U.S. patent application number 11/787803 was filed with the patent office on 2007-10-25 for optimization of a dual refrigeration system natural gas liquid plant via empirical experimental method.
This patent application is currently assigned to Saudi Arabian Oil Company. Invention is credited to Salah Al-Ali, Henry H. Chan, Othman A. Taha.
Application Number | 20070245770 11/787803 |
Document ID | / |
Family ID | 38625556 |
Filed Date | 2007-10-25 |
United States Patent
Application |
20070245770 |
Kind Code |
A1 |
Taha; Othman A. ; et
al. |
October 25, 2007 |
Optimization of a dual refrigeration system natural gas liquid
plant via empirical experimental method
Abstract
The current invention is an empirical optimization method based
on statistical modeling relating NGL plant process variables with
the refrigeration system's electricity usage. The method identifies
the key process control variables in an NGL plant to be optimized.
This method is applicable to an NGL plant that uses dual
refrigeration systems.
Inventors: |
Taha; Othman A.; (Dhahran,
SA) ; Chan; Henry H.; (Dhahran, SA) ; Al-Ali;
Salah; (Dammam, SA) |
Correspondence
Address: |
BRACEWELL & GIULIANI LLP
P.O. BOX 61389
HOUSTON
TX
77208-1389
US
|
Assignee: |
Saudi Arabian Oil Company
Dhahran
SA
|
Family ID: |
38625556 |
Appl. No.: |
11/787803 |
Filed: |
April 18, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60793111 |
Apr 19, 2006 |
|
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|
Current U.S.
Class: |
62/612 ;
62/228.1; 62/657 |
Current CPC
Class: |
F25J 3/0238 20130101;
F25J 2270/12 20130101; F25J 3/0233 20130101; G05B 13/021 20130101;
G05B 13/041 20130101; F25J 2270/60 20130101; F25J 2200/76 20130101;
G05B 13/048 20130101; F25J 2200/70 20130101; F25J 2290/50 20130101;
F25J 2280/50 20130101; F25J 2205/04 20130101; F25J 2290/10
20130101; F25J 2240/02 20130101; F25J 2200/02 20130101; F25J 3/0209
20130101; F25J 3/0295 20130101 |
Class at
Publication: |
062/612 ;
062/657; 062/228.1 |
International
Class: |
F25J 1/00 20060101
F25J001/00; F25B 49/00 20060101 F25B049/00; F25J 3/00 20060101
F25J003/00 |
Claims
1. A method for optimizing the production of NGL outlet stream from
an NGL plant, the method comprising the steps of: feeding a natural
gas feed stream to a first chilling unit to produce a chilled rich
gas stream and a chilled liquid stream; feeding the chilled rich
gas stream to a second chilling unit to produce a second chilled
rich gas stream and a second chilled liquid stream; feeding the
second chilled rich gas stream to a third chilling unit to produce
a third chilled liquid stream; feeding the chilled liquid stream
and the second chilled liquid stream and the third chilled liquid
stream to a demethanizer column, the demethanizer column producing
an overhead stream and a bottoms stream, the bottoms stream having
a bottom product specification, the overhead stream defining an
overhead propane concentration; feeding the overhead stream through
an overhead valve having an overhead valve outlet pressure;
providing heat exchange through a first propane refrigeration
system to the first chilling unit, the first chilling unit having a
first chiller, the first chilling unit having a first chill down
separator, the first propane refrigeration system having a propane
compressor, the propane compressor defining a propane compressor
power output and a propane compressor suction pressure, providing
heat exchange through a second propane refrigeration system
operable for providing cooling to the second chilling unit, the
second chilling unit having a second chill down separator, the
second chilling unit including a primary second chiller, the second
propane refrigeration system including a second propane compressor
defining a second propane compressor power output and a second
propane compressor suction pressure; providing heat exchange to the
third chilling unit through an ethane refrigeration system having
an ethane compressor, the ethane compressor defining an ethane
compressor suction pressure; and minimizing a refrigeration load
while maintaining the bottom ratio within a predetermined bottom
ratio range and while maintaining the overhead propane
concentration within a predetermined overhead propane concentration
range, the refrigeration load being the electricity required to
operate the first propane refrigeration system, the second propane
refrigeration system and the ethane refrigeration system.
2. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the propane compressor power output within a
predetermined propane compressor power output range.
3. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the propane compressor suction pressure within a
predetermined propane compressor suction pressure range.
4. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining a propane compressor scraper output pressure within a
predetermined propane compressor output pressure range.
5. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the first tray temperature within a predetermined first
tray temperature range.
6. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the overhead valve output pressure of the overhead
stream within a predetermined overhead valve output pressure
range.
7. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the first chiller output level within a predetermined
first chiller output level range.
8. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the ethane compressor suction pressure within a
predetermined ethane compressor suction pressure range.
9. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the ethane compressor power output within a
predetermined ethane compressor power output range.
10. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining an overhead temperature of the overhead stream within a
predetermined overhead temperature range.
11. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the first chill down separator temperature within a
predetermined first chill down separator temperature range.
12. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining a second propane exchanger temperature of a second
propane exchanger within a predetermined second propane exchanger
temperature range.
13. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining a second propane compressor suction pressure within a
predetermined second propane compressor suction pressure range.
14. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the second propane compressor power output within a
predetermined second propane power output range.
15. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the primary second chiller output level within a
predetermined primary second chiller output level.
16. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the subsequent second chiller output level within a
predetermined subsequent second chiller output level range.
17. The method of claim 1 wherein the step of minimizing the
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range further comprises the step of
maintaining the second chill down separator within a second chill
down separator temperature range.
