Abstract:
A method is described for removing carbon dioxide during Liquid Natural Gas production from natural gas at gas pressure letdown stations. The above method removes carbon dioxide from a Liquid Natural Gas production stream by using hydrocarbon fractions taken from a gas for consumption stream as a carbon dioxide stripping adsorption agent for a stripping column used to remove carbon dioxide.

Description:
FIELD 
       [0001]    There is described a method of removing carbon dioxide during production of Liquid Natural Gas (LNG) from natural gas at gas pressure letdown stations. 
       BACKGROUND 
       [0002]    In Canadian Patent 2,536,075 entitled “Method of conditioning natural gas in preparation for storage”, there is disclosed a method in which natural gas is divided into a primary stream and a secondary stream. Through a series of heat exchanges a temperature of the primary stream is raised in preparation for consumption and a temperature of the secondary stream is lowered in to produce Liquid Natural Gas (LNG). 
         [0003]    A serious problem not addressed in this patent is the presence of carbon dioxide (CO 2 ) in the LNG . In the production of LNG , cryogenic temperatures are reached where the carbon dioxide can form dry ice which can plug lines and equipment. When producing LNG at gas pressure letdown stations the carbon dioxide must be removed to prevent the formation of dry ice and plugging of lines and equipment on the production plant. Traditionally, this concern is addressed by employing mol sieves to absorb and remove the carbon dioxide from the LNG production gas stream. These mol sieves are the largest component of a LNG plant and are energy intensive to regenerate. There will hereinafter be described an alternative method of addressing carbon dioxide removal. 
       SUMMARY 
       [0004]    There is provided a method of removing carbon dioxide during Liquid Natural Gas production from natural gas at a gas pressure let down station. The method involves passing high pressure natural gas through a first heat exchanger to pre-cool the high pressure natural gas entering the pressure let down station. The pre-cooled high pressure natural gas is then passed through a separator to remove condensates from the high pressure natural gas exiting the first heat exchanger. The high pressure natural gas is then passed through a natural gas dewatering unit to remove water from the high pressure natural gas exiting the separator. The dewatered high pressure natural gas then is passed through a second heat exchanger to pre-cool the dewatered high pressure natural gas. A step is then taken of splitting the dewatered high pressure natural gas into a Liquid Natural Gas production stream and a gas for consumption stream. The Liquid Natural Gas production steam is passed through a third heat exchanger to pre-cool the Liquid Natural Gas production stream. The Liquid Natural Gas production stream is passed through a carbon dioxide stripping column to remove carbon dioxide. The gas for consumption stream is passed through a first pressure reduction unit to depressurize the gas for consumption stream. The gas for consumption stream is passed through a second separator to recover condensed hydrocarbon fractions from the gas for consumption stream. The condensed hydrocarbon fractions from the gas for consumption stream are routed to the stripping column for use as a carbon dioxide stripping adsorption agent. The Liquid Natural Gas production stream is then passed through one or more further heat exchangers to further cool the Liquid Natural Gas production stream to facilitate Liquid Natural Gas production. The Liquid Natural Gas production stream is passed through a second pressure reduction unit to depressurize the Liquid Natural Gas production stream. A final step is then taken of passing the Liquid Natural Gas production stream through a third separator to achieve separation of Liquid Natural Gas from vapours. 
         [0005]    The above described method achieves the objective of removal of carbon dioxide from the Liquid Natural Gas production stream by using hydrocarbon fractions taken from the gas for consumption stream as a carbon dioxide stripping adsorption agent for the stripping column used to remove carbon dioxide. There is much less cost and maintenance associated with this method, as compared to the use of a mole sieve. 
         [0006]    The pressure reduction units used can be gas expanders or J.T. (Joules-Thomson) valves. The use of gas expanders will be described and illustrated with reference to  FIG. 2  and the use of a J.T. valve will be described and illustrated with reference to  FIG. 3 . The use of gas expanders is preferred as they are more efficient and produce colder temperatures. IN addition, when a gas expander is used with an associated generator, energy is produced that can be used for other purposes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein: 
           [0008]      FIG. 1  (labelled as “PRIOR ART”) is a schematic diagram of a pressure letdown station equipped with JT valves for pressure controlled letdown, a heater and a heat exchanger. 
           [0009]      FIG. 2  is a schematic diagram of a LNG production process at added to an existing gas pressure letdown station and equipped with; gas pre-treatment units, heat exchangers, a stripping column, gas expanders, KO drums, pumps and LNG storage. The process natural gas stream is supplied from high pressure natural gas transmission pipeline. 
           [0010]      FIG. 3  is a schematic diagram of a LNG production process involving the use of J.T. valves in place of gas expanders, but in all other respects identical to  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The method will now be described with reference to  FIG. 1  through  FIG. 3 . 
