Method of removing nitrogen from a nitrogen containing stream

Abstract

 The present invention provides a method of removing nitrogen from a nitrogen-containing stream (10), the method comprising at least the steps of:
(a) providing a nitrogen-containing stream (10);
(b) heating the nitrogen-containing stream (10) in a first heat exchanger (2) thereby obtaining a heated nitrogen-containing stream (20);
(c) compressing the heated nitrogen-containing stream (20) thereby obtaining a compressed nitrogen-containing stream (30);
(d) splitting the compressed nitrogen-containing stream (30) into a first split stream (40) and a second split stream (50);
(e) cooling the second split stream (50) in the first (2) and a second heat exchanger (3) thereby obtaining a cooled second split stream (70);
(f) expanding the cooled second split stream (70) thereby obtaining an expanded second split stream (80);
(g) separating the expanded second split stream (80) in a first gas/liquid separator (5) thereby obtaining a gaseous nitrogen-enriched stream (90) and a liquid nitrogen-depleted stream (110);
(h) heating the gaseous nitrogen-enriched stream (90) in the first or second heat exchanger (2,3) thereby obtaining a heated nitrogen-enriched stream (100).

Description

[0001] The present invention relates to a method of removing nitrogen from a nitrogen-containing stream.

[0002] Methods of removing nitrogen from a nitrogen-containing stream are known in the art. As an example, it is known to remove nitrogen using an NRU (Nitrogen Rejection Unit). A problem of using an NRU for removing nitrogen is the high expenditure associated with it.

[0003] It is an object of the present invention to solve or at least minimize the above problem.

[0004] It is a further object of the present invention to provide a simpler and more cost-effective method of removing nitrogen from a nitrogen-containing stream, in particular a methane-rich stream such as obtained during the liquefaction of natural gas.

[0005] One or more of the above or other objects are achieved according to the present invention by providing a method of removing nitrogen from a nitrogen-containing stream, the method comprising at least the steps of:

(a) providing a nitrogen-containing stream;

(b) heating the nitrogen-containing stream in a first heat exchanger thereby obtaining a heated nitrogen-containing stream;

(c) compressing the heated nitrogen-containing stream thereby obtaining a compressed nitrogen-containing stream;

(d) splitting the compressed nitrogen-containing stream into a first split stream and a second split stream;

(e) cooling the second split stream in the first and a second heat exchanger thereby obtaining a cooled second split stream;

(f) expanding the cooled second split stream thereby obtaining an expanded second split stream;

(g) separating the expanded second split stream in a first gas/liquid separator thereby obtaining a gaseous nitrogen-enriched stream and a liquid nitrogen-depleted stream;

(h) heating the gaseous nitrogen-enriched stream in the first or second heat exchanger thereby obtaining a heated nitrogen-enriched stream.

[0006] An advantage of the method according to the present invention is that it has a surprisingly simple design and can be standardized to treat and liquefy a wide range of feed gas compositions. Further, there is relatively limited utility and chemicals requirement resulting in a significant OPEX and CAPEX reduction.

[0007] In step (a), a nitrogen-containing stream is provided. Although the nitrogen-containing stream is not particularly limited, it preferably is a methane-rich gas stream. Preferably, the nitrogen-containing stream comprises at least 5 mol% nitrogen, preferably at least 10 mol%, more preferably at least 15 mol%. Typically, the nitrogen-containing stream contains less than 80 mol% nitrogen. According to a preferred embodiment, the nitrogen-containing stream comprises at least 20 mol%, preferably at least 40 mol% methane, and typically below 85 mol%. Preferably, the nitrogen-containing stream comprises at most 1.0 mol% C₂₊, preferably at most 0.5 mol%, more preferably at most 0.25 mol%, even more preferably at most 0.1 mol%. In the context of the present invention, "C₂₊" refers to C₂₊-hydrocarbons, i.e. hydrocarbons containing 2 or more carbon atoms per molecule.

