Abstract:
A process and an apparatus for liquefying a portion of a natural gas stream are disclosed. The natural gas stream is cooled under pressure and divided into a first stream and a second stream. The first stream is cooled, expanded to an intermediate pressure, and supplied to a lower feed point on a distillation column. The second stream is expanded to the intermediate pressure and divided into two portions. One portion is cooled and then supplied to a mid-column feed point on the distillation column; the other portion is used to cool the first stream. The bottom product from this distillation column preferentially contains the majority of any hydrocarbons heavier than methane that would otherwise reduce the purity of the liquefied natural gas, so that the overhead vapor from the distillation column contains essentially only methane and lighter components. This overhead vapor is cooled and condensed, and a portion of the condensed stream is supplied to a top feed point on the distillation column to serve as reflux. A second portion of the condensed stream is expanded to low pressure to form the liquefied natural gas stream.

Description:
[0001]    This invention relates to a process and apparatus for processing natural gas to produce liquefied natural gas (LNG) that has a high methane purity. In particular, this invention is well suited to production of LNG from natural gas found in high-pressure gas transmission pipelines. The applicants claim the benefits under Title 35, United States Code, Section 119(e) of prior U.S. Provisional Application No. 61/086,702 which was filed on Aug. 6, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Natural gas is typically recovered from wells drilled into underground reservoirs. It usually has a major proportion of methane, i.e., methane comprises at least 50 mole percent of the gas. Depending on the particular underground reservoir, the natural gas also contains relatively lesser amounts of heavier hydrocarbons such as ethane, propane, butanes, pentanes and the like, as well as water, hydrogen, nitrogen, carbon dioxide, and other gases. 
         [0003]    Most natural gas is handled in gaseous form. The most common means for transporting natural gas from the wellhead to gas processing plants and thence to the natural gas consumers is in high-pressure gas transmission pipelines. In a number of circumstances, however, it has been found necessary and/or desirable to liquefy the natural gas either for transport or for use. In remote locations, for instance, there is often no pipeline infrastructure that would allow for convenient transportation of the natural gas to market. In such cases, the much lower specific volume of LNG relative to natural gas in the gaseous state can greatly reduce transportation costs by allowing delivery of the LNG using cargo ships and transport trucks. 
         [0004]    Another circumstance that favors the liquefaction of natural gas is for its use as a motor vehicle fuel. In large metropolitan areas, there are fleets of buses, taxi cabs, and trucks that could be powered by LNG if there were an economical source of LNG available. Such LNG-fueled vehicles produce considerably less air pollution due to the clean-burning nature of natural gas when compared to similar vehicles powered by gasoline and diesel engines (which combust higher molecular weight hydrocarbons). In addition, if the LNG is of high purity (i.e., with a methane purity of 95 mole percent or higher), the amount of carbon dioxide (a “greenhouse gas”) produced is considerably less due to the lower carbon:hydrogen ratio for methane compared to all other hydrocarbon fuels. 
         [0005]    The present invention is generally concerned with the liquefaction of natural gas such as that found in high-pressure gas transmission pipelines. A typical analysis of a natural gas stream to be processed in accordance with this invention would be, in approximate mole percent, 89.4% methane, 5.2% ethane and other C 2  components, 2.1% propane and other C 3  components, 0.5% iso-butane, 0.7% normal butane, 0.6% pentanes plus, and 0.6% carbon dioxide, with the balance made up of nitrogen. Sulfur containing gases are also sometimes present. 
         [0006]    There are a number of methods known for liquefying natural gas. For instance, see Finn, Adrian J., Grant L. Johnson, and Terry R. Tomlinson, “LNG Technology for Offshore and Mid-Scale Plants”, Proceedings of the Seventy-Ninth Annual Convention of the Gas Processors Association, pp. 429-450, Atlanta, Ga., Mar. 13-15, 2000 for a survey of a number of such processes. U.S. Pat. Nos. 5,363,655; 5,600,969; 5,615,561; 6,526,777; and 6,889,523 also describe relevant processes. These methods generally include steps in which the natural gas is purified (by removing water and troublesome compounds such as carbon dioxide and sulfur compounds), cooled, condensed, and expanded. Cooling and condensation of the natural gas can be accomplished in many different manners. “Cascade refrigeration” employs heat exchange of the natural gas with several refrigerants having successively lower boiling points, such as propane, ethane, and methane. As an alternative, this heat exchange can be accomplished using a single refrigerant by evaporating the refrigerant at several different pressure levels. “Multi-component refrigeration” employs heat exchange of the natural gas with a single refrigerant fluid composed of several refrigerant components in lieu of multiple single-component refrigerants. Expansion of the natural gas can be accomplished both isenthalpically (using Joule-Thomson expansion, for instance) and isentropically (using a work-expansion turbine, for instance). 
         [0007]    While any of these methods could be employed to produce vehicular grade LNG, the capital and operating costs associated with these methods have generally made the installation of such facilities uneconomical. For instance, the purification steps required to remove water, carbon dioxide, sulfur compounds, etc. from the natural gas prior to liquefaction represent considerable capital and operating costs in such facilities, as do the drivers for the refrigeration cycles employed. This has led the inventors to investigate the feasibility of producing LNG from natural gas that has already been purified and is being transported to users via high-pressure gas transmission pipelines. Such an LNG production method would eliminate the need for separate gas purification facilities. Further, such high-pressure gas transmission pipelines are often convenient to metropolitan areas where vehicular grade LNG is in demand. 
         [0008]    In accordance with the present invention, it has been found that LNG with methane purities in excess of 99 percent can be produced from natural gas, even when the natural gas contains significant concentrations of carbon dioxide. The present invention, although applicable at lower pressures and warmer temperatures, is particularly advantageous when processing feed gases in the range of 600 to 1500 psia [4,137 to 10,342 kPa(a)] or higher. 
     
