U.S. patent application number 11/560527 was filed with the patent office on 2007-06-28 for sour natural gas pretreating method.
Invention is credited to Raphael Huyghe, Francois Lallemand, Fabrice Lecomte, Eric Lemaire.
Application Number | 20070144943 11/560527 |
Document ID | / |
Family ID | 36808751 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070144943 |
Kind Code |
A1 |
Lemaire; Eric ; et
al. |
June 28, 2007 |
Sour Natural Gas Pretreating Method
Abstract
The invention relates to a method of pretreating a natural gas,
water-saturated or not, essentially comprising hydrocarbons, a
substantial amount of hydrogen sulfide and possibly carbon dioxide.
The method according to the invention comprises an H.sub.2S-rich
stream recycling stage.
Inventors: |
Lemaire; Eric; (Anse,
FR) ; Huyghe; Raphael; (Lyon, FR) ; Lecomte;
Fabrice; (Paris, FR) ; Lallemand; Francois;
(Morlaas, FR) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET
SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
36808751 |
Appl. No.: |
11/560527 |
Filed: |
November 16, 2006 |
Current U.S.
Class: |
208/208R |
Current CPC
Class: |
B01D 53/002 20130101;
C10L 3/10 20130101; C10L 3/102 20130101; B01D 53/1462 20130101 |
Class at
Publication: |
208/208.00R |
International
Class: |
C10G 45/00 20060101
C10G045/00; C10G 17/00 20060101 C10G017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 18, 2005 |
FR |
05/11.695 |
Claims
1) An improved method of pretreating a natural gas under pressure
containing hydrocarbons, at least one of the acid compounds
hydrogen sulfide and carbon dioxide, and water, comprising: a)
cooling the natural gas so as to produce a liquid phase and a gas
phase, b) contacting in a distillation column the gas phase
obtained in stage a) with a liquid phase obtained in stage c) so as
to produce a gas phase and a liquid phase, c) cooling the gas phase
obtained in stage b) so as to produce a liquid phase and a gas
phase, characterized in that at least part of the liquid phase
obtained in stage b) at the column bottom is recycled upstream from
natural gas cooling stage a).
2) A method as claimed in claim 1, wherein at least part of the
liquid phase obtained in stage b) at the column bottom is directly
recycled into said distillation column.
3) A method as claimed in claim 2, wherein at least part of the
liquid phase obtained in stage b) at the column bottom is directly
recycled into said distillation column at a lower level than the
gas phase intake.
4) A method as claimed in claim 2, wherein at least part of the
liquid phase obtained in stage b) at the column bottom is directly
recycled into said distillation column at a higher level than the
gas phase intake.
5) A method as claimed in claim 2, wherein at least part of the
liquid phase obtained in stage b) at the column bottom is directly
recycled into said distillation column at the same level as the gas
phase intake.
6) A method as claimed in claim 1, wherein said recycle stream is
subjected to a thermal exchange stage in order to be heated.
7) A method as claimed in claim 6, wherein said recycle stream is
subjected to a thermal exchange stage in order to be heated between
50.degree. C. and 150.degree. C., preferably between 75.degree. C.
and 120.degree. C.
8) A method as claimed in claim 1, wherein said recycle stream is
determined in such a way that, after mixing with the input gas, the
H.sub.2S content of the effluent entering the column ranges between
15% and 50% by mole, preferably between 20% and 45% by mole.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method of pretreating a
water-saturated natural gas containing a substantial amount of
hydrogen sulfide and possibly carbon dioxide and other sulfur
compounds. The invention mainly comprises a stage of recycling a
stream containing a large amount of hydrogen sulfide.
BACKGROUND OF THE INVENTION
[0002] Natural gas treatment generally requires a method in three
successive stages. The first stage generally consists in reducing
the proportion of sour gases such as hydrogen sulfide and carbon
dioxide. This first stage, also known as deacidizing stage, is
often followed by a water removal or dehydration stage, and by a
consecutive stage of heavy hydrocarbon recovery or stripping.
