Abstract:
An improved refrigeration cycle is provided. The cycle provides for improved thermal performance by incorporating a refrigerant makeup tank that introduces liquid refrigerant with expanded refrigerant upstream of the heat exchanger, thereby providing a savings in either electricity costs or refrigerant.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and device for improving the thermal performance of a nitrogen refrigeration cycle. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gas liquefaction is a known process used to liquefy gases and commonly employs a nitrogen refrigeration cycle to provide the necessary refrigeration. The nitrogen refrigeration cycle, in its most basic terms, includes the following steps: nitrogen gas is compressed, cooled, and then expanded in a turbo booster. During this expansion, the temperature of the gas drops and the expanding gas moves the turbine, which in turn performs work on the compressor of the turbo booster. The cold gas is then used to provide the necessary cooling to liquefy the target stream. 
         [0003]    One problem with refrigeration cycles is that there are refrigerant losses. The mechanical equipment used in the process contains seals, which have gas losses to the atmosphere that must be made up. Although mechanical designs are available which recover a portion of these seal gas losses back to a low pressure stream within the refrigeration cycle, this does not completely eliminate the losses and can incur additional capital cost. One solution to this problem is shown in  FIG. 1 . 
         [0004]      FIG. 1  represents typical nitrogen refrigeration cycle  1  that is useful for providing the necessary refrigeration to liquefy the target gas. Nitrogen gas  2  is partially compressed in turbo booster  10  (which is a combination of compressor  20  and turbine  30  in which the turning of turbine  30  helps to turn compressor  20 ) and then further compressed in high pressure compressor  40  and then partially cooled in a heat exchanger  50 . Cooled compressed nitrogen  41  then exits the heat exchanger  50  and is expanded in turbine  30  of the turbo booster  10 . Expanded refrigerant  32  is then used to provide refrigeration for a gas stream (not shown). During this set up, nitrogen is lost in the turbo booster  10  and high pressure compressor  40 . These losses are made up by the introduction of gaseous nitrogen from LIN tank  60 . Liquid nitrogen  62  is withdrawn from LIN tank  60  and vaporized in ambient vaporizer  64 . Often, particularly in cold environments, ambient vaporizer  64  is supplemented with an electric heater which requires at least some utility cost or other type of vaporizer such as steam sparged water bath, natural gas fired water bath, electric, etc . . . Therefore, methods known heretofore may require additional utility cost. A first portion  66  is used to provide the makeup losses for high pressure compressor  40  and turbo booster  10  by injecting first portion  66  into the warm end of heat exchanger  50 . A second portion can be used as utility nitrogen for other users (e.g., instrument, purge/utility gas for both this plant or another nearby plant). 
         [0005]    However, there is a need for an improved refrigeration cycle that requires less power and/or makeup fluid. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is directed to a device and a method that satisfies at least one of these needs. Certain embodiments of the present invention relate to the use of an improved refrigeration cycle as part of a liquefaction process, in which makeup refrigerant is introduced on the cool side of the heat exchanger as opposed to the warm side of the heat exchanger. 
         [0007]    In one embodiment, the method for producing a liquefied gas stream from an inlet gas feed stream can include the steps of: 
         [0008]    cooling at least a portion of the inlet gas feed stream by heat exchange contact with an expanded refrigerant to produce the liquefied gas stream, wherein the expanded refrigerant is circulated in a refrigeration cycle; and 
         [0009]    introducing a liquid refrigerant makeup stream into the refrigeration cycle by adding the liquid refrigerant makeup stream to the expanded refrigerant prior to heat exchange contact with the inlet gas feed stream to provide for makeup losses in the refrigeration cycle and to provide additional refrigeration to the refrigeration cycle. 
         [0010]    In another embodiment, the inlet gas feed stream to be liquefied is natural gas. In another embodiment, the refrigeration cycle is a nitrogen refrigeration cycle and the liquid refrigerant makeup stream originates from a liquid nitrogen tank containing liquid nitrogen. In another embodiment, the expanded refrigerant is nitrogen. In another embodiment, the expanded refrigerant is expanded in a device selected from the group consisting of an expansion valve, a turbo-expander, and a liquid (or dense fluid) expander. In another embodiment, the method can further includes the steps of withdrawing a utility nitrogen stream from the nitrogen vapor stream subsequent to heat exchange contact with the inlet gas feed stream, and introducing the utility nitrogen stream to a second user for use as utility nitrogen. 
         [0011]    In another embodiment, the method producing a liquefied gas stream from an inlet gas feed stream can include the steps of: 
         [0012]    cooling at least a portion of the inlet gas feed stream by heat exchange contact with a nitrogen refrigeration cycle to produce the liquefied gas stream; and 
         [0013]    adding a liquid refrigerant makeup stream into the expanded refrigerant prior to heat exchange contact with the inlet gas feed stream to provide for makeup losses in the nitrogen refrigeration cycle and to provide additional refrigeration to the nitrogen refrigeration cycle. In one embodiment, the nitrogen refrigeration cycle can include the steps of: expanding a nitrogen stream to a cold nitrogen vapor stream, cooling at least a portion of the inlet feed gas stream by heat exchange contact with the cold nitrogen vapor stream, compressing the cold nitrogen vapor stream to form a compressed nitrogen vapor stream, and cooling at least a portion of the compressed nitrogen vapor stream by heat exchange contact with the cold nitrogen vapor stream. 
