Abstract:
A system and process for liquefying a gas, comprising introducing a feed stream into a liquefier comprising at least a warm expander and a cold expander; compressing the feed stream in the liquefier to a pressure greater than its critical pressure and cooling the compressed feed stream to a temperature below its critical temperature to form a high pressure dense-phase stream; removing the high pressure dense-phase stream from the liquefier, reducing the pressure of the high pressure dense-phase stream in an expansion device to form a resultant two-phase stream and then directly introducing the resultant two-phase stream into a storage tank; and combining a flash portion of the resultant two-phase stream with a boil-off vapor from a liquid in the storage tank to form a combined vapor stream, wherein the temperature of the high pressure dense-phase stream is lower than the temperature of a discharge stream of the cold expander.

Description:
BACKGROUND 
       [0001]    Nitrogen liquefiers are well known in the art and are generally linked to a nitrogen generator, for example, or an Air Separation Unit (ASU). Liquefiers may be used to liquefy low pressure gaseous nitrogen from an ASU, for example. Liquefiers may also take at least a part of their feed from the ASU at higher pressure and/or at cryogenic temperatures for liquefying purposes. 
         [0002]    In traditional liquefaction processes, high pressure nitrogen is cooled to cryogenic temperatures to form a dense-phase fluid (i.e., a fluid below its critical temperature and above its critical pressure) and then reduced in pressure, normally through the use of a valve or dense fluid expander, so that it forms mostly liquid with some flash vapor. This two-phase mixture is then fed to a separator. A cold expander also typically discharges a vapor or slightly liquefied stream into the separator. Vapor from the separator is re-warmed to ambient temperature and then recycled in the process, whilst the liquid is subcooled before being fed to an insulated liquid storage tank, for example. This subcooling may take place by pressure reduction in a second separator at a lower pressure or indirectly in a subcooler by heat exchange against a boiling liquid at low pressure. The use of a subcooler allows enough pressure to be maintained in the liquid to transfer it to storage without using pumps, for example. 
         [0003]    Portions of the liquid produced in the liquefier may be stored, for example, in an insulated liquid storage tank for future use or be exported by road tanker while other portions of the liquid may be returned to the ASU to provide refrigeration, for example. 
         [0004]    If a second separator is used, the second separator must be elevated above the level of the storage tank if the use of additional pumps is to be avoided. 
         [0005]    Storage of the liquid in insulated liquid storage tanks is, however, not a simple solution. Heat ultimately leaks into the insulated liquid storage tank from the surroundings due to imperfect insulation, for example. Also, part of the liquid stored in the insulated liquid storage tanks evaporates and requires the production of additional liquid to compensate for such loss. Traditionally, the cold vapor that is formed as a result of the liquid evaporating in the insulated liquid storage tank is vented to atmosphere to avoid the pressure of the insulated liquid storage tank from rising, however, refrigeration is then lost in the process. 
         [0006]    Previously disclosed nitrogen liquefiers linked to ASU plants were, therefore, problematic for several reasons. First, recovery of flash or cold boil-off vapor from the insulated liquid nitrogen tank required use of a cold blower. Cold blowers were used to pressurize the flash or cold boil-off vapor from the tank so that it was at sufficient pressure to be sent back to the liquefier or ASU to allow for its refrigeration to be recovered. Only part of the refrigeration can be recovered, however, when a blower is used because the blower&#39;s power is ultimately added to the cold stream of boil-off vapor as heat. Moreover, blowers are inconvenient and expensive to install and maintain, and add further complexity to these systems and processes, thus, making use of blowers uneconomical 
         [0007]    Second, use of cold end liquid nitrogen separators add complexity to the process, and make it more costly to implement as they must all be enclosed within an insulated cold box. Large and complex cold boxes are difficult to deal with when scheduling shipping routes because certain destination locations may be hard or even impossible to reach with such large pre-insulated loads (i.e., cold box loads). 
         [0008]    Third, liquefaction processes have typically included subcoolers to reduce the flash gas formed in the tank. Such subcoolers also add undesirable cost and complexity to the process. 
