U.S. patent application number 17/089402 was filed with the patent office on 2021-05-06 for process and apparatus for separating hydrocarbon.
This patent application is currently assigned to TOYO ENGINEERING CORPORATION. The applicant listed for this patent is TOYO ENGINEERING CORPORATION. Invention is credited to Keisuke SASAKURA, Shoichi YAMAGUCHI, Taisei YAMAMOTO.
Application Number | 20210131728 17/089402 |
Document ID | / |
Family ID | 1000005238022 |
Filed Date | 2021-05-06 |
![](/patent/app/20210131728/US20210131728A1-20210506\US20210131728A1-2021050)
United States Patent
Application |
20210131728 |
Kind Code |
A1 |
SASAKURA; Keisuke ; et
al. |
May 6, 2021 |
PROCESS AND APPARATUS FOR SEPARATING HYDROCARBON
Abstract
To provide a process for separating hydrocarbons capable of
recovering ethane or propane, including improved cold heat recovery
enabling a reduction in compressor power. A process for separating
hydrocarbons, in which a residual gas enriched with methane or
ethane and a heavy fraction enriched with a lower volatile
hydrocarbon are separated, includes: a) partially condensing the
feed gas by cooling using the residual gas and another refrigerant
as a refrigerant, followed by vapor-liquid separation; b)
depressurizing and supplying the liquid obtained from step (a) to
the distillation column; c) expanding a part or all of the gas
obtained from step (a) by an expander to cause partial
condensation, followed by vapor-liquid separation; d) feeding the
liquid obtained from step (c) to the distillation column after
using it as the further refrigerant in step (a); e) feeding a part
or all of the gas obtained from step (c) to the distillation
column; and f) obtaining the residual gas from the top of the
distillation column and the heavy fraction from the bottom of the
distillation column.
Inventors: |
SASAKURA; Keisuke;
(Narashino-shi, JP) ; YAMAMOTO; Taisei;
(Narashino-shi, JP) ; YAMAGUCHI; Shoichi;
(Narashino-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYO ENGINEERING CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
TOYO ENGINEERING
CORPORATION
Tokyo
JP
|
Family ID: |
1000005238022 |
Appl. No.: |
17/089402 |
Filed: |
November 4, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2215/64 20130101;
F25J 3/0219 20130101; F25J 2200/38 20130101; F25J 2270/06 20130101;
F25J 2200/02 20130101; F25J 3/0209 20130101; F25J 2210/12 20130101;
F25J 2215/62 20130101; F25J 3/0238 20130101; F25J 2210/60 20130101;
F25J 2205/04 20130101; F25J 2200/70 20130101; F25J 3/0295 20130101;
F25J 3/0242 20130101; F25J 2240/02 20130101 |
International
Class: |
F25J 3/02 20060101
F25J003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 5, 2019 |
JP |
2019-200772 |
Claims
1. A process for separating hydrocarbons, wherein a feed gas
containing at least methane and a hydrocarbon less volatile than
methane is separated into a residual gas enriched with methane and
lean in a hydrocarbon less volatile than methane and a heavy
fraction lean in methane and enriched with a hydrocarbon less
volatile than methane using a distillation column, the process
comprising: a) partially condensing the feed gas by cooling using
the residual gas and another refrigerant as a refrigerant, followed
by vapor-liquid separation; b) depressurizing and supplying the
liquid obtained from step (a) to the distillation column; c)
expanding a part or all of the gas obtained from step (a) by an
expander to cause partial condensation, followed by vapor-liquid
separation; d) feeding the liquid obtained from step (c) to the
distillation column after using it as the further refrigerant in
step (a); e) feeding a part or all of the gas obtained from step
(c) to the distillation column; and f) obtaining the residual gas
from the top of the distillation column and the heavy fraction from
the bottom of the distillation column.
2. A process for separating hydrocarbons, wherein a feed gas
containing at least ethane and a hydrocarbon less volatile than
ethane is separated into a residual gas enriched with ethane and
lean in a hydrocarbon less volatile than ethane and a heavy
fraction lean in ethane and enriched with a hydrocarbon less
volatile than ethane using a distillation column, the process
comprising: a) partially condensing the feed gas by cooling using
the residual gas and another refrigerant as a refrigerant, followed
by vapor-liquid separation; b) depressurizing and supplying the
liquid obtained from step (a) to the distillation column; c)
expanding a part or all of the gas obtained from step (a) by an
expander to cause partial condensation, followed by vapor-liquid
separation; d) feeding the liquid obtained from step (c) to the
distillation column after using it as the further refrigerant in
step (a); e) feeding a part or all of the gas obtained from step
(c) to the distillation column; and f) obtaining the residual gas
from the top of the distillation column and the heavy fraction from
the bottom of the distillation column.
3. The process according to claim 1, wherein all of the gas
obtained from step (a) is supplied to step (c), and all of the gas
obtained from step (c) is supplied to step (e).
4. The process according to claim 1, wherein a portion of the gas
obtained from step (a) is supplied to step (c), the remainder of
the gas obtained from step (a) is cooled by heat exchange with the
residual gas to be totally condensed, and the totally condensed
liquid is depressurized and supplied to the distillation
column.
5. The process according to claim 1, wherein a portion of the gas
obtained from step (c) is supplied to step (e), the remainder of
the gas obtained from step (c) is compressed and cooled by heat
exchange with the residual gas to be totally condensed, and the
totally condensed liquid is depressurized and supplied to the
distillation column.
6. A separation apparatus for hydrocarbons, wherein a feed gas
containing at least methane and a hydrocarbon less volatile than
methane is separated into a residual gas enriched with methane and
lean in a hydrocarbon less volatile than methane and a heavy
fraction lean in methane and enriched with a hydrocarbon less
volatile than methane, the separation apparatus comprising: a
distillation column discharging the residual gas from the top of
the distillation column and the heavy fraction from the bottom of
the distillation column; a heat exchange means for partially
condensing the feed gas by cooling, comprising a refrigerant flow
path in which the residual gas flows as a refrigerant, and another
refrigerant flow path in which another refrigerant flows; a first
vapor-liquid separator for vapor-liquid separation of the partially
condensed feed gas obtained from the heat exchange means; a line
for supplying the liquid obtained from the first vapor-liquid
separator to the distillation column via a pressure reducing valve;
an expander for expanding and partially condensing part or all of
the gas obtained from the first vapor-liquid separator; a second
vapor-liquid separator connected to an outlet of the expander; a
line for supplying the liquid obtained from the second vapor-liquid
separator to the distillation column via said another refrigerant
flow path; and a line for supplying part or all of the gas obtained
from the second vapor-liquid separator.
7. A separation apparatus for hydrocarbons, wherein a feed gas
containing at least ethane and a hydrocarbon less volatile than
ethane is separated into a residual gas enriched with ethane and
lean in a hydrocarbon less volatile than ethane and a heavy
fraction lean in ethane and enriched with a hydrocarbon less
volatile than ethane, the separation apparatus comprising: a
distillation column discharging the residual gas from the top of
the distillation column and the heavy fraction from the bottom of
the distillation column; a heat exchange means for partially
condensing the feed gas by cooling, comprising a refrigerant flow
path in which the residual gas flows as a refrigerant, and another
refrigerant flow path in which another refrigerant flows; first
vapor-liquid separator for vapor-liquid separation of the partially
condensed feed gas obtained from the heat exchange means; a line
for supplying the liquid obtained from the first vapor-liquid
separator to the distillation column via a pressure reducing valve;
an expander for expanding and partially condensing part or all of
the gas obtained from the first vapor-liquid separator; a second
vapor-liquid separator connected to an outlet of the expander; a
line for supplying the liquid obtained from the second vapor-liquid
separator to the distillation column via said another refrigerant
flow path; and a line for supplying part or all of the gas obtained
from the second vapor-liquid separator.
