Abstract:
An improved process for liquid nitrogen production by cryogenic air separation using a distillation column system to enhance the product recovery.

Description:
BACKGROUND OF THE INVENTION 
     This invention concerns a new and efficient process for producing liquid nitrogen. 
     Liquid nitrogen is normally produced as a by-product of oxygen production. While there are sizable market demands for nitrogen by itself, in fields such as glass making, chemical inverting, electronics, and food preparation, these are end demands for the gas product, and the liquid form is merely a convenience for transportation and storage. 
     Large or stand alone demands for nitrogen gas are conventionally supplied by nitrogen gas generators, which often involve cryogenic distillation but produce no meaningful amounts of nitrogen in liquid form. When there is a large demand for the liquid form, without the simultaneous requirement of oxygen, nitrogen gas generators are typically coupled with a separate nitrogen liquefaction unit to fulfill this requirement. 
     SUMMARY OF THE INVENTION 
     It is a major object of the invention to provide an improved process for the direct production of liquid nitrogen, for example as a sole product in an integrated process, saving both power and capital. Basically, the improved process is enabled by use of systems containing distillation columns, operating in series at different pressure levels, to extract a higher yield of nitrogen per unit of compressed air feed processed, as will be seen. The process basically includes: 
     a) passing a portion of pressurized air feed through a cold expander and feeding a part of the air exhaust from said expander as a gaseous air stream into the bottom of a first column having a top condenser, 
     b) passing another portion of the pressurized air feed through the main heat exchanger for cooling and liquefying, passing said portion of air through a valve and feeding as a liquid air stream into the middle of the first column; 
     c) separating the said gaseous and said liquid air streams in the first column into a first liquid nitrogen stream and a first oxygen-enriched liquid stream; 
     d) feeding the first oxygen-enriched liquid stream into the middle of a second column having a bottom reboiler, in which the liquid is evaporated due to indirect heat exchange with the nitrogen vapor in the first column top condenser, and a top condenser; 
     e) separating the first oxygen-enriched liquid stream in the second column into a second liquid nitrogen stream and a second oxygen-enriched liquid stream; 
     f) feeding the second oxygen-enriched liquid stream into the middle of a third column having a bottom reboiler, in which the liquid is evaporated due to indirect heat exchange with the nitrogen vapor in the second column top condenser, and a top condenser, or into an upper reboiler; 
     g) separating the second oxygen-enriched liquid stream in the third column into a third liquid nitrogen stream and a third oxygen-enriched liquid stream, and feeding the third oxygen-enriched liquid stream into the upper reboiler; 
     h) evaporating the oxygen-enriched liquid stream in the upper reboiler due to indirect heat exchange with the nitrogen vapor in the third or second column top condenser; 
     i) removing the first liquid nitrogen stream from the first column and feeding it into the top of the third or second column; 
     j) removing the second liquid nitrogen stream from the second column and feeding it into the top of the third column or using it as a liquid nitrogen product; 
     k) removing the third liquid nitrogen stream from the third column and using it as a liquid nitrogen product; 
     l) removing the evaporated oxygen-enriched stream from the upper reboiler, warming it and removing from the process. 
     As will be seen, the cleaned and pressurized air feed is split in two streams, the first stream is passed through a warm expander, the second stream is further compressed in boosters by using expander power, previously cooled in a main heat exchanger and split in two portions, one portion of this air feed is passed through a cold expander and the other portion is further cooled and liquefied in the heat exchanger; using a tripe or double distillation columns system to enhance recovery of liquid nitrogen from air enabling substantial reduction in the feed air compressor, and absorber size and power. 
     A further object is to provide for use of these multiple distillation column systems to enhance recovery of nitrogen from air, and to permit use of an air recycle process to produce refrigeration. Significant reductions in main heat exchanger size and cost are enabled. 
     Yet another object is provision of a complete process including provision of distillation columns, condenser-reboilers, heat exchangers and compressors, operating as disclosed herein. 
    