[0003] French patent FR-2,814,378 describes a natural gas
pretreating method allowing to obtain, at a lower cost, a
methane-rich gas depleted in hydrogen sulfide and freed of
substantially all the water that said natural gas initially
contained. Simultaneously, a hydrocarbon-depleted aqueous liquid
containing a large part of the hydrogen sulfide is obtained and
generally injected into an underground reservoir, an oil production
well for example. Thus, the method described in this French patent
allows, in a single stage, to remove or to significantly reduce the
water initially contained in the natural gas while reducing the
acid constituent contents. The method described in this patent also
allows to obtain a liquid phase containing mainly hydrogen sulfide
that can be readily pressurized prior to being injected into the
well.
[0004] However, French patent application FR-2,814,378 does not
allow the hydrogen sulfide and carbon dioxide content of the gas to
be treated to be reduced to an acceptable level as regards
commercial requirements. It is therefore often necessary to reduce
this sour gas content by means of a post-treatment. The methods
generally used for these post-treatments are chemical absorption
methods using, for example, solvents containing amines and carried
out at high temperatures or temperatures close to the ambient
temperature. These post-treatment methods allow to achieve natural
gas deacidizing: the chemical solvent absorbs the acid constituents
by chemical reaction. However, they have the drawback of lading the
deacidized gas with water, the chemical solvent being used in
aqueous solution. Thus, using a chemical solvent requires a third
treatment for removing the water contained in the deacidized gas in
order to prevent hydrate formation.
[0005] The present invention thus relates to a method that has been
improved in relation to the method of the prior art described in
document FR-2,814,378 filed by the applicant in that the proportion
of acid components at the bottom of the separation column is
notably increased.
SUMMARY OF THE INVENTION
[0006] The present invention therefore relates to a method of
pretreating a natural gas under pressure containing hydrocarbons,
at least one of the acid compounds hydrogen sulfide and carbon
dioxide, and water, comprising:
[0007] a) cooling the natural gas so as to produce a liquid phase
and a gas phase,
[0008] b) contacting in a distillation column the gas phase
obtained in stage a) with a liquid phase obtained in stage c) so as
to produce a gas phase and a liquid phase,
[0009] c) cooling the gas phase obtained in stage b) so as to
produce a liquid phase and a gas phase.
[0010] According to the invention, at least part of the liquid
phase obtained in stage b) at the column bottom is recycled
upstream from the gas phase intake into said column.
[0011] At least part of the liquid phase obtained in stage b) at
the column bottom can be recycled upstream from natural gas cooling
stage a).
[0012] At least part of the liquid phase obtained in stage b) at
the column bottom can be directly recycled into said distillation
column.
[0013] At least part of the liquid phase obtained in stage b) at
the column bottom can be directly recycled into said distillation
column at a lower level than the gas phase intake.
[0014] At least part of the liquid phase obtained in stage b) at
the column bottom can be directly recycled into said distillation
column at a higher level than the gas phase intake.
[0015] At least part of the liquid phase obtained in stage b) at
the column bottom can be directly recycled into said distillation
column at the same level as the gas phase intake.
[0016] The recycle stream can be subjected to a heat exchange stage
so as to be heated.
[0017] The recycle stream can be subjected to a heat exchange stage
so as to be heated between 50.degree. C. and 150.degree. C.,
preferably between 75.degree. C. and 120.degree. C.
[0018] The recycle stream can be determined in such a way that,
after mixing with the input gas, the H.sub.2S content of the
effluent entering the column ranges between 15% and 50% by mole,
preferably between 20% and 45% by mole.
BRIEF DESCRIPTION OF THE FIGURES
[0019] Other features and advantages of the present invention will
be clear from reading the description hereafter, given by way of
non limitative example, with reference to the accompanying figures
wherein:
[0020] FIG. 1 shows an example of a process flowsheet according to
the invention,
[0021] FIG. 2 shows an advantageous variant of the process
flowsheet according to the invention.
DETAILED DESCRIPTION
[0022] The object of the present invention shows that it is
possible, under suitable thermodynamic conditions, to concentrate
the initial natural gas in methane while removing the major part of
the sour gases and substantially all of the water it contains.
Substantially all of the water means that the amount of water
present in the final gas is below 50 ppm by mole, preferably below
10 ppm by mole, and more preferably below 5 ppm by mole.
