Abstract:
A vertical tubular reactor for converting ammonia synthesis loop purge gas to ammonia; a method for converting ammonia synthesis loop purge gas to form additional ammonia; and a method for retrofitting a conventional ammonia plant having a synthesis loop using an iron-based synthesis catalyst and having a purge gas stream, the method including a supplemental ammonia converter for the purge gas stream. The supplemental ammonia converter is a shell and tube reactor. The tubes are filled with a catalyst comprising a platinum group metal such as ruthenium. The tubes are maintained in a substantially isothermal condition by boiling water in the shell side. As a retrofit modification to an existing ammonia synthesis plant, the purge stream is passed through the supplemental ammonia converter on a once-through basis to form additional ammonia and reduce the amount of purge gas. Advantages of the retrofit modification include lower energy consumption, lower purge rates and higher ammonia production rates.

Description:
FIELD OF THE INVENTION 
     This invention relates to an isothermal ammonia converter, and more particularly to an ammonia converter and method for converting ammonia synthesis loop purge gas containing nitrogen and hydrogen to form additional ammonia. 
     BACKGROUND OF THE INVENTION 
     Ammonia is commonly manufactured by reacting nitrogen and hydrogen in a synthesis loop including a compressor, an ammonia synthesis reactor and ammonia condensation and recovery. The unreacted synthesis gas mixture is typically recycled from the ammonia separator to the compressor and back to the reactor. Make-up synthesis gas is continuously added to the synthesis loop to provide fresh hydrogen and nitrogen. Because the synthesis gas contains argon, methane and other inert components, a purge stream is usually taken from the synthesis loop to avoid the excessive buildup of the inerts in the synthesis loop. The purge gas is typically processed in a hydrogen recovery unit, and a hydrogen-enriched stream is recycled to the synthesis loop. In some cases, the purge gas is used directly in the fuel system with or without any additional treatment or hydrogen recovery. 
     A significant technological advance in the manufacture of ammonia has been the use of a highly active synthesis catalyst comprising a platinum group metal such as ruthenium on a graphite-containing support as described in U.S. Pat. Nos. 4,055,628; 4,122,040; and 4,163,775; all of which are hereby incorporated herein by reference. Also, reactors have been designed to use this more active catalyst, particularly the catalytic reactor bed disclosed in U.S. Pat. No. 5,250,270 which is hereby incorporated herein by reference. Other ammonia synthesis reactors include those disclosed in U.S. Pat. Nos. 4,230,669; 4,696,799; and 4,735,780; and the like. 
     Ammonia synthesis schemes have also been developed based on the highly active catalyst. In U.S. Pat. No. 4,568,530, stoichiometrically hydrogen-lean synthesis gas is reacted in a synthesis reactor containing the highly active catalyst in the synthesis loop. 
     In U.S. Pat. No. 4,568,532, an ammonia synthesis reactor based on the highly active catalyst is installed in series in the synthesis loop downstream from a reactor containing the more conventional iron-based synthesis catalyst. 
     In U.S. Pat. No. 4,568,531, the purge gas removed from the primary synthesis loop is introduced into a second synthesis loop using the more active synthesis catalyst to produce additional ammonia from the purge stream. Another purge stream, significantly reduced in size, is taken from the second synthesis loop to avoid the excessive buildup of inerts. The second synthesis loop, like the primary synthesis loop, employs a recycle compressor to recycle synthesis gas to the active catalyst converters in the second synthesis loop. 
     It would be very desirable to convert hydrogen and nitrogen in the purge stream from a conventional ammonia synthesis loop into additional ammonia using a once-through reactor which does not require staged cooling and a synthesis gas recycle compressor. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an ammonia converter which can be used to convert ammonia synthesis loop purge gas containing nitrogen and hydrogen to form additional ammonia. The ammonia converter is a shell-and-tube reactor using a platinum group metal ammonia synthesis catalyst in the tubes which are maintained in essentially an isothermal condition by boiling water or another heat transfer fluid on the shell side. The ammonia converter allows the ammonia synthesis process to produce additional ammonia from the synthesis loop purge gas by passing the purge gas through the isothermal ammonia converter. The ammonia converter can be installed as a retrofit modification of an existing ammonia synthesis plant to pass the purge stream or a combination of purge streams from several plants through the isothermal ammonia converter on a once-through basis to form additional ammonia, and reduce the size of the purge gas stream which is either processed further in a hydrogen recovery unit or sent to the fuel system directly. 
