Abstract:
A process for separating nitrogen and hydrocarbons from a mixture of gases by splitting the mixture into a plurality of separate streams and throttling the flow of each stream to achieve a selected variable flow rate therebetween. The plurality of separate streams are individually cooled by exchanging heat with a plurality of different process streams. The cooled streams are combined and expand into a separation column where nitrogen ascends the column and exits as a process stream while hydrocarbon descends the column to a reboiler thereof and exits as a process stream. The reboiler is used for cooling one of the separate streams. The hydrocarbon from the bottom of the column is expanded and used to cool a reflux condenser located inside the column and thereafter cools another of the streams before it is discharged from the process. The nitrogen process stream is used to cool another of the separated streams, and then is discharged from the process.

Description:
RELATED PATENT APPLICATIONS 
     This patent application is a continuation-in-part of my co-pending patent application Ser. No. 07/932,867, filed Aug. 20, 1992, now U.S. Pat. No. 5,257,505, issued Nov. 2, 1993 which in turn is a continuation-in-part of my patent application Ser. No. 07/682,287, filed Apr. 9, 1991, now U.S. Pat. No. 5,141,544, issued Aug. 25, 1992. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention discloses a novel high efficiency nitrogen rejection unit by which varying amounts of excess nitrogen are removed from a natural gas stream. Transporting pipelines usually accept natural gas containing up to a maximum of four mole percent total inerts. In this disclosure, total inerts are calculated as the sum of carbon dioxide, nitrogen, helium and other non-hydrocarbon gases. Carbon dioxide is easily removed by various commercial methods, for example as taught in my co-pending patent application Ser. No. 07/932,867 filed Aug. 20, 1992 and my U.S. Pat. No. 5,141,544 issued Aug. 25, 1992, and by U.S. Pat. No. 4,762,543. 
     However, nitrogen, helium and argon are not as chemically reactive and, therefore, cannot be removed as easily or generally by the same methods as carbon dioxide. Nitrogen, helium, argon and other atomically light gases physically act in similar manners at very low temperatures, therefore it will be understood that reference only to nitrogen in the remainder of this description also includes these other gases. 
     Prior to the above intellectual property matters, commercial removal of nitrogen usually was accomplished by fractionation under cryogenic conditions, as seen, for example in U.S. Pat. Nos. 4,451,275, 4,526,595, 4,675,035, and 4,609,390. These previous nitrogen extraction methods achieve a high degree of nitrogen purity, but at a high cost in initial plant equipment and refrigeration horsepower. Examples of these and other processes are shown in the accompanying Prior Art Statement. 
     The nitrogen removal method and apparatus presented herein uses no external refrigeration equipment and is considerably less expensive than previously known conventional methods. The process of this invention utilizes a thermal drive mechanism comprising a series of Joule-Thomson expansion valves (sometimes hereinafter referred to as a JT valve), the optimum physical placement of cross heat exchangers, and computer-based automatic control of cross heat exchanger loading and temperature monitoring. 
     This invention differs from my above mentioned patents and patent application by the provision of method and apparatus that includes a modified thermal drive mechanism which utilizes a series of Joule-Thomson expansion valves and the optimum physical placement of cross heat exchangers, and computer-based automatic control of cross heat exchanger loading and temperature monitoring. 
     SUMMARY OF THE INVENTION 
     The present invention provides both method and apparatus for separating nitrogen and hydrocarbon vapor from a mixture thereof wherein the mixture enters the system at a relatively high pressure and provides the energy for effecting the separation by the employment of the Joule-Thomson effect to selected process streams. 
     More specifically, the process, according to the invention, comprises separation of a feed gas that is a mixture of nitrogen and hydrocarbon vapor. The feed gas is split into a plurality of separate streams, each of which is throttled to achieve a selected variable flow rate therebetween. Each of the split streams is cooled by exchanging heat with one or more of an exiting process stream. The cooled split streams, save one, are recombined and then the recombined cooled split streams expand to the internal pressure of a nitrogen rejection column where the nitrogen and hydrocarbon are separated and exit in separate streams therefrom. The one split stream is cooled by exchanging heat with other process streams, and expands to the internal pressure of the nitrogen rejection column at a location spaced several trays above the introduction of the recombined split streams. The separated nitrogen and hydrocarbon exit in separate streams from the nitrogen rejection column. The separated streams include a nitrogen outlet line, a low pressure sales gas outlet line, and a high pressure sales gas outlet line. 
