Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR COOLING DOWN A CRYOGENIC HEAT
EXCHANGER AND
METHOD OF LIQUEFYING A HYDROCARBON STREAM
FIELD OF THE INVENTION
The present invention relates to a method and apparatus
for cooling down a cryogenic heat exchanger.
In various embodiments specifically disclosed herein, the
cryogenic heat exchanger is adapted to liquefy a hydrocarbon
stream, such as a natural gas stream.
In another aspect, the present invention relates to a
method of liquefying such a hydrocarbon stream.
BACKGROUND
Several types of cryogenic heat exchangers are known.
Such cryogenic heat exchangers may be used in methods of
liquefying a natural gas stream to produce liquefied natural
gas (LNG). In such a case, the cryogenic heat exchanger is
generally able to receive the hydrocarbon stream to be
liquefied, to heat exchange the hydrocarbon stream against an
at least partly evaporating refrigerant thereby at least
partially liquefying the hydrocarbon stream, and to discharge
the at least partially liquefied hydrocarbon stream.
Depending on the type of hydrocarbons in the stream, and
the pressure level under which the hydrocarbon stream passes
through the cryogenic heat exchanger, a typical temperature
at which for instance natural gas starts to liquefy may be at
-135 C.
However, before it is ready for normal operation of
cooling and/or liquefying the hydrocarbon stream, the
cryogenic heat exchanger needs to be cooled down, e.g. as
part of a plant start-up routine.
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In order to prevent damage to the cryogenic heat
exchanger, including for instance leaks that may result from
thermal expansion and contraction distributions over the
cryogenic heat exchanger, operators and manufacturers of such
cryogenic heat exchangers typically recommend to avoid as
much as possible to exceed a certain specified maximum
temperature rate of change over time.
On the other hand, in order to minimize the non-
productive or sub-optimal productive period of the cryogenic
heat exchanger, operators typically want to cool down their
cryogenic heat exchanger at the highest rate possible.
US Patent 4,809,154 describes an automated control system
for the control of mixed refrigerant-type liquefied natural
gas production facilities, wherein functional parameters are
optimized. Optimization is accomplished by adjusting
parameters including mixed refrigerant inventory,
composition, compression ratio, and compressor turbine speeds
to achieve the highest product output value for each unit of
energy consumed by the facility.
In more detail, process controller system of US Pat. '154
is implemented in a parallel processing computer system
allowing parallel control processes to be executed on
multiple processors having access to a common storage wherein
values representative of the current state of every sensor
and every controller associated with the production facility
are stored. To manage the parallel control processes, a
request queue and a return queue are maintained, as well as a
priority table, which is used to resolve contention among
parallel operating process loops.
The process controller system of US Pat. '154 may work
satisfactorily to optimize or keep optimal quantity or
quality of the liquefied gas being produced while the
liquefaction process runs. However, the process controller
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s yst em of US Pat. '154 is not suitable for controlling the
cryogenic heat exchanger during initial cooling down at start
up, because that requires a sequence of steps to be carried
out which cannot be handled using the system of priority
tables and request and return queues.
W02009/098278 describes a method and apparatus for
cooling down a cryogenic heat exchanger. Cooling down is done
in an automated manner and allows to cool down at the highest
rate possible without exceeding the specified maximum rate of
temperature change.
SHORT DESCRIPTION
It is an object of the invention to provide an apparatus
and method for cooling down a cryogenic heat exchanger in a
more flexible and time efficient manner, in particular in
situations wherein operation of the cryogenic heat exchanger
is restarted after an interruption, while the refrigerant is
still well below ambient temperature. This may for instance
be the case if operation has been interrupted for a
relatively short period of time (for instance for
maintenance, after a compressor induced trip, pit stop or a
shutdown normally required) or even after longer
interruptions (days) during which proper box-up is done to
maintain the low temperature as much as possible.
The present invention provides an apparatus for cooling
down a cryogenic heat exchanger adapted to liquefy a
hydrocarbon stream, such as a natural gas stream, which
cryogenic heat exchanger is arranged to receive the
hydrocarbon stream to be liquefied and a refrigerant, to
exchange heat between the hydrocarbon stream and the
refrigerant, thereby at least partially liquefying the
hydrocarbon stream, and to discharge the at least partially
liquefied hydrocarbon stream and spent refrigerant that has
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passed through the cryogenic heat exchanger, the apparatus
comprising
- a refrigerant recirculation circuit to recirculate spent
refrigerant back to the cryogenic heat exchanger, the
refrigerant recirculation circuit comprising at least a
compressor, a compressor recycle valve, a cooler, and a first
JT valve;
- a programmable controller arranged to perform a comparison
step (502), comprising:
(i) receive one or more refrigerant temperature
indications, providing an indication of the temperature
of the refrigerant,
(ii) compare the one or more refrigerant temperature
indications with one or more associated predetermined
threshold values, and
(iii) based on the outcome of the comparison under (ii)
select one of an automated warm cooling down procedure of
the cryogenic heat exchanger and an automated cold
cooling down procedure of the cryogenic heat exchanger.
After step (iii) the programmable controller is arranged
to execute the selected cooling down procedure. It is noted
that the warm and the cold cooling down procedure are similar
procedures which may involve similar actions, but the actions
are executed in a different way, in particular differing in
one of the following aspects: different step sizes for
opening/closing valves, different timing for opening/closing
valves, different threshold values for deciding on further
opening/closing valves, in particular different TROC-values.
In particular, the cold cooling down procedure differs
from the warm cooling down procedure as the warm cooling down
procedure comprises a step in which it is ensured that the JT
valve is closed (closed automatically or by prompting an
operator to close the JT valve), while the cold cooling down
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procedure allows to start with an opened JT valve. The cold
cooling down procedure allows to start reducing the cryogenic
heat exchanger temperature from the (still) cold condition
(e.g. -80 C to -130 C) down to LNG production point
(approximately -165 C). This will be explained in more detail
below.
