Floating liquefied natural gas pretreatment system

Abstract

A pretreatment system and method for a floating liquid natural gas (“FLNG”) facility are presented. The inlet natural gas stream flows through a membrane system to remove carbon dioxide and a heat exchanger, producing first and second cooled CO.sub.2-depleted non-permeate streams. The first cooled CO.sub.2-depleted non-permeate stream is routed to additional pretreatment equipment, while the second cooled CO.sub.2-depleted non-permeate stream is routed directly to a LNG train. Alternatively, the inlet natural gas stream may flow through a membrane system to produce a single cooled CO.sub.2-depleted non-permeate stream that is routed to the LNG train after sweetening and dehydration. Because the pretreatment system delivers the incoming gas stream to the LNG train at a lower temperature than conventional systems, less energy is needed to convert the gas stream to LNG. In addition, the pretreatment system has a smaller footprint than conventional pretreatment systems.

Claims

1. A pretreatment system for a floating liquid natural gas (“FLNG”) facility, the pretreatment system comprising: one or more membranes configured to receive an inlet natural gas stream; a heat exchanger configured to receive and cross-exchange heat between a cooled permeate stream and a cooled non-permeate stream from the one or more membranes and a water-removed natural gas outlet stream to produce: a first heated permeate stream, a first cooled non-permeate stream, and a second cooled non-permeate stream; an additional pretreatment process equipment; means to route the first cooled non-permeate stream to the additional pretreatment process equipment; and means to route the second cooled non-permeate stream to an LNG train.

2. A pretreatment system according to claim 1, wherein the additional pretreatment process equipment includes a gas dehydrator configured to provide the water-removed natural gas outlet stream to the heat exchanger.

3. A pretreatment system according to claim 1 wherein the one or more membranes includes a CO.sub.2 removal membrane.

4. A pretreatment system according to claim 1 further comprising: a mercury remover configured to receive the inlet natural gas stream and provide a mercury-removed natural gas stream to the one or more membrances.

5. A pretreatment system according to claim 4 wherein the mercury remover includes a mercury/H.sub.2S removal bed.

6. A pretreatment system according to claim 4 wherein at least one of the first and second cooled non-permeate streams is used to cool the mercury-removed natural gas stream.

7. A pretreatment system according to claim 1 wherein the additional pretreatment process equipment is chosen from the group consisting of a gas sweetener and a gas dehydrator.

8. A pretreatment system according to claim 7 wherein the gas sweetener includes: a membrane configured to removing carbon dioxide; and an amine unit configured to remove hydrogen sulfide.

9. A pretreatment system according to claim 1 wherein the second cooled non-permeate stream has a temperature ranging from about 15° C. to 40° C. (60° F. to 104° F.) when it is delivered to the LNG train.

10. A pretreatment system according to claim 9 wherein the second cooled non-permeate stream has a temperature ranging from about 15° C. to 21° C. (60° F. to 70° F.) when it is delivered to the LNG train.

11. A pretreatment system according to claim 1 wherein at least one of the first and second cooled non-permeate streams is used to cool the inlet natural gas stream.

12. A pretreatment system for a floating liquid natural gas (“FLNG”) facility, the pretreatment system comprising a mercury remover configured to receive an inlet natural gas stream; one or more membranes configured to receive a mercury-removed natural gas stream from the mercury removal system; a heat exchanger configured to receive and cross-exchange heat between a cooled permeate stream and a cooled non-permeate stream from the one or more membranes and water-removed natural gas outlet stream to produce: a first heated permeate stream, a first cooled non-permeate stream, and a second cooled non-permeate stream; a gas sweetener configured to receive the first cooled non-permeate stream; and a gas dehydrator configured to receive a sweetened natural gas stream and produce the water-removed natural gas outlet stream.

13. A pretreatment system according to claim 12 wherein the one or more membranes includes a CO.sub.2 removal membrane.

14. A pretreatment system for a floating liquid natural gas (“FLNG”) facility, the pretreatment system comprising one or more membranes configured to receive an inlet natural gas stream; and a heat exchanger configured to receive and cross-exchange heat between a cooled permeate stream and a cooled non-permeate stream from the one or more membranes and a water-removed natural gas outlet stream to produce: a first heated permeate stream, a first cooled non-permeate stream, and a second cooled non-permeate stream.

15. A pretreatment system according to claim 14 further comprising: a mercury remover configured to provide a mercury-removed inlet natural gas stream to the one or more membranes.

16. A pretreatment system according to claim 14 further comprising: a gas sweetener configured to receive the first cooled non-permeate stream.

17. A pretreatment system according to claim 16 further comprising: a gas dehydrator configured to receive a sweetened natural gas stream and produce the water-removed natural gas outlet stream.

18. A pretreatment system according to claim 14 wherein the one or more membranes includes a CO.sub.2 removal membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) So that the manner in which the above recited features can be understood in detail, a more particular description may be had by reference to embodiments, some of which are illustrated in the appended drawings, wherein like reference numerals denote like elements. It is to be noted, however, that the appended drawings illustrate various embodiments and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments.

