U.S. patent application number 12/136372 was filed with the patent office on 2008-12-18 for method of refining natural gas and natural gas refining system.
Invention is credited to Takashi Asano, Yuuko HINO, Mamoru Kamoshida.
Application Number | 20080307827 12/136372 |
Document ID | / |
Family ID | 40131087 |
Filed Date | 2008-12-18 |
United States Patent
Application |
20080307827 |
Kind Code |
A1 |
HINO; Yuuko ; et
al. |
December 18, 2008 |
METHOD OF REFINING NATURAL GAS AND NATURAL GAS REFINING SYSTEM
Abstract
A method and system of refining natural gas that improves the
quality of liquefied natural gas and enables separation and
recovery of hydrocarbons other than methane. The method of refining
natural gas containing methane; any other hydrocarbon selected from
the group consisting of ethane, ethylene, propane, propylene,
n-butane, isobutane, 1-butene, n-pentane, and isopentane; carbon
dioxide; and hydrogen sulfide, includes adjusting a pressure and
temperature of the natural gas so that the methane is in the gas
phase, the other hydrocarbon in the liquid phase, and the carbon
dioxide and the hydrogen sulfide in the solid phase, respectively;
separating the natural gas, of which the pressure and temperature
has been adjusted, into a gas containing the methane and a
suspension liquid; and separating the separated suspension liquid
into a liquid containing the other hydrocarbon and a solid
containing the carbon dioxide and the hydrogen sulfide.
Inventors: |
HINO; Yuuko; (Hitachi,
JP) ; Asano; Takashi; (Hitachinaka, JP) ;
Kamoshida; Mamoru; (Hitachi, JP) |
Correspondence
Address: |
MATTINGLY, STANGER, MALUR & BRUNDIDGE, P.C.
1800 DIAGONAL ROAD, SUITE 370
ALEXANDRIA
VA
22314
US
|
Family ID: |
40131087 |
Appl. No.: |
12/136372 |
Filed: |
June 10, 2008 |
Current U.S.
Class: |
62/634 ;
62/618 |
Current CPC
Class: |
F25J 2210/62 20130101;
B01D 21/267 20130101; F25J 2205/10 20130101; F25J 2205/20 20130101;
F25J 2220/64 20130101; F25J 3/0615 20130101; F25J 2220/66 20130101;
F25J 2240/40 20130101; F25J 2270/904 20130101; Y02C 20/40 20200801;
Y02C 10/12 20130101; F25J 2210/04 20130101; F25J 2215/04 20130101;
F25J 3/0635 20130101; B01D 2221/04 20130101; F25J 2210/02 20130101;
F25J 3/064 20130101; F25J 3/067 20130101; F25J 3/061 20130101 |
Class at
Publication: |
62/634 ;
62/618 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 11, 2007 |
JP |
2007-153832 |
May 9, 2008 |
JP |
2008-123344 |
Claims
1. A method of refining natural gas containing methane; any other
hydrocarbon selected from the group consisting of ethane, ethylene,
propane, propylene, n-butane, isobutane, 1-butene, n-pentane, and
isopentane; carbon dioxide; and hydrogen sulfide, the method
comprising the steps of: adjusting a pressure and temperature of
the natural gas so that the methane is in the gas phase, the other
hydrocarbon in the liquid phase, and the carbon dioxide and the
hydrogen sulfide in the solid phase, respectively; separating the
natural gas, of which the pressure and temperature has been
adjusted, into a gas containing the methane and a suspension
liquid; and separating the separated suspension liquid into a
liquid containing the other hydrocarbon and a solid containing the
carbon dioxide and the hydrogen sulfide.
2. The method of refining natural gas according to claim 1, wherein
the temperature of the natural gas is adjusted to a value less than
a boiling point of ethylene but equal to or higher than a melting
point of n-pentane.
3. The method of refining natural gas according to claim 1, wherein
the pressure of the natural gas is adjusted in a range from 1 to 10
atm.
4. The method of refining natural gas according to claim 1, wherein
the pressure and temperature of the natural gas is adjusted by
adiabatic-expanding the natural gas with expanding means.