The term warm cooling down procedure is used to refer to a
cool down procedure where the initial temperature of the
refrigerant is relatively high, e.g. above the predetermined
threshold value, possibly requiring a pre-cool down procedure.
An example of a warm cooling down procedure is explained in
detail in W02009/098278.
The term cold cooling down procedure is used to refer to a
cool down procedure where the initial temperature of the
refrigerant is relatively low, e.g. below the predetermined
threshold value and no pre-cool down procedure is required.
A warm cooling down procedure is typically employed when
first starting a newly built apparatus or at the end of a
relatively long maintenance period. However, during the
lifespan of the apparatus, relatively short maintenance
operations are to be carried out, at the end of which, the
refrigerant is still relatively cold, i.e. well below ambient
temperatures. Instead of waiting for the refrigerant to reach a
temperature allowing carrying out a warm cooling down
procedure, a cold cooling down procedure is now proposed which
makes it possible to cool down the refrigerant from a
relatively cold starting point. This is relatively time
efficient. It also adds flexibility since it allows for an
automated cool down for warm and cold conditions.
After the cryogenic heat exchanger has been cooled down
with the method as defined above and/or using the apparatus
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defined above, the hydrocarbon stream may be liquefied in one
or more steps including heat exchanging the hydrocarbon stream
in the cryogenic heat exchanger, in order to produce a
liquefied hydrocarbon product.
In another aspect, the invention provides a method of
cooling down a cryogenic heat exchanger adapted to liquefy a
hydrocarbon stream, such as a natural gas stream, comprising
the steps of
- providing a cryogenic heat exchanger arranged to receive the
hydrocarbon stream to be liquefied and a refrigerant, to
exchange heat between the hydrocarbon stream and the
refrigerant, thereby at least partially liquefying the
hydrocarbon stream, and to discharge the at least partially
liquefied hydrocarbon stream and spent refrigerant that has
passed through the cryogenic heat exchanger,
- providing a refrigerant recirculation circuit to recirculate
spent refrigerant back to the cryogenic heat exchanger, the
refrigerant recirculation circuit comprising at least a
compressor, a compressor recycle valve, a cooler, and a first
JT valve;
- performing a comparison step, the comparison step comprising:
(i) receiving input signals representing sensor signals of
one or more refrigerant temperature indications, providing an
indication of the temperature of the refrigerant,
(ii) comparing the one or more refrigerant temperature
indications with one or more associated predetermined threshold
values, and
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(iii) based on the outcome of the comparison under (ii)
selecting one of an automated warm cooling down procedure of
the cryogenic heat exchanger and an automated cold cooling down
procedure of the cryogenic heat exchanger.
In accordance with another aspect there is provided an
apparatus for cooling down a cryogenic heat exchanger adapted
to liquefy a hydrocarbon stream, such as a natural gas stream,
which cryogenic heat exchanger is arranged to receive the
hydrocarbon stream to be liquefied and a refrigerant, to
exchange heat between the hydrocarbon stream and the
refrigerant, thereby at least partially liquefying the
hydrocarbon stream, and to discharge the at least partially
liquefied hydrocarbon stream and spent refrigerant that has
passed through the cryogenic heat exchanger, the apparatus
comprising
- a refrigerant recirculation circuit to recirculate spent
refrigerant back to the cryogenic heat exchanger, the
refrigerant recirculation circuit comprising at least a
compressor, a compressor recycle valve, a cooler, and a first
JT valve;
- a programmable controller arranged to perform a comparison
step, comprising:
(i) receive one or more refrigerant temperature
indications, providing an indication of the temperature of the
refrigerant,
(ii) compare the one or more refrigerant temperature
indications with one or more associated predetermined threshold
values, and
(iii) based on the outcome of the comparison under (ii)
select one of an automated warm cooling down procedure of the
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cryogenic heat exchanger and an automated cold cooling down
procedure of the cryogenic heat exchanger
wherein the automated warm and cold cooling down procedure
comprise an initial opening step, the initial opening step
comprises imposing an initial opening of the first JT valve,
wherein the initial opening step of the first JT valve
according to the automated warm cooling down procedure differs
from the initial opening step of the first JT valve according
to the automated cold cooling down procedure; and
wherein the programmable controller is arranged to, as part of
the initial opening step, perform a TROC step comprising
adjusting the opening of the first JT valve based on a
determined temperature rate of change (TROC) of the refrigerant
over the first JT valve in accordance with an adjustment
scheme, wherein the automated warm cooling down procedure and
the automated cold cooling down procedure comprise different
adjustment schemes.
In accordance with yet another aspect there is provided a
method of cooling down a cryogenic heat exchanger adapted to
liquefy a hydrocarbon stream, such as a natural gas stream,
comprising the steps of
- providing a cryogenic heat exchanger arranged to receive the
hydrocarbon stream to be liquefied and a refrigerant, to
exchange heat between the hydrocarbon stream and the
refrigerant, thereby at least partially liquefying the
hydrocarbon stream, and to discharge the at least partially
liquefied hydrocarbon stream and spent refrigerant that has
passed through the cryogenic heat exchanger,
- providing a refrigerant recirculation circuit to recirculate
spent refrigerant back to the cryogenic heat exchanger, the
refrigerant recirculation circuit comprising at least a
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compressor, a compressor recycle valve, a cooler, and a first
JT valve, ;
- performing a comparison step by a programmable controller,
the comparison step comprising:
(i) receiving input signals representing sensor signals of
one or more refrigerant temperature indications, providing an
indication of the temperature of the refrigerant,
(ii) comparing the one or more refrigerant temperature
indications with one or more associated predetermined threshold
values, and
(iii) based on the outcome of the comparison under (ii)
selecting one of an automated warm cooling down procedure of
the cryogenic heat exchanger and an automated cold cooling down
procedure of the cryogenic heat exchanger
wherein the automated warm and cold cooling down procedure
comprise an initial opening step, the initial opening step
comprises imposing an initial opening of the first JT valve,
wherein the initial opening step of the first JT valve
according to the automated warm cooling down procedure differs
from the initial opening step of the first JT valve according
to the automated cold cooling down procedure; and
as part of the initial opening step, performing a TROC step
comprising adjusting the opening of the first JT valve based on
a determined temperature rate of change (TROC) of the
refrigerant over the first JT valve in accordance with an
adjustment scheme, wherein the automated warm cooling down
procedure and the automated cold cooling down procedure
comprise different adjustment schemes.
