AU2018392161B2

AU2018392161B2 - Method for liquefying a natural gas stream containing nitrogen

Info

Publication number: AU2018392161B2
Application number: AU2018392161A
Authority: AU
Inventors: Sébastien LICHTLE; Marie MUHR; Henri Paradowski
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2017-12-21
Filing date: 2018-12-17
Publication date: 2024-07-18
Anticipated expiration: 2038-12-17
Also published as: AU2018392161A1; WO2019122656A1; RU2020121187A3; FR3075940B1; FR3075940A1; RU2020121187A

Abstract

A method for liquefying a natural gas feed stream comprising the following steps: Step a): Cooling the feed gas stream in order to obtain a liquefied natural gas stream at a temperature T1 and a pressure P1b; Step b): Introducing the stream from step a) into a denitrogenation column at a pressure P2 and a temperature T2 lower than T1 in order to produce, at the bottom, a denitrogenated liquefied natural gas stream and, at the head, a nitrogen-enriched vapour stream; Step c): Condensing the nitrogen-enriched vapour stream from step b) in a heat exchanger in order to produce a two-phase stream; Step d): Introducing the two-phase stream from step c) into a phase separator vessel in order to produce a liquid stream and a nitrogen-enriched gas stream; characterised in that at least part of the liquid stream from step b) is used during step c) to cool the vapour stream from step b) in said heat exchanger.

Description

METHOD FOR LIQUEFYING A NATURAL GAS STREAM CONTAINING NITROGEN

[0001] The present invention relates to the field of liquefying natural gas. The liquefaction of

natural gas consists in condensing natural gas and in subcooling it to a temperature that is low

enough for it to be able to remain liquid at atmospheric pressure. It is then transported in methane tankers.

[0002] At the present time, the international market for liquid natural gas (LNG) is growing rapidly, but the whole LNG production chain requires substantial investments. Reducing the level

of these investments per ton of LNG produced is thus a prime objective. It is also important to reduce the carbon footprint by reducing the fuel consumption.

[0003] US 6 105 389 proposes a liquefaction process including two coolant mixtures circulating in two independent closed circuits. Each of the circuits functions by means of a compressor

communicating to the coolant mixture the power required to cool the natural gas. Each

compressor is driven by a gas turbine which is chosen from the standard ranges proposed on the market. However, the power of the gas turbines that are currently available is limited.

[0004] US 6 763 680 describes a liquefaction process in which the liquefied natural gas under pressure is expanded in at least two steps so as to obtain at least two gas fractions. The liquefied

natural gas under pressure is cooled while ensuring the reboiling of adeazotization column.

[0005] At the column outlet, a first nitrogen-depleted liquid fraction and a first nitrogen

enriched gas fraction are obtained. This liquid fraction is again expanded to give a nitrogen depleted liquefied natural gas and a second gas fraction. At least one gas fraction is recompressed

and then mixed with the natural gas before condensation.

[0006] Moreover, a process for liquefying natural gas as described in the prior art is unsuitable when said natural gas to be liquefied comprises an excessive content of nitrogen, i.e. greater

than 4%. The present invention proposes to improve the process disclosed in US 6 763 680.

[0006a] The discussion of documents, acts, materials, devices, articles and the like is included

in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

[0006b] Unless the context requires otherwise, where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be

interpreted as specifying the presence of the stated features, integers, steps or components, but

not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0007] One of the aspects of the present invention is to enable a reduction in the investment cost required for a liquefaction plant. Another aspect of the present invention is to achieve, under

better conditions, separation of the nitrogen which may be contained in the gas.

[0008] Thus, the inventors of the present invention have developed a process for producing, starting with a fixed amount of natural gas, nitrogen-depleted liquefied natural gas, the flow rate

of which gas is increased while at the same time minimizing the costs required for the deployment of processes of this type.