5. The method of refining natural gas according to claim 1, wherein
the pressure and temperature of the natural gas is adjusted by
mixing liquefied natural gas in the natural gas with mixing
means.
6. The method of refining natural gas according to claim 1, wherein
the suspension liquid is separated into the liquid containing the
other hydrocarbon and the solid containing the carbon dioxide and
the hydrogen sulfide with a cyclone.
7. The method of refining natural gas according to claim 1, wherein
in the range in which the pressure of the natural gas is 1 atm or
more and the temperature of the natural gas is a value less than a
melting point of the hydrogen sulfide but equal to or higher than a
melting point of n-pentane, the pressure and temperature of the
natural gas is adjusted so that a concentration of methane in the
gas phase is 90% or more.
8. The method of refining natural gas according to claim 7, wherein
the temperature of the natural gas is adjusted to a value less than
a boiling point of ethylene.
9. The method of refining natural gas according to claim 7, wherein
the pressure of the natural gas is adjusted to 10 atm or below.
10. The method of refining natural gas according to claim 7,
wherein the temperature of the natural gas is adjusted to a value
less than a boiling point of ethylene and the pressure of the
natural gas is adjusted to 10 atm or below.
11. A natural gas refining system for refining gas containing
methane; any other hydrocarbon selected from the group consisting
of ethane, ethylene, propane, propylene, n-butane, isobutane,
1-butene, n-pentane, and isopentane; carbon dioxide; and a hydrogen
sulfide, the refining system comprising: pressure/temperature
adjusting means for adjusting a pressure and temperature of the
natural gas so that the methane is in the gas phase, the other
hydrocarbon in the liquid phase, and the carbon dioxide and the
hydrogen sulfide in the solid phase, respectively; gas separating
means for separating the natural gas, of which the pressure and
temperature adjusted by the pressure/temperature adjusting means,
into a gas containing the methane and a suspension liquid; and
solid/liquid separating means for separating the suspension liquid
separated by the gas separating means into a liquid containing the
other hydrocarbon and a solid containing the carbon dioxide and the
hydrogen sulfide.
12. The natural gas refining system according to claim 11, wherein
the pressure/temperature adjusting means adjusts the temperature of
the natural gas to a value less than a boiling point of ethylene
and equal to or higher than a melting point of n-pentane.
13. The natural gas refining system according to claim 11, wherein
the pressure/temperature adjusting means adjusts the pressure of
the natural gas in a range from 1 to 10 atm.
14. The natural gas refining system according to claim 11, wherein
the pressure/temperature adjusting means has expanding means for
adiabatic-expanding the natural gas.
15. The natural gas refining system according to claim 11, wherein
the pressure/temperature adjusting means has mixing means for
mixing liquefied natural gas in the natural gas.
16. The natural gas refining system according to claim 11, wherein
the solid/liquid separating means is a cyclone.
17. The natural gas refining system according to claim 11, wherein
in the range in which the pressure of the natural gas is 1 atm or
more and the temperature of the natural gas is a value less than a
melting point of the hydrogen sulfide but equal to or higher than a
melting point of n-pentane, the pressure/temperature adjusting
means adjusts the pressure and temperature of the natural gas so
that a concentration of methane in the gas phase is 90% or
more.
18. The natural gas refining system according to claim 17, wherein
the pressure/temperature adjusting means adjusts the temperature of
the natural gas to a value less than a boiling point of
ethylene.
19. The natural gas refining system according to claim 17, wherein
the pressure/temperature adjusting means adjusts the pressure of
the natural gas to 10 atm or below.
20. The natural gas refining system according to claim 17, wherein
the pressure/temperature adjusting means adjusts the temperature of
the natural gas to a value less than a boiling point of ethylene
and also adjusts the pressure of the natural gas to 10 atm or
below.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of refining
natural gas mined in a natural gas field or an oil field, and to a
natural gas refining system.
