell and tube heat exchanger. A partition plate in the annular housing is disposed between the upper catalyst volume and the lower catalyst volume. An upper discharge plenum is formed between the partition plate and a catalyst support below the upper catalyst volume. An intermediate inlet plenum is formed between the partition plate and the lower catalyst volume. A gas bypass is provided for diverting a portion of the downward gas flow from the gas inlet zone past the upper catalyst volume to the intermediate inlet plenum above the lower catalyst volume. There is a lower discharge plenum below a catalyst support at a lower end of the lower catalyst volume. A discharge passage is in fluid communication between each of the upper and lower discharge plenums and a shell-side fluid inlet to the heat exchanger. A shell-side fluid outlet from the heat exchanger is in fluid communication with the gas outlet zone. When it is desired to use the ammonia converter, the catalyst volumes are filled with a suitable ammonia conversion catalyst. 
     The gas bypass preferably includes a first plurality of tubes passing through the upper catalyst volume and upper discharge plenum. A second plurality of tubes can pass through the intermediate inlet plenum and lower catalyst volume, and communicate between the upper and lower discharge plenums. The outer shroud can depend from an inverted support cone secured between the shell and an upper end of the outer shroud. The discharge passage can include an annulus between the inner shroud and a concentric intermediate shroud having a larger diameter. 
     Alternatively, the gas bypass preferably includes an annulus between the outer shroud and the shell and a plurality of openings in the outer shroud into the intermediate inlet. The outer shroud can be supported on a support cone secured between the shell and a lower end of the outer shroud. The discharge passage can include an annulus between the inner shroud and a concentric intermediate shroud having a larger diameter. A plurality of openings can be formed in the intermediate shroud between the upper discharge plenum and the discharge passage. The fixed bed zone is preferably constructed as a modular pre-assembly attached to the shell via the support cone. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic elevation shown partly in section of a split-flow, fixed-bed vertical ammonia converter according to one embodiment of the invention showing the use of pipes for splitting the gas flow between the catalyst volumes in the catalyst zone. 
         FIG. 2  is a schematic elevation shown partly in section of a split-flow, fixed-bed vertical ammonia converter according to anther embodiment of the invention showing the use of an annular passage for splitting the gas flow between the catalyst volumes in the catalyst zone. 
         FIG. 3  is a schematic elevation shown partly in section of a vertical ammonia converter according to another embodiment of the invention showing a plurality of parallel split-flow, fixed-bed catalyst zones below a conventional top bed. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings wherein the same reference numerals are used to refer to similar parts,  FIG. 1  shows a catalyst zone  100  disposed within the vertical shell  102  of an ammonia converter according to one embodiment of the invention. Manways  104 ,  106  are provided for access at the respective gas inlet zone  108  and gas outlet zone  110 . 
     A housing  112  is disposed concentrically about a shell-and-tube heat exchanger  114 . The housing  112  has inner and outer concentric shrouds  116 ,  118 . An intermediate shroud  140  is disposed outwardly of the inner shroud  116 . The shrouds  118 ,  140  are disposed on either side of annular upper and lower catalyst volumes  120 ,  122 . As used herein, the expression “catalyst volume” refers to the space intended to contain the ammonia conversion catalyst whether it actually contains the catalyst or has not yet been filled with catalyst. An annular partition plate  124  is disposed between the catalyst volumes  120 ,  122 . Catalyst supports  126 ,  127  below each catalyst volume  120 ,  122  are made of a wire mesh, profile wire screen (e.g. trade designation Johnson Screen), or other structure well known in the art for supporting fixed catalyst beds. A similar screen  128 ,  129  is disposed at the top of each catalyst bed  120 ,  122 . Each catalyst volume  120 ,  122  preferably has essentially the same volume, i.e. essentially the same inside diameter, outside diameter and depth, to facilitate the same extent of ammonia conversion handling essentially the same volume of gas supplied to each catalyst bed as described in more detail below. 
     An annular upper discharge plenum  130  is formed between the partition plate  124  and catalyst support  126 . An annular lower discharge plenum  132  is similarly formed between the catalyst support  127  and an annular bottom panel  134  of the housing  112 . An annular intermediate inlet plenum  136  is formed between the partition plate  124  and an upper end of the lower catalyst volume  122 . 