         [0012]    Referring to  FIG. 2 , this method was developed with a view to pre-treat and produce LNG at gas pressure letdown stations. The disclosed invention utilized a different approach in a unique and innovative variant of the methane expansion cycle, which to date is used in commercial applications known as letdown plants. The system here described takes advantage of the gas streams delivered to end users at pressure letdown stations. The inventors, have previously been granted a patent for LNG production at pressure letdown stations employing expanders/generators, heat exchangers and separators to generate and recover refrigeration to produce LNG. This invention allows for an improved method of producing LNG at gas pressure letdown stations. This method allows for LNG to be pre-treated for the removal of carbon dioxide using the condensed heavy hydrocarbon fractions as a stripping agent in a stripping column. This is an improvement on the existing practice of mol sieves for carbon dioxide removal. The stripping agents employed in the stripping column are the hydrocarbon fractions condensed and recovered in a separator downstream of the expander/generator on the continuous natural gas stream. These hydrocarbon fractions are ideal stripping agents in terms of temperature and composition for carbon dioxide stripping in a stripping column. This new feature is an improvement on the writer previous patented LNG production process at gas pressure letdown stations. The description of application of the method should, therefore, be considered as an example. 
         [0013]    Referring to  FIG. 1 , a typical gas pressure letdown station in a natural gas transmission pipeline. Natural gas is delivered from an high pressure transmission pipeline, gas stream  1  is first pre-heated in heat exchanger  2 , the heated gas stream  3  is depressurized through a JT valve  4  to pipeline  5  pressure setting  7  and then routed to end users. A gas stream  8  provides the fuel required to heater  9 . A closed recycling loop glycol/water  10  transfers the heat from heater  9  to gas heat exchanger  2  to pre-heat the gas. A temperature setting  6  controls the gas pre-heat requirements. This simplified process arrangement as shown is  FIG. 1  constitutes a standard operation at gas pressure letdown stations. The purpose of pre-heating the gas before decreasing the pressure at the pressure letdown station is to prevent the formation of hydrates due to the presence of water in the gas composition. 
         [0014]    Referring to  FIG. 2 , the proposed invention process arrangement is added to an existing pressure letdown station as shown operating in parallel. In the proposed invention, stream  13  is first pre-cooled in heat exchanger  14 , the cooled stream  15  enters separator  16  where condensate is removed through stream  17 . The vapour stream  18  is de-watered in pre-treatment unit  19 . The dried gas stream  20  is further cooled in heat exchanger  21 . The cooler gas stream  22  is split into streams  23  and  24 . Stream  23  is the continuous natural gas stream to end users, it is reduced in pressure at expander/generator  25 / 26  to meet the pressure requirements of end users. The dry, depressurized, very cold, gas stream  27  enters separator  28  where the condensed hydrocarbon fraction is separated from the vapour fraction. Stream  24  is further cooled in heat exchanger  31  before entering CO2 stripper column  41 . The separated very cold gas stream  29  is split into streams  30  and  35 . Stream  30  is warmed up in heat exchanger  31 ,  21  and  14  before distribution to end users. Stream  35  is warmed up through heat exchangers  46  and  14  before distribution to end users. The very cold condensate stream  38  enters pump  39  and is pumped to stripper column  41  as an adsorption stream  42  to strip CO2 from stream  24 . A mixture of CO2 and heavy hydrocarbon fractions exit the stripping column  41  through stream  43  and pump  44 . The CO2 stripped gas stream  45  is further cooled in heat exchangers  46  and  48  before entering expander/generator  50 / 51  and entering separator  53  through line  52 . The liquid fraction LNG exits separator  53  to storage through line  54 . The cryogenic vapour  55  is warmed up in heat exchanger  48 , enters compressor  56 , is routed through line  57  and mixed with stream  58  and delivered to end users through line  59 . 
         [0015]    The inventive step in this process is the generation and recovery of coolth energy in conjunction with a diverted gas stream  24  to pre-treat and produce LNG using a CO2 stripper column at gas pressure letdown stations. The use of expanders/generators in pressure reduction processes to generate the Joule Thompson effect is well understood and in practice in the gas industry in various forms. The advantage of the proposed invention is the process configuration which utilizes produced condensed hydrocarbon fractions as a stripping agent in a stripping column at a pressure letdown station to strip the CO2 fraction from the LNG production stream. Typically pressure letdown stations operate as shown in  FIG. 1 , requiring the use of a portion of the gas flow through the station (generally about 1% of the gas flow) to pre-heat the gas to prevent the formation of hydrates. The proposed invention eliminates the present practice of combusting gas for gas pre-heating. It eliminates the need to use the industry standard mol sieve technology at a pressure letdown station to remove CO2 from a natural gas LNG producing stream. The CO2 stripping adsorption agents are the hydrocarbon fractions condensed in the process from the natural gas stream to end users. The amount of adsorption agent required can be met through a controlled recycled stream supplied from stream  44  until it reaches steady state since there is a continuous new supply of hydrocarbon fractions from stream  38 . 
         [0016]      FIG. 3  shows the same method as that illustrated in  FIG. 2 , with all reference numerals indicating identical elements. The only difference between  FIG. 2  and  FIG. 3 , is that in  FIG. 2  the pressure reduction units used are gas expanders  25  and  50 , whereas in  FIG. 3  the pressure reducing units are gas expander  25  and J.T. Valve  60  are used as pressure reduction units in place of gas expander  50 . 
         [0017]    In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. 
         [0018]    The scope of the claims should not be limited by the illustrated embodiments set forth as examples, but should be given the broadest interpretation consistent with a purposive construction of the claims in view of the description as a whole.