In accordance with still yet another aspect there is
provided a method of cooling down a cryogenic heat exchanger
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adapted to liquefy a hydrocarbon stream, the method comprising
the steps of:
receiving, by the cryogenic heat exchanger, the hydrocarbon
stream to be liquefied and a refrigerant, to exchange heat
between the hydrocarbon stream and the refrigerant, thereby at
least partially liquefying the hydrocarbon stream, and to
discharge the at least partially liquefied hydrocarbon stream
and a spent refrigerant that has passed through the cryogenic
heat exchanger,
recirculating, by a refrigerant recirculation circuit, the
spent refrigerant back to the cryogenic heat exchanger, wherein
the refrigerant recirculation circuit comprises at least a
compressor, a compressor recycle valve, a cooler, and a first
Joule Thomson (JT) valve;
performing a comparison step by a programmable controller, said
comparison step comprising:
(i) receiving one or more refrigerant temperature indications,
and providing an indication of a temperature of the
refrigerant,
(ii) comparing the one or more refrigerant temperature
indications with one or more associated predetermined threshold
values, and
(iii) based on an outcome of the comparison under (ii)
selecting one of an automated warm cooling down procedure of
the cryogenic heat exchanger and an automated cold cooling down
procedure of the cryogenic heat exchanger;
wherein the automated cold cooling down procedure is adapted to
reduce a temperature of the cryogenic heat exchanger from a
cold condition down to LNG production point and is allowed to
start with an opened first JT valve,
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wherein the automated warm cooling down procedure and the
automated cold cooling down procedure comprise an initial
opening step, the initial opening step comprises imposing an
initial opening of the first JT valve, wherein the initial
opening step of the first JT valve according to the automated
warm cooling down procedure differs in size of the initial
opening from the initial opening step of the first JT valve
according to the automated cold cooling down procedure; and
wherein as part of the initial opening step, performing, by the
programmable controller, a TROC step comprising adjusting the
opening of the first JT valve based on a determined temperature
rate of change (TROC) of the refrigerant over the first JT
valve in accordance with an adjustment scheme, wherein the
automated warm cooling down procedure and the automated cold
cooling down procedure comprise a plurality of different
adjustment schemes.
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SHORT DESCRIPTION OF THE DRAWINGS
The present invention will now be illustrated by way of
example only, and with reference to embodiments and the
accompanying non-limiting schematic drawings in which:
Fig. 1 schematically shows a cryogenic heat exchanger
arrangement according to one embodiment;
Fig. 2 schematically shows a cryogenic heat exchanger
arrangement according to another embodiment;
Fig. 3 schematically shows a block diagram for
automatically cooling down the cryogenic heat exchanger of
Fig. 1 or Fig. 2
Fig. 4 schematically shows a main cryogenic heat
exchanger arrangement according to another embodiment of the
invention as used in a test;
Fig. 5 schematically shows the line-up of Fig. 4
illustrating monitored temperatures and pressures;
Fig. 6 shows a block diagram for automatically cooling
down the cryogenic heat exchanger of Fig. 4 or Fig. 5.
DETAILED DESCRIPTION
For the purpose of this description, a single reference
number will be assigned to a line (conduit) as well as a
stream carried in that line (conduit). Same reference numbers
refer to similar components, streams or lines (conduits).
Described are methods and apparatuses employing a
programmable controller that is arranged to receive input
signals, such as user input and measurement readings, to
process the input signals and produce control signals, such
as data output and valve control signals.
The programmable controller may be formed as a computer
comprising an input/output device for receiving/transmitting
signals, a memory arranged to store data and a processor
arranged to communicate with the input/output device and
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memory (reading, writing). The processor is arranged to read
and execute program lines, e.g. stored in the memory, to
perform the method as described. The memory may also be
(partially) located as a separate unit which is accessible by
the programmable controller.
The programmable controller may be embedded in a
distributed control system (DOS), wherein for instance
modules provide output via an interface server, such as an
OLE (object-linking and embedding) for process control (OPO)
that may communicate between the computer program and various
interface blocks that may be present in the DOS. In such an
arrangement, the DOS can take back control without waiting
for the programmable controller to transfer control as may be
desired during emergencies or the like.
Automated cooling down of a cryogenic heat exchanger
advantageously facilitates cooling down the cryogenic heat
exchanger at the highest rate possible without exceeding the
specified maximum rate of temperature change. When cooling
down the cryogenic heat exchanger under manual control, an
operator typically has to maintain a wider margin between the
rate of temperature change and the specified maximum.
Moreover, thanks to the automation as described in this
document, an even more time-efficient cooling down is
provided.
Moreover, the methods and apparatuses disclosed herein
may also be used to avoid one or more spatial temperature
gradients in or around the cryogenic heat exchanger to exceed
a recommended maximum value(s).
The advantages of the methods and apparatuses described
herein are more pronounced for cooling down counter-current
cryogenic heat exchangers, preferably using an external
refrigerant, wherein the evaporating refrigerant flows
counter-currently relative to the stream or streams that
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i s /are to be cooled in the cryogenic heat exchanger against
the evaporating refrigerant, than for cooling down co-current
cryogenic heat exchangers.
As will be appreciated by the person skilled in the art,
the maximum temperature rate of change and/or maximum spatial
temperature gradient is generally dependent on the type
and/or specific design of the heat exchanger that is subject
to the process of cooling down. Specific recommendations
regarding such values may be provided by the manufacturer.