[0003] 2. Description of the Related Art
[0004] Natural gas mined in a natural gas field or an oil field is
generally cooled by liquefaction equipment and converted to
liquefied natural gas (LNG). Natural gas generally contains methane
as a main component of liquefied natural gas and also impurities
such as hydrocarbons other than methane, carbon dioxide, sulfur
compounds, and mercury. When natural gas is cooled and liquefied
without removal of the impurities, the problems arise, for
instance, that the impurities are solidified and liquefaction
equipment (such as heat transfer equipment) is blocked, or that
liquefied natural gas decreases in quality. Therefore, it is
necessary to remove the impurities. Examples of a method of
removing the impurities, especially carbon dioxide or hydrogen
sulfide which solidify during the liquefaction process of
liquefying natural gas, include a chemical absorption method or a
physical adsorption method in which such impurities as carbon
dioxide or hydrogen sulfide are separated in the gas phase.
[0005] For instance, JP-A-2005-515298 describes the method of
separating natural gas to a solid phase and a liquid phase after
adjusting a pressure and temperature of the natural gas so that
methane contained in the natural gas is converted to the liquid
phase and the impurities such as carbon dioxide and hydrogen
sulfide to the solid phase (namely, natural gas to a suspension
liquid). In the conventional technique described in
JP-A-2005-515298, natural gas is expanded by a Joule Thompson valve
to adjust a pressure to a range from 150 to 250 psia (a range from
1.0 MPa to 1.7 MPa) and a temperature to a range from about -100 to
about -125.degree. C., and the expanded gas is supplied to a
cryogenic tank. Further, liquefied natural gas in the sub-cool
state is supplied to the cryogenic tank and the natural gas is
cooled to -140.degree. C. therein. With the operations described
above, liquefied natural gas in the pressurized state is produced.
Then the liquefied natural gas (suspension liquid) taken out from a
lower side of the cryogenic tank is separated, for instance, with a
cyclone, from a solid containing carbon dioxide, hydrogen sulfide,
and the like.
SUMMARY OF THE INVENTION
[0006] In the conventional technique described in JP-A-2005-515298,
a temperature and pressure of natural gas is adjusted so that
methane contained in natural gas is converted to the liquid phase
and impurities contained therein such as carbon dioxide and
hydrogen sulfide are converted to the solid phase to realize
separation between the solid phase and the liquid phase. However,
some hydrocarbons other than methane contained in natural gas may
remain in the liquid state and be contained in liquefied natural
gas in the liquid phase state. In this case, the product quality
may be deteriorated because the combustion efficiency is affected
depending on a content of hydrocarbons other than methane in the
liquefied natural gas. On the other hand, the hydrocarbons (such as
ethane or propane) other than methane are useful materials, and
therefore it has been desired to separate and recover the
hydrocarbons from the liquefied natural gas.
[0007] An object of the present invention is to provide a method of
refining natural gas and a natural gas refining system which can
improve quality of liquefied natural gas and enable separation and
recovery of the hydrocarbons other than methane.
[0008] (1) To achieve the object described above, the present
invention provides a method of refining natural gas containing
methane; any other hydrocarbon selected from the group consisting
of ethane, ethylene, propane, propylene, n-butane, isobutane,
1-butene, n-pentane, and isopentane; carbon dioxide; and hydrogen
sulfide, and the method comprises the steps of adjusting a pressure
and temperature of the natural gas so that the methane is in the
gas phase, the other hydrocarbon in the liquid phase, and the
carbon dioxide and the hydrogen sulfide in the solid phase,
respectively; separating the natural gas, of which the pressure and
temperature has been adjusted, into a gas containing the methane
and a suspension liquid; and separating the separated suspension
liquid into a liquid containing the other hydrocarbon and a solid
containing the carbon dioxide and the hydrogen sulfide.
[0009] (2) In the method described in (1) above, preferably the
temperature of the natural gas is adjusted to a value less than a
boiling point of ethylene but equal to or higher than a melting
point of n-pentane.
[0010] (3) In the method described in (1) above, preferably the
pressure of the natural gas is adjusted in a range from 1 to 10
atm.
[0011] (4) In the method described in (1) above, preferably the
pressure and temperature of the natural gas is adjusted by
adiabatic-expanding the natural gas with expanding means.
[0012] (5) In the method described in (1), preferably the pressure
and temperature of the natural gas is adjusted by mixing liquefied
natural gas in the natural gas with mixing means.
[0013] (6) In the method described in (1) above, preferably the
suspension liquid is separated into the liquid containing the other
hydrocarbon and the solid containing the carbon dioxide and the
hydrogen sulfide with a cyclone.
