U.S. patent application number 12/350717 was filed with the patent office on 2009-05-07 for method of removing solid carbon dioxide.
Invention is credited to Robert AMIN.
Application Number | 20090113932 12/350717 |
Document ID | / |
Family ID | 27810079 |
Filed Date | 2009-05-07 |
United States Patent
Application |
20090113932 |
Kind Code |
A1 |
AMIN; Robert |
May 7, 2009 |
METHOD OF REMOVING SOLID CARBON DIOXIDE
Abstract
The invention provides a method of removing solid carbon dioxide
from cryogenic equipment, including the steps of: (a) introducing a
stream including ethane to the cryogenic equipment to convert solid
carbon dioxide to liquid form whereby a mixture of liquid ethane
and liquid carbon dioxide is formed; and (b) removing the mixture
of liquid ethane and liquid carbon dioxide from the cryogenic
equipment. In particular, the method can be used in a liquefied
natural gas (LNG) plant wherein cryogenic equipment contains LNG,
and the method includes the steps of: (a') removing the LNG from
the cryogenic equipment; (a) introducing a stream including ethane
to convert solid carbon dioxide to liquid form whereby a mixture of
liquid ethane and liquid carbon dioxide is formed; and (b) removing
the mixture of liquid ethane and liquid carbon dioxide from the
cryogenic equipment. The result is an effective cleaning method for
fouled LNG equipment.
Inventors: |
AMIN; Robert; (Bentley,
AU) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
27810079 |
Appl. No.: |
12/350717 |
Filed: |
January 8, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10525476 |
Aug 26, 2005 |
7493779 |
|
|
PCT/EP03/09575 |
Aug 27, 2003 |
|
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|
12350717 |
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Current U.S.
Class: |
62/620 ;
137/15.05; 62/611; 62/618 |
Current CPC
Class: |
F25J 2220/66 20130101;
C01B 32/55 20170801; Y10T 137/0424 20150401; F25J 2205/20 20130101;
Y02C 20/40 20200801; F25J 1/0022 20130101; Y02C 10/12 20130101;
F25J 3/0295 20130101; F25J 1/0262 20130101; B08B 9/08 20130101;
Y02P 20/152 20151101; B08B 9/032 20130101; F25J 2280/40 20130101;
C09K 5/042 20130101; F28G 13/00 20130101; Y10S 62/928 20130101;
B08B 9/00 20130101; C09K 2205/106 20130101; F25J 1/0248 20130101;
Y02P 20/151 20151101 |
Class at
Publication: |
62/620 ;
137/15.05; 62/611; 62/618 |
International
Class: |
F25J 3/00 20060101
F25J003/00; B08B 3/08 20060101 B08B003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 27, 2002 |
AU |
2002951005 |
Claims
1. A method of removing solid carbon dioxide from cryogenic
equipment, comprising the steps of: (a) introducing a stream
including ethane to said cryogenic equipment to convert solid
carbon dioxide to liquid form whereby a mixture of liquid ethane
and liquid carbon dioxide is formed; and (b) removing the mixture
of liquid ethane and liquid carbon dioxide from the cryogenic
equipment.
2. The method of claim 1, wherein the stream including ethane
contains at least 35% mol ethane.
3. The method of claim 1, wherein the stream including ethane is
liquid.
4. The method of claim 1, wherein the cryogenic equipment is taken
off-line before introducing the stream including ethane.
5. The method of claim 1, in which the cryogenic equipment is used
to produce liquefied natural gas.
6. The method of claim 1, in which the stream including ethane
contains carbon dioxide up to 65% mol.
7. The method of claim 1, in which the method further comprises the
step of separating the mixture of liquid ethane and liquid carbon
dioxide to form a first product enriched in ethane and a second
product enriched in carbon dioxide.
8. The method of claim 7, in which the first and second product are
separated by distillation, extraction, absorption, crystallisation,
decanting, multi-stage extraction or other chemical treatments or
any combination thereof.
9. The method of claim 1, in which the mixture of liquid ethane and
liquid carbon dioxide is azeotropic, and is separated to form a
first product enriched in ethane and a second product enriched in
carbon dioxide by extractive distillation or membrane-based
separation techniques or a combination thereof.