     The nitrogen rejection column includes a novel internal reflux condenser at the upper end thereof with the lower end thereof terminating in a reboiler. The internal reflux condenser is supported interiorly within the upper end of the column and includes a chamber formed between parallel plate members. A first and second plurality of vertical tubes extend through the plate members. The first plurality of tubes communicate the interior of the rejection column immediately above and below the plate members and form a condensing surface. The second plurality of vertical tubes extend through the lower plate member and down the column to a vapor trap and forms a one way flow path for the descending liquid. 
     Accordingly, a primary object of the present invention is the provision of both method and apparatus for the separation of nitrogen and hydrocarbons from a mixture thereof, including a thermal drive mechanism for the process which utilizes a series of Joule-Thomson expansion valves and the judicious physical placement of cross heat exchangers. 
     Another object of the present invention is the provision of a system by which a separation process is carried out and wherein nitrogen and hydrocarbons are separated from a mixture thereof while utilizing the pressure drop of the various process streams for the thermal drive of the system. 
     A further object of this invention is the provision of a system for separating nitrogen and hydrocarbons from a relatively high pressure mixture thereof by splitting the mixture into a single stream and a plurality of streams, cooling each split stream of the mixture by expansion of various downstream process streams which exchange heat with the split streams, and then effecting a separation in an improved separation column by introducing the split streams at selected locations within the column. 
     A still further object of this invention is the provision of a method of separating nitrogen and hydrocarbons from a high pressure mixture thereof by utilizing the pressure drop of various process streams thereof for the thermal drive of the system and judiciously controlling the various flow rates throughout the process. 
     Another and still further object of this invention is the provision of a process by which nitrogen is removed from produced compressible fluid obtained from a wellbore by splitting the compressible fluid into a plurality of streams, cooling each split stream of the mixture by expansion of various downstream process streams which exchange heat with the split streams, and thereafter effecting a separation of the nitrogen from the residual compressible fluid in a separation column. 
     These and other objects and advantages of the present invention will become readily apparent to those skilled in the art upon reading the following detailed description and claims and by referring to the accompanying drawings. 
     The above objects are attained in accordance with the present invention by the provision of a method for use with apparatus fabricated in a manner substantially as described herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 of the drawing is a diagrammatical representation of a system made in accordance with the present invention for removing nitrogen and hydrocarbons from a mixture thereof; 
     FIG. 2 is an enlarged, broken, diagrammatical representation showing the details of part of the apparatus of FIG. 1; and, 
     FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The figures of the drawings disclose apparatus made in accordance with this invention for removal of nitrogen from natural gas streams. As particularly seen diagrammatically illustrated in FIG. 1, a natural gas stream 1 enters a water dehydration and CO2 removal apparatus. A clean, dry mixture of nitrogen and hydrocarbons continues at feed gas stream 2, and through divider device V1 where part of the main flow is split away from feed gas stream 2. The remaining feed gas stream continues to a diverter valve device V2 where the main flow is split into a plurality of streams illustrated herein as three separate, parallel streams 3, 4 and 5. Heat exchangers A, B and C are connected in parallel respective to one another with the downstream side 6, 7 and 8 thereof being recombined at collection point V3. Heat exchangers D and E are series connected respective to one another and are connected to a first JT expansion device I, while heat exchangers A, B and C are connected to a second JT expansion device F. 
     A nitrogen rejection column G includes a novel internal reflux condenser K supported within the upper end thereof and is made in accordance with the present invention. The lower end of column G terminates in a reboiler, illustrated for convenience as the before mentioned exchangers D and E. Heat exchanger H has a secondary series connected respective to a third JT expansion device J, with the outlet thereof being connected to the novel reflux condenser K. 
     As seen in FIG. 2, the internal reflux condenser K is disclosed diagrammatically in its simplest form. The condenser is supported interiorly within the upper marginal end of the column G and includes a chamber formed between spaced, parallel plate members BB and CC. Hence the interior wall surface of the column and the confronting faces of the plate members form a heat exchanger chamber within which a first and second plurality of vertical tubes AA and DD are exposed. Opposed ends of tubes AA extend through plate members BB, CC and communicate the interior of the tower immediately below plate CC and with the interior of the tower immediately above plate member BB. The upper ends of the plurality of vertical tubes AA extend a few inches above the plate member BB to trap liquid and in order to provide a low vapor velocity area to facilitate liquid-vapor separation. 
     A second plurality of vertical tubes DD each have an inlet end M that lays flush with the upper plate member BB, and an outlet end N at the lower end thereof that extends well below the lower plate member CC and into a liquid trap EE which is in the form of an upwardly opening container having overflow edge FF. The outlet end N of tubes DD is submerged within liquid contained within trap EE. 
