Method to produce LNG at gas pressure letdown stations in natural gas transmission pipeline systems

Abstract

There is described a method to produce LNG at gas pressure letdown stations. A high pressure gas stream is pre-cooled, dewatered, and then divided into two streams: a diverted LNG production stream (LNG stream) and a gas to end users stream (User stream). Carbon dioxide is removed from the LNG stream and the LNG stream is compressed. The LNG stream is then precooled by passing through one or more heat exchangers. Hydrocarbon condensate is removed from the LNG steam by passing the LNG stream through a first Knock Out drum. The LNG stream is then depressured by passing through a JT valve to depressurize the gas vapour exiting the first Knock Out drum and discharge it into a second Knock Out drum where the LNG is captured.

Description

FIELD OF THE INVENTION

The present invention relates to a method that produces LNG at gas pressure letdown stations in natural gas transmission pipeline systems using the refrigeration generated from the expansion of the gas stream to distribution.

BACKGROUND OF THE INVENTION

Canadian Patent 2,536,075 describes a process for producing Liquid Natural Gas (LNG) at Pressure letdown stations. There will hereinafter be described an alternative method of producing LNG at gas pressure letdown stations.

SUMMARY OF THE INVENTION

There is described a method to produce LNG at gas pressure letdown stations. A first step involves pre-cooling a high pressure gas stream entering a gas pressure letdown station. A second step involves dewatering the high pressure natural gas stream after precooling. A third step involves splitting the dewatered high pressure natural gas stream into two streams: a diverted (LNG production) stream and a gas to end users stream. A fourth step involves removing carbon dioxide from the diverted (LNG production) stream. A fifth step involves compressing the diverted (LNG production) stream, which has been dewatered and had carbon dioxide removed. A sixth step involves precooling the diverted (LNG production) stream by passing the diverted (LNG production) stream through one or more heat exchangers downstream of the compressor. A seventh step involves removing hydrocarbon condensate from the diverted (LNG production) steam by passing the diverted (LNG production) stream through a first Knock Out drum so that the diverted (LNG production) stream exiting the first Knock Out drum is a gas vapour stream. An eighth step involves depressurizing the diverted (LNG production) stream by passing the diverted (LNG production) stream through a JT valve to depressurize the gas vapour exiting the first Knock Out drum and discharge it into a second Knock Out drum. A ninth step involves removing LNG from the diverted (LNG production) stream in the second Knock Out drum.

The disclosed invention provides a method for production of LNG at gas pressure letdown stations.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawing, the drawing is for the purpose of illustration only and is not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein:

FIG. 1 is a schematic diagram of a method to produce LNG at gas pressure letdown stations in natural gas transmission pipeline systems.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method to produce LNG at gas pressure letdown stations in natural gas transmission pipeline systems will now be described with reference to FIG. 1.

This alternative method of producing LNG at gas pressure letdown stations allows for LNG to be produced at a lower capital cost but at a higher operating cost than the method described in Canadian Patent 2,536,075.

Referring to FIG. 1, a typical gas pressure letdown station in a natural gas transmission pipeline. Natural gas is delivered through a high pressure transmission pipeline 1. Stream 2 is a gas stream that is first pre-heated in heater 3 before it is depressurized through JT valve 4 (typically down to 100 psi) and then routed to end users through line 6. A gas stream 5 provides the fuel required for heater 3. This simplified process arrangement as enclosed in the cloud constitutes a standard operation at gas pressure letdown stations. In the proposed invention, stream 7 is first pre-cooled in heat exchanger 8, the cooled stream 9 is then de-watered in pre-treatment unit 10. The dryed gas stream 12 is reduced in pressure at JT valve 13 at an approximate rate of 7 F for every 100 psi pressure drop. The dry, depressurized, cool, gas stream 14 is mixed with cryogenic vapors stream 35 and stream 39 to form a cooler mixture stream 15. The cold gas stream 15 is warmed in heat exchanger 16. The warmer stream 17 gains further heat through exchanger 18 and the now yet warmer stream 19 enters heat exchanger 8 for further heating. Stream 20 is now dry and at an equivalent temperature as stream 7. Nevertheless, stream 20 is further heated at exchanger 21 before being routed through stream 22 to end users stream 6.

The dry stream 11, the diverted stream is first pretreated in pre-treatment unit 23 to remove carbon dioxide. The dry, carbon dioxide free stream 24 is then compressed in compressor 25. The compressed stream 26 enters heat exchanger 21 where it is cooled. The compressed and cooled stream 27 is further cooled in heat exchanger 18. The compressed cooled stream 28 is yet further cooled in heat exchanger 16 and the colder compressed stream 29 enters knock out drum 30 to separate the condensed fraction. The vapour stream 31 is then depressurized through JT valve 32 and the two phase stream 33 enters knock out drum 34 to where a condensed LNG stream 36 is routed to storage and a cryogenic vapour stream 35 is routed and mixed with gas stream 14. The condensed fraction stream 37 is depressurized through JT valve 38 and the two phase stream 39 is mixed with streams 14 and 35 to form a mixture stream 15. The inventive step in this process is the generation and recovery of cold in conjunction with compression of a diverted gas stream to produce LNG using JT valves at gas pressure letdown stations. The use of compression and pressure reduction to generate the Joule Thompson effect is well understood and in practice in the gas industry in various forms. The advantage of the proposed invention is the process configuration which omits the use of gas expanders and replaces it with selective compression and JT valves, allowing for a lower capital cost LNG production at gas pressure letdown stations.

In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.

The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given a broad purposive interpretation consistent with the description as a whole.

Claims

1. A method to produce LNG at gas pressure letdown stations, comprising: pre-cooling a pressurized natural gas stream entering a gas pressure letdown station;dewatering the pressurized natural gas stream after pre-cooling;splitting the dewatered pressurized natural gas stream into a diverted LNG production stream and a gas to end users stream;cooling the diverted LNG production stream to produce LNG by: depressurizing the gas to end users stream by passing the gas to end users stream through an end user JT valve, the gas to end users stream being unexpanded between the pressurized natural gas stream entering the gas pressure letdown station and the end user JT valve;warming the gas to end users stream by passing the gas to end users stream through a series of heat exchangers;removing carbon dioxide from the diverted LNG production stream;compressing the diverted LNG production stream, which has been dewatered and had carbon dioxide removed, in a compressor, the series of heat exchangers being directly downstream of the compressor;precooling the diverted LNG production stream by passing the diverted LNG production stream through the series of heat exchangers directly downstream of the compressor;removing hydrocarbon condensate from the diverted LNG production stream by passing the diverted LNG production stream through a first Knock Out drum so that the diverted LNG production stream exiting the first Knock Out drum is a first gas vapour stream;depressurizing the diverted LNG production stream by passing the diverted LNG production stream directly from the first Knock Out drum through a JT valve and discharging the first gas vapour stream into a second Knock Out drum; andremoving the produced LNG from the diverted LNG production stream in the second Knock Out drum;wherein cooling the diverted LNG production stream to produce LNG consists of using reduced temperatures generated by depressurizing the gas to end users stream and the diverted LNG production stream.