Method for liquefying natural gas and nitrogen

Abstract

A method for producing liquefied natural gas and a stream of liquid nitrogen including step a): producing gaseous nitrogen in an air separation unit; step b): liquefying a stream of natural gas in a natural gas liquefaction unit including a main heat exchanger and a system for producing cold; step c): liquefying the nitrogen stream resulting from step a) in the main exchanger of the natural gas liquefaction unit in parallel with the liquefied natural gas in step b); wherein all the cold necessary for liquefying the stream of nitrogen and for liquefying the natural gas is supplied by the system for producing cold of the natural gas liquefaction unit.

Claims

1. A method for producing liquefied natural gas and a stream of liquid nitrogen, comprising at least the following steps: Step a): producing gaseous nitrogen in an air separation unit; Step b): liquefying a stream of natural gas in a natural gas liquefaction unit comprising a main heat exchanger and a refrigeration system; Step c): liquefying the nitrogen stream resulting from step a) in the main heat exchanger of the natural gas liquefaction unit in parallel with the liquefied natural gas in step b) and exporting at least a portion of the liquefied nitrogen stream as a product; wherein all the refrigeration necessary for liquefying the stream of nitrogen and for liquefying the natural gas is supplied by said refrigeration system of the natural gas liquefaction unit.

2. The method as claimed in claim 1, wherein said refrigeration system comprises at least one compressor and at least one turbine-booster system.

3. The method as claimed in claim 1, wherein the liquefaction unit comprises a refrigeration cycle supplied with a refrigerant stream comprising at least one constituent, wherein at least one constituent is selected from the group consisting of nitrogen, methane, ethylene, ethane, butane and pentane.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein: The sole FIGURE illustrates the scheme of a particular embodiment of an implementation of a method according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(2) In the FIGURE, a stream of natural gas 1 is fed into the main exchanger 2 of a natural gas liquefaction unit 3 in order to be liquefied. A stream 20 of liquid natural gas is withdrawn from the liquefaction unit 3. A refrigerant stream circulates in closed cycle in this heat exchanger 2, in order to supply the cold necessary for liquefying said stream 1 of natural gas.

(3) In particular, the present FIGURE describes a liquefaction cycle using nitrogen.

(4) However, other types of natural gas liquefaction cycles may be employed, for example a reverse Brayton cycle (notably supplied with nitrogen, but it is also possible to use the NG cycle itself) or a cycle based on one or more mixed refrigerants.

(5) At the same site, an air separation unit (ASU) 4 containing at least one so-called high-pressure column 6 and a so-called low-pressure column 5 produces a gaseous nitrogen stream 7. This nitrogen stream 7 is fed into the system 8 for producing cold of the liquefaction unit 3 via a compressor 9. At the outlet of the compressor, the nitrogen stream is fed into at least one booster 10 in series with the compressor 9. At least part of the flow from this at least one booster 10 is connected to at least one turbine 11, a turbine 11 connected to a booster 10 forming what is called a turbine/booster system in the present application. At the outlet of the booster 10, the nitrogen stream is fed into the main heat exchanger 2 to be cooled in parallel with the stream 1 of liquefied natural gas in this exchanger 2. A part 12 of the gaseous stream thus cooled is withdrawn from the exchanger 2 at an intermediate level 13 in order to be fed into the turbine 11 connected to the booster 10 from which the gaseous stream previously fed into the exchanger 2 is obtained. At the outlet of the turbine 11, the nitrogen stream is fed back into the heat exchanger 2 at its coldest end (i.e. an inlet 14 whose temperature level is the lowest of the temperature levels of the exchanger 2). The nitrogen stream thus fed into the exchanger is then heated as far as the outlet 15 of the exchanger 2 whose temperature level is the highest, and then is sent to the compressor 9 in order to follow the same path as stream 7.

(6) The other part 16 of the nitrogen stream at the outlet of booster 10 fed into the heat exchanger 2, which is not withdrawn at the intermediate level 13, is liquefied in parallel with the natural gas stream 1. Once liquefied, a stream 17 of liquid nitrogen is split into at least two streams 18 and 19. Stream 18 of liquid nitrogen is recycled to the air separation unit 4 by being fed in at the top of the low-pressure column 5 of unit 4. For its part, the stream of liquid nitrogen 19 is intended for production.

(7) A variant of the method according to the invention consists of feeding at least one part 7 of the stream of gaseous nitrogen 7 withdrawn from the air separation unit 4 directly into the main heat exchanger 2 in order to be liquefied in parallel with the natural gas stream 1 and to be withdrawn in liquid form at an outlet 21 of the exchanger whose temperature level is the lowest and thus rejoin the stream 19 intended for production.

(8) It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.