[0008] Typically, the nitrogen-containing stream is a relatively cold stream having a temperature of at most (i.e. below) -90°C, preferably at most -140°C. Preferably, the nitrogen-containing stream has a temperature of at most -150°C, preferably at most -160°C, more preferably at most -162°C. Further, the nitrogen-containing stream typically has a pressure of below 10.0 bara, preferably below 5.0 bara, more preferably below 2.0 bara. In case the nitrogen-containing stream has an elevated pressure (i.e. above 6.0 bar such as from 6.0 to 25 bara), it preferably has a temperature of from -95°C to -140°C.

[0009] In step (b), the nitrogen-containing stream is heated in a first heat exchanger thereby obtaining a heated nitrogen-containing stream. Preferably, the heated nitrogen-containing stream obtained in step (b) has a temperature of from -140°C to 20°C, preferably below 10°C, more preferably below -20°C and preferably above - 80°C, more preferably above -50°C. In case the heated nitrogen-containing stream has an elevated pressure (i.e. above 6.0 bara, such as from 6.0 to 25 bara), it typically has a temperature of from 20°C to -90°C, preferably below -55°C.

[0010] In step (c), the heated nitrogen-containing stream is compressed thereby obtaining a compressed nitrogen-containing stream. Typically, the compressed nitrogen-containing stream typically has a pressure of above 3.0 bara to 45.0 bara, preferably below 30 bara and more preferably from 3 to 10 bara. If desired, the compressed nitrogen-containing stream may be cooled before splitting in step (d), e.g. in an air cooler.

[0011] In step (d), the compressed nitrogen-containing stream is split into a first split stream and a second split stream, typically at a volume ratio [first split stream/second split stream] of 0.10 to 0.40. Typically, the first split stream is reused.

[0012] In step (e), the second split stream is cooled in the first and a second heat exchanger thereby obtaining a cooled second split stream. Preferably, in step (e) the second split stream is cooled first in the second heat exchanger and then in the first heat exchanger to obtain the cooled second split stream.

[0013] Typically the cooled second split stream has a temperature of from -155°C to -160°C, preferably about - 158°C. Further, the cooled second split stream typically has a pressure of from 3 to 10 bara. The person skilled in the art will readily understand that the typical temperature of the cooled second split stream may depend on the pressure of the stream; in case the cooled second split stream has an elevated pressure (e.g. above 10 bara, such as in the range of from 17 to 45 bara) the temperature of said stream will typically be higher (i.e. warmer), preferably in the range of from -85°C to -145°C.

[0014] In step (f), the cooled second split stream is expanded thereby obtaining an expanded second split stream. Although the expander as used in step (f) according to the present invention is not particularly limited (and may include a JT valve an orifice, a common expander, etc.), it is preferred that in the expander enthalpy is withdrawn from the cooled second split stream. A suitable expander for withdrawing enthalpy whilst expanding is a turbo-expander.

[0015] Typically, the expanded second split stream typically has a pressure of from 1.5 to 6.0 bara, preferably about 2.0 bara. Preferably, the expanded second split stream obtained in step (f) has a temperature of at most -150°C, preferably at most -155°C, more preferably at most -160°C, even more preferably at most -165°C. Again, the person skilled in the art will readily understand that the typical temperature of the expanded second split stream may depend on the pressure of the stream. In case the expanded second split stream has an elevated pressure (e.g. above 6.0 bara) the temperature of said stream will typically be higher (i.e. warmer); in the latter case, the pressure is preferably between 6.0 and 25 bara and the temperature between - 115°C and -145°C.

[0016] In step (g), the expanded second split stream is separated in a first gas/liquid separator thereby obtaining a gaseous nitrogen-enriched stream and a liquid nitrogen-depleted stream. Preferably, the gaseous nitrogen-enriched stream obtained in step (g) comprises from 30 to 75 mol% nitrogen, preferably above 40 mol%, more preferably above 50 mol% and preferably less than 72 mol%. Further it is preferred that the liquid nitrogen-depleted stream obtained in step (g) comprises at least 90 mol% methane, preferably at least 92 mol% and typically less than 98 mol%, preferably less than 95 mol%. Typically, the liquid nitrogen-depleted stream obtained in step (g) comprises less than 10 mol% nitrogen.

