U.S. patent application number 11/865847 was filed with the patent office on 2008-01-31 for process and apparatus for separation of hydrocarbons from liquefied natural gas.
This patent application is currently assigned to TOYO ENGINEERING CORPORATION. Invention is credited to Akhilesh Kumar, Susumu Ohara, Shoichi Yamaguchi, Nobuhiro YOSHIDA.
Application Number | 20080022717 11/865847 |
Document ID | / |
Family ID | 35053116 |
Filed Date | 2008-01-31 |
United States Patent
Application |
20080022717 |
Kind Code |
A1 |
YOSHIDA; Nobuhiro ; et
al. |
January 31, 2008 |
PROCESS AND APPARATUS FOR SEPARATION OF HYDROCARBONS FROM LIQUEFIED
NATURAL GAS
Abstract
An apparatus and process for separating a feed liquefied natural
gas containing at least methane and a hydrocarbon less volatile the
methane, into a product natural gas enriched with methane and lean
in hydrocarbon less volatile than methane and a heavier fraction
lean in methane and enriched with hydrocarbon less volatile than
methane. The process includes heating the feed liquefied natural
gas in a heat exchanger, passing the heated fluid into a
distillation column, withdrawing the heavier fraction from a bottom
of the column, and withdrawing a residue gas from a top of the
column. The process also includes liquefying at least part of the
residue gas in the heat exchanger, refluxing a part of the liquid
portion of the fluid obtained in the liquefying step into the
column, and withdrawing, as the product natural gas, the remainder
of the liquid portion.
Inventors: |
YOSHIDA; Nobuhiro; (Chiba,
JP) ; Yamaguchi; Shoichi; (Chiba, JP) ; Ohara;
Susumu; (Chiba, JP) ; Kumar; Akhilesh; (Chiba,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
TOYO ENGINEERING
CORPORATION
Tokyo
JP
|
Family ID: |
35053116 |
Appl. No.: |
11/865847 |
Filed: |
October 2, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10942842 |
Sep 17, 2004 |
|
|
|
11865847 |
Oct 2, 2007 |
|
|
|
Current U.S.
Class: |
62/630 ;
62/620 |
Current CPC
Class: |
F25J 2230/60 20130101;
F25J 3/0214 20130101; F25J 3/0238 20130101; F25J 2205/02 20130101;
C10G 5/00 20130101; B01D 3/14 20130101; F25J 2235/60 20130101; F25J
3/0233 20130101; F25J 2200/02 20130101; F25J 2230/08 20130101 |
Class at
Publication: |
062/630 ;
062/620 |
International
Class: |
F25J 3/02 20060101
F25J003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 5, 2004 |
JP |
2004-111236 |
Jun 4, 2004 |
JP |
2004-167236 |
Claims
1. A process for separating a liquefied natural gas into a product
natural gas and a heavier fraction, said process comprising: (a)
heating the liquefied feed natural gas in a first heat exchanger;
(b) passing the heated feed natural gas into a distillation column;
(c) heating liquid from the heated liquefied feed natural gas that
is received within a bottom part of the distillation column using a
reboiler; (d) withdrawing the heavier fraction from the bottom part
of the distillation column; (e) withdrawing a residue gas from a
top part of the distillation column; (f) liquefying at least a part
of the residue gas in the first heat exchanger to form a fluid
having a liquid portion; (g) refluxing a part of the liquid portion
into the distillation column; and (h) withdrawing, as the product
natural gas, a remainder of the liquid portion, which has not been
fed into the distillation column in step (g).
2. The process according to claim 1, further comprising: (i)
heating the heated liquefied feed natural gas from step (a), in a
second heat exchanger, prior to step (b).
3. The process according to claim 2, wherein a part of the residue
gas is liquefied in step (f), and further comprising: (j)
separating a gas portion from the fluid obtained in step (f) and
then pressurizing the gas portion; (k) liquefying the gas portion
obtained in step (j), in the second heat exchanger; and (l)
withdrawing the liquid obtained in step (k) as the product natural
gas.
4. The process according to claim 3, wherein an entirety of the gas
portion obtained in step (j) is liquefied in step (k) and withdrawn
in step (l).
5. The process according to claim 2, further comprising heating the
liquefied feed natural gas from step (i), prior to step (b).
6. The process according to claim 5, wherein a part of the residue
gas is liquefied in step (f), and further comprising: (j)
separating a gas portion from the fluid obtained in step (f) and
then pressurizing the gas portion; (k) liquefying the gas portion
obtained in step (j), in the second heat exchanger; and (l)
withdrawing the liquid obtained in step (k) as the product natural
gas.
7. The process according to claim 6, wherein an entirety of the gas
portion obtained in step (j) is liquefied in step (k) and withdrawn
in step (l).
8. The process according to claim 1, wherein the distillation
column is a demethanizer.
9. The process according to claim 1, further comprising supplying
to the first heat exchanger, as the liquefied feed natural gas, a
feed natural gas containing at least methane and a hydrocarbon less
volatile than methane, wherein the heavier fraction withdrawn in
step (d) is lean in methane and enriched with the hydrocarbon less
volatile than methane, and wherein the product natural gas
withdrawn in step (h) is enriched with methane and lean in the
hydrocarbon less volatile than methane.
10. The process according to claim 1, further comprising supplying
to the first heat exchanger, as the liquefied feed natural gas, a
feed natural gas containing at least methane, ethane, and a
hydrocarbon less volatile than ethane, wherein the heavier fraction
withdrawn in step (d) is lean in methane and ethane, and enriched
with the hydrocarbon less volatile than ethane, and wherein the
product natural gas withdrawn in step (h) is enriched with methane
and ethane, and lean in the hydrocarbon less volatile than
ethane.
11. An apparatus for separating a liquefied feed natural gas into a
product natural gas and a heavier fraction, said apparatus
comprising: a first heat exchanger configured to receive and heat
the liquefied feed natural gas; a distillation column configured to
receive a heated liquefied feed natural gas via said first heat
exchanger, said distillation column having a bottom part with a
lower outlet configured to discharge the heavier fraction
therefrom, said distillation column having a top part with an upper
outlet configured to discharge a residue gas to said first heat
exchanger such that said first heat exchanger can cool at least a
part of the residue gas to form a fluid including a liquid portion
thereof; a reflux device configured to feed at least a part of the
liquid portion of the residue gas into said distillation column; a
line configured to discharge a remainder of the liquid portion of
the residue gas as the product natural gas; and a reboiler
configured to heat liquid from the heated liquefied feed natural
gas that is received within said bottom part of said distillation
column.
12. The apparatus according to claim 11, further comprising a
second heat exchanger configured to receive and heat the liquefied
feed natural gas from said first heat exchanger, wherein said
distillation column is configured to receive the heated liquefied
feed natural gas via said first heat exchanger and said second heat
exchanger.
