Patent Document

REFERENCE TO PENDING APPLICATIONS 
     This application is not based upon any pending domestic or international patent application. 
     REFERENCE TO MICROFICHE APPENDIX 
     This application is not referenced in any microfiche appendix. 
     FIELD OF THE INVENTION 
     The present invention relates to a method and system for treating a hydrocarbon mixture to remove CO 2  along with attendant sulfur compounds using distillation and membrane separation systems with critically arranged distillation reflux paths. 
     BACKGROUND OF THE INVENTION 
     Much of the world&#39;s natural gas supply is contaminated with unacceptably high levels of carbon dioxide (CO 2 ). In some cases, in addition to excessive CO 2 , the natural gas may also contain excessive levels of sulfur compounds. Such sulfur compounds include hydrogen sulfide and carbonyl sulfide. In many cases, the carbon dioxide and sulfur contaminants lower the BTU value of natural gas making such gas unsuitable for use as a fuel or unsuitable to be transported in a pipeline carrier. Various commercial technologies including low temperature distillation, amine scrubbing and membrane separation, have been developed to upgrade natural gas containing excessive CO 2  or sulfur compounds. All of the above-mentioned technologies typically produce a useable natural gas stream and a carbon dioxide/sulfur compound stream. The distillation separation of CO 2  from hydrocarbon gas is a very energy and capital-intensive process. The present invention is an improvement on distillation technology that reduces the energy and capital requirement, producing a hydrocarbon product more efficiently. 
     Background information relating to the extraction of CO 2 , with or without accompanying sulfur compounds, from hydrocarbon gas may be found in the following publications: 
     (1)  Process Can Efficiently Treat Gases Associated With CO   2    Miscible Flood—Oil  &amp;  Gas Journal,  Jul. 18, 1983. 
     (2) U.S. Pat. No. 4,936,887—Distillation Plus Membrane Processing of Gas Streams, Waldo et al., Jun. 26, 1990. 
     (3) Canadian Patent No. 1,253,430—Process and Apparatus for Fractionation of a Gaseous Mixture, Burr, May 2, 1989. 
     (4) U.S. Pat. No. 4,417,449—Process for Separating Carbon Dioxide and Acid Gases From a Carbonaceous Off-Gas, Hagarty et al., Nov. 29, 1983. 
     (5) U.S. Pat. No. 4,602,477—Membrane-Aided Distillation for Carbon Dioxide and Hydrocarbon Separation, Lucadamo, Jul. 29, 1986. 
     (6) U.S. Pat. No. 4,444,571—Energy Efficient Process for the Stripping of Gases from Liquids, Matson, Apr. 24, 1984. 
     (7) U.S. Pat. No. 4,374,657—Process of Separating Acid Gases from Hydrocarbons, Feb. 22, 1983. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a membrane/distillation system for producing a CO 2  product, or a sour CO 2  product and a hydrocarbon product. The system is comprised of: (a) ancillary equipment for dehydrating, cooling, and temperature conditioning the inlet gas; (b) a distillation system for separating the conditioned inlet gas into a CO 2  liquid stream and a distillation overhead stream; (c) a primary condenser and reflux drum for separating the distillation overhead into a primary reflux stream and a hydrocarbon vapor stream, (d) a membrane system for separating the vapor stream into a hydrocarbon product and a permeate stream that is compressed, cooled and condensed to form additional reflux for the distillation column. The inlet hydrocarbon stream may be a natural gas stream or associated gas stream and may have liquid hydrocarbon components and which contains carbon dioxide and/or sulfur compounds. The hydrocarbon product may be a stream consisting predominantly of light hydrocarbons. The hydrocarbon product may include insignificant amounts of CO 2 , sulfur containing species and other components. The CO 2  product may include insignificant amounts of hydrocarbon and other components, or the CO 2  product may be pure CO 2 . 
     In one embodiment of the invention, the inlet gas stream is preconditioned for the separation by ancillary equipment. If required, inlet temperature, and pressure of the dehydrated hydrocarbon mixture are adjusted. After conditioning, the conditioned inlet stream is subjected to distillation. The distillation column produces an overhead stream and a CO 2  bottom product. The distillation overhead is further processed by a primary reflux system. The primary reflux system partially condenses the stream in a condenser. The partially condensed stream is separated by the primary reflux drum into a liquid reflux and hydrocarbon overhead. The liquid reflux is returned to the column. The hydrocarbon-enriched overhead vapor from the primary reflux drum is further separated by the membrane system. The membrane system separates the reflux drum vapor into a hydrocarbon vapor stream and permeate stream. The permeate stream is compressed to a pressure greater than the distillation overhead. The compressed permeate stream is combined with the distillation overhead. This combined stream (distillation overhead and permeate stream) comprises the primary condenser inlet stream. This combined condenser inlet is fed to the primary reflux system which ultimately provides liquid reflux and membrane feed as described above. 