[0017] In step (h) the gaseous nitrogen-enriched stream is heated in the first or second heat exchanger thereby obtaining a heated nitrogen-enriched stream. Preferably, in step (h) the gaseous nitrogen-enriched stream is heated in the second heat exchanger.

[0018] Typically the heated nitrogen-enriched stream has a temperature of from 10°C to 30°C, preferably about 15°C. Further, the gaseous nitrogen-enriched stream typically has a pressure of from 1.5 to 6.0 bara.

[0019] According to an especially preferred embodiment according to the present invention, the method further comprises the steps of:

(i) expanding the liquid nitrogen-depleted stream obtained in step (g) thereby obtaining an expanded nitrogen-depleted stream; and

(j) separating the expanded nitrogen-depleted stream in a second gas/liquid separator thereby obtaining a second gaseous nitrogen-enriched stream and a second liquid nitrogen-depleted stream.

[0020] Typically, the expanded nitrogen-depleted stream obtained in step (i) is a multiphase stream (in particular containing gas and liquid) and has a temperature of from -166°C to -173°C, preferably about - 171°C. Again, the person skilled in the art will readily understand that the typical temperature of the expanded nitrogen-depleted stream may depend on the pressure of the stream; in case the expanded nitrogen-depleted stream has an elevated pressure the temperature of said stream will typically be higher.

[0021] Preferably, the second gaseous nitrogen-enriched stream is used as at least a part of the nitrogen-containing stream provided in step (a). Preferably, all of the second gaseous nitrogen-enriched stream is used for the nitrogen-containing stream provided in step (a); of course, the nitrogen-containing stream provided in step (a) may be composed of more than only the second gaseous nitrogen-enriched stream.

[0022] According to a preferred embodiment, the second liquid nitrogen-depleted stream obtained in step (j) is stored in a storage tank, typically an LNG storage tank. Preferably, a boil-off gas stream from said storage tank is fed into the second gas/liquid separator.

[0023] Hereinafter the invention will be further illustrated by the following non-limiting drawings. Herein shows:

Fig. 1 schematically a process scheme for performing the method according to the present invention; and

Fig. 2 schematically an alternative process scheme for performing the method according to the present invention, at an elevated pressure.

[0024] For the purpose of this description, same reference numbers refer to same or similar components.

[0025] Fig. 1 schematically shows a process scheme for performing a method of removing nitrogen from a nitrogen-containing stream. The process scheme is generally referred to with reference number 1.

[0026] The process scheme 1 comprises a heat exchanger 2 ("the first heat exchanger") and a heat exchanger 3 ("the second heat exchanger"), a compressor 4, a first gas/liquid separator 5, a second gas/liquid separator 6, an LNG storage tank 7 and JT-valves 8 and 9. The process scheme may comprise further heat exchangers in addition to the first heat exchanger 2 and second heat exchanger 3. Preferably, the first heat exchanger 2 and second heat exchanger 3 are separate heat exchangers.

[0027] The particular embodiment as shown in Fig. 1 shows the nitrogen rejection from a boil-off stream 140 coming from the LNG tank 7.

[0028] During use of the process scheme 1 according to the present invention, a nitrogen-containing stream 10 is provided. Typically the nitrogen-containing stream 10 is a cold stream. The nitrogen-containing stream 10 is heated in a first heat exchanger 2 thereby obtaining a heated nitrogen-containing stream 20 and subsequently compressed in the compressor 4 thereby obtaining a compressed nitrogen-containing stream 30. The compressed nitrogen-containing stream 30 is split into a first split stream 40 and a second split stream 50.

[0029] The second split stream 50 is cooled in the first 2 and a second heat exchanger 3 thereby obtaining a cooled second split stream 70. The first and second heat exchangers 2 and 3 are indirect heat exchangers; hence no direct contact between the streams takes place, but only heat exchanging contact. In the embodiment of Figure 1, the second split stream 50 is cooled first in the second heat exchanger 3 and then in the first heat exchanger 2 to obtain the cooled second split stream 70.

