Abstract:
A method to recover natural gas liquids (NGLs) from natural gas streams at NGL recovery plants. The present disclosure relates to methods using liquid natural gas (LNG) as an external source of stored cold energy to reduce the energy and improve the operation of NGL distillation columns. More particularly, the present disclosure provides methods to efficiently and economically achieve higher recoveries of natural gas liquids at NGL recovery plants.

Description:
FIELD 
       [0001]    The present disclosure relates to a method for production of liquid natural gas (LNG) at midstream natural gas liquids (NGLs) recovery plants. More particularly, the present disclosure provides methods to efficiently and economically produce LNG at NGL recovery plants. 
       BACKGROUND 
       [0002]    Natural gas from producing wells contain natural gas liquids (NGLs) that are commonly recovered. While some of the needed processing can be accomplished at or near the wellhead (field processing), the complete processing of natural gas takes place at gas processing plants, usually located in a natural gas producing region. In addition to processing done at the wellhead and at centralized processing plants, some final processing is also sometimes accomplished at Midstream NGLs Recovery Plants “straddle plants.” These plants are located on major pipeline systems. Although the natural gas that arrives at these straddle plants is already of pipeline quality, there still exists quantities of NGLs, which are recovered at these straddle plants. 
         [0003]    The straddle plants essentially recover all the propane and a large fraction of the ethane available from the gas before distribution to consumers. To remove NGLs, there are three common processes; refrigeration, lean oil absorption, and cryogenic. 
         [0004]    The cryogenic processes are generally more economical to operate and more environmentally friendly; current technology generally favors the use of cryogenic processes over refrigeration and oil absorption processes. The first-generation cryogenic plants were able to extract up to 70% of the ethane from the gas; modifications and improvements to these cryogenic processes over time have allowed for much higher ethane recoveries (&gt;90%). 
       SUMMARY 
       [0005]    The present disclosure provides a method for maximizing NGLs recovery at straddle plants and produces LNG. The method involves producing LNG and using the produced LNG as an external cooling source to control the operation of a de-methanizer column. According to at least one embodiment, the method furthers the production of ethane and generates LNG. 
         [0006]    As will hereinafter be further described, the production of LNG is determined by the flow of a slipstream from the de-methanizer overhead stream in an NGL recovery plant. An NGLs recovery plant de-methanizer unit typically operates at pressures between 300 and 450 psi. When the de-methanizer is operated at higher pressures, the objective is to reduce re-compression costs, resulting in lower natural gas liquids recoveries. At lower operating pressures in the de-methanizer, natural gas liquids yields and compression costs are increased. The typical selected mode of operation is based on market value of natural gas liquids. The proposed method allows for an improvement in de-methanizer process operations and production of additional sources of revenue, LNG, and electricity. This method permits selective production of LNG and maximum recovery of natural gas liquids. The LNG is produced by routing a slipstream from the de-methanizer overhead stream through an expander generator. When the pressure is reduced through a gas expander, the expansion of the gas results in a considerable temperature drop of the gas stream, liquefying the slipstream. The nearly isentropic gas expansion also produces torque and therefore shaft power that can be converted into electricity. A portion of the produced LNG is used as a reflux stream in the de-methanizer, to control tower overhead temperature and hence ethane recovery. Moreover, generating an overhead de-methanizer stream substantially free of natural gas liquids is made possible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    These and other features of the disclosure 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 in any way limit the scope of the invention to the particular embodiment or embodiments shown. 
           [0008]      FIG. 1  is a schematic diagram of a facility equipped with a gas expander installed after the de-methanizer overhead stream to produce LNG; and 
           [0009]      FIG. 2  is a schematic diagram of a facility equipped with a JT valve after the de-methanizer overhead stream to produce LNG. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The method will now be described with reference to  FIG. 1 . 
