METHOD AND APPARATUS TO AVOID LNG FLASH WHEN EXPANDING TO THE LNG STORAGE FACILITY

Abstract

Process for eliminating the evaporation of a liquefied natural gas stream during the transfer thereof into a storage facility, comprising the following steps: Step a): liquefaction, by means of a refrigeration cycle, of a natural gas stream and of a nitrogen stream in a main heat exchanger; Step b): cooling of the liquefied natural gas stream from step a) in a second heat exchanger by circulation of said liquefied natural gas stream countercurrent to a liquid nitrogen flow that is vaporized while cooling said liquefied natural gas stream; wherein the liquid nitrogen flow used in step b) is from step a).

Claims

1. A process for eliminating the evaporation of a liquefied natural gas stream during the transfer thereof into a storage facility, the process comprising the steps of: Step a): liquefying, by use of a refrigeration cycle, of a natural gas stream and of a nitrogen stream in a main heat exchanger; Step b): cooling the liquefied natural gas stream from step a) in a second heat exchanger by circulation of said liquefied natural gas stream countercurrent to a liquid nitrogen flow (14) that is vaporized while cooling said liquefied natural gas stream; and Step c): expanding the liquid nitrogen stream from step a) after leaving the main exchanger at its coldest level then introduction of said thus expanded stream into the second heat exchanger during step b), wherein the liquid nitrogen flow used in step b) is from step a), and wherein the liquid nitrogen stream from step a) is subcooled in the second heat exchanger before step c).

2. The process according to claim 1, wherein the nitrogen stream from step b) supplies the refrigeration cycle used in step a), after having cooled the liquefied natural gas stream, by being introduced at the coldest level of said main heat exchanger, then by circulating countercurrent to the streams to be liquefied during step a) up to the hottest level of said main heat exchanger where said nitrogen stream is vaporized.

3. The process according to claim 1, wherein at least one portion of said of vaporized nitrogen stream forms the nitrogen stream to be liquefied in the main exchanger used in step a).

4. The process according to claim 1, wherein the refrigeration cycle is a Turbo-Brayton nitrogen cycle.

5. The process according to claim 1, wherein the natural gas introduced in step a) comprises at least 50% by volume of methane.

6. The process according to claim 1, wherein the cooled liquefied natural gas from step b) is transferred to a storage facility.

7. The process according to claim 1, wherein the parameters of the refrigeration cycle are adjusted during the process as a function of the temperature desired for the liquefied natural gas stream from step b) and as a function of the composition of said natural gas stream.

8. The process according to claim 1, wherein parameters of the refrigeration cycle are adjusted during the process as a function of the nitrogen content of said natural gas stream.

9. The process according to claim 1, wherein the natural gas stream to be liquefied is introduced during step a) at the hottest level of the main heat exchanger and is discharged in liquid form at the coldest level of said main exchanger, then is introduced during step b) at the hottest level of the second heat exchanger and is then discharged at the coldest level of said second heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The present invention will be well understood, and its advantages will also become apparent in the light of the following description, provided purely by way of non-limiting example with reference to the appended drawings, in which:

[0047] In the only FIGURE:

[0048] The FIGURE shows a diagram in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0049] The invention will be described in a more detailed manner by referring to the FIGURE which illustrates the diagram of one particular embodiment of an implementation of a process according to the invention.

[0050] In the FIGURE, a flow 1 of natural gas optionally previously pretreated (typically having undergone a separation from a portion of at least one of the following constituents: water, CO.sub.2, methanol, sulphur-containing compounds) is introduced into a main heat exchanger 2 in order to be liquefied.

[0051] The FIGURE therefore shows a process for liquefaction of a feed flow 1. The feed stream 1 may be a pretreated natural gas stream, in which one or more substances, such as sulphur, carbon dioxide, water, are reduced so as to be compatible with cryogenic temperatures, as is known in the prior art.

[0052] Optionally, the feed stream 1 may have undergone one or more precooling steps as is known in the prior art. One or more of the precooling steps may comprise one or more refrigeration circuits. By way of example, a natural gas feed stream is generally treated starting from an initial temperature of 30° C.-50° C. Following one or more precooling steps, the temperature of the natural gas feed stream may be reduced to −30° C. to −70° C.