13. The apparatus according to claim 12, wherein said first heat
exchanger is configured to liquefy a part of the residue gas,
further comprising: a gas-liquid separator configured to receive
the fluid from said first heat exchanger and separate the fluid
into the liquid portion and a gas portion; a pressurization device
configured to pressurize the gas portion and feed the pressurized
gas portion to said second heat exchanger so that said second heat
exchanger can liquefy the pressurized gas portion; and a line
configured to discharge liquid liquefied from the pressurized gas
portion in said second heat exchanger as the product natural
gas.
14. The apparatus according to claim 13, wherein said second heat
exchanger is configured to liquefy an entirety of the pressurized
gas portion.
15. The apparatus according to claim 12, further comprising a
heater configured to receive and heat the liquefied feed natural
gas from said second heat exchanger, wherein said distillation
column is configured to receive the heated liquefied feed natural
gas via said first heat exchanger, said second heat exchanger, and
said heater.
16. The apparatus according to claim 15, wherein said first heat
exchanger is configured to liquefy a part of the residue gas,
further comprising: a gas-liquid separator configured to receive
the fluid from said first heat exchanger and separate the fluid
into the liquid portion and a gas portion; a pressurization device
configured to pressurize the gas portion and feed the pressurized
gas portion to said second heat exchanger so that said second heat
exchanger can liquefy the pressurized gas portion; and a line
configured to discharge liquid liquefied from the pressurized gas
portion in said second heat exchanger as the product natural
gas.
17. The apparatus according to claim 16, wherein said second heat
exchanger is configured to liquefy an entirety of the pressurized
gas portion.
18. The apparatus according to claim 11, wherein said distillation
column is a demethanizer.
19. The apparatus according to claim 11, wherein: said first heat
exchanger is configured to receive, as the liquefied feed natural
gas, a feed natural gas containing at least methane and a
hydrocarbon less volatile than methane; said lower outlet of said
distillation column is configured to discharge, as the heavier
fraction, a heavier fraction lean in methane and enriched with the
hydrocarbon less volatile than methane; and said line is configured
to discharge, as the product natural gas, a product natural gas
enriched with methane and lean in the hydrocarbon less volatile
than methane.
20. The apparatus according to claim 11, wherein: said first heat
exchanger is configured to receive, as the liquefied feed natural
gas, a feed natural gas containing at least methane, ethane, and a
hydrocarbon less volatile than ethane; said lower outlet of said
distillation column is configured to discharge, as the heavier
fraction, a heavier fraction lean in methane and ethane, and
enriched with the hydrocarbon less volatile than ethane; and said
line is configured to discharge, as the product natural gas, a
product natural gas enriched with methane and ethane, and lean in
the hydrocarbon less volatile than ethane.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority under 35 USC .sctn.120 from U.S. Ser. No. 10/942,842,
filed Sep. 17, 2004, and under 35 USC .sctn.119 from Japanese
Patent Application Nos. 2004-111236, filed Apr. 5, 2004, and
2004-167236, filed on Jun. 4, 2004, the entire contents of which
are incorporated herein by reference.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to a process and apparatus for
separation of hydrocarbons, used for separating and recovering
hydrocarbons such as ethane, propane and the like from a liquefied
natural gas.
[0004] 2. Background of the Invention
[0005] It is being conducted generally to liquefy a natural gas at
about -162.degree. C. at around atmospheric pressure, send the
liquefied natural gas to a marketplace by marine transportation,
vaporize the liquefied natural gas, then feed it into a natural gas
pipeline as a town gas or as a fuel for thermal power generation.
Incidentally, a natural gas liquefied at around atmospheric
pressure is called liquefied natural gas (LNG). The liquefied
natural gas received at the marketplace contains, in some cases, a
large amount of hydrocarbons of 2 to 5 carbon atoms. Such a
liquefied natural gas has a high calorific value and therefore may
not meet the natural gas specification required by the marketplace.
Or, the hydrocarbons of 2 to 5 carbon atoms (e.g. ethane and
propane) contained in liquefied natural gas can be used as a raw
material for petrochemical plants and therefore have, in some
cases, a higher commercial value than when used as a town gas or as
a fuel for thermal power generation. Hence, it has been desired to
separate and recover hydrocarbons of 2 or more carbon atoms from a
liquefied natural gas received by the marketplace before the
natural gas is fed into a natural gas pipeline.
[0006] For separation and recovery of hydrocarbons such as ethane,
propane and the like from a liquefied natural gas, it is possible
to apply a process described in U.S. Pat. No. 6,510,706 (Patent
Literature 1). This process is concerned with a technique of
liquefying, for liquid transportation, a natural gas at a
temperature exceeding -112.degree. C. (-170.degree. F.) and
sufficient for the liquefied natural gas at or below its bubble
point. Incidentally, a natural gas liquefied in a pressurized state
is called pressurized liquid natural gas (PLNG) and is
distinguished from the above-mentioned liquefied natural gas (LNG).
According to the technique, hydrocarbons less volatile than methane
can be removed from a pressurized liquid natural gas (PLNG). This
technique may be applied to a process for separating and recovering
hydrocarbons such as ethane, propane and the like from a liquefied
natural gas (LNG) transported at around atmospheric pressure at
about -162.degree. C. In the literature, a process is described
including heating feed PLNG in a heat exchanger, thereby vaporizing
at least a portion of the PLNG; passing the partially vaporized
PLNG to a fractionation column; withdrawing a liquid stream
enriched with hydrocarbons less volatile than methane from a lower
portion of the fractionation column; withdrawing a vapor stream
from an upper portion of the fractionation column; and passing the
vapor stream to the heat exchanger to condense the vapor to produce
PLNG lean in hydrocarbons less volatile than methane. In the
literature, it is also described that when higher recovery rate of
ethane and propane is desired, a reflux effect is obtained by
withdrawing part of the feed PLNG and feeding it into the top of
the distillation column in a liquid state without the vaporization
by the heat exchanger, whereby ethane and propane can be obtained
at a higher recovery rate.
[0007] For separation and recovery of hydrocarbons such as ethane,
propane and the like from a liquefied natural gas, there can also
be used a process disclosed in U.S. Pat. No. 2,952,984 (Patent
Literature 2). In the literature, there is described a process
including feeding a liquefied natural gas into the middle portion
of a fractionation column; heating the contents of the lower
portion of the fractionation column to produce methane-enriched
vapors in the upper portion of the fractionation column;
withdrawing vapors from the upper portion of the fractionation
column and directly passing the vapors in heat exchange relation
with the liquefied natural gas being fed to the fractionation
column, to heat the feed and cool the vapors; separating condensed
liquid from the vapors; refluxing the condensed liquid to the upper
portion of the fractionation column; and withdrawing a heavier
hydrocarbon from the lower portion of the fractionation column.