Where the cryogenic heat exchanger comprises a shell side
for evaporating refrigerant and a tube side for auto-cooling
the refrigerant, the selected spatial temperature gradient
may reflect the temperature differential between a shell side
of the cryogenic heat exchanger and a refrigerant-containing
tube side.
There are other preferred temperature gradients to be
used, for instance in line-ups wherein downstream of the
cooler and upstream of the first JT valve a liquid/vapour
separator is provided in the refrigerant recirculation
circuit, to receive a partly condensed refrigerant and
separate the partly-condensed refrigerant stream into a
liquid heavy refrigerant fraction and a gaseous light
refrigerant fraction and to discharge the liquid heavy
refrigerant fraction via a liquid outlet and the gaseous
light refrigerant fraction via a gas outlet, which fractions
are passed to the cryogenic heat exchanger, wherein the first
JT valve is arranged to control passage of one of these
fractions, preferably the light refrigerant fraction.
The selected spatial temperature gradient may in such a
line-up reflect one or more of: the temperature differential
between the spent refrigerant and the refrigerant between the
gas outlet and the gaseous refrigerant inlet of the cryogenic
heat exchanger; and the temperature differential between
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spent refrigerant and the refrigerant between the liquid
outlet and the liquid refrigerant inlet of the cryogenic heat
exchanger.
Other possible controlled variables include variables
indicative of operating conditions of one or more
compressors, such as surge conditions. A so-called surge
deviation parameter may be determined based on sensor data to
quantify the deviation between surge and actual operating
condition of the compressor. Typical sensor data that is
taken into account for determining the surge deviation
parameter includes the flow through the relevant compressor
stage and inlet and discharge pressure of the relevant stage.
For automatically cooling a cryogenic heat exchanger, the
one or more manipulated variables may comprise one or both
of: a first JT valve setting that represents a measure of
amount of opening of the first JT valve; and a compressor
recycle valve setting that represents a measure of amount of
closing of the compressor recycle valve. The amount of
opening of the first JT valve quite directly affects the rate
of cooling of the cryogenic heat exchanger because it is one
of the factors that determine the Joule-Thomson effect that
the JT valve has on the refrigerant stream as it passes
through the JT valve, which determines the cooling power of
the refrigerant. The amount of closing of the compressor
recycle valve also affects the rate of cooling of the
cryogenic heat exchanger because it also influences the JT
effect at the first JT valve because it is one way of
controlling the pressure and flow rate of the refrigerant.
Of course, there are other manipulated variables that can
control the pressure and/or flow rate of the refrigerant,
such as compressor speed. Thus compressor speed may also be
used as one of the manipulated variable(s). However, in
contrast to speed, a valve is a very suitable item to
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manipulate in a control sequence that has relatively
immediate effect on the pressure.
The methods and apparatuses disclosed herein may be used
in a method of liquefying a hydrocarbon stream such as a
natural gas stream. In such a case, the cooling down of the
cryogenic heat exchanger is followed by normal operation
wherein the hydrocarbon stream is cooled in the cryogenic
heat exchanger until it is liquefied, preferably followed by
sub-cooling in the cryogenic heat exchanger or in a
subsequent heat exchanger.
It is desirable to liquefy 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 smaller volume
and does not need to be stored at a high pressure.
Usually natural gas, comprising predominantly methane,
enters an LNG plant at elevated pressures and is pre-treated
to produce a purified feed stock 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 is then optionally further
cooled, and expanded through one or more expansion stages to
final atmospheric pressure suitable for storage and
transportation. The flashed vapour from each expansion stage
can be used as a source of plant fuel gas.
It is remarked that US 2006/0213223 Al discloses a
liquefaction plant and method for producing liquefied natural
gas. Control of the plant may be fully or partially
automated, such as by using an appropriate computer, a
programmable logic circuit (PLC), using closed-loop and open-
loop schemes, using proportional, integral, derivative (PID)
control. However, US 2006/0213223 does not teach a computer
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program or an algorithm as described in the present
application.
As schematically shown in Fig. 1, there is provided a
cryogenic heat exchanger 1 arranged to receive, via conduit 2
and hydrocarbon stream inlet 7, the hydrocarbon stream that
is to be liquefied, in order to exchange heat between the
hydrocarbon stream and an at least partly evaporating
refrigerant 3. As a result of the heat exchanging, the
hydrocarbon stream may be at least partially liquefied. The
preferably at least partially liquefied hydrocarbon stream is
discharged via hydrocarbon stream outlet 8 into conduit 4. In
the embodiment as drawn, conduit 2 and conduit 4 connect via
a tube side 29. However, other types of heat exchangers are
possible.
The cryogenic heat exchanger 1 comprises a refrigerant
inlet 5 for an external refrigerant and a refrigerant outlet
6 for spent refrigerant that has passed through the cryogenic
heat exchanger. A refrigerant recirculation circuit 10 is
provided to recirculate spent refrigerant back to the inlet
5. The refrigerant recirculation circuit 10 comprises, at
least, a compressor 11, a compressor recycle valve 12, a
cooler 13, and a first Joule-Thompson (first JT) valve 14.
In practical embodiments of the invention, a JT valve may
be used in combination with an expander. However, in
particular during the cooling down of the heat exchanger, the
JT valve is preferably used for controlling the cooling.
In practical embodiments of the invention, the compressor
may consist of a plurality of compression stages, for
instance 15 compression stages or more. A number of these
stages, for instance 15 of these stages, may be provided in
the form of an axial compressor or centrifugal compressor in
one casing. Each stage may comprise a dedicated recycle
valve, and/or a single recycle valve may be shared by any
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number of subsequent stages. Several compressors or
compressor casings may be arranged in series one after
another to form a compressor train. Each casing (or
compressor stage) may be followed by any number of optional
coolers (or intercoolers), and optional knock-out drums to
remove any liquid from the compressed vapour before passing
the compressed vapour to the next compression stage. After
the last compression stage, the compressed refrigerant stream
may be cooled.
