U.S. patent application number 14/365171 was filed with the patent office on 2014-10-23 for system and method for producing a liquefied hydrocarbon stream and method of operating a compressor.
The applicant listed for this patent is SHELL INTERNATIONAL RESEARCH MAATSCHAPPIJ B.V., SHELL OIL COMPANY. Invention is credited to Michiel Gijsbert Van Aken.
Application Number | 20140311183 14/365171 |
Document ID | / |
Family ID | 47358181 |
Filed Date | 2014-10-23 |
United States Patent
Application |
20140311183 |
Kind Code |
A1 |
Van Aken; Michiel Gijsbert |
October 23, 2014 |
SYSTEM AND METHOD FOR PRODUCING A LIQUEFIED HYDROCARBON STREAM AND
METHOD OF OPERATING A COMPRESSOR
Abstract
At least part of a fluid is compressed in a compressor driven by
an electric motor. The compressor has variable inlet guide vanes of
which an angle can be adjusted. The electric motor is powered using
a power supply network, and a signal representative of a condition
of the power supply network is monitored. From the signal, it is
automatically determined whether additional load shedding is
needed, by comparing the signal to a predetermined criterion. The
variable inlet guide vanes angle is automatically adjusted when the
criterion is satisfied and additional load shedding is needed. This
automatically reduces the loading of the compressor. The compressor
and the method of operating it may be employed as part of a system
for producing a liquefied hydrocarbon stream and/or in the course
of producing a liquefied hydrocarbon stream, in which case the
compressor can be a refrigerant compressor and the fluid a
refrigerant fluid.
Inventors: |
Van Aken; Michiel Gijsbert;
(The Hague, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SHELL INTERNATIONAL RESEARCH MAATSCHAPPIJ B.V.
SHELL OIL COMPANY |
The Hague
Houston |
TX |
NL
US |
|
|
Family ID: |
47358181 |
Appl. No.: |
14/365171 |
Filed: |
December 13, 2012 |
PCT Filed: |
December 13, 2012 |
PCT NO: |
PCT/EP2012/075314 |
371 Date: |
June 13, 2014 |
Current U.S.
Class: |
62/611 ;
417/53 |
Current CPC
Class: |
F04D 27/002 20130101;
F25J 1/0022 20130101; F04D 27/0246 20130101; F25J 2280/20 20130101;
F25J 1/0298 20130101; F25J 1/0284 20130101 |
Class at
Publication: |
62/611 ;
417/53 |
International
Class: |
F25J 1/00 20060101
F25J001/00; F04D 27/00 20060101 F04D027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 15, 2011 |
EP |
11193688.6 |
Claims
1. A method of producing a liquefied hydrocarbon stream,
comprising: circulating a refrigerant fluid through a refrigerant
circuit, comprising compressing at least part of the refrigerant
fluid in a refrigerant compressor driven by an electric motor
wherein the refrigerant compressor comprises variable inlet guide
vanes of which an angle compared to a reference position can be
adjusted; removing heat from an initially vaporous hydrocarbon
stream thereby condensing at least part of the initially vaporous
hydrocarbon stream to form the liquefied hydrocarbon stream, said
removing heat comprising heat exchanging at least said part of the
initially vaporous hydrocarbon stream against at least part of the
refrigerant fluid circulating through the refrigerant circuit;
powering the electric motor using a power supply network;
monitoring a signal representative of a condition of the power
supply network; automatically determining from the signal whether
additional load shedding is needed by comparing the signal to a
predetermined criterion; and, automatically adjusting the variable
inlet guide vanes angle thereby reducing the loading of the
refrigerant compressor when the criterion is satisfied and
additional load shedding is needed.
2. The method of claim 1, further comprising maintaining the
variable inlet guide vanes at an optimized target angle to optimize
one or both of efficiency and operating envelope of the refrigerant
compressor when the criterion is not satisfied.
3. The method of claim 2, wherein said adjusting the variable inlet
guide vanes angle comprises deliberately changing the variable
inlet guide vanes angle away from the optimized target angle.
4. The method of claim 1, wherein said condition of the power
supply network represents power available on the power supply
network relative to power being consumed, and wherein the
predetermined criterion is satisfied when the available power,
according to the monitored signal, drops below a predetermined
value.
5. The method of claim 1, wherein the power supply network operates
at a network frequency and wherein the signal representative of the
condition of the power supply network represents the network
frequency at which the power supply network operates.
6. The method of claim 1, wherein the initially vaporous
hydrocarbon stream comprises natural gas and wherein the liquefied
hydrocarbon stream is a liquefied natural gas stream.