         [0014]    In another aspect of the invention, an apparatus for producing a liquefied gas stream from an inlet gas feed stream is provided. In one embodiment, the apparatus can include a heat exchanger, a refrigeration cycle, and a makeup liquid refrigerant tank. In one embodiment, the heat exchanger has a cold side and a warm side and is configured to receive the inlet gas feed stream and to produce the liquefied gas stream. In one embodiment, the refrigeration cycle is configured to provide refrigeration to the heat exchanger. Preferably, the refrigeration cycle further includes a compressor and an expander, wherein the compressor is configured to compress a refrigerant to a compression pressure, wherein the expander is configured to receive the compressed refrigerant and expand the compressed refrigerant to an expanded pressure to produce a cold refrigerant at an outlet of the expander, and wherein the outlet of the expander is in fluid communication with the cold side of the heat exchanger. In one embodiment, the makeup liquid refrigerant tank is configured to introduce a makeup liquid refrigerant with the cold refrigerant prior to heat exchange with the inlet gas feed stream. 
         [0015]    In another embodiment, the makeup liquid refrigerant tank is in fluid communication with the refrigeration cycle at a point downstream the outlet of the expander and upstream the cold side of the heat exchanger. In another embodiment, the expander is a device selected from the group consisting of an expansion valve, a turbo-expander, and a liquid/dense fluid expander. In an additional embodiment, the apparatus can further include a utility nitrogen withdrawal line configured to withdraw a nitrogen stream from the refrigeration cycle at a point downstream the warm side of the heat exchanger and introduce the nitrogen stream to a second user for use as utility nitrogen. 
         [0016]    Certain embodiments of the present invention advantageously do not require supplemental energy for the vaporization of the make up fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it can admit to other equally effective embodiments. 
           [0018]      FIG. 1  shows a refrigeration cycle in accordance with the prior art. 
           [0019]      FIG. 2  shows a refrigeration cycle in accordance with an embodiment of the invention. 
           [0020]      FIG. 3  shows a liquefaction process in accordance with the prior art. 
           [0021]      FIG. 4  shows a liquefaction process in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims. 
         [0023]    Now turning to  FIG. 2 . Nitrogen refrigeration cycle  1  of  FIG. 2  is largely similar to nitrogen refrigeration cycle  1  of  FIG. 1 , with the exception of the makeup nitrogen. In  FIG. 2 , LIN tank  60  is in fluid communication with the cold side of the heat exchanger. As such, liquid nitrogen  62  travels from LIN tank  60  and is combined with gaseous nitrogen  32  where liquid nitrogen  62  vaporizes. In one embodiment, the flow rate of liquid nitrogen  62  is about 1% of the flow rate of expanded refrigerant  32 , thereby allowing liquid nitrogen  62  to vaporize when combining with expanded refrigerant  32  before going through heat exchanger  50 . A small amount of second portion  68  can be withdrawn on the warm side of heat exchanger  50  in order to provide a makeup stream for nearby utilities. By combining liquid nitrogen  62  on the cold side of heat exchanger  50 , additional refrigeration is provided to nitrogen refrigeration cycle  1 , whereas the prior art loses this refrigeration in ambient vaporizer  64 , which often must be supplemented with electric power (utility cost) in the form of an electric trim heater. 
       EXAMPLES 
       [0024]      FIG. 3  represents another example as known in the prior art while  FIG. 4  represents another embodiment of the present invention. These figures are used to show the comparative advantages of embodiments of the present invention. 
         [0025]    Now turning to  FIG. 3 . Compressed nitrogen  42  exits high pressure compressor  40  and is split into two portions. First portion  44  is compressed in first turbo-booster  15  and then further compressed in second turbo-booster  25 . Fully compressed nitrogen  27  is cooled in heat exchanger  50  and then expanded in second turbo-booster  25  to provide expanded first portion  34 . Expanded first portion  34  is introduced into the cool side of heat exchanger  50  where it provides refrigeration to feed gas  48  in order to produce liquid feed  52 . Feed gas  48  can be any gas that is desirable to liquefy, for example, natural gas. Expanded first portion  34  exits heat exchanger  50  and is recompressed in high pressure compressor  40  to start the cycle over again. During the same cycle, second portion  46  enters heat exchanger  50  and is only partially cooled before being expanded in first turbo-booster  15  to reduce both its pressure and temperature. Expanded second portion  36  is introduced to heat exchanger  50  to provide additional cooling before it combines with expanded first portion  34  on its way back to high pressure compressor  40 . 
         [0026]    As in  FIG. 1 , nitrogen is lost in first turbo-booster  15 , second turbo-booster  25 , and high pressure compressor  40 . These losses are made up by the introduction of gaseous nitrogen  66  from LIN tank  60 . Liquid nitrogen  62  is withdrawn from LIN tank  60  and vaporized in ambient vaporizer  64 . A first portion  66  is used to provide the makeup losses for high pressure compressor  40  and turbo booster  10  by injecting first portion  66  into the warm end of heat exchanger  50 . A second portion  68  can be used as utility nitrogen for other users (e.g., for makeup losses for a nearby air separation plant) or as instrument gas and/or purge gas for this plant and/or another nearby facility. 
         [0027]    Now turning to  FIG. 4 . Nitrogen refrigeration cycle  3  of  FIG. 4  is largely similar to nitrogen refrigeration cycle  3  of  FIG. 3 , with the exception of the makeup nitrogen. In  FIG. 4 , LIN tank  60  is in fluid communication with the cold side of the heat exchanger  50 . As such, liquid nitrogen  62  travels from LIN tank  60  and is combined with expanded refrigerant  34 , where it vaporizes. 
         [0028]    Simulations were run for the processes shown in  FIG. 3  and  FIG. 4  for liquefying a natural gas stream having a flow rate of 3,265 Nm 3 /hr. The results can be found in Table I below: 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Simulation Results 
               
