U.S. patent application number 11/632137 was filed with the patent office on 2008-03-20 for treating liquefied natural gas.
Invention is credited to Johan Jan Barend Pek, Cornelis Buijs, Robert Klein Nagelvoort.
Application Number | 20080066493 11/632137 |
Document ID | / |
Family ID | 34930484 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080066493 |
Kind Code |
A1 |
Buijs; Cornelis ; et
al. |
March 20, 2008 |
Treating Liquefied Natural Gas
Abstract
Method of treating liquefied natural gas (1) to obtain a liquid
stream (21) having a reduced content of components having low
boiling points comprising expanding (3) the liquefied gas to expand
to obtain expanded two-phase fluid; introducing the two-phase fluid
into a column (10) below a gas-liquid contacting section (14);
withdrawing from the bottom (16) a liquid-stream (17) having a
reduced content of components having low boiling points;
withdrawing from the top (23) of the column (10) a gaseous stream
(25) enriched in components having low boiling points; heating the
gaseous stream in a heat exchanger (27); compressing (30) the
stream to fuel gas pressure to obtain fuel gas (33); separating a
recycle stream (34a) from the fuel gas; at least partly condensing
(27) the recycle stream to obtain a reflux stream (34b); and
introducing the reflux stream (34b) into the column (10) above the
contacting section (14).
Inventors: |
Buijs; Cornelis; (The Hague,
NL) ; Nagelvoort; Robert Klein; (The Hague, NL)
; Barend Pek; Johan Jan; (The Hague, NL) ; Buijs;
Cornelis; (The Hague, NL) ; Nagelvoort; Robert
Klein; (The Hague, NL) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
34930484 |
Appl. No.: |
11/632137 |
Filed: |
July 12, 2005 |
PCT Filed: |
July 12, 2005 |
PCT NO: |
PCT/EP05/53319 |
371 Date: |
January 10, 2007 |
Current U.S.
Class: |
62/620 |
Current CPC
Class: |
F25J 2200/76 20130101;
F25J 2200/02 20130101; F25J 2240/30 20130101; F25J 2270/02
20130101; F25J 2215/04 20130101; F25J 2200/78 20130101; F25J
2235/60 20130101; F25J 3/0209 20130101; F25J 2230/60 20130101; F25J
2205/02 20130101; F25J 2245/90 20130101; F25J 2270/04 20130101;
F25J 2270/42 20130101; F25J 3/0233 20130101; F25J 2290/62 20130101;
F25J 2200/40 20130101; F25J 3/0257 20130101; F25J 2290/40
20130101 |
Class at
Publication: |
062/620 |
International
Class: |
F25J 3/02 20060101
F25J003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 12, 2004 |
EP |
04254152.4 |
Claims
1. A method of treating liquefied natural gas supplied at
liquefaction pressure containing components having low boiling
points to obtain a liquid product stream having a reduced content
of components having low boiling points, which method comprises the
steps of: (a) allowing the liquefied gas to expand to separation
pressure to obtain an expanded two-phase fluid; (b) introducing the
expanded two-phase fluid into a column below a gas-liquid
contacting section arranged in the column; (c) collecting in the
bottom of the column liquid from the two-phase fluid and
withdrawing from the bottom of the column a liquid stream having a
reduced content of components having low boiling points;
introducing the liquid stream into a flash vessel at a low
pressure; removing a second gaseous stream from the top of the
flash vessel; and removing from the bottom of the flash vessel a
liquid stream to obtain the liquid product stream; (d) allowing
vapor from the two-phase fluid to flow through the contacting
section; (e) withdrawing from the top of the column a gaseous
stream that is enriched in components having low boiling points;
(f) heating the gaseous stream obtained in step (e) in a heat
exchanger to obtain a heated gaseous stream; (g) compressing the
heated gaseous stream obtained in step (f) to fuel gas pressure to
obtain fuel gas; (h) separating a recycle stream from the fuel gas
obtained in step (g); (i) at least partly condensing the recycle
stream obtained in step (h) to obtain a reflux stream; and (j)
introducing the reflux stream obtained in step (i) at separation
pressure into the column above the contacting section.