[0030] Then, the cooled second split stream 70 is expanded in JT-valve 8 thereby obtaining an expanded second split stream 80, which expanded second split stream 80 is separated in the first gas/liquid separator 5 to obtain a gaseous nitrogen-enriched stream 90 and a liquid nitrogen-depleted stream 110. The gaseous nitrogen-enriched stream 90 is heated (in the embodiment of Figure 1) in the second heat exchanger 3 thereby obtaining a heated nitrogen-enriched stream 100. The heated nitrogen-enriched stream 100 is sent to a duct (or auxiliary) burner of a gas turbine to be used as a fuel gas.

[0031] In the embodiment of Figure 1, the liquid nitrogen-depleted stream 110 is expanded thereby obtaining an expanded nitrogen-depleted stream 120. Subsequently, the expanded nitrogen-depleted stream 120 is separated in the second gas/liquid separator 6 thereby obtaining a second gaseous nitrogen-enriched stream and a second liquid nitrogen-depleted stream 130. In the embodiment of Figure 1, the second gaseous nitrogen-enriched stream is used as (part of) the nitrogen-containing stream 10 heated in the first heat exchanger 2.

[0032] The second liquid nitrogen-depleted stream 130 is stored in an LNG storage tank 7. A boil-off gas stream 140 from said storage tank 7 is fed into the second gas/liquid separator 6 and reused. Of course, the LNG storage tank 7 may contain further inlets than shown in Figure 1. Also, other streams may be fed into the second gas/liquid separator 6.

[0033] In the embodiment of Fig. 2, an alternative process scheme is shown that is in particular suitable for performing the removal nitrogen at an elevated pressure (i.e. stream 10 at above 6.0 bara). Apart from different temperatures and pressures, Fig. 2 differs from the embodiment shown in Fig. 1 that it does not comprise the separator 6 and JT-valve 9. Also stream 110 feeds directly feeding into the tank 7. Further, nitrogen-containing stream 10 does not originate from the separator 6 (which is absent in Fig. 2), but may be drawn from elsewhere.

Example 1

[0034] Table 1 below shows an actual non-limiting example, providing information on conditions and composition of the various streams, whilst using the scheme of Figure 1 for removing nitrogen from a nitrogen-containing stream. In the embodiment of Table 1, nitrogen-containing stream 10 was at about atmospheric pressure and contained trace amounts (less than 0.1 mol%) of C₂₊.

Table 1. Composition and properties of various streams

Stream	Pressure [bara]	Temp. [°C]	State	Amount of CH4 [mol%]	Amount of N2 [mol%]
10	1.05	-162.7	Gas	82	18
20	0.85	-134.9	Gas	82	18
30	4.5	-38.5	Gas	82	18
40	4.5	-38.5	Gas	82	18
50	4.5	-38.5	Gas	82	18
60	4.5	-61.1	Gas	82	18
70	4.5	-158.5	Gas/liquid	82	18
80	2.0	-165.9	Gas/liquid	82	18
90	2.0	-165.9	Gas	30	70
100	1.8	-41.5	Gas	30	70
110	2.0	-165.9	Liquid	93	7
120	1.05	-171.1	Gas/liquid	93	7
130	1.05	-162.7	Liquid	96	4
140	1.05	-161.7	Gas	83	17

[0035] As can be seen from Table 1, the present invention allows for an effective removal of nitrogen (via stream 100) from stream 10; stream 130 being returned to the LNG storage tank 7 contained 4 mol% N₂.

Example 2

[0036] In a further non-limiting example, exemplified by Table 2 and whilst using the scheme of Figure 2, nitrogen-containing stream 10 had a higher pressure (17 bara) than in Example 1 and contained a small amount (0.2 mol%) of C₂₊.

Table 2. Composition and properties of various streams

Stream	Pressure [bara]	Temp. [°C]	State	Amount of CH4 [mol%]	Amount of N2 [mol%]	Amount of C2+ [mol%]
10	17.0	-111.3	Gas	92.1	7.7	0.2
20	16.65	-70.8	Gas	92.1	7.7	0.2
30	36.4	20.0	Gas	92.1	7.7	0.2
40	36.4	20.0	Gas	92.1	7.7	0.2
50	36.4	20.0	Gas	92.1	7.7	0.2
60	36.2	-108.8	Gas	92.1	7.7	0.2
70	36.0	-109.4	Gas	92.1	7.7	0.2
80	17.0	-118.7	Gas/ liquid	92.1	7.7	0.2
90	17.0	-118.7	Gas	76.4	23.6	0
100	16.8	-111.3	Gas	76.4	23.6	0
110	17.0	-118.7	Liquid	94.4	5.4	0.2
120	N.A.
130	N.A.
140	N.A.

