Method And Apparatus for High Purity Liquefied Natural Gas

Abstract

A novel method and system for liquefying and distilling natural gas into high purity liquid methane (LNG) and NGL product streams. Heat exchangers and distillation towers are configured to produce high purity liquefied natural gas (LNG) and NGL product streams, while also rejecting excess nitrogen contained in the inlet gas stream, utilizing liquid nitrogen as the process refrigerant. A molecular sieve pretreatment system is configured to utilize the vaporized nitrogen stream for regeneration of the molecular sieve beds which are designed for removing water and carbon dioxide from the inlet gas stream

Description

CROSS-REFERENCE

[0001] This application is a continuation in part to U.S. patent application Ser. No. 12/765,750, filed on Apr. 22, 2010, priority of which is claimed herein and the entirety of which is hereby incorporated by reference.

DESCRIPTION OF RELATED ART

[0002] The present application relates to liquefaction of natural gas, more particularly, to a more environmentally friendly method and system for providing high purity Liquefied Natural Gas (LNG) with reduced cost by utilizing liquid nitrogen (LIN) as the LNG condensing medium and process refrigerant.

[0003] Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.

[0004] The advent of high cost in transportation fuels such as gasoline and diesel (based on high petroleum costs) and the coincident very low cost in natural gas has prompted the transportation industry to switch fuels to compressed and liquefied natural gas. Besides the lower cost, another benefit of natural gas as a fuel is that it is a cleaner and safer burning fuel, as it is lighter than the air.

[0005] Natural gas is generally converted into liquefied form for storage and transportation. Liquefying reduces the volume of natural gas by a factor of 600, allowing for high efficiency and reduced cost in these areas. However, the liquefaction of natural gas into a Liquefied Natural Gas (LNG) product is currently performed using large and capital expenditure intensive equipment. The inlet gas is typically treated with an amine facility and molecular sieves to remove excess sulfur, CO.sub.2 and water; cold scrubber columns in the cold box are used to remove heavy hydrocarbons sufficiently to prevent freezing and fouling in the passages of the cryogenic heat exchangers. The cryogenic refrigeration used in conventional LNG facilities can be provided by various technologies but generally utilize a refrigerant that requires compression, heat rejection using large air coolers or cooling water, expansion, vaporization of proprietary refrigerants, etc. The typical natural gas liquefier must be constructed in a large scale to be economical. More importantly, the LNG from a typical liquefier will generally only meet an industrial or commercial grade standard sufficient for gas utility of a power plant use, not sufficient high quality enough for vehicle use.

[0006] The transportation industry requires a LNG product that has a much higher concentration of methane and lower concentrations of ethane, propane, butanes, pentanes, hexanes, etc. than industrial or commercial grade LNG. Also a more distributed production of LNG, in smaller quantities, is preferred to achieve a higher degree of market penetration by reducing cost in LNG fuel transportation. The cost of trucking LNG fuel from a large distant LNG facility is prohibitive.

[0007] There is a need to produce high quality LNG in an environmentally friendly way as well as at a reduced cost. There is also a need to recover other components, such as ethane and heavier hydrocarbon components from natural gas as a Natural Gas Liquids (NGL product).

[0008] Nitrogen is the most abundant element in air and is distilled, purified and liquefied in air separation facilities throughout the World, making it generally available anywhere a LNG Plant is built. The cryogenic properties of liquid nitrogen render it a much safer and environmentally friendlier alternative refrigerant to the current refrigerants used in the liquefying and recovery of natural gas. A system and method is needed to take advantage of these properties of liquid nitrogen.

SUMMARY

[0009] The present application discloses new approaches to take advantage of a commonly available and widely distributed industrial gas, nitrogen in liquid form. The invention takes advantage of liquid nitrogen (LIN) for use as a refrigerant for liquefaction of natural gas (LNG) and to recover other components as natural gas liquid products (NGL).