However, for the purpose of illustrating the present
invention, a schematically simplified compressor line-up is
depicted in Figs. 1 and 2, with only one compressor drawn in
and one recycle valve. An example comprising two compression
stages will be described in more detail with reference to
Fig.'s 4 - 6.
In operation, spent (at least partly evaporated)
refrigerant is drawn from the heat exchanger 1 via outlet 6,
and at least a part of it is passed to a suction inlet of
compressor 11 via conduit 25.
The gaseous part of the spent refrigerant stream in
conduit 25 is compressed to yield a compressed refrigerant
stream 16 that is subsequently cooled in one or more coolers,
here depicted as cooler 13, thereby at least partially
condensing the compressed refrigerant stream 16 to form an at
least partially condensed refrigerant stream 17. The at least
partially condensed refrigerant stream 17 is expanded over
first JT valve 14 and subsequently led into the heat
exchanger 1 via inlet 5.
As shown in Fig. 1, the refrigerant stream flows co-
currently with the hydrocarbon stream (from left to right)
through the heat exchanger 1. However, the flow may be
arranged counter-currently instead, such as is for example
the case in Fig. 2.
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In Fig. 2 an alternative cryogenic heat exchanger
arrangement is shown that comprises the same elements as the
embodiment of Fig. 1, and in addition includes a refrigerant
tube side 15 for auto-cooling the refrigerant. Both the
hydrocarbon stream 2 and the refrigerant are heat exchanged
against the evaporating refrigerant in the heat exchanger 1.
The compressed refrigerant stream 16 is subsequently cooled
in one or more coolers, here depicted as cooler 13, followed
by cooling in the heat exchanger 1, via tube side 15, thereby
at least partially condensing the compressed refrigerant
stream 16 to form the at least partially condensed
refrigerant stream 17. The auto-cooled, at least partially
condensed refrigerant stream 17, is drawn from the heat
exchanger at outlet 18 and led through first JT valve 14
before it is passed, via inlet 5, into the heat exchanger 1,
where it is allowed to at least partially evaporate.
Optionally, a refrigerant make-up system may be provided
which is capable of changing the inventory of the refrigerant
in particular in the case of a mixed refrigerant.
The current invention relates to an apparatus or method
for cooling down a cryogenic heat exchanger adapted to
liquefy a hydrocarbon stream 2, 7, 29, 8, 4, such as a
natural gas stream, which cryogenic heat exchanger 1 is
arranged to receive the hydrocarbon stream to be liquefied
and a refrigerant, to exchange heat between the hydrocarbon
stream and the refrigerant, thereby at least partially
liquefying the hydrocarbon stream, and to discharge the at
least partially liquefied hydrocarbon stream and spent
refrigerant that has passed through the cryogenic heat
exchanger, the apparatus comprising
- a refrigerant recirculation circuit to recirculate spent
refrigerant back to the cryogenic heat exchanger, the
refrigerant recirculation circuit comprising at least a
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compressor 11, a compressor recycle valve 12, a cooler 13,
and a first JT valve 14.
The refrigerant recirculation circuit may circulate a
single component refrigerant, such as methane, ethane,
propane, or nitrogen; or a multi-component mixed refrigerant,
sometimes referred to simply as mixed refrigerant (MR), based
on two or more components. These components may preferably be
selected from the group comprising nitrogen, methane, ethane,
ethylene, propane, propylene, butane and pentane.
The refrigerant circuit may involve any number of
separate lines or streams of refrigerant to cool different
hydrocarbon streams, and any number of common elements or
features, including compressors, coolers, expanders, etc.
Some refrigerant streams may be common and some may be
separate.
In a particular embodiment of the present invention, the
described method of cooling down a cryogenic heat exchanger
is part of a method of liquefying a hydrocarbon stream such
as natural gas from a feed stream. Likewise, the apparatus as
described herein may be used in such a method of liquefying a
hydrocarbon stream.
The hydrocarbon stream may be any suitable hydrocarbon-
containing, preferably methane-containing, stream to be
liquefied, but is usually drawn from 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 natural gas is comprised substantially of
methane. Preferably the feed stream comprises at least
60 mol% methane, more preferably at least 80 mol% methane.
A hydrocarbon feed stream may be liquefied by passing it
through a number of cooling stages. Any number of cooling
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stages can be used, and each cooling stage can involve one or
more heat exchangers, as well as optionally one or more
steps, levels or sections. Each cooling stage may involve two
or more heat exchangers either in series, or in parallel, or
a combination of same.
Various types of suitable heat exchangers able to cool
and liquefy a hydrocarbon feed stream are known in the art
and the present invention may be applied to any one of them.
Examples of such heat exchanger types are heat exchangers
available from Air Products & Chemicals Inc. and Linde AG,
typically comprising one, or two, or three, or more bundles.
Various arrangements of suitable heat exchangers able to
cool and liquefy a feed stream such as a hydrocarbon stream
such as natural gas are known in the art, including single
mixed refrigerant (SMR) arrangements, dual mixed refrigerant
(DMR) arrangements, propane-mixed refrigerant arrangements
(C3-MR), arrangements based on three or more cycles, such as
e.g. a so-called APX arrangement launched by Air Products &
Chemicals Inc. based on C3-MR-N2 cycles, and cascade
arrangements including those with a sub-cooling cycle. The
present invention may be applied to any heat exchanger in any
of such arrangements, and other suitable arrangements, with
some minor modifications that are within the reach of the
person skilled in the art.
In various arrangements, the cooling and liquefying of
the hydrocarbon feed stream involves two (or more) cooling
stages comprising a pre-cooling stage and a main cooling
stage. Typically, the pre-cooling stage cools the hydrocarbon
stream to below 0 C, for instance to a temperature in the
range -10 C to -35 C, and the second stage, which may be
referred to as a main cryogenic stage from -10 C to -35 C
down to -145 C to -160 C or even -170 C to liquefy the
hydrocarbon stream.