         [0011]    Referring to  FIG. 1 , a pressurized natural gas stream  1  is routed to heat exchanger  2  where the temperature of the feed gas stream is reduced by indirect heat exchange with counter-current cool streams  6 ,  29 ,  30 ,  32 , and  36 . The cooled stream  1  enters feed separator  3  where it is separated into vapour and liquid phases. The liquid phase stream  4  is expanded through valve  5  and pre-heated in heat exchanger  2  prior to introduction into de-methanizer column  11  through line  6 . The gaseous stream  7  is routed to gas expander  8 . The expanded and cooler vapor stream  9  is mixed with LNG for temperature control and routed through stream  10  into the upper section of distillation column  11 . The overhead stream  12  from de-methanizer column  11  is split into streams  13  and  32 . Stream  13  is routed to gas pre-treatment unit  14  to remove CO 2 , then through stream  15  enters gas expander  16 . Stream  15  pressure is dropped at gas expander  16 , the expansion of the gas results in a considerable temperature drop of the gas stream causing it to liquefy upon exiting gas expander  16 . The nearly isentropic expansion across the gas expander produces torque and therefore shaft power. The result of this energy conversion process is that the horsepower extracted from the natural gas stream is then transmitted to a shaft that drives an electrical generator  17  to produce electricity. The condensed stream  18  enters vessel  19 , the LNG receiver. The gaseous fraction in vessel  19  is routed through stream  36  into heat exchanger  2  to give up its cold, enters compressor  37  and the compressed gas stream  38  is mixed with compressed gas stream  34  to become stream  35  for distribution. LNG is fed through line  20  into pump  21 . The pressurized LNG stream  22  feeds streams  23  and  24 . Stream  23  is routed to LNG storage. The pressurized LNG stream  24  is routed through reflux temperature control valve  25  providing the reflux stream  26  to de-methanizer column  11 . A slipstream from the pressurized LNG stream  24  provides temperature control to stream  9  through temperature control valve  27 , temperature controlled stream  10  enters the upper section of de-methanizer column  11 . The controlled temperature of stream  10  by addition of LNG enables operation of the de-methanizer column at higher pressures to compensate for the loss of cool energy generated by the expander at higher backpressures. A second slipstream from pressurized LNG stream  24  provides methane for carbon dioxide stripping through flow control valve  28 , this LNG stream  29  is pre-heated in heat exchanger  2  before introduction into the lower section of the distillation column  11  as a stripping gas. The liquid fraction stream  30  is reboiled in heat exchanger  2  and routed back to the bottom section of de-methanizer column  11 , to control NGL product stream  31 . The distilled stream  32 , primarily methane, is pre-heated in heat exchanger  2  and routed to compressor  33  for distribution and or recompression through line  34 . 
         [0012]    Referring to  FIG. 2 , the main difference from  FIG. 1  is the substitution of a gas expander to a JT valve  39  to control the pressure drop of stream  15 . This process orientation provides an alternative method to produce LNG at NGLs recovery plants albeit less efficient than when using an expander as shown in  FIG. 1 . A pressurized natural gas stream  1  is routed to heat exchanger  2  where the temperature of the feed gas stream is reduced by indirect heat exchange with counter-current cool streams  30 ,  29 ,  6 ,  32  and  36 . The cooled stream  1  enters feed separator  3  where it is separated into vapour and liquid phases. The liquid phase stream  4  is expanded through valve  5  and pre-heated in heat exchanger  2  prior to introduction into distillation column  11  through line  6 . The gaseous stream  7  is routed to gas expander  8 , the expanded and cooler vapor stream  9  is temperature controlled by LNG addition valve  27 , the cooler stream  10  is routed into the upper section of de-methanizer column  11 . The overhead stream  12  from de-methanizer column  11  is split into streams  13  and  32 . Stream  13  is routed to gas pre-treatment unit  14  to remove CO 2 , then through stream  15  enters JT valve  39 . Stream  15  pressure is dropped through JT valve  39 , the expansion of the gas results in a temperature drop of the gas stream causing it to partially condense upon exiting JT valve  39 . The partially condensed stream  18  enters vessel  19 , the LNG receiver, where the liquid components are separated from the gaseous phase components. The liquid phase stream, LNG, is fed through line  20  into pump  21 . The pressurized LNG stream  22  feeds streams  23  and  24 . Stream  23  is routed to LNG storage. The pressurized LNG stream  24  is routed through reflux temperature control valve  25  providing the reflux stream  26  to de-methanizer column  11 . A slipstream from the pressurized LNG stream  24  provides temperature control to stream  9  through temperature control valve  27 , temperature controlled stream  10  enters the upper section of de-methanizer column  11 . The controlled temperature of stream  10  by addition of LNG enables operation of the de-methanizer column at higher pressures to compensate for the loss of cool energy generated by the expander at higher backpressures. A slipstream from pressurized LNG stream  24  provides methane for carbon dioxide stripping through flow control valve  28 , the LNG stream  29  is pre-heated in heat exchanger  2  before introduction into the lower section of the de-methanizer column  11  as a stripping gas. The liquid fraction stream  30  is reboiled in heat exchanger  2  and routed back to the bottom section of de-methanizer column  11 , to control NGL product stream  31 . The gaseous stream  36  exits the LNG receiver  19  and is pre-heated in heat exchanger  2 , the now warmed gas stream enters compressor  37  and exits through line  38  and mixes with compressed gas stream  34  into natural gas distribution line  35 . The distilled stream  32 , primarily methane, is pre-heated in heat exchanger  2  and routed to compressor  33  the compressed gas stream  34  is mixed with compressed gas stream  38  for distribution and or recompression through line  35 . 
         [0013]    In the preferred method, LNG is produced through a gas expander. A portion of the produced LNG provides cold energy that improves the operation and efficiency of NGL de-methanizer columns. Moreover, the gas expander generates electricity which reduces the energy required for recompression of gas for distribution. 
         [0014]    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. 
         [0015]    The following claims are to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, and what can be obviously substituted. The scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.