[0010] The system uses liquid nitrogen refrigerant for the purpose of assisting in the total liquefaction of an inlet hydrocarbon gas stream and the distillation of said liquid stream(s) into Natural Gas Liquids (NGL's) and vehicle quality Liquefied Natural Gas (LNG). The system recovers >99% of the ethane (C2) component into the NGL product stream, while meeting all pipeline specifications for other contaminants. The NGL product is also known as Y-Grade product. The liquid nitrogen refrigerant is capable of sub-cooling the reflux stream to the distillation tower (Demethanizer), resulting in an overhead vapor product stream that contains <0.1% of the C2 component.

[0011] The pretreatment system can be regenerated using the vaporized refrigerant, which is then vented safely to the atmosphere or recycled for re-use. The volume of nitrogen is sufficient to the extent that the regeneration of the pretreatment system can be performed at significantly lower temperatures than what is typically seen for pretreatment systems. The pretreatment system does not require a Regeneration Gas Cooler or a Regeneration Gas Scrubber as part of the pretreatment system. High temperature switching valves are no longer required for the pretreatment system.

[0012] The disclosed system can be utilized to remove light end contaminants (such as nitrogen) from the LNG product stream (to meet pipeline and transport specifications) through use of a secondary fractionation column (NRU Column). Liquid refrigerant can be used as the condensing medium in the NRU column condenser. Condenser may be integral or external to column. Light end contaminant removal system (NRU) can be operated such that hydrocarbon content in the overhead vapor stream is <0.1%, while providing a bottoms liquid product that contains as little or as much contaminant as desired.

[0013] The liquid LNG product can be further sub-cooled, using liquid refrigerant as the cooling source, before being fed to LNG storage. The liquid LNG product can be pumped to pipeline pressure before being vaporized in the process exchanger and sent to gas sales. The need for residue gas compression can be eliminated. The system allows for fully integrated heat exchange using Process Heat Exchangers and a minimal number of other pieces of process equipment, thus maintaining a minimal Plot Area. And since all feed streams to the Demethanizer are liquid, the tower is smaller and there is no need for a "bell section" as is typical with other process designs.

[0014] The system does not use any turbo expander machinery or any equipment typically associated with this type of gas processing facility. The entire gas stream can be liquefied at relatively low pressures (250-350 psig preferred), allowing inlet compression requirements to be reduced 30% to 100% compared to other typical processes used for LNG and NGL recovery. As the refrigerant system is an "open loop" design with the refrigerant as a consumable, there is no need for refrigerant compression within the plant design. Vaporized refrigerant and/or nitrogen from the NRU overhead can be captured and routed back to the refrigerant supplier for reclamation. The system provides almost a 100 percent recovery of all components while minimizing energy input and reducing overall atmospheric emissions. This invention can also apply to recovery of ethane plus without recovering methane (LNG) as a liquid. As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given. It is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

[0015] This method and system will meet and exceed all of the functions of a typical large LNG facility but on a smaller scale. This production system eliminates the processes of compression, expansion of special refrigerants, recycling of regeneration gas, etc. from the liquefaction and fractionation process and takes advantage of the low cost LIN from the air separation facilities. Being an inert gas, a process that utilizes nitrogen refrigeration may be regarded as inherently safer than plants that use flammable and explosive liquid hydrocarbon refrigerants.

[0016] In one embodiment, a system for liquefying and distilling natural gas includes a molecular sieve pretreatment system that utilizes vaporized nitrogen refrigerant for regeneration. The pretreatment system is designed to remove water and carbon dioxide (CO.sub.2) from the inlet gas in order to prevent freezing in the LNG liquefaction process. The system also includes heat exchangers and towers that provide heat exchange and fractionation of LNG (to reach over 99% methane purity), to recover other NGL products and reject nitrogen from the product streams.

[0017] In one embodiment, a system for liquefying and distilling natural gas includes a NGL processing facility to further store and recover other natural gas components as liquid products (NGL).

[0018] In one embodiment, a Nitrogen Rejection Unit (NRU) is designed to remove nitrogen from the LNG product. Recovered nitrogen gas from the NRU may be recycled into the liquid nitrogen feed stream.

[0019] In one embodiment, a system for liquefying and distilling natural gas allows for the recovery of more than 99% of the contained ethane, and also allows for the recovery of other natural gas components as a liquid NGL product that contains less than 0.5 LV % methane in ethane.