8. The apparatus according to claim 6, comprising a line for
feeding all of the gas obtained from the first vapor-liquid
separator to the expander, and a line for supplying all of the gas
obtained from the second vapor-liquid separator to the distillation
column.
9. The apparatus according to claim 6, comprising: a line for
feeding part of the gas obtained from the first vapor-liquid
separator to the expander; a condenser for cooling and totally
condensing the remainder of the gas obtained from the first
vapor-liquid separator by heat exchange with the residual gas; a
pressure reducing valve for decompressing the totally condensed
liquid in the condenser; and a line connecting an outlet of the
pressure reducing valve for decompressing the totally condensed
liquid in the condenser to the distillation column.
10. The apparatus according to claim 6 comprising: a line for
supplying part of the gas obtained from the second vapor-liquid
separator to the distillation column; a compressor for compressing
the remainder of the gas obtained from the second vapor-liquid
separator; a condenser for cooling and totally condensing the gas
compressed by the compressor by heat exchange with the residual
gas; a pressure reducing valve for decompressing the totally
condensed liquid in the condenser; and a line connecting an outlet
of the pressure reducing valve for decompressing the totally
condensed liquid in the condenser to the distillation column.
11. The apparatus according to claim 7, comprising a line for
feeding all of the gas obtained from the first vapor-liquid
separator to the expander, and a line for supplying all of the gas
obtained from the second vapor-liquid separator to the distillation
column.
12. The apparatus according to claim 7, comprising: a line for
feeding part of the gas obtained from the first vapor-liquid
separator to the expander; a condenser for cooling and totally
condensing the remainder of the gas obtained from the first
vapor-liquid separator by heat exchange with the residual gas; a
pressure reducing valve for decompressing the totally condensed
liquid in the condenser; and a line connecting an outlet of the
pressure reducing valve for decompressing the totally condensed
liquid in the condenser to the distillation column.
13. The apparatus according to claim 7 comprising: a line for
supplying part of the gas obtained from the second vapor-liquid
separator to the distillation column; a compressor for compressing
the remainder of the gas obtained from the second vapor-liquid
separator; a condenser for cooling and totally condensing the gas
compressed by the compressor by heat exchange with the residual
gas; a pressure reducing valve for decompressing the totally
condensed liquid in the condenser; and a line connecting an outlet
of the pressure reducing valve for decompressing the totally
condensed liquid in the condenser to the distillation column.
14. The process according to claim 2, wherein all of the gas
obtained from step (a) is supplied to step (c), and all of the gas
obtained from step (c) is supplied to step (e).
15. The process according to claim 2, wherein a portion of the gas
obtained from step (a) is supplied to step (c), the remainder of
the gas obtained from step (a) is cooled by heat exchange with the
residual gas to be totally condensed, and the totally condensed
liquid is depressurized and supplied to the distillation
column.
16. The process according to claim 2, wherein a portion of the gas
obtained from step (c) is supplied to step (e), the remainder of
the gas obtained from step (c) is compressed and cooled by heat
exchange with the residual gas to be totally condensed, and the
totally condensed liquid is depressurized and supplied to the
distillation column.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a process and an apparatus
for separating a hydrocarbon used for separating and recovering
ethane or propane from, for example, natural gas,
petroleum-associated gas, or off-gas from a refinery or
petrochemical plant.
Description of Related Art
[0002] Conventionally, separation of methane and hydrocarbons
having 2 or more carbon atoms and separation of ethane and
hydrocarbons having 3 or more carbon atoms have been carried
out.
[0003] For example, as a process of recovering ethane or propane
from natural gas, a process including cooling natural gas and
distilling and separating a light component and an ethane (or
propane) and a heavy hydrocarbon component in a demethanizer (in
the case of propane recovery, a deethanizer) is widely used. In the
process, a propane refrigeration system and a turboexpander are
used to cool the natural gas to the temperature required for
separation.
[0004] WO 2005/009930 A1 discloses a process of recovering ethane
or propane from a feed gas such as natural gas using a distillation
column. The process includes the following steps:
(a) a step of cooling and partly condensing the feed gas to
separate into gas and liquid; (b) a step of supplying the liquid
obtained in step (a) to a distillation column; (c) a step of
expanding the gas obtained in step (a) by an expander, condensing a
part of the expanded gas to separate into gas and liquid; (d) a
step of feeding the liquid obtained in step (c) to a distillation
column; (e) a step of dividing the gas obtained in step (c) into a
first portion and a second portion; (f) a step of feeding the first
portion to the distillation column; (g) a step of compressing and
cooling the second portion to be condensed, and then reducing
pressure and feeding to the distillation column as a reflux; (h) a
step of obtaining residual gas from the top of the distillation
column and obtaining a heavy fraction from the bottom of the
distillation column.
SUMMARY OF THE INVENTION
[0005] In the process described in WO 2005/009930 A1, the liquid
obtained in step (c) is directly supplied to the distillation
column. Therefore, there is room for improvement from the viewpoint
of cold heat recovery, and a relatively large compressor power is
required for the recovery of ethane or propane.
[0006] It is an object of the present invention to provide a
process for separating hydrocarbons capable of recovering ethane or
propane, including an improved cold heat recovery allowing
reduction in compressor power. It is another object of the present
invention to provide an apparatus for separating a hydrocarbon,
suitable for carrying out this process.
[0007] An aspect of the present invention provides,
[0008] a process for separating hydrocarbons, wherein a feed gas
containing at least methane and a hydrocarbon less volatile than
methane is separated into a residual gas enriched with methane and
lean in a hydrocarbon less volatile than methane and a heavy
fraction lean in methane and enriched with a hydrocarbon less
volatile than methane using a distillation column, the process
comprising:
[0009] a) partially condensing the feed gas by cooling using the
residual gas and another refrigerant as a refrigerant, followed by
vapor-liquid separation;
[0010] b) depressurizing and supplying the liquid obtained from
step (a) to the distillation column;
[0011] c) expanding a part or all of the gas obtained from step (a)
by an expander to cause partial condensation, followed by
vapor-liquid separation;
[0012] d) feeding the liquid obtained from step (c) to the
distillation column after using it as the further refrigerant in
step (a);
[0013] e) feeding a part or all of the gas obtained from step (c)
to the distillation column; and
[0014] f) obtaining the residual gas from the top of the
distillation column and the heavy fraction from the bottom of the
distillation column.
[0015] Another aspect of the present invention provides,
[0016] a process for separating hydrocarbons, wherein a feed gas
containing at least ethane and a hydrocarbon less volatile than
ethane is separated into a residual gas enriched with ethane and
lean in a hydrocarbon less volatile than ethane and a heavy
fraction lean in ethane and enriched with a hydrocarbon less
volatile than ethane using a distillation column, the process
comprising:
[0017] a) partially condensing the feed gas by cooling using the
residual gas and another refrigerant as a refrigerant, followed by
vapor-liquid separation;
[0018] b) depressurizing and supplying the liquid obtained from
step (a) to the distillation column;
[0019] c) expanding a part or all of the gas obtained from step (a)
by an expander to cause partial condensation, followed by
vapor-liquid separation;
[0020] d) feeding the liquid obtained from step (c) to the
distillation column after using it as the further refrigerant in
step (a);
[0021] e) feeding a part or all of the gas obtained from step (c)
to the distillation column; and
[0022] f) obtaining the residual gas from the top of the
distillation column and the heavy fraction from the bottom of the
distillation column.