    
     
         [0009]    For a better understanding of the present invention, reference is made to the following examples and drawings. Referring to the drawings: 
           [0010]      FIG. 1  is a flow diagram of an LNG production plant in accordance with the present invention; and 
           [0011]      FIG. 2  is a flow diagram illustrating an alternative means of application of the present invention to an LNG production plant. 
       
    
    
       [0012]    In the following explanation of the above figures, tables are provided summarizing flow rates calculated for representative process conditions. In the tables appearing herein, the values for flow rates (in moles per hour) have been rounded to the nearest whole number for convenience. The total stream rates shown in the tables include all non-hydrocarbon components and hence are generally larger than the sum of the stream flow rates for the hydrocarbon components. Temperatures indicated are approximate values rounded to the nearest degree. It should also be noted that the process design calculations performed for the purpose of comparing the processes depicted in the figures are based on the assumption of no heat leak from (or to) the surroundings to (or from) the process. The quality of commercially available insulating materials makes this a very reasonable assumption and one that is typically made by those skilled in the art. 
         [0013]    For convenience, process parameters are reported in both the traditional British units and in the units of the Système International d&#39;Unités (SI). The molar flow rates given in the tables may be interpreted as either pound moles per hour or kilogram moles per hour. The energy consumptions reported as horsepower (HP) and/or thousand British Thermal Units per hour (MBTU/Hr) correspond to the stated molar flow rates in pound moles per hour. The energy consumptions reported as kilowatts (kW) correspond to the stated molar flow rates in kilogram moles per hour. The LNG production rates reported as gallons per day (gallons/D) and/or pounds per hour (Lbs/hour) correspond to the stated molar flow rates in pound moles per hour. The LNG production rates reported as cubic meters per hour (m 3 /H) and/or kilograms per hour (kg/H) correspond to the stated molar flow rates in kilogram moles per hour. 
       DESCRIPTION OF THE INVENTION 
       [0014]      FIG. 1  illustrates a flow diagram of a process in accordance with the present invention adapted to produce an LNG product with a methane purity in excess of 99%. 
         [0015]    In the simulation of the  FIG. 1  process, inlet gas taken from a natural gas transmission pipeline enters the plant at 100° F. [38° C.] and 900 psia [6,205 kPa(a)] as stream  30 . Stream  30  is cooled in heat exchanger  10  by heat exchange with cool LNG flash vapor at −15° F. [−82° C.] (stream  43   c ), cool expanded vapor at −57° F. [−49° C.] (stream  35   a ), and cool flash vapor and liquid at −15° F. [−82° C.] (stream  46 ). The cooled stream  30   a  at −52° F. [−47° C.] and 897 psia [6,185 kPa(a)] is divided into two portions, streams  31  and  32 . Stream  32 , containing about 32% of the inlet gas, enters separator  11  where the vapor (stream  33 ) is separated from the condensed liquid (stream  34 ). 
         [0016]    Vapor stream  33  from separator  11  enters a work expansion machine  13  in which mechanical energy is extracted from this portion of the high pressure feed. The machine  13  expands the vapor substantially isentropically to slightly above the operating pressure of LNG purification tower  17 , 435 psia [2,999 kPa(a)], with the work expansion cooling the expanded stream  33   a  to a temperature of approximately −108° F. [−78° C.]. The typical commercially available expanders are capable of recovering on the order of 80-85% of the work theoretically available in an ideal isentropic expansion. The work recovered is often used to drive a centrifugal compressor (such as item  14 ), that can be used to compress gases or vapors, like stream  35   b  for example. The expanded and partially condensed stream  33   a  is divided into two portions, streams  35  and  36 . 