[0008] When the process described in the Patent Literature 1 is
applied for separation and recovery of hydrocarbons such as ethane
and the like from a liquefied natural gas, no high reflux effect is
obtained because a feed liquefied natural gas which is low in
methane concentration is used as a reflux for distillation column,
and the recovery rate of ethane is considered to remain at about
92%. Here, the ethane recovery rate means a proportion at which the
ethane contained in feed liquefied natural gas is separated from a
product liquefied natural gas and recovered as a component of NGL
(natural gas liquid); that is, the ethane recovery rate is a value
obtained by dividing the ethane amount in NGL by the ethane amount
in feed liquefied natural gas. Such an ethane recovery rate, i.e.,
at most about 92%, may be sufficient in order to adapt the product
LNG to the natural gas specification of the marketplace; however,
in order to obtain hydrocarbons of 2 to 5 carbon atoms as a raw
material gas for a petrochemical plant, recovery of a higher amount
of ethane for effective ethane utilization is desired from an
economical standpoint. Thus, further improvement in ethane or
propane recovery rates has been desired.
[0009] In the process described in the Patent Literature 2, the
condensed liquid of an overhead gas of distillation column in which
methane is concentrated, is fed as a reflux for the distillation
column; therefore, there is an advantage of high refluxing effect.
In this process, the gas from the upper portion of the distillation
column is cooled, the resulting condensed liquid is separated, a
residue natural gas is withdrawn in a gaseous state and then
compressed by a compressor to a pressure required for pipeline
transportation. Therefore, there has been a problem in that a large
energy is required for the gas compression.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide a process
and apparatus that can conduct separation of hydrocarbons from a
liquefied natural gas more effectively. Particularly, an object of
the present invention is to provide a process and apparatus for
separation of hydrocarbons using a distillation column, wherein an
improved separation efficiency is obtained and thereby a high
ethane or propane recovery rate is attainable. Another object of
the present invention is to provide a process and apparatus for
separation of hydrocarbons, wherein the required energy can be made
relatively small.
[0011] The present invention provides a process for separating a
feed liquefied natural gas containing at least methane and a
hydrocarbon less volatile than methane, into a product natural gas
enriched with methane and lean in the hydrocarbon less volatile
than methane and a heavier fraction lean in methane and enriched
with the hydrocarbon less volatile than methane, using a
distillation column. The process includes the steps of:
[0012] (a) heating the feed liquefied natural gas in a heat
exchanger;
[0013] (b) passing the fluid heated in step (a) into a distillation
column;
[0014] (c) withdrawing the heavier fraction from the bottom part of
the distillation column;
[0015] (d) withdrawing a residue gas from the top part of the
distillation column;
[0016] (e) liquefying at least a part of the residue gas in the
heat exchanger;
[0017] (f) refluxing a part of the liquid portion of the fluid
obtained in step (e) into the distillation column; and
[0018] (g) withdrawing, as the product natural gas, the remainder
of the liquid portion of the fluid obtained in step (e), which has
not been fed into the distillation column in step (f).
[0019] The present invention also provides a process for separating
a feed liquefied natural gas containing at least methane, ethane
and a hydrocarbon less volatile than ethane, into a product natural
gas enriched with methane and ethane and lean in the hydrocarbon
less volatile than ethane and a heavier fraction lean in methane
and ethane and enriched with the hydrocarbon less volatile than
ethane, using a distillation column. The process includes the steps
of:
[0020] (a) heating the feed liquefied natural gas in a heat
exchanger;
[0021] (b) passing the fluid heated in step (a) into a distillation
column;
[0022] (c) withdrawing the heavier fraction from the bottom part of
the distillation column;
[0023] (d) withdrawing a residue gas from the top part of the
distillation column;
[0024] (e) liquefying at least a part of the residue gas in the
heat exchanger;
[0025] (f) refluxing a part of the liquid portion of the fluid
obtained in step (e) into the distillation column; and
[0026] (g) withdrawing, as the product natural gas, the remainder
of the liquid portion of the fluid obtained in step (e), which has
not been fed into the distillation column in step (f).
[0027] The above processes may further include the step of (h)
heating the fluid heated in step (a), in a heat exchanger different
from the heat exchanger used in step (a), prior to step (b).
[0028] In this process, a part of the residue gas may be liquefied
in step (e), and the process may further include the steps of:
[0029] (i) separating a gas portion from the fluid obtained in step
(e) and then pressuring the gas portion;
[0030] (j) liquefying the whole portion of the fluid obtained in
step (i), in the heat exchanger used in step (h); and
[0031] (k) withdrawing the fluid obtained in step (j) as a product
natural gas.
[0032] The present invention also provides an apparatus for
separating a feed liquefied natural gas containing at least methane
and a hydrocarbon less volatile than methane, into a product
natural gas enriched with methane and lean in the hydrocarbon less
volatile than methane and a heavier fraction lean in methane and
enriched with the hydrocarbon less volatile than methane, using a
distillation column. The apparatus includes:
[0033] a distillation column to which a heated feed liquefied
natural gas is fed, from the bottom part of which the heavier
fraction is withdrawn, and from the top part of which a residue gas
is withdrawn;
[0034] a heat exchanger in which the feed liquefied natural gas and
the residue gas are heat exchanged to heat the feed liquefied
natural gas and to cool the residue gas and liquefy at least a part
of the residue gas;
[0035] a refluxing means for refluxing a part of the liquid portion
of the residue gas which has been liquefied at least partially in
the heat exchanger, into the distillation column; and
[0036] a line for withdrawing the remainder of the liquid portion
of the residue gas which has been liquefied at least partially in
the heat exchanger, as a product natural gas.
[0037] The present invention also provides an apparatus for
separating a feed liquefied natural gas containing at least
methane, ethane and a hydrocarbon less volatile than ethane, into a
product natural gas enriched with methane and ethane and lean in
the hydrocarbon less volatile than ethane and a heavier fraction
lean in methane and ethane and enriched with the hydrocarbon less
volatile than ethane, using a distillation column. The apparatus
includes:
[0038] a distillation column to which a heated feed liquefied
natural gas is fed, from the bottom part of which the heavier
fraction is withdrawn, and from the top part of which a residue gas
is withdrawn;
[0039] a heat exchanger in which the feed liquefied natural gas and
the residue gas are heat exchanged to heat the feed liquefied
natural gas and to cool the residue gas and liquefy at least a part
of the residue gas,
[0040] a refluxing means for refluxing a part of the liquid portion
of the residue gas which has been liquefied at least partially in
the heat exchanger, into the distillation column; and
[0041] a line for withdrawing the remainder of the liquid portion
of the residue gas which has been liquefied at least partially in
the heat exchanger, as a product natural gas.
[0042] These apparatuses may further include between the heat
exchanger and the distillation column, at least one second heat
exchanger for further heating the liquefied natural gas.