2. The method according to claim 1, further comprising heating the
second gaseous stream in the heat exchanger; compressing the second
gaseous stream to fuel gas pressure; and adding the second gaseous
stream to the recycle stream.
3. The method according to claim 1, wherein at least partly
condensing the recycle stream comprises indirectly heat exchanging
the recycle stream with the gaseous stream(s) in the heat
exchanger.
4. The method according to claim 1, wherein compressing the heated
gaseous stream to fuel gas pressure to obtain fuel gas further
includes removing the heat of compression.
5. The method according to claim 1, wherein the recycle stream
separated from the fuel gas is compressed to an elevated pressure
before it is at least partly condensed.
6. The method according to claim 2, wherein at least partly
condensing the recycle stream comprises indirectly heat exchanging
the recycle stream with the gaseous stream(s) in the heat
exchanger.
7. The method according to claim 2, wherein compressing the heated
gaseous stream to fuel gas pressure to obtain fuel gas further
includes removing the heat of compression.
8. The method according to claim 3, wherein compressing the heated
gaseous stream to fuel gas pressure to obtain fuel gas further
includes removing the heat of compression.
9. The method according to claim 2, wherein the recycle stream
separated from the fuel gas is compressed to an elevated pressure
before it is at least partly condensed.
10. The method according to claim 3, wherein the recycle stream
separated from the fuel gas is compressed to an elevated pressure
before it is at least partly condensed.
11. The method according to claim 4, wherein the recycle stream
separated from the fuel gas is compressed to an elevated pressure
before it is at least partly condensed.
Description
[0001] The present invention relates to treating liquefied natural
gas, and in particular treating liquefied natural gas that contains
components having boiling-points lower than methane. An example of
such a component is nitrogen. In the specification and in the
claims the expressions `low boiling point components` and
`components having low boiling points` will be used to refer to
components having boiling points lower than methane. The treatment
is directed to removing low boiling point components from the
liquefied natural gas in order to obtain a liquefied natural gas
having a reduced content of components having low boiling points.
The improved method can be applied in two ways: (1) to treat the
same amount of liquefied natural gas as in a conventional method,
or (2) to treat a larger amount of liquefied natural gas as in a
conventional method. When applied in the first way, the content of
low boiling point components in the liquefied gas treated with the
method of the present invention is lower than that in liquefied gas
treated with a conventional method. When applied in the second way,
the content of low boiling point components is maintained and the
amount of liquefied gas is increased.
[0002] U.S. Pat. No. 6,199,403 discloses a method to remove a
high-volatility component such as nitrogen from a feed stream rich
in methane. According to U.S. Pat. No. 6,199,403 the expanded
liquefied natural gas stream enters a separation column at an
intermediate level, i.e. not below a gas-liquid contacting
section.
[0003] U.S. Pat. No. 5,421,165 relates to a process for
denitrogenation of a feedstock of a liquefied mixture of
hydrocarbons. To this end U.S. Pat. No. 5,421,165 suggests a
relatively complicated process using a denitrogenation column
comprising a plurality of theoretical fractionation stages.
[0004] Another relatively complicated process has been described in
International patent application publication No. WO 02/50483. WO
02/50483 discloses several methods of removing components having
low boiling points from liquefied natural gas. According to WO
02/50483 a liquid product stream having a reduced content of
components having low boiling points is obtained.
[0005] A problem of the above processes described in WO 02/50483 is
that the liquid product stream contains an undesirable high content
of components having low boiling points.
[0006] It is an object of the present invention to minimize the
above problem.
[0007] It is a further object of the present invention to provide
an alternative process.
[0008] It is an even further object of the present invention to
provide a simplified process to reduce the amount of components
having low boiling points in a liquefied natural gas stream.