         [0009]    Moreover, while early liquefiers (i.e., liquefiers used prior to the liquefiers traditionally used today) employed a single expander and utilized only a single separator device, these early liquefiers were relatively inefficient. To increase the efficiency of the liquefiers, later liquefier designs used multiple expanders and multiple separators to recover flash vapors at intermediate pressures. Recovery of the flash vapors at intermediate pressures was thought, for many years and to this very day, to be necessary because flash vapor formed as a result of a liquid product entering a liquid storage tank was not desirable and, thus, would have been vented to the atmosphere to control the pressure of the storage tank. Such venting would, of course, result in loss of the valuable refrigeration from the flash vapor. 
         [0010]    Thus, there was a need in the industrial gases industry for a simple and low cost liquefaction process with the efficiency benefit of tank flash and boil-off vapor recovery without the complexity of cold blowers, cold end separators, or subcoolers. 
       SUMMARY 
       [0011]    The described embodiments satisfy the need in the art by providing a simplified and efficient liquefier using a liquid storage tank as a flash separator and recovering the flash and boil-off vapor from storage through the liquefier. Separators and subcoolers may be eliminated from the liquefier design and process. As the cold portion of the liquefier is essentially only a heat exchanger and piping, it may be insulated directly and the separate cold box structure eliminated. The described embodiments utilize a design and process that is opposed to conventional wisdom for the construction of efficient liquefier designs and processes. 
         [0012]    Production of liquid in a separate liquefier rather than in an ASU plant has operational advantages such as being easy to turn on and off according to demand, but has the significant disadvantages of the high capital cost and lower efficiency associated with a separate process unit. In general, increasing process efficiency will increase capital cost, and capital cost has to be increased to improve efficiency. The process and system described allows this capital cost to be reduced at the same time as improving the efficiency. 
         [0013]    In one embodiment, a process for liquefying a gas is disclosed, comprising introducing a feed stream into a liquefier comprising at least a warm expander and a cold expander; compressing the feed stream in the liquefier to a pressure greater than its critical pressure and cooling the compressed feed stream to a temperature below its critical temperature to form a high pressure dense-phase stream; removing the high pressure dense-phase stream from the liquefier and reducing the pressure of the high pressure dense-phase stream in an expansion device to form a resultant two-phase stream and then directly introducing the resultant two-phase stream into a storage tank; and combining a flash portion of the resultant two-phase stream with a boil-off vapor from a liquid in the storage tank to form a combined vapor stream, wherein the temperature of the high pressure dense-phase stream is lower than the temperature of a discharge stream of the cold expander. 
         [0014]    In another embodiment, a system for liquefying an atmospheric gas is disclosed, comprising: a first conduit for accepting a feed stream; a liquefier fluidly connected to the first conduit for compressing and cooling the feed stream to form a high pressure dense phase fluid, wherein the liquefier comprises at least a warm expander, a cold expander, a compressor for compressing the feed stream to a pressure greater than its critical pressure, and a heat exchanger, for cooling the compressed feed stream to a temperature below its critical temperature; a second conduit fluidly connected to the liquefier for accepting the high pressure dense-phase stream from the liquefier; a first expansion device fluidly connected to the second conduit to reduce the pressure of the high pressure dense-phase stream to form a resultant two-phase stream; a third conduit fluidly connected to the first expansion device for accepting the two-phase expanded stream; and a storage tank fluidly connected to the third conduit for accepting and storing the two-phase expanded stream, wherein the storage tank is designed to operate at a pressure at or below 1.5 bara, and wherein the heat exchanger is designed such that the temperature of the high pressure dense-phase stream is lower than the temperature of a discharge stream of the cold expander. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0015]    The foregoing summary, as well as the following detailed description of exemplary embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating embodiments, there is shown in the drawings exemplary constructions; however, the invention is not limited to the specific methods and instrumentalities disclosed. In the drawings: 
