U.S. patent application number 16/833872 was filed with the patent office on 2020-07-16 for process for producing liquefied natural gas.
The applicant listed for this patent is GASCONSULT LIMITED. Invention is credited to Anthony Dwight Maunder, Geoffrey Frederick Skinner.
Application Number | 20200224966 16/833872 |
Document ID | / |
Family ID | 54292047 |
Filed Date | 2020-07-16 |
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United States Patent
Application |
20200224966 |
Kind Code |
A1 |
Skinner; Geoffrey Frederick ;
et al. |
July 16, 2020 |
Process for Producing Liquefied Natural Gas
Abstract
A process for liquefying methane-rich gases comprising: (a)
providing a stream of methane-rich feed gas containing higher
hydrocarbons comprising C.sub.5+ hydrocarbons and/or aromatic
compounds at a feed gas pressure of from 40 bar to 120 bar; (b)
providing a stream of methane-rich recycle gas at a recycle gas
pressure of from 40 bar to 120 bar; (c) mixing the feed gas with a
first part of the recycle gas to form a mixture; (d) passing the
resulting mixture to a first gas expander having an outlet, the
first expander outlet having a first gas expander outlet pressure
of between 3 bar and 50 bar and less than the feed gas and recycle
gas pressures, to form a first gas expander outlet stream
comprising a mixture of vapor and a condensed liquid containing the
higher hydrocarbons; (e) separating the first gas expander outlet
stream into a liquid stream and a vapor stream; (f) reheating and
compressing the first vapor stream to a first vapor stream pressure
of from 40 bar to 120 bar to form a first constituent of the
recycle gas; (g) cooling a second part of the recycle gas to a
temperature higher than an outlet temperature of the first gas
expander; and, (h) liquefying said cooled second part of the
recycle gas to form liquefied methane, wherein a content of C5+
hydrocarbons is about 0.1 mol % or less in the liquefied methane
and a content of aromatic compounds is below 1 mol ppm in the
liquefied methane. In an embodiment, the cooled second part of the
recycle gas is completely or substantially liquefied in step (h).
In another embodiment, the second part of the recycle gas is
liquefied to form liquefied methane and a second vapor stream in
step (h), and the second vapor stream is reheated and compress in a
step (i) to a second vapor stream pressure of from 40 bar to 120
bar to form a second constituent of the recycle gas.
Inventors: |
Skinner; Geoffrey Frederick;
(Reading, GB) ; Maunder; Anthony Dwight;
(Caversham, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GASCONSULT LIMITED |
Twickenham |
|
GB |
|
|
Family ID: |
54292047 |
Appl. No.: |
16/833872 |
Filed: |
March 30, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15739179 |
Dec 22, 2017 |
10641548 |
|
|
PCT/GB2016/000127 |
Jun 23, 2016 |
|
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16833872 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2270/06 20130101;
F25J 1/0035 20130101; F25J 1/0022 20130101; F25J 1/0202 20130101;
F25J 2290/12 20130101; F25J 1/0254 20130101; F25J 2220/64 20130101;
F25J 1/004 20130101; F25J 1/0037 20130101 |
International
Class: |
F25J 1/00 20060101
F25J001/00; F25J 1/02 20060101 F25J001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 2015 |
GB |
1514932.1 |
Claims
1. A process for liquefying methane-rich gases, the process
comprising: (a) providing a stream of methane-rich feed gas
containing higher hydrocarbons comprising C.sub.5+ hydrocarbons
and/or aromatic compounds at a feed gas pressure of from 40 bar to
120 bar; (b) providing a stream of methane-rich recycle gas at a
recycle gas pressure of from 40 bar to 120 bar; (c) mixing the feed
gas with a first part of the recycle gas to form a mixture; (d)
passing the resulting mixture to a first gas expander having an
outlet, the first gas expander outlet having a first gas expander
outlet pressure of between 3 bar and 50 bar and less than said feed
gas and recycle gas pressures, to form a first gas expander outlet
stream comprising a mixture of vapor and a condensed liquid
containing said higher hydrocarbons; (e) separating the first gas
expander outlet stream into a liquid stream and a first vapor
stream; (f) reheating and compressing said first vapor stream to a
first vapor stream pressure of from 40 bar to 120 bar to form a
first constituent of said recycle gas; (g) cooling a second part of
said recycle gas to a temperature higher than an outlet temperature
of said first gas expander; (h) liquefying said cooled second part
of the recycle gas to form liquefied methane and a second vapor
stream, wherein a content of C.sub.5+ hydrocarbons is about 0.1 mol
% or less in the liquefied methane and a content of aromatic
compounds is below 1 mol ppm in the liquefied methane; and, (i)
reheating and compressing said second vapor stream to a second
vapor stream pressure of from 40 bar to 120 bar to form a second
constituent of said recycle gas.