    
     
       DRAWING DESCRIPTION 
         FIG. 1  is a schematic showing of a process for producing liquid nitrogen from air with a triple distillation column system; 
         FIG. 2  is a schematic showing a process for producing liquid nitrogen from air with a double distillation column system; 
         FIG. 2 a    is a schematic with a modified double distillation column system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the schematic of  FIG. 1 , air feed at  10  is filtered in  101 , and compressed at  102  to a pressure of 8 to 10 bara, cooled in  103 , and after removal in adsorber  104  of water and carbon dioxide, the air is mixed with the recycle stream  22  removed from the main heat exchanger  113  and at  13  fed to the compressor  105 , where it is further compressed to about 40 bara (+/−5), and cooled in  106 . 
     A portion  16  (or all) of the compressed air stream  14  is then boosted in one or two compressors  107  and  109 , driven by one or two turbo expanders  112  and  111 , to a pressure between 70 and 90 bara at  18 . The other portion  15  of the compressed air is fed in the warm turbo expander  111  and then to the heat exchanger  113  at  17 . 
     The boosted air is then cooled in the heat exchanger  113 , and a portion is fed at  19  to the cold turbo expander  112 , the remainder  23  being further cooled and liquefied, then expanded in a valve  114  and fed to the middle of the first distillation column  115  as a liquid air stream. 
     The exhaust  20  from the cold turbo expander is split. One portion returns at  21  to provide cooling in the aforementioned heat exchanger  113  while the split remainder to fed at  25  into the bottom of the first distillation column  115  as a gaseous air stream. In that first column, operated about from 7.5 to 9 bara, the air is distilled into pure nitrogen (from 1.0 to 0.0001 mol % of O 2 ) at  26 , and condensed in the top condenser  116 , a portion of the condensate being removed at  27  as a first liquid nitrogen stream, cooled in the heat exchanger  121  and passed through a valve  122  into the top of the third column  119 . The remainder returns at  28  to the first column as reflux. 
     The bottom liquid in the first column is rich in oxygen (24 to 26% O 2 ). This first oxygen-enriched liquid, removed at  29 , is cooled in exchanger  121  and fed through a valve  123  into the middle of the second column (operated from about 5.0 to 6.5 bara), where it joins with liquid descending in this column. This liquid descends countercurrent to the vapor generated by the bottom reboiler  116 . 
     The vapor ascending in the second column is progressively rectified until a pure nitrogen is achieved on the highest rectification stage. This nitrogen stream is condensed in the top condenser  118  and a portion removed at  31  and passed through valve  125  into the top of the third column  119 , while the remainder descends as reflux. 
     The bottom liquid in the second column is richer yet in oxygen (32-33% of O 2 ), removed at  30  as a second oxygen-enriched liquid, throttled at  124  and fed into the middle of the third column  119  (operated at 3-3.6 bara), where it joins with liquid descending in this column. This liquid descends countercurrent to the vapor generated by bottom reboiler  118 . 
     The vapor ascending in the third column is progressively rectified until a pure nitrogen is achieved on the highest rectification stage. This nitrogen stream is condensed in the top condenser  120  and a portion removed at  32  as a liquid nitrogen product while the remainder descends as reflux. 
     The bottom liquid in the third column is richer yet in oxygen (50-52% of O 2 ), removed at  33  as a third oxygen-enriched liquid, throttled in  126  to about one atmosphere and transferred to the upper reboiler  120 . 
     A very small amount of the oxygen rich (approximately 78% O 2 ) liquid is removed at  34  from the upper reboiler  120  to guard against build up of a dangerous substances in that reboiler as contaminants. 
     The vapor exiting the upper reboiler (waste) moves through the sub-cooling heat exchanger  121  and enters at  36  the main heat exchanger  113  where the refrigeration is recovered. At the exit of the main exchanger some of the waste is vented, the rest being used at  38  to regenerate the adsorber  104  associated with water and carbon dioxide removal. 
     The example stream parameters of the process for producing liquid nitrogen at 88.9 K corresponding to the pressure in the third column 3.27 bara are shown in the Table 1. Using the process with the triple column system allows increasing the liquid nitrogen output (LIN) from 0.33 mol/mol of processed air (p.a.) for the existent process to 0.59 mol/mol p.a. (The processed air flow rate is equal to the feed air flow rate). The increase in liquid nitrogen recovery enables a 20% reduction in the specific power and substantial reduction in the feed air compressor and adsorber size and cost. 
     The liquid nitrogen product can be subcooled in  127  from the temperature about 86-89 K at  32  to the temperature about 79-81 K at  40  by evaporating a part of liquid nitrogen stream  41  in  128  at reduced pressure close to atmospheric. The evaporating a part of liquid air stream at reduced pressure for the preliminary subcooling can be also used. 
