Patent Publication Number: US-2007101762-A1

Title: Method for designing a cryogenic air separation plant

Description:
TECHNICAL FIELD  
      This invention relates generally to cryogenic air separation and, more particularly,. to designing a cryogenic air separation plant.  
     BACKGROUND ART  
      A cryogenic air separation plant is a very complicated process plant which includes many different units and subsystems such as distillation columns, condensers, reboilers, prepurification systems, air compression systems, liquid pumps, heat exchangers, storage tanks, process control systems, buildings and other infrastructure. Accordingly, the designing of a cryogenic air separation plant is a complicated and thus costly endeavor which adds significantly to the overall costs of the plant beyond the equipment and construction costs. This is because in most cases each particular cryogenic air separation plant must be specifically designed. Rarely will an existing design for a previously constructed cryogenic air separation plant be exactly suited for use in the construction of a new cryogenic air separation plant. Any method which can reduce the complexity, time and cost for designing a new cryogenic air separation plant would be very useful.  
     SUMMARY OF THE INVENTION  
      A method for designing a cryogenic air separation plant comprising:  
      (A) electing a plant classification from a set of plant classifications;  
      (B) defining a group of predesigned subsystems for the elected plant classification;  
      (C) initiating the design of a specific cryogenic air separation plant within the elected plant classification by choosing at least one predesigned subsystem to form the base system of the cryogenic air separation plant; and  
      (D) completing the design of the cryogenic air separation plant by adding to the base system an auxiliary system comprising at least one subsystem designed specifically for the cryogenic air separation plant.  
      As used herein the term “predesigned subsystem” means an arrangement comprising a plurality of engineered components and wherein each engineered component is connected to at least one other engineered component of the subsystem.  
      As used herein the term “engineered component” means a fully designed unit that performs at least one process step that is part of an overall process comprising heat exchange, distillation, compression and/or purification. Examples of engineered components include feed pretreatment units (for example prepurifiers), distillation columns, reboiler/condensers, heat exchangers, direct contact coolers, chillers, liquid pumps, gas compressors, cooling water towers, fluid expanders, process control units, liquid storage vessels, motors and electrical switch gears.  
      As used herein the term “fully designed” means design work, such as materials of construction specification, process equipment sizing and selection, major valves sizing and selection, equipment arrangement and location of all permanent support and loads, process piping sizing and routing, process instrumentation and controls specification, process analyzers selection and specification, vent and pressure relief devices sizing and specification, drain valves and headers sizing and specification, casing sizing and specifications, static and operating weights estimations, and shipping outline, has been completed.  
      As used herein the term “connected” means related by way of material transfer, energy transfer, and/or data transfer. 
    
    
     BRIEF DEXRIPTION OF THE DRAWINGS  
       FIG. 1  is a simplified schematic representation of one embodiment of a cryogenic air separation plant which may benefit from the application of the method of this invention.  
       FIG. 2  is a schematic representation of a predesigned subsystem which may be used with the plant illustrated in  FIG. 1 .  
       FIG. 3  is a schematic representation of another predesigned subsystem which may be used with the plant illustrated in  FIG. 1 . 
    
    
      The numerals in the Drawings are the same for the common elements.  
     DETAILED DESCRIPTION  
      In the method of this invention cryogenic air separation plant classifications based on specific requirements such as plant size, i.e. capacity, product slate (oxygen, nitrogen, argon and/or clean dry air, etc.), product type (gas and/or liquid), product specification (purity and/or pressure) and location (back-up needs, ambient conditions, local factors etc.), are defined. The desired cryogenic air separation plant fits into one of the classifications and is designed by fitting together at least one predesigned subsystem for that classification with one or more subsystems specifically designed for that particular cryogenic air separation plant.  