18. A liquefied natural gas plant for maximizing the production of
NGL from an inlet gas feed stream, the liquefied natural gas plant
comprising: a first chilling unit for cooling at least a portion of
the inlet gas feed stream by heat exchange contact with first and
second expanded refrigerants to produce from the first chilling
unit a chilled rich gas stream and a chilled liquid stream, the
first chilling unit having a first chiller, the first chiller
defining a first chiller output level, and the first chiller unit
having a first chill down separator, the first chill down separator
defining a first chill down separator temperature; a second
chilling unit to receive chilled rich gas stream and to further
chill the chilled rich gas stream to produce a second chilled rich
gas stream and a second chilled liquid stream, the second chilling
unit comprising a second chill down separator defining a second
chill down separator temperature, a subsequent second chiller
defining a subsequent second chiller output level, and a primary
second chiller; a third chilling unit to receive second chilled
rich gas stream and further chill second chilled rich gas to
produce a third chilled liquid stream; a demethanizer column for
receiving the chilled liquid stream and the second chilled liquid
stream and the third chilled liquid stream, the demethanizer column
producing an overhead stream and a bottoms stream, the demethanizer
column having a top tray in an upper section of the demethanizer
column and a mid-tray in a middle section of the demethanizer
column, the top tray having a top tray temperature, the bottoms
stream having a bottom ratio defined by methane concentration of
the bottom stream divided by ethane concentration of the bottom
stream, the overhead stream defining an overhead propane
concentration; an overhead valve receiving the overhead stream, the
overhead valve having an overhead valve outlet pressure; a first
propane refrigeration system operable to provide heat exchange with
the first chilling unit, the first propane refrigeration system
having a propane compressor, the propane compressor defining a
propane compressor power output and a propane compressor suction
pressure, a second propane refrigeration system operable to provide
heat exchange to the second chilling unit, the second propane
refrigeration system including a second propane compressor; an
ethane refrigeration system operable to provide heat exchange to
the third chilling unit, the ethane refrigeration system having an
ethane compressor, the ethane compressor defining an ethane
compressor suction pressure; and means for minimizing a
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range and while maintaining the overhead
propane concentration within a predetermine overhead propane
concentration range, the refrigeration load being the electricity
required to operate the first propane refrigeration system, the
second propane refrigeration system and the ethane refrigeration
system.
19. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
propane compressor power output within a predetermined propane
compressor power output range.
20. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
propane compressor suction pressure within a predetermined propane
compressor suction pressure range.
21. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining a
propane compressor scraper output pressure within a predetermined
propane compressor output pressure range.
22. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
first tray temperature within a predetermined first tray
temperature range.
23. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
overhead valve output pressure of the overhead stream within a
predetermined overhead valve output pressure range.
24. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
first chiller output level within a predetermined first chiller
output level range.
25. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
ethane compressor suction pressure within a predetermined ethane
compressor suction pressure range.
26. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining ethane
compressor power output within a predetermined ethane compressor
power output range.
27. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining an
overhead temperature of the overhead stream within a predetermined
overhead temperature range.
28. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
first chill down separator temperature within a predetermined first
chill down separator temperature range.
29. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining a second
propane exchanger temperature of a second propane exchanger within
a predetermined second propane exchanger temperature range.
30. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining a second
propane compressor suction pressure within a predetermined second
propane compressor suction pressure range.
31. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
second propane compressor power output within a predetermined
second propane compressor power output range.
32. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
primary second chiller output level within a predetermined primary
second chiller output level.
33. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
subsequent second chiller output level within a predetermined
subsequent second chiller output level range.
34. The liquefied natural gas plant of claim 18 for maximizing the
production of NGL from an inlet gas feed stream, the liquefied
natural gas plant further comprising means for maintaining the
second chill down separator 90 within a second chill down separator
temperature range.
Description
PRIORITY APPLICATION
[0001] This application is related to and claims priority and
benefit of U.S. Provisional Patent Application Ser. No. 60/793,111
filed Apr. 19, 2006, which is incorporated herein by reference in
its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention generally relates to the field of
optimization of control variables to maximize production of Natural
Gas Liquids ("NGL") in a gas plant while minimizing the
refrigeration system power usage.
[0004] 2. Description of the Related Art
[0005] Gas plants produce fuel gas, Natural Gas Liquids (NGL) and
other solid components such as sulfur. Such plants typically
include distillation columns, heat exchangers, and refrigeration
systems. The NGL product must meet certain specifications in order
to be a saleable product, but variation within these boundaries is
acceptable. Early efforts to improve NGL quality have been directed
toward maximizing the amount of refrigeration used to achieve
better recovery of heavier components. As energy costs have
increased, this approach is no longer economical.
[0006] U.S. Pat. No. 6,332,336, issued to Mirsky, teaches a method
and apparatus for maximizing the productivity of a natural gas
liquids production plant using a method that specifically
manipulates a turboexpander that drives a recompressor, with the
objective of maximizing NGL production.
[0007] U.S. Pat. No. 5,488,561, issued to Berkowitz, generally
described a multivariable process control method and apparatus,
which may be used in a fractionation process involving natural gas
liquids or other process. It includes an algorithm which controls
multiple variables during the process based upon control equations
utilizing information from rigorous process simulations and actual
plant performance. Historical data is used for calibration purposes
through the routines.
[0008] U.S. Pat. No. 5,791,160, issued to Mandler, discusses a
dynamic method and apparatus for regulatory control of production
and temperature in a mixed refrigerant liquefied natural gas
facility utilizing a generic system modeling program and an
optimization computer program which may use simulation
techniques.
[0009] U.S. Pat. No. 4,616,308, Morshedi et al., teaches a method
of dynamically controlling a process having a plurality of
independently controlled, manipulated variables and at least one
controlled variable that is dependent upon the manipulated
variables. This method applies generally to process control.
[0010] U.S. Pat. Nos. 6,116,050 and 5,992,175, the Yao patents,
discuss NGL recovery processes utilizing various controllers to
control the recovery machinery. The patent teaches physically
manipulating the temperature profile within the column to obtain
desired separation results.
[0011] U.S. Pat. No. 4,164,452, issued to Funk, discusses a
pressure responsive fractionation control system and method for
utilizing various controllers and simulation techniques. This is
applies to a propane recovery system that includes a de-methanizer
tower followed by a de-ethanizer tower.
[0012] It would be advantageous to develop a new method and
apparatus that provides improvement in the recovery of the valuable
NGL products while minimizing energy requirements. It would be
advantageous to allow for the optimization of the process variables
within allowable quality variations and equipment constraints while
minimizing the electricity or energy usage.