[0043] In this apparatus, the heat exchanger for cooling the
residue gas may liquefy a part of the residue gas, and the
apparatus may further include a gas-liquid separation means for
separating the partially liquefied fluid obtained in the heat
exchanger for cooling the residue gas, into a liquid portion and a
gas portion; and a pressurization means for pressurizing the gas
portion, said second heat exchanger liquefying the whole portion of
the pressurized gas portion. And the apparatus may still further
include a line for withdrawing the fluid liquefied in the second
heat exchanger, as a product natural gas.
[0044] In the present invention, the overhead gas of a distillation
column (a demethanizer in the case of ethane recovery and a
deethanizer in the case of propane recovery) is cooled and
condensed and the resulting liquefied natural gas high in methane
or ethane concentration is used as a reflux. Thereby, the methane
or ethane concentration in the overhead gas of distillation column
is increased; there is obtained a higher separation efficiency; and
a higher ethane or propane recovery rate is obtained. Further in
the present invention, at least a part or the whole of the residue
natural gas withdrawn as a product can be in a liquid state.
Thereby, when the product natural gas is fed into, for example, a
pipeline, the power required for pressurization of the natural gas
can be relatively small.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] FIG. 1 is a flow diagram for explaining an embodiment of the
present process for separation of hydrocarbons from liquefied
natural gas.
[0046] FIG. 2 is a flow diagram for explaining another embodiment
of the present process for separation of hydrocarbons from
liquefied natural gas.
[0047] FIG. 3 is a flow diagram for explaining a further embodiment
of the present process for separation of hydrocarbons from
liquefied natural gas.
[0048] The numerals used in these Figures indicate the following:
[0049] 1: Feed liquefied natural gas; [0050] 2: Pump for feed
liquefied natural gas; [0051] 3: Feed liquefied natural gas reflux;
[0052] 4: Reflux condenser; [0053] 5: Demethanizer feed
(deethanizer feed in the case of propane; recovery plant); [0054]
6: Demethanizer (deethanizer in the case of propane recovery
plant); [0055] 7: Residue gas; [0056] 8: NGL (natural gas liquid);
[0057] 9: Reflux drum; [0058] 10: Reflux pump; [0059] 11: Liquefied
residue gas reflux; [0060] 12: Product liquefied natural gas;
[0061] 13: Reboiler; [0062] 14: Demethanizer feed preheater; [0063]
15: Second residue gas; [0064] 16: Residue gas compressor; [0065]
17: Pressurized residue gas; [0066] 18: Pressurized residue gas
condenser; [0067] 19: Liquefied pressurized residue gas; [0068] 21:
Fluid obtained by cooling of residue gas by reflux condenser;
[0069] 22: Fluid obtained by heating of feed liquefied natural gas
by reflux condenser; [0070] 23: Fluid obtained by heating of fluid
22 by pressurized residue gas condenser; and [0071] 24: Liquefied
residue gas withdrawn as a product
DETAILED DESCRIPTION OF THE INVENTION
[0072] FIG. 1 shows a process for separation of hydrocarbons from a
liquefied natural gas according to the present invention. A
description is herein made of a process for recovery of ethane from
a liquefied natural gas. The process for recovery of ethane from a
liquefied natural gas is a process for separating, by distillation,
the hydrocarbon components contained in a feed liquefied natural
gas, into methane and heavier components of hydrocarbons less
volatile than methane. Incidentally, FIG. 1 is only to explain a
preferred embodiment of the present invention, and the present
invention is in no way limited thereby.
[0073] For example, feed liquefied natural gas 1 consisting
substantially of methane and hydrocarbons of 2 or more carbon atoms
is sent to feed liquefied natural gas pump 2 and is pressurized to
a pressure which is a operation pressure of demethanizer 6 plus a
head required for reflux condenser 4, pressure loss of pipe and
demethanizer feed. The liquefied natural gas used as a raw material
can be obtained by liquefying a natural gas at around atmospheric
pressure, for example, at least 99 kPa (A) and at most 150 kPa (A),
and at its bubble point or a lower temperature (for example, at
least -170.degree. C. and at most -150.degree. C.). The "(A)" as
unit of pressure indicates an absolute pressure.
[0074] Part of the pressurized feed liquefied natural gas is sent
to the demethanizer 6 as feed liquefied natural gas reflux 3, and
the remainder is sent to the reflux condenser (heat exchanger) 4.
This feed liquefied natural gas reflux is preferable for
improvement in separation efficiency. The ratio of dividing into
the feed liquefied natural gas reflux and the feed liquefied
natural gas sent into the reflux condenser is preferred to be
determined so that the ethane recovery rate in the demethanizer
(the propane recovery rate in deethanizer) is maximized. For
increasing the ethane recovery rate (the propane recovery rate in
deethanizer), the above dividing ratio is preferably about 1:20 (1
is the reflux and 20 is sent to the reflux condenser) to 1:5.
[0075] The feed liquefied natural gas is subjected to heat exchange
with a demethanizer overhead gas 7 in reflux condenser 4 and
heated, and the resulting fluid is fed into the demethanizer 6 as
demethanizer feed 5 through a pipe, by a pressure difference
between the outlet of feed liquefied natural gas pump 2 and the
demethanizer 6. At the outlet of the reflux condenser 4, this fluid
5 may be partially or wholly a gas, or may be wholly a liquid.
[0076] When there is given, to the reflux condenser 4, a thermal
load capable of condensing the whole amount of the demethanizer
overhead gas 7, it is easy to make the whole amount of a product
natural gas to be withdrawn, into a liquid.
[0077] Fluid 5 at the outlet of the reflux condenser 4, when fed
into the demethanizer 6, is preferably a gas at least partially
from the viewpoint of reducing the load of reboiler 13. Hence, as
shown in FIG. 2, when feed liquefied natural gas 22 heated at the
reflux condenser 4 is a liquid wholly, or when required, it is
possible to provide, between the reflux condenser and a
demethanizer, at least one heating means such as another heat
exchanger (second heat exchanger) 14 or the like to further heat
fluid 22, increase a gas-phase proportion in the fluid and then
feed the fluid into the demethanizer.
[0078] As the reflux condenser and the second heat exchanger, there
can be used a known heat exchanger of multi-tubular type, plate
type or the like depending upon the operation conditions. The
material therefor can be selected appropriately from known
materials for the heat exchanger, such as stainless steel and the
like.
[0079] As the demethanizer 6, there can be used a known
demethanizer capable of separating methane from a liquefied natural
gas. The demethanizer has, for example, trays or packings inside
the column and separates more volatile components from less
volatile components by distillation. An appropriate operation
pressure of the demethanizer differs depending upon the composition
of the feed liquefied natural gas used and the required
specification for demethanizer bottom liquid but is preferably
about at least 1.0 MPa (A) and at most 4.5 MPa (A).