[0014] (7) In the method described in (1) above, preferably in the
range in which the pressure of the natural gas is 1 atm or more and
the temperature of the natural gas is a value less than a melting
point of the hydrogen sulfide but equal to or higher than a melting
point of n-pentane, the pressure and temperature of the natural gas
is adjusted so that a concentration of methane in the gas phase is
90% or more.
[0015] (8) In the method described in (7) above, preferably the
temperature of the natural gas is adjusted to a value less than a
boiling point of ethylene.
[0016] (9) In the method described in (7) above, preferably the
pressure of the natural gas is adjusted to 10 atm or below.
[0017] (10) In the method described in (7) above, preferably the
temperature of the natural gas is adjusted to a value less than a
boiling point of ethylene and the pressure of the natural gas is
adjusted to 10 atm or below.
[0018] (11) To achieve the object described above, the present
invention provides a natural gas refining system for refining gas
containing methane; any other hydrocarbon selected from the group
consisting of ethane, ethylene, propane, propylene, n-butane,
isobutane, 1-butene, n-pentane, and isopentane; carbon dioxide; and
a hydrogen sulfide, and the system comprises pressure/temperature
adjusting means for adjusting a pressure and temperature of the
natural gas so that the methane is in the gas phase, the other
hydrocarbon in the liquid phase, and the carbon dioxide and the
hydrogen sulfide in the solid phase, respectively; gas separating
means for separating the natural gas, of which the pressure and
temperature adjusted by the pressure/temperature adjusting means,
into a gas containing methane and a suspension liquid; and
solid/liquid separating means for separating the suspension liquid
separated by the gas separating means into a liquid containing the
other hydrocarbon and a solid containing the carbon dioxide and the
hydrogen sulfide.
[0019] (12) In the system described in (11) above, preferably the
pressure/temperature adjusting means adjusts the temperature of the
natural gas to a value less than a boiling point of ethylene but
equal to or higher than a melting point of pentane.
[0020] (13) In the system described in (11) above, preferably the
pressure/temperature adjusting means adjusts the pressure of the
natural gas in a range from 1 to 10 atm.
[0021] (14) In the system described in (11) above, preferably the
pressure/temperature adjusting means has expanding means for
adiabatic-expanding the natural gas.
[0022] (15) In the system described in (11) above, preferably the
pressure/temperature adjusting means has mixing means for mixing
liquefied natural gas in the natural gas.
[0023] (16) In the system described in (11) above, preferably the
solid/liquid separating means is a cyclone.
[0024] (17) In the system described in (11) above, preferably in
the range in which the pressure is 1 atm or more and the
temperature of the natural gas is a value less than a melting point
of the hydrogen sulfide but equal to or higher than a melting point
of n-pentane, the pressure/temperature adjusting means adjusts the
pressure and temperature of the natural gas so that a concentration
of methane in the gas phase is 90% or more.
[0025] (18) In the system described in (17) above, preferably the
pressure/temperature adjusting means adjusts the temperature of the
natural gas to a value less than a boiling point of ethylene.
[0026] (19) In the system described in (17) above, preferably the
pressure/temperature adjusting means adjusts the pressure of the
natural gas to 10 atm or below.
[0027] (20) In the system described in (17) above, preferably the
pressure/temperature adjusting means adjusts the temperature of the
natural gas to a value less than a boiling point of ethylene and
the pressure of the natural gas to 10 atm or below.