10. The method of claim 7, in which one or more alkanes or their
isotropes are introduced to the mixture prior to the separation
step.
11. The method of claim 7, in which the stream that includes ethane
comprises the first product that is recycled to the step (a).
12. A method of removing solid carbon dioxide from cryogenic
equipment, wherein the cryogenic equipment contains liquefied
natural gas, the method comprising the steps of: (a') removing the
liquefied natural gas from the cryogenic equipment; (a) introducing
a stream including ethane to convert solid carbon dioxide to liquid
form whereby a mixture of liquid ethane and liquid carbon dioxide
is formed; and (b) removing the mixture of liquid ethane and liquid
carbon dioxide from the cryogenic equipment.
13. The method of claim 12, wherein the stream including ethane
contains at least 35% mol ethane.
14. The method of claim 12, wherein the stream including ethane is
liquid.
15. The method of claim 12, wherein the cryogenic equipment is
taken off-line before introducing the stream including ethane.
16. The method of claim 12, in which the cryogenic equipment is
used to produce liquefied natural gas.
17. A method of producing liquefied natural gas, wherein natural
gas is introduced in cryogenic equipment and is cooled down to form
liquefied natural gas, and further wherein solid carbon dioxide is
removed from the cryogenic equipment by: (a) introducing a stream
including ethane to the cryogenic equipment to convert solid carbon
dioxide to liquid form whereby a mixture of liquid ethane and
liquid carbon dioxide is formed; and (b) removing the mixture of
liquid ethane and liquid carbon dioxide from the cryogenic
equipment.
18. The method of claim 17, wherein the stream including ethane
contains at least 35% mol ethane.
19. The method of claim 17, wherein the stream including ethane is
liquid.
20. The method of claim 17, wherein the cryogenic equipment is
taken off-line before introducing the stream including ethane.
21. The method of claim 17, wherein the stream including ethane
contains carbon dioxide up to 65% mol.
22. The method of claim 17, wherein the liquefied natural gas is
removed from the cryogenic equipment prior to the step (a).
23. A method of producing liquefied natural gas, wherein natural
gas is introduced in cryogenic equipment and is cooled down to form
liquefied natural gas, and further wherein solid carbon dioxide is
removed from the cryogenic equipment by: (a) introducing a stream
including ethane to the cryogenic equipment to convert solid carbon
dioxide to liquid form whereby a mixture of liquid ethane and
liquid carbon dioxide is formed; and (b) removing the mixture of
liquid ethane and liquid carbon dioxide from the cryogenic
equipment, wherein the solid carbon dioxide is already present on a
surface of said cryogenic equipment prior to said introducing said
stream.
24. The method of claim 23, wherein the stream including ethane
contains at least 35% mol ethane.
25. The method of claim 23, wherein the stream including ethane is
liquid.
26. The method of claim 23, wherein the cryogenic equipment is
taken off-line before introducing the stream including ethane.
27. The method of claim 23, further comprising the step of
adjusting the relative percentages of ethane and carbon dioxide for
a given pressure and temperature such that the mixture of liquid
ethane and liquid carbon dioxide is near azeotropic.
28. The method of claim 23, in which the stream including ethane
contains carbon dioxide up to 65% mol.
29. The method of claim 23, in which the method further comprises
the step of separating the mixture of liquid ethane and liquid
carbon dioxide to form a first product enriched in ethane and a
second product enriched in carbon dioxide.
30. The method of claim 29, in which the first and second product
are separated by distillation, extraction, absorption,
crystallisation, decanting, multi-stage extraction or other
chemical treatments or any combination thereof.
31. The method of claim 23, in which the mixture of liquid ethane
and liquid carbon dioxide is azeotropic, and is separated to form a
first product enriched in ethane and a second product enriched in
carbon dioxide by extractive distillation or membrane-based
separation techniques or a combination thereof.
32. The method of claim 29, in which one or more alkanes or their
isotropes are introduced to the mixture prior to the separation
step.
33. The method of claim 29, in which the stream that includes
ethane comprises the first product that is recycled to the step
(a).