     In one aspect, then, the present invention provides an ammonia converter for converting ammonia synthesis loop purge gas containing nitrogen and hydrogen to form additional ammonia. The ammonia converter is a shell and tube reactor having upright tubes. A source of feed gas contains nitrogen and hydrogen for supply to an inlet of the tubes. Ammonia synthesis catalyst in the tubes is adapted to convert the nitrogen and hydrogen to ammonia as the gas passes through the tubes. A source of saturated boiler feed water supplies boiling water to a shell-side of the reactor to maintain a substantially isothermal shell-side condition and remove heat from the tubes. A tube-side outlet is provided for recovering product gas having an increased ammonia content relative to the feed gas. The catalyst preferably comprises a platinum group metal such as ruthenium supported on graphite. The tubes are preferably sized for containing a catalyst volume, and present an area for heat transfer to the boiling water, to maintain the feed and product gases at a temperature in the range from 315° C. to 435° C. at a reaction pressure from 60 to 210 bar. The pressure of the shell-side boiling water is preferably from 60 to 150 bar. The feed gas preferably comprises synthesis loop purge gas having an ammonia content less than 4 mole percent, and the product gas preferably has an ammonia content from about 15 to about 40 mole percent. The converter can further include an ammonia separator for removing ammonia from the product gas to form an ammonia-lean stream, a hydrogen recovery unit for removing hydrogen from the ammonia-lean stream to form a nitrogen-rich stream, and a compressor for recycling a portion of the nitrogen-rich stream to the feed gas source. 
     In another aspect, the present invention provides a method for converting ammonia synthesis loop purge gas containing nitrogen and hydrogen to form additional ammonia. The method includes the steps of supplying the synthesis loop purge gas to the inlet of the tubes of the shell and tube reactor of the ammonia converter described above, operating the ammonia converter, and recovering ammonia from the product gas to form an ammonia-lean stream. The method can also include the step of preheating the synthesis purge gas in heat exchange with the product gas. The ammonia recovery step preferably includes cooling the product gas to condense ammonia and separating the liquid ammonia from the ammonia-lean stream. The method can also include the steps of supplying the ammonia-lean stream to a hydrogen recovery unit to form a nitrogen-rich stream and a hydrogen-rich stream, compressing a portion of the nitrogen-rich stream and recycling the compressed nitrogen-rich stream into the preheated synthesis loop purge gas, and recycling the hydrogen-rich stream to the synthesis loop. 
     In a further aspect of the invention, there is provided a method for retrofitting an ammonia plant having a synthesis loop and a purge gas loop. The retrofit method is particularly applicable to retrofitting an ammonia plant wherein fresh ammonia synthesis gas containing hydrogen and nitrogen is combined in the synthesis loop with first and second recycle streams to form a combined ammonia synthesis gas, the combined ammonia synthesis gas is reacted over ammonia synthesis catalyst to form a converted gas, and a purge gas stream and ammonia are removed from the converted gas to form the first recycle stream; and wherein the purge gas stream is processed in a hydrogen recovery unit to form a nitrogen-rich stream and a hydrogen-rich stream which is supplied to the synthesis loop as the second recycle stream. The retrofit method includes installing a shell and tube reactor having upright tubes containing ammonia synthesis catalyst for once-through conversion of nitrogen and hydrogen in a purge gas feed stream, including the purge gas stream from the synthesis loop, into additional ammonia in a reactor effluent stream. Boiler feed water is supplied to a shell side of the reactor to remove heat from the tubes and maintain a substantially isothermal condition on the shell side. Heat exchangers and a vapor-liquid separator are installed for condensing and recovering ammonia from the reactor effluent stream and forming an ammonia-lean stream. The ammonia-lean stream is passed to the hydrogen recovery unit. 
     The retrofit method can also include installing a compressor for combining a portion of the nitrogen-rich stream from the hydrogen recovery unit with the purge gas stream from the synthesis loop to form the purge gas feed stream. The heat exchangers installed to condense ammonia from the reactor effluent stream preferably include a heat exchanger for preheating the purge gas stream from the synthesis loop against the reactor effluent stream. The step of supplying boiler feed water preferably includes installation of a steam drum for receiving saturated steam and water from the shell side of the reactor, forming a saturated steam stream, and recycling condensate to the shell side of the reactor. The molar ratio of hydrogen to nitrogen in the purge gas feed stream is preferably less than 2.2. The isothermal reactor preferably operates at a tube-side temperature from 315° C. to 435° C. and pressure from 60 to 210 bar. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic flow sheet of an isothermal ammonia converter installed according to the present invention. 