7. System for producing a liquefied hydrocarbon stream, comprising:
a refrigerant circuit arranged to circulate a refrigerant fluid,
comprising a refrigerant compressor for compressing at least part
of the refrigerant fluid and an electric motor engaged to the
refrigerant compressor to drive the refrigerant compressor wherein
the refrigerant compressor comprises variable inlet guide vanes
having an adjustable angle compared to a reference position; a heat
exchanger train comprising at least one heat exchanger, said heat
exchanger train arranged to remove heat from an initially vaporous
hydrocarbon stream thereby condensing at least part of the
initially vaporous hydrocarbon stream to form the liquefied
hydrocarbon stream, said least one heat exchanger arranged to
accommodate at least said part of the initially vaporous
hydrocarbon stream and at least part of the refrigerant fluid
circulating through the refrigerant circuit in a mutual heat
exchanging relationship; a power supply network connected to the
electric motor for powering the electric motor; and, a load
shedding controller arranged to monitor a signal representative of
a condition of the power supply network, to automatically determine
from the signal whether additional load shedding is needed by
comparing the signal to a predetermined criterion, and, when the
criterion is satisfied and additional load shedding is needed, to
adjust the variable inlet guide vanes angle to a position wherein
the refrigerant compressor is unloaded relative to the loading in a
previous condition whereby the variable inlet guide vanes angle was
in a previous position when the criterion was not satisfied.
8. The system of claim 7, further comprising a process controller
arranged to maintain the variable inlet guide vanes at an optimized
target angle to optimize one or both of efficiency and operating
envelope of the refrigerant compressor when the criterion is not
satisfied.
9. The system of claim 7, wherein said condition of the power
supply network represents power available on the power supply
network relative to power being consumed, and wherein the
predetermined criterion is satisfied when the available power,
according to the monitored signal, drops below a predetermined
value.
10. The system of claim 7, wherein the power supply network
operates at a network frequency and wherein the signal
representative of the condition of the power supply network
represents the network frequency at which the power supply network
operates and wherein the predetermined criterion is satisfied when
the network frequency drops below a pre-determined nominal network
frequency.
11. Method of operating a compressor, comprising: compressing at
least part of a fluid in a compressor driven by an electric motor,
wherein the compressor comprises variable inlet guide vanes of
which an angle can be adjusted; powering the electric motor using a
power supply network; monitoring a signal representative of a
condition of the power supply network; automatically determining
from the signal if additional load shedding is needed by comparing
the signal to a predetermined criterion; and, automatically
adjusting the variable inlet guide vanes angle thereby reducing the
loading of the compressor when the criterion is satisfied and
additional load shedding is needed.
12. The method of claim 11, further comprising maintaining the
variable inlet guide vanes at an optimized target angle to optimize
one or both of efficiency and operating envelope of the compressor
when the criterion is not satisfied.
13. The method of claim 12, wherein said adjusting of the variable
inlet guide vanes angle comprises deliberately changing the
variable inlet guide vanes angle away from the optimized target
angle.
14. The method of claim 11, wherein said condition of the power
supply network represents power available on the power supply
network relative to power being consumed, and wherein the
predetermined criterion is satisfied when the available power,
according to the monitored signal, drops below a predetermined
value.
15. The method of claim 11, wherein the power supply network
operates at a network frequency and wherein the signal
representative of the condition of the power supply network
represents the network frequency at which the power supply network
operates and wherein the predetermined criterion is satisfied when
the network frequency drops below a pre-determined nominal network
frequency.
16. The method of claim 11, wherein the compressor is a refrigerant
compressor, whereby the fluid is a refrigerant fluid.
Description
[0001] In a first aspect, the present invention relates to a method
of producing a liquefied hydrocarbon stream. In a second aspect,
the present invention relates to a system for producing a liquefied
hydrocarbon stream. In a further aspect, the present invention
relates to a method of operating a compressor.
[0002] A common liquefied hydrocarbon stream in the industry is
liquefied natural gas (LNG), which may be obtained by liquefying a
natural gas stream. It is desirable to liquefy natural gas for a
number of reasons. As an example, natural gas can be stored, and
transported over long distances, more readily in the form of LNG
than in gaseous form, because as LNG it occupies a smaller volume
and does not need to be stored at a high pressure.
[0003] US pre-grant patent application publication No. 2010/0257895
describes an all electric LNG plant wherein LNG is produced, in
which compressors in the form of refrigerant compressors are
employed to refrigerate the natural gas. The refrigerant
compressors are driven by electric motors. A power plant supplies
the electrical power for these motors. The power plant contains a
plurality of power generation units, each based on an electric
generator driven by a gas or steam turbine.
