Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF COOLING A HYDROCARBON STREAM AND AN APPARATUS
THEREFOR
The present invention relates to a method of cooling
a hydrocarbon stream, and an apparatus therefor.
An important example of such a hydrocarbon stream to
be cooled is a natural gas stream. The cooling may
include liquefying the hydrocarbon stream to produce a
liquefied hydrocarbon stream, such a liquefied natural
gas (LNG) stream in case when the hydrocarbon stream to
be cooled is a natural gas stream.
Natural gas is a useful fuel source, as well as being
a source of various hydrocarbon compounds. It is often
desirable to liquefy natural gas in a liquefied natural
gas (LNG) plant at or near the source of a natural gas
stream for a number of reasons. As an example, natural
gas can be stored and transported over long distances
more readily as a liquid than in gaseous form because it
occupies a small volume and does not need to be stored at
high pressure.
Usually, natural gas, comprising predominantly
methane, enters an LNG plant at elevated pressures and is
pre-treated to produce a purified feed stream suitable
for liquefaction at cryogenic temperatures. The purified
gas is processed through a plurality of cooling stages
using heat exchangers to progressively reduce its
temperature until liquefaction is achieved. The liquid
natural gas cam then be further cooled and expanded to
final atmospheric pressure suitable for storage and
transportation.
In addition to methane, natural gas usually includes
some heavier hydrocarbons and impurities, including but
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not limited to carbon dioxide, sulphur, hydrogen sulphide
and other sulphur compounds, nitrogen, helium, water,
other non-hydrocarbon acid gases, ethane, propane,
butanes, C5+ hydrocarbons and aromatic hydrocarbons.
These and any other common or known heavier hydrocarbons
and impurities either prevent or hinder the usual known
methods of liquefying the methane, especially the most
efficient methods of liquefying methane. Most known or
proposed methods of liquefying hydrocarbons, especially
liquefying natural gas, are based on reducing as far as
possible the levels of at least most of the heavier
hydrocarbons and impurities prior to the liquefying
process.
Hydrocarbons heavier than methane and usually ethane
are typically condensed and recovered as natural gas
liquids (NGLs) from a natural gas stream. The methane is
usually separated from the NGLs in a high pressure scrub
column, and the NGLs are then subsequently fractionated
in a number of dedicated distillation columns to yield
valuable hydrocarbon products, either as products streams
per se or for use in liquefaction, for example as a
component of a refrigerant.
Meanwhile, the methane from the scrub column is
subsequently liquefied to provide LNG.
US Patent Application No. 2003/0046953 discloses a
method of controlling the production of LNG which permits
continuous maximum utilization of the available power to
drive the refrigeration cycle, because the operator can
manipulate the set point of the flow rate of one of the
refrigerants and the ratio of the flow rates of the heavy
mixed refrigerant to the light mixed refrigerant.
The above method cannot prevent the overcooling of a
heat exchanger to below its minimum temperature limits or
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avoid excessive mechanical stress (thermal shocks) of the
heat exchanger caused when the temperature drops too
quickly. Should this occur, leaks in the heat exchanger
may develop. The present invention seeks to address this
and other problems associated with the cooling of a
hydrocarbon stream.
In a first aspect, the present invention provides a
method of cooling a hydrocarbon stream in a heat
exchanger, comprising at least the steps of:
(a) providing a hydrocarbon stream;
(b) heat exchanging the hydrocarbon stream in a first
heat exchanger against at least one refrigerant stream
having a refrigerant stream flow rate, to provide a
hydrocarbon stream having a hydrocarbon stream flow rate
and at least one return refrigerant stream;
(c) inputting a first set point for the refrigerant
stream flow rate; and
(d) adjusting the refrigerant stream flow rate and the
hydrocarbon stream flow rate until the set point is
achieved, wherein
(1) if the first set point is greater than the
refrigerant stream flow rate, then the hydrocarbon stream
flow rate is increased before the refrigerant stream flow
rate is increased;
(2) if the first set point is less than the refrigerant
stream flow rate, then the refrigerant stream flow rate
is decreased before the hydrocarbon stream flow rate is
decreased; and
(3) if the hydrocarbon stream flow rate decreases, then
the refrigerant stream flow rate is decreased.