[0023] Another aspect of the present invention provides,
[0024] a separation apparatus for hydrocarbons, wherein a feed gas
containing at least methane and a hydrocarbon less volatile than
methane is separated into a residual gas enriched with methane and
lean in a hydrocarbon less volatile than methane and a heavy
fraction lean in methane and enriched with a hydrocarbon less
volatile than methane, the separation apparatus comprising:
[0025] a distillation column discharging the residual gas from the
top of the distillation column and the heavy fraction from the
bottom of the distillation column;
[0026] a heat exchange means for partially condensing the feed gas
by cooling, comprising a refrigerant flow path in which the
residual gas flows as a refrigerant, and another refrigerant flow
path in which another refrigerant flows;
[0027] a first vapor-liquid separator for vapor-liquid separation
of the partially condensed feed gas obtained from the heat exchange
means;
[0028] a line for supplying the liquid obtained from the first
vapor-liquid separator to the distillation column via a pressure
reducing valve;
[0029] an expander for expanding and partially condensing part or
all of the gas obtained from the first vapor-liquid separator;
[0030] a second vapor-liquid separator connected to an outlet of
the expander;
[0031] a line for supplying the liquid obtained from the second
vapor-liquid separator to the distillation column via said another
refrigerant flow path; and
[0032] a line for supplying part or all of the gas obtained from
the second vapor-liquid separator.
[0033] Another aspect of the present invention provides,
[0034] a separation apparatus for hydrocarbons, wherein a feed gas
containing at least ethane and a hydrocarbon less volatile than
ethane is separated into a residual gas enriched with ethane and
lean in a hydrocarbon less volatile than ethane and a heavy
fraction lean in ethane and enriched with a hydrocarbon less
volatile than ethane, the separation apparatus comprising:
[0035] a distillation column discharging the residual gas from the
top of the distillation column and the heavy fraction from the
bottom of the distillation column;
[0036] a heat exchange means for partially condensing the feed gas
by cooling, comprising a refrigerant flow path in which the
residual gas flows as a refrigerant, and another refrigerant flow
path in which another refrigerant flows;
[0037] first vapor-liquid separator for vapor-liquid separation of
the partially condensed feed gas obtained from the heat exchange
means;
[0038] a line for supplying the liquid obtained from the first
vapor-liquid separator to the distillation column via a pressure
reducing valve;
[0039] an expander for expanding and partially condensing part or
all of the gas obtained from the first vapor-liquid separator;
[0040] a second vapor-liquid separator connected to an outlet of
the expander;
[0041] a line for supplying the liquid obtained from the second
vapor-liquid separator to the distillation column via said another
refrigerant flow path; and
[0042] a line for supplying part or all of the gas obtained from
the second vapor-liquid separator.
[0043] According to one aspect of the present invention, there is
provided a process of separating hydrocarbons capable of recovering
ethane or propane, including an improved cold heat recovery
allowing reduction in compressor power. According to another aspect
of the present invention, there is provided an apparatus for
separating a hydrocarbon, suitable for carrying out the
process.
BRIEF DESCRIPTION OF DRAWINGS
[0044] FIG. 1 is a process flow diagram showing an ethane recovery
process according to a first embodiment of the present
invention.
[0045] FIG. 2 is a process flow diagram showing the ethane recovery
process of Comparative Example 1.
[0046] FIG. 3 is a process flow diagram showing an ethane recovery
process according to a second embodiment of the present
invention.
[0047] FIG. 4 is a process flow diagram showing the ethane recovery
process of Comparative Example 2.
[0048] FIG. 5 is a process flow diagram showing an ethane recovery
process according to a third embodiment of the present
invention.
[0049] FIG. 6 is a process flow diagram showing the ethane recovery
process of Comparative Example 3.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0050] The following description and drawings are merely
illustrative of preferred embodiments of the present invention, and
the invention is not limited thereto. Note that, in a narrow sense,
"reflux" means a liquid which is obtained by condensing the top gas
of a distillation column to be returned to the distillation column
again, but in addition to this, "reflux" broadly includes a liquid
to be supplied to the top of a distillation column for the purpose
of rectification. In this specification, "reflux" is used in the
broad sense and also includes a liquid having a rectification
effect for being supplied to a distillation column.
Embodiment 1
[0051] The present invention relates to an ethane recovery process
and a propane recovery process. With respect to the embodiment 1 of
the present invention, an example of the ethane recovery process
will be described using the process flow diagram shown in FIG. 1.
As used herein, the ethane recovery process is a process in which a
hydrocarbon component contained in a feed gas is separated into
methane, and ethane and a heavy component by distillation. The
ethane recovery process has a distillation column (demethanizer)
and a facility for cooling the feed gas to a temperature necessary
for distillation.
[0052] In this process, a feed gas containing at least methane and
a hydrocarbon having a lower volatility than methane is separated
into a residual gas enriched with methane and lean in the
hydrocarbon less volatile than methane, and a heavy fraction lean
in methane and enriched with a hydrocarbon less volatile than
methane. For this purpose, a demethanizer 11 is used as a
distillation column which discharges residual gas from the top of
the column and discharges the heavy fraction from the bottom of the
column. This process performs steps (a) to (f).
[0053] a) Step of Partially Condensing the Feed Gas by Cooling
Using the Residual Gas and Another Refrigerant as a Refrigerant,
Followed by Vapor-Liquid Separation.
[0054] In this step, a heat exchange means for partially condensing
by cooling the feed gas is used, the heat exchange means including
a refrigerant flow path in which the residual gas flows as a
refrigerant, and another refrigerant flow path in which another
refrigerant flows. In addition, a first vapor-liquid separator for
vapor-liquid separation of the partially condensed feed gas
obtained from the heat exchange means is used. The heat exchange
means may include one or more heat exchangers. If the heat exchange
means includes two or more heat exchangers, a refrigerant flow path
through which the residual gas flows and another refrigerant flow
path through which another refrigerant flows may be provided in the
same heat exchanger, and they may be provided separately in
different heat exchangers. Further, each of the plurality of heat
exchangers may have a refrigerant flow path through which the
residual gas flows. Each of the plurality of heat exchangers may
have another refrigerant flow path. It is possible to use a
plurality of refrigerants as another refrigerant, for example, one
heat exchanger may have a plurality of "another refrigerant flow
path" through which a plurality of refrigerant flows
respectively.
[0055] The feed gas, e.g. natural gas, is cooled by heat exchange
means and partially condensed. The partially condensed feed gas is
separated in a first vapor-liquid separator 4, also called a
low-temperature separator. To increase the recovery rate of ethane,
the lower the temperature of the low-temperature separator 4, the
more preferable. Further, the ratio at which the natural gas is
condensed varies depending on the composition of the natural gas
(the ratio of hydrocarbons having 2 or more carbon atoms), and is
about 5 mol % or more and about 20 mol % or less. As the heat
exchanger used for cooling the feed gas, it is possible to
appropriately use a known heat exchanger such as a plate fin heat
exchanger or shell and tube heat exchanger. The low-temperature
separator 4 may be a vertical or horizontal vessel (a cylindrical
vessel having end plates at both ends), and a mist eliminator may
be provided inside the vessel in order to improve the separation
efficiency of vapor and liquid.
[0056] In the example shown in FIG. 1, the first feed gas cooler 1,
the feed gas chiller 2, and the second feed gas cooler 3 are used
as a heat exchanger in the step (a). The feed gas is cooled in the
first feed gas cooler 1 by heat exchange with the residual gas and
the side stream F1 of the demethanizer, then cooled by propane
refrigeration in the feed gas chiller 2, and then cooled again in
the second feed gas cooler 3 by heat exchange with the residual
gas, the side stream F3 of the demethanizer, and the condensate
(line 104) condensed in the turboexpander outlet separator 7
(second vapor-liquid separator). A partially condensed feed gas
(vapor-liquid two-phase flow) is obtained from the second feed gas
cooler 3. Note that the side streams F1 and F3 are returned to the
demethanizer 11 after the heat exchange described above,
respectively (the flow returned is shown as F2 and F4,
respectively). That is, the condensate (liquid obtained from step
(c)) 104 condensed in the turboexpander outlet separator 7, the
side streams F1 and F3 of the demethanizer, and the propane of the
propane refrigeration system are used as "another refrigerant" in
step (a).