         [0017]    Stream  36 , containing about 35% of the effluent from expansion machine  13 , is further cooled in heat exchanger  18  by heat exchange with cold LNG flash vapor at −153° F. [−103° C.] (stream  43   b ) and cold flash vapor and liquid at −153° F. [−103° C.] (stream  45 ). The further cooled stream  36   a  at −140° F. [−96° C.] is thereafter supplied to distillation column  17  at a mid-column feed point. The second portion, stream  35 , containing the remaining effluent from expansion machine  13 , is directed to heat exchanger  15  where it is warmed to −57° F. [−49° C.] as it further cools the remaining portion (stream  31 ) of the cooled stream  30   a . The further cooled stream  31   a  at −82° F. [−64° C.] is then flash expanded through an appropriate expansion device, such as expansion valve  16 , to the operating pressure of fractionation tower  17 , whereupon the expanded stream  31   b  at −126° F. [−88° C.] is directed to fractionation tower  17  at a lower column feed point. 
         [0018]    Distillation column  17  serves as an LNG purification tower. It is a conventional distillation column containing a plurality of vertically spaced trays, one or more packed beds, or some combination of trays and packing. This tower recovers nearly all of the hydrocarbons heavier than methane present in its feed streams (streams  36   a  and  31   b ) as its bottom product (stream  38 ) so that the only significant impurity in its overhead (stream  37 ) is the nitrogen contained in the feed streams. Equally important, this tower also captures in its bottom product nearly all of the carbon dioxide feeding the tower, so that carbon dioxide does not enter the downstream LNG cool-down section where the extremely low temperatures would cause the formation of solid carbon dioxide, creating operating problems. Stripping vapors for the lower section of LNG purification tower  17  are provided by the vapor portion of stream  31   b , which strips some of the methane from the liquids flowing down the column. 
         [0019]    Reflux for distillation column  17  is created by cooling and condensing the tower overhead vapor (stream  37  at −143° F. [−97° C.]) in heat exchanger  18  by heat exchange with streams  43   b  and  45  as described previously. The condensed stream  37   a , now at −148° F. [−100° C.], is divided into two portions. One portion (stream  40 ) becomes the feed to the LNG cool-down section. The other portion (stream  39 ) enters reflux pump  19 . After pumping, stream  39   a  at −148° F. [−100° C.] is supplied to LNG purification tower  17  at a top feed point to provide the reflux liquid for the tower. This reflux liquid rectifies the vapors rising up the tower so that the tower overhead vapor (stream  37 ) and consequently feed stream  40  to the LNG cool-down section contain minimal amounts of carbon dioxide and hydrocarbons heavier than methane. 
