Abstract:
The present invention provides for a process for providing cooling and controlling the refrigeration in a cooling loop used in the production of liquefied natural gas. A cooling loop in contact with a heat exchanger contains a refrigerant composition and by controlling the amount of a component in the refrigerant composition, the necessary level of cooling provided to the heat exchanger can be maintained.

Description:
[0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 60/919,998 filed Mar. 26, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention provides for a method to continuously monitor and control the refrigerant composition in a closed cycle mixed refrigerant system. 
         [0003]    Liquefied natural gas (LNG) is produced by both small scale and large scale plants. The overall heating value of the LNG can be affected by the percentage of heavier hydrocarbon components found in the natural gas and hence, heavier hydrocarbons may need be removed prior to liquefaction. Some natural gases will also require removal of the heavy ends to prevent operating problems in the liquefaction cycle 
         [0004]    U.S. Pat. No. 6,530,240 B1 teaches a method for controlling a mixed refrigerant based, natural gas liquefier system that utilizes an exchange of system refrigerant between the system and an external storage tank whereby the use of extremely high pressures in the compressor discharge that is employed in conventional systems is circumvented. This reference is directed to the control of mixed refrigerant based natural gas liquefiers using low cost HVAC components due to the risk of exceeding the pressure and temperature requirements of the HVAC components. The pressure limitations are avoided by adjusting the pressure in the refrigerant circulation circuit to below about 175 psig by exchange of refrigerant with the refrigerant storage circuit. 
         [0005]    MRC Cycle for LNG/Air Separation Saves Money, D. T. Linnett, Tenth Australian Chemical Engineering Conference, 1982, Sydney, 24-26 August teaches alternative cycles for liquefaction of natural gas and the advantages of mixed refrigerant cascade cycle. The MRC cycle uses a single multi-component refrigerant comprising five components, nitrogen, methane, ethylene, propane, and butane. The mixture composition is such that it condenses over a very wide temperature range. The mixture is compressed in a single compressor and then partially condensed against cooling water. The liquid is separated, sub cooled, expanded to a common low pressure, then evaporated and recycled to the compressor. The uncondensed vapor is further cooled and further partially condensed and the same procedure is repeated. 
         [0006]    Closed cycle mixed refrigerant cycle (MRC) based liquefaction systems are commonly used for the liquefaction of natural gas. These systems have been identified to offer improved efficiency by way of lower power consumption compared to a single component nitrogen based system. An MRC system for natural gas liquefaction uses three or more components. One such mixture specified by the owners of the &#39;240 patent for their small scale LNG system uses a five component refrigerant mixture consisting of nitrogen, methane, ethane, iso-butane and iso-pentane. In this mixture, the iso-pentane is liquid at the warm end or ambient temperature. A number of factors such as ambient temperature, discharge pressure of the high pressure refrigerant mixture in the cycle, two phase flow distribution in the heat exchanger, and refrigerant component solubility in oil in an oil-flooded compressor system, affect the overall cooling efficiency of the closed cycle refrigeration system. When the cooling efficiency of a system is affected, the power consumption for the liquefaction of natural gas will also be affected. 
         [0007]      FIG. 1  illustrates a prior mixed refrigerant system. Natural gas is fed to the main heat exchanger  10  through line  1  and product liquefied is recovered via valve  3  through lines  2  and  4 . A mixed refrigerant of fixed composition will enter the suction of a compressor as vapor only. The compressed refrigerant is fed through line  6  and is cooled in an evaporative cooler  20  and partially condensed. The two-phase high pressure mixture is then fed through line  7  into the main plate fin heat exchanger  10  and completely condensed as it passes down to the Joule-Thompson valve. The high pressure liquid refrigerant is expanded to a pressure close to the compressor suction pressure and produces cooling by flashing and phase change. This high pressure liquid refrigerant is fed through line  11  into the heat exchanger  10 . This gas-liquid refrigerant mixture is completely vaporized in the low pressure passage of the main heat exchanger  10  and provides refrigeration to liquefy the high pressure refrigerant and the natural gas. The warm gaseous refrigerant leaving the exchanger is fed through line  5  to the compressor  15  thereby completing the refrigerant cycle. 
         [0008]    This is a closed refrigerant system, so the total holdup/inventory is constant and is distributed between the gas and the liquid phase. This is true for all closed refrigerant cycles. The refrigerant in the gas phase is a function of the system pressure with the balance being in the liquid phase. The primary liquid holdup is at the cold end of the main heat exchanger. This suggests that the system pressure will determine the liquid holdup. In this case, the primary liquid holdup will be at the cold end upstream of the Joule-Thomson valve. The refrigerant composition everywhere in the loop is the same. 
         [0009]    The present invention addresses the inefficiencies of these earlier system by exploiting the difference in the liquid and gas phase compositions at different locations in the loop. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides for a method of controlling the liquid inventory in a closed loop refrigeration system in order to increase the quantity or relative percentage of the heavy component, iso-pentane, which is a liquid at ambient conditions (all other components are vapor), in the refrigerant mixture so that the warm end heat exchange in the refrigeration system heat exchanger is optimized. This will improve liquefaction efficiency. 
         [0011]    In a first embodiment of the present invention there is disclosed a method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop containing a refrigerant composition is directed through a heat exchanger comprising controlling said refrigerant composition continuously by changing the liquid level in a warm end phase separator. 
         [0012]    The refrigerant composition contains iso-pentane amongst other components. The liquid level in the warm end phase separator decreases as the heat transfer required of the heat exchanger increases. The liquid level is controlled on-line typically by a PLC in communication with the warm end phase separator. As the liquid level in the warm end phase separator increases, the heat transfer required of the heat exchanger, which is in fluid communication therewith will decrease. 
         [0013]    The cooling loop and the heat exchanger are also in fluid communication and the control of the refrigerant composition is performed by changing the amount of iso-pentane present therein. 
         [0014]    In another embodiment of the present invention there is disclosed a method for providing cooling to a process for producing liquefied natural gas comprising the steps:
       a) contacting a stream of liquefied natural gas with a heat exchanger;   b) contacting the heat exchanger with a cooling loop containing a refrigerant composition;   c) adjusting the composition of the refrigerant by controlling the amount of a heavier condensable component in the refrigerant composition in response to performance variations in the heat exchanger; and   d) recovering liquefied natural gas.       
 