     Referring to the schematic of  FIG. 2 , air feed at  10  is filtered in  101 , and compressed in  102  to a pressure of 5 to 7.5 bara, cooled in  103  and after removal in adsorber  104  of water and carbon dioxide, the air is mixed with the removed from the main heat exchanger  113  recycle stream  22  and at  13  fed to the compressor  105 , where it is further compressed to about 30 bara (+/−5), and cooled in  106 . 
     A portion  16  (or all) of the compressed air stream  14  is then boosted in one or two compressors  107  and  109 , driven by one or two turbo expanders  112  and  111 , to a pressure between 55 and 75 bara at  18 . The other portion  15  of the compressed air is fed in the warm turbo expander  111  and then in the heat exchanger  113  at  17 . 
     The boosted air is then cooled in the heat exchanger  113 , and a portion  19  is fed to the cold turbo expander  112 , the remainder  23  being further cooled and liquefied, then expanded in a valve  114  and fed in the middle of the first (lower) distillation column  115  as a liquid air stream. 
     The exhaust  20  from the cold turbo expander is split. One portion returns at  21  to provide cooling in aforementioned heat exchanger  113  while the split remainder  25  is passed through a throttling valve  119 , wherein the pressure is decreased by up to 2 bar (for example, from 6.5 to 4.5 bara), and then at  33  fed into the bottom of the first (lower) column as a gaseous air stream. 
     In that first (lower) column, operated about from 4.5 to 6.5 bara, the air is distilled into pure nitrogen (from 1.0 to 0.0001 mol % of O 2 ) at  26 , and condensed in the top condenser  116 , a portion of the condensate being removed at  27 , as a first liquid nitrogen stream, cooled in the heat exchanger  121  and passed through a valve  122  into the top of the second (upper) column. The remainder returns at  28  to the lower column as reflux. 
     The bottom liquid in the lower column is rich in oxygen (27 to 28% O 2 ). This first oxygen-enriched liquid removed at  29  is cooled in  121  and fed through a valve  123  into the middle of the upper column (operated from about 2.7 to 3.3 bara), where it joins with liquid descending in this column. This liquid descends countercurrent to the vapor generated by the bottom reboiler  116 . 
     The vapor ascending in the upper column  117  is progressively rectified until a pure nitrogen is achieved on the highest rectification state. This nitrogen stream is condensed in the top condenser  118  and a portion removed at  32  as a liquid nitrogen product while reminder descends as reflux. 
     The bottom liquid in the upper column is richer yet in oxygen (43-45% of O 2 ), removed at  30  as a second oxygen-enriched liquid throttled in  124  to about one atmosphere and transferred to the upper reboiler  118 . 
     A very small amount of the oxygen rich (approximately 73% O 2 ) liquid is removed at  34  from the upper reboiler to guard against build up of a dangerous substances in that reboiler as contaminants. 
     The vapor exiting the upper reboiler (waste) moves through the sub-cooling heat exchanger  121  and enters at  36  the main heat exchanger  113  where the refrigeration is recovered. At the exit of the main exchanger some of the waste is vented, the rest being used at  38  to regenerate the adsorber  104  associated with water and carbon dioxide removal. 
     The example stream parameters of the process for producing liquid nitrogen an 88.1 K corresponding to the pressure 3.05 bara in the upper column are shown in the Table 2. Using the process with the double column system allows increasing the liquid nitrogen output (LIN) from 0.33 mol/mol of processed air (p.a.) for the existent process to 0.52 mol/mol p.a. The increase in liquid nitrogen recovery enables a 15% reduction in the specific power and substantial reduction in the feed air compressor and adsorber size and cost. 
     The part of the air exhaust from the cold expander is passed into the lower column through a throttling valve using for the expander exhaust pressure control and allowing also to additional increasing the liquid nitrogen recovery. For example, if the pressure in this valve is decreased by 2 bar (from 6.5 to 4.5 bara), the liquid nitrogen output (LIN) is increased by 0.021 mol/mol p.a., that is 3.8% (0.020/0.522=1.038). 
     The pressure of the liquid nitrogen product can be increased by using a liquid column. For example, if the difference in elevation between the top of the upper column and the place of withdrawal of the liquid nitrogen product is equal to 16 m, the pressure can be increased by 1.2 bar. 
     As previously noted, the liquid nitrogen product can be subcooled in  127  from the temperature about 86-89 K at  32  to the temperate about 79-81 K at  40  by evaporating a part of the liquid nitrogen stream  41  at reduced pressure close to atmospheric. 