[0023] The invention relates to an improved method allowing to
prevent hydrate formation in all the stages of the device allowing
said methane concentration.
[0024] According to the present invention, after treating according
to the present method the natural gas coming from the production
well, a final gas containing the major part of the hydrocarbons
contained in said gas is recovered. What is understood to be the
major part of the hydrocarbons is at least 90% hydrocarbons,
preferably at least 95% hydrocarbons and more preferably at least
97% hydrocarbons.
[0025] Furthermore, the present invention advantageously allows to
save, under stabilized conditions, using an anti-hydrate such as
methanol whose transport, use and/or recovery can be expensive
and/or complex.
[0026] In general terms, the invention relates to a method of
pretreating a natural gas under pressure containing hydrocarbons,
at least one of the acid compounds hydrogen sulfide and carbon
dioxide, and water, comprising:
[0027] a) cooling the natural gas so as to produce a liquid phase
and a gas phase,
[0028] b) contacting in a distillation column the gas phase
obtained in stage a) with a liquid phase obtained in stage c) so as
to produce a gas phase and a liquid phase,
[0029] c) cooling the gas phase obtained in stage b) so as to
produce a liquid phase and a gas phase.
[0030] The improvement of the present invention lies in that part
of the liquid phase obtained in stage b) is recycled upstream from
the distillation column intake so that, after mixing with the input
gas, the H.sub.2S content of the effluent entering the column
ranges between 15% and 50% by mole, preferably between 20% and 45%
by mole.
[0031] In stage c) of the method according to the invention, the
gas phase obtained in stage b) can be cooled by means of a heat
exchanger and/or of an expander.
[0032] The method according to the invention can comprise the
following stage:
[0033] d) cooling the gas phase obtained in stage c) by means of an
expander so as to produce a gas phase and a liquid phase that is
recycled to stage b).
[0034] The method according to the invention can comprise the
following stage:
[0035] e) compressing at least one of the gas phases obtained in
stage c) and in stage d) using the energy recovered from the
expander.
[0036] In stage c) of the method according to the invention, the
gas phase obtained in stage b) can be cooled by means of a venturi
throat, said liquid phase being collected at the level of the
venturi throat and said gas phase being recovered at the outlet of
the divergent tube of the venturi throat. The liquid phase
collected at the venturi throat can be cooled to produce the liquid
recycled to stage b) and a gas phase.
[0037] The gas phases obtained in stage c) and in stage d) can be
used to cool the gas phase obtained in stage b) and/or to cool the
natural gas in stage a).
[0038] The method according to the invention can comprise the
following stage:
[0039] f) vaporizing at least part of the liquid phase obtained in
stage b) and feeding said vaporized at least part of the liquid
phase into the distillation column to create an ascending vapour
flow within said column.
[0040] According to the present invention, part of the heat of the
liquid phase obtained in stage b) can be used to heat the gas phase
obtained in stage a).
[0041] In stage a) of the method according to the invention, the
liquid phase and the gas phase can be separated in a drum.
[0042] The operating conditions of the method according to the
invention can be as follows:
[0043] Distillation Column of Stage b)
[0044] T.degree. C.=-20.degree. C. to 100.degree. C., preferably
-15.degree. C. to 70.degree. C.
[0045] P>1 MPa abs., preferably 4 to 10 MPa abs.
[0046] Cooling Pressure and Temperature in Stage c)
[0047] T.degree. C.=-100.degree. C. to +30.degree. C., preferably
-40.degree. C. to 0.degree. C.
[0048] P>1 MPa, preferably 4 to 10 MPa
[0049] Cooling Temperature of Said Natural Gas in Stage a)
[0050] 0.degree. C. to 50.degree. C., preferably 20.degree. C. to
40.degree. C.
[0051] According to the present invention, the hydrogen sulfide
partial pressure in the natural gas can be at least 0.5 MPa,
preferably at least 1.5 MPa. The distillation column can comprise
at least 3 theoretical stages, preferably 4 to 6. In stage a), the
natural gas can be at a pressure ranging between 6.5 MPa and 12
MPa, and at a temperature above 15.degree. C.