[0009] One or more of the above or other objects are achieved
according to the present invention by providing a method of
treating liquefied natural gas supplied at liquefaction pressure
containing components having low boiling points to obtain a liquid
product stream having a reduced content of components having low
boiling points, which method comprises the steps of: [0010] (a)
allowing the liquefied gas to expand to separation pressure to
obtain an expanded two-phase fluid; [0011] (b) introducing the
expanded two-phase fluid into a column below a gas-liquid
contacting section arranged in the column; [0012] (c) collecting in
the bottom of the column liquid from the two-phase fluid and
withdrawing from the bottom of the column a liquid stream having a
reduced content of components having low boiling points;
introducing the liquid stream into a flash vessel at a low
pressure; removing a second gaseous stream from the top of the
flash vessel; and removing from the bottom of the flash vessel a
liquid stream to obtain the liquid product stream; [0013] (d)
allowing vapor from the two-phase fluid to flow through the
contacting section; [0014] (e) withdrawing from the top of the
column a gaseous stream that is enriched in components having low
boiling points; [0015] (f) heating the gaseous stream obtained in
step (e) in a heat exchanger to obtain a heated gaseous stream;
[0016] (g) compressing the heated gaseous stream obtained in step
(f) to fuel gas pressure to obtain fuel gas; [0017] (h) separating
a recycle stream from the fuel gas obtained in step (g); [0018] (i)
at least partly condensing the recycle stream obtained in step (h)
to obtain a reflux stream; and [0019] (j) introducing the reflux
stream obtained in step (i) at separation pressure into the column
above the contacting section.
[0020] Applicants have found that the liquid product stream
obtained according to the present invention contains a smaller
content of components having low boiling points than one would
expect.
[0021] An important advantage of the method according to the
present invention is that it can be suitably used for large
liquefaction plants.
[0022] The present invention will now be illustrated by way of
example in more detail with reference to the non-limiting
accompanying drawings, wherein:
[0023] FIG. 1 shows schematically a process flow scheme
illustrating a part of an embodiment of the method of the present
invention (not including a flash vessel as required according to
the present invention);
[0024] FIG. 2 shows schematically an alternative of the process of
FIG. 1;
[0025] FIG. 3 shows schematically a process flow scheme of a fully
elaborated embodiment of the method of the present invention,
including a flash vessel;
[0026] FIG. 4 shows schematically an alternative of the process of
FIG. 3;
[0027] FIG. 5 shows schematically and not to scale an alternative
to part V of the process flow scheme of FIG. 4; and
[0028] FIG. 6 shows the process according to FIG. 4 having two
contacting zones.
[0029] Reference is made to FIG. 1. Liquefied natural gas
containing components having low boiling points is supplied at
liquefaction pressure through conduit 1 to an expansion device in
the form of expansion engine 3 and Joule-Thompson valve 5 in the
discharge conduit 6 of expansion engine 3. In the expansion device,
the liquefied gas is allowed to expand to separation pressure, and
an expanded two-phase fluid is obtained. The liquefaction pressure
is suitably in the range of from 3 to 8.5 MPa and the separation
pressure is suitably in the range of from 0.1 to 0.5 MPa.
[0030] The expanded two-phase fluid is passed through conduit 9 to
a column 10. The expanded two-phase fluid is introduced into the
column 10 at separation pressure via a suitable inlet device, such
as vane inlet device 12. The vane inlet device, also known as
schoepentoeter, allows efficient separation of gas and liquid.
[0031] The column 10 is provided with a gas-liquid contacting
section 14. The contacting section 14 may comprise any suitable
means for contacting a gas and a liquid, such as trays and
packings. Preferably, the contacting section 14 consists of between
two and eight horizontal contacting trays 15. The expanded
two-phase fluid is introduced into the column 10 below the
gas-liquid contacting section 14. The person skilled in the art
will readily understand that the column may comprise two or more
contacting sections 14.
[0032] In the bottom 16 of the column 10 liquid from the two-phase
fluid is collected, and a liquid stream having a reduced content of
components having low boiling points is removed from the bottom 16
through conduit 17 and pumped by pump 18 to a storage tank 20. From
the storage tank 20 a liquid product stream is removed through
conduit 21 and a gaseous stream through conduit 22. The gaseous
stream is also known as boil-off gas.
[0033] Vapor from the two-phase fluid to flow through the
contacting section 14. From the top 23 of the column 10 a gaseous
stream that is enriched in components having low boiling points is
removed through conduit 25. The gaseous stream is heated in a heat
exchanger 27 to obtain a heated gaseous stream that is passed
through conduit 28 to a compressor 30. In compressor 30 the heated
gaseous stream is compressed to fuel gas pressure to obtain fuel
gas. The fuel gas is removed through conduit 31 and cooled in heat
exchanger 32 to remove the heat of compression. The fuel gas is
passed away through conduit 33. The fuel gas pressure is in the
range of from 1 to 3.5 MPa.