[0004] If, for some reason, the available electrical power suddenly
goes down, or is partially interrupted, the LNG production process
will fail and it can take at least a number of hours before the
refrigerant compressor(s) have started up again and before the LNG
production process has regained it stability of operation.
[0005] This risk can be reduced by installing an over-capacity of
power generation (based on the so-called N+1 principle) in the form
of stand-by power generation units, or by operating a plurality of
power generation units at less than full capacity (sometimes
referred to as maintaining "spinning reserve"). These solutions
have in common that failure of one generation unit can be afforded
because, compared to the N power generation units minimally
required to deliver the total power demand of the LNG plant, an
additional power generation unit is provided in accordance with the
N+1 principle.
[0006] US 2010/0257895 further proposes that in the event of the
failure of a power generation unit in the power plant, the
(rotational) speed of the compressor drive will preferably be
lowered if a previously determined overall positive load reserve is
smaller than the power which was being supplied by the power
generation unit before its failure. According to the quadratic load
characteristic curve of a turbine compressor, the power drawn from
the electric motors reduces as the cube of the rotational speed.
Only if the actual energy demand of the LNG plant is not covered
even taking into account the reduction in the compressor drive
speed, it is expedient to switch off at least one predetermined
electrical consumer in the gas liquefaction plant.
[0007] One drawback of the load shedding solution proposed in US
2010/0257895 is that the response time for load shedding to take
effect is limited by the total rotational inertia of rotating parts
of the motors, compressors and drive shafts. Another drawback is
that the solution of US 2010/0257895 requires the compressor(s) to
be driven by variable-speed motors.
[0008] In a first aspect, the present invention provides a method
of producing a liquefied hydrocarbon stream, comprising: [0009]
circulating a refrigerant fluid through a refrigerant circuit,
comprising compressing at least part of the refrigerant fluid in a
refrigerant compressor driven by an electric motor wherein the
refrigerant compressor comprises variable inlet guide vanes of
which an angle compared to a reference position can be adjusted;
[0010] removing heat from an initially vaporous hydrocarbon stream
thereby condensing at least part of the initially vaporous
hydrocarbon stream to form the liquefied hydrocarbon stream, said
removing of heat comprising heat exchanging at least said part of
the initially vaporous hydrocarbon stream against at least part of
the refrigerant fluid circulating through the refrigerant circuit;
[0011] powering the electric motor using a power supply network;
[0012] monitoring a signal representative of a condition of the
power supply network; [0013] automatically determining from the
signal whether additional load shedding is needed by comparing the
signal to a predetermined criterion; [0014] automatically adjusting
the variable inlet guide vanes angle thereby reducing the loading
of the refrigerant compressor when the criterion is satisfied and
additional load shedding is needed.
[0015] In a further aspect, the invention provides a system for
producing a liquefied hydrocarbon stream, comprising: [0016] a
refrigerant circuit arranged to circulate a refrigerant fluid,
comprising a refrigerant compressor for compressing at least part
of the refrigerant fluid and an electric motor engaged to the
refrigerant compressor to drive the refrigerant compressor wherein
the refrigerant compressor comprises variable inlet guide vanes
having an adjustable angle compared to a reference position; [0017]
a heat exchanger train comprising at least one heat exchanger, said
heat exchanger train arranged to remove heat from an initially
vaporous hydrocarbon stream thereby condensing at least part of the
initially vaporous hydrocarbon stream to form the liquefied
hydrocarbon stream, said least one heat exchanger arranged to
accommodate at least said part of the initially vaporous
hydrocarbon stream and at least part of the refrigerant fluid
circulating through the refrigerant circuit in a mutual heat
exchanging relationship; [0018] a power supply network connected to
the electric motor for powering the electric motor; [0019] a load
shedding controller arranged to monitor a signal representative of
a condition of the power supply network, to automatically determine
from the signal whether additional load shedding is needed by
comparing the signal to a predetermined criterion, and, when the
criterion is satisfied and additional load shedding is needed, to
adjust the variable inlet guide vanes angle to a position wherein
the refrigerant compressor is unloaded relative to the loading in a
previous condition whereby the variable inlet guide vanes angle was
in a previous position when the criterion was not satisfied.
[0020] In still another aspect, the invention provides a method of
operating a compressor, comprising: [0021] compressing at least
part of a fluid in a compressor driven by an electric motor,
wherein the compressor comprises variable inlet guide vanes of
which an angle can be adjusted; [0022] powering the electric motor
using a power supply network; [0023] monitoring a signal
representative of a condition of the power supply network; [0024]
automatically determining from the signal if additional load
shedding is needed by comparing the signal to a predetermined
criterion; [0025] automatically adjusting the variable inlet guide
vanes angle thereby reducing the loading of the compressor when
additional load shedding is needed.