[0028] With the present invention, product quality of liquefied
natural gas can be improved, and in addition hydrocarbons other
than methane can be separated and recovered.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic diagram illustrating the configuration
of a natural gas refining system according to an embodiment of the
present invention;
[0030] FIG. 2 is a view showing boiling points and melting points
of components contained in natural gas at 1 atm;
[0031] FIG. 3 is a schematic diagram illustrating the configuration
of a first modification of the natural gas refining system
according to the present invention;
[0032] FIG. 4 is a schematic diagram illustrating the configuration
of a second modification of the natural gas refining system
according to the present invention;
[0033] FIG. 5 is a schematic diagram illustrating the configuration
of a third modification of the natural gas refining system
according to the present invention;
[0034] FIG. 6 is a view showing the composition of natural gas when
the natural gas contains n-pentane;
[0035] FIG. 7 is a view showing a methane concentration in refined
gas obtained from the natural gas containing n-pentane;
[0036] FIG. 8 is a view showing ranges of a pressure and
temperature required when a methane concentration of refined gas
obtained from the natural gas containing n-pentane is 90% or
more;
[0037] FIG. 9 is a view showing ranges of a pressure and
temperature required when a methane concentration of refined gas
obtained from natural gas containing ethylene is 90% or more;
[0038] FIG. 10 is a view showing ranges of a pressure and
temperature required when a methane concentration of refined gas
obtained from natural gas containing ethylene is 90% or more in the
case where the natural gas refining system according to the present
invention is configured with low pressure devices;
[0039] FIG. 11 is a view showing the composition of natural gas
when the natural gas contains various hydrocarbons;
[0040] FIG. 12 is a view showing methane concentrations in refined
gas obtained from natural gas containing various hydrocarbons;
and
[0041] FIG. 13 is a view showing ranges of a pressure and
temperature required when a methane concentration of refined gas
obtained from natural gas containing various hydrocarbons is 90% or
more.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] An embodiment of the present invention is described below
with reference to the drawings.
[0043] FIG. 1 is a schematic diagram illustrating a natural gas
refining system according to an embodiment of the present
invention.
[0044] In FIG. 1, the natural gas refining system is installed in a
plant for producing liquefied natural gas (LNG) from natural gas
mined in a natural gas field or in an oil field. The natural gas
refining system comprises a separation tank 1 (a gas/solid
separating unit), a nozzle 2 (a pressure/temperature adjusting
unit) for injecting natural gas into the separation tank 1, a
natural gas feed line 3, a liquefied natural gas feed line 4, a gas
takeoff line 5, a suspension liquid takeoff line 6, and a cyclone 7
(a solid/liquid separating unit). The natural gas feed line 3
supplies natural gas (more specifically, natural gas just mined in
a natural gas field or in an oil field, or natural gas pressurized
to about 60 atm (6 MPa)) to the nozzle 2. The liquefied natural gas
feed line 4 supplies a portion of the liquefied natural gas
produced in the plant to the nozzle 2. The gas takeoff line 5 is
connected to an upper portion of the separation tank 1. The
suspension liquid takeoff line 6 is connected to a lower portion of
the separation tank 1. The cyclone 7 is connected to the suspension
liquid takeoff line 6.
[0045] The nozzle 2 is a mixing and expanding type nozzle (a mixing
unit and an expanding unit), although the details thereof are not
shown in the figure. The nozzle 2 mixes liquefied natural gas (LNG)
supplied through the liquefied natural gas feed line 4 in natural
gas supplied from the natural gas feed line 3, and diffusively
injects the resulting mixture into the separation tank 1 to subject
the mixture to adiabatic expansion so that a pressure and a
temperature of the natural gas are deceased. With this operation,
the pressure of the natural gas is adjusted to a range from 1 to 10
atm (a range from 0.1 to 1 MPa) (preferably to a range from 1 to 2
atm (a range from 0.1 to 0.2 MPa)). The temperature of the natural
gas is adjusted to a value less than a boiling point of ethylene
but equal to or higher than a melting point of n-pentane. FIG. 2 is
a view showing boiling points and melting points of components
contained in natural gas (for instance, methane, ethane, ethylene,
propane, propylene, n-butane, isobutane, 1-butene, n-pentane,
isopentane, carbon dioxide, and hydrogen sulfide) at 1 atm (0.1
MPa). As clearly understood from FIG. 2, when temperature of
natural gas is adjusted to a value less than a boiling point of
ethylene but equal to or higher than a melting point of n-pentane
(for instance, to a range from a value less than -104.degree. C. to
a value equal to or equal to or higher than -129.7.degree. C. at 1
atm (0.1 MPa)), methane contained in the natural gas is converted
to the gas phase, other hydrocarbon (more specifically, any of
ethane, ethylene, propane, propylene, n-butane, isobutane,
1-butene, n-pentane, and isopentane) to the liquid phase, and
carbon dioxide and hydrogen sulfide to the solid phase.