     An annular discharge passage  138  is formed between the inner shroud  116  and a concentric intermediate shroud  140  spaced outwardly therefrom. There is a passage for gas between the lower end of the intermediate shroud  140  and the bottom panel  134 . A shell-side fluid inlet  142  to an upper end of the heat exchanger  114  is provided by perforations at the upper end of the inner shroud  116 . 
     A first plurality of pipes  144  is disposed to pass through the upper catalyst volume  120  and the partition plate  124 . A second plurality of pipes  146  is disposed to pass through the partition plate  124  and the lower catalyst volume  122 . If desired, the pipes  144 ,  146  can be evenly spaced in a circular configuration, but each set desirably presents essentially the same cross-sectional flow area and hydraulic radius to facilitate an even 50—50 split of gas supplied to each catalyst volume. If desired, any heat transfer to the gas in pipes  144 ,  146  can be minimized by using an appropriate diameter to minimize surface area and to obtain an appropriate heat transfer coefficient (i.e. wall thickness, double pipe construction and/or insulation). Where heat transfer is significant enough to heat the gas passing through the pipes  144  and/or  146 , the depth of the upper and/or lower catalyst volumes  120 ,  122  can be adjusted slightly to compensate. 
     The heat exchanger  114  is familiar to those familiar with similar interstage heat exchangers employed in the prior art radial-flow ammonia converters. The shell is formed by the inner shroud  116 . The tubes  148  are supported at either end by tube sheets  150 ,  152  at respective inlet and outlet heads  154 ,  156  and pass through conventional baffles  157 . Cooling fluid, which can usually comprise feed gas, is introduced via inlet pipe  158  connected to the inlet head  154 . The inlet head  154  preferably has an outside diameter that is less than that of the inner shroud  116  to provide an annular passage for cooled shell-side gas to enter the gas outlet zone  110 . The outlet head  156  preferably has an outside diameter about that of the inner shroud  116 . Heated cooling fluid is exhausted from the outlet head  156  via outlet pipe  162 . 
     The intermediate shroud  140  is supported from the outlet head  156  by means of conical ring  164 . The outer shroud  118  is secured at its upper end to the shell  102  by means of conical support ring  166 . The rings  164 ,  166  seal the housing  112  to prevent gas from bypassing the catalyst zone  100 . 
     Catalyst is introduced into and/or removed from the upper catalyst bed  120  in a conventional manner. Catalyst can be introduced into and/or removed from the lower catalyst bed  122  by inserting hose(s) (not shown) through the pipes  144 . The catalyst loading can also be facilitated by employing removable top hold down screens  128 ,  129  and providing personnel access ways (not shown) through the upper bed support grid  126  and partition plate  124 . This allows the lower bed to be loaded and the hold down grid  129  installed, after which the hatch ways in the personnel access openings are installed, the upper bed loaded, and the hold down grid  128  installed. 
     In one example of the  FIG. 1  embodiment, the shell  102  could have an inside diameter of 12 feet, the outer shroud  118  a diameter of 11.5 feet, the intermediate shroud  140  a diameter of 5 feet, and inner shroud  116  a diameter of 4 feet. The inlet and outlet pipes  158 ,  162  can have a nominal diameter of 12 inches, and the tubes  148  a length of 12 feet. The plenums  130 ,  132 ,  136  can have a height of 1 foot, and the catalyst supports  126 ,  127  and screens  128 ,  129  a thickness of approximately 3 inches. In this example, 4 ten-inch pipes  144 ,  146  can be used through each of the beds  120 ,  122 , which each have a depth of 3.5 feet. The total catalyst volume is 567 cubic feet and the pressure drop (excluding the heat exchanger) is estimated at 6.7 psi. 
       FIG. 2  shows a catalyst zone  200  similar to the embodiment of  FIG. 1 , but it uses an external bypass to supply the feed gas to the lower catalyst bed  122  rather than the internal bypass pipes of  FIG. 1 . The annulus  201  between shell  102  and outer shroud  118  has an open upper end in fluid communication with the gas inlet zone  108 . A support cone  202  securing a lower end of the housing  112  to the shell  102  forms a fluid-tight seal at the lower end of the annulus  201  against the gas outlet zone  110 . A plurality of perforations  204  is formed in the outer shroud ⅛ to provide fluid communication between the annulus  201  and the intermediate inlet plenum  136 . A plurality of perforations  206  is similarly formed in the intermediate shroud  140  to provide fluid communication from the outlet plenum  130  into the discharge passage  138 . The perforations  204 ,  206  should be sized and numbered to match the respective fluid flow resistance to provide an essentially even 50—50 split of feed gas between the upper and lower catalyst beds  120 ,  122 . 
