Abstract:
A method to recover natural gas liquids from natural gas streams at NGL recovery plants. The present invention 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 invention provides methods to efficiently and economically achieve higher recoveries of natural gas liquids at NGL recovery plants.

Description:
FIELD 
       [0001]    The present invention relates to methods for recovery of natural gas liquids (NGLs) from methane rich gases using liquid natural gas (LNG). More particularly, the present invention provides methods to efficiently and economically achieve higher recoveries of natural gas liquids 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 ‘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 overtime have allowed for much higher ethane recoveries &gt;90%. This increase in recovery comes with consumption of relatively large quantities of energy due to their compression requirements. Prior art has taught that use of lean reflux streams reduce energy consumption and achieves high ethane recoveries, Moreover, methane gas has been proven to be a superior stripping gas to control carbon dioxide concentrations in NGL product. Many patents exist disclosing improved designs for generation of lean reflux to recover ethane and heavier components in NGL plants. they typically involve significant capital expenditures and increased operational costs. A need exists for an efficient ethane and NGL recovery process that is capable of achieving very high ethane recoveries at a lower energy consumption and a lower capital cost when compared to prior art. 
       SUMMARY 
       [0005]    The present invention provides a method for recovery of natural gas liquids from natural gas streams in a NGL recovery plant. The method involves the use of LNG as a reflux stream, a feed mixer and a stripping gas in the operation of a LNG recovery plant. The use of LNG as stored cold energy to control a NGL distillation column temperature profile and operation, increases the efficiency and recovery of NGLs in natural gas streams. Moreover, LNG, primarily methane, is an ideal stripping gas to control carbon dioxide concentration in the NGL product stream. 
         [0006]    As will hereinafter be further described, the interacting step can be either direct or indirect. Direct interaction is achieved by injecting LNG as a liquid reflux to the distillation column to control overhead temperature, by direct mix with expanded gas stream to control distillation column pressure and as a stripping gas for carbon dioxide control in NGL product stream. Indirect interaction is achieved by, first cooling the distillation column overhead stream in a heat exchanger and then used as a reflux in the distillation column. The condensate generated from overhead stream is used as a second reflux stream for a dual reflux operation, increasing NGLs recovery. 
     
    
     
       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  is a schematic diagram of a facility equipped with LNG storage and supply for direct cooling in accordance with the teachings of the present invention. 
           [0009]      FIG. 2  is a schematic diagram of a facility equipped with LNG storage and supply for indirect cooling in a heat exchanger to generate a second reflux stream. 
       
    
    
     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  24 ,  19 ,  6  and  21 . 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  20  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  17  into the upper section of distillation column  20 . A LNG storage drum  10 , supplies LNG through line  11  to LNG pump  12 . The pressurized LNG stream  13  is routed through temperature control valve  14  providing the reflux stream to distillation column  20 . A slipstream from the pressurized LNG stream  13  provides temperature control to stream  9  through temperature control valve  16 , temperature controlled stream  17  enters the upper section of distillation column  20 . The controlled temperature of stream  17  by addition of LNG enables operation of the distillation column at higher pressures to compensate for the loss of coolth energy generated by the expander at higher backpressures. A second slipstream from pressurized LNG stream  13  provides methane for carbon dioxide stripping through flow control valve  18 , the LNG is pre-heated in heat exchanger  2  before introduction into the lower section of the distillation column  20  as a stripping gas. The distilled stream  21 , primarily methane, is pre-heated in heat exchanger  2  and routed to compressor  22  for distribution and or recompression through line  23 , The liquid fraction stream  24  is reboiled in heat exchanger  2  and routed back to the bottom section of distillation column  20 , to control NGL product stream  25 . 
         [0012]    Referring to  FIG. 2 , the coolth energy of LNG is used to first condense the overhead stream of the distillation column generating a second reflux stream before its use as the primary reflux stream, allowing for an increase in efficiency in plant operations. 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  28 ,  18 ,  6  and  25 . The cooled stream  1  enters feed separator) 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  19  through line  6 . The gaseous stream  7  is routed to gas expander  8 , the expanded and cooler vapor stream  9  is routed into the upper section of distillation column  20 . A LNG storage drum  10 , supplies LNG through line  11  to LNG pump  12 . The pressurized LNG stream  13  enters heat exchanger  11  and is routed through temperature control valve  15  as reflux stream  16  to distillation column  19 . A slipstream from pressurized LNG stream  13  provides methane for carbon dioxide stripping through flow control valve  17 , the LNG is pre-heated in heat exchanger  2  before introduction into the lower section of the distillation column  19  as a stripping gas. The distilled stream  20 , primarily methane, is cooled in heat exchanger  14  and discharged into overhead separator  21 . The condensed stream  22  feed reflux pump  23 , the pressurized reflux stream  24  enters distillation column  19  as a second reflux stream for a dual reflux distillation column operation. The vapour stream  25  is pre-heated in heat exchanger  2  and routed to compressor  26  for distribution and or recompression through line  27 . The liquid fraction stream  28  is reboiled in heat exchanger  2  and routed back to the bottom section of distillation column  19 , to control NGL product stream  29 . 
         [0013]    In the preferred method, LNG provides stored cold energy that improves the operation and efficiency of NGL distillation columns. The above described method uses this stored cold energy to condense natural gas liquids from natural gas streams by direct mixing. This direct mixing provides better heat transfer and reduces the energy requirements to condense NGLs. It also 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 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. Those skilled in the art will appreciate that various adaptations and modifications of the described embodiments can be configured without departing, from the scope of the claims. The illustrated embodiments have been set forth only as examples and should not be taken as limiting the invention. It is to be understood that, within the scope of the following claims, the invention may be practiced other than as specifically illustrated and described.