[0052] The liquid phases obtained in stages a) and b) can be fed
into a well.
[0053] Thus, according to the present invention, the method
describes control of the thermodynamic conditions (pressure and
temperature for example) depending on the nature of the gas treated
(notably the water content thereof), said control allowing
progressive drainage of the water contained in said gas while
preventing hydrate formation. In general, according to the present
method, a distillation column allowing progressive drainage of the
water from the bottom to the top of the column is used, so as to
recover at the top of said column a substantially water-free gas,
i.e. comprising an amount of water that is lower than the hydrate
formation limit at the lowest temperature reached during next stage
c) of condensation by cooling and by expansion. In particular,
according to the invention, the water-saturated gas from stage a)
is introduced at a sufficiently low level of the column, i.e. at a
sufficiently high temperature to prevent hydrate formation. The
column must therefore contain a rather large number of theoretical
stages to allow drainage of the water and to obtain a temperature
gradient between the cold top and the bottom of the column.
Furthermore, addition of a reboiler advantageously allows to
maintain a rather high temperature in the column and therefore to
prevent hydrate formation, and to minimize and/or control
hydrocarbon losses.
[0054] The essential improvement provided by the present invention
lies in that part of the H.sub.2S-rich liquid phase obtained in
stage b) is recycled upstream from the distillation column intake
so that, after mixing with the input gas, the H.sub.2S content of
the effluent entering the column ranges between 15% and 50% by
mole, preferably between 20% and 45% by mole. The examples
hereafter show the improvement efficiency.
EXAMPLE 1
[0055] The process flowsheet described hereafter corresponds to
FIG. 1. The compositions and the properties of the streams are
numbered from 1 to 10 on the lines of the process flowsheet and
they are defined in Table 1. The crude gas (1) is available at
30.degree. C. and 62.3 bars. It contains approximately 18.8% by
mole H.sub.2S and 1000 ppm by mole water. It is mixed with recycled
stream (9). Injection of the H.sub.2S-rich liquid stream is carried
out in the direction of flow of the stream through a branch line at
the top of the line. The recycled stream is very rich in H.sub.2S
(about 78% by mole), at a higher temperature than the crude gas
because it directly comes from reboiler E2 of the column. A
gas/liquid mixture (2) that is homogenized by passage through a
mixer M1 is obtained.
[0056] At the outlet of mixer M1, the stream is heated up to
30.degree. C. This stream is fed into separating drum B1. The gas
phase (4) obtained is saturated with water, the excess water (5)
condensing in drum B1. Gas phase (4) thus obtained is fed into
separation column C1. This column allows significant removal of the
H.sub.2S while limiting hydrocarbon losses. The gas obtained at the
top of the column is cooled in exchanger E3, then in exchanger E4,
so as to reach a temperature of -30.degree. C. The liquid/gas
mixture obtained is separated in drum B2. Liquid phase (6) is used
as reflux for column C1 after passage through pump P1. The gas
phase is used for precooling the gas from the top of the column in
exchanger E3.
[0057] The gas obtained (7) contains 10% by mole H.sub.2S, i.e.
removal of about 50% of the H.sub.2S contained in the crude gas. It
no longer contains water (content below 5 ppm by mole). The liquid
at the bottom of column C1 is passed through pump P2. At the outlet
of pump P2, part of the liquid is sent to reboiler E2, another part
(9) is recycled to the gas feed point (1) of the plant, and the
last part is sent to reinjection pump P3. The liquid stream (5)
obtained at the bottom of drum B1 is added to stream (8) prior to
reinjection. Recycling of H.sub.2S-rich liquid stream (9) allows
the H.sub.2S content of the input gas to be greatly increased. In
this example, 20% recycling allows the H.sub.2S concentration to be
raised by 18% to 30% by mole.
EXAMPLE 2
[0058] The process flowsheet described below corresponds to FIG. 2.
The compositions and the properties of the streams are numbered
from 1 to 11 on the lines of the process flowsheet and they are
defined in Table 2.