         [0019]    In this embodiment, the heavier condensable component is iso-pentane. The performance variations in the heat exchanger are selected from the group consisting of a temperature increase in the heat exchanger and a temperature decrease in the heat exchanger. The amount of the heavier condensable component in the refrigerant composition will increase in response to an increase in temperature in the heat exchanger. Accordingly the amount of the heavier condensable component in the refrigerant composition will decrease in response to a decrease in temperature in the heat exchanger. 
         [0020]    In another embodiment of the present invention, there is disclosed a method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop comprising two warm end separators and a cold end separator are in fluid connection with each other comprising controlling the liquid level in the warm end separators. 
         [0021]    In this embodiment, the cooling loop contacts a heat exchanger. The refrigerant composition contains iso-pentane and the control of the liquid level is performed by changing the amount of iso-pentane present in the refrigerant composition. The liquid level in the warm end separators decreases as the heat transfer required of the heat exchanger increases. 
         [0022]    The liquid level is controlled on-line typically by a PLC in communication with the warm end phase separator. A first warm end separator is in fluid communication with a second warm end separator. The liquid level in the warm end separators increases as the heat transfer required of the heat exchanger decreases. 
         [0023]    In another embodiment of the present invention there is disclosed a method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop comprising a warm end separator and a cold end separator are in fluid connection with each other comprising the steps:
       a) controlling the liquid level in the warm end separator;   b) removing vapor from the cold end separator;   c) heat exchanging the vapor with a separate liquid nitrogen storage system; and   d) returning the liquid from the exchange of step c) to a heat exchanger.       
 