[0008a] A further aspect of the present invention is a process for liquefying a natural gas feed stream, comprising the following steps:

Step a): cooling the feed gas stream to obtain a liquefied natural gas stream at a temperature T1and a pressure Pib;

Step b): introducing the stream obtained from step a) into adeazotization column at a pressure P2 and a temperature T2 below T1 to produce, in the said column, adeazotized

liquefied natural gas stream, and, at the top of said column, a nitrogen-enriched vapor stream;

Step c): at least partially condensing the nitrogen-enriched vapor stream obtained from step b) in a heat exchanger to produce a two-phase stream;

Step d): introducing the two-phase stream obtained from step c) into a phase separating vessel to produce at least two phases including a liquid stream and a nitrogen

enriched gas stream; Step e): the part of the liquid stream obtained from step b) which is not used in

step c) is cooled by indirect heat exchange with a second gas fraction obtained in step f) to obtain a cooled liquid fraction and a second heated gas fraction,

2a

Step f): the cooled liquid fraction obtained in step e) is expanded and is then introduced into a second phase-separating vessel, to obtain a liquefied natural gas and the

second gas fraction,

Step g): at least part of said second heated gas fraction is compressed to a pressure P1;

wherein at least part of the liquid stream obtained from step b) is used in step c) to cool the vapor stream obtained from step b) in said heat exchanger.

[0009] One subject of the present invention is a process for liquefying a natural gas feed stream, comprising the following steps: Step a): cooling the feed gas stream to obtain a liquefied natural gas stream at a

temperature T1and a pressure Pib;

Step b): introducing the stream obtained from step a) into adeazotization column at a pressure P2 and a temperature T2 below T1to produce, in the vessel of said column, adeazotized

Step c): at least partially condensing the nitrogen-enriched vapor stream obtained

from step b) in a heat exchanger to produce a two-phase stream;

Step d): introducing the two-phase stream obtained from step c) into a phase

separating vessel to produce at least two phases including a liquid stream and a nitrogen enriched gas stream;

characterized in that at least part of the liquid stream obtained from step b) is used in step c) to

cool the vapor stream obtained from step b) in said heat exchanger.

[0010] According to other embodiments, a subject of the invention is also:

- A process as defined above, characterized in that, during step a), said natural gas feed stream and a second coolant mixture are cooled by indirect heat exchange with at least one

first coolant mixture to obtain a cooled natural gas and a second cooled coolant mixture, and the cooled natural gas is then condensed and cooled by indirect heat exchange with the second

2b

cooled coolant mixture and with at least some of the gas stream obtained in step d) to obtain a liquefied natural gas.

- A process as defined above, preceding, characterized in that, prior to step b), the

stream obtained from step a) is cooled in a reboiling means of saiddeazotization column down to the temperature T2.

- A process as defined above, characterized in that the stream cooled to the temperature T2 is expanded in an expansion means before being introduced into the

deazotization column.

- A process as defined above, characterized in that at least part of the liquid stream obtained from step d) is used as reflux at the top of thedeazotization column.

- A process as defined above, characterized in that it comprises the following steps:

Step e): the part of the liquid stream obtained from step b) which is not used in step c) is cooled by indirect heat exchange with a second gas fraction obtained in step f) to

obtain a cooled liquid fraction and a second heated gas fraction,

Step f): the cooled liquid fraction obtained in the step is expanded and is then

introduced into a second phase-separating vessel, to obtain a liquefied natural gas and the

second gas fraction,

Step g): at least part of said second heated gas fraction is compressed to a

pressure P1.

- A process as defined above, characterized in that the nitrogen content of the

nitrogen-enriched gas stream obtained from step d) is greater than 30 mol%.

- A process as defined above, characterized in that T1 is between -140°C and

120°C.

- A process as defined above, characterized in that P2 is between 3 bar abs and 10

bar abs, P1 is between 4 MPa and 7 MPa.