     In a separate embodiment of the invention, the inlet hydrocarbon fluid mixture is initially preconditioned and separated by the distillation system. If required, the inlet temperature and pressure of the dehydrated hydrocarbon mixture are adjusted. After conditioning, the inlet stream is subjected to fractional distillation. The distillation column produces an overhead stream and a CO 2  bottom product. The distillation overhead is further processed by a primary reflux system. The primary reflux system partially condenses the condenser inlet stream in the primary condenser. The partially condensed stream is separated by the primary reflux drum into a liquid reflux and hydrocarbon overhead. The overhead hydrocarbon vapor from the primary reflux drum provides a partial feed to the membrane system. The membrane system separates the membrane inlet stream into a hydrocarbon product and a permeate stream. The permeate stream is compressed to a pressure greater than the primary reflux. The permeate stream is partially condensed in a secondary condenser and a secondary reflux drum is used to separate the two phase fluid. The pressurized liquid from the secondary reflux drum is added to the primary reflux downstream of the primary condenser. The hydrocarbon vapor from the secondary reflux drum is combined with the hydrocarbon vapor stream from the primary reflux drum. This combined stream comprises the membrane inlet stream. 
     In either of the above embodiments of the invention, the CO 2  bottom product from the fractional distillation is processed identically. The stream is partially vaporized in a reboiler heater. A reboiler separator produces a vapor for re-introduction into the column and a CO 2  liquid product. A portion of the CO 2  liquid product may optionally be used to satisfy the cooling requirements of the process. In this mode of operation, a CO 2  gas product is also produced. 
     The conditioned inlet gas required as feed to this invention may be obtained by a variety of methods well-known to those skilled in the art. The dehydrating system may be a glycol absorption system, a desiccant absorption system or a membrane dehydration system. For purposes of this invention, a dehydrating system is defined as a system that removes water from the stream to a dew point of less than the lowest temperature observed in the system. 
     A cooling system for the purposes of this invention may be a heat exchange system, a gas expansion system, a turbo expander system, a valve expansion system, or a mechanical refrigeration system. The heat exchange system is defined as one or more heat exchangers which utilize ambient temperature, or temperature of internal process streams, to decrease the temperature of the specified stream. A heat exchange system may consist of aerial-type exchangers, shell and tube, or plate and frame-type exchangers, which transfer heat from one process stream to another. An expansion system, either gas or liquid, is the expansion of a process stream to a condition of lower pressure. A turbo expander system is the expansion of this process stream through a turbo expander. In a turbo expander system, the expansion or pressure reduction of the gas stream is used to generate mechanical energy and effect a cooling of the process stream. A valve expansion system is the expansion or pressure reduction of this process stream through a valve or an orifice. The pressure reduction causes the gas stream to cool. A mechanical refrigeration system is the reduction of a process temperature by use of cooling derived from a refrigeration source that is ancillary to the process streams. In a mechanical refrigeration system, a refrigerant is contained in a closed loop. The refrigerant is subjected to pressurization, expansion and condensation. On expansion, the pressurized refrigerant vaporizes and cools. This cooling is utilized in a cross exchanger to reduce the temperature of the process stream. The heat loss from the cross exchange causes condensation of the refrigerant stream. The condensed refrigerant is again pressurized and the cycle repeated. 