           [0016]      FIG. 1  is a flow diagram of an exemplary process for using a liquid storage tank as a flash separator and recovering the flash and boil-off vapor from storage through the liquefier, in accordance with the present invention; 
           [0017]      FIG. 2  is a flow diagram of an alternative exemplary process incorporating a different liquefier configuration; 
           [0018]      FIG. 3  is a flow diagram of a previously disclosed process with the same expander configuration as shown in  FIG. 1 , wherein the process includes a cold end separator and subcooler, but comprises no flash vapor or boil-off recovery from the tank; and 
           [0019]      FIG. 4  is a flow diagram illustrating various ways to integrate the exemplary process of  FIG. 1  with an Air Separation Unit where any other process according to the invention may be integrated with the Air Separation Unit in a similar fashion. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates an exemplary system and process for using a liquid storage tank  170  as a flash separator and recovering the flash and boil-off vapor from the liquid storage tank  170  through the liquefier  101 .  FIG. 1  discloses low pressure nitrogen feed stream  100  being combined with warmed tank flash and boil-off vapor stream  102  to form combined stream  104 . The low pressure feed stream  100  may be nitrogen, or it may be another gas or gas mixture such as air, oxygen, argon, carbon monoxide, neon, ethylene, helium, or hydrogen, for example. The combined stream  104  is then compressed in the feed compressor  106  to about 6 bara to form compressed stream  108 . Compressed stream  108  is then cooled in an aftercooler  110  to form cooled stream  112 . Cooled stream  112  is then combined with recycle stream  114  to form stream  116 . Stream  116  is then compressed in recycle compressor  118  to about 32 bara resulting in compressed stream  120 . Stream  120  is then cooled in an aftercooler  122  to form stream  124 . Stream  124  is then split into streams  126  and  128 . 
         [0021]    Stream  126  is (optionally) cooled in the heat exchanger  130  to form stream  132 . Stream  132  is then expanded in warm expander  134  to around 6 bara to form warm expanded stream  136 . 
         [0022]    Stream  128  is further compressed in the warm compander compressor  138  to form stream  140 . Stream  140  is then cooled in the warm compander aftercooler  142  to form cooled stream  144 . Cooled stream  144  is then compressed again in cold compander compressor  146  to about 65 bara to form compressed stream  148 . Compressed stream  148  is then cooled again in the cold compander compressor aftercooler  150  to form high pressure stream  152 . This high pressure stream  152  is cooled in the heat exchanger  130  to an intermediate temperature of about 182 K, producing streams  154  and  156 . 
         [0023]    Stream  156  is expanded in a cold expander  158  to form discharge stream  160 . Discharge stream  160  is returned to the cold end of the heat exchanger  130  where it is warmed and mixed with the exhaust stream  136  from the warm expander  134  to form stream  162 . Stream  162  is warmed in heat exchanger  130  to form recycle stream  114 . Recycle stream  114  is then mixed with compressed feed stream  112  and fed to the suction of the recycle compressor  118 . 
         [0024]    Stream  154  is further cooled in the heat exchanger  130  to form a high pressure dense-phase stream  164 . High pressure dense-phase stream  164  is withdrawn from the cold end of the heat exchanger  130  at a temperature of about  96  K, reduced in pressure across one or more expansion devices  166  to form stream  168 , where stream  168  is fed directly into a liquid storage tank  170 . As used herein, the term “fed directly” shall mean that the designated stream, after exiting the one or more expansion devices  166  is provided to the liquid storage tank  170  via a conduit without encountering any further apparatus that would alter the composition, temperature, or pressure of the designated stream. Moreover, as used herein “directly connected” shall mean that a first device or piece of an apparatus is connected to a second device or piece of an apparatus without any intermediate devices or pieces of apparatus that would alter the composition, temperature, or pressure of a stream passing through, for example, the first device to the second device. 