         [0028]    The controlling of the liquid level is performed on-line. The cooling loop will contact a heat exchanger and the liquid level in the warm end separator is controlled by changing the amount of iso-pentane present in the liquid. The liquid level in the warm end separators decreases as the heat transfer required of the heat exchanger increases. 
         [0029]    In a further embodiment of the present invention, there is disclosed a method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop comprising a warm end separator and a cold end separator are in fluid communication with each other comprising controlling the liquid level in the warm end separator and controlling the flow of natural gas from a separator to a heat exchanger. 
         [0030]    The liquid level in the warm end separator is controlled by changing the amount of iso-pentane present in the liquid and can be performed on-line. The flow of natural gas from a separator to a heat exchanger is controlled by the temperature of the separator where a gas stream will enter the heat exchanger and a bottom liquid stream is directed from the separator for reentry into the separator. 
         [0031]    The bottom liquid stream comprises butane, propane and pentane. The gas stream from the separator after exiting the heat exchanger is directed into the separator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a schematic of a liquefied production process with a heat exchanger. 
           [0033]      FIG. 2  is a schematic of a liquefied production process where the liquid level in the warm end separator is continuously controlled. 
           [0034]      FIG. 3  is a schematic of a liquefied production process where second warm end storage is provided at the suction side of the refrigerant compressor. 
           [0035]      FIG. 4  is a schematic of a liquefied production process where there is an additional cold end control by removing vapor from the cold end separator. 
           [0036]      FIG. 5  is removal of the heavy hydrocarbon contaminants in the natural gas by an intermediate draw. 
           [0037]      FIG. 6  is a graph showing LNG production for three cases with different warm end temperatures and adjusted refrigerant compositions. 
           [0038]      FIG. 7  is a graph showing the heat exchanger temperature profile for the design case 1 in Table 1 and  FIG. 6 . 
           [0039]      FIG. 8  is a graph showing the heat exchanger temperature profile with increased warm end temperature for case 2 in Table 1 and  FIG. 6 . 
           [0040]      FIG. 9  is a graph showing the heat exchanger temperature profile for the case with the increased warm end temperature and adjusted refrigerant composition for case 3 in Table 1 and  FIG. 6   
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]      FIG. 2  shows a mixed refrigerant system employing the methods of the present invention. In this system, both warm  30  and cold end  35  phase separators are present. There are different refrigerant compositions in each of the phase separators and at the bottom of the main heat exchanger upstream of the Joule-Thompson valve. 
         [0042]    For purposes of  FIGS. 2 ,  3 ,  4  and  5 , common components compressor, evaporative cooler, main heat exchanger, natural gas feed and liquefied product recovery have been designated with the same numbers throughout. 
         [0043]    By changing the relative liquid holdup in the separators and the heat exchanger backup, the refrigerant composition in the loop can be adjusted. The liquid level in the warm end separator is continuously controlled online in order to regulate the iso-pentane in the five-component refrigerant mixture depending on various input conditions. If a higher level of warm end heat transfer is required in the main heat exchanger, a higher amount of higher boiling iso-pentane is required in the refrigerant mixture and this is achieved by decreasing the level in the warm end separator. 
         [0044]    The increased heavy components such as iso-pentane in the refrigerant mixture can help to counter the impact due to an ambient temperature increase, a discharge pressure decrease or due to increased solubility of iso-pentane in the compressor oil. 
         [0045]    In  FIG. 2 , there is represented one aspect of the present invention. Natural gas is fed into line  1  and enters the main heat exchanger  10 . The liquefied natural gas will leave the main heat exchanger through line  2  where its flow will be controlled by valve  3  and recovered via line  4 . 
         [0046]    The refrigerants will circulate through line  12  and enter the compressor  15  and travel through line  13  to an evaporative cooler. The colder and compressed gas stream of refrigerants will travel through line  14  to a warm end separator  30  where the warmed stream of refrigerants will leave the warm end separator  30  through line  29  and enter the top of the main heat exchanger  10 . The now cooled stream of refrigerants will leave the main heat exchanger  10  through line  21  and enter a Joule-Thompson valve  22  and travel through line  23  to a cold end separator  25 . The gaseous portion from the cold end separator  25  will leave through line  24  and reenter the main heat exchanger  10 . The cold bottoms from the cold end separator  25  will leave either through valve  26  or valve  27  and line  28  where they will reenter the main heat exchanger  10 . 
         [0047]    The cold end bottoms from the warm end separator  30  will be pumped out through pump  16  and line  15  as well as be withdrawn through line  17  to valve  18 . Opening of the valve  18  will allow the colder bottoms to travel through line  19  back to the main heat exchange  10 . 