[0034] A recycle stream from the fuel gas and supplied to the heat
exchanger 27 through conduit 34a. In the heat exchanger 27 the
recycle stream is at least partly condensed to obtain a reflux
stream, which is passed to the column 10 through the conduit 34b
provided with Joule-Thompson valve 37. The reflux stream is
introduced at separation pressure into the column 10 via inlet
device, such as vane inlet device 39 above the contacting section
14.
[0035] Table 1 summarizes the result of a hypothetical example,
wherein the method of FIG. 1 is compared to a base case. In the
base case the recycle stream and the feed are introduced into the
column at the same level, so that the liquid phases of the two
streams are mixed before introduction thereof in the column and the
column has no contacting section. It was found that the liquid
stream withdrawn through conduit 17 for the base case contains more
nitrogen than the same stream for the present invention.
TABLE-US-00001 TABLE 1 Summary of hypothetical example with the
embodiment of FIG. 1. Embodiment of Base case Number of trays in 3
-- contacting section Flow rate feed 190.86 kg/s 190.86 kg/s
through conduit 9 Temperature of -145.degree. C. -145.degree. C.
feed introduced through inlet device 12 Nitrogen content 3.05 mol %
3.05 mol % in feed Recycle flow rate 26 kg/s 26 kg/s Temperature of
-165.6.degree. C. -165.2.degree. C. recycle introduced through
inlet device 39 Nitrogen content Vapour phase Total recycle of
recycle stream contains 33 mol % stream contains Liquid phase 22
mol % contains 1.7 mol % Flow rate of 169.25 kg/s 169.19 kg/s
product in conduit 21 Nitrogen content 0.65 mol % 0.82 mol % of
product in conduit 21 Flow rate of fuel 20.51 kg/s 20.59 kg/s gas
in conduit 33 Nitrogen content 24 mol % 22 mol % of fuel gas Power
required for 30.8 MW 31.2 MW compressor 30
[0036] Table 1 shows that a lower nitrogen content in the product
stream is obtained with the method of the present invention.
[0037] In an alternative embodiment the recycle stream separated
from the fuel gas is additionally compressed in an auxiliary
compressor to an elevated pressure before it is at least partly
condensed in heat exchanger 27. The high-pressure recycle stream
can be used in several ways, which will be discussed with reference
to FIG. 2. The parts that were already discussed with reference to
FIG. 1 have got the same reference numerals.
[0038] The auxiliary compressor included in conduit 34a is referred
to with reference numeral 35. The auxiliary compressor 35 can be
provided with a cooler (not shown) to remove the heat of
compression for the compressed recycle stream. The compressed
recycle stream is at least partly condensed by cooling it in heat
exchanger 27. Part of the cold that is needed is provided by the
gaseous stream that is enriched in components having low boiling
points that is passed through conduit 25. The remainder is provided
by the recycle stream. Cold from the recycle stream can be obtained
by expanding a part of the recycle stream to an intermediate
pressure in Joule-Thompson valve 38, using the expanded fluid to
cool the recycle stream in conduit 34a and supplying the expanded
fluid through conduit 38a to the compressor 30. The intermediate
pressure to which the part of the recycle stream is expanded is in
the range of from the suction pressure to the discharge pressure of
the compressor 30 (ends of the range included). The stage at which
the expanded recycle stream enters the compressor 30 is so selected
that the pressure of the expanded recycle stream matches the
pressure of the fluid in the compressor 30 in that stage.
[0039] The remainder of the recycle stream is expanded by the
Joule-Thompson valve 37 and introduced as reflux in the column 10
as discussed with reference to FIG. 1.
[0040] An advantage of the embodiment discussed with reference to
FIG. 2 is that the recycle stream is expanded from a larger
pressure and thus cooled to a lower temperature. This allows a
warmer feed stream, for example a feed stream at -142.degree. C.,
compared to a feed stream temperature of -145.degree. C. (in the
above example). Thus the temperature of the liquefied gas from the
main cryogenic heat exchanger can be higher and therefore, for the
same amount of energy, more gas can be liquefied.
