Abstract:
Apparatus for condensing hydrocarbons, such as but not limited to butane and propane, from a stream of natural gas includes as its primary components a pre-cooler assembly, a chiller assembly, a refrigerant compressor/condenser assembly, and separator assembly. In the pre-cooler assembly warm gas entering the apparatus is cooled by counter-current flow heat exchange with cooled natural gas exiting the apparatus. The chiller assembly of the apparatus the hydrocarbon stream is cooled in co-current flow heat exchange with a first refrigerant tube, carrying a first refrigerant which is itself cooled in co-current heat exchange with a second refrigerant tube disposed in coaxial relationship with the first refrigerant tube. Both first and second refrigerant tubes are disposed in coaxial relationship with an outer jacket conveying the gas stream through the chiller assembly. Condensed hydrocarbons are separated from the gas stream in the separator assembly.

Description:
RELATED DATA APPLICATION 
     This application claims the benefits of U.S. Provisional Patent Application Ser. No. 60/724,998, filed Oct. 7, 2005. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the extraction of hydrocarbons from natural gas, and in its preferred embodiments more specifically relates to apparatus and methods for condensing hydrocarbons from natural gas by indirect heat exchange with a refrigerant. 
     BACKGROUND OF THE INVENTION 
     Natural gas is commonly considered to consist of methane, but the gas produced from a well normally contains higher hydrocarbons such as propane and butane. Those hydrocarbons are in gaseous form when the gas is collected from the well, but because of their higher boiling points are more readily condensed to liquids. It is desirable to remove higher hydrocarbons from the methane before the natural gas is introduced into high pressure pipelines to prevent condensate from forming in the pipeline and in associated equipment during transport. Those hydrocarbons also have economic value to the well owner or operator. 
     Approaches known in the prior art for removing higher hydrocarbons from methane at the wellhead include expansion of the produced gas, and the use of refrigerant, to cool the gas and condense some of the higher hydrocarbons. Although these approaches do result in removal of some of the higher hydrocarbons from the predominantly methane stream, their effectiveness is limited because of limitations of the temperature drop that can be achieved. 
     There remains a need for an efficient and effective process and apparatus for removing hydrocarbons such as propane and butane from a natural gas stream at the wellhead. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of and apparatus for removing hydrocarbon liquids from natural gas streams to control the hydrocarbon dewpoint of gas upstream of transmission pipelines. The method and apparatus utilize a highly effective refrigeration system, and in the preferred embodiment the apparatus is constructed and provided as modular units for ease of transport and installation. Though the scope of the invention is not limited in either minimum or maximum capacity, the apparatus of the invention is known to be commercially feasible and economically effective for processing gas volumes ranging from 50 mcfg/day to 1,000 mcfg/day. The apparatus is capable of achieving temperatures of −20° F. using commercially available refrigerants, and at operating pressures ranging from ambient to at least 1,000 psig. 
     In the process or method of the invention the incoming gas stream enters a pre-cooler heat exchanger, where the gas stream is pre-cooled by cold outlet gas. The gas stream passes through a chiller section where the temperature of the gas stream is lowered to that required to condense the designated hydrocarbons from the gas by indirect contact with refrigerant. The chiller section utilizes a unique dual refrigerant design, in which a first, super-cooled, refrigerant line is disposed within a second refrigerant line. The second refrigerant line is disposed within the gas piping, so that the gas is in heat exchange contact with the second refrigerant line, and the refrigerant flowing through the second line is in heat exchange contact with the first refrigerant line. From the chiller section the gas and condensed vapors next flow into a separator unit, where the condensed hydrocarbons are collected. The gas, free of the liquid hydrocarbons, flows from the separator unit, passes through the pre-cooler, and exits the apparatus. The recovered hydrocarbon liquids flow, or are pumped, from the bottom of the separator to storage. 
     The apparatus of the invention and the method of the invention will be described in more detail with reference to the accompany drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a preferred embodiment of the apparatus of the invention and a flow diagram showing fluid flows through the apparatus in accordance with the method of the invention. 
         FIG. 2  is a schematic illustration of a preferred embodiment of the pre-cooler assembly of the apparatus of the invention. 
         FIG. 3  is a schematic illustration of a preferred embodiment of the chiller assembly of the apparatus of the invention. 
         FIG. 4  is a cross-sectioned illustration showing the concentric disposition of the lines carrying the three fluids flowing through the chiller assembly of the apparatus of the invention. 
         FIG. 5  is a schematic illustration of a preferred embodiment of the separator assembly of the apparatus of the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In the preferred embodiment the basic components of the apparatus of the invention include a pre-cooler assembly  10 , a chiller assembly  11 , at least one refrigerant compressor/condenser assembly  12 , and separator assembly  13 , with associated conduits and piping for routing gas, liquid, and refrigerant streams, and associated controls. 