In a second aspect, the present invention provides an
apparatus for operating a heat exchanger, comprising at
least:
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- a first heat exchanger having a first inlet for a
hydrocarbon stream and first outlet for a cooled
hydrocarbon stream, at least a second inlet for a at
least one refrigerant stream and a second outlet for a
return refrigerant stream;
- a refrigerant flow controller to measure a signal
proportional to the refrigerant stream flow rate of at
least one refrigerant stream to provide a refrigerant
flow signal which is transmitted to a high selector, said
refrigerant flow stream controller operating a
refrigerant valve to control the flow rate of the
refrigerant stream;
- a hydrocarbon flow controller to measure a signal
proportional to the hydrocarbon stream flow rate of the
hydrocarbon stream to provide a hydrocarbon flow signal
which is transmitted to a low-selector, said hydrocarbon
stream flow controller operating a hydrocarbon stream
valve to control the flow rate of the hydrocarbon stream;
- a flow setter to input a set point to provide a set
point signal which is transmitted to the low selector and
the high selector;
- the low selector transmitting the lowest of the set
point signal and hydrocarbon stream flow signal to the
refrigerant flow controller; and
- the high selector transmitting the highest of the set
point signal and the refrigerant stream flow signal to
the hydrocarbon flow controller.
Embodiments and examples of the present invention
will now be described by way of example only and with
reference to the accompanying non-limited drawings in
which:
Figure 1 shows schematically a flow scheme for a
apparatus for cooling a hydrocarbon stream provided with
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means for carrying out an embodiment of the present
invention;
Figure 2 shows a control scheme for a method of
cooling a hydrocarbon stream according to an embodiment
5 of the present invention;
Figure 3 shows a control scheme for a method of
cooling a hydrocarbon stream according to a further
embodiment of the present invention.
As described herein, overcooling of a heat exchanger
may be prevented by adjusting the refrigerant stream flow
rate and the hydrocarbon stream flow rate in the
following manner, until the set point is achieved:
(1) if the first set point is greater than the
refrigerant stream flow rate, then the hydrocarbon stream
flow rate is increased before the refrigerant stream flow
rate is increased;
(2) if the first set point is less than the refrigerant
stream flow rate, then the refrigerant stream flow rate
is decreased before the hydrocarbon stream flow rate is
decreased; and
(3) if the hydrocarbon stream flow rate decreases, then
the refrigerant stream flow rate is decreased.
In this way, it is ensured that there is always
sufficient hydrocarbon in the hydrocarbon stream to
accept the cold from the refrigerant in the refrigerant
stream, thereby preventing overcooling of the heat
exchanger.
It is preferred that these steps are carried out
automatically i.e. without, or with minimal human
intervention after the first set point is provided, for
instance in a fully automated control system.
Figure 1 provides an apparatus for cooling,
preferably liquefying, a hydrocarbon stream 45. The
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hydrocarbon stream may be any suitable gas stream to be
cooled, but is usually a natural gas stream obtained from
natural gas or petroleum reservoirs. As an alternative
the natural gas stream may also be obtained from another
source, also including a synthetic source such as a
Fischer-Tropsch process.
Usually the natural gas stream is comprised
substantially of methane. Preferably the feed stream
comprises at least 60 mol% methane, more preferably at
least 80 mol% methane.
Depending on the source, the natural gas may contain
varying amounts of hydrocarbons heavier than methane such
as ethane, propane, butanes and pentanes as well as some
aromatic hydrocarbons. Natural gas may also contain non-
hydrocarbons such as H20, N2, C02, H2S and other sulphur
compounds, and the like.
If necessary, the hydrocarbon stream containing the
natural gas may be pre-treated before use. This pre-
treatment may comprise removal of undesired components
such as C02 and H2S or other steps such as pre-cooling,
pre-pressurizing or the like. As these steps are well
known to the person skilled in the art, they are not
further discussed here.