     A preferred temperature range of the cooling procedure of step (a) is from about −30° F. to about 150° F. and more preferably between −20° F. and 60° F. 
     The system of the present invention may comprise a depressurizing device for optimizing the properties of inlet streams for separation by components of this invention. Distillation and membrane separation are the primary components. Typical depressurizing devices are a compressor, a turbo expander, and an expansion valve. The separation system of the present invention may also comprise a pump and a compressor. The pressure of the pressuring adjusting step (b) is from about 200 psia to about 1200 psia, and preferably from 350 psia to 800 psia and most preferably between 550 and 650 psia. 
     The distillation system is defined as a separation device that utilizes differences in boiling point and relative volatility to effect separation of components. The distillation system may have a plurality of distillation columns and the columns may be in a series or recycle configuration. Typical distillation columns employ trays and weirs to effect the successive steps of rectification and equilibration required for distillation. The column has a reflux produced by an overhead reflux system (condenser and separator drum) and reboiler vapor produced by a bottom fluid boiler and separator drum. 
     The membrane system is defined as a system which utilizes a selective barrier that is capable of separating components on the basis of size, shape or solubility. The membrane system separates a high-pressure feed stream into a high-pressure non-permeate stream and a lower pressure permeate stream. Membranes that preferentially permeate CO 2  faster than hydrocarbons are useful for this invention. Membranes of this type are typically comprised of a glassy polymer. A glassy polymer is a polymer that is applied at a temperature lower than the glass transition. Examples of polymer families that are typically employed as glassy polymer membranes include: cellulose acetate, polyaramides, polybenzoxazoles, polycarbonates, polyimides, and polysulfones. Structural modification of the base polymer backbone is often used to enhance the gas separation performance of a given polymer family. These structural variants are also useful in this invention. 
     The membrane system has at least one membrane unit. The membrane system can have a plurality of membrane units. Often, the plurality of membrane units are arranged in a series configuration. The series configuration leads to improved performance when the membrane module performance is less than predictions based on an ideal membrane unit. A recycle configuration of the membrane modules can also be used to reduce hydrocarbon losses. 
     In one embodiment, the process comprises the step of recovering energy from the stream of CO 2  liquid from the bottom of the distillation column. By flashing all or a part of the liquid across an expansion valve, sufficient refrigeration can be achieved to meet or exceed the cooling requirements of the system. Furthermore, this mode of operation eliminates the necessity of ancillary mechanical refrigeration. 
     In another embodiment, the present invention relates to a process for producing high levels of CO 2  liquid and a hydrocarbon product. The process comprises the steps of (a) cooling dehydrated hydrocarbon fluid mixture; (b) adjusting the pressure of the hydrocarbon fluid mixture; (c) distilling the hydrocarbon fluid mixture to produce a CO 2  liquid and a hydrocarbon byproduct containing CO 2  and/or sour gas; and (d) utilizing a membrane system to further separate the hydrocarbon byproduct to produce a recoverable hydrocarbon product and an additive for distillation column reflux. In this embodiment, mechanical refrigeration is used for the cooling step (c) and, the CO 2  liquid from the bottom of the distillation column (c) is collected as product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a schematic flow diagram of a preferred embodiment of the present invention. 
         FIG. 2  represents a schematic flow diagram of the system of the present invention wherein a permeate stream from a membrane system is condensed in a separate condenser and added to the vapor from the distillation column condenser to provide feed for the membrane system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Major elements of the invention are indicated in the drawings by numerals as follows: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 14 
                 Inlet gas stream 
               
