Abstract:
A modified purifier process, includes supplying a first stream of a feed gas containing hydrogen and nitrogen in a mol ratio of about 2:1, and also containing methane and argon, then cryogenically separating the feed gas into the following:
       f) a second stream of synthesis gas containing hydrogen and nitrogen in a mol ratio of about 3:1,   g) waste gas containing principally nitrogen, and also containing some hydrogen and all of the methane supplied in the first stream,
 
and splitting the waste gas into:
   h) a third stream of hydrogen/nitrogen gas   i) a fourth stream of high concentrated nitrogen   j) a fifth stream of methane rich gas, to be used as fuel.
 
The combined second and third streams typically are passed to an ammonia synthesis process.

Description:
[0001]    This application is a continuation-in-part of pending U.S. application Ser. No. 12/586,350, filed Sep. 22, 2009, which is a regular application converted from Provisional application Ser. No. 61/192,556, filed Sep. 22, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates generally to purification of feed gas used for the manufacture of ammonia, and more particularly to improvements in processing of feed gas from which hydrogen rich ammonia synthesis gas and waste gas are derived. The invention specifically concerns treatment of the waste gas to derive useful gas streams, one of which is hydrogen/nitrogen rich, another is nitrogen rich, and another is methane rich. In the prior purifier process, synthesis gas is separated from the waste gas, which contains excess nitrogen from the feed gas, a small amount of hydrogen, all of the incoming methane and about 600 of the incoming argon. Such waste gas is typically utilized as fuel in a primary reformer. 
         [0003]    Improvements in treatment of the waste gas are needed for enhanced overall process efficiency. 
       SUMMARY OF THE INVENTION 
       [0004]    It is a major object of the invention to provide improvements in treatment of such waste gas, as will be seen. Basically, the improved process of the invention derives three product streams from the waste gas, one of which is hydrogen/nitrogen rich, another is basically nitrogen rich, and another which is methane rich, with a higher heating value than in processes employed so far, more suitable for use as a fuel, with less nitrogen going up the stack and eventually full recovery of hydrogen. The overall process includes the steps:
       1) supplying a first stream of a feed gas containing hydrogen and nitrogen in a MOL ratio of about 2/1, and also containing methane and argon,   2) cryogenically separating the feed into the following:
           a) a second stream of a synthesis gas containing hydrogen and nitrogen in a MOL ratio of about 3/1,   b) waste gas containing principally nitrogen, and also containing substantially all of the methane supplied in the first stream,   
           3) and splitting the waste gas into:
           c) a third stream of hydrogen/nitrogen gas,   d) a fourth stream of highly concentrated nitrogen,   e) a fifth stream of methane rich gas, useful as fuel.   
               
 
         [0013]    In that overall process, the second, third, fourth and fifth streams are typically delivered as product streams; and the second plus third product streams of synthesis gas may be advantageously delivered to an ammonia synthesis process. 
         [0014]    Another object is to provide the split into a third, fourth and fifth streams, through cryogenic separation in such manner that
       a) the amount of hydrogen of the third stream equals the hydrogen content of the waste gas   b) the amount of methane of the fifth stream equals the methane of the waste gas.       
 
         [0017]    Accordingly, the prior “Purifier” process is modified and improved through these measures, in that
       a) all incoming hydrogen is completely recovered towards synthesis gas   b) the heating value of the methane rich gas is increased, typically from 165 BTU/SCF (LHV) to about 700 BTV/SCF (LHV).
 
The methane rich gas is used as fuel and increased heating value improves the combustion.
       