[0026] The present invention will now be further illustrated by way
of example, and with reference to the accompanying non-limiting
drawings, in which:
[0027] FIG. 1 schematically shows a system for producing a
liquefied hydrocarbon stream; and
[0028] FIG. 2 schematically shows an illustrative non-limiting
example of an embodiment of variable inlet guide vanes in a
centrifugal compressor.
[0029] For the purpose of this description, a single reference
number will be assigned to a line as well as a stream carried in
that line. The same reference numbers refer to similar components,
streams or lines.
[0030] The present disclosure describes methods and systems for
producing a liquefied hydrocarbon stream. In the course of
producing the liquefied hydrocarbon stream, a compressor is
employed which is driven by an electric motor. At least part of a
fluid is compressed in the compressor. The compressor has variable
inlet guide vanes of which an angle can be adjusted. The electric
motor is powered using a power supply network, and a signal
representative of a condition of the power supply network is
monitored. From the signal, it is automatically determined whether
additional load shedding is needed, by comparing the signal to a
predetermined criterion. The variable inlet guide vanes angle are
automatically adjusted when the criterion is satisfied and
additional load shedding is needed, thereby automatically reducing
the loading of the compressor.
[0031] By adjusting the variable inlet guide vanes angle, the power
demand can be reduced without relying on reducing the motor speed.
Therefore, the presently proposed method of load shedding can be
employed irrespective of whether a variable speed electric drive is
used or a fixed speed electric drive.
[0032] Moreover, the inertia of the rotating mass such as rotating
parts of the motor, the compressor and the drive shaft, does not
influence the response time of a load shedding action. Inlet guide
vanes have much less inertia than the rotating parts of the
motor/compressor system, and therefore it is envisaged that the
response time associated with adjusting the variable inlet guide
vanes can be much lower.
[0033] For instance, the compressor is unloaded by adjusting the
variable inlet guide vanes angle when the monitored signal
indicates that the available power drops below a predetermined
value. The condition of the power supply network represented by the
signal may thus represent power available on the power supply
network relative to power being consumed, whereby the predetermined
criterion is satisfied when the available power, according to the
monitored signal, drops below a predetermined value.
[0034] This way the power supply network can be protected by
quickly unloading the compressor thereby imposing a quick relief of
the power being demanded from the power supply network.
[0035] If the predetermined criterion is not satisfied, no
additional load shedding is needed.
[0036] In one group of embodiments the signal representative of the
condition of the power supply network is representative of a
network frequency at which the power supply network operates. The
need for additional load shedding can be inferred from such a
signal if the network frequency deviates from a pre-determined
nominal network frequency. Typically, if the actual network
frequency is lower than the nominal network frequency load shedding
may be needed in order to reduce the power demand on the network
which helps to bring the actual network frequency back to the
nominal network frequency. Automatically determining from the
signal whether additional load shedding is needed may comprise
comparing the actual network frequency to a pre-determined nominal
network frequency. The predetermined criterion to determine whether
additional load shedding is needed may include the nominal network
frequence and the criterion is satisfied when the actual network
frequence drops below the predetermined nominal network
frequency.
[0037] In preferred embodiments, the compressor is provided in the
form of a refrigerant compressor, whereby the fluid is a
refrigerant fluid, such as may be employed in a system and/or
process for producing a liquefied hydrocarbon stream. Suitably, the
compressor is driven only by the electric motor.
[0038] The proposed load shedding method can also be used to
prevent overloading that could result from an increase in ambient
temperature. This may be useful particularly if the compressor is
provided in the form of a refrigerant compressor employed for
compressing at least part of a refrigerant fluid, as may be done in
the course of operating a process of producing as liquefied
hydrocarbon stream. An increase in ambient temperature generally
increases the power demand for liquefying the hydrocarbon stream.
In addition, if the power supply network is powered via one or more
gas turbines, the available power will reduce as a result of the
increase in ambient temperature.
[0039] The proposed load shedding method can be used in connection
with compressors relying on so-called "island-mode" power
generation, where the power supply network is powered by a
dedicated power plant, as well as those powered by imported power
such as imported power from a domestic grid or from an industrial
grid to which other consumers of power are connected as well.
[0040] Provided that the compressor comprises variable inlet guide
vanes of which an angle can be adjusted, implementation of the
proposed load shedding method does not require significant
adaptations of equipment. Implementation may be achieved by
changing the existing control system which is typically already in
place for controlling the inlet guide vane settings, or by adding a
dedicated control system.