     The example conditions of the process with the double column system ( FIG. 2 ) for liquid nitrogen production at 81 K are illustrated in the Table 3. In this case the process with the double column system allows increasing the liquid nitrogen output (LIN) from 0.303 mol/mol of processed air (p.a.) for the existent process to 0.448 mol/mol p.a. The increase in liquid nitrogen recovery enables a 12% reduction in the specific power and also substantial reduction in the feed air compressor and adsorber size and cost. 
     Using the evaporating part of the liquid air stream at reduced pressure for the preliminary subcooling of liquid nitrogen product enables 13% reduction in the specific power as compared with the existent process. 
     The advantages of the air recycle are lower size (by 33%) of the main heat exchanger as compared with the nitrogen recycle. 
     The booster compressors  107  and  109  can operate in series as shown in  FIG. 2 , or in parallel. Series connection reduces the specific power by 1% compared to the parallel. 
     Feeding the liquid air stream into the lower column allows to increase the LIN output as compared with feeding this stream into the upper column. As is seen from the Table 4, LIN is increased by 4% (0.522/0.502=1.04). In addition, feeding the liquid air stream into the lower column allows reducing of the throttling valve and pipe size. 
     The liquid air stream fed into the lower column should be equal to at least 40% of the processed air. 
     The reflux ratio in the columns and correspondingly the number of trays makes a greater impact on the liquid nitrogen product yield and other parameters, that affect the energy and equipment costs. It is estimated that the optimal relationship between the reflux ratio and the minimum reflux ratio for the columns is approximately from 1.1. to 1.2, if the liquid nitrogen product contains about 0.01% of oxygen. 
     The waste gas removed from the main heat exchanger  113  contains from 38% O 2  (Table 3) to 43.4% O 2  (Table 2). For the purpose of decreasing the oxygen content in the regeneration gas, one or more trays can be added above the upper reboiler to provide two separate streams: a regeneration gas and a waste stream with increased oxygen content (Table 3). 
     This method of decreasing the oxygen content in the regeneration gas result in increasing the boiling temperature in the upper reboiler and the pressure in the columns and therefore leads to reducing the liquid nitrogen recovery. Another method in which the pressure in the columns is not increased is discussed below. 
     Referring to the schematic of  FIG. 2 a   , the oxygen-enriched liquid from the upper column is removed at  30 , throttled in  124  and transferred to the first upper reboiler  118 , wherein this liquid is partially evaporated. The vapor that contains less oxygen is removed at  35 , then heated in the exchangers  121  and  113  and used at  38  as a regeneration gas for the adsorber  104 . The remainder liquid is removed from  118  at  39 , throttled in  125  and fed into the second upper reboiler  120 . The vapor exiting  120  at  42  is heated in the exchangers  121  and  113  and vented at  37  as a waste gas. 
     A very small amount of the oxygen rich (approx 80% O 2 ) liquid is removed at  34  from the second upper reboiler  120  to guard against build up of a dangerous substances in that reboiler as contaminants. 
     As is seen from the Table 5, the oxygen content in the regeneration gas is equal to 25%, that is much less than in case of using one upper reboiler (43.4% O 2 ), Table 2). The temperature difference in the first upper reboiler is equal to 3.74 K and in the second upper reboiler—1.17 K (Table 5), whereas the temperature difference in the case of using one upper reboiler is equal to 1.20 K (Table 2). The total surface of the first and second upper reboiler is less by 17% than the surface in case of using one upper reboiler. 
     The part  25  of the air exhaust from the cold expander  112  can be passed into the lower column  115  through an additional expander  119  ( FIG. 2 a   ) using for the receiving an additional refrigeration capacity. For example, if the pressure is expanded by 2 bar (from 6.5 to 4.5 bara), the additional refrigeration capacity is equal to 2% of the total capacity, and the specific power can be decreased by 1.4% due to reducing the recycle air flow rate. It should be noted that the cold (and warm) expander exhaust pressure decrease is inexpedient, since it leads to a decrease in the efficiency of the recycle system. 
     ALTERNATIVE ARRANGEMENTS 
     a. The bottoms from the columns can be passed to the upper reboiler; 
     b. The liquid air stream can be fed to either column; 
     c. The first and second liquid nitrogen stream or the first liquid nitrogen stream can be used as a liquid nitrogen product; 
     e. The portions of the liquid nitrogen product removing from the distillation columns are passed through throttling valves into a liquid separator, from which the liquid is removed as a liquid nitrogen product at the temperature about 79-81 K and the vapor is passed through heat exchangers and removed from the process. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The stream parameters of the process with the  
               