     FIG. 2 shows a process flow diagram of an ammonia plant synthesis loop and purge loop in which the purge gas from two synthesis loops is converted to additional ammonia in an isothermal ammonia converter installed according to the principles of one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, there is shown a process  10  for the once-through isothermal conversion of hydrogen and nitrogen in a purge gas stream to additional ammonia. The purge gas in stream  12  is preheated in feed/effluent exchanger  14  for feed to the tube side of a reactor  16 . A fired heater  17  can be used for additional heating of the purge gas feed stream and/or for startup. The tubes in the reactor  16  are filled with catalyst, and the reactor  16  is kept essentially isothermal by boiling water on the shell side of the reactor. A steam drum  18  is provided to maintain the reactor  16  in a flooded condition. Condensate is circulated to the shell side of the reactor  16  via line  20 , and steam and condensate are returned to the steam drum  18  via line  22 . Make-up boiler feed water is supplied via line  24 . The pressure for steam generation in the reactor  16  is desirably selected to be consistent with the maximum pressure of the boiler feed water available, to minimize the temperature difference between the tube and shell sides of the reactor  16 . As the purge gas passes through the catalyst in the tubes of the reactor  16 , the ammonia concentration is increased from a low inlet concentration, typically from 1 to 10 percent ammonia, to an outlet concentration of at least about 20 percent ammonia, to as high as about 40 percent or more. The ammonia product is recovered by cooling reactor effluent stream  28  in the feed/effluent exchanger  14 , with cooling water in exchanger  30 , and then with ammonia refrigerant in the exchanger  32 . Condensed ammonia is recovered from separator  34  via line  36 . Remaining purge gas separated from the ammonia product is sent to optional hydrogen recovery unit  38  via line  40 . The hydrogen recovery unit  38  is operated conventionally and can also receive additional purge gases, such as, for example, compressed medium pressure flash gases from the ammonia recovery of the main synthesis loop, via line  42 . The hydrogen recovery unit  38  typically produces hydrogen stream  44 , argon stream  46 , fuel gas stream  48 , ammonia stream  50  and nitrogen stream  52 . A portion of the nitrogen stream  52  can be recycled to the suction of the synthesis gas compressor (not shown) via line  54 , and the remaining portion is optionally recycled to the reactor  16  by compression in nitrogen compressor  56 . Recycling the nitrogen to the reactor  16  results in a relatively low H/N ratio, preferably less than 2.2, more preferably about 1.7-1.9, which allows a significant reduction in catalyst volume in reactor  16 . 
     With reference to FIG. 2, there is shown a schematic process diagram for a two-train ammonia plant retrofitted by installing the once-through ammonia converter of the present invention to convert hydrogen and nitrogen in the combined purge streams from the two trains into additional ammonia. The process  100  includes a compression step  102  in which makeup gas  104 , recycle hydrogen  106 , recycle nitrogen  108  and recycle syngas  110  are compressed to form a feed  112  for conversion step  114  employing a conventional compressor and magnetite catalyst converters. Effluent  116  from the conversion step  114  is cooled and passed through a separator in a high pressure separation step  118 . A purge stream  120  is taken from the vapor phase from the high pressure separation step  118 , and the remainder is recycled to the compression step  102  as the recycle syngas  110  as described above. Liquid from the high pressure separation step  118  is processed in a low pressure separation step  122  to form liquid ammonia  124  and vapor  126  which is processed in ammonia scrubbing step  128 . Vapor  130  essentially free of ammonia is compressed in compression step  132  to produce vapor  134  at a suitable pressure for hydrogen recovery. 
     Similarly, in a second train, makeup gas  136  and syngas recycle  138  are compressed in compression step  140  to form a feed  142  to a magnetite conversion step  144 . Effluent  146  from the magnetite conversion step  144  is cooled and separated in high pressure separation step  148 , a purge stream  150  is taken off from the vapor from the high pressure separation step  148 , and the remainder recycled as recycle syngas  138  to the compression step  140 . Liquid from the high pressure separation step  148  is processed in low pressure separation step  152  to obtain liquid ammonia  154  and an ammonia-lean vapor  156  for feed to ammonia scrubbing step  158  to form an essentially ammonia-free vapor  160 . Vapor  160  is compressed in compression step  162  to form a vapor  164  at a suitable pressure for hydrogen recovery. 
     Recycled nitrogen  166  is added to purge gas  120  and purge gas  150  to form feed  168  to a supplemental ammonia conversion step  170 . The supplemental ammonia conversion step  170  includes passing the feed  168  through an isothermal ammonia converter installed according to the present invention, and obtains an effluent  172  containing additional ammonia. The effluent  172  is cooled and ammonia  174  separated therefrom in separation step  176 . Vapor  178  from the separation step  176  is fed to hydrogen recovery unit  180  which also receives vapor  134  and vapor  164  from the respective compression steps  132  and  162 . The hydrogen recovery step includes cryogenic processing or membrane-based recovery to obtain a nitrogen-rich stream  182  and a hydrogen-rich stream  184 . An optional nitrogen-rich product  186  can be taken off from the stream  182 , and another portion  188  is preferably supplied to compression step  190  to produce the nitrogen recycle  166  as describe above. The remaining nitrogen  108  is supplied to the compression step  102  as described above. A helium purge  192  may be taken off from the hydrogen-rich stream  184  and remaining hydrogen  106  is recycled to the compression step  102  as described above. The hydrogen recovery step  180  can also produce a conventional argon-rich stream  194  and a fuel gas stream  196 . Any ammonia  198  obtained from the hydrogen recovery step  180  is supplied to an ammonia storage step  200  with ammonia  125 ,  155  and  174 . An ammonia product  202  is obtained from the ammonia storage  200 . 