[0041] The elevated pressure of the fluid discharged from the
auxiliary compressor 35 is so selected that the costs of the power
required to drive the auxiliary compressor 35 are less than the
value of the increased amount of gas that is liquefied.
[0042] In the above we discussed an embodiment in which the
expansion is done in the expansion valves 37 and 38. However, it
will be understood that the expansion of the recycle stream can be
done in two stages, at first in an expansion device, such as
expander 36 and subsequently in the Joule-Thompson valves 37 and
38.
[0043] Instead of supplying the expanded fluid through conduit 38a
to the compressor 30, the expanded fluid can be supplied to an
inlet (not shown) of the compressor 35.
[0044] In the embodiments discussed with reference to FIGS. 1 and
2, the liquid from the two-phase fluid is collected in the bottom
16 of the column 10, and from the bottom 16 a liquid stream 17 is
withdrawn having a reduced content of components having low boiling
points to obtain the liquid product stream. In an alternative
embodiment of the invention, this step comprises collecting in the
bottom of the column liquid from the two-phase fluid and
withdrawing from the bottom of the column a liquid stream having a
reduced content of components having low boiling points;
introducing the liquid stream into a flash vessel at a low
pressure; removing a second gaseous stream from the top of the
flash vessel; and removing from the bottom of the flash vessel a
liquid stream to obtain the liquid product stream.
[0045] This embodiment according to the present invention including
a flash vessel will now be discussed with reference to FIG. 3. The
parts that were already discussed with reference to FIG. 1 have got
the same reference numerals.
[0046] The column 10' comprises an upper part 10u and a lower part
10l, wherein the upper part performs the function of the column 10
in FIG. 1 and the lower part 10l is a flash vessel operating at a
pressure that is below the pressure in the upper part 10u. Suitably
the pressure in the upper part 10u is in the range of from 0.2 to
0.5 MPa and the pressure in the flash vessel 10l in the range of
from 0.1 to 0.2 MPa. The person skilled in the art will readily
understand that the flash vessel 10l may be a component that is
physically separated from the column 10l (i.e. at a certain
distance).
[0047] During normal operation, liquid from the two-phase fluid
supplied through conduit 9 is collected in the bottom 16' of the
upper part 10u of the column 10'. From that bottom 16' is withdrawn
a liquid stream having a reduced content of components having low
boiling points through conduit 17'. This stream is then introduced
into the flash vessel 10l at a low pressure. The pressure reduction
is achieved by means of Joule-Thompson valve 40 in conduit 17'.
Consequently a two-phase mixture is formed and that is introduced
via inlet device 41 into the flash vessel 10l.
[0048] Through conduit 17'' a liquid stream having a reduced
content of components having low boiling points is removed, which
is passed to the storage tank 20.
[0049] From the top 23'' of the flash vessel 10l a second gaseous
stream is removed.
[0050] Suitably the second gaseous stream is passed through conduit
42 to heat exchanger 27, in which the second gaseous stream is
heated by heat exchange with the recycle stream supplied through
conduit 34a. The heated stream is compressed in compressor 45, the
heat of compression is removed in heat exchanger 48 and passed
through conduit 49 to add the compressed second gaseous stream to
the recycle stream in conduit 34a.
[0051] It will be understood that compressors 45 and 30 can be
combined into one compressor (not shown). In that case, conduit 42
is connected to the suction end of that compressor, conduit 28 to
an intermediate inlet and conduit 32 is connected to the discharge
end of that compressor.
[0052] An advantage of this method is that it can be used for large
liquefaction plants.
[0053] As with the embodiment discussed with reference to FIG. 1,
the embodiment discussed with reference to FIG. 3 can as well be
provided with an auxiliary compressor to compress the recycle
stream separated from the fuel gas to an elevated pressure before
it is at least partly condensed in heat exchanger 27. The
high-pressure recycle stream can be used in several ways, which
will be discussed with reference to FIG. 4. The parts that were
already discussed with reference to FIG. 3 have got the same
reference numerals.