               
                 triple column system (FIG. 1) for producing  
               
               
                 liquid nitrogen at 88.9 K (example) 
               
             
          
           
               
                   
                 Flow 
                   
                   
                   
                 Content 
               
               
                   
                 rate, 
                   
                   
                 Vapor 
                 of 
               
               
                   
                 mol/mol 
                 Temperature, 
                 Pressure, 
                 mole 
                 oxygen, 
               
               
                 No 
                 p.a.* 
                 K. 
                 bara 
                 fraction 
                 % mol 
               
               
                   
               
             
          
           
               
                 11 
                 1.0 
                 300.0 
                 8.70 
                 1.0 
                 20.95 
               
               
                 12 
                 1.0 
                 280.0 
                 8.35 
                 1.0 
                 20.95 
               
               
                 13 
                 3.29 
                 291.0 
                 8.30 
                 1.0 
                 20.95 
               
               
                 14 
                 3.29 
                 300.0 
                 39.9 
                 1.0 
                 20.95 
               
               
                 15 
                 1.0 
                 300.0 
                 39.9 
                 1.0 
                 20.95 
               
               
                 16 
                 2.29 
                 300.0 
                 39.9 
                 1.0 
                 20.95 
               
               
                 18 
                 2.29 
                 300.0 
                 82.9 
                 1.0 
                 20.95 
               
               
                 19 
                 1.613 
                 202.0 
                 82.8 
                 1.0 
                 20.95 
               
               
                 20 
                 1.613 
                 105.72 
                 8.45 
                 1.0 
                 20.95 
               
               
                 22 
                 2.29 
                 296.0 
                 8.35 
                 1.0 
                 20.95 
               
               
                 23 
                 0.677 
                 108.89 
                 82.7 
                 0.0 
                 20.95 
               
               
                 24 
                 0.677 
                 103.66 
                 8.43 
                 0.0572 
                 20.95 
               
               
                 25 
                 0.323 
                 105.72 
                 8.45 
                 1.0 
                 20.95 
               
               
                 27 
                 0.1641 
                 100.96 
                 8.30 
                 0.0 
                 0.01 
               