- A process as defined above, in which, in step b), the natural gas mixture and the

second coolant mixture are cooled to a temperature of between -70°C and -35°C by heat exchange with the first coolant mixture.

- A process as defined above, in which the first coolant mixture includes, as a mole

fraction, the following components:

o Ethane:30%to70%

o Propane: 30% to 70% o Butane: 0% to 20%.

- A process as defined above, in which the second coolant mixture includes, as a

mole fraction, the following components:

o Nitrogen: 0% to 20%

o Methane: 30% to 70% o Ethane: 30% to 70%

o Propane: 0% to 10%.

[0011] The process according to the invention effectively makes it possible to

substantially increase the production capacity while adding a limited number of additional items of equipment.

[0012] The process according to the invention is particularly advantageous when each of

the cooling circuits uses a coolant mixture which is entirely condensed, expanded and vaporized.

[0013] The term "feed stream" as used in the present patent application relates to any composition containing hydrocarbons, including at least methane.

[0014] The heat exchanger may be any heat exchanger, any unit or other arrangement suitable for allowing the passage of a certain number of streams, and thus allowing direct or

indirect heat exchange between one or more coolant fluid lines and one or more feed streams.

[0015] Other features and advantages of the invention will be better understood and will

emerge more clearly on reading the description given below with reference to the figure schematically representing a liquefaction process according to the invention.

[0016] In the figure, a natural gas feed stream 1 is introduced into a heat exchanger unit

El at a temperature T1.

[0017] Typically, the feed stream 1 may contain methane, ethane, propane, hydrocarbons containing at least four carbon atoms. This stream may contain traces of contaminants, for example H20 0-1 ppm, H2S 4 ppm, C02 50 ppm.

[0018] The natural gas feed stream strictly comprises more than 1 mol% of nitrogen, but the specifications impose a maximum content of 1 mol% of nitrogen in LNG.

[0019] Typically, the unit El may contain several heat exchangers (2, 3, 112).

[0020] According to the natural gas liquefaction process represented schematically in the figure, the natural gas stream 1 arrives via pipe 51, for example at a pressure of between 4

MPa and 7 MPa and at a temperature of between °C and 60°C. The natural gas circulating in pipe 51 is combined with the gas 50 to form a natural gas mixture circulating in pipe 51.

The gas circulating in pipe 51, a first coolant mixture 201 and a second coolant mixture 109 enter the exchanger El and circulate therein in parallel and co-current directions.

[0021] The natural gas leaves the exchanger El via pipe 4, for example at a temperature of between -35°C and -70°C. The second coolant mixture leaves totally condensed from the

exchanger El via pipe 100, for example at a temperature of between -35°C and -70°C.

[0022] In the exchanger El, three fractions (202, 212, 222) of the first coolant mixture in the liquid phase are successively withdrawn. The fractions are expanded (207, 217 and 227)

through expansion valves 211, 221 and 231 to three different pressure levels, and are then vaporized in the exchanger El by heat exchange with the natural gas stream circulating in

pipe 51, the second coolant mixture and a part of the first coolant mixture. The three vaporized fractions (208, 218, 228) are sent to different stages (257, 253, 250) of the

compressor K1. The vaporized fractions are compressed in the compressor K1 and then condensed in the condenser E101 by heat exchange with an external cooling fluid, for

example water or air. The first coolant mixture 201 obtained from the compressor E101 is

sent to the exchanger El. The pressure of the first coolant mixture at the outlet of the compressor K1 may be between 2 MPa and 6 MPa. The temperature of the first coolant

mixture at the outlet of the condenser E101 may be between 10°C and 55°C.

[0023] The first coolant mixture may be formed by a mixture of hydrocarbons such as a mixture of ethane and propane, but may also contain methane, butane and/or pentane. The proportions,

in mole fractions (%), of the components of the first coolant mixture may be:

o Ethane: 30% to 70%

o Propane: 30% to 70%

o Butane: 0% to 20%.