SUMMARY OF THE INVENTION
[0013] The current invention is an empirical optimization method
based on statistical modeling relating NGL plant process variables
with the refrigeration system's electricity usage. The method
identifies the key process control variables in an NGL plant to be
optimized. This method is applicable to an NGL plant that uses dual
refrigeration systems. It describes methods to calculate the key
optimal targets for the process control settings. These key optimal
targets can be fed to a multivariable controller algorithm that
controls the NGL plants, or can be implemented directly by the NGL
plant operators inputting the calculated optimal targets in the NGL
plant's distributed control system (DCS).
[0014] The process in consideration is an NGL plant that uses dual
refrigeration systems. The optimization is done by developing
empirical statistical models relating NGL plant process variables
with the compressors' electricity load. These models are then used
to calculate optimum values for the process control settings. These
optimal targets can be fed to a multivariable controller algorithm
that controls the NGL plants, or can be implemented directly by the
NGL plant operators inputting set-points in distributed control
systems (DCS).
[0015] The method herein describes a strategy to optimize the dual
refrigeration system compressors to minimize the electricity
consumption while maintaining the NGL recovery at a desired level.
This is done by modeling the plant using experimental design. The
method is also applicable to systems where only one refrigeration
system is available. In this case, the benefit is less because the
number of degrees of freedom is reduced.
[0016] The methodology described herein optimizes the control of
load distribution in the NGL chillers to minimize compressor power
usage versus NGL recovery.
[0017] The present invention advantageously includes a method to
improve the efficiency of NGL product recovery by utilizing
empirical mathematical representations of the NGL plant, capturing
the behavior of the different process equipments.
[0018] The method herein describes optimizing the usage of dual
refrigeration systems. The method manages the load distribution in
the chill-down equipments to allow for increasing feed processing
capacity, best recovery conditions, and optimizing the electrical
energy. This method would allow the optimization of a dual
refrigeration system to minimize the electricity consumption while
maintaining the NGL recovery at desired level. This is done
empirically by carrying out plant tests in operating NGL plants and
is based on design of experiment.
[0019] A method is described for controlling the load distribution
in the NGL plant chilldown equipments which will result in an
optimal temperature profile to achieve the desired NGL recovery
level with minimal power required by the refrigeration system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] So that the manner in which the features, advantages and
objects of the invention, as well as others which will become
apparent, may be understood in more detail, more particular
description of the invention briefly summarized above may be had by
reference to the embodiment thereof which is illustrated in the
appended drawings, which form a part of this specification. It is
to be noted, however, that the drawings illustrate only a preferred
embodiment of the invention and is therefore not to be considered
limiting of the invention's scope as it may admit to other equally
effective embodiments.
[0021] FIG. 1 shows one preferred embodiment of the gas plant of
the invention. The refrigeration systems shown separately as part
of FIG. 1 integrate into the entire plant as discussed in the
following description.
[0022] FIG. 2 shows one preferred statistical optimization
embodiment for implementing the optimization scheme discussed in
the following description. The Statistical Optimizer 1 entitled
RT-MSPC resides in a computer where optimization programs and
process models are used to calculate. These targets are then
communicated electronically to the NGL plant Decentralized Control
System 3, also called "DCS", or to plant Multivariable Control
System 2, also called "MPC", which in turn sends control signals to
the NGL plant's actuators.
[0023] FIG. 3 shows one preferred statistical optimizer embodiment
with steps for either online or offline use of the NGL
Optimizer.
[0024] FIG. 4 and FIG. 5 show the process variables trends with
normalized electricity cost and suction pressures been manipulated
during the design of the experiment.
DETAILED DESCRIPTION
[0025] The invention includes a method for optimizing the
production of NGL product stream 303 from a Natural Gas Liquids
("NGL") plant. The invention provides a method for optimizing the
utilization of the available refrigeration capacity. The method
honors process equipment and product quality constraints such as
the NGL product specification, an upper limit of the percent of
ethane and propane (mole percent) in the residue gases, a maximum
pressure drop across the top section of the demethanizer column and
a predetermined operating range for the refrigerant compressors
suction pressures. Natural gas feed stream 9 is fed to first
chilling unit 12 to produce chilled rich gas stream 37 and chilled
liquid stream 36. Pressure and flow monitoring devices are useful
for determining or controlling the pressure and flow of the feed
stream. Residue gas stream 31, in combination with other residue
from the demethanizer 200, is collected as sales gas. Pressure of
stream 31 is measured and monitored. Valve 131 on stream 31 is used
to control the unit pressure. Flow of stream 31 is measured,
typically after valve 131. Chilled rich gas stream 37 and chilled
liquid stream 36 have different compositions as a result of
separation of natural gas feed stream 9. Natural gas feed stream 9
contains sweet gas that has been submitted to a sweetening process
to remove hydrogen sulfide and carbon dioxide. Natural gas stream 9
is dehydrated in molecular sieve beds to reduce moisture levels.
Natural gas feed stream 9 is preferably in a pressure range of
200-1000 psig or is compressed to reach this range. Chilled gas
stream 37 is fed to second chilling unit 18 to produce second
chilled gas stream 92 and second chilled liquid stream 91. The
second chilled gas stream 92 is fed to the third chilling unit 22
to produce third chilled liquid stream 116.
[0026] In an embodiment of the invention, a method is provided for
optimizing the production of NGL outlet stream 303. The invention
includes maximizing the utilization of the available refrigeration
capacity while minimizing the operating column pressure and
temperature of the demethanizer column. Similarly, propane losses
to the overhead product from the demethanizer are minimized. This
is achieved within the constraints of the NGL bottom product
specification, an upper constraint of the percent of propane (mole
percent) in the residue gases, a maximum pressure drop across the
top section of the demethanizer column and a predetermined
operating range for the propane and ethane refrigerant compressors
suction pressure. Natural gas feed stream 9 is fed to first
chilling unit 12 to produce chilled rich gas stream 37 and chilled
liquid stream 36. Pressure and flow monitoring devices are useful
for determining or controlling the pressure and flow of the feed
stream. Lean gas stream 31 can also be removed and, alone or in
combination with other residue from the demethanizer, be collected.