[0046] In the separation tank 1, because of a difference in the
specific gravity, a gas containing methane moves upward, while a
suspension liquid in which a liquid containing other hydrocarbon
and a solid containing carbon dioxide and hydrogen sulfide are
mixed moves downward, so that the gas and the suspension liquid are
separated from each other. Then, the gas separated in the
separation tank 1 is supplied to liquefaction equipment (more
specifically, heat transfer equipment for cooling the gas
containing methane or a compressor for which the gas containing
methane is used as a cooling medium) is supplied via the gas
takeoff line 5. Furthermore, the suspension liquid separated in the
separation tank 1 is supplied via the suspension liquid takeoff
line 6 to the cyclone 7.
[0047] The cyclone 7 is a known unit available for the purpose as
described above, and separates the suspension liquid into a liquid
containing other hydrocarbon and a solid containing carbon dioxide
and hydrogen sulfide. The separated liquid is recovered via a
liquid takeoff line 8, while the separated solid is recovered via a
solid takeoff line 9. The other hydrocarbon recovered as described
above is further separated to respective components by, for
instance, distillation equipment (not shown) and used for other
purposes.
[0048] Operations of and effects by the natural gas refining system
having the configuration as described above according to this
embodiment will be described below.
[0049] In this embodiment, liquefied natural gas is mixed in
natural gas with the mixing and expanding type of nozzle 2, and the
resulting mixture is injected into the separation tank 1 and
subjected to adiabatic expansion there to decrease the pressure and
the temperature of the natural gas. In this step, the pressure of
the natural gas is adjusted to a range from 1 to 10 atm (a range
from 0.1 to 1 MPa) and also the temperature of the natural gas is
adjusted to a value less than a boiling point of ethylene but equal
to or higher than a melting point of n-pentane. Therefore methane
contained in the natural gas is converted to the gas phase, other
hydrocarbon (more specifically, any of ethane, ethylene, propane,
propylene, n-butane, isobutane, 1-butene, n-pentane, and
isopentane) to the liquid phase, and carbon dioxide and hydrogen
sulfide to the solid phase. Then, the natural gas is separated to
the gas containing methane and the suspension liquid, and the
separated gas phase is supplied to the liquefaction equipment to
produce liquefied natural gas. With the process, not only carbon
dioxide and hydrogen sulfide contained in the natural gas, but also
the other hydrocarbon can be separated to produce liquefied natural
gas with improved product quality. On the other hand, the
suspension liquid separated in the separation tank 1 is
continuously processed with the cyclone 7 to be separated to and
recovered as a liquid containing the other hydrocarbon and a solid
containing carbon dioxide and hydrogen sulfide. In this way,
hydrocarbons other than methane can be separated and recovered, and
hydrocarbons other than the recovered methane can be utilized for
various purposes.
[0050] Furthermore, in this embodiment, the natural gas is adjusted
with the nozzle 2 to the pressure range from 1 to 10 atm (a range
from 0.1 to 1 MPa), and therefore as compared to a case where the
pressure is adjusted to a value higher than 10 atm (1 MPa), the
conditions for pressure resistance can be mitigated. Accordingly,
it enables cost reduction in manufacturing the refining system.
[0051] Although not specifically described in the embodiment above,
a pressure and temperature of natural gas may be adjusted by
controlling a flow rate of liquefied natural gas to be supplied to
the nozzle 2. In other words, although not shown, the natural gas
refining system may include a pressure sensor for detecting a
pressure in the separation tank 1 and a temperature sensor for
detecting a temperature in the separation tank 1. Further the
natural gas refining system may include a flow control valve
provided on the liquefied natural gas feed line 4 for controlling a
flow rate of liquefied natural gas, and a control unit for
controlling the flow control valve so that values detected by the
pressure sensor and the temperature sensor are set to target
values. Also in this configuration, the effects as described above
are provided.
[0052] The embodiment has been described with reference to a case
where the mixing and expanding type nozzle 2 is employed as an
expanding unit for adiabatic-expanding the natural gas and also as
a mixing unit for mixing liquefied natural gas in natural gas, but
the present invention is not limited to the configuration.