 
         [0020]    These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: 
     
    
     
       DRAWING DESCRIPTION 
         [0021]      FIG. 1  is a diagram showing the separation of a feed gas into synthesis gas and waste gas as in the Purifier process. 
           [0022]      FIG. 2  is a diagram showing the additional split of the waste gas into hydrogen/nitrogen gas, nitrogen rich gas and methane rich gas, 
           [0023]      FIGS. 3 and 4  show detailed processes. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In  FIG. 1 , feed gas, such as hydrogen, nitrogen, argon and methane is fed at  10  to a purification or separation process  11 . The feed gas typically has an H/N ratio of about 2. Separated hydrogen is fed at  12  (in a stream with a H/N ratio of about 3) from the process  11 , and delivered for example as synthesis gas to a conversion process producing ammonia. Separated “waste” gas is fed at  13  from the process  11 , and contains nitrogen, methane, and about 60% if the incoming argon at  10 , usable as a low grade fuel for combustion and heating, for example to the primary reformer or to a boiler. The typical heating value of the waste gas  13  is approximately 160 BTU/SCF (LHV). See in this regard U.S. Pat. No. 3,442,613 to Grotz. 
         [0025]    In a preferred and improved prior Purifier process as represented in  FIG. 2  and in more detail in  FIG. 3  feed gas is delivered at  110  to a cryogenic separation process indicated generally at  111 . Synthesis gas is withdrawn from the process at  112 . Nitrogen rich gas and methane rich gas are separated in the process and delivered as streams  113  and  114  respectively. The methane rich gas  114  is typically used as a (high grade) fuel for instance in the primary reformer upstream of  111 . 
         [0026]    Referring in detail to process  111  in  FIG. 3  coldbox  115   a  and columns  130  and  140  are additions to an existing coldbox  115  with an existing column  116 . The streams  110 ,  112   c  and  131  flow through the existing coldbox or refrigerated heat exchanger  115  for heat exchange as shown via coils  110   a ,  110   b ,  112   a  and  126   a . As in the purifier process expander C 4  provides refrigeration between coils  110   a  and  110   b . An existing separation column  116  receives the refrigerated feed via line  117  and synthesis gas is taken from the top of this column and passed through the existing top mounted refluxed condenser  119 . Synthesis gas is taken overhead via line  121  and passed to coil  112   a  in the existing box  115  for delivery at line  112   c.    
         [0027]    Waste gas is taken from the bottom of the existing column  116  and is passed via line  122  to the existing Joule Thompson valve  123 . A typical pressure drop through the JT valve is 300 to 350 psi. 
         [0028]    Cooled waste gas then passes via line  125  to provide refrigeration for the existing condenser  119 . It passes through line  126  and coil  126   a  in the existing coldbox  115  for delivery via line  131  to coil  114   a  in an additional coldbox  115   a  and exits via line  131   b  as feed to an additional second column  130 . Column  130  is provided with a top mounted refluxed condenser  135 . 
         [0029]    Methane rich gas leaves the bottom of column  130  via line  133  to flow to coil  145   a  in the additional coldbox  115   a  to deliver at line  134 . If needed, the pressure of the methane rich gas is boosted in a single stage blower C 1  and methane rich gas is delivered at  114 . 
         [0030]    Overhead gas is taken via line  132  to a third additional column  140 . The separation in column  130  is such that all of the incoming hydrogen via line  131   b  but none of the incoming methane via line  131   b  goes overhead via line  132 . 
         [0031]    The additional third column  140  is provided with a top mounted refluxed condenser  145 . Nitrogen rich gas leaves the bottom of column  140  via line  143  to flow to coil  113   a  in the additional coldbox  115   a , and to deliver at line  113 . Nitrogen rich gas (typically 97 + % nitrogen, with the remainder being Argon) may be rejected to the atmosphere. 
         [0032]    Overhead gas from the additional column  140  is taken via line  142  to coil  140   a  in the additional coldbox  115   a  to deliver at line  146 . The separation in column  140  is such that all of the incoming hydrogen via line  132  goes overhead at column  140 . Hydrogen/nitrogen delivered at line  146  is recompressed in compressor C 2  and combined with the synthesis gas at line  112   c , and is delivered at line  112 . 
         [0033]    Refrigeration for the refluxed condensers  135  and  145  is provided by a refrigeration compressor C 3 . The discharge of compressor C 3  delivers via line  151  to coil  150   a  in the additional cold box  115   a . The cold refrigerant leaves via line  152  and is expanded via valve  153  to line  154 . Refrigerant to refluxed condenser  135  is delivered via line  155 ; refrigerant to refluxed condenser  145  is delivered via line  156 . Refrigerant returns from the refluxed condenser  135  via line  157  and from refluxed condenser  145  via line  158 . The combined refrigerant returns via line  159  into coil  150   b  in the additional coldbox  115   a , and leaves via line  160  to the suction of the refrigerant compressor C 3 . 
         [0000]    Following data are representative for  FIG. 3   
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Feedgas 
                 Synthesis gas 
                 waste gas 
                 NZ rich gas 
                 CH9 rich gas 
               