Pressure of stream 31 is measured through a pressure monitor. Valve
131 on stream 31 is useful to control the unit pressure. Flow of
stream 31 is measured, typically after valve 131. Chilled rich gas
stream 37 and chilled liquid stream 36 have different compositions
as a result of separation of natural gas feed stream 9. Natural gas
feed stream 9 preferably contains sweet gas, such as gas that has
been submitted to a sweetening process to remove hydrogen sulfide
or carbon dioxide. Natural gas stream 9 is preferably dehydrated in
molecular sieve beds to reduce moisture levels to less than 1 ppm
water. Natural gas feed stream 9 is preferably in a pressure range
of 800-1000 psia or is compressed to reach this range. Chilled rich
gas stream 37 is fed to second chilling unit 18 to produce second
chilled rich gas stream 92 and second chilled liquid stream 91,
also having different compositions. Second chilled rich gas stream
92 is fed to third chilling unit 22 to produce third chilled liquid
stream 116.
[0027] The three liquid streams, namely, chilled liquid stream 36,
second chilled liquid stream 91 and third chilled liquid stream
116, are fed to the demethanizer column 200. Demethanizer column
200 produces overhead stream 201 and bottoms stream 202.
Demethanizer column 200 is a trayed column. Bottoms stream 202 is
controlled to a specified bottoms product specification. The
overhead stream 201 is characterized by an overhead ethane and
propane concentration. The overhead valve 32 can be described by
the percent open of the valve, with 100% being fully open. It can
also be described by pressure drop, such as PSI.
[0028] In an embodiment of the invention, the three liquid streams,
namely, chilled liquid stream 36, second chilled liquid stream 91
and third chilled liquid stream 116, are fed to demethanizer column
200. Demethanizer column 200 produces overhead stream 201 and
bottoms stream 202. Demethanizer column 200 has a top tray 33 in an
upper section of the demethanizer column and mid-tray 44 in a
middle section of the demethanizer column. The top tray defines a
top tray temperature and the demethanizer defines a column
operating pressure. Bottoms stream 202 is characterized by having a
bottom ratio defined by methane concentration of the bottom stream
divided by ethane concentration of the bottom stream. The overhead
stream is characterized by having an overhead propane
concentration.
[0029] Demethanizer column 200 overhead stream 201 is fed through
an overhead valve 32. This valve 32 is used to control the
operating pressure of the Demethanizer column 200.
[0030] In an embodiment of the invention, overhead stream 201 is
fed through an overhead valve 32 at overhead valve outlet pressure
or operating pressure. The overhead valve 32 can be described by
the percent open of the valve, with 100% being fully open. It can
also be described by pressure drop, such as PSI.
[0031] First propane refrigeration system 34 is operated to provide
cooling to first chiller 30, second chiller 70 and third chiller
80. The first chilling unit 12 includes first chiller 30 and first
chill down separator 38. The second chilling unit 18 includes
second chiller 70, third chiller 80 and separator 90. The third
chilling unit 22 includes fourth chiller 105 and separator 115. The
fourth chiller is refrigerated by ethane refrigeration system
64.
[0032] First propane refrigeration system 34 includes propane
compressor 800A. The propane compressor defines a propane
compressor power output and the propane compressor suction
pressures.
[0033] Second propane refrigeration system 54 is operated to
provide cooling to the same equipment as system 34. It can be
implemented in parallel with first propane refrigeration system 34
that can be operated independently, or it can be used as a backup
system when the first propane refrigeration system 34 is out of
service. Second propane refrigeration system 54 includes a second
propane compressor 800B. Second propane compressor 800B defines a
second propane compressor power output and a second propane
compressor suction pressure.
[0034] Second chilling unit 18 includes second chiller 70, second
chill down separator 90 and the third chiller 80. In an embodiment,
the second chill down separator 90 defines a second chill down
separator temperature, and the subsequent second chiller 80 defines
a subsequent second chiller output level. Level instruments are
installed in second chiller 70 and subsequent second chiller
80.
[0035] Ethane refrigeration system 64 provides heat exchange to
fourth chiller 105. Ethane refrigeration system 64 includes ethane
compressor 900. Ethane compressor 900 defines an ethane compressor
suction pressure. One preferred embodiment includes controlling the
compressor suction pressure of the ethane compressor 900, which in
turn controls the heat exchange to the chilled gas stream 92.
[0036] The method of the invention includes optimizing
refrigeration load while maintaining the bottom product
specification. This optimization is also performed while staying
within a prescribed range of the overhead ethane and propane
concentration. The refrigeration load is defined as the electricity
required, or similar energy requirements, to operate the first
propane refrigeration system 34, the second propane refrigeration
system 54 and the ethane refrigeration system 64. The current
invention is an empirical optimization method based on statistical
modeling relating NGL plant process variables with the
refrigeration system's electricity usage. The method identifies the
key process control variables in an NGL plant to be optimized. This
method is applicable to an NGL plant that uses dual refrigeration
systems. It describes methods to calculate the key optimal targets
for the process control settings. These key optimal targets can be
fed to a multivariable controller algorithm (such as MPC) that
controls the NGL plants, or can be implemented directly by the NGL
plant operators inputting the calculated optimal targets in the NGL
plant's distributed control system (DCS).
[0037] Model Predictive Control, ("MPC), is an advanced control
method for process industries that improves on standard feedback
control by predicting how a process, such as distillation, will
react to inputs such as heat input. This means that reliance on
feedback can be reduced since the effects of inputs will be derived
from mathematical empirical models. Feedback can still used to
correct for model inaccuracies. The MPC controller relies on an
empirical model of a process obtained, for example, by plant
testing to predict the future behavior of dependent variables of a
dynamic system based on past moves of independent variables. It
usually relies on linear models of the process. Commercial
suppliers of MPC software useful in this invention include
AspenTech (DMC+), Honeywell (RMPCT) and Shell Global Solutions
(SMOC).
