Arctic cascade method for natural gas liquefaction in a high-pressure cycle with pre-cooling by ethane and sub-cooling by nitrogen, and a plant for its implementation

Abstract

A technology liquefies natural gas. The natural gas liquefaction method pre-cools treated natural gas by ethane evaporation, sub-cools liquefied gas using cooled nitrogen as a refrigerant, reduces liquefied gas pressure, separates non-liquefied gas and diverts liquefied natural gas. Before pre-cooling the natural gas is compressed, ethane is evaporated during the multi-stage pre-cooling of liquefied gas with simultaneous evaporation of ethane using cooled ethane as a refrigerant. Ethane generated by evaporation is compressed, condensed and used as a refrigerant during the cooling of liquefied gas and nitrogen, with nitrogen being compressed, cooled, expanded and fed to the natural gas sub-cooling stage. The natural gas liquefaction unit contains a natural gas liquefaction circuit, an ethane circuit and a nitrogen circuit. The natural gas liquefaction circuit includes a natural gas compressor, a cooler unit, ethane vaporizers, a closed-end subcooling heat exchanger, and a separator, connected in series.

Claims

1. A natural gas liquefaction method, which comprises compression of treated natural gas, multi-stage pre-cooling the compressed natural gas by means of evaporation of ethane in vaporizers connected in series, the ethane being used as a first refrigerant, sub-cooling the pre-cooled natural gas using cooled nitrogen as a second refrigerant, reducing pressure of the sub-cooled natural gas to form a reduced pressure mixture, separating the reduced pressure mixture into a non-liquefied natural gas and a liquefied natural gas, wherein the ethane after the evaporation is compressed, condensed and then used for the evaporation, the nitrogen is compressed, cooled by alternate feeding to the vaporizers and to nitrogen-nitrogen heat exchangers, expanded and then used for sub-cooling of the pre-cooled natural gas, further the nitrogen after the sub-cooling is used as a cooling agent in the nitrogen-nitrogen heat exchangers.

2. The method according to claim 1, wherein during the multi-stage pre-cooling the natural gas is in a single-phase state, preventing phase transition processes.

3. The method according to claim 1, wherein before the multi-stage pre-cooling the natural gas is cooled by ambient air or water of a water basin from Arctic, Antarctic, or close regions.

4. The method according to claim 1, wherein during the sub-cooling the non-liquefied natural gas in a single-phase critical state is used as a third refrigerant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other objects and features of the invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention.

(2) In the drawings,

(3) The sole FIGURE presents a schematic diagram of the proposed plant, explaining the proposed method of natural gas liquefaction.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) A natural gas liquefaction line comprises a natural gas compressor 2, an air cooler 5, ethane vaporizers 7, a closed-end sub-cooling heat exchanger 9, for example, a multithread one, and a separator 11, connected in series.

(5) An ethane circuit comprises at least one ethane compressor 4 (two compressors 4 connected in series are shown in the sole FIGURE), an air cooler 13, and said vaporizers 7, outlets of which are connected to inputs of at least one compressor 4, connected in series. As is shown on the diagram, an outlet of the first vaporizer 7 is connected to an inlet of the second compressor 4, while outlets of remaining vaporizers 7 are connected to steps of the first compressor 4.

(6) A nitrogen circuit includes at least one nitrogen compressor 3 (two compressors 3 connected in series are shown in the sole FIGURE), an air-cooler 14, said ethane vaporizers 7, between which nitrogen-nitrogen heat exchangers 8 are connected, a turboexpander of an expander-compressor unit 10, said closed-end sub-cooling heat exchanger 9, said nitrogen-nitrogen heat exchangers 8 and a turbocompressor of the expander-compressor unit 10 connected to an inlet of the first nitrogen compressor 3.

(7) A BOG outlet of a separator 11 is connected with the closed-end sub-cooling heat exchanger 9 which has its BOG outlet connected to a BOG compressor 15.

(8) Further, a drive of all compressors 2, 3, 4 is a gas turbine engine 1 connected to a multiplier 6 that distributes power to each compressor 2, 3, 4.

(9) The natural gas liquefaction method is as follows.

(10) The natural gas (NG) pretreated for liquefaction (with vapors of water, carbon dioxide and other contaminants removed) is fed to the natural gas compressor 2, compressed to required pressure, cooled by the ambient cold in the air or water cooler unit or units 5, to a temperature c. +10° C. and sent to the ethane vaporizers 7 for pre-cooling. After sequential cooling in the vaporizers 7, the gas with a temperature c. −84° C. is fed to the closed-end gas sub-cooling heat exchanger 9 where it is sub-cooled with nitrogen and BOG to a temperature of c. −137° C. Then the gas pressure is reduced at the throttle to c. 0.15 MPag, while its temperature drops to c. −157° C., after which the gas-liquid stream enters the end separator 11. From the separator 11 LNG is supplied to storage tanks by a pump 12, while the non-liquefied part of the gas is delivered to the end heat exchanger 9, dissipates cold to the liquefied gas stream and is compressed by the BOG compressor 15 to a pressure of c. 3.0 MPag. Part of the boil-off gas is delivered to a unit fuel system, while another part goes to recycling at the start of the liquefaction process.

(11) The pre-cooling circuit uses ethane as the refrigerant. Gaseous ethane from vaporizers 7 with different pressures enters the multistage compressor 4 (compressors), where it is compressed to a pressure of c. 3 MPag, and is condensed in air coolers 13 at a temperature of +10° C. and lower. Liquid ethane is sent to the vaporizers 7, where nitrogen cools the gas to a temperature of c. —84° C., at various pressure levels. The gaseous ethane from the vaporizers 7 is fed to the compressor 4 (compressors) and further along the cycle.

(12) The nitrogen compressed by compressors 3 to c. 10 MPa is cooled in air-coolers 14, alternately enters ethane vaporizers 7 and nitrogen-nitrogen heat exchangers 8, and, cooled by the nitrogen return stream and in ethane vaporizers 7 to a temperature of c. −84° C., enters the turboexpander, where the nitrogen booster turbocompressor serves as a load in the expander-compressor unit 10. After reducing the expander pressure to 2.6 MPa and cooling to −140° C., the nitrogen enters the closed-end multithread sub-cooling heat exchanger 9. After dissipating cold to the liquefied gas stream, the nitrogen passes through recuperative nitrogen-nitrogen heat exchangers 8, enters the turbocompressor of the expander-compressor unit 10, is compressed to a pressure of c. 3 MPag, enters the inlet of the compressor 3, is additionally compressed to 10 MPag and is sent to the cycle.

(13) The process operates in nominal mode at an ambient temperature of +5° C. and below. At temperatures above +5° C., the performance of the process train starts declining. Since the technology is developed for the Arctic and Antarctic latitudes, the waters of the Arctic or Antarctic seas, bays and other water bodies, which have low temperatures even in summer, can also be used for ethane condensation in units 13 in a hot summer period.

(14) In order to optimize the kinematic circuit and to reduce the quantity of rotating equipment, all the compressors 2, 3, 4 used for gas, ethane and nitrogen compressing can be driven by a single gas turbine engine 1, with power to be distributed to each compressor through the multiplier 6.

(15) The estimated energy consumption of LNG production using the Arctic Cascade technology is about 220 kW per ton.

(16) Although only a few embodiments of the present invention have been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.