[0059] In the diagram of FIG. 2, recycling (9) is achieved directly
into separation column C1 with the stream bearing reference number
(11). This recycling to the column is shown entering below gas feed
point (4), but in other variants shown by dotted lines (11'),
(11''), recycling can be achieved at any other level of the column,
above or at the feed point. The stream directly fed into column
(11) can represent all of or only part of recycled stream (9), and
the other part (11''') can be sent to gas intake (1) for
example.
[0060] FIG. 2 illustrates the layout of a thermal exchanger E5 in
relation to liquid recycled stream (9) from the column bottom. This
exchanger allows the liquid to be more or less heated, to obtain
either a gas/liquid mixture, or a gas at the dew point thereof, or
an "overheated" gas (i.e. at a temperature above the dew point
thereof). Typically, the temperature at the outlet of this
exchanger can range between 50.degree. C. and 150.degree. C.,
preferably between 75.degree. C. and 120.degree. C. This exchanger
can be used whatever the recycling inlet point.
[0061] The main advantages of this variant of the present invention
are as follows: [0062] The change to the vapour phase of the
recycled liquid can allow better mixing with the gas supplied,
whatever the recycling inlet point, [0063] the possibility of
bringing calories upon recycling provides operating flexibility to
the plant, notably as regards control of the thermodynamic
conditions, the temperature increase of the recycled stream can
allow to prevent hydrate formation risks.
[0064] The advantages of recycling from the column bottom to the
intake in relation to the SPREX.TM. process can be described as
follows: [0065] During the starting stages, the methanol is used to
prevent hydrate formation in the facilities; recycling allows to
minimize the amount of methanol to be used. In fact, it is mainly
present in the liquid from the column bottom and it is also
recycled into the input gas, [0066] recycling requires no
additional equipment because pump P2 at the column bottom is
essential to provide forced circulation in reboiler E2 of
thermosiphon type and thus to be able to exceed 30% vaporization in
the reboiler, [0067] recycling allows to reach a sufficient
H.sub.2S concentration to prevent hydrate formation. In fact, if
the H.sub.2S content of the gas is too low, the liquid/vapour
traffics and the thermal levels in the column can then favour
hydrate formation, [0068] recycling only reduces very slightly the
treating capacity of the process; in fact, the recycled stream is
extremely rich in H.sub.2S, [0069] the recycled stream is very rich
in water (more than 4000 ppm by mole), however drum B1 allows to
condense this water, the gas entering the column therefore does not
contain more water than without recycling, which is very important
to minimize hydrate formation risks, [0070] the recycled stream is
collected prior to mixing with liquid stream (5) from the bottom of
drum B1, which allows to prevent the water from "going round in
circles" and the content from increasing too much, [0071] the
recycling system allows the H.sub.2S content to be adjusted to the
desired value if that of the crude gas varies with the evolution of
the reservoir or the management of the various production site
wells. It also allows to treat natural gases whose H.sub.2S content