[0038] In an embodiment of the invention, the method further
includes maintaining the propane compressor power output within a
predetermined propane compressor power output range while
continuing to minimize the refrigeration load and maintain the
bottom ratio within a predetermined bottom ratio range. Thus, the
minimum refrigeration load can vary from the embodiment where the
propane compressor power output is not limited given that the
propane compressor power output is an additional constraint on the
system. As with all constraints to the optimization, the embodiment
is preferably calculated using rigorous simulation techniques in
combination with optimization algorithms. Alternately, simulation
using historical data can be used. A combination of rigorous
simulation corrected by historical data information can also be
used. The simulation solution is preferably implemented using
control systems. In a preferred embodiment, real time dynamic
simulation is used. These models are then used in conjunction with
an optimization package to perform real time calculations of the
process variables targets. Advantageously, the invention allows for
the increase of NGL production using a real time optimization of
the liquefied natural gas plant. This can be accomplished by
developing mathematical models of all the process units from
plants, tests, and laboratory experiments. This simulation can be
used in real time to receive information from monitoring equipment
in the liquefied natural gas plant which is then used to simulate
the plant and calculate the optimization with manipulation of the
identified control variables. Upon determination of the setting of
the control variables in order to optimize the conditions, the
plant can be modified preferably through the use of dynamic
controllers. The calculations can be repeated to verify
optimization with the new operating parameters. Such repetition can
be as needed or on a regular basis. One example would be gather
dynamic data, perform the simulation calculations and optimizations
and provide instructions to controller means every four
minutes.
[0039] The current invention is also applicable to an NGL plant
with a single refrigeration system by using the same empirical
optimization method based on statistical modeling relating NGL
plant process variables with the refrigeration system's electricity
usage. The method identifies the key process control variables in
an NGL plant to be optimized.
[0040] In a preferred embodiment of the invention, the method
includes optimizing the suction pressures of the compressors which
in turn impact the power usage of the compressors. This preferred
embodiment of the invention allows maintaining the constraints of
the compressors and the associated process variables within
allowable ranges while at the same time continues to minimize the
refrigeration load and maintain the Demethanizer Column bottom
products specifications.
[0041] In a preferred embodiment of the invention, the method
includes maintaining the propane compressor power output within a
predetermined propane compressor power output range while
continuing to minimize the refrigeration load and maintain the
Demethanizer Column bottom products specifications. Thus, the
minimum refrigeration load can vary from the embodiment where the
propane compressor power output is not limited given that the
propane compressor power output is an additional constraint on the
system. All target process variable values are preferably
determined using experimental techniques in combination with
optimization algorithms. The resulting solution is preferably
implemented using modern electronic control systems.
Advantageously, the invention allows for the increase of NGL
production using a real time optimization of the NGL plant. The
invention also advantageously minimizes the energy use of the
refrigeration system. This can be accomplished by developing
mathematical models of all the process units from data collected
via plants tests. The plant tests are performed by varying the
process operating conditions and collecting the operating data in
electronic media for analysis and modeling.
[0042] The method of the invention includes minimizing
refrigeration load while maintaining the bottom ratio within a
predetermined bottom ratio range to meet specification. This
minimization is also performed while maintaining the overhead
propane concentration within a predetermined overhead propane
concentration range. The refrigeration load is defined as the
electricity required, or similar energy requirements, to operate
the first propane refrigeration system 34, the second propane
refrigeration system 54 and the ethane refrigeration system 64.
This minimization is calculated through simulation of the LNG
plant. The simulation is then optimized using numerical methods
known in the art to obtain the minimum refrigeration load while
maintaining the product streams with the appropriate
characteristics such that specifications are met. The method
described herein uses mathematical models obtained from process
testing and optimization method to drive the process to the best
economic conditions. Simulation can be used to determine the
optimum levels and control systems operable to implement such
levels can be used.
[0043] In a preferred embodiment of the invention, the method
further includes maintaining the propane compressor power output
within a predetermined propane compressor power output range while
continuing to minimize the refrigeration load and maintain the
bottom ratio within a predetermined bottom ratio range. Thus, the
minimum refrigeration load can vary from the embodiment where the
propane compressor power output is not limited given that the
propane compressor power output is an additional constraint on the
system. As with all constraints to the optimization, the embodiment
is preferably calculated using rigorous simulation techniques in
combination with optimization algorithms. Alternately, simulation
using historical data can be used. A combination of rigorous
simulation corrected by historical data information can also be
used. The simulation solution is preferably implemented using
control systems. In a preferred embodiment, real time dynamic
simulation is used. These models are then used in conjunction with
an optimization package to perform real time calculations of the
process variables targets. Advantageously, the invention allows for
the increase of NGL production using a real time optimization of
the liquefied natural gas plant. This can be accomplished by
developing mathematical models of all the process units from
plants, tests, and laboratory experiments. This simulation can be
used in real time to receive information from monitoring equipment
in the liquefied natural gas plant which is then used to simulate
the plant and calculate the optimization with manipulation of the
identified control variables. Upon determination of the setting of
the control variables in order to optimize the conditions, the
plant can be modified preferably through the use of dynamic
controllers. The calculations can be repeated to verify
optimization with the new operating parameters. Such repetition can
be as needed or on a regular basis. One example would be gather
dynamic data, perform the simulation calculations and optimizations
and provide instructions to controller means every four
minutes.
[0044] In a preferred embodiment, the optimization variables in the
NGL plant are to be selected via statistical design of experiment
(DOE) which allows the selection of the variables that have the
most impact on the energy use of the refrigeration systems.
[0045] In another embodiment, the experimental data obtained using
DOE are analyzed using multivariate statistical process control
(MVSPC) techniques to determine the principal components analysis
(PCA) models. Several commercially available tools of PCA are
available such as MATLAB from MATHWORKS, Inc. or SIMCA-P from
UMETRICS, Inc.
[0046] In this embodiment, the energy load of the refrigeration
system utilized in the NGL process described earlier is optimized
using the statistical modeling techniques can be minimized while
maintaining product qualities.