     In FIG. 1, sensors S1, S2, S3, S4, S5, and S6 are connected to measure the temperature and pressure of the respective heat exchangers. Sensors S7, S8, and S9 are connected to measure the temperature and pressure of the respective JT expansion valves. The sensors and the stream splitters are connected to the computer means and control device CMCD for regulating the temperature, pressure, and flow rates of the appropriate process streams. 
     The nitrogen rejection unit produces no toxic or dangerous by products and often the feed stock, stream 1, is received at an elevated pressure so that little energy is consumed in the process. 
     OPERATION 
     This invention discloses an original technique for the efficient removal of nitrogen from natural gas streams without requiring rotating equipment or multiple fractionation columns. This technique includes a novel and useful apparatus by which a mixture of nitrogen and hydrocarbons are separated in a new and unobvious process. 
     According to this invention, nitrogen may be reduced from over 50 percent to less than 0.5 percent by volume in natural gas streams. The nitrogen reject stream discharged from this process typically has a purity of approximately 95 percent by volume. 
     Natural gas typically contains carbon dioxide and water vapor naturally occurring from the production reservoir. The water and carbon dioxide must first be removed before introduction into the nitrogen removal unit. This system is represented as stream 1 in FIG. 1. After the carbon dioxide and water are removed using conventional methods, it is represented as feed gas stream 2. 
     Feed gas stream 2 is now split into a plurality of streams including the one split stream at 28 and a plurality of streams 3, 4, and 5, which represent the main flow of the feed gas, all of which is controlled by computerized flow control techniques, using computer means known to those skilled in the art. The plurality of streams include a first split stream 3 which enters the primary side of heat exchanger A where heat is removed from the first stream 3 by being absorbed into the nitrogen rich stream 26, as will be explained later on in this disclosure. A second split stream 4 enters heat exchanger B where heat is rejected to a low pressure residue gas stream 20 from heat exchanger H. A third split stream 5 enters heat exchanger C where heat is removed or absorbed into the high pressure residue stream 14 from the column bottom. Streams 1, 2, 3, 4, 5, and 28 are at a pressure between 700 and 1200 PSIA (pounds per square inch absolute) and a temperature between 80 to 120 degrees F. 
     Streams 6, 7, 8, and 11 exist at between -60 degrees F. and -150 degrees F. and at a pressure only slightly lower than in streams 3, 4, and 5, respectively. Stream 6, 7, and 8 recombine at V3 to form stream 11 which enters pressure reducing JT valve F. Pressure reduced in JT valve F reduces the pressure from the inlet 700 to 1200 PSIA to approximately 315 PSIA and exits pressure control valve F as stream 12. This further cools stream 12 due to the JT effect as stream 11 expands to the column internal pressure. 
     Fourth split stream 28 exits the dividing device V1 and is routed to heat exchanger D where heat is removed from stream 28 and rejected into stream 22. Stream 22 enters heat exchanger D at a temperature of between -100 degrees F. to -200 degrees F. Stream 10 exits heat exchanger D and enters heat exchanger E where heat is again rejected from stream 10 and absorbed into stream 24. Stream 10 exits heat exchanger E as stream 29 at a temperature of -125 degrees F. to -200 degrees F. 
     Stream 29 continues to pressure reducing JT device I where pressure and temperature are further reduced. Stream 29 exits the device I as stream 30 at a temperature of between -200 degrees F. and -250 degrees F., and at a pressure of approximately 315 PSIA. Stream 30 then enters the tower G at an intermediate location that is above the tower entrance of stream 12 and below reflux condenser K. 
     Streams 12 and 30 enter at the illustrated intermediate feed stream locations on the nitrogen rejection tower G and are spaced at least one and preferably three trays apart. The nitrogen rejection tower G utilizes the before mentioned internal reflux condenser seen at K. Streams 12 and 30 enter column G as two phase fluid streams that are partly liquid and partly vapor. The liquid naturally falls by gravity downward inside tower G where the liquid is stripped of nitrogen by contact with the rising vapor generated and introduced lower in the column. Approximately 3 separation stages or trays T2 are located in the column between the feed location of stream 30 and the feed location of stream 12. The illustrated liquid draw tray TD1 enables stream 24 to exit the tower. Stream 24 enters heat exchanger E where heat is absorbed into stream 24 from stream 10. Temperature in stream 24 is approximately -200 degrees F. to -225 degrees F. and stream 25 is -180 degrees F. to -215 degrees F. Stream 25 reenters tower G below the liquid draw tray TD1 as a two phase fluid. The vapor continues up the tower to strip the nitrogen from the falling liquid from streams 12 and 30 as mentioned above. 