     The principles of the invention are illustrated by way of the following example: 
     EXAMPLE 
     With reference to FIG. 2, an existing two-train ammonia process was modeled using an ASPEN process simulator. The model was subsequently altered to include a supplemental ammonia conversion step  170  to study a simulation of a retrofit to the existing plant. It was presumed that such a retrofit would reduce the purge rate; reduce energy costs; and increase ammonia production; and by study and calculation, the presumption was confirmed. In the following example, pressures are approximate and pressure drops are mostly ignored. 
     The supplemental ammonia conversion step  170  that was simulated is based on tubular reactor  16  having vertical tubes as shown in FIG.  1 . In the simulation, feed  168  is preheated to 360° C. in a feed/effluent heat exchanger. On start-up, a fired heater provides the required preheat. Feed  168  (FIG. 2) is fed to the top of the reactor  16  (FIG. 1) and flows downward through reactor tubes filled with a ruthenium-impregnated catalyst. 
     Since the conversion of nitrogen and hydrogen to ammonia is a highly exothermic reaction, the tubular reactor  16  is designed to absorb the heat generated. Further, it is desirable to maintain the reaction at a constant temperature. Isothermal conditions are closely approximated by maintaining the shell side in a flooded condition with pressurized water at its boiling point. Referring again to FIG. 1, a steam drum  18  is provided in an elevated position relative to the reactor  16  to maintain the reactor  16  in the flooded condition. The heat of the reaction is absorbed by the water and converted to steam for energy efficiency. 
     By conducting a catalyst optimization study, the preferred volume of catalyst in the tubular reactor was determined to be 2.35 m 3 . The required catalyst volume is relatively constant when the concentration of ammonia in the outlet ranges between 20 and 30 mole percent. In this simulation, the concentration in the reactor effluent  172  was 21.94%. The catalyst volume is further optimized by recycling nitrogen directly to the supplemental ammonia conversion step  170 . The required catalyst volume is minimized when recycled nitrogen  166  flow is controlled to maintain a hydrogen/nitrogen ratio of 1.82 in feed  168 . 
     The reactor  16  was sized to accommodate 2.35 m 3  of catalyst and to transfer the heat of the reaction, which was 8,714.3 MJ/hr, to pressurized water boiling on the shell side. The pressure for steam generation was chosen to be consistent with the maximum pressure at which boiler feed water was available from the existing plant. By operating the shell side at this practical maximum pressure, the temperature difference between the tube and shell sides of the reactor is minimized. Typically, the shell side would be operated at a pressure between 60 and 150 bar. 
     The reactor  16  for supplemental ammonia conversion step  170  was modeled as a shell and tube exchanger similar to TEMA type BEM with fixed tube sheets and low chrome tubes with INCONEL safe-ends on both ends. The shell was carbon steel, and the channels and tube sheets were low chrome overlayed with stainless steel. The design pressure for the tubes having a minimum wall thickness of 11.43 mm was 203.9 kg/cm 2 . By simulation it was determined that a reactor  16  having 179 tubes of 102 mm in diameter and about 3 m long would accommodate the preferred catalyst volume and provide sufficient area for heat transfer. 
     It was determined that the shell extending the length of the tubes would contain an adequate amount of pressurized boiler feed water to absorb the heat of reaction of the simulated ammonia production rate. For boiler feed water at this pressure, the design pressure of the shell is 140.6 kg/cm 2 . The pressure drop through the catalyst is 0.5 kg/cm 2 . Typically, the feed and product gases in the reactor  16  are maintained in a temperature range between 315° C. to 435° C. and in a pressure range between 60 to 210 bar. In this simulation, the feed entered the reactor  16  at 360° C. and exited at 404° C. and 180 bar. 
     Reactor effluent  172  is cooled to 71° C. in a feed/effluent exchanger; to 38° C. by cooling water; and to 5° C. by ammonia refrigerant, and then directed to separation step  176 , where 79 metric tons/day (mtpd) of ammonia  174  are recovered at 98.6% purity. 
     According to the principles developed in this simulation, an ammonia plant retrofitted with reactor  16  will have lower purge rates, lower energy costs and higher ammonia production rates. The key advantage of reactor  16  is that it operates on a once-through basis, eliminating the need for multiple reactors with interstage cooling. This is possible because the vertical tube reactor is operated essentially isothermally by boiling pressurized water on the shell side. 
     The results of the ASPEN simulation are summarized in Table 1. The stream numbers correspond to FIG.  2  and the detailed description of the invention. 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Stream 
                 104 
                 106 
                 108 
                 110 
                 112 
                 116 
                 120 
               