[0047] In another preferred embodiment of the invention, the method
further includes maintaining the refrigeration compressors suction
pressures within the predetermined refrigeration compressors
suction pressures ranges while minimizing the refrigeration load
and maintaining the bottom ratio within a predetermined bottom
ratio range. The refrigeration compressor suction pressure ranges
are typically the manufacturers' recommended ranges defined to
avoid surging, stonewalling or mechanical damage to the
compressors. In yet another alternate embodiment, the compressor
power usage can also be maintained within predetermined limits as
described above.
[0048] In another preferred embodiment of the invention, the method
further includes maintaining the propane compressor suction
pressure within a predetermined propane compressor suction pressure
range while minimizing the refrigeration load and maintaining the
bottom ratio within a predetermined bottom ratio range. The propane
compressor suction pressure range is typically the manufacturers'
recommended range defined to avoid surging, stonewalling or
mechanical damage to the compressors. In yet another alternate
embodiment, the propane compressor power output can also be
maintained within predetermined limits as described above. In yet
another alternate embodiment, the constraints discussed below can
also be used in conjunction with the restraints of this embodiment.
It is recognized that the larger the number of constraints, the
more difficult and time consuming it is to solve the optimization
algorithm. It is also recognized that it is possible to provide
constraints that allow for no solution set. Ranges and limits for
constraints for all embodiments of the current invention must allow
for a possible solution while taking into account the
constraints.
[0049] In an alternate embodiment, the method further includes
maintaining a propane compressor scraper output pressure within a
predetermined propane compressor output pressure range while
minimizing the refrigeration load and maintaining the bottom ratio
within a predetermined bottom ratio range.
[0050] In another alternate embodiment, the method further includes
maintaining the first tray temperature within a predetermined first
tray temperature range while minimizing the refrigeration load and
maintaining the bottom ratio within a predetermined bottom ratio
range.
[0051] In another alternate embodiment, the method further includes
maintaining the overhead valve output pressure of the overhead
stream within a predetermined overhead valve output pressure range
while minimizing the refrigeration load and maintaining the bottom
ratio within a predetermined bottom ratio range. It is known in the
art of multivariable control to use multivariable control so that
the constraints can be maintained. In a preferred embodiment, the
multivariable controllers are the preferred means to maintain the
process constraints within the specified engineering ranges. The
empirical optimization described in this invention provides the
optimization techniques for the refrigeration compressors and
provide the optimal targets for the optimization variables and
these targets can be sent to the multivariable controllers which
drive the process to reach these targets.
[0052] In another alternate embodiment, the method further includes
maintaining the first chiller output level within a predetermined
first chiller output level range while minimizing the refrigeration
load and maintaining the bottom ratio within a predetermined bottom
ratio range.
[0053] In yet another embodiment, the efficiency of the NGL process
is calculated and named the Coefficient of Performance (COP).
Statistical models relating the optimization variables and the
coefficient of performance (COP) are developed. The optimization
tools in the solution are then used to calculate the optimal
targets for the optimization variables in order to optimize the
COP. One formulation of COP for the NGL plant is the ratio of the
amount of heat removed from the feed gas to the total power
input.
[0054] In yet another embodiment, the heat duty for each stage of
the cooling process can be computed and used for the overall
optimization. Then the models relating the statistical optimizer
variables to these heat duties are developed and used to calculate
the targets for the optimization solution. The heat duty for each
stage of the cooling process can be computed either from the
process side or the refrigeration side.
[0055] In yet another embodiment, an alternative method to actually
carry out the experiments obtained from the design of experiment
(DOE) can be substituted using NGL plant simulators. Examples of
this type of simulators are rigorous steady-state or dynamic or
operator training simulators.
[0056] In a preferred embodiment, the statistical optimizer
disclosed in the present invention can be used as an on-line
optimizer, which continuously sends targets to the process control
systems.
[0057] In a preferred embodiment, the statistical optimizer
disclosed in the present invention can be used as an off-line
advisory statistical optimizer, which computes the optimal targets
of the NGL plant and advises the operators on the best settings in
order to achieve the optimal economic benefits.
[0058] In a preferred embodiment, the statistical optimizer
disclosed in the present invention can be used as a design tool to
compute the optimal targets for design purposes using a high
fidelity simulator.
[0059] In another alternate embodiment, the method further includes
maintaining the ethane compressor suction pressure within a
predetermined ethane compressor suction pressure range while
minimizing the refrigeration load and maintaining the bottom ratio
within a predetermined bottom ratio range.
[0060] In yet another alternate embodiment, the method further
includes maintaining the ethane compressor power output within a
predetermined ethane compressor power output range while minimizing
the refrigeration load and maintaining the bottom ratio within a
predetermined bottom ratio range. In another preferred alternate
embodiment, the method further includes maintaining an overhead
temperature of the overhead stream within a predetermined overhead
temperature range while minimizing the refrigeration load and
maintaining the bottom ratio within a predetermined bottom ratio
range.
[0061] In yet another alternate embodiment, the method further
includes maintaining the first chill down separator temperature
within a predetermined first chill down separator temperature range
while minimizing the refrigeration load and maintaining the bottom
ratio within a predetermined bottom ratio range. In yet another
alternate embodiment, the method further includes maintaining a
second propane exchanger temperature of a second propane exchanger
58 within a predetermined second propane exchanger temperature
range while minimizing the refrigeration load and maintaining the
bottom ratio within a predetermined bottom ratio range.
[0062] In yet another alternate embodiment, the method further
includes maintaining a second propane compressor suction pressure
within a predetermined second propane compressor suction pressure
range while minimizing the refrigeration load and maintaining the
bottom ratio within a predetermined bottom ratio range. In yet
another alternate embodiment, the method further includes
maintaining the second propane compressor power output within a
predetermined second propane power output range while minimizing
the refrigeration load and maintaining the bottom ratio within a
predetermined bottom ratio range.
[0063] In yet another alternate embodiment, the method further
includes maintaining the primary second chiller output level within
a predetermined primary second chiller output level while
minimizing the refrigeration load and maintaining the bottom ratio
within a predetermined bottom ratio range. In yet another alternate
embodiment, the method further includes maintaining the subsequent
second chiller output level within a predetermined subsequent
second chiller output level range while minimizing the
refrigeration load and maintaining the bottom ratio within a
predetermined bottom ratio range.