In addition to methane, natural gas contains various
amounts of ethane, propane and heavier hydrocarbons. The
composition varies depending upon the type and location
of the gas. Hydrocarbons heavier than methane generally
are removed from natural gas to various extends, for
several reasons, such as in case of C5+ hydrocarbons
having freezing temperatures above the liquefaction
temperature of methane, that may cause them to block
parts of a methane liquefaction plant. C2-4 hydrocarbons
can be used as a source of Liquefied Petroleum Gas (LPG).
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Thus, the hydrocarbon stream 45 refers to a
composition having been partly, substantially or wholly
treated for the reduction and/or removal of one or more
compounds or substances, including but not limited to
sulphur, sulphur compounds, carbon dioxide, water, and
C2+ hydrocarbons.
If hydrocarbon stream 45 comprises natural gas, it
may have been pre-treated to separate out any heavier
hydrocarbons and impurities such as carbon dioxide,
nitrogen, helium, water, sulphur and sulphur compounds,
including but not limited to acid gases.
The hydrocarbon stream 45 may have been pre-cooled in
a pre-cooling stage to reduce the temperature of the
hydrocarbon stream. The provision of cooling in a pre-
cooling stage is known to the person skilled in the art.
The pre-cooling may be part of a liquefaction process or
a separate process. Cooling of a hydrocarbon feed stream
to provide the hydrocarbon stream 45 may involve reducing
the temperature of the feed stream to below 0 C, for
example in the range of -10 C to -70 C to provide a
cooled initial hydrocarbon stream.
The cooled hydrocarbon feed stream can be passed to a
separator such as a condensate stabilisation column,
usually operating at an above ambient pressure in a
manner known in the art. The condensate stabilisation
column provides an overhead mixed hydrocarbon stream,
preferably having a temperature below 0 C, and a heavy
condensate stream. The overhead mixed hydrocarbon stream
is an enriched-methane stream compared to the cooled
hydrocarbon feed stream.
The term "mixed hydrocarbon stream" as used herein
relates to a stream comprising methane (Cl) and at least
5 mol% of one or more hydrocarbons selected from the
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group comprising: ethane (C2), propane (C3), butanes
(CO, and C5+ hydrocarbons. Typically, the proportion of
methane in the mixed hydrocarbon stream 8 is 30-50 mol%,
with significant fractions of ethane and propane, such as
5-10 mol% each.
In NGL recovery, it is desired to recover the methane
in a mixed hydrocarbon stream for further cooling, such
as liquefaction in a LNG plant, and to provide at least a
C2+ stream, optionally one or more of a C2 stream, a C3
stream, a C4 stream, and a C5+ stream.
At least a fraction, usually all, of the mixed
hydrocarbon stream is passed into a NGL recovery system.
The NGL recovery system usually involves one or more
gas/liquid separators such as distillation columns to
separate the mixed hydrocarbon stream into at least a Cl
stream and one or more C2+ streams, commonly at low
pressure, for example in the range of 20 to 35 bar. An
example of a suitable first gas/liquid separator is a
"demethanizer" designed to provide a methane-enriched
overhead stream, and one or more liquid streams at or
near the bottom enriched in C2+ hydrocarbons.
As the mixed hydrocarbon stream 8 is usually provided
from a high pressure 40 to 70 bar initial hydrocarbon
stream, it may require to be expanded, for instance to
reduce the temperature, prior to the first gas/liquid
separator.
The first gas/liquid separator is adapted to separate
the liquid and vapour phases, so as to provide a Cl
overhead stream (as the hydrocarbon stream 45
subsequently used herein), and a C2+ bottom stream. The
Cl overhead stream (which is the hydrocarbon stream 45)
may still comprise a minor (<10 mol%) amount of C2+
hydrocarbons, but is preferably >80 mol%, more preferably
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>95 mol% methane. The C2+ bottom stream 50 can be >90 or
>95 mol% ethane and heavier hydrocarbons, and can be
subsequently fractionated or otherwise used in a manner
known in the art for an NGL stream.