[0057] The first feed gas cooler 1 has a refrigerant flow path
through which the residual gas flows and has a refrigerant flow
path through which the side stream F1 flows as the "another
refrigerant flow path". The second feed gas cooler 3 has a
refrigerant flow path through which the residual gas flows as a
refrigerant, and has a refrigerant flow path through which the
liquid obtained from the second vapor-liquid separator (line 104)
flows and a refrigerant flow path through which the side stream F3
flows as the "another refrigerant flow path". The feed gas chiller
2 has a refrigerant flow path through which propane flows of the
propane refrigeration system.
[0058] b) Step of Supplying the Liquid Obtained from Step (a) to
the Distillation Column Under Reduced Pressure
[0059] In this step, a line 101 for supplying the condensate
obtained from the low-temperature separator (first vapor-liquid
separator) 4 to the demethanizer 11 is used. A pressure reducing
valve 14 may be provided in this line. Typically, the pressure of
the condensate is reduced by the pressure reducing valve 14 to a
pressure obtained by adding a pressure loss at the time of feeding,
to the operating pressure of the feed stage of the demethanizer (in
the case of propane recovery, the deethanizer), and a part of the
condensate is vaporized into a vapor-liquid two-phase flow. In
addition, the temperature decreases with this vaporization (in the
case of Example 1 corresponding to Embodiment 1, the temperature
decreases to -84.6.degree. C.)
[0060] c) Step of Expanding a Part or all of the Gas Obtained from
Step (a) by an Expander to Partially Condense the Gas, Followed by
Vapor-Liquid Separation
[0061] In this step, an expander which expands and partially
condenses part or all of the gas obtained from the low-temperature
separator (first vapor-liquid separator) 4, in particular a
turboexpander 5, is used. A turbo expander outlet separator 7
connected to the turbo expander 5 outlet is also used as a second
vapor-liquid separator.
[0062] In the present example, all of the low-temperature separator
4 outlet gas (line 110) is sent to the turbo expander 5, and
typically the pressure at the outlet of the turbo expander 5 is
reduced to a pressure obtained by adding a pressure loss at the
time of feeding, to the operating pressure of the feed stage of the
demethanizer (in the case of propane recovery, the deethanizer). At
this time, due to the effect of the isentropic expansion, the
outlet gas of the turboexpander 5 becomes extremely low temperature
(in the case of Example 1, -85.2.degree. C.) and partially
condenses (in the case of Example 1, 27.9 mol % is liquefied). It
is also possible to recover the energy which the gas loses during
expansion as the power of the compressor 6.
[0063] The gas partially condensed at the outlet of the
turboexpander 5 is separated in a turboexpander outlet separator 7
(second vapor-liquid separator).
[0064] The turboexpander outlet separator 7 may be a vertical or
horizontal vessel (a cylindrical vessel having end plates at both
ends) and may have a mist eliminator therein to increase the
separation efficiency of vapor and liquid.
[0065] d) Step of Feeding the Liquid Obtained from Step (c) to the
Distillation Column after Using it as the "Another Refrigerant" in
Step (a);
[0066] In this step, a line for supplying the liquid obtained from
the turboexpander outlet separator 7 to the demethanizer
(distillation column) 11 via the above-mentioned "another
refrigerant flow path" is used (lines 104 and 102). In the present
example, "another refrigerant flow path" for flowing the liquid
obtained from the turbo expander outlet separator 7 is one of the
refrigerant flow paths provided in the second feed gas cooler 3
located most downstream based on the flow direction of the feed gas
among the heat exchangers used for cooling of the step (a). In
Example 1, the liquid in line 104 is warmed to -39.0.degree. C. by
being used as "another refrigerant", resulting in a vapor-liquid
two-phase flow.
[0067] e) Step of Supplying a Part or all of the Gas Obtained from
Step (c) to the Distillation Column
[0068] In this step, a line for supplying a part or all of the gas
obtained from the turboexpander outlet separator (second
vapor-liquid separator) 7 to the demethanizer (distillation column)
11 is used.
[0069] In this example, all of the gas obtained from the
turboexpander outlet separator (second vapor-liquid separator) 7 is
supplied to the demethanizer 11 (line 103).
[0070] The demethanizer 11 equips, for example, trays or packings
inside the column, and separates the high volatile component and
the low volatile component by a distillation operation. The
pressure of the demethanizer is preferably as high as possible as
long as a predetermined ethane recovery rate can be achieved in
order to reduce the power required for the compression of the
residual gas downstream, and is preferably 1.5 MPa or more and 3.5
MPa or less from this viewpoint, and more preferably 2.5 MPa or
more and 3.5 MPa or less.
[0071] In this example, three types of fluids are fed to the
demethanizer 11. The top of the column is fed with condensate
separated by a low-temperature separator 4 as a reflux via a
pressure reducing valve 14 (line 101), below the feed location the
outlet gas of the turbo expander outlet separator 7 is fed (line
103), and further below the feed location the liquid separated in
the turbo expander outlet separator 7 is fed after heat exchange
with the feed gas in a second feed gas cooler 3 (line 102). In FIG.
1, the liquid separated in the low-temperature separator 4 is fed
as a reflux (line 101), but the liquid separated in the
turboexpander outlet separator 7 may be used as a reflux after heat
exchange with the feed gas. The more detailed location of the feed
to the demethanizer can be appropriately determined according to
the temperature and methane concentration of each feed.
[0072] A reboiler 12 is installed at the bottom of the demethanizer
to volatilize methane in the bottom liquid of the column, and heat
is applied so that the concentration of methane in the bottom
liquid of the column becomes equal to or lower than a predetermined
value.
[0073] f) Step of obtaining a residual gas from the top of the
distillation column and a heavy fraction from the bottom of the
distillation column. From the top of the demethanizer, a residual
gas containing methane as a main component, from which components
such as ethane and propane have been removed, is separated and
utilized for heat exchange with the feed gas. Thereafter, if
necessary, the residual gas is compressed to a predetermined
pressure by a compressor 6 driven by the turbo compressor and a
compressor (residual gas compressor) 13 driven by a motor or the
like. From the bottom of the demethanizer 11, ethane, propane and
heavy components are separated as NGLs (natural gas-liquid). The
obtained NGL is separated into respective components, for example,
in an NGL separation step which is further provided downstream.
[0074] As the feed gas, a natural gas containing methane and
hydrocarbons having lower volatility than methane is preferred. The
raw material feed gas may be a petroleum-associated gas or an
off-gas from a refinery or petrochemical plant.
[0075] The higher the concentration of the hydrocarbons having
lower volatility than methane in the feed gas, the greater the
difference between the methane concentration in the inlet gas of
the turboexpander 5 and the methane concentration in the outlet gas
of the turboexpander outlet gas separator 7, and accordingly the
effect of improving the reflux tends to be produced. Therefore,
when the concentration of hydrocarbons having lower volatility than
methane in the feed gas is 5 mol % or more and 50 mol % or less,
further, when the concentration is 10 mol % or more and 50 mol % or
less, the effect of the present invention is particularly
remarkable.
[0076] Further, since the lower the ethane concentration in the
residual gas means a higher ethane recovery rate, the ethane
concentration in the residual gas is preferably as low as possible,
preferably 5 mol % or less, and more preferably 1 mol % or
less.
[0077] The NGL is composed of hydrocarbons having lower volatility
than the liquefied and recovered methane, and is sent to an NGL
fractionation facility which is further provided downstream, for
example, and is separated into products such as ethane, propane,
and butane. In such a case, methane in NGL is preferably low to
such an extent that the criteria of the ethane product can be
satisfied, and is preferably 2 mol % or less, more preferably 1 mol
% or less.
[0078] In the case of the propane recovery process, the same
principle as in the above example is used, and a deethanizer is
used instead of the demethanizer 11, and a residual gas containing
methane and ethane as main components is separated from the top of
the deethanizer, and propane and heavy components are separated as
NGL from the bottom of the deethanizer.
Embodiment 2
[0079] With respect to the second embodiment of the present
invention, an example of an ethane recovery process will be
described using the process flow diagram shown in FIG. 3.