               
                 16 
                 Inlet cross heat exchanger 
               
               
                 18 
                 Cooled inlet stream 
               
               
                 20 
                 Reboiler cross heater 
               
               
                 22 
                 Conditioned inlet stream 
               
               
                 24 
                 Distillation column 
               
               
                 26 
                 CO 2  bottom product stream 
               
               
                 28 
                 Distillation overhead stream 
               
               
                 30 
                 Permeate stream 
               
               
                 32 
                 Combined condenser inlet stream 
               
               
                 34 
                 Primary condenser 
               
               
                 36 
                 Primary condenser outlet stream 
               
               
                 38 
                 Primary reflux drum 
               
               
                 40 
                 Hydrocarbon vapor stream 
               
               
                 42 
                 Primary reflux liquid stream 
               
               
                 44 
                 Primary reflux pump 
               
               
                 46 
                 Pumped primary reflux liquid stream 
               
               
                 48 
                 Membrane unit 
               
               
                 49 
                 Membrane inlet 
               
               
                 50 
                 Permeate cross heat exchanger 
               
               
                 52 
                 Hydrocarbon gas product stream 
               
               
                 54 
                 Permeate stream 
               
               
                 56 
                 Compressor 
               
               
                 58 
                 Compressed permeate stream 
               
               
                 60 
                 First permeate cross heat exchanger feed stream 
               
               
                 62 
                 Second permeate cross heat exchanger feed stream 
               
               
                 64 
                 Permeate cross heat exchanger outlet stream 
               
               
                 66 
                 Hydrocarbon product cross heat exchanger 
               
               
                 68 
                 Hydrocarbon product cross heat exchanger outlet stream 
               
               
                 70 
                 CO 2  bottom product pump 
               
               
                 72 
                 Pumped CO 2  bottom product stream 
               
               
                 74 
                 Reboil/separator 
               
               
                 76 
                 Reboiler separator inlet stream 
               
               
                 78 
                 Reboiler separation vapor stream 
               
               
                 80 
                 Reboiler separation liquid stream 
               
               
                 82 
                 Primary CO 2  refrigerant stream 
               
               
                 84 
                 CO 2  liquid product 
               
               
                 86 
                 Primary refrigerant pressure reduction device 
               
               
                 88 
                 Primary condenser refrigerant inlet stream 
               
               
                 90 
                 Primary condenser refrigerant outlet stream 
               
               
                 92 
                 CO 2  gas product 
               
               
                 94 
                 Hydrocarbon gas product 
               
               
                 96 
                 Secondary reflux drum 
               
               
                 98 
                 Secondary condenser 
               
               
                 102 
                 Secondary condenser outlet stream 
               
               
                 104 
                 Secondary reflux liquid stream 
               
               
                 106 
                 Combined reflux liquid stream 
               
               
                 108 
                 Secondary CO 2  refrigerant stream 
               
               
                 110 
                 Secondary refrigerant pressure reduction device 
               
               
                 112 
                 Secondary condenser refrigerant inlet stream 
               
               
                 114 
                 Secondary condenser refrigerant outlet stream 
               
               
                 116 
                 Combined refrigerant outlet stream 
               
               
                 118 
                 Secondary hydrocarbon vapor stream 
               
               
                   
               
             
          
         
       
     