A scheme for cooling, preferably liquefying, a
hydrocarbon stream such as natural gas is shown in
Figure 1. The hydrocarbon stream 45 is passed through a
main cooling stage 1 having first heat exchanger 50, to
provide a cooled, preferably liquefied hydrocarbon stream
55, which can be liquefied natural gas.
The main cooling stage 1 comprises at least one,
preferably cryogenic, first heat exchanger 50. The first
heat exchanger 50 may be a plate and fin or shell and
tube heat exchanger, more preferably a kettle heat
exchanger. The first heat exchanger 50 has a shell side
51. In the shell side can be arranged three tube bundles
53, 57, 59. The main cooling stage 1 further comprises a
refrigerant circuit 100 comprising a refrigerant
compressor 110, a suitable refrigerant driver 120, a
refrigerant cooler 130 and a separator 140.
There can be various arrangements for the hydrocarbon
stream 45 and the refrigerant stream in the main cooling
stage 1. Such arrangements are known in the art. These
can involve one or more heat exchangers 50, optionally at
different pressure levels, and optionally within one
vessel such as the cryogenic heat exchanger shown.
In the embodiment shown in Figure 1, hydrocarbon
stream 45 is passed through the first heat exchanger 50
in first tube bundle 52. The first heat exchanger 50
reduces the temperature of the hydrocarbon stream 45 to
provide a cooled, preferably liquefied hydrocarbon stream
55 such as a LNG stream, which could be at a temperature
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of about or lower than -90 C, preferably lower than
-120 C.
The liquefied hydrocarbon stream 55 can be passed to
an expansion device, such as a cooled hydrocarbon stream
valve 60 which is a flow control valve, optionally
preceded by an expansion turbine (not shown), to control
the flow rate of the cooled hydrocarbon stream 55. The
cooled hydrocarbon stream valve 60 can lower the pressure
of the cooled hydrocarbon stream 55, for instance to
allow the storing of a LNG stream at about atmospheric
pressure.
At least one refrigerant stream 145b, 185b is used to
remove heat from the hydrocarbon stream 45 in the first
heat exchanger 50. A refrigerant, preferably being a
mixed refrigerant, is cycled in the refrigerant circuit
100. Figure 1 shows a closed refrigerant cycle.
In one embodiment of the present invention, the mixed
refrigerant for the refrigerant circuit 100 comprises:
> 30 mol% of a compound selected from the group
consisting of ethane and ethylene or a mixture thereof;
and
> 30 mol% of a compound selected from the group
consisting of propane and propylene or a mixture thereof.
In general, the second refrigerant may be any suitable
mixture of components including two or more of nitrogen,
methane, ethane, ethylene, propane, propylene, butane,
pentane, etc.
A gaseous refrigerant is drawn from the shell side 51
of first heat exchanger 50 as return refrigerant stream
105. This is compressed by refrigerant compressor 110 to
provide a compressed refrigerant stream 115. The
refrigerant compressor 110 is driven by a suitable driver
refrigerant compressor driver 120.
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Compressed refrigerant stream 115 is passed to a
cooler 130, such as an air cooler, in which the heat of
compression is removed together with heat absorbed in the
first heat exchanger 50, and the mixed refrigerant can
thus be partially condensed providing a cooled compressed
refrigerant stream 135. The cooling and partial
condensation of the compressed refrigerant stream 115 may
also be carried out in one of more heat exchangers.
The cooled compressed refrigerant stream 135 is
passed to a separator 140, which splits the cooled
compressed refrigerant stream 135 into one or more
fractions. Figure 1 shows the cooled compressed
refrigerant stream 135 split into two fractions, an
incoming refrigerant stream 143 and a second incoming
refrigerant stream 147. Preferably, separator 140 splits
the cooled compressed refrigerant stream 135 into bottoms
heavy mixed refrigerant (HMR) 143 and an overhead light
mixed refrigerant (LMR) 147. If separator 140 is a
gas/liquid separator, the HMR fraction can be a liquid
product and the LMR fraction can be a vapour product.