[0041] FIG. 1 illustrates the method of operating the compressor in
the context of a method and system for producing a liquefied
hydrocarbon stream. However, the teachings provided herein below
regarding the operation of the compressor are not necessarily
restricted to, or limited to, embodiments wherein the compressor is
a refrigerant compressor and/or wherein the fluid is a refrigerant
fluid.
[0042] The system illustrated in FIG. 1 employs at least one
refrigerant circuit, including a first refrigerant circuit 100
arranged to circulate a refrigerant fluid 110. Each of the at least
one refrigerant circuits comprises a compressor in the form of a
refrigerant compressor 120, for compressing at least part of the
refrigerant fluid 110 being circulated in the refrigerant circuit
100. Each refrigerant compressor is engaged with an electric motor
130, typically via a mechanical drive shaft 125 extending between
the respective refrigerant compressor 120 and the electric motor
130 to drive the rotor of the respective refrigerant compressor 120
into rotation.
[0043] The electric motor 130 is connected to a power supply
network 400, for powering the electric motor 130. The power supply
network comprises a power source, typically in the form of a power
plant 410 and a distribution network 420 connected to the power
source. The power plant can be of the "island-mode" type, which is
a dedicated power plant for powering the hydrocarbon liquefaction
facility, or it can be an external power source from which power is
imported into the facility. The distribution network 420 may be
connected to a power busbar 430, arranged to feed power to the at
least one electric motor 130 via a power feed line 140.
[0044] In the embodiment of FIG. 1, the at least one refrigerant
circuit comprises an optional second refrigerant circuit 200 as
well, to circulate a second refrigerant fluid 210. It comprises a
second refrigerant compressor 220; a second electric motor 230; a
second power feed line 240; and a second mechanical drive shaft 125
all similarly interrelated as described above with reference to the
first refrigerant circuit 100.
[0045] The system depicted in FIG. 1 further comprises a heat
exchanger train 300. The heat exchanger train 300 is very
schematically shown, as many different types of such heat exchanger
trains are known in the art. The heat exchanger train 300 is
arranged to remove heat from an initially vaporous hydrocarbon
stream 10, thereby condensing at least part of the initially
vaporous hydrocarbon stream 10 to form a liquefied hydrocarbon
stream 90. The heat exchanger train typically comprises at least
one heat exchanger that is arranged to accommodate at least said
part of the initially vaporous hydrocarbon stream 10 in a mutual
heat exchanging relationship with at least part of the refrigerant
fluid 110 circulating through the refrigerant circuit 100.
[0046] The compressor(s) may be of any type that is provided with
variable inlet guide vanes, including axial compressors (for
example the AN 200 axial compressor manufactured by General
Electric) and centrifugal compressors.
[0047] Inlet guide vanes are often installed on commercially
available refrigerant compressors to increase efficiency and to
extend the operating envelope. Inlet guide vanes are typically
installed on the first compression stage, but for instance in case
of integrally geared compressors, with multiple compressor stages,
inlet guide vanes can also be installed on one or more subsequent
stages such as the second stage.
[0048] Inlet guide vanes are typically provided in the form of
radially positioned aerofoils in the vapour flow of the compressor.
Suitably the inlet guide vanes are positioned inside the suction
duct. Generally and under normal operating conditions such inlet
guide vanes serve to guide the refrigerant vapour into the
subsequent compression stage at the most efficient direction onto
the vanes or impellers of the subsequent compression stage.
[0049] Variable inlet guide vanes, such as may be employed in the
context of the present invention, are normally rotatable about
their mounting axes. Under normal operating conditions, different
refrigerant entry speeds can be accommodated by rotating the
variable inlet guide vanes into different positions. Rotation may
be imparted on the variable inlet guide vanes by a vane adjustment
mechanism coupled to an actuator.
[0050] The invention is expressly not limited to any particular
type of inlet guidvane geometry and or vane adjustment mechanism.
There are various suitable ways for acting on the vanes, including
rotating ring concepts, lever concepts, hydraulic pistons concepts,
all acting on the variable inlet guide vanes. Examples of variable
inlet guide vanes and possible mechanisms for adjusting their angle
are shown in for instance US patent application publication
2010/0172745 and U.S. Pat. No. 7,520,716. In these examples the
vapour flows generally inwardly towards the rotating axis of the
compressor. US patent application publication 2010/0329898 shows an
example wherein the vapour generally flows axially, in the
direction along the rotating axis. US patent application
publications 2010/0172745 and 2010/0329898, and U.S. Pat. No.
7,520,716 are incorporated in the present disclosure by
reference.