     Pre-cooler assembly  10  is a heat exchanger in which heat is transferred from the warm gas stream entering the apparatus to the cooled gas stream exiting the apparatus. The gas streams are in countercurrent flow through the pre-cooler, which is structured with outerjackets  14  and an inner tube or pipe  15  of smaller diameter, extending coaxially through the interior of jackets  14 . In the preferred embodiment the warm gas stream flows through jackets  14  and the cooled gas stream flows through pipe  15 , but the routing of the gas streams through the pre-cooler is not critical, because the volume of the warm gas is greater than the volume of the cooled gas exiting the apparatus. The diameter of jackets  14  and the diameter of pipe  15  are selected to accommodate the gas volume and flow rates through the system. 
     In the preferred embodiment a plurality of jackets  14  are staged in a compact stacked orientation, as shown in  FIG. 2 , connected by short connector conduits  16 . Pipe  15  preferably extends continuously in a sinuous curve through the stacked series of jackets. The compact arrangement of the pre-cooler jackets is utilized to minimize the footprint of the complete apparatus and facilitate making the apparatus available in portable units. It is to be understood that the arrangement is not critical to the heat exchange function of the pre-cooler, and that other orientations may be used within the scope of the invention. 
     Although the invention is not limited to any particular temperature change in the pre-cooler, in a typical installation the temperature of the incoming gas is reduced approximately thirty to forty degrees F. For example, if the temperature of the warm gas stream entering the pre-cooler is one hundred degrees F., the temperature of that gas stream exiting the pre-cooler may be expected to be reduced to approximately sixty degrees F. 
     Chiller assembly  11  of the apparatus, illustrated in  FIG. 3 , comprises an elongate heat exchange assembly, preferably formed in the same general configuration as pre-cooler  10 , with a plurality of outerjackets  17 , a first refrigerant tube  18 , and a second refrigerant tube  19 , of smaller diameter than refrigerant tube  18 . Refrigerant tube  18  extends through the interior of outerjackets  19  in coaxial relation with jackets  17 , and refrigerant tube  19  extends through the interior of refrigerant tube  18  in concentric, coaxial relation with that tube. The coaxial relationship among jacket  17 , tube  18 , and tube  19  is illustrated in  FIG. 4 . In the illustrated embodiment each jacket  17  is formed in a narrow U-shaped configuration, joined by short gas conduits  20 , similar to conduits  16  in the pre-cooler construction. Refrigerant tubes  18  and  19  preferably extend continuously through each jacket  17  and between jackets. Tubes  18  and  19  are disposed in the interior of jackets  17  through the curvature at the base of the U-shape of each of jackets  17 , but do not extend through conduits  20  between jackets. Rather, the refrigerant tubes exit from the end of one of the legs of the U-shaped jacket and enter the end of the adjacent leg of the next jacket, following a smooth curvature between jackets. The configuration and stacked arrangement of jackets and refrigerant tubes in the preferred embodiment of the chiller assembly is used in order to achieve a compact design and facilitate the construction of portable units of the apparatus. Other spacial orientations of the heat exchange components may be used if, for example, portability of the apparatus is not a factor. 
     The hydrocarbon gas flowing through jackets  17  is cooled by heat exchange between the gas and a first refrigerant in first refrigerant tube  18 , through the wall of tube  18 . The refrigerant flowing through tube  18  is itself cooled by heat exchange with a second refrigerant in second refrigerant tube  19 , through the wall of tube  19 . In the preferred embodiment, the flow of gas through the chiller assembly is co-current with the flow of the first refrigerant through tube  18 , and the flow of the first refrigerant through tube  18  is also co-current with the flow of the second refrigerant through tube  19 . 
     This two stage cooling is a unique feature of the apparatus and method of the invention, and achieves a significant increase in the efficiency of the system in comparison to single stage refrigerant cooling. There are a number of variables involved in cooling, or removing heat energy from, a first fluid stream in heat transfer contact with a second fluid stream, including the temperature difference between the two fluids, the area of contact between the two streams, and the relative volumes of the two streams. In the present situation, and in chiller assembly  11 , two stages of heat transfer occur simultaneously, throughout the chiller assembly. In the first stage the first fluid is the natural gas flowing through jackets  17  and the second fluid is the refrigerant flowing through tube  18 . To achieve sufficient cooling of the natural gas to condense hydrocarbons such as propane and butane from the stream in a heat exchanger of reasonable size for use in a wellhead application requires a certain volume of refrigerant, a certain area of contact, and a certain temperature difference, which parameters can be readily determined. The problem inherent in the use of single stage refrigerant systems is efficiently and economically reducing the temperature of the refrigerant sufficiently to provide the required cooling capacity. The use of conventional compressor/condenser technology to achieve the required temperature drop in the required volume of refrigerant is economically impractical. 