[0024] The natural gas circulating in pipe 4 may be fractionated, i.e. a portion of the hydrocarbons containing at least two carbon atoms is separated from the natural gas, using a

device previously described in the prior art. The fractionated natural gas is sent to a heat exchanger E2. The C2+ hydrocarbons collected are sent to fractionation columns including a

deethanizer. The light fraction collected at the top of the deethanizer may be mixed with the natural gas circulating in pipe 4. The liquid fraction collected at the bottom of the deethanizer is

sent to a depropanizer.

[0025] The gas circulating in pipe 4 and the second coolant mixture circulating in pipe 100

enter the exchanger E2 and circulate therein in parallel and co-current directions.

[0026] The second coolant mixture leaving the exchanger E2 via pipe 101 is expanded by the expansion member T3. The expansion member T3 may be a turbine, a valve or a combination of

a turbine and a valve. The second expanded coolant mixture obtained from the turbine T3 is sent via pipe 102 to the exchanger E2 to be vaporized while cooling counter-currentwise the natural

gas and the second coolant mixture.

[0027] On leaving the exchanger E2, the second vaporized coolant mixture 103 is compressed

by the compressor K2 and then cooled in the indirect heat exchanger C2 by heat exchange with an external cooling fluid, for example water or air. The second coolant mixture 109 obtained from

the exchanger C2 is sent to the exchanger. The pressure of the second coolant mixture at the outlet of the compressor K2 may be between 2 MPa and 8 MPa. The temperature of the second

coolant mixture at the outlet of the exchanger C2 may be between 10°C and 55°C.

[0028] In the process described with reference to the figure, the second coolant mixture is not split into separate fractions, but, to optimize the approach in the exchanger E2, the second coolant mixture may also be separated into two or three fractions, each fraction being expanded to a different pressure level and then sent to different stages of the compressor K2.

[0029] The second coolant mixture is formed, for example, by a mixture of hydrocarbons and of nitrogen such as a mixture of methane, ethane and nitrogen, but may also contain

propane and/or butane. The proportions, in mole fractions (%), of the components of the

second coolant mixture may be:

o Nitrogen: 0% to 20%

o Methane: 30% to 70% o Ethane:30%to70%

o Propane: 0% to 10%.

[0030] The natural gas leaves liquefied from the heat exchanger E2 via pipe 10 at a temperature preferably at least 10°C higher than the bubble temperature of the liquefied

natural gas produced at atmospheric pressure (the bubble temperature denotes the

temperature at which the first vapor bubbles form in a liquid natural gas at a given pressure) and at a pressure P1b identical to the inlet pressure P1 of the natural gas, pressure losses

aside. For example, the natural gas leaves the exchanger E2 at a temperature of between 105°C and -145°C and at a pressure of between 4 MPa and 7 MPa. Under these temperature

and pressure conditions, the natural gas does not remain entirely liquid after expansion up to atmospheric pressure.

[0031] The natural gas circulating in pipe 10 is cooled in the reboiler E4 of adeazotization column CO. The natural gas 12 is cooled by heating the bottom (25, 26) of the column CO by

indirect heat exchange, and is then expanded in the expansion member V1. The two-phase

mixture 13 obtained at the outlet of the member V1 is introduced into the column CO at a level N1. A nitrogen-enriched gas fraction 38 is recovered at the top of the column CO. It is

sent to be cooled 39 in a heat exchanger E5 and then separated in a phase-separating vessel B2 in the form of a gas fraction 21 and a liquid fraction 21'.

[0032] The gas fraction 21 evacuated from the vessel B2 is introduced into the exchanger

E2. In the exchanger E2, the gas fraction cools counter-currentwise the natural gas stream 4 and is then directed via pipe 22 to the compressor K4. The liquid fraction 21' evacuated from the vessel B2 is used as reflux at the top of the column CO.