             
          
           
               
                 Stream # 
                 110 
                 112c 
                 146 
                 112 
                 131b 
                 113 
                 134 
               
               
                   
               
             
          
           
               
                 Temp ° F. 
                 40 
                 35 
                 35 
                 34 
                 30 
                 35 
                 35 
               
               
                 pressure psia 
                 399 
                 348 
                 40 
                 348 
                 40 
                 15 
                 25 
               
               
                 Comp. MOL % H2 
                 66.1 
                 76.2 
                 20.5 
                 73.8 
                   5.5 
                 — 
                 — 
               
               
                 Comp. MOL % N2 
                 31.0 
                 23.6 
                 79.5 
                 26.0 
                   75.5 
                 96.8 
                 28.9 
               
               
                 Comp. MOL % Ar 
                 0.5 
                 0.2 
                 — 
                 0.2 
                   2.1 
                 3.2 
                 2.5 
               
               
                 Comp. MOL % CH 4   
                 2.4 
                 — 
                 — 
                 — 
                   16.9 
                 — 
                 68.6 
               
               
                   
                 100 
                 100 
                 100 
                 100 
                 100  
                 100 
                 100 
               
               
                 LHV BTU/SCF 
                 — 
                 — 
                 — 
                 — 
                  17- 
                 — 
                 625 
               
               
                   
               
               
                 Temperatures 
               
               
                 T1 = −286° F. 
               
               
                 T2 = −295 
               
               
                 T5 = −284 
               
               
                 T6 = −304 
               
               
                 T8 = −325 
               
               
                 T10 = −321 
               
               
                 T13 = 308 
               
               
                 Pressures 
               
               
                 P1 = 385 psia 
               
               
                 P2 = 22 
               
               
                 P3 = 20 
               
               
                 P4 = 15 
               
               
                 P5 = 172 
               
             
          
         
       