         [0025]    Stream  168  is flashed into the liquid storage tank  170  to produce mostly liquid with some vapor. The liquid from stream  168  will add to the liquid already present in the liquid storage tank  170 , whilst the flash vapor will combine with boil-off vapor already present in the liquid storage tank  170 . A combined vapor stream  172  composed of flash vapor and boil-off vapor is withdrawn from the liquid storage tank  170 , and, during normal operation, is fed to the heat exchanger  130  of the liquefier  101  as stream  174 . Stream  174  is warmed in the heat exchanger  130  to form warmed tank flash and boil-off vapor stream  102  and mixed with the low pressure feed  100  to form combined stream  104  entering the make-up compressor  106  of the liquefier  101 . 
         [0026]    If the liquefier  101  is not operating, the liquid storage tank  170  boil-off vapors can be removed from the liquid storage tank  170  as combined vapor stream  172 ,  176 , reduced in pressure across one or more expansion devices  178  to form stream  180 , and vented to the atmosphere to control the pressure of the liquid storage tank  170 . 
         [0027]    One of the significant benefits of this system arrangement is the simplified design. Heat exchanger  130 , expanders  134 ,  158  and the associated piping may be insulated separately, for example, with an insulating material such as mineral wool, polyurethane foam, foamglass, “cryogel,” or a suitable alternative, or installed in small local cold boxes connected by insulated piping. Reducing the size requirements of the cold box is especially important when dealing with and scheduling shipping routes because certain destination locations may be hard or impossible to reach with larger pre-insulated loads (i.e., cold box loads). 
         [0028]    Further, contrary to traditional belief, recovery of the boil-off vapor from the liquid storage tank  170  surprisingly improves the overall efficiency of the liquefier  101  and storage system by around 0.5-1.0% (depending on the relative sizes of the liquid storage tank  170  and liquefier  101  and the quality of tank insulation) compared to previous designs where the boil-off gas was not recovered, as its cold is used to partially cool the product and reduce the power required by the liquefier  101  rather than being wasted by venting it directly to atmosphere. In addition, the required nitrogen feed flow is reduced (as the previously vented nitrogen is recovered) which could lead to use of smaller ASUs. 
         [0029]    If the low pressure nitrogen feed stream  100  to the liquefier  101  is at a pressure high enough to provide the low pressure nitrogen feed stream  100  directly into the suction of the recycle compressor  118 , the feed compressor  106  may also be eliminated, and in that case, the warmed tank flash and boil-off vapor stream  102  may be vented to the atmosphere through a valve to simply control the pressure of the liquid storage tank  170 . 
         [0030]    With surprising and unexpected result, Applicants found that if high pressure dense-phase stream  164  is cooled below the temperature of discharge stream  160  through indirect heat exchange against the recovered combined vapor stream  174  in heat exchanger  130 , then reduction of the pressure of high pressure dense-phase stream  164  to the pressure of discharge stream  160  would not result in the generation of significant amounts of flash vapor, thus, the efficiency of the liquefier  101  is not reduced by eliminating the additional separator and its related components. In fact, one skilled in the art will appreciate that this exemplary embodiment eliminates the need for separators and subcoolers (for example separator  304  and subcooler  310  of  FIG. 3 ) while maintaining a high level of efficiency. For example, while traditional systems and processes may have used two or more separators to recover the flash vapors at high and reduced pressures, the disclosed system and process achieves the same result minus substantial capital cost and substantial transport planning while achieving equal or better efficiencies. 
         [0031]    In another embodiment, and as illustrated in  FIG. 2 , a similar system and process to  FIG. 1  is disclosed; however this embodiment comprises a different expander arrangement. In this system/process, stream  124  from the recycle compressor aftercooler  122  is split into two streams  226  and  228  that feed the compressor ends of the warm and cold companders  238  and  246  arranged in parallel. The respective outlet streams  240  and  248  of the warm and cold companders  238  and  246  are combined into stream  249  and cooled in aftercooler  250  before being fed to heat exchanger  130  as stream  252 . Stream  252  is cooled to a first intermediate temperature in heat exchanger  130  before being split into streams  232  and  253 . 