[0020] In the present system, the use of vaporized nitrogen refrigerant for the regeneration of the pre-treatment system eliminates the requirement of a Regeneration Gas Cooler or a Regeneration Gas Scrubber typically found in a conventional system.

[0021] Because nitrogen is environmentally safe, the system can operate under an "open loop" design, and the need for refrigerant compression within the plant design can therefore be eliminated. However, the nitrogen can also be recycled back to a Nitrogen Liquefier or other system for reuse.

[0022] The present system will provide almost a 100% recovery of all components of natural gas while minimizing energy input and reducing overall atmospheric emissions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The disclosed application will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:

[0024] FIG. 1 schematically shows an exemplary process for the condensation, distilling and fractionation of natural gas to produce high quality LNG and pipeline quality Y-Grade NGL product, along with rejection of nitrogen from the LNG product, using liquid nitrogen as a refrigerant in accordance with this application.

DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS

[0025] The numerous innovative teachings of the present application will be described with particular reference to presently preferred embodiments (by way of example, and not of limitation). The present application describes several embodiments, and none of the statements below should be taken as limiting the claims generally.

[0026] For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and description and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale, some areas or elements may be expanded to help improve understanding of embodiments of the invention.

[0027] The terms "first," "second," "third," "fourth," and the like in the description and the claims, if any, may be used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable. Furthermore, the terms "comprise," "include," "have," and any variations thereof, are intended to cover non-exclusive inclusions, such that a process, method, article, apparatus, or composition that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, apparatus, or composition.

[0028] The necessary materials and facilities for gas feeding and gas pipes, heat exchange material, controlling valves are known arts in the field. Other enabling descriptions may be found in the US Patent Application Publication US 2011/0259044 A1 the entirety of which is hereby incorporated by reference.

[0029] FIG. 1 illustrates an exemplary embodiment of a natural gas liquefaction system 100 that includes a molecular sieve pretreatment system 101, liquefaction heat exchanger system 103 NB, cold separator 105, demethanizer 107 and nitrogen rejection unit 109.

[0030] The molecular sieve pretreatment system 101 is designed to remove water and up to two percent (2%) carbon dioxide (CO2) from the inlet gas in order to prevent freezing in the LNG liquefaction process. Inlet gas 1 enters the Inlet Gas Filter/Coalescer to remove any possible solids materials and/or entrained liquids that may be contained in the feed gas 1.

[0031] The pretreatment system 101 uses molecular sieve beds and is regenerated by vaporized nitrogen refrigerant. Due to the volume and mass of nitrogen available for regeneration, only minimal heat needs to be added to fully regenerate the mol sieves.

[0032] The treated inlet gas 2 exits the molecular sieve vessel, then flows to one of the Dust Filters which remove molecular sieve fines. The filtered gas 2 then flows to the liquefaction plant.

[0033] The sieve beds are regenerated by the vaporized nitrogen refrigerant stream 3. The methane filled beds are first depressured, with the methane gas being routed through the Regeneration Gas Purge Compressor and back to the inlet of the Inlet Gas Filter/Coalescer. Once the pretreatment bed has been depressured, a slip stream of nitrogen gas stream 3 is introduced into the bed to act as a system purge, to remove the last remnants of methane. The purge nitrogen/methane gas is then routed through the Regeneration Gas Purge Compressor and sent back to the entrance of the plant. After the purge is complete, the full stream of nitrogen regeneration gas 3 is routed to the Regeneration Gas Heater where it is heated to between 200.degree. F. and 400.degree. F. The heated nitrogen gas stream 3 then flows through the bed, stripping the adsorbed water and CO.sub.2. The vaporized nitrogen stream 3 is also used to cool the bed after regeneration. The exiting nitrogen gas stream may be vented directly to atmosphere with no environmental impacts. Alternatively, the nitrogen could flow to a Regeneration Gas Cooler and on to a Nitrogen Recycle Compressor where it is compressed and cooled for return to the Air Separation Plant/Nitrogen Liquefier.