Description
[0001] This application is a divisional of U.S. application Ser.
No. 10/525,476 having a 35 U.S.C 371 date of Aug. 26, 2005 that was
PCT filed Aug. 27, 2003 having PCT No. PCT/EP03/09575 claiming
priority from Australian Patent Application No. 2002951005 filed
Aug. 27, 2002, which are all incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a method of removing solid
carbon dioxide (CO.sub.2) from cryogenic equipment, in particular
from cryogenic equipment used in gas conditioning or gas
deep-extraction processes, and more particularly from cryogenic
equipment used in the production of Liquefied Natural Gas
(LNG).
BACKGROUND OF THE INVENTION
[0003] Natural gas contains a wide range of species which are
capable of forming solids during the cryogenic process of producing
LNG known as liquefaction. One of the species that causes
considerable problems to LNG producers is carbon dioxide. In a
conventional LNG facility, pretreatment of the natural gas is
conducted to decrease the carbon dioxide content to between 50 and
125 ppm prior to the natural gas entering the liquefaction
process.
[0004] On average, carbon dioxide compositions in a natural gas
feed stream can range between 0.5% and 30% mol and can be as high
as 70% mol in commercially viable reservoirs like Natuna,
Indonesia. Carbon dioxide is typically removed using chemical
reactions such as reversible absorption processes with amine
solvents. These are expensive and complex processes and commonly
encounter operational problems such as foaming, corrosion, blocked
filters and amine degradation. Losses of amine, water and
hydrocarbons are commonly encountered. These processes also consume
significant amounts of energy to regenerate and pump the
solvent.
[0005] LNG refrigeration systems are expensive because so much
refrigeration is needed to liquefy natural gas. A typical natural
gas stream enters a LNG plant at pressures from about 40 bar to
about 76 bar and temperatures from about 20.degree. C. to about
40.degree. C. Natural gas, which is predominantly methane, cannot
be liquefied by simply increasing the pressure, as is the case with
heavier hydrocarbons used for energy purposes. The critical
temperature of methane is -82.5.degree. C. This means that methane
can only be liquefied below that temperature regardless of the
pressure applied. Since natural gas is a mixture of gases, it
liquefies over a range of temperatures. The critical temperature of
natural gas is typically between about -85.degree. C. and
-62.degree. C. Natural gas compositions at atmospheric pressure
will typically liquefy in the temperature range between about
-165.degree. C. and -155.degree. C. Since refrigeration equipment
represents such a significant part of the LNG facility cost,
cleaning of this equipment is important.
[0006] In conventional LNG plants, the natural gas is typically
cooled in one or more heat exchangers. If insufficient carbon
dioxide is removed prior to the natural gas entering the heat
exchangers, carbon dioxide precipitates as a solid and accumulates
on the cold surfaces of the heat exchangers and other plant
equipment eventually rendering these items inoperable. When fouling
has reached a critical level, the vessel must be taken off-line for
the carbon dioxide solids to be removed. This can be achieved by
warming-up the affected equipment. However, this causes
considerable downtime and energy-loss for the plant. Alternatively,
the solid carbon dioxide may be removed mechanically. In such a
case of mechanical defouling of equipment, the vessel, baffles
and/or pipework may be damaged, which only encourages further
fouling in the next production cycle. Moreover, solids condensing
on metal surfaces form an insulating film reducing the thermal
efficiency of the heat exchanger.
[0007] There is a need for a simpler, more economical process for
the removal of solid carbon dioxide that has fouled plant equipment
under cryogenic conditions.
SUMMARY OF THE INVENTION
[0008] According to one aspect of the present invention, there is
provided a method of removing solid carbon dioxide from cryogenic
equipment, the method comprising the steps of:
(a) introducing a stream including ethane to said cryogenic
equipment to convert solid carbon dioxide to liquid form whereby a
mixture of liquid ethane and liquid carbon dioxide is formed; and
(b) removing the mixture of liquid ethane and carbon dioxide from
the cryogenic equipment.