[0033] The nitrogen-depleted liquid fraction 31 evacuated from the vessel of the column

CO is separated into two parts 32 and 34. A first part 32 is cooled in a heat exchanger E3 and is then expanded in an expansion member 33' at a pressure of between 0.05 MPa and 0.5

MPa. The second part 34 of the liquid fraction 31 is expanded 35 in an expansion member

34' and then feeds a heat exchanger E5. Vaporization of this stream 35 gives a stream 36 and represents the majority of the cooling necessary for cooling the gas stream 38 obtained

from the top of the column CO in the heat exchanger E5.

[0034] The expansion members such as V1, 33' and 34' may be an expansion turbine, an

expansion valve or a combination of a turbine and a valve. The two-phase mixture obtained at the outlet of the expansion member 33' is mixed with the stream 36 to give the two

phase mixture 37. The stream 37 is separated in a phase-separating vessel Bl in the form of a gas fraction 41 and a liquid fraction 61. The gas fraction 41 is introduced into the

exchanger E3. In the exchanger E3, the gas fraction 41 cools the liquid fraction 32 obtained

from the liquid stream 31 recovered in the vessel of the column C1 and is then directed via pipe 42 to the compressor K3. The gas mixture 49 leaving the compressor K3 is sent to a

heat exchanger C3 to be cooled by air or water. The gas mixture 50 leaving the exchanger C3 is then mixed with the natural gas stream 1 circulating in pipe 51.

[0035] The liquid fraction 61 evacuated from the tank B1 forms the liquefied natural gas (LNG) produced. The gas 28 circulating via pipe 27 can serve as fuel gas, a source of energy

for the functioning of a liquefaction plant.

[0036] More particularly, the deazotized LNG stream 31 produced at the bottom of the

column COis divided into two parts:

SA first minor part, stream 34, is expanded in the valve 34' down to a low

pressure P3 such that the stream 36 and the two-phase mixture at the outlet of the expansion member 33' are at close pressures before mixing. This

pressure P3 is thus between 0.05 MPa and 0.5 MPa, pressure losses aside.

• The stream 35 is obtained and feeds the exchanger E5. Vaporization of this

stream which gives the stream 36 provides the majority of the cooling

necessary for cooling the head vapor in the exchanger E5.

• A second major part, stream 32, is cooled counter-currentwise relative to the

flash gas, stream 41, to give the stream 33 which is expanded to a pressure P3 to be mixed with the stream 36 and to give the stream 37 which feeds the

LNG flash tank B1.

[0037] In order to further illustrate the implementation of a process as represented

schematically in the figure and as described previously, the data for the implementation of

said process according to the invention have been compared with those corresponding to the implementation of a process according to the invention described in the prior art and

illustrated by figure 2 of patent US 6 763 680.

[0038] These data have been collated in the following table.

[0039] The two processes were performed with the same methodology so that the advantage of the process according to the present invention is clearly evidenced.

[0040] The natural gas arrives via line 01 at a pressure of 60 bar and a temperature of 15°C. The composition of this gas, in mole fractions, is as follows:

o Methane: 91%

o Ethane: 2.5% o Propane: 1%

o Isobutane: 0.3% o n-Butane: 0.2%

o Nitrogen: 5%

[0041] The coolant mixture of the pre-cooling cycle (PR) is the same for the two processes: 50% ethane and 50% propane. It is used in the same way, only the flow rates are

adapted according to the needs. The coolant which produces the subcooling of the natural gas is denoted (LR).