[0034] The inlet gas 2 from pretreatment is chilled and partially condensed in the first process heat exchanger(s) system 103 NB, exchanging heat with several product and process streams, before being pressure controlled to the Cold Separator 105. The vapor portion stream 4 off the Cold Separator 105 flows to the Demethanizer Reflux Condenser where the stream is condensed and sub-cooled then fed to the top of the Demethanizer 107. Cold liquids from the Cold Separator 105 are flashed, via level control, and are routed to an intermediate point of the Demethanizer 107. The vapor from this stream provides additional stripping gas to the upper sections of the tower and the liquids provide additional reflux to the lower section of the Demethanizer.

[0035] The Demethanizer 107 is a distillation tower with multiple sections that operates at approximately 250 psig. The tower is provided with a Side Reboiler and Bottom Reboiler. These exchangers are provided heat by the feed gas stream. The sub-cooled liquid reflux from Demethanizer Reflux Condenser allows the system to recover more than 99% of the contained ethane in the bottom product, while still generating an NGL product that contains less than 0.5 liquid volume percent (LV %) methane/ethane. The vapor overhead from the Demethanizer is more than 99% methane, dependent on the nitrogen content in the inlet gas 2.

[0036] Through utilization of multiple pass heat exchange in the heat exchangers 103 NB, and distillation/fractionation in the Demethanizer 107, pretreated inlet gas 2 is condensed, fractionated and distilled into a high quality overhead methane product 50 and a bottoms NGL liquid product 52. An example operation gas flow process is as follows. Between the Pretreatment System 101, heat exchanger system 103 NB, Cold Separator system 105, Demethanizer 107 and Nitrogen Rejection Unit (NRU) 109 several flow loops are formed. Inlet gas 2 from the Pretreatment System 101 is routed to the first Heat Exchanger 103A, where it is partially condensed using process streams and nitrogen refrigerant. The flow of vapor 4 out of the Cold Separator 105 is maintained by use of a temperature control valve around the first Heat Exchanger 103A. Process streams used to condense the inlet gas 2 in Heat Exchanger 103A include the Demethanizer side reboiler 30 and bottom reboiler 20 streams, the Residue Gas stream 51, the Y-Grade Liquid Product stream 52 and nitrogen refrigerant stream 25.

[0037] Liquid stream 10 recovered out of the bottom of the Cold Separator 105 is routed, via level control, to the middle section of the Demethanizer 107. Vapor stream 4 from the Cold Separator is sent to the second Heat Exchanger 103B where it is condensed and sub-cooled against other process streams including the NRU reboiler stream 60 and nitrogen refrigerant stream 62. The condensed, sub-cooled vapor stream 56 is fed to the top of the Demethanizer 107 as tower reflux. The Demethanizer 107 fractionates the various components into an overhead vapor stream 50 containing essentially all of the methane and nitrogen components and a bottoms liquid stream 52 that contains essentially all of the ethane and heavier hydrocarbon components contained in the inlet gas 2.

[0038] The Demethanizer overhead vapor stream 50 is sent to the Heat Exchanger 103B where it is condensed, using the NRU Reboiler stream 60 and Liquid Nitrogen Refrigerant stream 62 as the condensing medium, and fed to the NRU Column 109. Here the essentially binary methane and nitrogen feed stream is fractionated into individual nitrogen vapor stream 63 and a liquefied methane stream 64. The NRU 109 is provided to drive off nitrogen from the LNG product and uses a tower reboiler heated in Exchanger 103B by the Demethanizer overhead vapor stream 50 and Cold Separator overhead vapor stream 4.

[0039] The liquid product 64 from the bottom of the NRU reboiler is sub-cooled in the Heat Exchanger 103B, using liquid nitrogen refrigerant stream 62 as the cooling medium, and routed to LNG storage. As an alternative, the LNG product stream 64 can be routed from the NRU 109 to the first Heat Exchanger 103A and used as refrigerant to condense Inlet Gas 2, thus reducing the amount of nitrogen 25 required for the system. The vaporized LNG 70 can then be sold directly into a pipeline.

[0040] The NRU overhead stream is partially condensed, using liquid nitrogen refrigerant stream 65, to minimize the hydrocarbon content in the overhead vapor stream. The overhead stream 63 from the NRU 109 is then blended with the liquid nitrogen stream 65 feeding the NRU Condenser and subsequently combined with the remainder of the liquid nitrogen refrigerant utilized throughout the rest of the process.