[0009] According to another aspect of the present invention, there
is provided a method of removing carbon dioxide fouling of
cryogenic equipment containing LNG, the method comprising the steps
of:
(a') removing the LNG from the said cryogenic equipment; (a)
introducing a stream including ethane to convert solid carbon
dioxide to liquid form whereby a mixture of liquid ethane and
liquid carbon dioxide is formed; and (b) removing the mixture of
liquid ethane and liquid carbon dioxide from the cryogenic
equipment whereby the cryogenic equipment is defouled of solid
carbon dioxide and available for the reintroduction of the LNG
stream.
DETAILED DESCRIPTION OF THE INVENTION
[0010] Preferably the method comprises the step of adjusting the
relative percentages of ethane and carbon dioxide for a given
pressure and temperature such that the mixture of liquid ethane and
carbon dioxide is near azeotropic. By near azeotropic is understood
a composition wherein the carbon dioxide content varies between 5%
mol below or above the azeotropic composition. It is known that
ethane and carbon dioxide form an azeotrope. An azeotrope forms
because of a particular molecular interaction between two or more
components. When such components are mixed, the vapour and liquid
compositions at equilibrium are equal within a given pressure and
temperature range. The formation of an azeotrope is usually seen to
introduce an obstacle for the separation of the two components
within the liquid mixture and is thus something that is typically
avoided in chemical processing plants. The azeotropic composition
is dependent on the temperature and pressure, but is generally
about 65% mol carbon dioxide and 35% mol ethane.
[0011] Although the stream including ethane may be gaseous, the
stream including ethane is preferably liquid. This stream may
contain pure or substantially pure ethane. It may also comprise
other components. Preferably the stream including ethane contains
at least 35% mol ethane. Suitable components include other
hydrocarbons, such as propane and butane, and carbon dioxide. It is
preferred that the stream including ethane contains some carbon
dioxide already. The fastest and most efficient dissolution of
carbon dioxide solids occur under these conditions. The content of
carbon dioxide may suitably vary from 0 to about 65% mol.
[0012] Preferably, the method further comprises the step of
separating the mixture of liquid ethane and carbon dioxide to form
a first product rich in ethane and a second product rich in carbon
dioxide. More preferably, the first product rich in ethane is
available for recycling to step (a). More preferably, the second
product rich in carbon dioxide is also recovered and recycled. When
the carbon dioxide content is somewhat lower than the azeotropic
composition the separation between an ethane-rich product and a
carbon dioxide-rich product can easily be achieved by distillation.
The ethane-rich product may then be substantially pure ethane,
whereas the carbon dioxide-rich product has the azeotropic
composition.
[0013] When the mixture of liquid ethane and carbon dioxide has an
azeotropic composition, the method of separating the azeotropic
mixture may include distillation or membrane-based separation
techniques or a combination thereof. The method may include the
step of introducing one or more alkanes to the azeotropic mixture
prior to the separation step. The addition of one or more alkanes
has the effect of widening the two-phase liquid vapour equilibrium
area in the ethane-carbon dioxide system to allow easier separation
of ethane and carbon dioxide from the azeotropic ethane-carbon
dioxide liquid mixture.
[0014] Preferably, the cryogenic equipment is selected from the
list including heat exchangers, pipes, storage vessels, sub-cooling
vessels and/or separators.
[0015] The present invention will now be described in more detail
by means of the following example.
[0016] The present invention derives from observations made during
a series of tests conducted using a cryogenic vessel known as the
Sapphire Cell. The Sapphire Cell as the name suggests is
constructed of pure single crystal sapphire and allows hitherto
impossible direct observation of the phenomena occurring during LNG
liquefaction. Based on their observations of these phenomena, the
applicant has realised that liquid ethane can be used to remove
carbon dioxide fouling of cryogenic equipment.
[0017] During testing, the Sapphire Cell was used as a flash vessel
in fluid communication with a cryogenic chamber. Natural gas was
introduced to the Sapphire Cell and flashed down to 40 bar at
-82.degree. C. whereby LNG was formed. Under the conditions at
which liquefaction takes place, the carbon dioxide still present in
the natural gas feed stream will precipitate out in solid form
within the flash vessel.