(2) Preferred embodiments of the invention will now be described in further detail. Other features, aspects, and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and accompanying drawings (which are not to scale) where:

(3) FIG. 1 is a general layout of a FLNG facility showing a pretreatment system located upstream of the LNG train, according to an embodiment of the present invention.

(4) FIG. 2 is a simplified process flow diagram of a preferred embodiment of a pretreatment system, according to an embodiment of the present invention. The pretreatment system purifies and cools the inlet natural gas stream before it enters the LNG train, and is well-suited for use in the FLNG facility of FIG. 1.

(5) FIG. 3 is a simplified process flow diagram of another preferred embodiment of a pretreatment system, according to an embodiment of the present invention. The pretreatment system of FIG. 3 is also well-suited for use in the FLNG facility of FIG. 1.

DETAILED DESCRIPTION

(6) It is to be understood that the invention that is now to be described is not limited in its application to the details of the construction and arrangement of the parts illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. The phraseology and terminology employed herein are for purposes of description and not limitation.

(7) Elements shown by the drawings are identified by the following numbers: 10 From FLNG production/topside 19 Inlet natural gas stream 20 Mercury removal system 21 Substantially mercury-free natural gas stream 30 Membrane system 31 Cooled CO.sub.2-rich permeate stream 33 Cooled CO.sub.2-depleted non-permeate stream 40 Heat exchanger 41 First cooled CO.sub.2-depleted non-permeate stream 43 Second cooled CO.sub.2-depleted non-permeate stream 45 Heated CO.sub.2-rich permeate stream 50 Gas sweetening system 51 Sweetened natural gas stream 60 Gas dehydration system 61 Substantially water-free natural gas outlet stream 70 To LNG train 71 Single cooled CO.sub.2-depleted non-permeate stream 80 LNG gathering system

(8) In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.

(9) This invention describes a pretreatment system that delivers the incoming gas stream to the LNG train at a lower temperature than conventional systems. As a result, less energy is needed to convert the gas stream to LNG. In addition, the pretreatment system has a smaller footprint than conventional pretreatment systems.

(10) As shown in FIG. 1, a FLNG system aboard a sea-going vessel includes areas for power generation and utilities, pretreatment of the incoming natural gas stream, liquefaction of the natural gas into LNG, and LNG storage. The system also includes means for flaring excess natural gas and means, such as a cryogenic export system, for transferring LNG from the vessel to the next point of delivery. The pretreatment system may be located anywhere on the vessel but is generally placed near the LNG train. FIG. 2 and FIG. 3 show potential embodiments of the pretreatment system described herein.

(11) As shown in FIG. 2, a pretreatment system to cool and purify the inlet natural gas stream may include multiple components. One component may be a mercury removal system 20, which may be a PURASPEC.sub.JM™ mercury/H.sub.2S removal bed (Johnson Matthey Catalysts, Houston, Tex.) or its equivalent. The mercury removal system 20 removes the mercury from the inlet natural gas stream 19 to form a substantially mercury-free natural gas stream 21. The substantially mercury-free natural gas stream 21 may then be passed through a membrane system 30. Alternatively, the mercury/H.sub.2S removal bed may be installed downstream of the gas dehydration system 60.

(12) The membrane system 30 may be primarily responsible for removing carbon dioxide from the substantially mercury-free natural gas stream 21. As an example, the membrane system 30 is preferably one or more CYNARA® CO.sub.2 removal membrane systems (Cameron Process Systems, Houston, Tex.) or its equivalent. As the substantially mercury-free natural gas stream 21 passes through the membrane system 30, it is naturally cooled to produce a cooled CO.sub.2-rich permeate stream 31 and a cooled CO.sub.2-depleted non-permeate stream 33. Both non-permeate and permeate streams 31, 33 may then be fed to a heat exchanger 40.

(13) In the heat exchanger 40, heat from the cooled CO.sub.2-rich permeate stream 31 and the cooled CO.sub.2-depleted non-permeate stream 33 is cross-exchanged with a substantially water-free natural gas outlet stream 61 from a gas dehydration system 60 to produce a first cooled CO.sub.2-depleted non-permeate stream 41, a second cooled CO.sub.2-depleted non-permeate stream 43, and a heated CO.sub.2-rich permeate stream 45. The heated CO.sub.2-rich permeate stream 45 leaves the pretreatment system and is used as fuel or can be reinjected.

(14) After leaving the heat exchanger 40, the first cooled CO.sub.2-depleted non-permeate stream 41 may pass through a gas sweetening system 50 that produces a sweetened natural gas stream 51. As an example, the gas sweetening system 50 may be a hybrid system that includes a membrane system for removing carbon dioxide and hydrogen sulfide followed by an amine unit for removing carbon dioxide and hydrogen sulfide to the level required by the downstream refrigeration unit. The sweetened natural gas stream 51 may then pass through a gas dehydration system 60. The gas dehydration system 60 removes water from the sweetened natural gas stream 51 to produce a substantially water-free natural gas outlet stream 61 which is fed to the heat exchanger 40. The gas dehydration system 60 may include, but is not limited to, molecular sieves and hydrocarbon dew pointing.