[0037] As can be seen from Table 2, the present invention allows for an effective removal of nitrogen (via stream 100) from stream 10; stream 110 contained 5.4 mol% N₂.

[0038] The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention.

Claims

1. A method of removing nitrogen from a nitrogen-containing stream (10), the method comprising at least the steps of:

(a) providing a nitrogen-containing stream (10);

(b) heating the nitrogen-containing stream (10) in a first heat exchanger (2) thereby obtaining a heated nitrogen-containing stream (20);

(c) compressing the heated nitrogen-containing stream (20) thereby obtaining a compressed nitrogen-containing stream (30);

(d) splitting the compressed nitrogen-containing stream (30) into a first split stream (40) and a second split stream (50);

(e) cooling the second split stream (50) in the first (2) and a second heat exchanger (3) thereby obtaining a cooled second split stream (70);

(f) expanding the cooled second split stream (70) thereby obtaining an expanded second split stream (80);

(g) separating the expanded second split stream (80) in a first gas/liquid separator (5) thereby obtaining a gaseous nitrogen-enriched stream (90) and a liquid nitrogen-depleted stream (110);

(h) heating the gaseous nitrogen-enriched stream (90) in the first or second heat exchanger (2,3) thereby obtaining a heated nitrogen-enriched stream (100).

2. The method according to claim 1, wherein the nitrogen-containing stream (10) comprises at least 5 mol% nitrogen, preferably at least 10 mol%, more preferably at least 15 mol%.

3. The method according to claim 1 or 2, wherein the nitrogen-containing stream (10) comprises at most 1.0 mol% C₂₊, preferably at most 0.5 mol%, more preferably at most 0.25 mol%, even more preferably at most 0.1 mol%.

4. The method according to any one of claims 1-3, wherein the nitrogen-containing stream (10) has a temperature of at most -150°C, preferably at most -160°C, more preferably at most -162°C.

5. The method according to any one of claims 1-4, wherein in step (e) the second split stream (50) is cooled first in the second heat exchanger (3) and then in the first heat exchanger (2) to obtain the cooled second split stream (70).

6. The method according to any one of claims 1-5, wherein the expanded second split stream (80) obtained in step (f) has a temperature of at most -150°C, preferably at most -160°C, more preferably at most -165°C.

7. The method according to any one of claims 1-6, wherein the gaseous nitrogen-enriched stream (90) obtained in step (g) comprises from 30 to 75 mol% nitrogen, preferably above 40 mol%, more preferably above 50 mol% and preferably less than 72 mol%.

8. The method according to any one of claims 1-7, wherein the liquid nitrogen-depleted stream (110) obtained in step (g) comprises at least 90 mol% methane, preferably at least 92 mol%.

9. The method according to any one of claims 1-8, wherein in step (h) the gaseous nitrogen-enriched stream (90) is heated in the second heat exchanger (3).

10. The method according to any one of claims 1-9, further comprising the steps of:

(i) expanding the liquid nitrogen-depleted stream (110) obtained in step (g) thereby obtaining an expanded nitrogen-depleted stream (120); and

(j) separating the expanded nitrogen-depleted stream in a second gas/liquid separator (6) thereby obtaining a second gaseous nitrogen-enriched stream and a second liquid nitrogen-depleted stream (130).

11. The method according to claim 10, wherein the second gaseous nitrogen-enriched stream is used as at least a part of the nitrogen-containing stream (10) provided in step (a).

12. The method according to claim 10 or 11, wherein the second liquid nitrogen-depleted stream (130) obtained in step (j) is stored in a storage tank (7).

13. The method according to claim 12, wherein a boil-off gas stream (140) from said storage tank (7) is fed into the second gas/liquid separator (6).

Drawing