Descriptions of the same points as those in the embodiment 1 is
omitted.
[0080] In the embodiment 1, in the step (c), the entire amount of
the gas (line 110) obtained from the step (a), i.e. from the
low-temperature separator 4, is supplied to the turboexpander 5. In
the embodiment 2, line 110 is divided and only a portion of the gas
of line 110 (line 110a) is sent to turboexpander 5 for step (c).
The distribution ratio of line 110 is determined in view of the
required ethane recovery rate (line 110a:line 110b=70:30 (molar
ratio) in the case of example 2 corresponding to the embodiment 2).
The pressure at the outlet of the turboexpander 5 is reduced to a
pressure obtained by adding a pressure loss at the time of feeding,
to the operating pressure of the feed stage of the demethanizer (in
the case of propane recovery, the deethanizer). At this time, due
to the effect of the isentropic expansion, the outlet gas of the
turboexpander 5 becomes extremely low temperature (in Example 2,
-86.4.degree. C.) and partially condenses (in Example 2, 24.7% is
liquefied). It is also possible to recover the energy which the gas
loses during expansion as the power of the compressor 6.
[0081] The remainder (line 110b) of the gas in line 110 is cooled
and totally condensed by heat exchange with the residual gas
obtained from the top of the demethanizer in the condenser 10 (in
the case of Example 2, cooling to -90.8.degree. C.), the pressure
of the totally condensed liquid is reduced by the pressure reducing
valves 15 and the totally condensed liquid is supplied to the
demethanizer (distillation column) 11 (line 105). The pressure of
the totally condensed liquid is reduced by the pressure reducing
valve 15 to a pressure obtained by adding a pressure loss at the
time of feeding, to the operating pressure of the feed stage of the
demethanizer (distillation column) 11. Also, the totally condensed
liquid is partially vaporized by decompression, resulting in a
vapor-liquid two-phase flow, and the temperature decreases with
vaporization (in the case of example 2, -94.2.degree. C.).
[0082] For this purpose, the following apparatus are used:
[0083] line 110a, which feeds a portion of the gas obtained from
the low-temperature separator (first vapor-liquid separator) 4 to
the turboexpander 5;
[0084] condenser 10, which cools the remainder (line 110b) of the
gas obtained from the low-temperature separator (first vapor-liquid
separator) 4 by heat exchange with the residual gas to cause total
condensation;
[0085] pressure reducing valve 15 for depressurizing the totally
condensed liquid in the condenser 10; and
[0086] line 105, which connects the outlet of the pressure reducing
valve 15 to the demethanizer (distillation column) 11.
[0087] As the condenser 10, a heat exchanger for exchanging heat
between the gas in the line 110b and the residual gas can be used.
The condenser 10 can be disposed upstream of the feed gas coolers 1
and 3 and the feed gas chiller 2 with reference to the flow
direction of the residual gas.
[0088] In this example, four types of fluids are fed to the
demethanizer 11. At the top of the column, the liquid from line 105
is fed as a reflux, below the feed location the outlet gas of the
turbo expander outlet separator 7 is fed (line 103), below the feed
location the liquid from the low-temperature separator 4 is fed
after being decompressed with the pressure reducing valve 14 (line
101), and below the feed location the liquid from the turbo
expander outlet separator 7 is fed after heat exchange with the
feed gas (line 102).
[0089] With respect to the process flow, the embodiment 2 may be
the same as the embodiment 1 except that the above points. However,
the conditions such as the temperature and the pressure can be
appropriately changed in accordance with the difference in the
process flow.
Embodiment 3
[0090] With respect to the third embodiment of the present
invention, an example of an ethane recovery process will be
described using the process flow diagram shown in FIG. 5.
Descriptions of the same points as those in the embodiment 1 is
omitted.
[0091] In the embodiment 1, in step (e), the entire amount of gas
(line 103) obtained from step (c), i.e. from the turboexpander
outlet separator 7, is supplied to the demethanizer 11. In the
embodiment 3, line 103 is divided and only a portion of the gas of
line 103 (line 103a) is supplied to step (e), i.e., fed to
demethanizer 11. The distribution ratio of line 103 is determined
in view of the required ethane recovery rate (line 103a:line
103b=63:37 (molar ratio) in the case of example 3 corresponding to
the embodiment 3). The remainder of the gas in line 103 (line 103b)
is compressed (in the case of Example 3, 6.00 MPa), cooled by
heat-exchanging with the residual gas obtained from the top of the
demethanizer to cause total condensation (in the case of Example 3,
-94.2.degree. C.), and the totally condensed liquid is
depressurized and supplied to the demethanizer 11 (line 105). The
pressure of the totally condensed liquid is reduced by the pressure
reducing valve 15 to a pressure obtained by adding a pressure loss
at the time of feeding, to the operating pressure of the feed stage
of the demethanizer (distillation column) 11. Also, the totally
condensed liquid is partially vaporized by decompression, resulting
in a vapor-liquid two-phase flow, and the temperature decreases
with vaporization (in the case of example 3, -97.2.degree. C.).
[0092] For this purpose, the following apparatus are used:
[0093] line 103a, which feeds a portion of the gas obtained from
the turboexpander outlet separator 7 (second vapor-liquid
separator) to the demethanizer (distillation column) 11;
[0094] compressor 8, which compresses the remainder (line 103b) of
the gas obtained from the turbo expander outlet separator 7 (second
vapor-liquid separator);
[0095] condenser (reflux condenser) 10, which cool the gas
compressed by the compressor 8 by heat exchange with the residual
gas to cause total condensation;
[0096] pressure reducing valve 15 for depressurizing the totally
condensed liquid in the condenser 10; and
[0097] line 105, which connects the outlet of the pressure reducing
valve 15 to the demethanizer (distillation column) 11.
[0098] As the condenser 10, a heat exchanger for exchanging heat
between the gas in the line 103b and the residual gas can be used.
The condenser 10 can be disposed upstream of the feed gas coolers 1
and 3 and the feed gas chiller 2 with reference to the flow
direction of the residual gas.
In this embodiment, after the gas compressed by the compressor 8 is
cooled by a heat exchanger (reflux cooler) 9 using propane
refrigerant, the gas is cooled by heat exchange with residual gas
in a reflux condenser 10 and totally condensed. The reflux cooler 9
can be provided as required and is not required if cooling by the
reflux condenser 10 is sufficient.
[0099] In this example, four types of fluids are fed to the
demethanizer 11. At the top of the column, the liquid from line 105
is fed as reflux, below the feed location part of the outlet gas of
the turboexpander outlet separator 7 is fed (line 103a), below the
feed location the liquid from the low-temperature separator 4 is
fed (line 101) after being decompressed with the pressure reducing
valve 14, and below the feed location the liquid from the
turboexpander outlet separator 7 is fed after heat exchange with
the feed gas (line 102).
[0100] With respect to the process flow, Embodiment 3 may be the
same as Embodiment 1 except that the above points. However, the
conditions such as the temperature and the pressure can be
appropriately changed in accordance with the difference in the
process flow.
EXAMPLES
[0101] Hereinafter, the present invention will be described in more
detail based on Examples, but the present invention is not limited
thereto.
Example 1
[0102] A process simulation was carried out on an example of a case
where ethane recovery was carried out using a hydrocarbon
separation apparatus having the configuration shown in FIG. 1.
High-pressure raw natural gas from which water has been removed
beforehand is introduced into the hydrocarbon separation apparatus
under the conditions of 6.24 MPa and 17.1.degree. C. The
composition of the feed gas at this time is as shown in Table 1.
The flow rate is 13,700 kg-moles/hr (10.sup.3 moles/hr). Note that
Cn (n is a natural number) represents a hydrocarbon having n carbon
atoms. C5+ represents a hydrocarbon having 5 or more carbon
atoms.