The incoming refrigerant stream 143, which can be a
first or heavy mixed refrigerant, is passed through the
first heat exchanger 50 in second tube bundle 57, in
which it can be sub-cooled. The incoming second
refrigerant stream 147, which can be a second or light
mixed refrigerant, is passed through the first heat
exchanger 50 in third tube bundle 59, in which it can be
liquefied and sub-cooled.
The first or heavy mixed refrigerant exits the second
tube bundle 57 as cooled refrigerant stream 145. Cooled
refrigerant stream 145 is expanded in a refrigerant
expander 150 to provide an expanded refrigerant stream
145a. Refrigerant expander 150 can be driven by a
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suitable refrigerant expander driver 160. The expanded
refrigerant stream 145a can be passed through a
refrigerant valve 170, which can control the flow rate of
the expanded refrigerant stream 145a, to provide
refrigerant stream 145b, which is a controlled stream.
The refrigerant stream 145b is passed to the shell side
51 of the first heat exchanger 50 via second inlet 176 to
cool hydrocarbon stream 45 in first tube bundle 53.
Similarly, the second or light mixed refrigerant
exits the third tube bundle 59 as cooled second
refrigerant stream 185. Cooled second refrigerant stream
185 is expanded in a second refrigerant expander 190 to
provide an expanded second refrigerant stream 185a.
Second refrigerant expander 190 can be driven by a
suitable second refrigerant expander driver 200. The
expanded second refrigerant stream 185a can be passed
through a second refrigerant valve 210, which can control
the flow rate of the expanded cooled second refrigerant
stream 185a, to provide a second refrigerant stream 185b,
which is a controlled stream. The second refrigerant
stream 185b is passed to the shell side 51 of the first
heat exchanger 50 via third inlet 216 to cool hydrocarbon
stream 45 in first tube bundle 53.
The method of cooling the hydrocarbon stream 45 is
controlled in the following way:
A refrigerant stream flow rate, FR1, corresponding to
the flow rate of the refrigerant stream 145b is measured
by a refrigerant flow controller FC1 (340). Figure 1
shows the refrigerant flow controller 340 placed to
measure the flow rate FR1 of the cooled refrigerant
stream 145. However, the refrigerant flow controller 340
could be situated to measure the flow rate of any of the
streams 143, 145, 145a, 145b, as long as the flow
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controller 340 provides a signal proportional to the flow
rate FR1 of the refrigerant which is passed into the
shell side 51 of the first heat exchanger 50 to cool
hydrocarbon stream 45 e.g. the flow rate FR1 of
refrigerant stream 145b which is passed to the shell side
51 of the first heat exchanger 50 via second inlet 176 to
cool the hydrocarbon stream 45.
A hydrocarbon stream flow rate, FR2, corresponding to
the flow rate of the cooled hydrocarbon stream 55 is
measured by a hydrocarbon flow controller FC2 (350).
Figure 1 shows the cooled hydrocarbon flow controller 350
placed to measure the flow rate FR2 of the cooled
hydrocarbon stream 55. However, the hydrocarbon flow
controller 350 could be situated to measure the flow rate
FR2 of hydrocarbon stream 45, or any other hydrocarbon
stream as long as the flow controller 350 provides a
signal proportional to the flow rate FR2 of the
hydrocarbon stream passing through the first heat
exchanger 50.
The measurement of stream flow can be carried out by
any suitable apparatus, unit or device known in the art.
Non-limiting examples include orifice plates, venturi
tubes, flow nozzles, variable area meters, pilot tubes,
calorimetric meters, turbine meters, coriolis meters,
ultrasonic Doppler meters and vortex meters.
The flow controllers also control the operation of a
means for controlling the flow of a stream, preferably a
valve, such as a pneumatically, hydraulically or
electrically actuated valve.
A first set point, SP1, for the refrigerant stream
flow rate FR1 is selected and input to a flow setter HC
(300). The first set point, SP1, although provided in
terms of the refrigerant stream flow rate FR1 corresponds
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to the desired output of cooled hydrocarbon stream 55,
which is preferably a LNG stream, from the first heat
exchanger 50.