         [0032]    Stream  232  is expanded in warm expander  234  to form stream  236  and combined with warming discharge stream  160  forming stream  162  at an intermediate location of the heat exchanger  130 . Stream  253  is further cooled to a second intermediate temperature and split again into streams  256 ,  254 . Stream  256  is expanded in cold expander  258  to form discharge stream  160 . Discharge stream  160  is then warmed in the heat exchanger  130 . Stream  254  is further cooled in heat exchanger  130  to form the high pressure dense-phase stream  164  that is fed to the liquid storage tank  170  via expansion device  166 . 
         [0033]      FIG. 3  is a flow diagram of a previously disclosed prior art process with the same expander configuration as shown in  FIG. 1  but where the process comprises no flash vapor or boil-off recovery from the tank.  FIG. 3  is provided for exemplary purposes and to be used to compare with the system and process of  FIG. 1 . 
         [0034]    As illustrated in  FIG. 3 , a cold end separator  304  and subcooler  310  are incorporated in the liquefier  301  and there is no recovery of the flash or boil-off vapor from the liquid storage tank  170 . The high pressure dense-phase stream  164  from the cold end of the heat exchanger  130  is reduced in pressure in one or more expansion devices  300  and the resulting two-phase stream  302  is then fed to a separator  304  along with the cold expander discharge stream  160  that may contain some liquid. Vapor stream  306  from separator  304  is warmed in heat exchanger  130  to an intermediate temperature where it is combined with the warm expander exhaust stream  136  to form stream  162 . Liquid stream  308  from separator  304  is subcooled in subcooler  310  to about  78  K to form stream  312 . A portion  316  of subcooled liquid stream  312  is reduced in pressure in one or more expansion devices  318  and then evaporated in subcooler  310  to form vapor stream  320  and reheated in heat exchanger  130  to form stream  102 . The remaining portion  314  of subcooled liquid stream  312  is fed to the liquid storage tank  170  via one or more expansion devices  166  to form stream  168  where stream  168  is fed into the liquid storage tank  170 . Flash and boil-off vapor from the liquid storage tank  170  is vented via stream  176  through expansion device  178  to form stream  180  (to be vented to the atmosphere) to control the tank pressure. 
         [0035]      FIG. 4  is a flow diagram illustrating several exemplary options for integrating the liquefier system and process of  FIG. 1  with an ASU or nitrogen generator. For example, the low pressure nitrogen feed stream  100  from the warm end of the ASU may be completely or partly replaced by one or more of alternative feed streams  400 ,  404 , or  408 . 
         [0036]    A high pressure nitrogen stream  400  from the warm end of the ASU or nitrogen generator may also be mixed with stream  112  from the feed compressor aftercooler  110  to form stream  402  that may then be mixed with stream  114  to form stream  116  that is fed to the recycle compressor  118 . Alternatively, stream  400  may be mixed downstream of where stream  114  combined with stream  112 , or into an interstage location of the feed compressor  106  or recycle compressor  118 . 
         [0037]    A low pressure nitrogen stream  404  from a low pressure column or subcooler at the cold end of the ASU may be mixed with the returning low pressure stream  174  from the liquid storage tank  170  to form stream  406  that is then heated in the heat exchanger  130 . 
         [0038]    A cold high pressure nitrogen stream  408  from a high pressure column of the ASU or nitrogen generator or the single column of a single column nitrogen generator may be mixed with the discharge stream  160  from the cold expander  158  to form stream  410  that is then heated in heat exchanger  130 . 
         [0039]    Additionally, a divided portion stream  412  of the high pressure dense-phase stream  164  from the cold end of the liquefier may be fed directly to the ASU or nitrogen generator to provide refrigeration whilst the remaining portion  414  may be fed to the liquid storage tank  170 . As used herein a “divided portion” of a stream shall mean a portion having the same chemical composition as the stream from which it was taken. Divided portion stream  412  may be fed, for example, to the High Pressure (HP) column, the Low Pressure (LP) column, the subcooler, or the heat exchanger of an ASU. 
       EXAMPLE 
       [0040]    Tables 1 and 2 provide exemplary flow rates, temperatures, and pressures for the configurations/processes of  FIG. 1  and  FIG. 3 . The configuration/process disclosed in  FIG. 1  resulted in the data of Table 1, where 300 tonnes per day of liquid nitrogen was produced in liquid storage tank  170 . The configuration/process consumed approximately 5950 kW of electricity. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Stream 
               