               
                 29 
                 0.8359 
                 104.06 
                 8.45 
                 0.0 
                 25.06 
               
               
                 30 
                 0.6415 
                 99.69 
                 5.87 
                 0.0 
                 32.65 
               
               
                 31 
                 0.1943 
                 95.93 
                 5.72 
                 0.0 
                 0.01 
               
               
                 32 
                 0.5896 
                 88.91 
                 3.27 
                 0.0 
                 0.01 
               
               
                 33 
                 0.4104 
                 94.70 
                 3.42 
                 0.0 
                 51.03 
               
               
                 34 
                 0.0052 
                 87.78 
                 1.28 
                 0.0 
                 78.32 
               
               
                 35 
                 0.4052 
                 87.78 
                 1.28 
                 1.0 
                 50.68 
               
               
                   
               
               
                 *p.a. — processed air. 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 The stream parameters of the process with the  
               
               
                 double column system (FIG. 2) for producing liquid  
               
               
                 nitrogen at 88.1 K. (example) 
               
             
          
           
               
                   
                 Flow 
                   
                   
                   
                 Content 
               
               
                   
                 rate, 
                   
                   
                 Vapor 
                 of 
               
               
                   
                 mol/mol 
                 Temperature, 
                 Pressure, 
                 mole 
                 oxygen, 
               
               
                 No 
                 p.a.* 
                 K. 
                 bara 
                 fraction  
                 % mol 
               
               
                   
               
             
          
           
               
                 11 
                 1.0 
                 300.0 
                 6.40 
                 1.0 
                 20.95 
               
               
                 12 
                 1.0 
                 280.0 
                 6.05 
                 1.0 
                 20.95 
               
               
                 13 
                 3.03 
                 291.0 
                 6.0 
                 1.0 
                 20.95 
               
               
                 14 
                 3.03 
                 300.0 
                 31.0 
                 1.0 
                 20.95 
               
               
                 15 
                 0.88 
                 300.0 
                 31.0 
                 1.0 
                 20.95 
               
               
                 16 
                 2.15 
                 300.0 
                 31.0 
                 1.0 
                 20.95 
               
               
                 18 
                 2.15 
                 300.0 
                 64.2 
                 1.0 
                 20.95 
               
               
                 19 
                 1.566 
                 194.0 
                 64.1 
                 1.0 
                 20.95 
               
               
                 20 
                 1.566 
                 101.05 
                 6.15 
                 0.995 
                 20.95 
               
               
                 22 
                 2.03 
                 296.0 
                 6.05 
                 1.0 
                 20.95 
               
               
                 23 
                 0.584 
                 103.94 
                 64.0 
                 0.0 
                 20.95 
               
               
                 24 
                 0.584 
                 96.79 
                 5.18 
                 0.0772 
                 20.95 
               
               
                 25 
                 0.416 
                 101.05 
                 6.15 
                 0.995 
                 20.95 
               
               
                 33 
                 0.416 
                 98.96 
                 5.20 
                 0.999 
                 20.95 
               
               
                 27 
                 0.243 
                 94.15 
                 5.05 
                 0.0 
                 0.01 
               
               
                 29 
                 0.757 
                 97.35 
                 5.20 
                 0.0 
                 27.67 
               
               
                 30 
                 0.478 
                 92.97 
                 3.20 
                 0.0 
                 43.82 
               
               
                 32 
                 0.522 
                 88.12 
                 3.05 
                 0.0 
                 0.01 
               
               
                 34 
                 0.006 
                 86.92 
                 1.28 
                 0.0 
                 73.25 
               
               
                 35 
                 0.472 
                 86.92 
                 1.28 
                 1.0 
                 43.45 
               
               
                   
               
               
                 *p.a. — processed air. 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 The performance of the process with the  
               
               
                 double column system (FIG. 2) for producing  
               
               
                 liquid nitrogen at 81 K (example) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 # of case 
                 1.2A 
               
               
                   
                 Recycle 
                 Air 
               
               
                   