Overcooling of the first heat exchanger 50 can occur
when more cooling duty from the refrigerant is supplied
than is required by the hydrocarbon to be cooled. In
order to avoid the overcooling of the first heat
exchanger 50, the refrigerant stream flow rate FR1 and
the hydrocarbon stream flow rate FR2 are adjusted
according to the following cross-limiting control method.
In the event that the first set point SP1 for the
refrigerant stream flow rate is greater than the measured
refrigerant stream flow rate FR1 i.e. when the cooled
hydrocarbon output of the first heat exchanger 50 is to
be increased, then the hydrocarbon stream flow rate FR2
is increased before the refrigerant stream flow rate FR1
is increased.
As long as the increase in the flow rate of the
refrigerant stream FR1 follows the increase in the flow
rate of the hydrocarbon stream FR2 overcooling of the
first heat exchanger 50 can be avoided. However, any
period of time between increasing the flow rate of the
hydrocarbon stream FR2 and increasing the flow rate of
the refrigerant stream FR1 should be as short as possible
in view of the system response time in order to prevent
undercooling of the hydrocarbon stream.
In the event that the first set point SP1 for the
refrigerant stream flow rate is less than the measured
refrigerant stream flow rate FR1 i.e. when the cooled
hydrocarbon output of the first heat exchanger 50 is to
be decreased, then the refrigerant flow rate FR1 is
decreased before the hydrocarbon stream flow rate FR2 is
decreased.
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As long as the decrease in the flow rate of the
hydrocarbon stream FR2 follows the decrease in the flow
rate of the refrigerant stream FR1, overcooling of the
first heat exchanger 50 can be avoided. However, any
period of time between decreasing the flow rate of the
refrigerant stream FR1 and decreasing the flow rate of
the hydrocarbon stream FR2 should be as short as possible
in view of system response time in order to avoid
undercooling of the hydrocarbon stream.
In the event that the hydrocarbon stream flow rate
FR2 decreases, for instance during a tripping event in
which a user of the cooled hydrocarbon stream 55 is
withdrawn from operation or a supplier of the hydrocarbon
stream 45 is withdrawn from operation, then the
refrigerant stream flow rate FR1 should also be
decreased.
Preferably the refrigerant stream flow rate FR1 is
adjusted proportionally with the hydrocarbon stream flow
rate FR2, such that a constant temperature for the cooled
hydrocarbon stream 55 is maintained.
The refrigerant flow controller 340 can control the
flow rate of the refrigerant stream FR1 by operating
refrigerant stream valve 170. In an alternative
embodiment (not shown), the flow rate of the refrigerant
stream FR1 could be controlled by the presence of a
control valve in any of the streams 143 or 145 as long as
the control valve effected the flow of refrigerant into
the shell side 51 of the first heat exchanger 50.
Similarly the cooled hydrocarbon flow controller FC2
can control the flow rate of the cooled hydrocarbon
stream 55 (and thereby the flow rate of the hydrocarbon
stream 45) by operating cooled hydrocarbon valve 60. In
an alternate embodiment (not shown), the flow rate of the
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cooled hydrocarbon stream 55 could be controlled by the
presence of a control valve in the hydrocarbon stream 45.
It will be apparent from the system of Figure 1 that
the flow rate of any second refrigerant stream FR3 must
also be controlled. Figure 1 shows a second refrigerant
flow controller FC3 (360) which operates a second
refrigerant flow valve 210 in order to control the flow
rate FR3 of the second refrigerant stream 185b. The
second refrigerant flow valve 210 is shown after the
second refrigerant expander 190, but can be placed in
another second refrigerant stream, such as 147 or 185 as
long as the flow rate of second refrigerant into the
shell side 51 of the first heat exchanger 50 can be
controlled.
The flow rate FR3 of the second refrigerant stream
185b is adjusted relative to, and proportionally with,
the flow rate of the refrigerant stream FR1. A
refrigerant controller 330 can be used to adjust the
position of the second refrigerant flow valve 210 based
upon the instructions provided to the first refrigerant
stream flow controller FC1. The second refrigerant flow
controller FC3 measures the second refrigerant flow rate
FR3 in order to ensure that the system is providing the
correct cooling duty to the first heat exchanger 50.
