Process for separating nitrogen from a natural gas stream with nitrogen stripping in the production of liquefied natural gas

• US 10,113,127 B2
• Filed: 04/16/2010
• Issued: 10/30/2018
• Est. Priority Date: 04/16/2010
• Status: Active Grant

• Current Assignee: Black & Veatch International Company (Black & Veatch Corporation)
• Original Assignee: Black & Veatch International Company (Black & Veatch Corporation)
• Inventors: Price, Brian C.
• Primary Examiner(s): Raymond, Keith
• Application Number:
  US12/799,061
• Publication Number:
  US 20110289963A1
• Time in Patent Office: 3,119 Days
• Field of Search: 62618, 62620, 62623, 62927, 62630
• CPC Class Codes: C10L 3/105 of nitrogen
• CPC Class Codes: F25J 1/0047 using an "external" refrigerant stream in a closed vapor compression cycle F25J1/0221, F25J1/0225 take precedence
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/04 in a dual pressure main column system
• CPC Class Codes: F25J 2200/70 Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
• CPC Class Codes: F25J 2200/78 Refluxing the column with a liquid stream originating from an upstream or downstream fractionator column
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2210/06 Splitting of the feed stream, e.g. for treating or cooling in different ways
• CPC Class Codes: F25J 2215/04 Recovery of liquid products
• CPC Class Codes: F25J 2220/62 Separating low boiling components, e.g. He, H2, N2, Air
• CPC Class Codes: F25J 2260/20 Integration in an installation for liquefying or solidifying a fluid stream
• CPC Class Codes: F25J 2270/12 External refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/66 Closed external refrigeration cycle with multi component refrigerant [MCR], e.g. mixture of hydrocarbons
• CPC Class Codes: F25J 3/0209 Natural gas or substitute natural gas
• CPC Class Codes: F25J 3/0233 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0257 separation of nitrogen from air F25J3/04, production of nitrogen in general C01B21/00

Abstract

A mixed single refrigerant process for separating a nitrogen gas stream from a natural gas stream containing nitrogen to produce a nitrogen gas stream from a liquefied natural gas stream wherein the separated nitrogen gas stream is used as a refrigerant for the natural gas stream and wherein the mixed refrigerant provides cooling for the process.

Claims

1. A process for producing liquefied natural gas from a nitrogen-containing natural gas stream, the process comprising:

a) cooling a stream of mixed refrigerant in a first heat exchanger of a single closed-loop mixed refrigerant system to provide a cooled mixed refrigerant stream;

b) expanding at least a portion of the cooled mixed refrigerant stream to produce an expanded mixed refrigerant stream;

c) cooling and at least partially condensing the natural gas stream in a first heat exchange passageway via indirect heat exchange with the expanded mixed refrigerant stream in the first heat exchanger to produce a cooled natural gas stream;

d) dividing the cooled natural gas stream withdrawn from the first heat exchange passageway into a first portion and a second portion;

e) introducing the first portion into a lower inlet of a first vapor-liquid separator;

f) subsequent to said dividing, subcooling the second portion in the first heat exchanger to provide a subcooled liquid portion, wherein said subcooling is carried out in a second heat exchange passageway via indirect heat exchange with said expanded mixed refrigerant;

g) subsequent to said subcooling, introducing the subcooled liquid portion into an upper inlet of the first vapor-liquid separator, wherein the upper inlet of the first vapor-liquid separator is located at a higher vertical elevation than the lower inlet;

h) withdrawing a methane rich liquid bottoms stream and a first nitrogen rich vapor overhead stream from the first vapor-liquid separator;

i) further cooling the methane rich liquid bottoms stream in the first heat exchanger in a third heat exchange passageway to provide a first liquid natural gas stream;

j) introducing at least a portion of the first liquid natural gas stream into an inlet of a nitrogen stripping column;

k) introducing at least a portion of the first nitrogen rich vapor overhead stream into another inlet of the nitrogen stripping column;

l) withdrawing a stream of nitrogen-depleted liquefied natural gas (LNG) from a lower portion of the nitrogen stripping column, wherein the LNG comprises less than 3 volume percent nitrogen;

m) recovering an overhead nitrogen rich vapor stream from a location near the top of the nitrogen stripping column, wherein the overhead nitrogen rich vapor stream comprises less than 3 volume percent methane;

n) introducing the overhead nitrogen rich vapor stream into the first heat exchanger; and

o) using at least a portion of the overhead nitrogen rich vapor stream as a refrigerant in the first heat exchanger to carry out at least a portion of the cooling of step (a) and/or at least a portion of the cooling of step (c), wherein the single closed-loop mixed refrigerant system is the only closed-loop refrigeration system used to cool the natural gas stream.

2. The process of claim 1, wherein the first nitrogen rich vapor overhead stream is withdrawn from the first vapor-liquid separator and cooled via indirect heat exchange with a fluid stream withdrawn from a lower portion of the nitrogen stripping column in a nitrogen stripping column reboiler.

3. The process of claim 2, wherein the first nitrogen rich vapor overhead stream withdrawn from the nitrogen stripping column reboiler is further cooled by heat exchange with the overhead nitrogen rich vapor stream withdrawn from the nitrogen stripping column.

4. The process of claim 1, wherein the first vapor-liquid separator is a distillation column.

5. The process of claim 1, wherein the cooling of step (c) further provides a vaporized stream of mixed refrigerant;

further comprising, compressing the vaporized stream of mixed refrigerant to provide a stream of compressed mixed refrigerant, wherein the stream of mixed refrigerant cooled in step (a) comprises the stream of compressed mixed refrigerant.