[0080] A reboiler 13 is provided at the bottom portion of the
demethanizer and heat is applied thereto to vaporize the methane
contained in the bottom liquid of the demethanizer and control the
methane concentration in the liquid at an intended level or lower.
Installation of the reboiler is preferred to increase separation
efficiency.
[0081] From the top part of the demethanizer is separated residue
gas 7 from which components such as ethane, propane and the like
are removed and which is composed mainly of methane. The residue
gas is sent to the reflux condenser 4 and liquefied by heat
exchange with the feed liquefied natural gas, and the liquefied
residue gas is sent to a reflux drum 9.
[0082] From the reflux drum 9 is withdrawn a part of the liquefied
residue gas as product liquefied natural gas 12. For example, the
product liquefied natural gas is withdrawn by a liquid
pressurization means such as pump or the like, vaporized and then
sent to a natural gas pipeline. The liquid pressurization means,
for example, can be selected from commercially available means
depending upon the pressure of natural gas pipeline. The remainder
liquid of the reflux drum 9 is pressurized by a liquid
pressurization means (e.g. reflux pump 10) appropriately provided
for pressure balance and is returned to the demethanizer as
liquefied residue gas reflux 11. This liquid pressurization means,
for example, can be selected from commercially available pumps such
as centrifugal pump or the like depending upon the conditions such
as flow amount, head and the like. The ratio of the amount of
withdrawn product liquefied natural gas and the amount of liquefied
residue gas reflux can be determined depending upon the required
ethane recovery ratio (propane recovery ratio in deethanizer). In
the case of, for example, an ethane recovery process, the ratio can
be set at about 8:2 (8 is product liquefied natural gas and 2 is
liquefied residue gas reflux) to 5:5 in order to obtain an ethane
recovery ratio of at least 90% and at most 98%.
[0083] The methane concentration in liquefied residue gas reflux 11
is higher than the methane concentration in feed liquefied natural
gas reflux 3; consequently, the efficiency of separation of methane
from ethane during distillation operation is high. Therefore, by
giving liquefied residue gas reflux 11, there can be obtained a
product liquefied natural gas of higher methane concentration than
in a process (for example, the process described in the Patent
Literature 1) in which only feed liquefied natural gas reflux 3 is
given as a reflux for demethanizer. Accordingly, the amount of
ethane contained in product liquefied natural gas and lost is
small, whereby a higher ethane recovery ratio is obtained.
[0084] Incidentally, "reflux" means, in a narrow sense, a liquid
which is a condensed liquid of the overhead gas of distillation
column and returned to the distillation column; however, in a broad
sense, it includes, in addition thereto, even a liquid fed into the
top part of distillation column for rectification. In the present
invention, "reflux" is used in a broad sense and includes even a
liquid fed into a distillation column and having a rectification
effect.
[0085] As mentioned above, the product natural gas obtained by
separation of heavier fraction from feed liquefied natural gas is,
in some cases, pressurized in order to, for example, send it to a
pipeline. In this case, the energy for pressurization is large if
the product natural gas is a gas. Therefore, the proportion of
liquid in the product natural gas withdrawn is preferred to be
higher. Hence, in the present invention, at least a part,
preferably the whole part of the product natural gas is allowed to
be a liquid. In this connection, in the above embodiment, residue
gas 7 is liquefied wholly in the reflux condenser and the product
withdrawn is wholly a liquid, that is, a product liquefied natural
gas.
[0086] The amount of heat exchange in the reflux condenser 4 can be
such an amount that the residue gas can be liquefied wholly.
However, as described in detail later with reference to FIG. 3, it
is not necessary to liquefy the residue gas wholly in the reflux
condenser 4.
[0087] The reflux drum 9 is preferably provided for easy operation
of pump 10. The reflux drum can be, for example, a cylindrical
pressure vessel having a head at each end. As the capacity thereof,
an appropriate capacity can be determined from the standpoint of
continuing stable operation of the pump. The reflux drum can
function also as a gas-liquid separator when the fluid fed from
reflux condenser 4 contains a gas. In this case, the dimension
(diameter.times.length) of the reflux drum can be determined
appropriately so that the velocity of gas becomes equal to or
smaller than the settling velocity of droplets for conducting
gas-liquid separation. The material of the reflux drum can be
selected from materials (e.g. stainless steel) resistant to low
temperatures because the reflux drum is operated at low
temperatures of, for example, about -80.degree. C. to -110.degree.
C.
[0088] In an embodiment shown in FIG. 1, the reflux means for
refluxing part of the liquefied residue gas to the distillation
column has pump 10, reflux drum 9 and piping appropriately
provided. To the reflux drum are connected a line extending to the
pump 10 and a withdrawal line 12 for withdrawing a product
liquefied natural gas. The withdrawal line may be formed
appropriately by, for example, piping.
[0089] To the demethanizer are fed three kinds of fluids (indicated
by 3, 5 and 11 in FIG. 1). Particular feeding positions of these
fluids can be determined appropriately depending upon the
temperature and methane concentration of each fluid.
[0090] From the bottom of demethanizer 6 are separated ethane,
propane and further heavier components as NGL (natural gas liquid)
8. The NGL is, for example, separated into individual components by
a NGL separation process provided further downstream.
[0091] A lower ethane concentration in overhead gas 7 means a
higher ethane recovery rate. Therefore, the ethane concentration in
the overhead gas is preferred to be as low as possible and is
preferably 5 mol % or less, more preferably 1 mol % or less.
[0092] The NGL is substantially composed of recovered hydrocarbons
of 2 or more carbon atoms, is sent to, for example, an NGL
separation facility provided further downstream, and is separated
into products of ethane, propane, butane, etc. In such a case, the
methane concentration in the NGL is preferred to be low to such an
extent that the specification of ethane product is satisfied, and
is preferably 2 mol % or less, more preferably 1 mol % or less.
[0093] In an embodiment shown in FIG. 2, a feed liquefied natural
gas is heated by reflux condenser 4, and resulting fluid 22 is
further heated by a heating means such as a second heat exchanger
14 or the like, provided in addition to the reflux condenser 4, and
becomes fluid 5. Each of fluid 22 and fluid 5 may be a gas
partially or wholly, or may be a liquid wholly. Use of the second
heat exchanger is particularly preferred when fluid 22 is a liquid
wholly and fluid 5 is made into a gas partially or wholly. By
making the fluid fed to a demethanizer into a gas at least
partially, the load of reboiler 13 can be reduced.
[0094] As the heating medium used in the second heat exchanger 14
for heating of fluid 22, there can be used an appropriate fluid
having a desired temperature level. There can be used a fluid
supplied from outside the separation apparatus of the present
invention, or a fluid inside the separation apparatus. The second
heat exchanger is provided in order to reduce the load of reboiler
13; therefore, when a fluid supplied from outside is used as the
heat source of the second heat exchanger, it is preferred to use,
as the heat source, one which requires a lower energy consumption
(e.g. sea water or an aqueous glycol solution heated by an air
heater) than the heat source of the reboiler 13, (e.g. steam, a
heat transfer oil or a heat generated by a heating furnace).