[0018] The LNG produced was stored in the cryogenic chamber and the
system was cooled down to -80.degree. C. using a multi-component
refrigerant system and with liquid nitrogen down to -161.degree. C.
The cryogenic chamber was maintained at the same pressure as the
flash vessel until equilibrium conditions were attained so that the
vapour-liquid equilibrium phase diagrams for a given range of
compositions could be generated.
[0019] The liquid level within each of the flash vessel and
cryogenic chamber was measured using simple volumetric calibration.
The liquid level within the Sapphire Cell could also be observed by
the eye through the transparent walls of the Cell.
[0020] The temperature of the system was monitored using
temperature sensors inside each of the chambers with a third
temperature sensor monitoring the air bath around the cryogenic
chamber and flash vessel. Pressure sensors were located outside the
air bath at the inlet and outlet of each of the cryogenic chamber
and the flash vessel. Multi-port sampling valves were provided for
each of the cryogenic chamber and flash vessel to allow on-line gas
chromatographic analysis of samples when desired.
[0021] The system was agitated using a vortex operated magnetically
until solid separation of the carbon dioxide was observed. The
vortex encouraged gravity separation of the more dense carbon
dioxide solids to the bottom of the chamber. The effect of creating
a vortex is to draw the solids formed within the vessel towards the
wall of the vessel where they migrate down towards the bottom of
the vessel. The vortex can be established by mechanical means using
a stirrer or by including a hydrocyclone at the base of the
vessel.
[0022] In the first series of tests natural gas of known
composition as outlined below in Table 1 was introduced through a
control valve into the Sapphire Cell.
TABLE-US-00001 TABLE 1 GC Analysis of the Feed Gas* Component Mole
Fraction 1 N.sub.2 2.54 2 CO.sub.2 2.39 3 C.sub.1 84.16 4 C.sub.2
7.08 5 C.sub.3 3.05 6 iC.sub.4 0.31 7 nC.sub.4 0.38 8 iC.sub.5 0.05
9 nC.sub.5+ 0.04 *(Gas includes ppm's of mercaptan)
[0023] In a second series of tests, additional carbon dioxide was
added to the chamber to bring the carbon dioxide content up to 25%
as outlined below in Table 2.
TABLE-US-00002 TABLE 2 GC Analysis of the Feed Gas with Addition of
Extra CO.sub.2* Component Mole Fraction 1 N.sub.2 1.939 2 CO.sub.2
24.95 3 C.sub.1 64.64 4 C.sub.2 5.493 5 C.sub.3 2.385 6 iC.sub.4
0.239 7 nC.sub.4 0.292 8 iC.sub.5 0.038 9 nC.sub.5+ 0.023 *(Gas
includes ppm's of mercaptan)
[0024] LNG was transferred into the cryogenic storage vessel
leaving behind a slush comprising a relatively small percentage of
LNG plus solid carbon dioxide crystals in the flash vessel. The
composition of the LNG produced during the liquefaction process is
outlined below in Table 3. From this table it can be seen that the
carbon dioxide composition has been reduced from 25% in Table 2 to
just 0.29% due to the freeze out of carbon dioxide solids.
TABLE-US-00003 TABLE 3 GC Analysis of the Produced LNG After
CO.sub.2 Separation at 10 bar -140.degree. C. 1 N.sub.2 1.28 2
CO.sub.2 0.29 3 C.sub.1 94.65 4 C.sub.2 4.48 5 C.sub.3 2.02 6
iC.sub.4 0.21 7 nNC.sub.4 0.27 8 iC.sub.5 0.04 9 nC.sub.5+ 0.03
Carbon dioxide content was increased to 30% in the original gas
composition shown in Table 1.
[0025] The flash vessel containing the slush was left for one hour
to achieve equilibrium. Liquid ethane was introduced at the same
conditions of -80.degree. C. and 26 bar. It was observed that the
solid carbon dioxide within the slush started to dissolve
immediately as liquid ethane was introduced.
[0026] In a third series of tests, a liquid mixture of 15% mol
carbon dioxide and 85% mol ethane was introduced into the Sapphire
Cell. The contents of the Sapphire Cell were agitated using the
magnetically induced vortex. The transparent walls of the Sapphire
Cell made it possible to observe the fine solid crystals of carbon
dioxide forming and dissolving in rapid succession.