         [0020]    The feed stream for the LNG cool-down section (condensed liquid stream  40 ) enters heat exchanger  51  at −148° F. [−100° C.] and is subcooled by heat exchange with cold LNG flash vapor at −169° F. [−112° C.] (stream  43   a ) and cold flash vapor at −164° F. [−109° C.] (stream  41 ). Subcooled stream  40   a  −150° F. [−101° C.] from heat exchanger  51  is flash expanded through an appropriate expansion device, such as expansion valve  52 , to a pressure of approximately 304 psia [2,096 kPa(a)]. During expansion a portion of the stream is vaporized, resulting in cooling of the total stream to −164° F. [−109° C.] (stream  40   b ). The flash expanded stream  40   b  enters separator  53  where the flash vapor (stream  41 ) is separated from the liquid (stream  42 ). The flash vapor (first flash vapor stream  41 ) is heated to −153° F. [−103° C.] (stream  41   a ) in heat exchanger  51  as described previously. 
         [0021]    Liquid stream  42  from separator  53  is subcooled in heat exchanger  54  to −168° F. [−111° C.] (stream  42   a ). Subcooled stream  42   a  is flash expanded through an appropriate expansion device, such as expansion valve  55 , to the LNG storage pressure (90 psia [621 kPa(a)]). During expansion a portion of the stream is vaporized, resulting in cooling of the total stream to −211° F. [−135° C.] (stream  42   b ), whereupon it is then directed to LNG storage tank  56  where the LNG flash vapor resulting from expansion (stream  43 ) is separated from the LNG product (stream  44 ). The LNG flash vapor (second flash vapor stream  43 ) is then heated to −169° F. [−112° C.] (stream  43   a ) as it subcools stream  42  in heat exchanger  54 . Cold LNG flash vapor stream  43   a  is thereafter heated in heat exchangers  51 ,  18 , and  10  as described previously, whereupon stream  43   d  at 95° F. [35° C.] can then be used as part of the fuel gas for the plant. 
         [0022]    Tower bottoms stream  38  from LNG purification tower  17  is flash expanded to the pressure of cold flash vapor stream  41   a  by expansion valve  20 . During expansion a portion of the stream is vaporized, resulting in cooling of the total stream from −133° F. [−92° C.] to −152° F. [−102° C.] (stream  38   a ). The flash expanded stream  38   a  is then combined with cold flash vapor stream  41   a  leaving heat exchanger  51  to form a combined flash vapor and liquid stream (stream  45 ) at −153° F. [−103° C.] which is supplied to heat exchanger  18 . It is heated to −119° F. [−84° C.] (stream  45   a ) as it supplies cooling to expanded stream  36  and tower overhead vapor stream  37  as described previously. 
         [0023]    The liquid (stream  34 ) from separator  11  is flash expanded to the pressure of stream  45   a  by expansion valve  12 , cooling stream  34   a  to −102° F. [−74° C.]. The expanded stream  34   a  is combined with heated flash vapor and liquid stream  45   a  to form cool flash vapor and liquid stream  46 , which is heated to 94° F. [35° C.] in heat exchanger  10  as described previously. The heated stream  46   a  is then re-compressed in two stages, compressor  23  and compressor  25  driven by supplemental power sources, with cooling to 120° F. [49° C.] between stages supplied by cooler  24 , to form the compressed first residue gas (stream  46   d ). 
         [0024]    The heated expanded vapor (stream  35   b ) at 95° F. [35° C.] from heat exchanger  10  is the second residue gas. It is re-compressed in two stages, compressor  14  driven by expansion machine  13  and compressor  22  driven by a supplemental power source, with cooling to 120° F. [49° C.] between stages supplied by cooler  21 . The compressed second residue gas (stream  35   e ) combines with the compressed first residue gas (stream  46   d ) to form residue gas stream  47 . After cooling to 120° F. [49° C.] in discharge cooler  26 , the residue gas product (stream  47   a ) returns to the natural gas transmission pipeline at 900 psia [6,205 kPa(a)]. 
         [0025]    A summary of stream flow rates and energy consumption for the process illustrated in  FIG. 1  is set forth in the following table: 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 (FIG. 1) 
               