is lower than a content commonly considered by the SPREX.TM.
process, for example below 20% by mole H.sub.2S,
[0072] recycling allows a general increase in the "ease of
operation" of the plant. TABLE-US-00001 TABLE 1 Name of stream 1 2
3 4 5 6 7 8 9 10 Phase Vapour Mixed Vapour Vapour Liquid Liquid
Vapour Liquid Liquid Liquid Flow Rate kg-mol/hr 0.0859 0.1628
0.1628 0.1184 0.0444 0.0056 0.0004 0.0412 0.0769 0.0856 H2O N2
0.2189 0.2192 0.2192 0.2192 0.0000 0.0237 0.2187 0.0001 0.0003
0.0001 CO2 8.5369 9.4330 9.4330 9.4329 0.0001 10.5320 8.0567 0.4802
0.8961 0.4803 H2S 15.2497 30.0068 30.0046 30.0037 0.0008 24.7374
7.3389 7.9077 14.7571 7.9086 METHANE 53.1022 53.6625 53.6635
53.6634 0.0000 15.9360 52.8029 0.3002 0.5603 0.3003 ETHANE 2.2700
2.6086 2.6088 2.6088 0.0000 2.6636 2.0887 0.1814 0.3386 0.1814
PROPANE 0.7215 1.4197 1.4208 1.4208 0.0000 1.2697 0.3484 0.3741
0.6982 0.3741 BUTANE 0.4135 1.1638 1.1638 1.1638 0.000 0.1226
0.0114 0.4021 0.7503 0.4021 PENTANE 0.4621 1.3237 1.3237 1.3237
0.000 0.0123 0.0004 0.4617 0.8616 0.4617 Total Flow Rate kg-mol/hr
81.0607 100.0000 100.0000 99.9547 0.0453 55.3030 70.8666 10.1488
18.9393 10.1941 Temperature .degree. C. 30.0000 19.6979 30.0000
30.0000 30.0000 -30.0000 -1.8150 68.5415 68.5415 67.3981 Pressure
BAR (GA) 62.3000 61.9000 61.4000 61.4000 61.4000 60.8000 60.3000
63.5000 63.5000 250.0000 Enthalpy mm kcal/hr 0.1266 0.1624 0.1894
0.1890 0.0000 -0.0205 0.0655 0.0192 0.0357 0.0157 Molecular weight
23.5928 26.1192 26.1191 26.1226 18.3694 30.8675 21.6857 36.9322
36.9322 36.8496 Vapour molar fraction 1.0000 0.9460 1.0000 1.0000
0.0000 0.0000 1.0000 0.0000 0.0000 0.0000 Liquid molar fraction
0.0000 0.0540 0.0000 0.0000 1.0000 1.0000 0.0000 1.0000 1.0000
1.0000
[0073] TABLE-US-00002 TABLE 2 Name of stream 1 2 3 4 5 6 7 8 9 10
11 Phase Vapour Mixed Vapour Vapour -- Liquid Vapour Liquid Liquid
Liquid Vapour Flow Rate kg-mol/hr 0.0901 0.0901 0.0901 0.0901 --
0.0068 0.0005 0.0886 0.1298 0.0886 0.1298 H2O N2 0.2295 0.2295
0.2295 0.2295 -- 0.0256 0.2295 0.0001 0.0001 0.0001 0.0001 CO2
8.9518 8.9518 8.9518 8.9518 -- 11.3746 8.6504 0.3006 0.4403 0.3006
0.4403 H2S 15.9907 15.9907 15.9907 15.9907 -- 25.7071 7.6956 8.3094
12.1692 8.3094 12.1692 METHANE 55.6828 55.6828 55.6828 55.6828 --
17.0788 55.5379 0.1444 0.2114 0.1444 0.2114 ETHANE 2.3803 2.3803
2.3803 2.3803 -- 2.9115 2.2605 0.1193 0.1747 0.1193 0.1747 PROPANE
0.7566 0.7566 0.7566 0.7566 -- 1.3714 0.3746 0.3812 0.5583 0.3812
0.5583 BUTANE 0.4336 0.4336 0.4336 0.4336 -- 0.0917 0.0086 0.4202
0.6153 0.4202 0.6153 PENTANE 0.4846 0.4846 0.4846 0.4846 -- 0.0071
0.0002 0.4786 0.7009 0.4786 0.7009 Total Flow Rate kg-mol/hr
85.0000 85.0000 85.0000 85.0000 -- 58.5747 74.7577 10.2423 15.0000
10.2423 15.0000 Temperature .degree. C. 30.0000 29.7399 30.0000
30.0000 -- -30.0000 -2.4000 76.9433 76.9433 75.6265 100.0000
Pressure BAR (GA) 62.3000 61.9000 61.4000 61.4000 -- 60.8000
60.3000 63.5000 63.5000 250.0000 63.5000 Enthalpy mm kcal/hr 0.1328
0.1328 0.1334 0.1334 -- -0.0217 0.0687 0.0218 0.0320 0.0175 0.0643
Molecular weight 23.5928 23.5928 23.5928 23.5928 -- 30.8217 21.7422
37.0688 37.0688 37.0688 37.0688 Vapour molar fraction 1.0000 1.0000
1.0000 1.0000 -- 0.0000 1.0000 0.0000 0.0000 0.0000 1.0000 Liquid
molar fraction 0.0000 0.0000 0.0000 0.0000 -- 1.0000 0.0000 1.0000
1.0000 1.0000 0.0000
* * * * *