               
                   
               
               
                 Components 
               
               
                 (mole percent) 
               
               
                 H2 
                 73.80 
                 82.27 
                 0.26 
                 62.27 
                 65.11 
                 54.60 
                 62.27 
               
               
                 N2 
                 24.86 
                 13.01 
                 99.74 
                 18.74 
                 20.13 
                 16.43 
                 18.74 
               
               
                 CH4 
                 1.04 
                 0.57 
                 — 
                 9.69 
                 7.59 
                 8.53 
                 9.69 
               
               
                 Ar 
                 0.28 
                 0.75 
                 — 
                 3.20 
                 2.50 
                 2.81 
                 3.20 
               
               
                 NH3 
                 — 
                 — 
                 — 
                 3.89 
                 2.94 
                 15.69 
                 3.89 
               
               
                 He 
                 0.02 
                 3.40 
                 — 
                 2.21 
                 1.72 
                 1.94 
                 2.21 
               
               
                 Total Flow 
                 64030 
                 2338 
                 700 
                 245480 
                 312540 
                 312540 
                 6706 
               
               
                 (kg/hour) 
               
               
                 Temp (° C.) 
                 100 
                 9 
                 23 
                 −1 
                 14 
                 406 
                 −1 
               
               
                 Press (kg/cm2) 
                 227.0 
                 227.0 
                 3.4 
                 227.0 
                 227.0 
                 227.0 
                 227.0 
               
               
                   
               
               
                 Stream 
                 125 
                 134 
                 136 
                 138 
                 142 
                 146 
                 150 
               
               
                   
               
               
                 Components 
               
               
                 (mole percent) 
               
               
                 H2 
                 0.04 
                 52.26 
                 74.27 
                 62.07 
                 65.31 
                 53.22 
                 62.07 
               
               
                 N2 
                 0.02 
                 17.57 
                 24.76 
                 20.60 
                 21.71 
                 17.67 
                 20.60 
               