Specifically, for instance, a mixing valve as a mixing unit and an
expansion valve as an expanding unit may be provided in place of
the nozzle 2. For instance, in the case where the target
temperature conditions for natural gas are achieved only by means
of adiabatic expansion, the configuration is allowable in which the
mixing unit is not provided and only the expanding unit is
provided. Also in this configuration, the effects as described
above are provided.
[0053] The embodiment described above is based on the configuration
in which the cyclone 7 continuously processes the suspension liquid
separated in the separation tank 1, but the present invention is
not limited to this configuration. Specifically, for instance, as
shown in FIG. 3, also the configuration is allowable in which a
switching valve 10 is provided on the suspension takeoff line 6 in
such a manner that it is opened or closed according to the
necessity. Also in a first modification, the same effects as those
described above can be provided. In this modification, the cyclone
7 can efficiently be operated according to a required processing
rate of natural gas and contents of components other than
methane.
[0054] In the embodiment above, an internal structure of the
separation tank 1 is not specifically described, but, for instance,
as shown in FIG. 4, a plurality of inclined trays (four shelves in
FIG. 4) may be provided inside the separation tank 1. Also in a
second modification, the same effects as those in the embodiment
described above can be provided. Furthermore, in this modification,
a partial pressure of methane in the gas decreases in the lower
trays (lower ones in FIG. 4) while methane in the liquid phase is
likely to be converted to the gas phase. As a result, improvement
in the recovery rate of methane is achieved. Furthermore, a
temperature control unit such as a heater may be provided so that a
temperature in the lower tray 11 is higher than that in the upper
tray 11. With the configuration, a vapor pressure of methane
becomes larger in the lower trays, and therefore methane in the
liquid phase is likely to shift to the gas phase. As a result,
improvement in the recovery rate of methane is achieved.
[0055] The above embodiment has been described with reference to
the case where the cyclone 7 is provided as a solid/liquid
separating unit for separating the suspension liquid separated in
the separation tank 1 to a solid phase and a liquid phase, but the
present invention is not limited to this configuration.
Specifically, for instance, filtration equipment or distillation
equipment may be provided in place of the cyclone 7. Furthermore,
description of the embodiment above has been made with reference to
the case where the separation tank 1 and the cyclone 7 are provided
independently, but the present invention is not limited to this
configuration. Specifically, as shown in FIG. 5, the cyclone 7 is
provided at a portion below the separation tank 1 in such a manner
that the separation tank 1 and the cyclone 7 are integrally formed.
Also in the third modification, the same effects as those in the
embodiment above can be obtained.
[0056] In the embodiment described above, a pressure and
temperature of natural gas in the separation tank 1 is adjusted so
that methane contained in natural gas is converted to the gas
phase, other hydrocarbons to the liquid phase, and the carbon
dioxide and hydrogen sulfide to the solid phase, respectively. This
does not always mean that all of methane contained in the natural
gas, all of the other hydrocarbons, and carbon dioxide and hydrogen
sulfide are converted to the gas phase, liquid phase, and solid
phase, respectively. Specifically, a concentration of methane in
the refined gas depends on a composition, a pressure, and a
temperature of natural gas in the separation tank 1.
[0057] Description is provided below for a method of setting a
pressure and temperature in the separation tank 1 for making higher
a concentration of methane in refined gas with reference to the
simulated natural gas as shown in FIG. 6. The simulated natural gas
comprises three components of methane, carbon dioxide, and
n-pentane having the highest melting point among the hydrocarbons
other than methane shown in FIG. 2.
[0058] FIG. 7 shows concentrations of methane in refined gas when
the simulated natural gas is separated at various pressures and
temperatures. A concentration of methane in liquefied natural gas
is required to be higher than a value slightly lower than 90%
indicating a percentage of methane contained in town gas when the
refined gas is used as town gas. Therefore, it is desirable that
the methane concentration in the refined gas be 90% or more. As
understood from the figure, when the pressure is high, it is
possible to raise to 90% the methane concentration in refined gas
at a higher temperature.