2. The process according to claim 1, comprising cooling the mixture
of feed gas and the first part of the recycle gas in a heat
exchanger before passing the mixture to the first gas expander.
3. The process according to claim 1, comprising heating or cooling
the first gas expander outlet stream in a heat exchanger prior to
separation to modify the quantity of said higher hydrocarbons in
the liquid.
4. The process as claimed in claim 1, wherein the methane-rich feed
gas pressure and the methane-rich recycle gas pressure are each
from 50 bar to 100 bar and/or the first gas expander pressure is
from 5 bar to 30 bar.
5. The process as claimed in claim 1, comprising at least partially
cooling the second part of the recycle gas by heat exchange with
the second vapor stream prior to compressing said second vapor
stream.
6. The process as claimed in claim 1, wherein the methane-rich gas
is natural gas.
7. A process for liquefying methane-rich gases, the process
comprising: (a) providing a stream of methane-rich feed gas
containing higher hydrocarbons comprising C.sub.5+ hydrocarbons
and/or aromatic compounds at a feed gas pressure of from 40 bar to
120 bar; (b) providing a stream of methane-rich recycle gas at a
recycle gas pressure of from 40 bar to 120 bar; (c) mixing the feed
gas with a first part of the recycle gas to form a mixture; (d)
passing the resulting mixture to a first gas expander having an
outlet, the first gas expander outlet having a first gas expander
outlet pressure of between 3 bar and 50 bar and less than the feed
gas and recycle gas pressures, to form a first gas expander outlet
stream comprising a mixture of vapor and a condensed liquid
containing said higher hydrocarbons; (e) separating the first gas
expander outlet stream into a liquid stream and a first vapor
stream; (f) reheating and compressing said first vapor stream to a
vapor stream pressure of from 40 bar to 120 bar to form a first
constituent of said recycle gas; (g) cooling a second part of said
recycle gas to a temperature higher than an outlet temperature of
said first gas expander; and, (h) completely or substantially
liquefying said cooled second part of the recycle gas to form
liquefied methane, wherein a content of C.sub.5+ hydrocarbons is
about 0.1 mol % or less in the liquefied methane and a content of
aromatic compounds is below 1 mol ppm in the liquefied methane.
8. The process according to claim 7, comprising cooling the mixture
of feed gas and the first part of the recycle gas in a heat
exchanger before passing the mixture to the first gas expander.
9. The process according to claim 7, comprising heating or cooling
the first gas expander outlet stream in a heat exchanger prior to
separation to modify the quantity of said higher hydrocarbons in
the liquid.
10. The process as claimed in claim 7, wherein the methane-rich
feed gas pressure and the methane-rich recycle gas pressure are
each from 50 bar to 100 bar and/or the first gas expander pressure
is from 5 bar to 30 bar.
11. The process as claimed in claim 7, comprising at least
partially cooling the second part of the recycle gas by heat
exchange with the second vapor stream prior to compressing said
second vapor stream.
12. The process as claimed in claim 7, wherein the methane-rich
feed gas is natural gas.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 15/739,179 having 35 U.S.C. 371(c) date of Dec. 22, 2017, which
is the U.S. National Stage of International Application No.
PCT/GB2016/000127 filed Jun. 23, 2016, which claims the priority
benefit of UK Application No. 1514932.1 filed Aug. 21, 2015, the
entire respective disclosures of which are incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a method for liquefying
methane-rich gas containing higher hydrocarbons.
BACKGROUND
[0003] In the production of liquid methane-rich gas, such as liquid
natural gas (LNG) it is generally desired to reduce its content of
C.sub.5+ hydrocarbons to around 0.1 mol % and of aromatic compounds
to below 1 mol ppm to avoid such materials solidifying in the heat
exchangers of the liquefaction process. The content of such higher
hydrocarbons is normally reduced by means such as cooling the feed
gas and removing the condensed liquid, or by washing the feed gas
with a suitable hydrocarbon liquid in a so-called "scrub column",
or by the use of a solid adsorbent.
[0004] However, when the pressure of the feed gas is much higher
than 50 bar, the above-mentioned techniques may be insufficient to
achieve the desired levels of residual higher hydrocarbons. In such
instances provision can be made for the pressure of the feed gas to
be reduced significantly, typically in a work expander, its heavy
hydrocarbon content then reduced by condensation or scrubbing, and
the depleted feed gas recompressed to near its original pressure
upstream for the liquefaction step.