     
         [0034]    For a completely new (grass roots) design the coldboxes  115  and  115   a  of  FIG. 3  can advantageously be combined into one coldbox  180 , and the expander C 4  can be eliminated, as shown in  FIG. 4 . 
         [0035]    Referring in detail to process  211  in  FIG. 4  the streams  110 ,  112   c ,  113  and  134  flows through a coldbox or refrigerated heat exchanger  116  for heat exchange as shown via coils  110   a ,  112   a ,  113   a  and  114   a . A first separation column  116  receives the refrigerated feed via line  117  and synthesis gas taken from the top of this column and passed through a top mounted refluxed condenser  119 . Synthesis gas is taken overhead via line  121  and passes to coil  112   a  in box  180  for delivery at line  112   c.    
         [0036]    Waste gas is taken from the bottom of the column  116  and is passed via line  122  to the Joule Thompson valve  123 . A typical pressure drop through the JT valve is 300 to 350 psi. 
         [0037]    Cooled waste gas then passes via line  125  to provide refrigeration for the condenser  119 . It passes through line  126  and coil  126   a  in the coldbox  180  for delivery via line  131  as feed to a second column  130 . Column  130  is provided with a top mounted refluxed condenser  135 . 
         [0038]    Methane rich gas leaves the bottom of column  130  via line  133  to flow to coil  114   a  in coldbox  180  to deliver at line  134 . If needed, the pressure of the methane rich gas is boosted in a single stage blower C 1  and methane rich gas is delivered at  114 . 
         [0039]    Overhead gas is taken via line  132  to a third column  140 . The separation in column  130  is such that all of the incoming hydrogen via line  131  but none of the incoming methane via line  131  goes overhead via line  132 . 
         [0040]    Third column  140  is provided with a top mounted refluxed condenser  145 . Nitrogen rich gas leaves the bottom of column  140  via line  143  to flow to coil  113   a  in coldbox  180 , and to delivery at line  113 . Nitrogen rich gas (typically 97% nitrogen, with the remainder being Argon) may be rejected to the atmosphere. 
         [0041]    Overhead gas from column  140  is taken via line  142  to coil  140   a  in coldbox  115  to deliver at line  146 . The separation in column  1450  is such that all of the incoming hydrogen via line  132  goes overhead at column  140 . Hydrogen/nitrogen delivered at line  146  is recompressed in compressor C 2  and combined with the synthesis gas at line  112   c , and is delivered at line  112 . 
         [0042]    Refrigeration for the refluxed condensers  135  and  145  is provided by a refrigeration compressor C 3 . The discharge of compressor C 3  delivers via line  151  to coil  150   a  in coldbox  180 . The cold refrigerant leaves via line  152  and is expanded via valve  153  to line  154 . Refrigerant to refluxed condenser  135  is delivered via line  155 ; refrigerant to refluxed condenser  145  is delivered via line  156 . Refrigerant returns from the refluxed condenser  135  via line  157  and from refluxed condenser  145  via line  158 . The combined refrigerant returns via line  159  into coil  150   b  in coldbox  180 , and leaves via line  160  to the suction of the refrigerant compressor C 3 . 
         [0000]    The following data are representative for  FIG. 4 . 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Feed 
                   
                 N2 Rich 
                 CH4 Rich 
               
               
                   
                 gas 
                 Synthesis gas 
                 gas 
                 gas 
               
             
          
           
               
                 stream # 
                 110 
                 112c 
                 146 
                 112 
                 113 
                 134 
               
               
                   
               
             
          
           
               
                 Temp. ° F 
                 38 
                 39 
                 32 
                 38 
                 40 
                 30 
               
               
                 psia 
                 391 
                 379 
                 18 
                 379 
                 17 
                 17 
               
               
                 comp. mol % H2 
                 67.3 
                 75.7 
                 31.3 
                 74.7 
                 — 
                 — 
               
               
                 comp. mol % N2 
                 29.7 
                 24.0 
                 68.7 
                 25.0 
                 97.9 
                 22.0 
               
               
                 comp. mol % Ar 
                 0.6 
                 0.3 
                 — 
                 0.3 
                 2.1 
                 5.3 
               
               
                 comp. mol % CH4 
                 2.4 
                 — 
                 — 
                 — 
                 — 
                 72.7 
               
               
                   
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 LHV BTU/SCF 
                 — 
                 — 
                 — 
                 — 
                 — 
                 660 
               
               
                   
               
             
          
         
       
     
         [0043]    The presentation of the coldboxes  115 ,  115   a  and  180  in  FIG. 3  and  FIG. 4  is schematic and each coldbox is characterized by the following:
       1) Heat is exchanged between the flowing process streams, and the temperatures change accordingly as indicated. The heat exchange between the warm and the cold streams is in balance.   2) The heat exchangers and columns are embedded in one common box, providing cold insulation to prevent ingression of heat to the equipment. The insulation side of the cold box interior has one common identical stagnant temperature, for the whole box interior.   3) The presentation in  FIG. 3  and  FIG. 4  indicates that heat exchange occurs directly between the warm and cold streams, inside the heat exchange device.   4) Accordingly, the cold box interior maintains, throughout the entirety of the gas purification process, the same temperature at which the indicated streams are passed through the cold box interior, after the cryogenic separation.       
 
         [0048]    The parameters, upstream of the coldbox as presented, are to be adjusted as to maintain the feed gas to the coldbox per  FIG. 3  and  FIG. 4  line  110 .