[0095] In FIG. 3 is shown another preferred embodiment of the
process for separation of hydrocarbons from liquefied natural gas
according to the present invention. In the embodiment shown in FIG.
3, there are added, to the embodiment of FIG. 1, a pressurized
residue gas condenser 18 and a residue gas compressor 16. In this
embodiment, as compared with when the embodiment shown in FIG. 1 is
used, it is possible to reduce the operation pressure of the
demethanizer 6 and accordingly reduce the manufacturing cost and
energy consumption of the demethanizer 6. Also in this embodiment,
there are used, as a second heat exchanger, two heat exchangers,
i.e. the pressurized residue gas condenser 18 and the demethanizer
feed preheater 14. There may be a modification of the FIG. 3
embodiment, in which the demethanizer feed preheater is removed
from the FIG. 3 embodiment and the pressurized residue gas
condenser 18 alone is used as the second heat exchanger.
[0096] In FIG. 3, a feed liquefied natural gas is heated in the
reflux condenser 4, and the resulting fluid is further heated in
the pressurized residue gas condenser 18, before being sent to the
demethanizer 6 as demethanizer feed 5. Residue gas 7 leaving the
top of the demethanizer is not liquefied wholly but is liquefied
only partially in the reflux condenser 4. Then, fluid 21 at the
outlet of the reflux condenser 4 is sent to the reflux drum 9 and
separated into a gas portion and a liquid portion. The reflux drum
in this embodiment functions also as a gas-liquid separation means
and line 15 for discharging the gas portion is connected to the
reflux drum. Gas 15 separated in the reflux drum 9 (the gas is
hereinafter referred to as second residue gas) is pressurized in
residue gas compressor 16. Pressurized gas (hereinafter referred to
as pressurized residue gas) 17 is cooled in the pressurized residue
gas condenser 18 by heat exchange with the feed liquefied natural
gas and is liquefied wholly. Liquefied pressurized residue gas 19
leaving the liquefied residue gas condenser 18 is withdrawn as
product liquefied natural gas 12 together with part of the liquid
separated in the reflux drum 9.
[0097] Here, fluid 23 obtained by heating of feed liquefied natural
gas may be a gas partially or wholly or may be a liquid wholly, at
the outlet of the pressurized residual gas condenser 18. Meanwhile,
in this embodiment, fluid 19 is made into a liquid wholly at the
outlet of the pressurized residual gas condenser 18 in order to
make small the power required for pressurization done for sending
fluid 19 into a pipeline as part of a product natural gas. Hence,
as the thermal load of pressurized residual gas condenser 18, there
is given a heat amount capable of condensing the whole amount of
pressurized residual gas 17.
[0098] As the proportion at which residue gas 7 leaving the top of
the demethanizer is liquefied in the reflux condenser 4 is smaller,
the amount of the gas separated in the reflux drum 9 is larger and
the thermal load required for complete condensation of pressurized
residual gas 17 in the pressurized residual gas condenser 18 is
larger. In this connection, the temperature when the feed liquefied
natural gas leaves the pressurized residual gas condenser 18, that
is, the temperature of fluid 23 is higher and its temperature
difference from the temperature of pressurized residue gas 17 is
smaller. Generally, in a heat exchanger, at least 2 to 3.degree. C.
is secured as the temperature difference between a high-temperature
fluid and a low-temperature fluid (this difference is called
temperature approach) for efficient heat exchange. In pressurized
residual gas condenser 18 as well, the Is temperature approach is
preferably at least 2.degree. C., more preferably at least
3.degree. C. Hence, the ratio at which residue gas 7 leaving the
demethanizer top is liquefied in the reflux condenser 4 (that is,
the molar ratio of liquid in fluid 21) is taken at a range in which
a temperature approach of preferably at least 2.degree. C., more
preferably at least 3.degree. C. is obtained in the pressurized
residue gas condenser 18 and, from this standpoint, is preferably
at least 0.4 and at most 0.9.
[0099] The pressure at the outlet of the residue gas compressor 16
is preferably a pressure at which the condensation temperature of
pressurized residue gas 17 is heightened and a temperature approach
of at least 2.degree. C. is easily secured in the pressurized
residue gas condenser 18, and is preferred to be higher by at least
0.2 MPa and at most 2.0 MPa than the pressure at the inlet of the
residue gas compressor 16.
[0100] In this embodiment, a gas compressor is used as the
pressurization means for pressurization of gas 15. However, other
known pressurization means capable of pressurizing a gas can be
used as well. As the gas compressor, there can be selected a known
compressor such as centrifugal gas compressor or the like,
depending upon the flow amount and pressure difference of gas. As
the driver for the compressor, there can be used appropriately a
motor, a steam turbine, a gas turbine or the like.
[0101] Also in the process for propane recovery, the principle is
the same as in the above-mentioned process for ethane recovery. A
deethanizer is used in place of the demethanizer 6; an overhead gas
composed mainly of methane and ethane is separated from the top
part of the deethanizer; from the bottom part of the deethanizer
are separated propane and further heavier components as NGL.
EXAMPLES
[0102] The present invention is described in more detail below by
way of Examples. However, the present invention is not restricted
by these Examples.
Example 1
[0103] In this Example is described a process for ethane recovery
from liquefied natural gas, having a flow shown in FIG. 1. Here,
feed liquefied natural gas 1 is introduced into the present process
under the saturated conditions (pressure: 0.1 MPa (A), temperature:
-161.degree. C.). The composition of this feed liquefied natural
gas is shown in Table 1. The flow rate of the gas is 39,451
kg-mol/hr (kg-mol/hr means 103 mol/hr). Incidentally, Cn (n is a
natural number) indicates a hydrocarbon having a carbon number of
n. C5+ indicates hydrocarbons of 5 or more carbon atoms.
TABLE-US-00001 TABLE 1 Composition of feed liquefied natural gas
(mol %) N.sub.2 0.2 C1 89.7 C2 6.2 C3 2.3 C4 1.5 C5+ 0.1 Total
100.0
[0104] The liquefied natural gas is pressurized to 4.15 MPa (A) by
the feed liquefied natural gas pump 2. A part of thereof is fed
into the fourth (from top) tray of the demethanizer as feed
liquefied natural gas flux 3, at a rate of 1,973 kg-mol/hr
(10.sup.3 mol/hr). The remaining 37,478 kg-mol/hr (10.sup.3 mol/hr)
is sent to the reflux condenser 4, undergoes heat exchange with
demethanizer overhead gas 7 and is heated to -92.1.degree. C., and
is fed into the tenth tray of the demethanizer 6 in a complete
liquid state.