      In the practice of this invention a base system is defined to comprise at least one, preferably two or more, predesigned subsystems that are common to meet different requirements regarding, for example, product type and purity. An auxiliary system is defined to comprise one or more subsystems that are designed specifically to provide the complete plant. For example the base system may provide for air compression, prepurification, heat exchange, refrigeration supply, cryogenic distillation, condensation/reboil, liquid pumping, liquefaction, process control; and the auxiliary system may provide for product compression, liquids storage, switchgear and transformers, cooling water, motor control, buildings and other infrastructure. Any base system can operate over a range of pre-determined conditions, and the specific application requirements will fall within the allowable limits. Generally the auxiliary system will address the specific application requirements associated with product specifications or location factors. These could include factors such as product purity, pressure, backup needs, or cooling water needs. Once the engineering work for the base system is completed, it can be reused for all of the specific applications that have similar requirements.  
      The invention will be more particularly described and exemplified with reference to the Drawings. Referring now to  FIGS. 1-3 , ambient air  61  from air suction filter  101  is compressed in main air compressor  102  which is driven.by motor  103 . Resulting air stream  63  is cooled in cooler  104 , and cooled stream  64  is subjected to free water removal in moisture removal system  105 . Resulting elevated pressure air stream  5  is then fed to prepurification system  107 , which is a continuously operating two bed pressure swing adsorption (PSA) process. One bed purifies the air of water, carbon dioxide, and most of the hydrocarbons in stream  5  while the other bed is being regenerated by waste nitrogen stream  47 . The exiting contents of the regenerating bed leaves prepurification system  107  as waste stream  50 . Prepurified air stream  6  then enters a dust filter (not shown) for the removal of any remaining solid particles. Dust free prepurified air stream is split into streams  8  and  11  and further compressed in compressors  109  and  113  respectively. Aftercoolers  110  and  114  remove the heat of compression in the resulting air streams. The compressors  109  and  113 , turbine  117 , and motor  116  can be configured as a single component or as combinations of one or more of them. Motor  116  can supply additional power if the work generated by turbine  117  is not sufficient to drive compressors  109  and  113 . Likewise, if turbine  117  generates more work than is required by compressors  109  and  113 , motor  116  removes the excess power from the system.  
      In primary heat exchanger  115 , stream  15  is condensed against boiling oxygen product and warming nitrogen gas, whereupon it exits the cold end of primary heat exchanger  115  as subcooled liquid air stream  17 . Stream  17  is split into streams  19  and  20 . Stream  19  is fed to medium pressure column  118  several stages from the bottom and stream  20  is fed to the middle of low pressure column  121 . Stream  10  is cooled in primary heat exchanger b  115  and removed from primary heat exchanger  115  at an intermediate point. Cooled air stream  16  is then fed to expansion turbine  117 , which supplies the refrigeration needs of the plant. Turbine discharge air stream  18  is then fed to the bottom of medium pressure column  118 . In column  118  the air is separated by cryogenic rectification into oxygen-enriched and nitrogen-enriched portions. Oxygen-enriched liquid  21  is removed from the bottom of the column and passed into heat exchanger  120  where it is cooled against warming nitrogen gas and from which it exits as a sub-cooled liquid  26 . Subcooled oxygen-enriched liquid stream  26  is split into streams  27  and  33 . Stream  27  is fed directly to low pressure column  121  l below the feed point for stream  20  but above the bottom of the column. Stream  33  is fed to the boiling side of condenser/reboiler  122  where it is partially vaporized. Oxygen-enriched vapor and liquid streams  29  and  30  exit condenser/reboiler  122  and are fed to an intermediate point of low pressure column  121 , below that point where stream  27  enters the column.  
      Nitrogen-enriched vapor  22  exits the top of the medium pressure column  118  and enters the condensing side of condenser/reboiler  119 . Stream  22  is liquefied against vaporizing bottoms liquid in column  121 . Liquid nitrogen  23  leaving condenser/reboiler  119  is split into two streams; stream  24  is returned to column  118  as reflux and stream  25  is sent to heat exchanger  120 . Stream  25  is subcooled against warming nitrogen vapor. Subcooled liquid nitrogen stream  31  is split into two streams; stream  32  enters low pressure column  121  at or near the top and stream  28  is sent liquid nitrogen storage vessel  127 .  