[0059] FIG. 8 shows a pressure and a temperature when refined gas
obtained from the simulated natural gas has a concentration of
methane of 90% or more (This figure is prepared from FIG. 7). In
this figure, .largecircle. represents a pressure at which the
methane concentration in the refined gas is maximized at each
temperature, .DELTA. (overlaying on .largecircle. in this figure)
represents an upper limit pressure when the methane concentration
in the refined gas is 90% or more at each temperature, and
.gradient. represents a lower limit pressure at which the methane
concentration in the refined gas is 90% or more at each
temperature. This means that when the pressure and temperature is
in the area from the upper limit to the lower limit, most of
methane is converted to a gas phase.
[0060] Since it is not necessary to provide a depressurizing unit
such as a vacuum pump in the separation tank 1, the pressure range
is preferably 1 atm (0.1 MPa) or more. Furthermore, because the
hydrocarbons other than methane are converted to the liquid phase
and carbon dioxide and hydrogen sulfide to the solid phase, the
temperature range is required to be less than a boiling point of
hydrogen sulfide and at the same time equal to or higher than a
melting point of n-pentane. Specifically, when the pressure and
temperature is in the hatched area in FIG. 8, the methane
concentration in the refined gas can be made higher so that methane
contained in the natural gas is converted to the gas phase, other
hydrocarbons to the liquid phase, and carbon dioxide and hydrogen
sulfide to the solid phase, respectively.
[0061] Next, description is provided for ranges of a pressure and
temperature when natural gas contains ethylene with reference to
FIG. 9. Ethylene is a compound having the lowest boiling point
among the components other than methane shown in FIG. 2. Because
most of ethylene is to be converted to the liquid phase, the
temperature range is required to be less than a bolding point of
ethylene. Specifically, when the pressure and temperature is in the
hatched area in the figure, the methane concentration in the
refined gas can be made higher so that methane contained in natural
gas is converted to the gas phase, other hydrocarbons to the liquid
phase, and carbon dioxide and hydrogen sulfide to the solid phase,
respectively.
[0062] As described above, when the pressure is set to 10 atm (1
MPa) or below, the conditions for pressure resistance can be
mitigated as compared to the case where the pressure is 10 atm (1
MPa) or more. Accordingly, it possible to reduce manufacturing cost
for the refining system. When the pressure and temperature is in
the hatched area in FIG. 10, it is possible to configure a natural
gas refining system with low-pressure devices and to make higher a
methane concentration in refined gas so that methane contained in
the natural gas is converted to the gas phase, other hydrocarbons
to the liquid phase, and carbon dioxide and hydrogen sulfide to the
solid phase, respectively.
[0063] A method of setting a pressure and temperature for raising a
methane concentration in refined gas when the natural gas contains
various hydrocarbons is described below with reference to FIGS. 11
to 13.
[0064] FIG. 11 is a view showing the composition of natural gas
when the natural gas contains various hydrocarbons. The methane
concentration in refined gas obtained from natural gas shown in
FIG. 11 is determined as shown in FIG. 12. When a pressure and
temperature required for the methane concentration of 90% or more
in the refined gas shown in FIG. 12 is determined, FIG. 13 can be
prepared.
[0065] Differences between FIG. 13 and FIG. 8 are described below.
When the natural gas contains various hydrocarbons, a pressure at
which the methane concentration in refined gas at each temperature
is maximized (represented by .largecircle. in the figure) is
separated from an upper limit pressure at which the methane
concentration in the refined gas is 90% or more at each temperature
(represented by .DELTA. in the figure). However, the upper limit
changes little.
[0066] On the other hand, a lower limit pressure at which the
methane concentration in refined gas at each temperature is 90%
(represented by .gradient.) in the figure becomes higher due to
effects by hydrocarbons other than methane and n-pentane.
Therefore, ranges of a pressure and a temperature (in the hatched
area in the figure) at which the methane concentration in the
refined gas can be made higher so that the methane contained in the
natural gas is converted to the gas phase, other hydrocarbons to
the liquid phase, and carbon dioxide and hydrogen sulfide to the
solid phase respectively are narrower as compared to those shown in
FIG. 8. In other words, the ranges of a pressure and a temperature
are included in the ranges of a pressure and a temperature shown in
FIG. 8.
* * * * *