     The liquid from stream 25 continues down the tower another approximate six stages or trays through T4 where the nitrogen is stripped by vapor rising up the column as generated in the reboiler, (heat exchanger D). The column liquid is removed from column G by means of the liquid draw tray TD2 and exits as stream 22 where it enters heat exchanger D and exits the exchanger as stream 23. Stream 23 is a two phase fluid and is routed back to the lower portion of the column below liquid draw tray TD2 for separation. The temperature of stream 22 is approximately -200 degrees F. to -225 degrees F. and the temperature in stream 23 is approximately -160 degrees F. to -195 degrees F. 
     Stream 13 is predominately hydrocarbon and exits the bottom of the nitrogen rejection column G where it is divided into streams 14 and 15. Stream 14 continues to heat exchanger C where heat is absorbed from stream 5. Stream 14 exits device C as stream 16 at a temperature of 60 to 100 degrees F. and a pressure of approximately 300 PSIA. The processed stream 16 is discharged from the system as high pressure sales gas outlet and represents the main product manufactured with this process. 
     Stream 15 continues to heat exchanger H where it is subcooled to approximately -200 degrees F. and exits as stream 17. Stream 17 then enters JT expansion valve J where the pressure is reduced to near 25 PSIA and at a temperature of approximately -250 degrees F. Stream 18 is then routed to the internal reflux condenser equipment K. The condenser equipment K is utilized to provide the required cooling to the nitrogen rejection tower by controlled overhead condensation or cooling. This equipment K absorbs heat from the tower overhead vapor and condenses hydrocarbon vapor entering the inlet of tubes AA at the lower part of the condenser K. 
     Referring to FIG. 2 for further details on the internal reflux condenser K, the column vapor enters the lower part or tube sheet of the heat exchanger CC. The vapor continues up the inside of the heat exchanger tubes AA where hydrocarbon condensation occurs on the internal wall surface of the tubes. During low inlet flow operation, the condensed liquid will flow counter current to the vapor flow and gravitate downward where it will fall to the column internals below tube sheet CC. 
     During higher flows, the liquid will be condensed and carried upward along with the gas vapor. The condenser tubes are designed to extend 3 to 4 inches beyond the top tube sheet labeled BB. This extension is necessary in order to provide a location below the upper ends of the tubes AA for separation of liquid and vapor. 
     In addition, in order for the trapped liquid to return to the column trays, a second set of tubes DD is provided and installed flush with the top tube sheet labeled BB. The lower marginal length of tubes DD extend below the lower tube sheet labeled CC. The purpose of tubes DD is to provide a flow path for condensate liquid to be transferred through the tube sheets BB and CC, as shown. The lower end of tubes DD are installed in a seal pan and form a liquid trap which is shown as EE on FIG. 2. The liquid trap EE maintains a liquid seal on the lower end of tubes DD to prevent upward liquid flow through tubes DD. The liquid trap EE preferably is upwardly opening as shown, and can overflow the edge FF as required. 
     Cooling is provided to the reflux condenser equipment K by absorbing heat into stream 18 which enters the lower part of the shell side of condenser equipment K near lower tube sheet CC. Heat is absorbed into this two phase fluid as explained earlier in conjunction with the reflux condenser K located at the top of tower G. 
     The fluid in stream 18 exits the reflux condenser K as stream 19. Stream 19 temperature is approximately -200 degrees F. Stream 19 enters heat exchanger H (FIG. 1) where heat is absorbed into stream 19 and exits exchanger H as stream 20. 
     Stream 20 continues to heat exchanger B where heat is absorbed from process stream 4. Stream 20 exits exchanger B as product stream 21. This stream 21 is one of two product streams 16 and 21. Stream 21 exits the nitrogen rejection column at near 20 PSIA and 60 to 100 degrees F., while stream 16 exits the plant at near 30 psi and 60 to 100 degrees F. Stream 26 exits the tower G overhead as the nitrogen rich or nitrogen reject stream. Stream 26 is routed to heat exchanger A where heat is absorbed from Stream 3. Stream 27 exits the exchanger A at approximately 100 degrees F. and near 300 PSIA. 
     Those skilled in the art, having digested this disclosure, will appreciate that employment of the reflux condenser K together with the four split streams 3, 4, 5, 28 which are arranged to enter column G at intermediate locations 12 and 30 provide a useful and heretofore unknown process by which nitrogen is removed from natural gas to thereby provide unexpected results that include a more useful product at a minimum investment in process equipment.