               
                 Stream Flow Summary - Lb. Moles/Hr [kg moles/Hr] 
               
             
          
           
               
                   
                   
                   
                   
                   
                 C. 
                   
               
               
                 Stream 
                 Methane 
                 Ethane 
                 Propane 
                 Butanes+ 
                 Dioxide 
                 Total 
               
               
                   
               
             
          
           
               
                 30 
                 1,178 
                 69 
                 27 
                 25 
                 8 
                 1,318 
               
               
                 31 
                 371 
                 22 
                 9 
                 8 
                 2 
                 415 
               
               
                 32 
                 807 
                 47 
                 18 
                 17 
                 6 
                 903 
               
               
                 33 
                 758 
                 36 
                 10 
                 4 
                 5 
                 820 
               
               
                 34 
                 49 
                 11 
                 8 
                 13 
                 1 
                 83 
               
               
                 35 
                 493 
                 24 
                 7 
                 3 
                 3 
                 533 
               
               
                 36 
                 265 
                 12 
                 3 
                 1 
                 2 
                 287 
               
               
                 37 
                 270 
                 0 
                 0 
                 0 
                 0 
                 277 
               
               
                 38 
                 474 
                 34 
                 12 
                 9 
                 4 
                 536 
               
               
                 39 
                 108 
                 0 
                 0 
                 0 
                 0 
                 111 
               
               
                 40 
                 162 
                 0 
                 0 
                 0 
                 0 
                 166 
               
               
                 41 
                 20 
                 0 
                 0 
                 0 
                 0 
                 21 
               
               
                 42 
                 142 
                 0 
                 0 
                 0 
                 0 
                 145 
               
               
                 43 
                 32 
                 0 
                 0 
                 0 
                 0 
                 35 
               
               
                 45 
                 494 
                 34 
                 12 
                 9 
                 4 
                 557 
               
               
                 46 
                 543 
                 45 
                 20 
                 22 
                 5 
                 640 
               
               
                 47 
                 1,036 
                 69 
                 27 
                 25 
                 8 
                 1,173 
               
               
                 44 
                 110 
                 0 
                 0 
                 0 
                 0 
                 110 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 Recoveries* 
               
             
          
           
               
                 LNG 
                 13,389 
                 gallons/D 
                 [111.7 
                 m 3 /D] 
               
               
                   
                 1,781 
                 Lbs/H 
                 [1,781 
                 kg/H] 
               
             
          
           
               
                 LNG Purity 
                 99.35% 
                   
                   
               
             
          
           
               
                 Power 
               
             
          
           
               
                 1 st  Residue Gas Compression 
                 428 
                 HP 
                 [704 
                 kW] 
               
               
                 2 nd  Residue Gas Compression 
                 145 
                 HP 
                 [238 
                 kW] 
               
               
                 Totals 
                 573 
                 HP 
                 [942 
                 kW] 
               
               
                   
               
               
                 *(Based on un-rounded flow rates) 
               
             
          
         
       