             
          
           
               
                   
                 100 
                 102 
                 114 
                 132 
                 152 
                 156 
                 160 
                 164 
                 174 
                 in tank 
               
               
                   
                   
               
             
          
           
               
                 Flow 
                 446 
                 122 
                 2386 
                 978 
                 1977 
                 1409 
                 1409 
                 569 
                 122 
                 446 
               
               
                 (kmol/hr) 
               
               
                 Temperature 
                 299 
                 299 
                 299 
                 267 
                 303 
                 182 
                 97 
                 96 
                 78 
                 78 
               
               
                 (K) 
               
               
                 Pressure 
                 1.03 
                 1.03 
                 6.00 
                 31.84 
                 64.80 
                 64.60 
                 6.20 
                 64.60 
                 1.10 
                 1.10 
               
               
                 (bar (abs)) 
               
               
                   
               
             
          
         
       
     
         [0041]    The configuration/process disclosed in  FIG. 3  resulted in the data of Table 2, where 300 tonnes per day of liquid nitrogen was also produced in liquid storage tank  170 . This configuration/process consumed approximately 6000 kW of electricity. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Stream 
               
             
          
           
               
                   
                 100 
                 102 
                 114 
                 132 
                 152 
                 156 
                 160 
                 164 
                 176 
                 312 
                 314 
                 in tank 
               
               
                   
                   
               
             
          
           
               
                 Flow 
                 460 
                 97 
                 2552 
                 1238 
                 1871 
                 1369 
                 1369 
                 502 
                 13 
                 557 
                 460 
                 446 
               
               
                 (kmol/hr) 
               
               
                 Temperature 
                 299 
                 299 
                 299 
                 254 
                 303 
                 174 
                 97 
                 99 
                 78 
                 79 
                 79 
                 78 
               
               
                 (K) 
               
               
                 Pressure 
                 1.03 
                 1.03 
                 6.00 
                 30.05 
                 64.80 
                 64.60 
                 6.20 
                 64.60 
                 1.10 
                 6.00 
                 6.00 
                 1.10 
               
               
                 (bar (abs)) 
               
               
                   
               
             
          
         
       
     
         [0042]    Importantly, the exemplary process of FIG.  1 /Table 1 produces the same net quantity (446 kmol/hr) of liquid nitrogen in the liquid storage tank, but uses 0.8% less power than the previously disclosed process of FIG.  3 /Table 2, has a 3% lower feed rate (stream  100 ) due to the recovery of flash and boiloff vapor from the liquid storage tank (stream  174 ) and elimination of tank boil-off losses to atmosphere (stream  176 ), and provides significant capital cost savings from the elimination of a first separator, a second separator or subcooler, and their associated valves, controls and insulating enclosure. As the cold portion of the liquefier comprises essentially only a heat exchanger and the associated piping, the liquefier equipment may be insulated directly and the separate cold box structure required to contain and insulate the first separator, the second separator or subcooler, and their associated valves, and controls may be eliminated, thus, significantly reducing the size of the cold box. Reducing the size requirements of the cold box is especially important when dealing with and scheduling shipping routes because certain destination locations may be hard or even impossible to reach with larger pre-insulated loads (i.e., cold box loads). 
         [0043]    While aspects of the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the claimed invention should not be limited to any single embodiment, but rather should be construed in breadth and scope in accordance with the appended claims.