[0053] In the FIGURE, the heat exchanger 2 is preferably a brazed aluminium plate cryogenic heat exchanger. Cryogenic heat exchanges are known in the prior art and may have various arrangements of their feed flow(s) and refrigerant streams. In addition, such heat exchangers may also have one or more lines to enable the passage of other flows, such as refrigerant streams for other steps of a cooling process, for example in liquefaction processes. These other lines or flows are not represented in the FIGURE for greater simplicity.

[0054] The feed stream 1 enters the heat exchanger 2 via a feed inlet 3 and passes through the heat exchanger via the line 4, then is extracted from the exchanger at the outlet 5 in order to provide a liquefied hydrocarbon flow 6. When the liquefied stream 6 is liquefied natural gas, the temperature may be around −150° C. to −170° C. The liquefaction of the feed stream 1 is carried out by means of a refrigerant fluid circuit 7. In this refrigerant circuit 7 a refrigerant, preferably nitrogen, circulates.

[0055] The liquefied natural gas flow 6 is then introduced into a second heat exchanger 15 via the inlet 24 at the hottest level of this second heat exchanger 15 in order to be subcooled to a temperature T3 lower than T2. The thus subcooled natural gas stream 26 is discharged from the heat exchanger 15 via the outlet 25 located at the coldest end of the exchanger 15. Typically, T3 is lower than T2, that is to say lower than −160° C., which temperature makes it possible to avoid the evaporation of the then subcooled liquefied natural gas 26, at the outlet 25.

[0056] In the arrangement of the operation of the heat exchanger 2 represented in the FIGURE, a gaseous refrigerant nitrogen stream 8 is introduced into the main exchanger 2 at an inlet 9 at the temperature T1 (for example between 0° C. and 40° C.), then it passes through this inlet and is liquefied and subcooled along the line 10 through the heat exchanger 2, to the outlet 11 in order to produce a liquid nitrogen stream 12.

[0057] The temperature T2 of the outlet 11 is lower than the temperature of the inlet 9 of the heat exchanger 2. T2 is typically between −80° C. and −175° C., for example −170° C. As it passes through the line 10, the gaseous refrigerant stream 8 is liquefied.

[0058] Thus, the nitrogen stream 8 and the natural gas stream 1 are liquefied in the same main heat exchanger 2 by one and the same refrigeration cycle 7.

[0059] The refrigerant nitrogen stream 12 is then expanded in an expander 13 for example using a valve, so as to provide a refrigerant stream at reduced pressure 14. This refrigerant stream 14 is then introduced into the lower part of a second heat exchanger 15 through the inlet 16 (at the coldest end of the exchanger 15). The temperature T3 of the inlet 16 is lower than T2. The introduction of the stream 14 into the heat exchanger 15 via the inlet 16 is then such that the passage of this refrigerant stream 14 through a line 17 in the heat exchanger 15 takes place in an ascending manner up to an outlet 18 of the heat exchanger 15. The temperature of this outlet 18 is substantially equal to T2.

[0060] The refrigerant stream 19 recovered at the outlet 18 of the heat exchanger 15 is then introduced via an inlet 20 into the coldest part of the main heat exchanger 2 at a temperature substantially equal to the temperature of the outlet 11. The refrigerant nitrogen stream is then reheated through the main heat exchanger 2 up to the outlet 21 at the temperature T1.

[0061] A gaseous refrigerant nitrogen stream 22 circulates in the refrigeration circuit 7 downstream of the outlet 21 of the main heat exchanger 2 at ambient temperature (that is to say the temperature measured in the space where the device for implementation of the process that is the subject of the present invention is placed. This temperature is for example between −20° C. and 45° C.).

[0062] A temperature substantially equal to another temperature is understood to mean a temperature equal to within ±5° C.

[0063] The cooled liquefied natural gas 26 at the end of the process that is the subject of the present invention may then, for example, be transferred to a storage or transport device.

[0064] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0065] The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

[0066] “Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

[0067] “Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0068] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0069] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0070] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.