Patent Publication Number: US-2022228079-A1

Title: Natural gas conditioning

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to and claims priority to the following co-pending application, the entirety of which is incorporated by reference herein: U.S. Provisional Patent Application Ser. No. 62/846,789, titled “NATURAL GAS CONDITIONING,” filed May 13, 2019. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to conditioning of rich natural gas and natural gas liquid (NGL) recovery. 
     BACKGROUND 
     Rich natural gas is produced along with crude oil when oil is produced by horizontal drilling and fracking. The produced natural gas is called associated gas and is comprised primarily of methane but also has heavier hydrocarbons including ethane, propane, butane, heptanes and hexane. The heavier hydrocarbons are often collectively called NGL. Unused gas-phase components are often wasted by the practice of flaring. 
     The rich natural gas causes two problems for the oil industry. First, the heavier hydrocarbons cause the gas energy content to be too high for internal combustion in engines. Specifically, the high energy content causes internal combustion engines to knock. Second, the high energy content makes the rich natural gas unsuitable for use as a compressed natural gas (CNG) fuel. The NGC process removes the valuable NGL hydrocarbons for sale and distribution while concurrently producing a lean natural gas suitable for internal combustion and CNG. Consequently, the associated gas may be put to good use instead of wasted up a flare stack. 
     Competing processes include mechanical refrigeration and Joule Thompson refrigeration. Mechanical refrigeration is relatively expensive and has reliability issues, particularly in cold weather. Mechanical refrigeration is also bulky and difficult to move from site to site as is often needed in oil fields. Conventional Joule Thompson refrigeration has the disadvantage of emulsifying the three-phase mixture from the depressurization valve. Consequently, NGL is often emulsified with the water. 
     SUMMARY 
     A natural gas conditioning process (NGC) described herein conditions rich natural gas for use as a fuel while simultaneously recovering valuable natural gas liquids (NGLs). Natural gas from a well at about 50 psig is compressed by a compressor, and then cooled in multiple steps, such as through cooling with conditioned natural gas, heat exchanged with an air cooler, heat exchanged with an expanded gas from a separator, and heat exchanged by an expanded NGL stream from a separator. Some embodiments do not include all of the coolers. Subsequently, gas, NGL liquid, and water are separated in a primary separator. NGL from the primary separator is expanded and cooled. The cooled gas/liquid mixture is then separated in a second separator. 
     The compressor, usually provided on a separate skid, is modified to work as an integrated part of the NGC system in two ways. First, the gas from the second separator is connected by piping to allow the second separator gas to be recycled to the compressor skid to improve recovery of NGL. Second, hot gas from the compressor is piped to the condition gas cooler to provide a heat sink upstream of an ambient air cooler. The resulting pre-cooled gas is returned to the ambient air cooler (the compressor aftercooler), allowing the ambient air cooler to operate closer to the ambient air temperature. 
     The NGC process is differentiated by traditional mechanical refrigeration or Joule Thompson cooling systems by several features. For example, inlet gas is indirectly cooled by a Joule Thompson expansion of the separator gas and separator NGL to preclude emulsification problems that are common in traditional Joule Thompson systems. Further, the heat integration of the conditioned gas cooler upstream of the ambient air cooler is different than traditional processes. Finally, no mechanical refrigeration system is required, which simplifies the process and lowers capital cost. 
     Up to 80% of propane and heavier components are recoverable, depending on the rich gas composition. The intensity of the flare can be reduced by up to 50%. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       (modified)  FIG. 1  is a process flow diagram for a natural gas conditioning process whereby rich natural gas is compressed and then cooled in four steps according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     An Embodiment of a Natural Gas Conditioning Process 
     (modified) Referring now to  FIG. 1 , in another embodiment of the system with multiple cooling steps. A first cooling step is by heat exchange with conditioned gas. Cooling step two is accomplished in an air cooler. Cooling step three is by a gas expansion cooler. Finally, cooling step four occurs in a NGL expansion cooler. NGL is separated from water and natural gas by first a high pressure three-phase separator, and then a low pressure two-phase separator. A rich natural gas  81  serves as a feed into the system and flows into a compressor  82 . The compressor  82  compresses the rich natural gas  81  to a pressure between about 600 psig and 1200 psig, and a temperature of the rich natural gas rises to about a temperature in the range of 200 to 300 deg. F. The hot, compressed gas is cooled in a conditioned gas cooler  103 . The cooled gas  104  from conditioned gas cooler  103  is further cooled in an air cooler  84 , such as by ambient air, to about a temperature between 50 deg. F. and 100 deg. F. A result is a cooled gas/NGL mixture  85 . The cooled gas/NGL mixture  85  from the air cooler  84  is further cooled by flowing the mixture  85  into the hot side of a gas expansion cooler  86 . The gas expansion cooler  86  turns the mixture  85  into a cooled gas/NGL/water mixture  87 , which then flows into a hot side of a NGL expansion cooler  88  and into a first separator  90  as an inbound gas/NGL/water mixture  89 . 
     Three streams exit the first separator  90 . Water  102  flows from a bottom of the first separator  90 . The water  90  can be then be put to other uses downstream directly or after treatment to remove any residual components from the separation. 
     Conditioned gas  91  exits the first separator  90  and flows through a gas expansion valve  92 . The gas expansion valve  92  facilitates keeping pressure inside the first separator  90  at a working pressure approximately between 600 psig and 1200 psig depending on a composition and characteristics of the inbound gas/NGL/water mixture  89 . Gas  93  from the gas expansion valve  92  flows through the cold side of the gas expansion cooler  86 . The conditioned gas  64  from expansion cooler  86  exits at a pressure approximately between 50 psig and 400 psig and at a temperature between 50 and 200 deg. F. Conditioned gas  105  flows into a cold side of the conditioned gas cooler  103  and leaves the system as conditioned gas  94 . 
     NGL  95  exits the first separator  90  and flows through a NGL valve  96 . A cooled NGL/gas mixture  97  from the NGL valve  96  flows into the cold side of NGL expansion cooler  88 . NGL/gas mixture  98  from the NGL expansion cooler  88  flows into a second separator  99  where gas  100  is separated from an output stream of NGL  101 . The NGL  101  exits at a pressure approximately between 100 psig and 250 psig and at a temperature between 20 and 80 deg. F. The recycle gas  100  exits at a pressure approximately between 100 psig and 250 psig and at a temperature between 20 and 80 deg. F. The gas  100  may be recycled to the compressor  81  or put to another use downstream directly or after treatment. Relative ratios of the three streams  91 ,  95  and  102  vary according to a composition of the rich natural gas  81 . 
     Embodiments and variations thereof, illustrated in the accompanying figure(s) are not to scale. The technical subject matter described above is merely illustrative, and, along with the figures, is not meant to limit the scope of the invention. It is to be appreciated that numerous other variations of the invention have been contemplated, as would be obvious to one ordinary skilled in the art given the benefit of the disclosure. All variations of the invention that read upon appended claims are intended and contemplated to be within the scope of the invention.