                 Type of scheme 
                 DCU 
               
               
                   
                 Feed air compressor 
                   
               
               
                   
                 flow rate, Nm{circumflex over ( )}3/h 
                 6920 
               
               
                   
                 suction pressure, bara 
                 0.99 
               
               
                   
                 discharge pressure, bara 
                 6.40 
               
               
                   
                 Recycle compressor 
                   
               
               
                   
                 flow rate, Nm{circumflex over ( )}3/h 
                 19151 
               
               
                   
                 Nm{circumflex over ( )}3Nm{circumflex over ( )}3 p.a. 
                 2.7677 
               
               
                   
                 suction pressure, bara 
                 6.05 
               
               
                   
                 discharge pressure, bara 
                 31.03 
               
               
                   
                 Main exchanger 
                   
               
               
                   
                 temperature, K. 
                   
               
               
                   
                 middle pressure air inlet 
                 300 
               
               
                   
                 temperature difference, K. 
                   
               
               
                   
                 warm end 
                 3.7 
               
               
                   
                 minimum 
                   
               
               
                   
                 warm section 
                 2.5 
               
               
                   
                 cold section 
                 1.6 
               
               
                   
                 U*A, kW/K. 
                 282 
               
               
                   
                 ‘Warm’ expander 
                   
               
               
                   
                 inlet pressure, bara 
                 53.2 
               
               
                   
                 outlet pressure, bara 
                 6.11 
               
               
                   
                 inlet temperature, K. 
                 300 
               
               
                   
                 outlet temperature, K. 
                 177.0 
               
               
                   
                 isentropic efficiency 
                 0.86 
               
               
                   
                 flow rate, Nm{circumflex over ( )}3/h 
                 4860 
               
               
                   
                 Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a.  
                 0.7024 
               
               
                   
                 ‘Cold’ expander 
                   
               
               
                   
                 inlet pressure, bara 
                 49.4 
               
               
                   
                 outlet pressure, bara 
                 6.15 
               
               
                   
                 inlet temperature, K. 
                 177.4 
               
               
                   
                 outlet temperature, K. 
                 101.1 
               
               
                   
                 vapor mole fraction 
                 0.993 
               
               
                   
                 isentropic efficiency 
                 0.88 
               
               
                   
                 flow rate, Nm{circumflex over ( )}3/h 
                 10786 
               
               
                   
                 Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 
                 1.5588 
               
               
                   
                 Lower column 
                   
               
               
                   
                 Pressure, top, bara 
                 5.95 
               
               
                   
                 Vapor flow rate, Nm{circumflex over ( )}3/h 
                 3415 
               
               
                   
                 Concentration 
                   
               
               
                   
                 liquid nitrogen, ppm O2 
                 3 
               
               
                   
                 kettle liquid, % mol O2 
                 38.0 
               
               
                   
                 Number of theoretical trays 
                 46 
               
               
                   
                 Condenser-reboiler 
                   
               
               
                   
                 temperature difference, K. 
                 2.7 
               
               
                   
                 Upper column 
                   
               
               
                   
                 Pressure, top, bara 
                 3.2 
               
               
                   
                 Vapor flow rate, Nm{circumflex over ( )}3/h 
                 3688 
               
               
                   
                 Concentration 
                   
               
               
                   
                 liquid nitrogen, ppm O2 
                 3 
               
               
                   
                 kettle liquid, % mol O2 
                 46.1 
               
               
                   
                 Number of theoretical trays 
                 40 
               
               
                   
                 Condenser-reboiler 
                   
               
               
                   
                 temperature difference, K. 
                 1.5 
               
               
                   
                 Liquid Nitrogen product 
                   
               
               
                   
                 LIN output, Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a.  
                 0.448 
               
               
                   
                 LIN capacity, Nm{circumflex over ( )}3/h 
                 3100 
               
               
                   
                 Temperature, K. 
                 81 
               
               
                   
                 Pressure, bara 
                 3.2 
               
               
                   
                 Regeneration gas and waste gas 
                   
               
               