         [0048]    In an alternative embodiment of the present invention,  FIG. 3  shows a second warm end storage being provided at the suction side of the refrigerant compressor and is connected to the discharge side warm end separator. This allows for a greater change in the iso-pentane quantity in the refrigerant mixture entering the main heat exchanger by allowing the back and forth transfer of liquid between the suction side separator and the discharge side warm end separator. 
         [0049]    In a further embodiment of the present invention, the primary control of the refrigerant composition is performed at the cold end. For small changes in refrigerant compositions, this is achieved by increasing or lowering the level in the cold end separator. When the level is increased, typically the concentrations of the lighter components such as nitrogen and methane in the five component refrigerant mixture is decreased and heavy components such as iso-pentane and iso-butane are increased. When the level is decreased, the concentrations of lighter components are increased. The continuous control of the cold end separator level allows the heat exchanger performance to be maximized when issues such as two phase maldistribution or varying refrigerant compressor discharge pressures are encountered. 
         [0050]    In  FIG. 3 , there is represented a further aspect of the present invention. Natural gas is fed into line  1  and enters the main heat exchanger  10 . The liquefied natural gas will leave the main heat exchanger through line  2  where its flow will be controlled by valve  3  and recovered via line  4 . 
         [0051]    Refrigerant will leave the main heat exchanger  10  through line  36  and travel to a suction side separator  35 . SP  35  provides additional valve to store liquid from SP  45 . Normally there is no liquid in stream through line  36 . The bottoms from the suction side separator will leave through line  38  and travel via pump  40  to line  41  or they can be recycled through valve  39  and line  37  back to the suction side separator  35 . If these bottoms are not recycled, they are transmitted via pump  42  and valve  33  back through line  44 A to the main heat exchanger  10 . 
         [0052]    The gas leaving the suction side separator  35  will leave through line  35 A to compressor  15  and evaporative cooler  20 . The cooled and compressed refrigerant stream will travel to the warm end separator  45  through line  45 A and through pump  42  where they can be transmitted through valve  44  and line  44 A to the main heat exchanger  10  or through valve  44  and line  43  back to the warm end separator  45 . The gas from the top of the warm end separator  45  will leave through line  46  and reenter the main heat exchanger  10 . 
         [0053]    Once they have traveled through the main heat exchanger  10 , the refrigerant mixture will leave through line  31  and their flow will be controlled by a Joule-Thompson valve  32 . Flow control could also be a suction pressure control. The refrigerant mixture will flow through line  33  to the cold end separator  40  where the liquid from the bottom will travel through valve  34  and line  34 A back to the main heat exchanger  10  or be recycled through line  34 B to the cold end separator  50 . The gas from the cold end separator  50  will leave through line  36  and reenter the bottom of the main heat exchanger  10 . 
         [0054]    In a further embodiment shown in  FIG. 4 , additional cold end control is achieved by removing vapor from the cold end separator and heat exchanging it with a separate liquid nitrogen storage system to liquefy a portion or all of it and returning the liquid to the cold side of the main heat exchanger. This allows for significant changes in the concentration of the lighter components such as nitrogen and methane in the five component refrigerant mixture. 
         [0055]    In  FIG. 4 , there is represented another aspect of the present invention. Natural gas is fed into line  1  and enters the main heat exchanger  10 . The liquefied natural gas will leave the main heat exchanger through line  2  where its flow will be controlled by valve  3  and recovered via line  4 . 
         [0056]    The refrigerant stream from the main heat exchanger  10  will leave through line  78  and connect with a suction side separator  55 . The bottoms from the suction side separator  55  will leave through line  57  and be pumped around through pump  60  where they will either reenter the suction side separator through line  61 , valve  58  and line  56  or be directed via line  62  to pump  63 , although typically there will be no bottoms. The suction side separator  55  will also utilize line  59  to create a connection with line  57  and pump  60  to recirculate as necessary through valve  58  some bottoms withdrawn from the suction side separator  55 . The refrigerant bottoms have passed through pump  63  and line  64  will travel through valve  66  and line  69  to reenter the main heat exchanger  10  at the top. 
         [0057]    The gaseous refrigerant mixture from the top of the suction side separator will leave through line  55 A through compressor  15  and line  13  and travel through the evaporative cooler  20  and line  14  into the warm end separator  65 . There the refrigerant stream will leave through the bottom and line  65 A and be returned through pump  63 , line  64 , valve  66  and line  69  to the top of the main heat exchanger  10 . A portion of this stream may travel via valve  66  into line  67  and be returned to the warm end separator  65 . 