[0051] An embodiment of variable inlet guide vanes of a centrifugal
compressor, derived from US patent application publication
2010/0172745, is shown in FIG. 2 as an illustrative non-limiting
example. It comprises a vane adjustment mechanism employing a
rotating ring 13 provided with a plurality of elongated slots 31,
and inlet guide vanes 11 positioned around a circumference of the
rotating ring 13. The inlet guide vanes 11 are pivotably attached
to a base plate (not shown, in the interest of clarity), such that
each one of the inlet guide vanes 11 can pivot around a shaft 45.
The inlet guide vanes 11 are each coupled to an end of one of a
plurality of lever arms 43 by the shaft 45. Each lever arm 43 is
provided with an outwardly protruding pin 35 in a direction
perpendicular to a plane of rotation of the lever arms 43 around
their shafts 45. Each pin 35 is configured to be positioned within
one of the elongated slots 31. As rotating ring 13 rotates relative
to the base plate, each inlet guide vane 11 is rotated by the same
amount.
[0052] Still referring to FIG. 2, the vane adjustment mechanism
also includes a rack and pinion drive mechanism 21 configured to
drive one of the plurality of inlet guide vanes 11, thereby
creating a drive vane 47. The rack and pinion drive mechanism
includes a pinion 53 coupled to an elongated shaft 55 of the drive
vane 47, which replaces the shaft 45, and a rack 57. The rack 57
includes a plurality of teeth 59 that is configured to engage with
a plurality of teeth 61 on the pinion 53, thereby operationally
coupling the rack 57 to the pinion 53. An end of the rack 57 is
coupled to a drive shaft 23, which may be actuated by for instance
hydraulic cylinder (not shown).
[0053] The drive shaft 23 is arranged to impart a linear motion 24
to the rack 57, which is converted to rotational motion 25 in the
pinion 53 thereby rotating drive vane 47 relative to the base
plate. Drive vane 47 transfers torque to the rotating ring 13 due
to the positioning of each pin 35 on each lever arm 43 within the
associate elongated slot 31 of the rotating ring 13 with which the
pin 35 interacts. The torque is thereby transmitted to the
remaining inlet guide vanes 11, as shown in FIG. 2, whereby the
respective inlet guide vanes 11 are caused to synchronously change
their angular position by the same amount. This way the variable
inlet guide vanes angle .alpha. compared to a reference position 16
can be adjusted. In FIG. 2, the radial position is taken to be the
reference position 16 as an example, but any suitable reference
position can be selected.
[0054] Referring again to FIG. 1, the system is further provided
with a load shedding controller C. The load shedding controller C
is arranged to monitor a signal that is representative of the
available power on the power supply network relative to the power
being consumed, and to adjust the variable inlet guide vanes angle
when the available power, as monitored, drops below a predetermined
value. In such a case, the variable inlet guide vanes angle is
adjusted to a position wherein the refrigerant compressor is
unloaded relative to the loading in a previous condition whereby
the variable inlet guide vanes angle was in a previous position. To
this end, the controller C may interact with the actuator of the
compressor 120.
[0055] A suitable signal to monitor is the network frequency. When
an AC power supply network is in stable operation it operates at a
(pre-determined) nominal frequency. The network frequency, which
may be defined as the frequency of the overall system associated
with the power network (including the active electric generators
and all the running loads that consume power), generally depends
directly on the amount of power that the generators can deliver to
the system compared to the power being demanded for consumption.
Gradual or sudden downward changes in generation capacity result in
frequency decline. The network frequency is thus a good indicator
for the need of load shedding, which is particularly suitable in
combination with island mode power generation. Preferably, the
controller C interacts with the actuator of the compressor 120 so
as to halt the drop in network frequency.
[0056] Particularly when operating on imported power, the signal
may suitably be an external signal, generated by the power provider
or one of the power providers providing power to the network, to
request load shedding.
[0057] Thus, the variable inlet guide vanes angle are preferably
adjusted whereby the refrigerant compressor is unloaded by a
portion of the original load that is equal to or greater than the
power generation deficiency. Herewith the balance between power
generation and the power demand will be restored where after the
power network can continue its stable operation.
[0058] The system may further comprise a process controller PC for
controlling the production of the liquefied hydrocarbon stream 90.
It may advantageously be arranged to maintain the variable inlet
guide vanes at an optimized target angle, to optimize one or both
of efficiency and operating envelope of the refrigerant compressor
120 when the available power as monitored is at or above the
predetermined value.
