Natural gas liquefaction with integrated nitrogen removal

Patent metadata

Field	Value
Publication number	US-11221176-B2
Application number	US-201816103479-A
Country	US
Kind code	B2
Filing date	Aug 14, 2018
Priority date	Aug 14, 2018
Publication date	Jan 11, 2022
Grant date	Jan 11, 2022

Abstract

A natural gas liquefaction method and system having integrated nitrogen removal. Recycled LNG gas is cooled in a separate and parallel circuit from the natural gas stream in the main heat exchanger. Cooled recycled gas and natural gas streams are directed to a nitrogen rectifier column after the warm bundle. The recycle stream is introduced to the rectifier column above the natural gas stream and at least one separation stage is located in the rectifier column between the recycle stream inlet and the natural gas inlet. The bottom stream from the rectifier column is directed to a cold bundle of the main heat exchanger where it is subcooled.

First claim

The invention claimed is:

1. A method for producing a nitrogen-depleted LNG product, the method comprising:
(a) passing a natural gas feed stream through a first circuit of a main heat exchanger to cool the natural gas feed stream and liquefy at least a portion of the natural gas stream against a first refrigerant, thereby producing a first cooled LNG stream;
(b) withdrawing the first cooled LNG stream from the main heat exchanger;
(c) expanding the first cooled LNG stream to form a first reduced pressure LNG stream;
(d) introducing the first reduced pressure LNG stream into a nitrogen rectifier column at a first location, the first location being located at a bottom end of the nitrogen rectifier column;
(e) withdrawing a first LNG bottoms stream from the bottom end of the nitrogen rectifier column;
(f) withdrawing an overhead stream from the nitrogen rectifier column;
(g) cooling the first LNG bottoms stream to create a subcooled LNG stream;
(h) directing at least a portion of the subcooled LNG stream to a flash drum or an LNG storage tank;
(i) collecting at least one selected from a group consisting of: a flash gas stream from the flash drum and a boil-off gas stream from the LNG storage tank to form a recycle stream;
(j) passing the recycle stream through a second circuit of the main heat exchanger to cool the recycle stream and liquefy at least a portion of the recycle stream, thereby producing an at least partially liquified recycle stream;
(k) withdrawing the at least partially liquified recycle stream from the second circuit of the main heat exchanger before the at least partially liquified recycle stream enters a cold bundle of the main heat exchanger and then expanding the at least partially liquified recycle stream to form a reduced pressure recycle stream; and
(l) introducing the reduced pressure recycle stream into the nitrogen rectifier column at a second location, the second location located below the rectifier column overhead and above the first location and at least one separation stage being located in the nitrogen rectifier column between the first location and the second location.

2. The method of claim 1 , further comprising:
(m) passing the subcooled liquid LNG stream through a condenser heat exchanger to form a mixed phase LNG stream, the condenser heat exchanger providing cooling duty via indirect heat exchange to the nitrogen column.

3. The method of claim 2 , further comprising:
(n) at least partially vaporizing the subcooled liquid LNG stream before performing step (m).

4. The method of claim 1 , further comprising:
(o) compressing and cooling the first refrigerant in a refrigeration loop;
(p) withdrawing a slip stream of the first refrigerant to provide cooling duty to the nitrogen rectifier column.

5. The method of claim 1 , further comprising:
(q) directing the at least a portion of the subcooled LNG stream to the flash drum.

6. The method of claim 1 , further comprising:
(r) compressing and cooling the recycle stream before performing step (j).

7. The method of claim 1 , further comprising:
(s) after performing step (b) and before performing step (d), further cooling the first LNG stream by indirect heat exchange against a reboil stream from the bottom end of the nitrogen rectifier column, thereby producing a warmed reboil stream;
(t) introducing the warmed reboil stream into the bottom end of the nitrogen rectifier column.

8. The method of claim 1 , wherein step (h) further comprises separating the subcooled liquid LNG stream into the liquid LNG product stream and the vapor LNG product stream in a nitrogen stripper column; and the method further comprises:
(u) withdrawing a nitrogen enriched vapor stream from an upper end of the nitrogen rectifier column, passing the nitrogen enriched vapor stream through a condenser heat exchanger located in the nitrogen stripper column to provide a boiling duty to the nitrogen stripper column, which produces an at least partially liquefied nitrogen enriched stream; and
(v) returning the at least partially liquefied nitrogen enriched stream to the upper end of the nitrogen rectifier column.

9. The method of claim 1 , further comprising:
(w) further cooling the overhead stream in an overhead hear exchanger and separating a further cooled overhead stream into a nitrogen-enriched stream and a hydrogen/helium-enriched stream;
(x) expanding nitrogen-enriched stream and using the expanded nitrogen-enriched stream to provide a refrigeration duty to the overhead heat exchanger.

10. The method of claim 1 , further comprising:
(y) separating the overhead stream into a nitrogen-enriched stream and a hydrogen/helium-enriched stream using a pressure swing adsorption or membrane unit.

11. The method of claim 1 , further comprising:
(z) separating the subcooled LNG stream into an LNG product stream and a vapor NG product stream;
wherein step (i) further comprises directing the LNG product stream to the LNG storage tank.

12. The method of claim 11 , further comprising:
(aa) combining the boil-off gas stream with the vapor NG product stream to form the recycle stream.

13. The method of claim 1 , wherein step (d) comprises introducing the first reduced pressure LNG stream into the nitrogen rectifier column at the first location, the first location being located below any separation stages located within the nitrogen rectifier column.

14. The method of claim 1 , wherein step (I) further comprises introducing the reduced pressure recycle stream in the nitrogen rectifier column at the second location, the second location being below at least one separation stage.

15. The method of claim 2 , wherein the mixed phase LNG stream is in upstream fluid flow communication with the recycle stream.

16. The method of claim 1 , wherein step g comprises:
cooling the first LNG bottoms stream in the cold bundle of the main heat exchanger to create a subcooled LNG stream wherein the cold bundle is colder than the second circuit of the main heat exchanger from which the at least partially liquified recycle stream is withdrawn.

Assignees

• Air Prod & Chem

Inventors

• Roberts Mark Julian
• Chen Fei
• Ott Christopher Michael
• Weist Annemarie Ott

Classifications

• F25J2205/60

  using adsorption on solid adsorbents, e.g. by temperature-swing adsorption [TSA] at the hot or cold end · CPC title

• F25J2200/74

  Refluxing the column with at least a part of the partially condensed overhead gas · CPC title

• F25J2245/90

  the recycled stream being boil-off gas from storage · CPC title

• F25J2205/40

  using hybrid system, i.e. combining cryogenic and non-cryogenic separation techniques · CPC title

• F25J1/0238

  Purification or treatment step is integrated within one refrigeration cycle only, i.e. the same or single refrigeration cycle provides feed gas cooling (if present) and overhead gas cooling · CPC title