[0064] In yet another alternate embodiment, the method further
includes maintaining the second chill down separator 90 within a
second chill down separator temperature range while minimizing the
refrigeration load and maintaining the bottom ratio within a
predetermined bottom ratio range.
[0065] The following examples are meant to illustrate in detail
manipulated variables and controlled variables according to
embodiments of the invention, but are in no way meant to limit the
scope of the invention. TABLE-US-00001 low High Equip or
Manipulated variables Units limit limit Stream Unit set point back
pressure PSIG 330 500 31 First chiller set point level % 11 12 30
Second chiller set point level % 20 35 70 Third chiller set point
level % 20 35 80 Final chiller set point level % 20 35 105 Propane
compressor A third section % 15 90 804-A scraper output pressure
Propane compressor B third section % 15 90 804-B scraper output
pressure DeCH4 overhead set point pressure PSIG 160 170 201 DeCH4
tray six temperature DEGF -55 -20 151 DeCH4 overhead bypass % 0 90
91 DeCH4 propane reboiler temperature % 0 75 151
[0066] TABLE-US-00002 Equip low hi or Controlled variables Units
limit limit Stream Unit feed pressure PSIG 350 500 31 Unit feed
flow MMSCFD 340 500 9 Unit pressure output % 20 97 31 First
separator temperature DEGF 63 80 35 First chiller output level % 20
95 30 Second chiller output level % 20 95 70 Third chiller output
level % 20 95 80 Final chiller output level % 20 95 105 Propane
compressor first section PSIG 14.7 23.8 801-A scraper output
pressure propane compressor 2nd section PSIG 15 40.7 802-A scraper
output pressure propane compressor A section PSIG 60 110 803-A
pressure propane compressor B section PSIG 60 110 803-B pressure
Ethan compressor suction PSIG 10 28 901 pressure propane compressor
A Amperes AMP 300 760 804-A Propane compressor B Ampere AMP 300 760
804-B Ethane compressor Ampere AMP 1500 4000 901 DeCH4 overhead
valve output % 10 95 201 pressure DeCH4 overhead temperature DEGF
-142 -70 201 Second exchanger temperature DEGF 85 150 65 DeCH4
overhead pressure drop PSID 0.2 0.9 201 DeCH4 tray six output TEMP
% 15 90 151 DeCH4 first tray TEMP DEGF 8 35 151 DeCH4 overhead
propane % 0.07 1.4 201 quality DeCH4 bottom ratio quality RATIO 0.4
2.5 303
[0067] The present invention also includes an apparatus
corresponding to the method of the invention. A preferred
embodiment of the apparatus is shown in FIG. 1. First propane
refrigeration system 34, second propane refrigeration system 54 and
ethane refrigeration system 64 are shown separately for clarity.
These refrigeration systems integrate with the process equipment of
the invention to provide heat exchange. The apparatus is a
liquefied natural gas plant for maximizing the production of NGL
from an inlet gas feed stream and includes means for controlling
specific portions of the plant within constraints.
[0068] The liquefied natural gas plant includes first chilling unit
12 for cooling at least a portion of the inlet gas feed stream by
heat exchange contact with first and second expanded refrigerants
to produce chilled rich gas stream 37 and chilled liquid stream 36
from first chilling unit 12. Chilled rich gas stream 37 and chilled
liquid stream 36 have different compositions as a result of the
separation. First chilling unit 12 includes first chiller 30 and
first chill down separator 38. First chiller 30 defines a first
chiller output level, and first chill down separator 38 defines a
first chill down separator temperature.
[0069] The liquefied natural gas plant also includes second
chilling unit 18 that receives chilled rich gas stream 37. Second
chilling unit 18 further chills the chilled rich gas stream 37 to
produce second chilled rich gas stream 92 and second chilled liquid
stream 91. Second chilled rich gas stream 92 and second chilled
liquid stream 91 have different compositions. Second chilling unit
18 includes second chill down separator 90, primary second chiller
70 and subsequent second chiller. Second chill down separator 90
defining a second chill down separator temperature. Subsequent
second chiller defines a subsequent second chiller output
level.
[0070] The liquefied natural gas plant includes third chilling unit
22, which receives second chilled rich gas stream 92. Third
chilling unit 22 includes third chiller 105 and is operable to
further chill second chilled rich gas 92 to produce third chilled
liquid stream 116.
[0071] Demethanizer column 200 receives chilled liquid stream 36,
second chilled liquid stream 91, and third chilled liquid stream
116 as feed streams to the column. Demethanizer column 200
producing overhead stream 201 and bottoms stream 202. Demethanizer
column 200 has top tray 33 in an upper section of the demethanizer
column 200 and mid-tray 44 in a middle section of demethanizer
column 200. The top tray has a top tray temperature that can be
monitored. The bottoms stream has a bottom ratio defined by methane
concentration of the bottom stream divided by ethane concentration
of the bottom stream. This bottom ratio can also be monitored. The
overhead stream defines an overhead propane concentration, which
can also be monitored, as can the other measured or constrained
properties. Overhead valve 32 receives overhead stream 201. The
overhead valve has an overhead valve outlet pressure and thereby
sets the pressure of the overhead stream.
[0072] First propane refrigeration system 34 is operable to provide
heat exchange with first chilling unit 12. First propane
refrigeration system 34 includes propane compressor 800 and can be
a typical propane refrigeration cycle. The propane compressor
defines a propane compressor power output and a propane compressor
suction pressure.
[0073] Second propane refrigeration system 54 is operable to
provide heat exchange to second chilling unit 18. Second propane
refrigeration system 54 includes second propane compressor 56.
[0074] Ethane refrigeration system 64 is operable to provide heat
exchange to third chilling unit 22. The ethane refrigeration system
has an ethane compressor 900. The ethane compressor defines an
ethane compressor suction pressure.
[0075] The liquefied natural gas plant includes means for
minimizing a refrigeration load while maintaining the bottom ratio
within a predetermined bottom ratio range and while maintaining the
overhead propane concentration within a predetermine overhead
propane concentration range, hereinafter referred to simply as
"means for minimizing refrigeration load". This means for
minimizing refrigeration load can include one or more controllers.