6. A system for liquefying a nitrogen-containing natural gas stream to provide an LNG product, the system comprising:

a first heat exchanger comprising a first heat exchange passageway and a second heat exchange passageway for cooling at least a portion of the natural gas stream and a nitrogen heat exchange passageway for warming a nitrogen-rich fluid stream, wherein the first heat exchange passageway comprises a warm natural gas inlet for receiving a vapor-phase natural gas feed stream and a cooled natural gas outlet for discharging the cooled natural gas stream, wherein the second heat exchange passageway comprises a cool natural gas inlet and a further cooled natural gas outlet for discharging a subcooled natural gas stream, and the nitrogen heat exchange passageway comprises a cool nitrogen inlet and a warmed nitrogen outlet;

a first cooled natural gas conduit having a first cooled gas inlet and a first cooled gas outlet, wherein the first cooled gas inlet is in fluid flow communication with the cooled natural gas outlet of the first heat exchange passageway and the first cooled gas outlet is in fluid flow communication with the cool natural gas inlet of the second heat exchange passageway;

a second cooled natural gas conduit having a second cooled gas inlet and a second cooled gas outlet, wherein the second cooled gas inlet is in fluid flow communication with the cooled natural gas outlet of the first heat exchanger;

a first vapor-liquid separator for separating at least a portion of the cooled natural gas stream into a nitrogen-rich vapor stream and a methane-rich liquid stream, wherein the first vapor-liquid separator comprises a first lower fluid inlet, a first upper fluid inlet, a first vapor outlet, and a first liquid outlet, wherein the first lower fluid inlet is positioned at a lower vertical elevation than the first upper fluid inlet, wherein the first lower fluid inlet is in fluid flow communication with the second cooled gas outlet of the second cooled natural gas conduit and the first upper fluid inlet is in fluid flow communication with the further cooled natural gas outlet of the second heat exchange passageway;

a second heat exchanger comprising a third heat exchange passageway for cooling at least a portion of the nitrogen-rich vapor stream withdrawn from the first vapor-liquid separator, wherein the third heat exchange passageway comprises a warm nitrogen-rich inlet and a cooled nitrogen-rich outlet, wherein the warm nitrogen-rich inlet is in fluid flow communication with the first vapor outlet of the first vapor-liquid separator;

a second vapor-liquid separator comprising a second upper fluid inlet, a second lower fluid inlet, a second vapor outlet, and a second liquid outlet, wherein the second upper fluid inlet is in fluid flow communication with the cooled nitrogen-rich outlet of the third heat exchange passageway, wherein the second lower fluid inlet of the second vapor-liquid separator is in fluid flow communication with the first liquid outlet of the first vapor-liquid separator, wherein the second vapor outlet is in fluid flow communication with the cool nitrogen inlet of the nitrogen heat exchange passageway disposed in the first heat exchanger; and

a single closed-loop mixed refrigerant system comprising—

a first refrigerant heat exchange passageway disposed within the first heat exchanger for cooling a stream of mixed refrigerant, wherein the first refrigerant heat exchange passageway comprises a warm refrigerant inlet and a cooled refrigerant outlet;

an expansion device for expanding the stream of cooled mixed refrigerant, wherein the expansion device comprises a high pressure fluid inlet and a low pressure fluid outlet, wherein the high pressure fluid inlet of the expansion device is in fluid flow communication with the cooled refrigerant outlet of the first refrigerant heat exchange passageway; and

a second refrigerant heat exchange passageway disposed within the first heat exchanger for warming the expanded stream of mixed refrigerant, wherein the second refrigerant heat exchange passageway comprises a cooled refrigerant inlet and a warmed refrigerant outlet, wherein the cooled refrigerant inlet is in fluid flow communication with the low pressure outlet of the expansion device, wherein the second refrigerant heat exchange passageway is configured to facilitate heat transfer between a stream of expanded mixed refrigerant in the second refrigerant heat exchange passageway and the streams passing through the first and second heat exchange passageways,

wherein the single closed-loop mixed refrigerant system is the only closed-loop mixed refrigeration system for cooling the natural gas stream.

7. The system of claim 6, further comprising a third heat exchanger for further cooling at least a portion of the cooled-nitrogen rich stream withdrawn from the cooled nitrogen-rich outlet of the third heat exchange passageway of the second heat exchanger, wherein the third heat exchanger comprises a fourth heat exchange passageway and a fifth heat exchange passageway,

wherein the fourth heat exchange passageway includes a warm nitrogen-rich fluid inlet and a cool nitrogen-rich fluid outlet and the fifth heat exchange passageway includes a cool nitrogen-rich gas inlet and a warm nitrogen-rich gas outlet, wherein the warm nitrogen-rich fluid inlet is in fluid flow communication with the cooled nitrogen-rich outlet of the third heat exchange passageway and the cool nitrogen-rich fluid outlet is in fluid flow communication with the second upper fluid inlet of the second vapor-liquid separator,

wherein the cool nitrogen-rich gas inlet of the fifth heat exchange passageway is in fluid flow communication with the second vapor outlet of the second vapor-liquid separator and the warm nitrogen-rich gas outlet is in fluid flow communication with the cool nitrogen inlet of the nitrogen heat exchange passageway disposed in the first heat exchanger.

8. The system of claim 6, wherein the first vapor-liquid separator is a distillation column.