     The present invention overcomes the problem inherent in the use of a single stage refrigeration system by utilizing a second refrigerant heat exchange stage to reduce the temperature of the first refrigerant. As referred to above, in the apparatus of the invention the second refrigerant flowing through tube  19  is in heat exchange contact with the first refrigerant flowing through tube  18 , which is in heat exchange contact with the refrigerant flowing through jackets  17 . The volume of refrigerant used in the second stage refrigerant system is significantly less than the volume of the refrigerant used in the first stage system, and the volumetric flow rate of refrigerant through second refrigerant tube  19  in the second stage is lower than the volumetric flow rate of refrigerant through first refrigerant tube  18  in the first stage. Accordingly, the capacity of the compressor/condenser equipment required to reduce the temperature of that second stage refrigerant volume to a given temperature is much lower than the capacity that would be required to equivalently reduce the temperature of the higher volume of refrigerant in the first stage. 
     In the apparatus of the invention, and in accordance with the method, the heat transferred from the natural gas to the first refrigerant, and the heat transferred from the first refrigerant to the second refrigerant, is removed using conventional refrigerant compressor/condenser equipment. In the preferred embodiment, as illustrated in  FIG. 3 , a two stage compressor/condenser assembly  12  is used to cool the two refrigerant streams. The first stage refrigerant and the second stage refrigerant remain discrete and separate from each other through the compressor/condenser assembly as well as through the chiller assembly. Compressor/condenser assembly  12  may be electrically driven, or may be gas engine driven, depending upon the availability of utilities at the site of operation and upon operator preference. 
     It has been found that effective and efficient cooling of the natural gas steam for condensation of propane and butane from the gas is achieved with co-current flow of the three fluid streams through chiller assembly  12 . In the context of flow characteristics, it is to be understood that the pressures and volumetric flow rates of the three streams are not the same through the chiller assembly. Operating pressures and flow rates may be varied for different apparatus capacities and conditions, to achieve optimum operating efficiencies, as may be determined by routine application of engineering principles. 
     It will be understood that the cooling of the first stage refrigerant by the second stage refrigerant through the chiller assembly of the apparatus reduces the temperature of the first stage refrigerant and maintains the temperature differential between the first stage refrigerant and the natural gas stream to a significantly greater degree that could be achieved using conventional apparatus and conventional methods. As a result, the apparatus and method of the invention is capable of achieving condensation of hydrocarbons such as propane and butane from the predominantly methane gas stream in the chiller assembly. 
     The stream exiting chiller assembly  11  is routed to separator assembly  13 , which is preferably located in close proximity to the outlet from the chiller assembly. Separator assembly  13 , illustrated in  FIG. 5 , comprises a vessel  21  with a substantially hollow interior. The flow from chiller assembly  11  enters vessel  21  approximately midway between the bottom closure  22  and the top closure  23 , and is dispersed into the interior of the vessel through flow diverter  24 . As the chilled hydrocarbon stream enters the interior of vessel  21  through the flow diverter the liquified constituents of the stream separate from the gaseous hydrocarbons. The gaseous constituents of the gas stream move upward in vessel  21 , and the liquid hydrocarbons drop to the bottom of the vessel. The gas moves upward through baffle plates  25  and a mist extractor  26  to assure removal of residual liquids from the gas before the gas exists vessel  21  through outlet  27 . Outlet  27  is connected to pipe  15  of pre-cooler assembly  10 , and the chilled gas leaving the separator assembly flows through the pre-cooler assembly in heat exchange with warm incoming gas as described above, and exits the apparatus to, e.g., a transmission pipeline. 
     Liquid hydrocarbons, also referred to as LPG (liquified petroleum gases) in the bottom of vessel  21  are withdrawn from the vessel through liquid outlet  28  to, typically, storage tanks where the LPG is held until collected and transported for sale or use. In the preferred embodiment the level of the LPG in vessel  21  is monitored by a controller  29 , through which a pump  30  is automatically activated when the LPG in the vessel reaches a selected level to move the LPG to storage. 
     The apparatus of the invention is preferably monitored and controlled by an automated control system, including but not limited to controller  29 , allowing operation of the apparatus without continuous operator attention. 
     The apparatus of the invention is readily adaptable to different produced gas flow rates and wellhead conditions. The duration of heat exchange contact between the various fluid streams may be varied by increasing the length of the heat exchanger components and/or by incorporation of additional heat exchange assemblies. For higher gas flows a second chiller assembly and a second compressor/condenser assembly may be added. The gas flow through a second chiller assembly may be routed in sequence from the first chiller assembly, or may be routed to the second chiller assembly in parallel with the first. 
     The foregoing description of the apparatus and the method of the invention is illustrative and is not intended to be limiting of the scope of either the apparatus or the method. The invention is susceptible to and the scope of the invention as generally defined by the following claims encompasses further variations and alternative embodiment that may be devised on the basis of the description provided herein.