[0105] The demethanizer is provided inside with trays of 19 stages
in terms of theoretical stage number and is operated under the
conditions of 3.75 MPa (A) and -89.1.degree. C. at the top and 3.80
MPa (A) and 46.1.degree. C. at the bottom. Here, the temperature of
the bottom is determined by an equilibrium temperature at which the
methane concentration in NGL 8 becomes 1 mol %. In order to operate
the bottom at this temperature, a heat of 70.22 MW is added by the
reboiler 13. The compositions of residue gas 7 separated from the
top of the demethanizer and NGL 8 separated from the bottom are
shown in Table 2. As to their flow rates, 50,676 kg-mol/hr
(10.sup.3 mol/hr) is for the residue gas and 3,978 kg-mol/hr
(10.sup.3 mol/hr) is for NGL. TABLE-US-00002 TABLE 2 Compositions
(mol %) of residue gas and NGL Residue gas NGL N.sub.2 0.3 0.0 C1
99.6 1.0 C2 0.1 60.3 C3 0.0 23.3 C4 0.0 14.8 C5+ 0.0 0.6 Total
100.0 100.0
[0106] Of the total ethane in the feed liquefied natural gas, 98.0%
is recovered as NGL. 100% of propane and heavier-than-propane
components are recovered as NGL.
[0107] Residue gas 7 leaving the top of the demethanizer undergoes
heat exchange in reflux condenser 4 and liquefied wholly, and then
is fed into the reflux drum 9. Then, 35,473 kg-mol/hr (10.sup.3
mol/hr) is withdrawn as product liquefied natural gas 12 and
remaining 15,203 kg-mol/hr (10.sup.3 mol/hr) is fed into the
uppermost tray of the demethanizer as liquefied residue gas reflux
11.
Comparative Example 1
[0108] There was employed the same process as in Example 1 except
that residue gas 7 was liquefied wholly in the reflux condenser 4
and was withdrawn as a product liquefied natural gas. That is, the
liquefied residue gas is taken out of the system with no returning
of liquefied residue gas to demethanizer as reflux.
[0109] Downstream of the pump 2, the ratio at which the feed
liquefied natural gas was divided into the feed liquefied natural
gas reflux 3 and a portion sent to the reflux condenser 4 was
varied to obtain the highest ethane recovery ratio.
[0110] The ethane recovery ratios in Example 1 and Comparative
Example 1 are shown in Table 3. An ethane recovery ratio of 98.0%
is achievable in Example 1, while in Comparative Example 1, the
highest ethane recovery ratio is 92.0%. The ethane recovery ratio
is dependent greatly upon the methane concentration in a reflux
returned to the demethanizer. In Example 1, a reflux (a liquefied
residue gas reflux) of methane concentration of 99.6 mol % is
feedable and a higher ethane recovery is achievable than in
Comparative Example 1. TABLE-US-00003 TABLE 3 Ethane recovery ratio
Comparative Example 1 Example 1 Ethane recovery ratio 92.0% 98.0%
Methane concentration in demethanizer reflux Feed liquefied natural
gas reflux 89.7 mol % 89.7 mol % Liquefied residue gas reflux --
99.6 mol %
Example 2
[0111] In this Example is described a process shown in FIG. 2, in
which the demethanizer feed preheater 14 is provided between the
reflux condenser 4 and the demethanizer 6 in order to reduce the
load of reboiler, and thereby a feed liquefied natural gas is
heated by a heating medium supplied from outside to increase the
proportion of gas in the demethanizer feed 5.
[0112] Here, feed liquefied natural gas 1 having the same
composition as in Example 1 is introduced into the present process
under the saturation conditions (pressure: 0.1 MPa (A),
temperature: -161.degree. C.). The flow rate is 39,451 kg-mol/hr
(10.sup.3 mol/hr) which is the same as in Example 1. Liquefied
natural gas 1 is pressurized to 4.15 MPa (A) by a feed liquefied
natural gas pump. A part thereof is fed into the fourth (from top)
tray of a demethanizer as a feed liquefied natural gas reflux 3 at
a rate of 1,973 kg-mol/hr (10.sup.3 mol/hr). The remaining 37,478
kg-mol/hr (10.sup.3 mol/hr) is sent to a reflux condenser 4,
undergoes heat exchange with demethanizer overhead gas 7 and is
heated to -91.9.degree. C., is further heated to -77.6.degree. C.
at the demethanizer feed preheater 14 by heat exchange with a
heating medium supplied from outside, and is fed into the tenth
tray of the demethanizer. At this time, the demethanizer feed
preheater 14 is provided with a heat of 25.76 MW which effectively
reduces the thermal load of the reboiler 13.
[0113] The demethanizer is provided inside with trays of 19 stages
in terms of theoretical stage number and is operated under the
conditions of 3.75 MPa (A) and -88.9.degree. C. at the top and 3.80
MPa (A) and 46.4.degree. C. at the bottom. Here, the temperature of
the bottom is determined by an equilibrium temperature at which the
methane concentration in NGL becomes 1 mol %. In order to operate
the bottom at this temperature, a heat of 34.65 MW is added from
the reboiler 13. The compositions of the residue gas separated from
the top of the demethanizer and the NGL separated from the bottom
are shown in Table 4. As to their flow rates, 50,713 kg-mol/hr
(10.sup.3 mol/hr) is for the residue gas and 3,952 kg-mol/hr
(10.sup.3 mol/hr) is for NGL. TABLE-US-00004 TABLE 4 Compositions
of residue gas and NGL (mol %) Residue gas NGL N.sub.2 0.2 0.0 C1
99.6 1.0 C2 0.2 60.0 C3 0.0 23.5 C4 0.0 14.9 C5+ 0.0 0.6 Total
100.0 100.0
[0114] As shown in Table 4, of the total ethane in the feed
liquefied natural gas, 96.9% is recovered as NGL. 100% of propane
and further heavier components are recovered as NGL.
[0115] The residue gas 7 leaving the top of the demethanizer 6
undergoes heat exchange in the reflux condenser 4 and is liquefied
wholly, and then is fed into the reflux drum 9. Then, 35,499
kg-mol/hr (10.sup.3 mol/hr) is withdrawn as product liquefied
natural gas 12 and remaining 15,214 kg-mol/hr (10.sup.3 mol/hr) is
fed into the uppermost tray of the demethanizer as liquefied
residue gas reflux 11.
[0116] By adding the demethanizer feed preheater 14, the ethane
recovery ratio is lower by 1.1% than in Example 1; however, the
thermal load of the reboiler 13 can be reduced by 35.57 MW.
Further, even when the heat amount (25.76 MW) added from the
demethanizer feed preheater 14 is deducted, the thermal load of the
whole process can be reduced by 9.81 MW.