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The present invention may involve one or more other or
further refrigerant circuits, for example in a pre-cooling
stage. Any other or further refrigerant circuits could
optionally be connected with and/or concurrent with the
refrigerant circuit for cooling the hydrocarbon stream.
As indicated above, according to the present embodiments,
an apparatus and method are provided for cooling down the
heat exchanger 1. This cooling down is needed before
operating the heat exchanger to actual liquefy the
hydrocarbon stream. The cooling down procedures may be
controlled by a programmable controller. Depending on the
temperature of the refrigerant different cooling down
procedures can be employed.
Fig. 3 schematically shows a block scheme representing
the steps that may be carried out. After a start signal is
generated in step 501 (see Fig. 3), a comparison step 502 is
performed which comprises:
(i) receiving one or more refrigerant temperature
indications, providing an indication of the temperature of
the refrigerant,
(ii) comparing the one or more refrigerant temperature
indications with one or more associated predetermined
threshold values, and
(iii) based on the outcome of the comparison under (ii),
selecting one of an automated warm cooling down procedure 503
of the cryogenic heat exchanger or an automated cold cooling
down procedure 504 of the cryogenic heat exchanger.
Step 502 may comprise a condition check, such as checking
whether an appropriate start signal is generated by the
distributed control system (DOS) and/or checking if a
heartbeat signal is present, i.e. checking if all the
relevant software modules are still active.
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Step (i) may comprise obtaining one or more refrigerant
temperature indications comprising at least one of a
refrigerant temperature indication of the refrigerant
- at a suction side of the JT valve 14;
- at a discharge side of the JT valve 14;
- at an entry side of the cryogenic heat exchanger 1;
- at a point inside the cryogenic heat exchanger 1;
- at a discharge side of the cryogenic heat exchanger 1.
For each received refrigerant temperature indication, one
or more suitable temperature sensors may be provided,
producing an indication of the temperature of the refrigerant
at that location.
Refrigerant temperature indications can be obtained by
performing temperature measurements on the refrigerant
directly.
Step (i) may further comprise obtaining an indication of
the temperature difference between shell side and tube side
of the heat exchanger and/or a bottom temperature of the heat
exchanger. These temperatures may be used throughout the
procedure. All actions may have a condition check, including
a temperature difference check.
For each received refrigerant temperature indication, a
predetermined threshold value is available and each received
refrigerant temperature indication is compared to the
associated threshold value. Comparing includes determining if
the received value is above or below the threshold value.
For instance, the temperature of the refrigerant at the entry
side of the cryogenic heat exchanger can be compared to a
threshold value of -50*0 to determine if the temperature is
above or below -50 C. Alternatively, the predetermined
threshold value may have any suitable value, e.g. -80 C or -
130 C.
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If the temperature is below the threshold value the
automated cold cooling down procedure 504 is selected and if
the temperature is above the threshold value the warm cooling
down procedure 503 is selected.
According to a further example, the temperature of the
refrigerant can be compared to a first and second threshold
value to determine if the temperature is between the first
and second threshold value. For instance, the temperature of
the refrigerant at the discharge side of the cryogenic heat
exchanger can be compared to a first threshold value of -15 C
and to a second threshold value of -55 C to determine if the
temperature is between -15 C and -55 C or not. This is done
to prevent too high temperature differences from occurring
between the refrigerant and the heat exchanger. Of course,
the exact values depend on the type of heat exchanger that is
used.
The warm cooling down procedure will not be described in
detail here. Reference is made to W02009/098278 which
provides a detailed explanation of the warm cooling down
procedure. The warm cooling down procedure comprises similar
steps as the cold cooling down procedure, but the warm and
cold cooling down procedures are not identical, as will be
explained in more detail below.
First action of the cold cooling down procedure is an
initial conditions definition action 505 in which the initial
conditions are defined. This action may use information on
critical and non-critical initial conditions, which may be
stored on the memory accessible by the programmable
controller.
In case of occurrence of a critical condition, the
programmable controller interrupts the procedure. The
procedure may be resumed and/or restarted after the critical
condition has been resolved or acknowledged, and the initial
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conditions have been acknowledged by an operator, either
manually or by running an automated control procedure to
restore the initial condition. In case of a non-critical
initial condition, a warning may be issued. This action 505
may further initiate the monitoring of critical variables.
Only once all critical variables are within predetermined
ranges, the next action (initial opening step 506) is
commenced.
Examples of critical initial conditions are:
- first JV valve 14 is not sufficiently closed (e.g. more
than 0.1 % open or other suitable number);
- pressure in the refrigerant circuit is lower than the
compressor 11 discharge;
- compressor 11 is not on-line and running, as can be
determined by measuring compressor speed (e.g. compressor
running at least 3400 rpm or other suitable speed) and
verifying that the suction and discharge valves on the
compressors are open;
- refrigerant pressure is too high (e.g. above 20 barg,
or other suitable figure);
- compressor inlet guide vane (IGV) is open.
Examples of non-critical initial conditions are:
- various actual temperatures, e.g. temperature of the
refrigerant directly upstream of and directly downstream of
the first JV valve 14, and/or temperature differentials;
- compressor recycle valves are not fully open (e.g. less
than 99 % open or any other suitable value); and
- compressed refrigerant pressure below a pre-determined
minimum value (as this may unnecessarily slow down the cool-
down processes). A typically suitable minimum value is 18
barg.
The warm and cold cooling down procedure comprise an
initial opening step 506, the initial opening step 506
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comprises imposing an initial opening of the first JT valve
14, wherein the initial opening step of the first JT valve 14
according to the automated warm cooling down procedure 503
differs from the initial opening step of the first JT valve
14 according to the automated cold cooling down procedure
504.