     Referring now to the drawings wherein like reference numerals designate identical or corresponding parts throughout the several views and more particularly to  FIG. 1  wherein the system and method of the present invention are illustrated. A dehydrated hydrocarbon fluid mixture gas stream inlet which contains high levels of carbon dioxide flows by way of inlet gas stream  14  and enters an inlet cross heat exchanger  16  for conditioning. The resulting cooled inlet stream  18  enters a reboiler cross heater  20  for further conditioning, producing a conditioned inlet stream  22 . Stream  22  may be further cooled using a chiller. If the pressure of conditioned inlet stream  22  exceeds the critical pressure, either a Joule-Thomson expander or a turbo-expander can be used to reduce the pressure of conditioned inlet stream  22 . The energy from the expander can be used for compression or for generating electricity. 
     Upon completion of the cooling process and pressure reduction processes, the hydrocarbon fluid mixture gas stream is properly conditioned for distillation separation. A distillation separation system that produces a high yield of liquid CO 2  is preferred. The primary reason for selecting distillation for the bulk removal of CO 2  is its ability to remove the CO 2  as a liquid. Conditioned inlet stream  22  is distilled in distillation column  24  producing a liquefied CO 2  bottom product stream  26  and a distillation overhead stream  28  (containing significant amounts of CO 2 ). The distillation overhead stream  28  is combined with permeate stream  30  from the membrane unit  48  producing combined condenser inlet stream  32 . This stream  32  is cooled by primary condenser  34  producing a primary condenser outlet stream  36 . This stream  36  enters a primary reflux drum  38  producing a hydrocarbon vapor stream  40  and a primary reflux liquid stream  42 . This liquid stream  42  flows back to distillation column  24  by gravity or is pumped by primary reflux pump  44  to enter a top tray of distillation column  24  as reflux. The hydrocarbon vapor stream  40  is sent to membrane unit  48  for further CO 2  removal. Hydrocarbon vapor stream  40  enters permeate cross heat exchange  5 Q and is warmed prior to entering membrane unit  48 . The membrane unit may be a single stage or multiple stages depending on the application, in addition, the permeate pressure of the membrane stages can be different to optimize compressing the permeate gas. Membrane separation produces a hydrocarbon product stream  52  and permeate stream  54 . For this example, permeate stream  54  is compressed in a compressor  56  producing a compressed permeate stream  58 . This stream  58  is divided into first and second permeate cross heat exchanger feed streams  60  and  62 . These streams are cooled by permeate cross heat exchanger  50  and hydrocarbon product cross heat exchanger  66  producing permeate cross heat exchanger outlet stream  64  and hydrocarbon product cross heat exchanger outlet stream  68  that combine to form permeate stream  30 . 
     Permeate stream  30  is then combined with distillation overhead stream  28  from the distillation column overhead to form combined condenser inlet stream  32 . Permeate stream  54  could also be removed for disposal or for further processing instead of being utilized for reflux enhancement. 
     The CO 2  bottom product stream  26  may be pumped to an elevated pressure using pump  70  into stream  72 . Thermal energy from the pumped CO 2  bottom product stream  72  is then recovered using reboiler cross heater  20  to cool inlet stream  18 . The reboiler separator inlet stream  76  enters a reboiler/separator  74 . The vapor from reboiler/separator  74 , stream  78 , is returned to the bottom of distillation column  24 . The liquid from reboiler/separator  74 , stream  80 , is split into a primary CO 2  refrigerant stream  82  for chilling, with the balance, stream  84  remaining as a CO 2  liquid product stream. Primary CO 2  refrigerant stream  82  is reduced in pressure with a primary refrigerant pressure reduction device  86  producing primary condensed refrigerant inlet stream  88 . This stream  88  enters primary condenser  34  providing cooling sufficient to produce the required reflux liquid stream  42 . Primary condenser refrigerant outlet stream  90  leaving primary condenser  34  enters inlet cross heat exhange  16  as an economizer to cool the inlet gas. The CO 2  gas stream leaving inlet cross heat exchange  16  as a gas stream  92  can be compressed to combine with liquid CO 2  product stream  84  or can be used as a CO 2  gas product stream. 
     For a typical application with an inlet gas of 58% CO 2  at 610 psia, the process, as shown in  FIG. 1 , produces a hydrocarbon product containing 10% CO 2  at 565 psia and recovers 89.9% of the hydrocarbon in the inlet gas stream. The CO 2  gas product stream contains 92.8% CO 2  and recovers 89.1% of the CO 2  at 200 psia. The CO 2  liquid product stream contains 92.8% CO 2  and recovers 3.7% of the CO 2  at 610 psia. This gives a total recovery of CO 2  for this example of 92.8%. A significant demand for energy in any CO 2  removal process producing gaseous CO 2  is compression of the CO 2 . CO 2  compression can be the limiting factor for projects requiring CO 2  at elevated pressures such as enhanced oil recovery, or re-injection of the CO 2  to eliminate venting to the atmosphere. The compression requirements for this process are less than that for traditional distillation processes, since the CO 2  product streams are produced at a relatively high pressure, and no external refrigeration is required. 