      Low pressure distillation column  121  further separates its feed streams into oxygen-rich and nitrogen-rich portions. An oxygen-rich liquid stream  34  is removed from the bottom of column  121 , where it is split into two streams; stream  35  is fed to liquid oxygen storage vessel  125  and stream  36  is fed to cryogenic oxygen pump  124  and raised to the pressure at which it will boil in primary heat exchanger  115 . High pressure liquid stream  37  is fed to the cold end of primary heat exchanger  115  where it is warmed and boiled against the condensing high pressure air stream  15 . Warmed, high pressure oxygen vapor product  48  exits the warm end of primary heat exchanger  115 .  
      Vapor stream  38  is removed from an intermediate point of low pressure column  121  and fed to the bottom of argon column  123 . Liquid stream  39  exits the bottom of argon column  123  and is returned to low pressure column  121  at the same point at which stream  38  was withdrawn. Argon-enriched liquid stream  40  is removed from the top of argon column  123  and fed to liquid argon storage vessel  126 . Also, argon-enriched vapor stream  41  exits the top of argon column  123  and is fed to the condensing side of condenser/reboiler  122 . Argon-enriched liquid stream  42  exits condenser/reboiler  122  and is returned to the top of argon column  123  as reflux.  
      Two nitrogen-rich streams are withdrawn from the top portion of low pressure column  121 . Product nitrogen-rich vapor  44  exits the top of the low pressure column  121 , is fed to heat exchanger  120 , is warmed against cooling streams, and leaves as superheated nitrogen vapor product stream  46 . Waste nitrogen-enriched vapor  43  is removed from low pressure column  121  a few stages from the top, is fed to heat exchanger  120 , is warmed against cooling streams, and leaves as superheated nitrogen vapor waste stream  45 . Both superheated nitrogen streams  45  and  46  are fed to the cold end of primary heat exchanger  115  where they are warmed against cooling air streams and exit primary heat exchanger  115  to form waste and product streams  47  and  49 , respectively.  
      As mentioned earlier, warm nitrogen-enriched waste stream  47  is fed to prepurifier vessel  107  in order to regenerate one of the PSA beds. If nitrogen product is desired at elevated pressure, it is compressed in compressor  134  (driven by motor  135 ) to form nitrogen product  53 . If the oxygen is boiled below its final delivery pressure, it is compressed in oxygen compressor  129  (driven by motor  130 ) to form oxygen product  51 .  
      In one embodiment of this invention the base system comprises two sets of pre-engineered and fully designed components (subsystems)  107  and  108 . The first subsystem  107  ( FIG. 2 ) comprises all of the pieces of equipment needed to treat stream  5  and produce stream  6 . The design of this portion of the plant is common to every particular application that has similar air throughput requirements regardless of any product slate variations or further air compression requirements. This subsystem includes process equipment, associated valves, piping, analyzers, electrical connections, and other infrastructure needed to remove contaminants of air.  
      The second subsystem  108  ( FIG. 3 ), a cryogenic processing unit, comprises contents of the air separation plant cold section such as in the cold box. Again, the design of this portion of the plant is common to applications that have similar product requirements. The equipment in this pre-engineered and fully designed subsystem includes distillation columns  118 ,  121 , and  123 , condenser/reboilers  119  and  122 , heat exchangers  115  and  120 , all of the associated valves and piping that are used to connect these pieces together, analyzers, and all the necessary electrical connections.  
      In any plant classification those subsystems required to complete the plant and are not included in the base system are part of an auxiliary system. Because of the differences in product slate, purities, delivery pressures and location issues, each auxiliary system is custom designed rather than operating the plant in an inefficient manner. For example, one plant may require more liquid back-up on account of its remote location, hence custom designing storage vessels  125 ,  126 , and  127  is preferred to over-designing the vessels to fit all liquid makes and including it in a base system. Likewise, main expansion turbine  117  will be custom designed for each application to account for the differences in liquid making requirements of each particular application. As another example, if one application requires the delivery of oxygen product at twice the pressure of another application, then compressors  113  and  130  and pump  124  will be custom designed for each plant. In another variation, if liquid air stream  17  is at a sufficiently high pressure for a particular application, then it would be advantageous to use a liquid turbine (not shown) to generate additional refrigeration from that liquid prior to feeding it to the two columns. Hence, the liquid turbine would be part of an auxiliary system of this different application.  