[0041] None of the description in the present application should be read as implying that any particular element, step, or function is an essential element which must be included in the claim scope: THE SCOPE OF PATENTED SUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none of these claims are intended to invoke paragraph six of 35 USC section 112 unless the exact words "means for" are followed by a participle.

[0042] The claims as filed are intended to be as comprehensive as possible, and NO subject matter is intentionally relinquished, dedicated, or abandoned.

Claims

1. A method of processing a hydrocarbon gas stream comprising an initial amount of methane, an initial amount of ethane and heavier hydrocarbon components, and an initial amount of contaminant comprising nitrogen, the method comprising: introducing the hydrocarbon gas stream to a first heat exchange unit; introducing a first stream of nitrogen refrigerant to at least the first heat exchange unit to aid in cooling of the hydrocarbon gas; flowing the cooled hydrocarbon gas from the first heat exchange unit to a separator and separating a liquid portion from a vapor portion; flowing the liquid portion from the separator to a demethanizer; liquefying all of the vapor portion from the separator by heat exchange with at least a second nitrogen refrigerant stream in a second heat exchange unit to achieve total liquefaction and subcooling, thereby forming all of the vapor portion from the separator into a totally subcooled liquid reflux stream to the demethanizer, wherein the first and second nitrogen refrigerant stream may be dependent or independent streams; flowing a vapor product from the demethanizer, wherein as the vapor product exits the demethanizer it comprises essentially all of the initial amount of methane and essentially all of the initial amount of contaminant; flowing a liquid NGL product from the demethanizer, wherein as the liquid NGL product exits the demethanizer it comprises essentially all of the initial amount of ethane and heavier hydrocarbon components; liquefying the vapor product of the demethanizer; flowing the liquefied vapor product to a nitrogen recovery unit; flowing a liquid LNG stream from the nitrogen recovery unit; flowing a vapor contaminant stream from the nitrogen recovery unit; combining the vapor contaminant stream from the nitrogen recovery unit into the second nitrogen refrigerant stream; and, cooling the liquid LNG stream from the nitrogen recovery unit in the second heat exchange unit.

2. The method of claim 1, further comprising passing the hydrocarbon gas stream through a pretreatment system prior to introducing the hydrocarbon gas to the first heat exchanger, and further comprising vaporizing at least a portion of the first or second nitrogen refrigerant streams and regenerating the pretreatment system with the vaporized nitrogen refrigerant.

3. The method of claim 2, further comprising regenerating the pretreatment system at a temperature of 400 degrees F. or less with the vaporized nitrogen refrigerant.

4. The method of claim 1, wherein the NGL product contains a percentage of ethane and methane.

5. The method of claim 4, further comprising recovering greater than 99% of the ethane from the NGL product

6. The method of claim 4, wherein the NGL product contains less than 0.5 liquid volume percent methane.

7. The method of claim 1, wherein liquefying the vapor' product of the first pressurized distillation tower comprises forming an LNG stream containing a percentage of methane.

8. The method of claim, wherein the LNG stream contains less than 0.1% ethane.

9. The method of claim 1, further comprising forming the LNG stream containing a percentage of nitrogen.

10. (canceled)

11. (canceled)

12. (canceled)

13. The method of claim 1, further comprising further cooling LNG of the LNG stream with the first or second nitrogen refrigerant streams prior to the LNG being stored in a storage unit.

14. The method of claim 1, further comprising pumping the LNG of the LNG stream to a transportation pipeline pressure, vaporizing the LNG, and transporting to sale.

15. The method of claim 1, further comprising the ability to liquefy the inlet hydrocarbons gas into LNG at a pressure of 350 psig or less.

16. The method of claim 1, further comprising vaporizing at least one of the nitrogen refrigerant streams through at least one of the heat exchange units, the demethanizer, the nitrogen recovery unit pressurized distillation towers, or a combination thereof.

17. The method of claim 16, further comprising recycling at least a portion of the vaporized nitrogen refrigerant for reclamation and re-use as nitrogen refrigerant.

18. The method of claim 16, further comprising venting at least a portion of the vaporized nitrogen refrigerant to the atmosphere.

19-21. (canceled)