         [0058]    The top from the warm end separator will leave through line  68  and reenter the main heat exchanger  10  at the top. Both refrigerant streams that leave the warm end separator  65  through either line  68  or  69  will be recovered from the bottom of the main heat exchanger  10  through line  71  where they will be drawn through a Joule-Thompson valve  72 . This stream will travel through line  73  into the cold end separator  70 . The gaseous mixture from the cold end separator  70  will leave through line  74  and enter line  79  where they will enter the liquid nitrogen refrigerant buffer  75 . This stream may also reenter the main heat exchanger  10  at the bottom. The cold ends of the cold end separator will travel to valve  77  and either be circulated through line  76  back to the cold end separator  70  or travel through line  78  into the bottom of the main heat exchanger  10 . 
         [0059]    The refrigerant stream that has entered the liquid nitrogen cooled refrigerant buffer  75  will be withdrawn through valve  85  and line  86  to reconnect with line  73  for reentry into the cold end separator  70 . 
         [0060]    The liquid nitrogen cooled refrigerant buffer  75  will use liquid nitrogen as the buffer and this enters through line  81  and valve  82  and will travel through line  83  and out through line  80  after it has absorbed heat from the refrigerant stream. The liquid nitrogen may also travel through valve  82  and line  84  where it can connect with valve  85  for either reentry into the liquid nitrogen cold refrigerant buffer  75  or pass through line  86  and line  73  to the cold end separator  70 . 
         [0061]      FIG. 5  demonstrates another aspect of the present invention where there is removal of pentane or hexane by regulating the temperature and the natural gas bypass flow. Natural gas is fed into line  1  and enters the main heat exchanger  10 . The liquefied natural gas will leave the main heat exchanger through line  2  where its flow will be controlled by valve  3  and recovered via line  4   
         [0062]    The natural gas feed can also travel through valve  106  and line  108  to a separator  105  where the top gaseous stream lean in the heavies will leave through line  109  and travel for entry into the main heat exchanger  10 . The bottom liquid stream enriched in heavy components such as propane, butane and pentane when present in the natural gas feed will leave through valve  112  and line  113  to a boiler (not shown). They can also be recycled through line  111  to the separator  105 . The natural gas feed may also be directed through valve  106  to lines  107  and  110  for entry into the separator  105 . 
         [0063]    The warm stream leaving the main heat exchanger  10  through line  96  will enter compressor  15  and through line  13  enter evaporative cooler  20 . The cooled and compressed refrigerant stream will enter the warm end separator  95  through line  14 . The top gaseous refrigerant stream from the warm end separator will leave via line  95 A for entry into the main heat exchanger  10 . The bottom liquid stream will leave via pump  99  and will travel to valve  100  where they may be recycle to the warm end separator through line  98 . 
         [0064]    The bottoms from the warm end separator may also continue through valve  100  and line  91  into the main heat exchanger  10 . This stream having passed through the main heat exchanger  10  will leave via line  91 A and pass through a Joule-Thompson valve  92  where it will enter the cold end separator  90  through line  93 . The bottoms from the cold end separator  90  will travel through valve  94  and either be recirculated through line  94 A to the cold end separator  90  or travel through line  96  for reentry into the main heat exchanger  10 . The top gaseous stream from the cold end separator  90  will travel through line  97  for entry back into the main heat exchanger  10 . 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Effect of warm end temperature and refrigerant 
               
               
                 composition on liquefier efficiency. 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
               
               
                   
                   
               
             
          
           
               
                   
                 Tmax 
                 ° C. 
                 25.5 
                 35 
                 35 
               
               
                   
                 x isopentane 
                 mol % 
                 12.9 
                 12.9 
                 15.5 
               
               
                   
                 LNG Prod. 
                 kg/hr 
                 745 
                 589 
                 695 
               
               
                   
                 Relative LNG 
                 % of 
                 100% 
                 79% 
                 93% 
               
               
                   
                 Production 
                 design 
               
               
                   
                   
               
               
                   
                 Case 1 is the design case, case 2 is a simulation for a 9.5K higher warm end temperature for the same refrigerant composition, and case 3 is also for the higher warm end temperature, but with an adjusted refrigerant composition. Only the amount of isopentane was increased in order to remove a temperature pinch at the warm end of the heat exchanger. FIGS. 7 through 9 show the effect of the increased warm end temperature and the compensating effect of the composition adjustment. 
               
             
          
         
       
     
         [0065]    While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.