     Referring now to  FIG. 2 , wherein like reference numerals designate identical or corresponding parts, a dehydrated hydrocarbon fluid mixture inlet gas stream  14  that contains carbon dioxide enters inlet cross heat exchanger  16  for cooling. The resulting cooled inlet stream  18  enters a reboiler cross heater  20  for further cooling, producing conditioned inlet stream  22  which may be further cooled using a chiller. If the pressure of conditioned inlet stream  22  exceeds the critical pressure, either a Joule-Thomson expander or a turbo expander can be used to reduce the pressure thereof Energy from an expander can be used for compression of the permeate gas or for generating electricity. 
     Upon completion of the cooling process and pressure reduction process, the hydrocarbon fluid mixture is properly conditioned for distillation separation. A distillation separation system that produces a high yield of liquid CO 2  is preferred. The primary reason for selecting distillation for the bulk removal of CO 2  is its ability to remove the CO 2  as a liquid. Conditioned inlet stream  22  is then distilled in distillation column  24  producing a CO 2  bottom product stream  26  and a distillation overhead stream  28 , which contains significant amounts of CO 2 . The distillation overhead stream  28  is cooled by primary condenser  34  producing primary condenser outlet stream  36  that enters primary reflux drum  38  producing a hydrocarbon vapor stream  40  and a primary reflux liquid stream  42 . This primary reflux liquid stream  42  is combined with secondary reflux liquid stream  104  from the secondary reflux drum  96 . The combined reflux liquid stream  106  flows to a top tray of distillation column  24  as a reflux. 
     Hydrocarbon vapor stream  40  from primary reflux drum  38  is combined with secondary hydrocarbon vapor stream  118  and enters permeate cross heat exchanger  50  and is warmed prior to entering membrane unit  48 . The membrane unit  48  may be single stage or multiple stages depending on the application. In addition, the permeate pressure of the membrane stages can be different to optimize compressing the permeate gas. Separation in membrane unit  48  produces a hydrocarbon product stream  52  and a permeate stream  54 . Stream  54  is then compressed in compressor  56  producing compressed permeate stream  58  that is cooled by heat exchangers  50  and  66  producing permeate stream  30 . The permeate stream  30  is then partially condensed using secondary condenser  98  producing secondary condenser outlet stream  102 . Secondary reflux drum  96  produces secondary hydrocarbon vapor stream  118  and secondary reflux liquid stream  104 . Vapor stream  118  is combined with vapor stream  40  from primary reflux drum  38 . The combined stream is feed to membrane unit  48 . Secondary reflux liquid stream  104  is combined with pumped primary reflux liquid stream from primary reflux drum  38  to provide the combined reflux liquid stream  106  that feeds onto an upper tray in distillation column  24 . 
     The liquefied CO 2  bottom product stream  26  may be pumped to an elevated pressure using pump  70 . Thermal energy from the pumped bottom product stream  72  is then recovered using heat exchanger  20  to cool inlet stream  18 . The high concentration reboiler separator inlet stream  76  leaving heat exchanger  20  enters reboiler/separator  74 . The vapor from reboiler/separator  74 , stream  78  is returned to the bottom of distillation column  24 . Liquid from reboiled/separator  74  is split into secondary CO 2  refrigerant stream  108  and reboiler separation liquid stream  80 . Stream  108  is reduced in pressure with a secondary refrigerant pressure reduction device  110  providing secondary condenser refrigerant stream  112  that enters secondary condenser  98  providing cooling sufficient to produce the required reflux stream  104  that is fed to distillation column  24 . The secondary refrigerant outlet stream  114  leaving secondary condenser  98  is combined with primary refrigerant outlet stream  90  and enters inlet cross heat exchange  16  as an economizer to cool the inlet gas to the process. CO 2  gas leaving heat exchange  16  as product  92  can be compressed to combine with liquid CO 2  stream  84  or retained as a CO 2  gas product stream. 
     For a typical application with an inlet gas of 58% CO 2  at 610 psia, the process as shown in the drawing produces a hydrocarbon gas product containing 10% CO 2  at 565 psia and recovers 91% of the methane in the inlet. The CO 2  product gas stream contains 92.8% CO 2  and recovers 88.2% of the CO 2  at 200 psia. The CO 2  liquid product stream contains 92.8% CO 2  and recovers 4.6% of the CO 2  at 610 psia. This gives a total recovery of CO 2  for this example of 92.8%. A significant demand for energy in any CO 2  removal process producing gaseous CO 2  is compression of the CO 2 . CO 2  compression can be the limiting factor for projects requiring the CO 2  at elevated pressure such as enhanced oil recovery, or re-injection of the CO 2  to eliminate venting to the atmosphere. The compression requirements for this process are less than that for a traditional distillation process since the CO 2  product streams are produced at a relatively high pressure and no external refrigeration is required. 
     While the invention has been described with a certain degree of particularity, it is manifest that many changes may be made in the details of construction and the arrangement of components of the equipment and systems used in the invention, as well as the steps and sequence thereof, of practicing the methods of the invention without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for purposes of exemplification, but is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which each element or step thereof is entitled.

Technology Category: 4