TABLE-US-00001 TABLE 1 Composition of Feed Gas(mole %) CO2 1.00 N2
0.54 C1 89.41 C2 4.91 C3 2.23 C4 1.29 C5+ 0.62 Total 100.00
[0103] The feed gas is heat-exchanged in the first feed gas cooler
1 with the residual gas of -39.0.degree. C. and the side stream F1
of the demethanizer 11 of -33.5.degree. C. to be cooled to
-24.6.degree. C. Thereafter, the feed gas is cooled to
-37.0.degree. C. by propane refrigerant in the feed gas chiller 2,
and cooled to -62.9.degree. C. in the second feed gas cooler 3 by
heat exchange with the residual gas of -84.6.degree. C., the side
stream F3 of the demethanizer 11 of -76.1.degree. C., and the
condensate (line 104) of the turboexpander outlet separator 7 of
-85.2.degree. C. Here the first feed gas cooler 1 and the second
feed gas cooler 3 are plate fin heat exchangers respectively, the
feed gas chiller 2 is a shell and tube heat exchanger of kettle
type.
[0104] Next, the feed gas is separated in the low-temperature
separator 4. The low-temperature separator 4 is a vertical vessel
having a mist eliminator therein (cylindrical container having a
mirror plate at both ends).
[0105] The entire amount of gas at the outlet of the
low-temperature separator 4 is sent to the turbo expander 5 and
depressurized to 3.47 MPa. The outlet gas is cooled down to
-85.2.degree. C. by the effect of isentropic expansion and provides
529 kW of power to the compressor 6 driven by the expander. The gas
at the outlet of the turboexpander 5 is separated in the
turboexpander outlet separator 7. The turbo expander outlet
separator 7 is a vertical vessel having a mist eliminator therein
(cylindrical container having a mirror plate at both ends).
[0106] The condensate at -85.2.degree. C. (line 104) separated by
the turboexpander outlet separator 7 is fed to the demethanizer 11
(line 102) after the temperature is raised to -39.0.degree. C. by
providing cold heat to the feed gas in the second feed gas cooler
3.
[0107] The demethanizer 11 has 40 trays installed therein, and the
gas at the outlet of the turboexpander outlet separator 7 is fed to
the tray of the third stage from the top of the column (line 103).
The liquid separated in the turboexpander outlet separator 7 passes
through the second feed gas cooler 3 and is fed to the tenth stage
from the top of the column (line 102). In addition, the liquid
separated in the low-temperature separator 4 is decompressed to
3.29 MPa with the pressure reducing valve 14, and then fed to the
first stage from the top of the column (line 101) as reflux.
[0108] Demethanizer 11 is operated under the conditions of 3.27 MPa
and -84.6.degree. C. at the top of the column, and is operated
under the conditions of 3.32 MPa and 39.8.degree. C. at the bottom
of the column. The temperature of the bottom of the column is
determined by the equilibrium temperature at which the methane
concentration in the NGL is less than or equal to 1 mol %, and 3.60
MW of heat is added from reboiler 12 in order to operate at that
temperature. The residual gas separated from the top of the
demethanizer 11 and the composition of NGL separated from the
bottom of the column are as shown in Table 2. The flow rates are
12,553 kg-moles/hr (10.sup.3 moles/hr) for residual gas and 1,147
kg-moles/hr (10.sup.3 moles/hr) for NGL. Note that "NC4" represents
normal butane, and "IC4" represents isobutane.
TABLE-US-00002 TABLE 2 Compositions of Residual Gas and NGL (mole
%) Residual Gas NGL CO2 0.52 6.30 N2 0.59 0.00 C1 97.49 1.00 C2
1.25 44.99 C3 0.14 25.13 NC4 0.01 10.86 IC4 0.01 4.33 C5+ 0.00 7.39
Total 100.00 100.00
[0109] Of the ethane in the feed gas, 76.7% is recovered as
NGL.
[0110] The residual gas leaving the top of the demethanizer 11 is
heat-exchanged with the feed gas to reach 15.1.degree. C. at the
outlet of the first feed gas cooler 1. Thereafter, the residual gas
is compressed to 3.25 MPa by the compressor 6 driven by the turbo
expander, and is compressed to 3.77 MPa by the residual gas
compressor 13. At this time, the required power of the residual gas
compressor 13 is 1031 kW.
Comparative Example 1
[0111] A process simulation was carried out on an example of a case
where ethane recovery was carried out using a hydrocarbon
separation apparatus having the configuration shown in FIG. 2. The
results are summarized in Table 3 together with the results of
Example 1.
[0112] In Example 1, from a vapor-liquid separated condensate (line
104) by the turbo-expansion outlet separator 7, cold heat is
recovered in the second feed gas cooler 3, and the condensate
becomes a vapor-liquid two-phase flow (line 102). At this time, the
methane fraction, which is a low boiling point component, is mainly
vaporized, so that the concentration of methane in the vapor-liquid
two phase stream of line 102 decreases. The higher the methane
concentration in the reflux liquid of the demethanizer 11, the
higher the reflux effect is, and hence in Example 1, the condensate
of the low-temperature separator 4 (the methane concentration is
higher than the vapor-liquid two-phase flow of line 102) is fed as
a reflux liquid to the first stage of the demethanizer.
[0113] On the other hand, in the configuration shown in FIG. 2,
from the condensate (line 102) separated in the turboexpander
outlet separator 7, cold heat is not recovered by the second feed
gas cooler 3 and its methane concentration is higher than the
methane concentration of the condensate of the low-temperature
separator 4, so that the condensate (line 102) is supplied as
reflux to the first stage of the demethanizer 11.
[0114] In the demethanizer 11, the gas from the outlet of the
turboexpander outlet separator 7 is fed to the tray of the fourth
stage from the top of the column (line 103). The liquid separated
in the low-temperature separator 4 is fed to the 14th stage from
the top of the column after being decompressed to 2.82 MPa by the
pressure reducing valve 14 (line 101).
[0115] With respect to the process flow, Comparative Example 1 is
the same as Example 1 except for the above points.
[0116] In Table 3, the "Refrigeration Load" is the thermal load of
the propane refrigeration system in the feed gas chiller 2. The
lowering of the refrigeration load means the lowering of the
propane refrigeration equipment capacity, and it is effective in
lowering of the energy consumed in the propane refrigeration
equipment and in lowering of the equipment cost of the propane
refrigeration.
[0117] The "Reboiler Heat Load" is the heat load of the reboiler 12
at the bottom of the demethanizer column. The reduction means a
reduction in the energy required for distillation, and there is an
effect of a reduction in the cost of utilities supplied from the
outside. Power of "Refrigeration Compressor" is a power consumed by
the compressor in the propane refrigeration system. Power of
"Residual Gas Compressor" is a power consumed by the residual gas
compressor 13.
[0118] As apparent from Table 3, Example 1 can reduce the total
compressor power and reboiler heat load even though the ethane
recovery rate is about the same as that in the case where ethane
recovery is performed in the configuration of Comparative Example
1.
TABLE-US-00003 TABLE 3 Comparison of Comparative Example 1 and
Example 1 Comparative Example 1 Example 1 Ethane Recovery Rate (%)
76.71 76.72 Refrigeration Load (MW) 3.68 3.48 Reboiler Heat Load
(MW) 4.45 3.60 Compressor Power Refrigeration Compressor (kW) 2,203
2,085 Residual Gas Compressor (kW) 2,116 1,032 Total Compressor
Power (kW) 4,319 3,117
Example 2
[0119] A process simulation was carried out on an example of a case
where ethane recovery was carried out using a hydrocarbon
separation apparatus having the configuration shown in FIG. 3. The
feed gas condition is the same as in Example 1
[0120] The feed gas is heat-exchanged in the first feed gas cooler
1 with the residual gas of -39.0.degree. C. and the side stream F1
of the demethanizer 11 of -39.3.degree. C. to be cooled to
-23.7.degree. C. Thereafter, the feed gas is cooled to
-37.0.degree. C. by propane refrigerant in the feed gas chiller 2,
and cooled to -60.4.degree. C. in the second feed gas cooler 3 by
heat exchange with the residual gas of -76.6.degree. C., the side
stream F3 of the demethanizer 11 of -77.7.degree. C., and the
condensate (line 104) of the turboexpander outlet separator 7 of
-86.4.degree. C. Here the first feed gas cooler 1 and the second
feed gas cooler 3 are plate fin heat exchangers respectively, and
the feed gas chiller 2 is a shell and tube heat exchanger of kettle
type.