     
         [0026]    The total compression power for the  FIG. 1  embodiment of the present invention is 573 HP [942 kW], producing 13,389 gallons/D [111.7 m 3 /D] of LNG. Since the density of LNG varies considerably depending on its storage conditions, it is more consistent to evaluate the power consumption per unit mass of LNG. For the  FIG. 1  embodiment of the present invention, the specific power consumption is 0.322 HP-H/Lb [0.529 kW-H/kg], which is similar to that of comparable prior art processes. However, the present invention does not require carbon dioxide removal from the feed gas prior to entering the LNG production section like most prior art processes do, eliminating the capital cost and operating cost associated with constructing and operating the gas treatment processes required for such processes. 
         [0027]    In addition, the present invention produces LNG of higher purity than most prior art processes due to the inclusion of LNG purification tower  17 . The purity of the LNG is in fact limited only by the concentration of gases more volatile than methane (nitrogen, for instance) present in feed stream  30 , as the operating parameters of LNG purification tower  17  can be adjusted as needed to keep the concentration of heavier hydrocarbons in the LNG product as low as desired. 
       Other Embodiments 
       [0028]    Some circumstances may favor splitting the feed stream prior to cooling in heat exchanger  10 . Such an embodiment of the present invention is shown in  FIG. 2 , where feed stream  30  is divided into two portions, streams  31  and  32 , whereupon streams  31  and  32  are thereafter cooled in heat exchanger  10 . 
         [0029]    In accordance with this invention, external refrigeration may be employed to supplement the cooling available to the feed gas from other process streams, particularly in the case of a feed gas richer than that described earlier. The particular arrangement of heat exchangers for feed gas cooling must be evaluated for each particular application, as well as the choice of process streams for specific heat exchange services. 
         [0030]    It will also be recognized that the relative amount of the feed stream  30  that is directed to the LNG cool-down section (stream  40 ) will depend on several factors, including feed gas pressure, feed gas composition, the amount of heat which can economically be extracted from the feed, and the quantity of horsepower available. More feed to the LNG cool-down section may increase LNG production while decreasing the purity of the LNG (stream  44 ) because of the corresponding decrease in reflux (stream  39 ) to LNG purification tower  17 . 
         [0031]    Subcooling of liquid stream  42  in heat exchanger  54  reduces the quantity of LNG flash vapor (stream  43 ) generated during expansion of the stream to the operating pressure of LNG storage tank  56 . This generally reduces the specific power consumption for producing the LNG by keeping the flow rate of stream  43  low enough that it can be consumed as part of the plant fuel gas, eliminating any power consumption for compression of the LNG flash gas. However, some circumstances may favor elimination of heat exchanger  54  (shown dashed in  FIGS. 1 and 2 ) due to higher plant fuel consumption than is typical, or because compression of the LNG flash gas is more economical. Similarly, elimination of the intermediate flash stage (expansion valve  52  and separator  53 , and optionally heat exchanger  51 , shown dashed in  FIGS. 1 and 2 ) may be favored in some circumstances, with the resultant increase in the quantity of LNG flash vapor (stream  43 ) generated, which could in turn increase the specific power consumption for the process. In such cases, expanded liquid stream  38   a  is directed to heat exchanger  18  (illustrated as stream  45 ), stream  40   a  is directed to expansion valve  55  (illustrated as stream  42   a ), and expanded stream  42   b  is thereafter separated to produce flash vapor stream  43  and LNG product stream  44 . 
         [0032]    In  FIGS. 1 and 2 , multiple heat exchanger services have been shown to be combined in common heat exchangers  10 ,  18 , and  51 . It may be desirable in some instances to use individual heat exchangers for each service, or to split a heat exchange service into multiple exchangers. (The decision as to whether to combine heat exchange services or to use more than one heat exchanger for the indicated service will depend on a number of factors including, but not limited to, LNG flow rate, heat exchanger size, stream temperatures, etc.) 
         [0033]    Although individual stream expansion is depicted in particular expansion devices, alternative expansion means may be employed where appropriate. For example, conditions may warrant work expansion of the further cooled portion of the feed stream (stream  31   a  in  FIG. 1  or stream  31   b  in  FIG. 2 ), the LNG purification tower bottoms stream (stream  38  in  FIGS. 1 and 2 ), and/or the subcooled liquid streams in the LNG cool-down section (streams  40   a  and/or  42   a  in  FIGS. 1 and 2 ). Further, isenthalpic flash expansion may be used in lieu of work expansion for vapor stream  33  in  FIGS. 1 and 2  (with the resultant increase in the power consumption for compression of the second residue gas). 
         [0034]    While there have been described what are believed to be preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto, e.g. to adapt the invention to various conditions, types of feed, or other requirements without departing from the spirit of the present invention as defined by the following claims.