Example 3
[0117] In this Example is described a process shown in FIG. 3,
intended for reductions in manufacturing cost of demethanizer as
well as in energy consumption, in which residue gas 7 is condensed
only partially and not wholly in the reflux condenser 4 and there
are added steps of separation of the gas in the reflux drum 9,
pressurization of the separated gas in the residue gas compressor
16 and liquefaction of the pressurized gas in the pressurized
residue gas condenser 18.
[0118] Here, feed liquefied natural gas 1 having the same
composition as in Examples 1 and 2 is introduced into the present
process under the saturation conditions (pressure: 0.1 MPa (A),
temperature: -161.degree. C.). The flow rate is 39,451 kg-mol/hr
(10.sup.3 mol/hr) which is the same as in Examples 1 and 2.
Liquefied natural gas 1 is pressurized to 3.26 MPa (A) by a feed
liquefied natural gas pump. A part thereof is fed into the fourth
(from top) tray of a demethanizer as a reflux for demethanizer, at
a rate of 1,973 kg-mol/hr (10.sup.3 mol/hr). The remaining 37,478
kg-mol/hr (10.sup.3 mol/hr) is sent to the reflux condenser 4,
undergoes heat exchange with demethanizer overhead gas 7 and is
heated to -99.9.degree. C. (the temperature approach is 3.degree.
C. in the condenser 4), and, in the pressurized residue gas
condenser 18, undergoes further heat exchange with pressurized
residue gas 17 and is heated to -89.9.degree. C. (the temperature
approach is also 3.degree. C. in the condenser 18).
[0119] Fluid 23 heated in the pressurized residue gas condenser 18
is further heated to -87.1.degree. C. in the demethanizer feed
preheater 14 by heat exchange with a heating medium supplied from
outside, and is fed into the tenth tray of the demethanizer. At
this time, the demethanizer feed preheater 14 is provided with a
heat of 19.38 MW which effectively reduces the heat amount given by
the reboiler 13.
[0120] The demethanizer is provided inside with trays of 19 stages
in terms of theoretical stage number and is operated under the
conditions of 2.86 MPa (A) and -96.9.degree. C. at the top and 2.91
MPa (A) and 30.7.degree. C. at the bottom. Here, the temperature of
the bottom is determined by an equilibrium temperature at which the
methane concentration in NGL becomes 1 mol %. In order to operate
the bottom at this temperature, a heat of 33.36 MW is added from
the reboiler 13. The compositions of the residue gas separated from
the top of the demethanizer and the NGL separated from the bottom
are shown in Table 5. As to their flow rates, 40,979 kg-mol/hr
(10.sup.3 mol/hr) is for the residue gas and 3,950 kg-mol/hr
(10.sup.3 mol/hr) is for NGL. TABLE-US-00005 TABLE 5 Compositions
of residue gas and NGL (mol %) Residue gas NGL N.sub.2 0.2 0.0 C1
99.6 1.0 C2 0.2 60.0 C3 0.0 23.5 C4 0.0 14.9 C5+ 0.0 0.6 Total
100.0 100.0
[0121] As shown in Table 5, of the total ethane in the feed
liquefied natural gas, 96.9% is recovered as NGL. 100% of propane
and heavier-than-propane components are recovered as NGL.
[0122] Residue gas 7 leaving the top of the demethanizer 6
undergoes heat exchange in the reflux condenser 4 and is liquefied
partially, is fed into the reflux drum 9, and is separated into a
gas and a liquid.
[0123] Of the liquid portion separated in the reflux drum 9, 25,256
kg-mol/hr (10.sup.3 mol/hr) is withdrawn as a product liquefied
natural gas and the remaining 5,478 kg-mol/hr (10.sup.3 mol/hr) is
fed into the uppermost tray of the demethanizer as liquefied
residue gas reflux 11. The gas portion separated in the reflux drum
9 is fed into the residue gas compressor 16 and pressurized to 4.01
MPa (A).
[0124] Pressurized residue gas 17 leaving the residue gas
compressor 16 undergoes heat exchange in the pressurized residue
gas condenser 18 and is liquefied wholly. Resulting liquefied
pressurized residue gas 19 is withdrawn as a product liquefied
natural gas. The flow rate of the liquefied pressurized residue gas
is 10,254 kg-mol/hr (10.sup.3 mol/hr). Liquefied residue gas 24
withdrawn from the reflux drum 9 and liquefied pressurized residue
gas 19 are combined to become product liquefied natural gas 12
having a flow rate of 35,501 kg-mol/hr (10.sup.3 mol/hr).
[0125] As shown in Table 6, in Example 3 compared with Example 2,
there are added pressurized residue gas condenser 18 and residue
gas compressor 16; however, the diameter of the demethanizer can be
reduced by about 1,200 mm while an ethane recovery rate of 96.9% is
being maintained. The length of the demethanizer is the same in
Example 2 and Example 3. The operation pressure as well is reduced
by 0.89 MPa as compared with that of Example 2 and, therefore, the
design pressure of the demethanizer can be reduced. Consequently,
the manufacturing cost of the demethanizer can be reduced.
TABLE-US-00006 TABLE 6 Comparison of demethanizer design Example 2
Example 3 Ethane recovery rate 96.9% 96.9% Demethanizer diameter
6,700 mm 5,500 mm Operation pressure 3.75 MPa (A) 2.86 MPa (A)
(column top) Manufacturing cost of demethanizer High Low
[0126] As shown in Table 7, in Example 3 compared with Example 2,
the pressure of the demethanizer can be made lower by 0.89 MPa;
therefore, the energy required for separation of methane and
hydrocarbons of 2 or more carbon atoms is smaller and the energy
consumption of the whole process can be reduced by 5.3 MW while an
ethane recovery rate of 96.9% is being maintained. Incidentally,
the compressor is driven by an electric motor; the energy
efficiency in generation of the electric power consumed by the
motor is regarded as 30%; and the energy consumption by the
compressor is converted into the heat amount of fuel gas used for
power generation. Also, the energy efficiency when the thermal load
consumed by the heating medium in the demethanizer feed preheater
and the reboiler is generated in a heating furnace, is regarded as
80%; and the thermal load is converted into the heat amount of fuel
gas used in the heating furnace. TABLE-US-00007 TABLE 7 Example 2
Example 3 Ethane recovery rate 96.9% 96.9% Compressor Compressor
power -- 1.3 MW Energy efficiency -- 30% Fuel gas consumption --
4.3 MW Heating medium load Thermal load of demethanizer feed 25.76
MW 19.38 MW preheater Thermal load of reboiler 34.65 MW 33.36 MW
Total thermal load of heating medium 60.41 MW 52.74 MW Energy
efficiency 80% 80% Fuel gas consumption 75.5 MW 65.9 MW Total of
fuel gas consumption 75.5 MW 70.2 MW
* * * * *