             
          
           
               
                   
                 FIG. 3 
                 FIG. 4 
                 % Change 
               
               
                   
                   
               
             
          
           
               
                 Nitrogen flow leaving High 
                 22613 
                 20797 
                 8% 
               
               
                 Pressure Compressor (Nm 3 /hr) 
               
               
                 Loss if First Turbo-booster 
                 68 
                 68 
                 — 
               
               
                 (Nm 3 /hr) 
               
               
                 Loss in Second Turbo-booster 
                 68 
                 68 
                 — 
               
               
                 (Nm 3 /hr) 
               
               
                 Loss in High Pressure 
                 102 
                 102 
                 — 
               
               
                 Compressor (Nm 3 /hr) 
               
               
                 Utility Usage (kW) 
                 1213 
                 1112 
                 8% 
               
               
                 Total Make-up LIN (Nm 3 /hr) 
                 338 
                 338 
                 — 
               
               
                 LIN added to Recycle (Nm 3 /hr) 
                 238 
                 238 
                 — 
               
               
                 LIN sent to Utilities (Nm 3 /hr) 
                 100 
                 100 
                 — 
               
               
                   
               
             
          
         
       
     
         [0029]    As is clearly shown in Table I, embodiments of the present invention can achieve approximately an 8% improvement over methods and apparatus of the prior art. This is extremely beneficial as embodiments of the present invention can provide a cost savings on both the capital cost because of the reduced size of the refrigeration cycle equipment and on the energy required to run the high pressure compressor. 
         [0030]    While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps or devices can be combined into a single step/device. 
         [0031]    The singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise. 
         [0032]    Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. 
         [0033]    Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.