The cross-limiting control system which can be used
in the method described herein can be automatic. Figure 1
shows Flow controllers FC1 (340), FC2 (350) and FC3 (360)
in operation of flow control valves 170, 60, 210
respectively. The first set point SP1 for the flow rate
of the first refrigerant stream flow rate (FR1) is input
to a flow setter HC (300). The flow setter 300 receives
the input of the first set point SP1 and transmits this
to low selector 310 and high selector 320.
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The low selector 310 receives the cooled hydrocarbon
stream flow rate as a signal from the cooled hydrocarbon
stream flow controller 350. The high selector 320
receives the refrigerant stream flow rate as a signal
from the refrigerant stream flow controller FC1.
The nature and operation of the high and low
selectors 310, 320 will now be discussed in greater
detail with respect to Figure 2. Figure 2 shows a
representation of a control scheme for a method of
cooling a hydrocarbon as described herein. A main cooling
stage 1 as described for Figure 1 can be used with this
embodiment. Only cooled hydrocarbon stream 55 and
corresponding control valve 60, and cooled refrigerant
stream 45, refrigerant compressor 150, compressed
refrigerant stream 145a, control valve 170 and
refrigerant stream 145b from Figure 1 are shown in Figure
2 for simplicity. However, the additional features of
Figure 1 may also be present.
The first set point SP1 for the refrigerant stream
flow rate FR1 is input into a flow setter HC (300) which
generates a set point signal SPS, which is transmitted to
the low selector 310 and the high selector 320.
Refrigerant flow controller 340 generates a
refrigerant flow signal FS1 which is proportional to the
flow rate FR1 of the refrigerant stream 145b. The
refrigerant flow signal FS1 is transmitted to a high
selector 320. High selector 320 also receives a first set
point signal SPS from the flow setter 300.
The refrigerant flow controller 340 operates the
refrigerant stream valve 170 to control the flow rate of
the refrigerant stream 145b.
Hydrocarbon flow controller 350 generates a
hydrocarbon flow signal FS2 which is proportional to the
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flow rate FR2 of the cooled hydrocarbon stream 55. The
hydrocarbon flow signal FS2 is transmitted to the low
selector 310. The low selector 310 also receives the
first set point signal SPS from the flow setter 300.
The cooled hydrocarbon flow controller 350 operates a
cooled hydrocarbon stream valve 60 to control the flow
rate of the cooled hydrocarbon stream 55.
The low selector 310 is programmed to pass the lowest
of the set point signal SPS and hydrocarbon flow signal
FS2 to the first refrigerant flow controller 340. In this
way, an increase in the first set point SPS will only
lead to an increase in the flow rate of the refrigerant
stream FR1 after the flow rate FR2 of the hydrocarbon
stream has been increased.
The high selector 320 is programmed to pass the
highest of the set point signal SPS and the refrigerant
flow signal FS1 to the hydrocarbon flow controller 350.
In this way, a decrease in the set point SPS will only
lead to a decrease in the flow rate FR2 of the
hydrocarbon stream after the flow rate FR1 of the
refrigerant stream has been decreased.
Thus, a method of cooling a hydrocarbon stream is
provided in which there is cross-limiting control between
the flow rate FR2 of the hydrocarbon stream and the flow
rate FR1 of the refrigerant stream such that overcooling
of the first heat exchanger 50 is prevented.
Figure 3 shows a representation of a control scheme
for a method of cooling a hydrocarbon as described herein
in which the temperature TC2 of the cooled hydrocarbon
stream 55 can be maintained by adjusting the ratio of the
flow rate FR1 of the refrigerant stream 145b, such as a
heavy mixed refrigerant, compared to the flow rate FR2 of
the cooled hydrocarbon stream 55. It will be apparent
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that in the embodiments of Figure 1 and Figure 2, there
was no means for adjusting this ratio and it was
therefore fixed.
The cooled hydrocarbon stream 55 is provided with a
temperature controller, TC2 (370). The temperature
controller 370 measures the temperature of the cooled
hydrocarbon stream 55 and transmits a signal TS2
proportional to the temperature.