According to an embodiment the initial opening of the
first JT valve is greater in the automated warm cooling down
procedure 503 than in the automated cold cooling down
procedure 504.
For instance, the initial opening imposed on the JT valve
14 according to the automated cold cooling down procedure 503
may be in the range of 1 - 2%, while the initial opening
imposed on the JT valve according to the automated warm
cooling down procedure 504 may be in the range of 3 - 5%.
In the warm cooling down procedure 503 the JT valve 14 is
initially opened relatively much, as to check if a Joule-
Thompson effect is actually present.
The opening of the valve is expressed in %, which
indicates the relative position of the valve plug (moveable
part of the valve) with respect to its valve seat (stationary
part of the valve). As will be understood, 0% means that the
valve is fully closed (valve plug against valve seat), 100%
means that the valve is fully opened (valve plug farthest
away from valve seat). It will understood that the relation
between the valve opening [%] and the flow rate depend on the
type of valve used (e.g. ball valve, butterfly valve, linear
globe type of valve, fast opening globe type of valve) and
may thus differ from a 1:1 relation.
According to an alternative embodiment the initial
opening step 506 of the first JT valve 14 in the automated
warm cooling down procedure 503 comprises imposing a
predetermined initial opening of the first JT valve 14 (e.g.
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3 - 5%), wherein the initial opening step 506 of the first JT
valve 14 in the automated cold cooling down procedure 504
comprises determining a current opening of the first JT valve
14 and imposing the determined current opening of the first
JT valve 14.
This allows starting the cold cooling down procedure
without adjusting the setting of the first JT valve. In any
case, the predetermined initial opening of the cold cooling
down procedure is smaller than the predetermined initial
opening of the warm cooling down procedure.
According to an embodiment the initial opening step 506
of the cold cooling down procedure 504 further comprises
opening the compressor recycle valve 12.
This forms a difference with the warm cooling down
procedure 503 wherein the compressor recycle valve 12 remains
closed or is actively closed in the initial opening step of
the warm cooling down procedure. In the cold cooling down
procedure, the compressor recycle valve 12 may already be
opened during the initial opening step 506 of the cold
cooling down procedure 504.
Opening the compressor recycle valve as part of the
initial opening step 506 will be done mainly in case of a
minor trip. Usually the compressor recycle valve will be
closed in the initial opening step 506.
According to an embodiment the programmable controller is
arranged to, as part of the initial opening step 506, perform
a TROC step comprising adjusting the opening of the first JT
valve 14 based on a determined temperature rate of change
(TROC) of the refrigerant over the first JT valve 14 in
accordance with an adjustment scheme, wherein the automated
warm cooling down procedure 503 and the automated cold
cooling down procedure 504 comprise different adjustment
schemes.
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Both the warm and cold cooling down procedures 503, 504
comprise comparable TROC steps, but both TROC steps use
different conditions to decide on how to adjust the opening
of the JT valve 14.
According to an embodiment determining the temperature
rate of change (TROC) of the refrigerant over the first JT
valve 14 is done by comparing two refrigerant temperature
indications obtained at a respective first t, and second t2
moment in time, the first and second moments in time being
separated by a predetermined time interval, wherein the
predetermined time interval according to the cold cooling
down procedure 504 is shorter than the predetermined time
interval according to the warm cooling down procedure 503.
The time interval according to the cold cooling down
procedure may be less than 50% of the time interval according
to the warm cooling down procedure.
The time interval according to the cold cooling down
procedure may typically be 2 minutes, while the time interval
according to the warm cooling down procedure may typically be
5 minutes. So, according to this example, the TROC according
to the warm cooling down procedure 503 is calculated as
follows: TROCwarm(t) = (Tt_5-Tt) *12 [ C/h], where the TROC
according to the cold cooling down procedure 504 is
calculated as follows: TROCcold(t) = (Tt-2-Tt) *30 [ C/h] r
wherein t, is time in minutes and T is temperature.
The determined TROC is compared to a predetermined TROC
threshold value to prevent too rapid cooling. For instance,
according to the cold cooling down procedure 503, the
predetermined TROC threshold value may be 28 C. The
adjustment scheme may prescribe that if the TROCcold is above
the predetermined TROC threshold value, e.g. above 28 C, the
first JT valve 14 will be closed with a certain predetermined
closing amount (e.g. 0.5%) and a predetermined waiting time
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is initiated (e.g. 5 minutes) before continuing with opening
the first JT valve 14 with a predetermined opening amount
(e.g. 0.2%), the predetermined closing amount being greater
than the predetermined opening amount.
The temperature measurements used to determine the
relevant temperature rate of change (TROC) can be obtained by
measuring the temperature of the refrigerant at one or more
of the following locations:
- at a suction side of the JT valve 14;
- at a discharge side of the JT valve 14;
- at an entry side of the cryogenic heat exchanger 1;
- at a point inside the cryogenic heat exchanger 1;
- at a discharge side of the cryogenic heat exchanger 1.
According to an embodiment the adjustment scheme of the
cold cooling down procedure comprises waiting a predetermined
time interval between imposing an initial opening of the
first JT valve and initiating the TROC step. The TROC step
comprises (as explained above) adjusting the opening of the
first JT valve 14 based on a monitored temperature rate of
change (TROC) of the refrigerant over the first JT valve 14.
Waiting a predetermined time interval is done to allow
pressure to seep through. A high pressure difference will
cause a high JT effect and further opening too fast could
cause a TROC which is too high.
The end of the initial opening step 506 can be determined
by determining the TROC and verify it is less than a
predetermined value or falls within a predetermined range.
Once the initial opening step 506 is finished the
automated cold cool down procedure comprises performing an
adjustment step 507 which comprising simultaneously
- adjusting and closing recycle valve (509) and
- further adjusting the first JT valve (508).
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As will be discussed in more detail below with reference
to Fig.'s 4 - 6, action 509 may comprise adjusting and
closing a plurality of recycle valves (509) and/or action 508
may comprise further adjusting a plurality of JT valves, in
particular a first and second JT valve.