[0054] The auxiliary compressor included in conduit 34a is referred
to with reference numeral 35. The auxiliary compressor 35 can be
provided with a cooler (not shown) to remove the heat of
compression for the compressed recycle stream. The compressed
recycle stream is partially condensed by cooling it in heat
exchanger 27. Part of the cold that is needed is provided by the
gaseous stream that is enriched in components having low boiling
points that is passed through conduit 25. The remainder is provided
by the recycle stream. Cold from the recycle stream can be obtained
by expanding a part of the recycle stream to an intermediate
pressure in Joule-Thompson valve 38, using the expanded fluid to
cool the recycle stream in conduit 34a and supplying the expanded
fluid through conduit 38a to the compressor 30. The intermediate
pressure to which the part of the recycle stream is expanded is in
the range of from the suction pressure to the discharge pressure of
the compressor 30 (ends of the range included). The point at which
the expanded recycle stream enters the compressor 30 is so selected
that the pressure of the expanded recycle stream matches the
pressure of the fluid in the compressor 30 at the inlet point.
[0055] The remainder of the recycle stream is expanded by the
Joule-Thompson valve 37 and introduced as reflux in the column 10
as discussed with reference to FIG. 1.
[0056] An advantage of this embodiment is that the recycle stream
is expanded from a larger pressure and thus cooled to a lower
temperature. This allows a warmer feed stream, for example a feed
stream at -142.degree. C., compared to a feed stream temperature of
-145.degree. C. (in the above example). Thus the temperature of the
liquefied gas from the main cryogenic heat exchanger can be higher
and therefore, for the same amount of energy, more gas can be
liquefied.
[0057] The elevated pressure of the fluid discharged from the
auxiliary compressor 35 is so selected that the costs of the power
required to drive the auxiliary compressor 35 are less than the
value of the increased amount of gas that is liquefied.
[0058] In the above we discussed an embodiment in which the
expansion is done in the expansion valves 37 and 38. However, it
will be understood that the expansion of the recycle stream can be
done in two stages, at first in an expansion device, such as
expander 36 and subsequently in the Joule-Thompson valves 37 and
38.
[0059] FIG. 4 also shows that the boil-off gas from the storage
tank 20 is provided via the conduit 22 to the suction end of the
compressor 45.
[0060] It will be understood that compressors 45 and 30 can be
combined into one compressor (not shown). In that case, conduit 42
(into which conduit 22 opens) is connected to the suction end of
that compressor, conduit 28 to an intermediate inlet and conduit 32
is connected to the discharge end of that compressor.
[0061] Instead of supplying the expanded fluid through conduit 38a
to the compressor 30, the expanded fluid can be supplied to an
inlet (not shown) of the compressor 35.
[0062] An alternative of the embodiment shown in FIG. 4 is shown in
FIG. 5, wherein a part of the recycle stream that is passed through
conduit 34a is separated therefrom and passed through conduit 50
through the heat exchanger 27. Then the cooled recycle stream is
expanded to the intermediate pressure in expander 51 and used to
cool the recycle stream in conduit 34a. The expanded stream is then
introduced into the compressor 30 at an intermediate stage.
[0063] Suitably, the recycle stream passed through conduit 34a is
between 10 and 90% by mass of the fuel gas that is passed through
conduit 31.
[0064] FIG. 6 shows the process according to FIG. 4 wherein the
column 10u comprises two contacting sections 14. The skilled person
will readily understand that more than two contacting sections 14
may be present.
[0065] From between the contacting sections 14, a stream is removed
via draw off device 63 and fed via line 60 to a heat exchanger 61,
wherein the stream is heat exchanged against the stream in line 1.
Subsequently the stream in line 60 is returned to the column 10u
and fed via vane inlet device 62.
[0066] In the embodiments discussed with reference to the Figures,
the contacting section 14 contains trays, however, any other
contacting means such as packing can be employed as well. The
length of the packed section is then preferably equivalent to
between two and eight contacting trays for the section above vane
inlet device 12 and between five and fifteen trays for the section
below draw-off device 63.
[0067] The method of the present invention provides a simple way of
reducing the amount of components having low boiling points in a
liquefied natural gas stream.
* * * * *