The refrigeration load is the electricity required to operate first
propane refrigeration system 34, second propane refrigeration
system 54 and ethane refrigeration system 64.
[0076] In another preferred embodiment, the liquefied natural gas
plant also includes means for maintaining the propane compressor
power output within a predetermined propane compressor power output
range. This is in addition to the means for minimizing
refrigeration load, which includes controlling the bottom ratio and
controlling the overhead propane concentration as described above.
This can also be in addition to any additional controlling means
discussed below or in the absence of such other controlling means.
Each of the controlling means discussed can be used alone or in
conjunction with each other.
[0077] In another preferred embodiment, the liquefied natural gas
plant also includes means for maintaining the propane compressor
suction pressure within a predetermined propane compressor suction
pressure range in addition to the means for minimizing the
refrigeration load. In yet another preferred embodiment, the
liquefied natural gas plant includes means for maintaining a
propane compressor scraper output pressure within a predetermined
propane compressor output pressure range.
[0078] In an alternate preferred embodiment, the liquefied natural
gas plant includes means for maintaining the first tray temperature
within a predetermined first tray temperature range. In another
embodiment, the liquefied natural gas plant includes means for
maintaining the overhead valve output pressure of the overhead
stream within a predetermined overhead valve output pressure
range.
[0079] In another embodiment, the liquefied natural gas plant
includes means for maintaining the first chiller output level
within a predetermined first chiller output level range. In yet
another embodiment, the liquefied natural gas plant includes means
for maintaining the ethane compressor suction pressure within a
predetermined ethane compressor suction pressure range.
[0080] In an embodiment, the liquefied natural gas plant includes
means for maintaining ethane compressor power output within a
predetermined ethane compressor power output range. In another
embodiment, the liquefied natural gas plant includes means for
maintaining an overhead temperature of the overhead stream within a
predetermined overhead temperature range. In still another
embodiment of the liquefied natural gas plant, the plant includes
means for maintaining the first chill down separator temperature
within a predetermined first chill down separator temperature
range. In an alternate embodiment, the liquefied natural gas plant
includes means for maintaining a second propane exchanger
temperature of a second propane exchanger 58 within a predetermined
second propane exchanger temperature range.
[0081] In another embodiment of the invention, the liquefied
natural gas plant includes means for maintaining a second propane
compressor suction pressure within a predetermined second propane
compressor suction pressure range. Another embodiment of the
liquefied natural gas plant includes means for maintaining the
second propane compressor power output within a predetermined
second propane compressor power output range. The liquefied natural
gas plant of claim includes means for maintaining the primary
second chiller output level within a predetermined primary second
chiller output level in another embodiment. The liquefied natural
gas plant includes means for maintaining the subsequent second
chiller output level within a predetermined subsequent second
chiller output level range in yet another embodiment.
[0082] In an alternate embodiment, the liquefied natural gas plant
includes means for maintaining the second chill down separator 90
within a second chill down separator temperature range.
[0083] Natural gas stream 9 is preferably a sweet gas, such as one
that has been submitted to a sweetening process. Dehydration of the
natural gas is also a common and desirable treatment. Molecular
sieve beds are commonly used to reduce moisture levels. The
moisture is preferably reduced to less than 1 ppm H.sub.2O by
volume.
[0084] The natural feed gas is cooled and chilled as described
above to a preselected temperature range, with a preferred
preselected temperature range being -80 to -120 degrees F. This
cooling/chilling can be accomplished by using refrigerants of
propane and ethane in the first propane refrigeration system,
second propane refrigeration system and ethane refrigeration
system. Multiple levels of propane and ethane temperatures are
appropriate. For example, propane refrigerants can be at 66 degrees
F. and 12 degrees F. for the first propane refrigeration system and
second refrigeration system respectively while ethane can be at -39
degrees F. This results in a high pressure residue gas and three
liquid streams of different compositions as described herein.
[0085] Overhead stream 201 is compressed to become residue gas
stream 42, which is a sales gas stream. In another embodiment (not
shown), the overhead stream 201 can be split, with compression
before or after the split, to produce the residue gas stream and a
recycle stream that is recycled into the demethanizer or other
unit. In an alternate embodiment, the overhead stream of the column
is low pressure residue gas, which can be combined with the high
pressure residue gas to produce a sales gas.
[0086] Bottom stream 202 can be split to provide NGL outlet stream
303. When alternate heat sources are available to the bottom of the
demethanizer and/or a stream containing at least partial vapor is
fed to the bottom of the demethanizer, then the entire bottom
stream 202 can be removed as NGL product. The three liquid streams
provide feed stream for the demethanizer column from which the NGL
product is drawn from the bottom.
[0087] Implementation of the invention advantageously includes
commercially available multivariable controllers. AspenTech
DMCPlus.TM. is one multivariable controller useful in
implementation of the invention. Process models are developed and
the control strategy implemented. The current invention results in
maximized incremental feed rate to the apparatus and maximized
yield of the valuable NGL product.
[0088] As an advantage of the present invention, the method and
apparatus of the invention allow for the optimization of the usage
of the propane and ethane refrigeration systems. The invention also
advantageously allows for managing the natural feed gas
distribution in between the chill-down units to allow for maximum
feed processing and best recovery conditions. The present invention
allows for the optimizing of the recovered NGL qualities so there
is less quality "give-away". In turn, this enhances the product
quality. The invention also advantageously allows for optimizing of
the electrical energy used by the refrigeration systems. The
invention also can allow for a greater throughput and decreased
over all power consumption. Furthermore, the method when using
automatic control feedback loop requires less intervention by a
console operator.
[0089] While the invention has been shown or described in only some
of its forms, it should be apparent to those skilled in the art
that it is not so limited, but is susceptible to various changes
without departing from the scope of the invention. For example,
this invention may be used in process design but is also useful in
conjunction with an existing process plant. This invention is
useful as a steady state tool and also for real time
optimization.
[0090] For example, splitters can be added to redirect amounts of
flow or to allow for control of amounts of flow. Recycle streams
can be used to enhance recovery or as a heat since for heat
exchangers. Other variation can also be made.
* * * * *