SUMMARY OF THE INVENTION
[0005] According to the invention there is provided:
[0006] a process for liquefying natural gas or other methane-rich
gases comprising [0007] providing a stream of feed methane-rich gas
at a pressure of from 40 to 120 bar and containing higher
hydrocarbons; [0008] providing a stream of methane-rich recycle gas
at a pressure of from 40 to 120 bar; [0009] mixing the feed gas
with a first part of the recycle gas; [0010] passing the resulting
mixture to a gas expander, the expander outlet having a pressure of
between 3 bar and 50 bar, so as to form a mixture of vapor and a
condensed liquid containing higher hydrocarbons (C.sub.5+
hydrocarbons and/or aromatic compounds); [0011] separating the
expander outlet stream into a liquid stream and a vapor stream;
[0012] reheating and compressing said vapor stream to a pressure of
from 40 to 120 bar to form a first constituent of the above-said
recycle gas; [0013] cooling a second part of the said recycle gas
to a temperature higher than the outlet temperature of the said gas
expander; [0014] passing said cooled second part of the recycle gas
into a liquefaction unit to form liquefied methane and a second
vapor stream; [0015] reheating and compressing said second vapor
stream to a pressure of from 40 to 120 bar to form a second
constituent of the above-said recycle gas.
[0016] The invention comprises an adaptation of methane expander
based LNG processes, and particularly of the dual methane expander
process described in WO 2012/172281, whereby the feed gas is
supplied to the said expander and the desired quantity of condensed
heavy hydrocarbons is separated from the expander outlet
stream.
[0017] The invention is applicable particularly to floating LNG
production, due to the potential for reducing weight and deck area,
and to small scale land-based LNG production from higher pressure
natural gases.
[0018] The pressure of the feed methane-rich gas is preferably from
50 to 100 bar in which case the recycle gas is preferably also
pressurized to 50 to 100 bar. The outlet pressure of the gas
expander is preferably from 5 to 30 bar.
[0019] Optionally, the mixture of feed gas and part of the recycle
gas is cooled in a heat exchanger before admission to the gas
expander. Optionally, the outlet stream from the gas expander may
be heated or cooled to vary the quantity of higher hydrocarbons in
the liquid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The invention will be further described with reference to
the accompanying drawings in which FIG. 1 represents a flow diagram
illustrating a process in accordance with the invention, FIG. 2
represents a flow diagram illustrating a process in accordance with
another embodiment of the invention, and FIG. 3 represents a flow
diagram illustrating a process in accordance with yet another
embodiment of the invention. Throughout the drawing figures, like
reference numerals identify like elements of the drawings.
[0021] The exact flow sheet will depend upon the feed gas
specification, but will generally contain these basic elements.
Where pressures are stated anywhere in this application as "bar",
these are bar absolute.
[0022] The feed natural gas (1) is passed through a pretreatment
stage A in which components such as acid gases, water vapor and
mercury may be removed to produce a pre-treated gas (2).
[0023] The pre-treated gas is mixed with a first part (4) of a
recycle gas (3), described below, comprising typically 30% to 60%
of the total recycle gas flow on a molar basis. In the resulting
mixture the ratio of the molar flow of the recycle gas to the molar
flow of feed gas is typically in the range of 0.5 to 2. The
resulting mixture (5), after optionally cooling (6) in cooler B,
flows to a gas expander machine C at a pressure of between 40 and
120 bar, more typically between 50 and 100 bar.
[0024] The outlet from expander C, stream (7), having a pressure of
between 3 bar and 50 bar and more typically between 5 bar and 30
bar, and may contain a condensate comprising C.sub.5+ and/or
aromatic compounds. Stream (7) may optionally be further cooled in
cooler D (stream 8) so as to increase the amount of condensate
formed.
[0025] The partially condensed stream (7 or 8) is separated into a
liquid (9) and a vapor (10) in separator E. Typically stream 9
contains lighter hydrocarbons in addition to the aforesaid
condensed heavy hydrocarbons. This stream will typically be removed
from the process for use as fuel, or may be separated into lighter
and heavier fractions, with the lighter fraction optionally
recycled. In a further option Separator E may form the upper part
of a demethanizer column. All these options for separation and
subsequent processing of Stream 9 do not form part of the
invention.
[0026] The vapor (10) from separator E is typically reheated in a
first cold passage of heat exchanger F and the stream (11)
compressed in compressor G to a pressure of 40 to 120 bar (stream
12) and then cooled in cooler H to form a first constituent of the
aforementioned recycle gas (3).
[0027] A second part (Stream 13) of the recycle gas (3) is cooled
(14) in a hot passage of heat exchanger F and is then passed into a
liquefaction unit N shown in dashed outline. The products of the
liquefaction unit are liquefied methane (LNG) and a vapor stream
(23). In the liquefaction unit the stream (14) is divided. A first
part (15), which typically comprises 25% to 35% of Stream 14, is
further cooled in a hot passage of heat exchanger I, to form a
methane-rich condensate or dense phase (16), which may be
depressurized in a valve or turbine J (Stream 17) to produce LNG
product.