               
                 CH4 
                 0.07 
                 25.34 
                 0.68 
                 10.79 
                 8.12 
                 9.28 
                 10.79 
               
               
                 Ar 
                 — 
                 4.12 
                 0.26 
                 4.38 
                 3.28 
                 3.75 
                 4.38 
               
               
                 NH3 
                 99.87 
                 — 
                 — 
                 1.70 
                 1.24 
                 15.69 
                 1.70 
               
               
                 He 
                 — 
                 0.71 
                 0.03 
                 0.46 
                 0.34 
                 0.39 
                 0.46 
               
               
                 Total Flow 
                 59834 
                 538 
                 64815 
                 225030 
                 289840 
                 289840 
                 4343 
               
               
                 (kg/hour) 
               
               
                 Temp (° C.) 
                 1 
                 180 
                 100 
                 −25 
                 5 
                 370 
                 −25 
               
               
                 Press (kg/cm2) 
                 19.1 
                 71.4 
                 199.0 
                 199.0 
                 199.0 
                 119.0 
                 199.0 
               
               
                   
               
             
          
           
               
                 Stream 
                 155 
                 164 
                 166 
                 168 
                 172 
                 174 
                 178 
                 186 
               
               
                   
               
               
                 Components 
               
               
                 (mole percent) 
               
               
                 H2 
                 0.03 
                 50.05 
                 0.26 
                 54.23 
                 36.25 
                 0.46 
                 44.06 
                 0.26 
               
               
                 N2 
                 0.01 
                 18.96 
                 99.74 
                 29.77 
                 25.98 
                 0.39 
                 31.55 
                 99.74 
               
               
                 CH4 
                 0.04 
                 24.76 
                 — 
                 8.81 
                 10.45 
                 0.48 
                 12.63 
                 — 
               
               
                 Ar 
                 0.01 
                 6.09 
                 — 
                 3.17 
                 3.77 
                 0.07 
                 4.58 
                 — 
               
               
                 NH3 
                 99.91 
                 — 
                 — 
                 2.67 
                 21.94 
                 98.60 
                 5.23 
                 — 
               
               
                 He 
                 — 
                 0.12 
                 — 
                 1.35 
                 1.60 
                 — 
                 1.95 
                 — 
               
               
                 Total Flow 
                 60146 
                 327 
                 4398 
                 15447 
                 15447 
                 3138 
                 12309 
                 258 
               
               
                 (kg/hour) 
               
               
                 Temp (° C.) 
                 −24 
                 143 
                 60 
                 −2 
                 380 
                 5 
                 5 
                 23 
               
               
                 press (kg/cm2) 
                 19.1 
                 71.4 
                 184.0 
                 184.0 
                 183.5 
                 182.5 
                 182.5 
                 3.4 
               
               
                   
               
             
          
           
               
                 Stream 
                 188 
                 194 
                 196 
                 198 
                 202 
               
               
                   
               
               
                 Components 
               
               
                 (mole percent) 
               
               
                 H2 
                 0.26 
                 — 
                 7.24 
                 — 
                 0.05 
               
               
                 N2 
                 99.74 
                 — 
                 15.85 
                 — 
                 0.02 
               
               
                 CH4 
                 — 
                 — 
                 74.88 
                 — 
                 0.07 
               
               
                 Ar 
                 — 
                 100.00 
                 1.60 
                 — 
                 0.01 
               
               
                 NH3 
                 — 
                 — 
                 0.17 
                 100.00 
                 99.85 
               
               
                 He 
                 — 
                 — 
                 0.26 
                 — 
                 — 
               
               
                 Total Flow 
                 4398 
                 1436 
                 2819 
                 748 
                 123870 
               
               
                 (kg/hour) 
               
               
                 Temp (° C.) 
                 23 
                 −100 
                 9 
                 30 
                 −11 
               
               
                 Press (kg/cm2) 
                 3.4 
                 3.41 
                 3.4 
                 16.0 
                 16.0 
               
               
                   
               
             
          
         
       
     
     The present invention is illustrated by way of the foregoing description and example. Various modifications will be apparent to those skilled in the art in view thereof. It is intended that all such variations within the scope and spirit of the appended claims be embraced thereby.