      Table 1 examples of four different plants belonging to the air separation process plant classification illustrated in  FIG. 1 . This table lists only some of the subsystems required to form a complete plant by the practice of this invention. The numerals in parentheses listed in first column correspond to the labels in  FIG. 1 . This table is not exhaustive with respect to plant subsystems and/or process variations, as there are many ways in which the invention could be applied. The plant subsystems are categorized as belonging to the base system (B) or belonging to an auxiliary system (A). By definition, any subsystem that is part of the base system (B) is necessarily the same for every plant belonging to a given classification. Additionally, by definition, every plant designed according to a given classification has to include each and every subsystem that is part of the base system (B). Therefore, Table 1 illustrates that the prepurification subsystem ( 107 ) and cryogenic processing subsystem ( 108 ) in all four plants are part of the base system. An auxiliary system in any plant will be different from that in any other plant belonging to the same classification. However, any subsystem that is part of an auxiliary system (A) by definition can have either the same design or a different design than that used in one or more of the other plants belonging to a given classification. Furthermore, any subsystem that is part of an auxiliary system (A) by definition does not have to be included in the design of every plant belonging to a given classification. For example, considering plant  1  as a base or a reference plant in a given classification, if the delivery pressure of product nitrogen from plant  2  is same or lower than that of product nitrogen leaving cryogenic processing subsystem ( 108 ), then the auxiliary system of plant  2  need not contain product nitrogen compression subsystem, hence this is left blank. Similarly, in plant  3  the product oxygen compression subsystem is not required, hence this is left blank. In plant  4  more liquid nitrogen production is required, hence the auxiliary system contains the nitrogen liquefier subsystem which was not required in plant  1 . Thus, in accordance with this invention, plant subsystems are categorized as belonging to a base system or an auxiliary system, and engineering costs are reduced by custom designing only the auxiliary system.  
                       TABLE 1                                      Plant Number                                     1   2   3   4               Product Nitrogen (49) Delivery Pressure   Base   Lower   Lower   Base       Product Oxygen (48) Delivery Pressure   Base   Base   Lower   Base       Liquid Nitrogen Make   Base   Base   Base   Higher                 Plant Subsystem/       Infrastructure Categorization                                 Ambient Air Filtration (101)   A   A   A   A       Feed Air Compression (102, 103, 104, 105)   A   A   A   A       Prepurification (107)   B   B   B   B       Air Compression for Oxygen Boiling (113, 114)   A   A   A   A       Air Compression for Refrigeration Supply (109, 110)   A   A   A   A       Air Expansion for Refrigeration Supply (117)   A   A   A   A       Cryogenic Processing (17, 120, 118, 119, 121, 122, 123)   B   B   B   B       Liquid Oxygen Pumping (124)   A   A   A   A       Oxygen Compression (129, 130)   A   A       A       Nitrogen Compression (134, 135)   A           A       Liquid Oxygen Storage (125)   A   A   A   A       Liquid Nitrogen Storage (127)   A   A   A   A       Liquid Argon Storage (126)   A   A   A   A       Cooling Tower (not shown)   A   A   A   A       Cooling Water Treatment/Supply (not shown)   A   A   A   A       Switchgear/Transformer (not shown)   A   A   A   A       Motor Control Center (not shown)   A   A   A   A       Compressor Building (not shown)   A   A   A   A       Controls Infrastructure (not shown)   A   A   A   A       Nitrogen Liquefier (not shown)               A                  
 
      Additionally, this invention can be applied to cryogenic air separation plants employing processes or classifications that are substantially different than the one illustrated in  FIGS. 1-3 .