     In one example of the  FIG. 2  embodiment, the shell  102  could have an inside diameter of 12 feet, the outer shroud  118  a diameter of 11 feet, the intermediate shroud  140  a diameter of 5 feet, and inner shroud  116  a diameter of 4 feet. The inlet and outlet pipes  158 ,  162  can have a nominal diameter of 12 inches, and the tubes  148  a length of 12 feet. The plenums  130 ,  136  can have a height of 15 inches, the plenum  132  a height of 12 inches, and the catalyst supports  126 ,  127  and screens  128 ,  129  a thickness of approximately 3 inches. The beds  120 ,  122  each have a depth of 3.75 feet. The total catalyst volume is 565 cubic feet and the pressure drop (excluding the heat exchanger) is estimated at 7.6 psi. 
     The present invention has the additional benefit of minimizing radial thermal stresses, confining these primarily to the axial dimension. The present invention also allows a modular construction. In the  FIG. 2  embodiment, for example, the accessibility of the support cone  202  allows a modular construction of the mechanical components of the zone  200  to be employed so that the assembled module, sans catalyst, can be lowered into the shell  102  and secured by welding the periphery of the support cone  202 . The  FIG. 1  embodiment is supported at the top so axial thermal expansion of the components, including the shrouds and exchanger tubes, is downward, and any slight differences in thermal expansion can be accounted for at the bottom. In addition, the  FIG. 1  embodiment has no open spaces at the top that would allow tools, parts, debris or the like to drop into it during loading or unloading. The  FIG. 2  design disposes the annular gas flow adjacent to the vessel shell with the result that the reactor length is minimized. 
     In  FIG. 3  there is illustrated one embodiment of an integrated multi-zone vertical ammonia converter  300  based on the principles of the present invention. The vessel has a vertical cylindrical shell  302  of uniform diameter and conventional domed top and bottom heads  304 ,  306 . First, second, third and fourth catalyst zones  308 ,  310 ,  312 ,  314  are vertically spaced within the vessel from top to bottom. The first zone  308  preferably contains magnetite catalyst, whereas the other zones  310 ,  312 ,  314  preferably contain high activity ammonia conversion catalyst well known to those skilled in the art, as disclosed, for example, in U.S. Pat. Nos. 4,055,628; 4,122,040; and 4,163,775; hereby incorporated herein by reference. In contrast to many prior art radial-flow vertical ammonia converters in which the shell has an enlarged diameter about the first catalyst zone, the present design facilitates fabrication by using a shell  302  that has a uniform diameter. 
     Feed gas is introduced to the top of the converter  300  via inlet nozzle  316 . The first magnetite catalyst zone  308  is preferably of a conventional radial flow design and contains first interstage heat exchanger  318  and associated cooling fluid inlet and outlet pipes  320 ,  322  passing through the shell  302  and top head  304 , respectively. The first catalyst bed  308  can be serviced via manway  324 . 
     Second, third and fourth catalyst zones  310 ,  312  and  314  contain high activity catalyst and are generally constructed in accordance with the design of  FIG. 1  as described above. The skilled artisan will readily appreciate that the design of  FIG. 2  could be used as an alternate. Second and third catalyst zones  310 ,  312  are associated with respective interstage heat exchangers  326 ,  328 , cooling fluid inlet pipes  330 ,  332 , and cooling fluid outlet pipes  334 ,  336 . The fourth catalyst zone  314  is preferably not associated with an interstage cooler since it is the terminal catalyst zone and does not need to be cooled within the reactor, but could be associated with a concentric heat exchanger (not shown), if desired. Manways  338 ,  340 ,  342  are provided above each of the respective catalyst zones  310 ,  312 ,  314  for catalyst addition and/or removal or other service. 
     The invention is illustrated by the foregoing description and examples. Those skilled in the art will develop various changes and modifications in view of the foregoing embodiments. It is intended that all such changes and modifications within the scope or spirit of the appended claims be embraced thereby.