[0121] Next, the feed gas is separated in the low-temperature
separator 4. The low-temperature separator 4 is a vertical vessel
having a mist eliminator therein (cylindrical container having a
mirror plate at both ends).
[0122] 70 mole % of the outlet gas of the low-temperature separator
4 is sent to the turboexpander 5 (line 110a) and depressurized to
3.20 MPa. The outlet gas is cooled to -86.4.degree. C. by the
effect of isentropic expansion, and accordingly a part of the gas
condenses into a vapor-liquid two-phase flow, and thereby 723 kW of
power is provided to the compressor 6 driven by the expander. The
gas (partially condensed) at the outlet of the turbo expander 5 is
separated in the turbo expander outlet separator 7. The turbo
expander outlet separator 7 is a vertical vessel having a mist
eliminator therein (cylindrical container having a mirror plate at
both ends).
[0123] The remaining 30 mole % of the outlet gas of the
low-temperature separator 4 is sent to a condenser (reflux
condenser) 10 (line 110b), heat-exchanged with the residual gas at
the top of the demethanizer 11, and is cooled to -90.8.degree. C.
to be totally condensed. The pressure of the condensate is reduced
to 3.00 MPa with the pressure reducing valve 15, and a part of the
condensate is vaporized into a vapor-liquid two-phase flow, and the
temperature is lowered to -94.2.degree. C. as the condensate is
vaporized. Thereafter, the two-phase flow is fed as a reflux liquid
to the first stage from the top (line 105). Here, the reflux
condenser 10 is a plate fin heat exchanger.
[0124] The demethanizer 11 has 40 trays installed therein, and the
gas at the outlet of the turboexpander outlet separator 7 is fed to
the tray of the fourth stage from the top of the column (line 103).
Further, the condensate of -86.4.degree. C. which is separated in
the turbo expander outlet separator 7 (line 104) is
temperature-elevated to -39.0.degree. C. by the cold heat recovery
in the second feed gas cooler 3 and thereby a part of the
condensate is vaporized to become a vapor-liquid two-phase flow,
and thereafter fed to the 20th stage from the top of the column
(line 102). Further, the liquid separated in the low-temperature
separator 4 is decompressed to 3.20 MPa with the pressure reducing
valve 14 and thereby a part thereof is vaporized to become a
vapor-liquid two-phase flow, and the temperature is lowered to
-84.2.degree. C. as the liquid is vaporized. Thereafter, the
two-phase flow is fed to the 14th stage from the top of the column
(line 101).
[0125] Demethanizer 11 is operated under the conditions of 3.00 MPa
and -92.8.degree. C. at the top of the column, and is operated
under the conditions of 3.05 MPa and 31.5.degree. C. at the bottom
of the column. The temperature of the bottom of the column is
determined by the equilibrium temperature at which the methane
concentration in the NGL is less than or equal to 1 mol %, and 3.65
MW of heat is added from reboiler 12 in order to operate at that
temperature. The residual gas separated from the top of the
demethanizer 11 and the composition of NGL separated from the
bottom of the column are as shown in Table 4. The flow rates are
12,444 kg-moles/hr (10.sup.3 moles/hr) for residual gas and 1,256
kg-moles/hr (10.sup.3 moles/hr) for NGL.
TABLE-US-00004 TABLE 4 Compositions of Residual Gas and NGL (mole
%) Residual Gas NGL CO2 0.42 6.74 N2 0.59 0.00 C1 98.34 1.00 C2
0.61 47.47 C3 0.03 23.99 NC4 0.00 10.02 IC4 0.00 4.02 C5+ 0.00 6.76
Total 100.00 100.00
[0126] Of the ethane in the feed gas, 88.7% is recovered as
NGL.
[0127] The residual gas leaving the top of the demethanizer 11 is
heat-exchanged with the feed gas to reach 15.1.degree. C. at the
outlet of the first feed gas cooler 1. Thereafter, the residual gas
is compressed to 3.17 MPa by the compressor 6 driven by the turbo
expander, and is compressed to 3.77 MPa by the residual gas
compressor 13. At this time, the required power of the residual gas
compressor 13 is 1859 kW.
Comparative Example 2
[0128] A process simulation was carried out on an example of a case
where ethane recovery was carried out using a hydrocarbon
separation apparatus having the configuration shown in FIG. 4. The
results are summarized in Table 5 together with the results of
Example 2.
[0129] In the configuration shown in FIG. 4, the condensate (line
102) separated by the turboexpander outlet separator 7 is directly
supplied to the demethanizer 11 without cold heat recovery by the
second feed gas cooler 3.
[0130] In Comparative Example 2, cold heat recovery using the
condensate of the turboexpander outlet separator 7 is not
performed, and accordingly the temperature of the stream flowing
into the low-temperature separator 4 is -52.0.degree. C., which is
8.4.degree. C. higher than in Example 2. Accordingly, the methane
concentration in the gas (line 110) separated in the
low-temperature separator 4 becomes lower as compared with Example
2, and eventually leads to a decrease in the reflux effect in the
distillation column.
[0131] In the demethanizer 11, the liquid from line 105 is fed to
the first stage from the top of the column as a reflux liquid. The
gas at the outlet of the turbo expander outlet separator 7 is fed
to the tray of the fourth stage from the top of the column (line
103). The liquid separated in the turboexpander outlet separator 7
is fed to the 14th stage from the top of the column (line 102).
Furthermore, the liquid separated in the low-temperature separator
4 is fed to the 20th stage from the top of the column after being
depressurized to 2.83 MPa with the pressure reducing valve 14 (line
101).
[0132] With respect to the process flow, Comparative Example 2 is
the same as in Example 2 except for the above points.
[0133] As is apparent from Table 5, Example 2 can obtain a higher
ethane recovery rate and can further reduce the total compressor
power and reboiler heat load as compared with the case where ethane
recovery is performed in the configuration of Comparative Example
2.
TABLE-US-00005 TABLE 5 Comparison of Comparative Example 2 and
Example 2 Comparative Example 2 Example 2 Ethane Recovery Rate (%)
86.80 88.67 Refrigeration Load (MW) 3.77 3.71 Reboiler Heat Load
(MW) 4.30 3.65 Compressor Power Refrigeration Compressor (kW) 2,256
2,221 Residual Gas Compressor (kW) 2,120 1,859 Total Compressor
Power (kW) 4,386 4,080
Example 3
[0134] A process simulation was carried out on an example of a case
where ethane recovery was carried out using a hydrocarbon
separation apparatus having the configuration shown in FIG. 5. The
feed gas condition is the same as in Example 1
[0135] The feed gas is heat-exchanged in the first feed gas cooler
1 with the residual gas of -39.0.degree. C. and the side stream F1
of the demethanizer 11 of -35.3.degree. C. to be cooled to
-22.6.degree. C. Thereafter, the feed gas is cooled to
-37.0.degree. C. by propane refrigerant in the feed gas chiller 2,
and cooled to -59.0.degree. C. in the second feed gas cooler 3 by
heat exchange with the residual gas of -68.0.degree. C., the side
stream F3 of the demethanizer 11 of -74.3.degree. C., and the
condensate (line 104) of the turboexpander outlet separator 7 of
-86.8.degree. C. Here the first feed gas cooler 1 and the second
feed gas cooler 3 are plate fin heat exchangers respectively, and
the feed gas chiller 2 is a shell and tube heat exchanger of kettle
type.