Stream Prior art According to

the invention Feed natural gas Kg/h 01 271000 271000 fixed

Nitrogen-rich gas produced Kg/h 28 30940 31040 LNG produced Kg/h 61 239950 239890

Nitrogen content of the LNG mol% 61 1 0.96

Nitrogen content of stream 28 mol% 43.5 43.3 Column pressure Barabs 3.6 4.5

Methane contained in stream 28 Kg/h 13200 13300 Flash gas recycle Kg/h 50 21070 35870

Compressor K1 power kW 22940 24020 Compressor K2 power kW 35390 32260

Compressor K3 power kW 2570 4500 Compressor K4 power kW 2350 2040

Total compressor power kW 63250 62820 -0.7%

NG temperature at E2 outlet °C 10 -141.1 -135.6 NG temperature at El outlet °C 04 -58.1 -58.4

Nitrogen in LR Nm3/h 100 9200 3750 Methane in LR Nm3/h 100 108000 100600

Ethane in LR Nm3/h 100 176500 180000 Propane in LR Nm3/h 100 990 1050

Total LR Nm3/h 100 294690 285400 PR total Nm3/h 201 413080 428270

Low pressure PR Nm3/h 227 117330 125230

Medium pressure PR Nm3/h 217 162860 173000 High pressure PR Nm3/h 207 132890 130040

Low pressure LR pressure Barabs 102 2.5 2.5 High pressure LR pressure Barabs 100 41.7 35.4

Tank B1 pressure Barabs 61 1.6 1.6

[0042] The novel process makes it possible to produce a nitrogen-depleted LNG while

saving energy.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A process for liquefying a natural gas feed stream, comprising the following steps:

liquefied natural gas stream, and, at the top of said column, a nitrogen-enriched vapor stream; Step c): at least partially condensing the nitrogen-enriched vapor stream obtained

from step b) in a heat exchanger to produce a two-phase stream; Step d): introducing the two-phase stream obtained from step c) into a phase

separating vessel to produce at least two phases including a liquid stream and a nitrogen

second gas fraction, Step g): at least part of said second heated gas fraction is compressed to a pressure

P1; wherein at least part of the liquid stream obtained from step b) is used in step c) to cool the vapor stream obtained from step b) in said heat exchanger.

2. The process as claimed in claim 1, wherein, during step a), said natural gas feed

stream and a second coolant mixture are cooled by indirect heat exchange with at least one first coolant mixture to obtain a cooled natural gas and a second cooled coolant mixture, and the

cooled natural gas is then condensed and cooled by indirect heat exchange with the second cooled coolant mixture and with at least some of the gas stream obtained in step d) to obtain a

liquefied natural gas.

3. The process as claimed in claim 1 or claim 2, wherein, prior to step b), the stream

obtained from step a) is cooled in a reboiling means of said deazotization column down to the temperature T2.

4. The process as claimed in claim 3, wherein the stream cooled to the temperature T2

is expanded in an expansion means before being introduced into thedeazotization column.

5. The process as claimed in any one of claims 1 to 4, wherein at least part of the liquid stream obtained from step d) is used as reflux at the top of thedeazotization column.

6. The process as claimed in any one of claims 1 to 5, wherein the nitrogen content of

the nitrogen-enriched gas stream obtained from step d) is greater than 30 mol%.

7. The process as claimed in any one of claims 1to 6, wherein T1is between -140°C and

-120°C.

8. The process as claimed in claim 6 or claim 7, wherein P2 is between 3 bar abs and 10 bar abs, P1 is between 4 MPa and 7 MPa.

9. The process as claimed in any one of claims 2 to 8, in which, in step b), the natural

gas mixture and the second coolant mixture are cooled to a temperature of between -70°C and 35°C by heat exchange with the first coolant mixture.

10. The process as claimed in claim 9, in which the first coolant mixture comprises, as a mole fraction, the following components:

o Ethane: 30% to 70%

o Propane: 30% to 70%

o Butane: 0% to 20%.

11. The process as claimed in claim 9 or claim 10, in which the second coolant mixture comprises, as a mole fraction, the following components:

o Nitrogen: 0% to 20%

o Methane: 30% to 70% o Ethane: 30% to 70% o Propane: 0% to 10%.

12. The liquefied natural gas feed stream produced according to the process as claimed in any one of claims 1 to 11.