[0028] While the example is based on a liquefaction unit N
generally in accordance with WO 2012/172281, other types of
liquefaction units could be substituted. In particular, a
liquefaction unit which achieved complete liquefaction of the said
second part of the recycle gas (14) so that the second vapor stream
(23) is zero could be employed.
[0029] To provide the most part of the necessary cooling in heat
exchanger I, a second part (18) is expanded in a second gas
expander K. Any liquid in the expander outlet (19) is separated
(20) in separator L and depressurized through valve or turbine M to
produce additional LNG product (21).
[0030] The vapor from separator L (22) is reheated in a cold
passage of heat exchanger I and stream (23) reheated in a second
cold passage of heat exchanger F. Stream (24) is then compressed in
compressor G to a pressure of from 40 to 120 bar to form a second
constituent of the aforementioned recycle gas (stream 3).
[0031] According to the invention the pressure of stream (24) may
be higher or lower than the pressure of stream (11).
[0032] An example of the removal of heavy hydrocarbon and aromatic
material is provided in Table 1. The benzene concentration of the
feed (2) of 1000 mol ppm is reduced to 1 mol ppm in stream (10).
Stream (10) has a composition close to the composition of the LNG
product.
TABLE-US-00001 TABLE 1 Stream No. 2 4 5 6 7 8 9 10 mol CO.sub.2
0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
0.000000 fraction N.sub.2 0.010399 0.017629 0.015034 0.015034
0.015034 0.015034 0.000567 0.015644 CH.sub.4 0.806366 0.935888
0.889394 0.889394 0.889394 0.889394 0.206702 0.918189
C.sub.2H.sub.6 0.101516 0.038661 0.061224 0.061224 0.061224
0.061224 0.215712 0.054708 C.sub.3H.sub.8 0.052817 0.007219
0.023588 0.023588 0.023588 0.023588 0.332095 0.010575
i-C.sub.4H.sub.10 0.006795 0.000283 0.002621 0.002621 0.002621
0.002621 0.054901 0.000416 n-C.sub.4H.sub.10 0.012252 0.000290
0.004584 0.004584 0.004584 0.004584 0.103162 0.000426
i-C.sub.5H.sub.12 0.002574 0.000016 0.000934 0.000934 0.000934
0.000934 0.022530 0.000023 n-C.sub.5H.sub.12 0.002986 0.000011
0.001079 0.001079 0.001079 0.001079 0.026281 0.000016 N
C.sub.6H.sub.14 0.001544 0.000001 0.000555 0.000555 0.000555
0.000555 0.013681 0.000001 M- 0.000412 0.000000 0.000148 0.000148
0.000148 0.000148 0.003648 0.000000 cyclopentane Benzene 0.001000
0.000001 0.000359 0.000359 0.000359 0.000359 0.008853 0.000001
cyclohexane 0.000206 0.000000 0.000074 0.000074 0.000074 0.000074
0.001824 0.000000 n-C.sub.7H.sub.16 0.000515 0.000000 0.000185
0.000185 0.000185 0.000185 0.004565 0.000000 M- 0.000206 0.000000
0.000074 0.000074 0.000074 0.000074 0.001826 0.000000 cyclohexane
toluene 0.000103 0.000000 0.000037 0.000037 0.000037 0.000037
0.000913 0.000000 n-C.sub.8H.sub.18 0.000206 0.000000 0.000074
0.000074 0.000074 0.000074 0.001826 0.000000 n-C.sub.9H.sub.20
0.000103 0.000000 0.000037 0.000037 0.000037 0.000037 0.000913
0.000000 H.sub.2O 0.000000 0.000000 0.000000 0.000000 0.000000
0.000000 0.000000 0.000000 .degree. C. 30.0 30.0 29.5 10.0 -63.3
-68.3 -68.3 -68.3 bar abs 65.0 64.9 64.9 64.8 14.0 13.9 13.9 13.9
kmol/h 5480 9786 15266 15266 15266 15266 618 14648 vapor fraction 1
1 1 1 0.968 0.960 0 1 mol C.sub.5+ 0.009854 0.000042 fraction
aromatic 0.001102 0.000001
[0033] FIG. 2 represents a flow diagram illustrating a process in
accordance with another embodiment of the invention wherein a heat
exchanger I completely or substantially liquefies the second part
of the recycle gas stream 14 to form liquefied methane 17.
[0034] FIG. 3 represents a flow diagram illustrating a process in
accordance with yet another embodiment of the invention wherein
there is no liquid in stream 19. Under such conditions, the
vapor/liquid separator L and the valve M of FIG. 1 are
redundant.
* * * * *