[0059] The load shedding controller C can be a separate dedicated
controller unit or it can be integrated with another controller,
for instance one that is arranged to control also other aspects of
the system, or it can be a hybrid controller whereby selected parts
of the load shedding controller C are provided as a separate
controller and other parts are provided integrated with the other
controller. In one example, the other controller can be the process
controller PC.
[0060] The system described above may be operated as follows.
[0061] The refrigerant fluid 110 is circulated through the
refrigerant circuit 100. In the course of this circulating, at
least part of the refrigerant fluid 100 is compressed in the
refrigerant compressor 120 to form a compressed refrigerant. The
refrigerant compressor 120 is driven by the electric motor 130,
which typically imparts a rotational motion to the mechanical drive
shaft 125 about its longitudinal axis. The electric motor 130 is
powered using power from the power supply network 400.
[0062] The compressed refrigerant is passed to the heat exchanger
train 300, where it is typically allowed to expand to a lower
pressure and evaporate by receiving heat from at least an initially
vaporous hydrocarbon stream 10. In many cases, but this is not a
requirement for every type of heat exchanger train 300, the
compressed refrigerant is condensed and preferably sub-cooled
before it is allowed to expand to said lower pressure. The
evaporated refrigerant is led back from the heat exchanger train
300 to the refrigerant compressor 120, to be recompressed. This
completes one cycle in the refrigerant circuit 100. Simultaneously
during the cycle, heat is removed from the initially vaporous
hydrocarbon stream 10 by at least part of the evaporating
refrigerant, by heat exchanging at least said part of the initially
vaporous hydrocarbon stream 10 against said at least part of the
refrigerant fluid circulating through the refrigerant circuit 100.
Ultimately at least part of the initially vaporous hydrocarbon
stream 10 is condensed as a result of removing heat from it by the
refrigerant fluid 120 and optional second and further refrigerant
fluids, to form the liquefied hydrocarbon stream 90.
[0063] Under normal stable operation, the variable inlet guide
vanes are set manually by the operator or automated by the process
controller PC and/or a compressor anti surge controller. The
variable inlet guide vanes are set at a selected angle, for
instance to achieve a desired operating window.
[0064] The available power on the power supply network 400 is
monitored, and the variable inlet guide vanes can be maintained at
the selected angle or moved to another selected angle as desired as
long as the available power as monitored is at or above a
predetermined value. In preferred embodiments of operation, the
variable inlet guide vanes are maintained at an optimized target
angle to optimize one or both of efficiency and operating envelope
of the refrigerant compressor as long as the available power as
monitored is at or above the predetermined value.
[0065] However, when the available power as monitored drops below
the predetermined value, the load shedding controller reacts by
adjusting the variable inlet guide vanes angle, thereby unloading
the refrigerant compressor 120. This can be done by quickly
changing the angle to a position different from the selected angle.
If the selected angle was at the optimized target angle, which
would be the case in preferred embodiments of operation, the
unloading of the refrigerant compressor 120 is achieved by
deliberately changing the variable inlet guide vanes angle away
from the optimized target angle.
[0066] Normally, the load demand by the refrigerant compressor is
reduced by closing the variable inlet guide vanes which in
accordance with convention in the art corresponds to moving the
position of the variable inlet guide vanes to increasingly negative
angles, whereby 0.degree. corresponds to the optimized target
angle.
[0067] The heat exchanger train 300 in the present specification
has been depicted very schematically. It can represent any suitable
hydrocarbon liquefaction process, in particular any natural gas
liquefaction process producing liquefied natural gas, and the
invention is not limited by the specific choice of heat exchanger
train. Examples of suitable heat exchanger trains are derivable
from single refrigerant cycle processes (usually single mixed
refrigerant--SMR--processes, such as PRICO described in the paper
"LNG Production on floating platforms" by K R Johnsen and P
Christiansen, presented at Gastech 1998 (Dubai), but also possible
is a single component refrigerant such as for instance the BHP-cLNG
process also described in the afore-mentioned paper by Johnsen and
Christiansen); double refrigerant cycle processes (for instance the
much applied Propane-Mixed-Refrigerant process, often abbreviated
C3MR, such as described in for instance U.S. Pat. No. 4,404,008, or
for instance double mixed refrigerant--DMR--processes of which an
example is described in U.S. Pat. No. 6,658,891, or for instance
two-cycle processes wherein each refrigerant cycle contains a
single component refrigerant); and processes based on three or more
compressor trains for three or more refrigeration cycles (an
example is described in U.S. Pat. No. 7,114,351).
[0068] Other examples of suitable heat exchanger trains are
described in: U.S. Pat. No. 5,832,745 (Shell SMR); U.S. Pat. No.