(15) The second cooled CO.sub.2-depleted non-permeate stream 43 may leave the heat exchanger 40 and be routed directly to a LNG train 70. Because the second cooled CO.sub.2-depleted non-permeate stream 43 has been cooled by the membrane system 30 and further cooled by the heat exchanger 40, the temperature at which it is delivered to the LNG train 70 is significantly reduced when compared to conventional pretreatment systems. For example, the temperature of the second cooled CO.sub.2-depleted non-permeate stream 43 may be about 15° C. to 21° C. (60° F. to 70° F.), while the temperature at which a conventional pretreatment system delivers natural gas to a LNG train is about 37° C. to 48° C. (100° F. to 120° F.). Delivering the second cooled CO.sub.2-depleted non-permeate stream 43 to the LNG train 70 at a cooler temperature reduces the overall LNG refrigeration load and the amount of energy required.

(16) As another example, the temperature of the outlet gas from a conventional pretreatment system may range from about 41° C. to 43° C. (105° F. to 110° F.). A dual-mixed refrigerant system with a design capacity of two million tons per annum (“MTPA”) LNG requires 62 megawatts (“MW”) of energy to convert outlet gas at that temperature to LNG. In contrast, as shown in the table below, the outlet gas temperature for the pretreatment system of the present invention ranges from about 26° C. to 35° C. (78° F. to 95° F.) as carbon dioxide inlet and outlet concentrations vary:

(17) TABLE-US-00001 Predicted Outlet Predicted Outlet Feed Gas CO.sub.2 Product Gas CO.sub.2 Gas Temperature Gas Temperature (mol %) (mol %) (° C.) (° F.) 20 7 26 78 15 7 29 84 10 7 32 89 7 3 33 91 5 3 35 95
Because the temperature of the outlet gas is lower, the LNG train requires less energy to convert the outlet gas to LNG. This energy savings when compared to the conventional pretreatment system is about five to fifteen percent.

(18) Like the second cooled CO.sub.2-depleted non-permeate stream 43, the first cooled CO.sub.2-depleted non-permeate stream 41 may also pass through the membrane system 30 and the heat exchanger 40, reducing its temperature before it enters the gas dehydration system 60. In addition, the second cooled CO.sub.2-depleted non-permeate stream 43 entirely bypasses the gas dehydration system 60. As a result, the gas dehydration system 60 has to treat less gas, and the gas that it does treat is at a lower temperature. The size of the gas dehydration system 60 may therefore be reduced, and the pretreatment system requires less floor space than conventional pretreatment systems.

(19) As an alternative, the first and second cooled CO.sub.2-depleted non-permeate streams 41, 43 may be used to cool the inlet natural gas stream 19, the substantially mercury-free natural gas stream 21, or both. If the natural gas is rich in heavy hydrocarbons, this cooling results in heavy hydrocarbon and water condensation in the inlet gas stream. This reduces the amount of water and heavy hydrocarbons that enter the pretreatment system, thereby reducing the overall processing load on the pretreatment section of the FLNG system.

(20) An alternate embodiment of a FLNG pretreatment system to cool and purify an inlet stream of natural gas is shown in FIG. 3. The inlet natural gas stream 19 from the LNG gathering system 80 may first be sent to a mercury removal system 20, which removes the mercury from the inlet natural gas stream 19 to form a substantially mercury-free natural gas stream 21. The substantially mercury-free natural gas stream 21 may then be passed to a membrane system 30, which is primarily responsible for removing carbon dioxide from the substantially mercury-free natural gas stream 21. The membrane system 30 also naturally cools the substantially mercury-free natural gas stream 21 to produce a single cooled CO.sub.2-depleted non-permeate stream 71. The single cooled CO.sub.2-depleted non-permeate stream 71 may then pass through a gas sweetening system 50 that produces a sweetened natural gas stream 51. The sweetened natural gas stream 51 may then pass through a gas dehydration system 60, which removes water from the sweetened natural gas stream 51 to produce a substantially water-free natural gag outlet stream 61 which is fed to the LNG train 70. Passing through the membrane system 30 reduces the temperature of the single cooled CO.sub.2-depleted non-permeate stream 71. As a result, downstream treatment equipment may be reduced in size and less energy is required to convert the substantially water-free natural gas outlet stream 61 to LNG.

(21) A FLNG pretreatment system and method have been disclosed. 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, the types and arrangement of components, and the number and order of pretreatment steps 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 claims, including the full range of equivalency to which each element thereof is entitled.

(22) Although the preceding description has been described herein with reference to particular means, materials and embodiments, it is not intended to be limited to the particulars disclosed herein; rather, it extends to all functionally equivalent structures, methods, and uses, such as are within the scope of the appended claims.