[0136] Next, the feed gas is vapor-liquid separated in the
low-temperature separator 4. The low-temperature separator 4 is a
vertical vessel having a mist eliminator therein (cylindrical
container having a mirror plate at both ends).
[0137] The entire amount of gas at the outlet of the
low-temperature separator 4 is sent to the turbo expander 5 and
reduced to 3.07 MPa. The outlet gas is cooled down to -86.8.degree.
C. by the effect of isentropic expansion and provides 1259 kW of
power to the compressor 6 driven by the expander. The gas at the
outlet of the turboexpander 5 is separated in the turboexpander
outlet separator 7. The turbo expander outlet separator 7 is a
vertical vessel having a mist eliminator therein (cylindrical
container having a mirror plate at both ends).
[0138] 37 mol % of the outlet gas (line 103) of the turbo expander
outlet separator 7 is pressurized by a compressor (low temperature
compressor) 8 to 6.00 MPa, then cooled to -94.2.degree. C. by a
heat exchanger (reflux cooler) 9 by propane refrigeration and a
condenser (reflux condenser) 10 for heat exchange with the residual
gas on the top of the demethanizer 11 to be totally condensed. The
pressure of the obtained condensate is reduced to 2.87 MPa with the
pressure reducing valve 15, and a part of the condensate is
vaporized into a vapor-liquid two-phase flow, and the temperature
is lowered to -97.2.degree. C. as the condensate is vaporized.
Thereafter, the two-phase flow is fed as a reflux liquid to the
first stage from the top (line 105). Here, the reflux cooler 9 is a
shell and tube heat exchanger of kettle type, the reflux condenser
10 is a plate fin heat exchanger. When the outlet temperature of
the reflux condenser 10 can be lowered to a temperature at which a
predetermined ethane recovery rate can be achieved by only
exchanging heat with the residual gas, the reflux cooler 9 may not
be installed in order to reduce the load of propane
refrigeration.
[0139] The demethanizer 11 has 40 trays installed therein, and a
part of the gas at the outlet of the turboexpander outlet separator
7 is fed to the tray of the ninth stage from the top of the column
(line 103a). Further, the condensate of -86.8.degree. C. which is
separated by the turbo expander outlet separator 7 (line 104) is
temperature-elevated to -39.0.degree. C. by the cold heat recovery
in the second feed gas cooler 3 and thereby a part of the
condensate is vaporized to become a vapor-liquid two-phase flow,
and thereafter fed to the 18th stage from the top of the column
(line 102). Further, the liquid separated in the low-temperature
separator 4 is decompressed to 2.89 MPa with the pressure reducing
valve 14, and thereby a part thereof is vaporized to become a
vapor-liquid two-phase flow, and the temperature is lowered to
-83.7.degree. C. as the liquid is vaporized. Thereafter, the
two-phase flow is fed to the 15th stage from the top of the column
(line 101).
[0140] Demethanizer 11 is operated under the conditions of 2.87 MPa
and -96.2.degree. C. at the top of the column, and is operated
under the condition of 2.92 MPa and 27.5.degree. C. at the bottom
of the column. The temperature of the bottom of the column is
determined by the equilibrium temperature at which the methane
concentration in the NGL is less than or equal to 1 mole %, and
3.35 MW of heat is added from reboiler 12 in order to operate at
that temperature. The residual gas separated from the top of the
demethanizer 11 and the composition of NGL separated from the
bottom of the column are as shown in Table 6. The flow rates are
12,388 kg-moles/hr (10.sup.3 moles/hr) for residual gas and 1,312
kg-moles/hr (10.sup.3 moles/hr) for NGL.
TABLE-US-00006 TABLE 6 Compositions of Residual Gas and NGL (mole
%) Residual Gas NGL CO2 0.38 6.84 N2 0.60 0.00 C1 98.78 1.00 C2
0.24 48.97 C3 0.00 23.25 NC4 0.00 9.60 IC4 0.00 3.86 C5+ 0.00 6.48
Total 100.00 100.00
[0141] Of the ethane in the feed gas, 95.5% is recovered as
NGL.
[0142] The residual gas leaving the top of the demethanizer 11 is
heat-exchanged with the feed gas to reach 15.1.degree. C. at the
outlet of the first feed gas cooler 1. Thereafter, the residual gas
is compressed to 3.19 MPa in the compressor 6 driven the turbo
expander, and is compressed to 3.77 MPa by the residual gas
compressor 13. At this time, the required power of the residual gas
compressor 13 is 1824 kW.
Comparative Example 3
[0143] A process simulation was carried out on an example of a case
where ethane recovery was carried out using a hydrocarbon
separation apparatus having the configuration shown in FIG. 6. This
process corresponds to the process disclosed in WO 2005/009930 A1.
The results are summarized in Table 7 along with the results of
Example 3.
[0144] In the configuration shown in FIG. 6, the condensate (line
102) separated in the turboexpander outlet separator 7 is directly
supplied to the demethanizer 11 without cold heat recovery by the
second feed gas cooler 3.
[0145] In Comparative Example 3, cold heat recovery using the
condensate of the turboexpander outlet separator 7 is not
performed, and accordingly the temperature of the stream flowing
into the low-temperature separator 4 is -44.1.degree. C., which is
14.9.degree. C. higher than in Example 3. Accordingly, the methane
concentration in the gas (line 110) separated in the
low-temperature separator 4 becomes lower as compared with Example
3, and eventually leads to a decrease in the reflux effect in the
distillation column.
[0146] In the demethanizer 11, the liquid from line 105 is fed to
the first stage from the top of the column as reflux. A part of the
gas at the outlet of the turboexpander outlet separator 7 is fed to
the tray of the ninth stage from the top of the column (line 103a).
The liquid separated in the turboexpander outlet separator 7 is fed
to the 12th stage from the top of the column (line 102).
Furthermore, the liquid separated in the low-temperature separator
4 is decompressed to 2.82 MPa with the pressure reducing valve 14,
and thereby a part thereof is vaporized to become a vapor-liquid
two-phase flow, and the temperature is lowered to -64.0.degree. C.
as the liquid is vaporized. Thereafter, the two-phase flow is fed
to the 15th stage from the top of the column (line 101).
[0147] With respect to the process flow, Comparative Example 3 is
the same as Example 3 except for the above points.
[0148] As is apparent from Table 7, Example 3 can obtain a higher
ethane recovery rate and can further reduce the total compressor
power and reboiler heat load as compared with the case where ethane
recovery is performed in the configuration of Comparative Example
3.
TABLE-US-00007 TABLE 7 Comparison of Comparative Example 3 and
Example 3 Comparative Example 3 Example 3 Ethane Recovery Rate (%)
92.62 95.53 Refrigeration Load (MW) 4.00 4.00 Reboiler Heat Load
(MW) 3.70 3.35 Compressor Power Refrigeration Compressor (kW) 2,396
2,393 Residual Gas Compressor (kW) 1,204 1,824 Low Temperature
Compressor (kW) 1,593 932 Total Compressor Power (kW) 5,192
5,149
REFERENCE SIGNS LIST
[0149] 1: first feed gas cooler [0150] 2: feed gas chiller [0151]
3: second feed gas cooler [0152] 4: low-temperature separator
[0153] 5: turbo expander [0154] 6: compressor driven by turbo
expander [0155] 7: turboexpander outlet separator [0156] 8:
low-temperature compressor [0157] 9: reflux cooler [0158] 10:
reflux condenser [0159] 11: demethanizer (in the case of propane
recovery, the deethanizer) [0160] 12: reboiler [0161] 13: residual
gas compressor [0162] 14: pressure reducing valve [0163] 15:
pressure reducing valve [0164] F1: side stream of demethanizer
[0165] F2: flow returned from side stream F1 [0166] F3: side stream
of demethanizer [0167] F4: flow returned from side stream F3
* * * * *