                   
                 flow rate, Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 
                 0.552 
               
               
                   
                 middle concentration, % mol O2 
                 37.9 
               
               
                   
                 Regeneration gas 
                   
               
               
                   
                 flow rate, Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 
                 0.25 
               
               
                   
                 concentration, % mol O2 
                 21.0 
               
               
                   
                 Waste gas 
                   
               
               
                   
                 flow rate, Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 
                 0.302 
               
               
                   
                 concentration, % mol O2 
                 51.9 
               
               
                   
                   
               
               
                   
                 p.a. — processed air. 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 The performance of the double column system at feeding 
               
               
                 the liquid air stream into the lower or upper column 
               
               
                 (example) 
               
             
          
           
               
                 # of case 
                 4.1 
                 4.2 
                 4.3 
               
               
                   
               
               
                 Feeding the liquid air stream into 
                 lower 
                 lower 
                 lower 
               
               
                   
                 column 
                 column 
                 column 
               
               
                 Feeding the bottom liquid 
                 upper 
                 upper 
                 upper 
               
               
                 from the lower column into 
                 column 
                 column 
                 column 
               
               
                 Liquid air stream, mol/mol p.a. 
                 0.584 
                 0.563 
                 0.563 
               
               
                 Lower column 
                   
                   
                   
               
               
                 Pressure (top), bara 
                 3.05 
                 3.05 
                 3.05 
               
               
                 Concentration, % mol O2 
                   
                   
                   
               
               
                 liquid nitrogen 
                 0.01 
                 0.01 
                 0.01 
               
               
                 kettle liquid 
                 27.67 
                 40.13 
                 40.13 
               
               
                 LIN output, mol/mol p. a. 
                 0.243 
                 0.2089 
                 0.2089 
               
               
                 Number of theoretical trays (NTT) 
                 36 
                 36 
                 36 
               
               
                 section 1 
                 32 
                 36 
                 36 
               
               
                 section 2 
                 4 
                   
                   
               
               
                 Upper column 
                   
                   
                   
               
               
                 Pressure (top), bara 
                 5.05 
                 5.05 
                 5.05 
               
               
                 Concentration, % mol O2 
                   
                   
                   
               
               
                 liquid nitrogen 
                 0.01 
                 0.01 
                 0.01 
               
               
                 kettle liquid 
                 43.82 
                 42 
                 43.8 
               
               
                 LIN output, mol/mol p. a. 
                 0.522 
                 0.5013 
                 0.5027 
               
               
                 Number of theoretical trays (NTT) 
                 36 
                 36 
                 36 
               
               
                 section 1 
                 32 
                 30 
                 32 
               
               
                 section 2 
                 4 
                 2 
                 4 
               
               
                 section 3 
                   
                 4 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 The stream parameters of the process in the  
               
               
                 first and second reboilers (FIG. 2a) (example) 
               
             
          
           
               
                   
                 Flow 
                   
                   
                   
                 Content 
               
               
                   
                 rate, 
                   
                   
                 Vapor 
                 of 
               
               
                   
                 mol/mol 
                 Temperature, 
                 Pressure, 
                 mole 
                 oxygen, 
               
               
                 No 
                 p.a.* 
                 K. 
                 bara 
                 fraction 
                 % mol 
               
               
                   
               
             
          
           
               
                 30 
                 0.478 
                 92.97 
                 3.20 
                 0.0 
                 43.82 
               
               
                 32 
                 0.522 
                 88.12 
                 3.05 
                 0.0 
                 0.01 
               
               
                 34 
                 0.006 
                 86.95 
                 1.13 
                 0.0 
                 80.71 
               
               
                 35 
                 0.17 
                 84.38 
                 1.28 
                 1.0 
                 24.86 
               
               
                 39 
                 0.308 
                 84.38 
                 1.28 
                 0.0 
                 54.27 
               
               
                 42 
                 0.302 
                 86.95 
                 1.13 
                 1.0 
                 53.73 
               
               
                   
               
               
                 *p.a. — processed air.