The temperature of the cooled hydrocarbon stream TC2
can be adjusted by providing a temperature set point TSP
to the temperature controller 370. The temperature TC2 of
the cooled hydrocarbon stream 55 can be decreased by
decreasing the flow rate FR2 of the cooled hydrocarbon
stream 55 relative to a refrigerant stream flow rate FR1.
Similarly, the temperature TC2 of the cooled hydrocarbon
stream 55 can be increased by increasing the flow rate
FR2 of the cooled hydrocarbon stream 55 relative to a
constant refrigerant stream flow rate FR1.
The signal TS2 from the temperature controller 370
can modulate the signal from the high selector 320 to the
cooled hydrocarbon stream controller FC2 to either
increase or reduce the flow rate of the cooled
hydrocarbon stream 55 compared to an unmodulated signal.
However, the modulation carried out to the signal from
the cooled hydrocarbon stream flow controller FC2 to the
low selector 310 is the inverse of the modulation carried
out on the signal from the high selector 320, such that
the cooled hydrocarbon stream flow signal FS2 reaching
the low selector 310 corresponds to the signal from the
hydrocarbon flow controller FC2 which would have occurred
if it was unmodulated by the cooled hydrocarbon stream
temperature controller TC2. In this way, the operation of
the low selector 310, and therefore the refrigerant flow
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controller FC1 is unaffected by the cooled hydrocarbon
stream temperature controller TC2.
Figure 3 shows one way in which the signal TS2 from
the temperature controller 370 can modulate the signal
from the High selector 320. The system has a given ratio
of the flow rate of the cooled hydrocarbon stream (LNG)
to the refrigerant stream (HMR), which provides the
cooled hydrocarbon stream 55 at a specific temperature.
This ratio is shown as (LNG/HMR) in Figure 3. In order to
be able to adjust the temperature of the cooled
hydrocarbon stream 55, the ratio of the flow rate of the
cooled hydrocarbon stream 55 to the refrigerant stream
must be altered from the given ratio. A parameter b
derived from the signal TS2 from the temperature
controller 370 can be used to scale the signal from the
high selector 320.
For instance, a parameter c derived from the signal
provided to the hydrocarbon flow controller 350 can be
determined as a function of a parameter a derived from
the signal from the high selector 320, the ratio of the
cooled hydrocarbon flow rate LNG to refrigerant flow rate
HMR i.e. (LNG/HMR) and a scaling factor (b/100)
determined from the parameter b derived from the signal
from the temperature controller 370. When parameter b
exceeds 100, for example when parameter b is in the range
of >100 to 150, parameter c of the signal provided to the
hydrocarbon flow controller 350 will increase
accordingly.
Similarly, a parameter e derived from the signal
provided to the Low selector 310 can be determined as a
function of the parameter d derived from the signal from
the hydrocarbon flow controller 350, the ratio of the
refrigerant flow rate HMR to the cooled hydrocarbon flow
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rate LNG i.e. (HMR/LNG) and the inverse of the scaling
factor (b/100) i.e. (100/b) determined from the parameter
b derived from the signal from the temperature controller
370.
The flow rate of any second refrigerant stream FR3
can be varied in proportion with that of the flow rate
FR1 of the refrigerant stream to maintain the ratio of
refrigerant to second refrigerant such as the ratio of
HMR to LMR. In a further embodiment, Figure 3 shows a
refrigerant bypass stream 225, which bypasses refrigerant
expander 150. The refrigerant bypass stream 225 is
controlled by refrigerant bypass valve 230 to provide
controlled refrigerant bypass stream 225a. Controlled
refrigerant bypass stream 225a can be combined with
refrigerant stream 145b to provide combined refrigerant
stream 245. The refrigerant bypass valve 230 can be
operated by a signal from the refrigerant flow controller
340 to enable the cooled refrigerant stream 145 to bypass
the refrigerant expander 150.
A person skilled in the art will readily understand
that the present invention may be modified in many ways
without departing from the scope of the appended claims.