[0027] In the first preferred embodiment of the present invention,
a heat exchanger or other cryogenic pipework fouled with carbon
dioxide is taken off-line. Liquid ethane is then introduced into
the heat exchanger or pipework. The carbon dioxide solids dissolve
as they convert back to liquid form. Dissolution of carbon dioxide
occurs at any composition of the stream including ethane. The
fastest rate of dissolution has been observed to occur when the
stream includes ethane and carbon dioxide, and in particular when
the ethane and carbon dioxide are present in such stream in
sufficient relative amounts at a given pressure and temperature to
form an azeotropic mixture. Under azeotropic conditions,
dissolution of the carbon dioxide solids is observed to happen at
its greatest speed and with greatest efficiency.
[0028] Having introduced the ethane and converted the solid carbon
dioxide to liquid form, it is preferable for the mixture of ethane
and carbon dioxide to be separated to recover and recycle the
ethane.
[0029] The most common method of separating homogeneous liquid
mixtures is the use of distillation, i.e. repeated vaporisation and
condensation whereby the vapour phase gradually becomes enriched in
the more volatile component. However, separation of a liquid
mixture by distillation depends on the fact that even when a liquid
is partially vaporised, the vapour and liquid compositions differ.
The vapour phase becomes progressively more enriched in the more
volatile component and is depleted in the less volatile component.
Repeated partial vaporisation is used to achieve the desired degree
of separation. An azeotrope, however, cannot be separated using
ordinary distillation since little enrichment of the vapour phase
occurs with each partial vaporisation step.
[0030] Therefore in most cases, azeotropic liquid mixtures require
special methods to facilitate separation of the component
species.
[0031] Separation of the azeotropic mixture may be effected using
techniques such as extraction, absorption, crystallisation,
decanting, multi-stage extraction or other chemical treatments or
any combination thereof. In order to use extractive distillation in
either a continuous or batch operation, it may be necessary to add
an entrainer such as propane, butane or other suitable alkane or a
combination thereof, the choice being dependent on the particular
phase behaviour of the system and available compounds. It is
envisaged that the alkane or alkanes would be recovered and
recycled to the system also.
[0032] Alternatively, membrane separation methods may be used prior
to or independently of distillation. Such methods include dialysis,
reverse osmosis, ultra-filtration, electrodialysis, helium
separation through glass, hydration separation through palladium
and alloy membranes, immobilised solvents and/or liquid-surfactant
membranes. The driving force for separation using membranes is
either a pressure or concentration difference across the membrane.
Membranes may be used to break azeotropic mixtures prior to feeding
the mixture to a subsequent continuous or batch distillation
separation process.
[0033] In the second preferred embodiment of the present invention,
the method can be used for removing solid carbon dioxide from
cryogenic equipment used in the production of LNG. The LNG would
first be drained from the system before introducing liquid ethane
in the manner outlined above.
[0034] A series of tests conducted using the Sapphire Cell have
confirmed that the presence of methane in the natural gas feed
stream has little or no effect on the formation of carbon dioxide
solids during LNG liquefaction nor the subsequent dissolution of
the carbon dioxide solids when the ethane is introduced.
[0035] It is proposed that this method of removing carbon dioxide
contaminants could be used for pipelines for carrying LNG, heat
exchangers, cryogenic cooling vessels, and any other plant
equipment used under cryogenic conditions where carbon dioxide
fouling occurs.
[0036] It will be readily apparent to a person skilled in the
relevant art that the present invention has significant advantages
over the prior art including, but not limited to, the
following:
(a) Existing LNG plants can be defouled without any requirement for
modification of the plant equipment; (b) Recycling of the ethane
will significantly contribute to reducing the cost of applying the
method according to the present invention for the removal of the
carbon dioxide solid contaminants; (c) The process is applicable to
a wide variation of feed gas compositions; and (d) The carbon
dioxide content of the natural gas can be adjusted in order to
assist in the removal of the carbon dioxide solids by the
ethane.
* * * * *