6,295,833; U.S. Pat. No. 5,657,643 (both are variants of Black and
Veatch SMR); U.S. Pat. No. 6,370,910 (Shell DMR). Another suitable
example of DMR is the so-called Axens LIQUEFIN process, such as
described in for instance the paper entitled "LIQUEFIN: AN
INNOVATIVE PROCESS TO REDUCE LNG COSTS" by P-Y Martin et al,
presented at the 22.sup.nd World Gas Conference in Tokyo, Japan
(2003). Other suitable three-cycle heat exchanger trains include
for example U.S. Pat. No. 6,962,060; WO 2008/020044; U.S. Pat. No.
7,127,914; DE3521060A1; U.S. Pat. No. 5,669,234 (commercially known
as optimized cascade process); U.S. Pat. No. 6,253,574
(commercially known as mixed fluid cascade process); U.S. Pat. No.
6,308,531; US application publication 2008/0141711; Mark J. Roberts
et al "Large capacity single train AP-X.TM. Hybrid LNG Process",
Gastech 2002, Doha, Qatar (13-16 Oct. 2002). These suggestions are
provided to demonstrate wide applicability of the invention, and
are not intended to be an exclusive and/or exhaustive list of
possibilities. Not all examples listed above employ electric motors
as refrigerant compressor drivers. It will be clear that any
drivers other than electric motors can be replaced for an electric
motor to be suitable for application in the context of the present
invention.
[0069] The initially vaporous hydrocarbon stream 10 to be
refrigerated, and ultimately preferably liquefied, may be derived
from any suitable gas stream to be refrigerated and optionally
liquefied. An often used example is a natural gas stream, obtained
from natural gas or petroleum reservoirs or coal beds. As an
alternative the initially vaporous hydrocarbon stream 10 may also
be obtained from another source, including as an example a
synthetic source such as a Fischer-Tropsch process.
[0070] When the initially vaporous hydrocarbon stream 10 is a
natural gas stream, it is usually comprised substantially of
methane. Preferably the gaseous hydrocarbon stream 10 comprises at
least 50 mol % methane, more preferably at least 80 mol %
methane.
[0071] Depending on the source, natural gas may contain varying
amounts of hydrocarbons heavier than methane such as in particular
ethane, propane and the butanes, and possibly lesser amounts of
pentanes and aromatic hydrocarbons. The composition varies
depending upon the type and location of the gas.
[0072] Conventionally, the hydrocarbons heavier than methane are
removed as far as needed to produce a liquefied hydrocarbon product
stream in accordance with a desired specification. Hydrocarbons
heavier than butanes (C4) are removed as far as efficiently
possible from the natural gas prior to any significant cooling for
several reasons, such as having different freezing or liquefaction
temperatures that may cause them to block parts of a methane
liquefaction plant.
[0073] The natural gas may also contain non-hydrocarbons such as
H.sub.2O, N.sub.2, CO.sub.2, Hg, H.sub.2S and other sulphur
compounds, and the like. Thus, if desired, the initially vaporous
hydrocarbon stream 10 comprising the natural gas may be
(pre-)treated before or during being refrigerated. This
(pre-)treatment may comprise reduction and/or removal of undesired
components such as CO.sub.2 and H.sub.2S or other steps such as
early cooling, pre-pressurizing or the like. As these steps are
well known to the person skilled in the art, their mechanisms are
not further discussed here.
[0074] In preferred embodiments disclosed herein, the initially
vaporous hydrocarbon stream 10 comprises natural gas, whereby the
liquefied hydrocarbon stream 90 is a liquefied natural gas
stream.
[0075] The compressor, such as the refrigerant compressor, used
herein may be exclusively driven by the electric motor, meaning the
electric motor is the only driver driving the compressor.
[0076] The person skilled in the art will understand that the
present invention can be advantageously applied in combination with
other load shedding techniques and/or power plant designs, in
particular including one or more of the elements as described in US
pre-grant patent application publication No. 2010/0257895; U.S.
Pat. No. 7,114,351; Gastech 2005 paper "All Electric Driven
Refrigeration Compressors in LNG Plants Offer Advantages" by Fritz
Kleiner and Steve Kaufmann. For instance, in case the presently
proposed system and method for producing a liquefied hydrocarbon
stream comprises two or more refrigerant compressors in parallel
operation (each compressing a portion of the total amount of
refrigerant flow) one or more of the compressors can be tripped
while keeping the remainder in operation. This may be particularly
considered if an extreme adjusting of the inlet guide vanes is
required and/or operation under significant reduced load is
anticipated for a prolonged period of time.
[0077] The person skilled in the art will understand that the
present invention can be carried out in many various ways without
departing from the scope of the appended claims.
* * * * *