In action 508 the first JT valve 14 is further adjusted.
In particular in the embodiment of Fig. 2, strong cooling may
cause condensation of the refrigerant. Just before
condensation occurs, the valve movements are preferably
slowed down, and the moment that condensation is detected the
valve may be closed partially to avoid too high a cooling
rate that would otherwise be caused by a sudden increase in
flow rate due to condensation (an increase of 100 tpd (tonnes
per day) 10 secs is not uncommon). After condensation is
detected, the valve opening may be normalized and continued
until the JT effect of the valve opening in diminished.
The JT effect may be monitored during the further opening
of the JT valve, for instance based on a temperature
difference between the temperature of the refrigerant
upstream of the JT valve and the temperature of the
refrigerant downstream of the JT valve. An assumption may be
made that the JT effect is present if the temperature
difference exceeds 8 C.
Condensation may be detected by deferment from one or
both of a temperature and flow measurement at the JT valve.
For the refrigerant that flows through the first JT valve 14,
the temperature of the refrigerant downstream of the JT valve
14 may be used and/or the flow through the JT valve, which in
turn may be estimated by determining a pressure differential
over the JT valve 14.
In preferred embodiments, the JT valve 14 can't be closed
further than a minimum opening corresponding to the opening
at the start of this module.
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The changes in JT effect upon further opening of the JT
valve may be small. However, at the same time the refrigerant
pressure is increased as simultaneously action 509 is
performed by manipulating the recycle valve 12 to meet a
target surge deviation of the compressor (or number of
compression stages). This module monitors the surge deviation
of the compressor 11, and closes the recycle valve 12 if the
surge deviation exceeds a pre-determined maximum deviation. A
suitable predetermined maximum deviation is 0.3.
If there are multiple recycle valves, e.g. on multiple
compressor stages, each recycle valve may be manipulated
individually (but simultaneously) taking into account a
dedicated surge deviation parameter for the corresponding
stage through which each particular recycle valve controls
the recirculation.
Since the closing of recycle valve 12 affects the
compressor suction pressure, this pressure is preferably
monitored to not go below a recommended limit, such as e.g.
1.8 barg. Closing the recycle valve decreases the suction
pressure as well. Therefore, the closing of the recycle valve
is made conditional to avoid causing the suction pressure to
go below predetermine target value. The objective is to
maintain a ramp (increase) on the discharge pressure by
closing the recycle valves steadily while monitoring surge
deviation. When the surge deviation is below the considered
minimum level (e.g. 0.1) then the module activity is stopped.
However surge deviation is monitored throughout the whole
final cool down procedure, and the recycle valves closed when
allowed by the surge deviation and the suction pressure is
within a predetermined range.
When the temperature of the cryogenic heat exchanger 1
has met its operating temperature, an end signal is
generated. This may be done as part of actions 508 and 509,
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each action generating a separate end signal, or a single end
signal may be generated as part of one of the actions or by a
separate action (not shown). When the end signal (s) is/are
triggered, the programmable controller may end the automated
cold cooling down procedure (action 510 in Fig. 3).
End action 510 may fully close the recycle valve 12 as
much as possible, provided that the surge deviation does not
stop this from occurring. If the surge deviation prevents
further closing of the recycle valve, in case the surge value
is too low (typically below 0.1), a warning message may be
generated and outputted to alert the operator that an IGV
adjustment may be necessary. An IGV movement has a similar
effect as the closing of the recycle valve 12. However, any
IGV movement may be constrained by the molecular weight of
the passing refrigerant that must exceed a pre-determined
minimum value. A typical MR minimum molecular weight is 24
g/mol. Obviously this warning signal may not be a useful
option if no IGV is present on the compressor in use.
Since an IGV movement is considered to be a last
resource, it has been contemplated to only alert the operator
to the possible necessity of an IGV movement instead of
attempting to execute any IGV movement under the control of
the automatic procedure as described herein.
Once the recycle valve is fully closed or closed
sufficiently, control may be handed over an operator and/or
present a status output or generate an operator alerting
signal to inform the operator that normal operation of the
cryogenic heat exchanger may proceed, or the like.
Alternatively, a subsequent control procedure or the like may
be started, e.g. normal operating control such as advanced
process control as described in e.g. US Pat. 7,266,975 and/or
US Pat. 6,272,882 or any other type of module.
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Figure 4 shows a larger type of cryogenic heat exchanger
100, embedded in a system of various pre-cooling heat
exchangers, serviced by such a further refrigerant circuit,
and other equipment, as may be found in a hydrocarbon
liquefaction plant. The further refrigerant circuit may
hereinafter be referred to as the "pre-cooling refrigerant
circuit" or "pre-cooling refrigerant cycle". Likewise, items
such as compressors and the refrigerant may also be referred
to as "pre-cooling refrigerant compressor" or "pre-cooling
refrigerant".
The cryogenic heat exchanger 100 of this embodiment will
hereinafter be referred to as the main cryogenic heat
exchanger 100, to distinguish it from any other heat
exchangers present in the embodiment. The main cryogenic heat
exchanger 100 comprises a warm end 33, a cold end 50 and a
mid-point 27. The wall of the main cryogenic heat exchanger
100 defines a shell side 110. In the shell side 110 are
located:
- a first tube side 29 extending from the warm end 33 to the
cold end 50, preferably extending between a hydrocarbon
stream inlet 7 and a hydrocarbon stream outlet 8;
- a second tube side 28 extending from the warm end 33,
preferably from a gaseous refrigerant inlet 49a at the warm
end 33, to the mid-point 27; and
- a third tube side 15 extending from the warm end 33,
preferably from a liquid refrigerant inlet 49b at the warm
end 33, to the cold end 50.
A refrigerant compressor train, as shown here
symbolically comprising first and second compressors 30 and
31, is provided to compress the refrigerant. Each of these
compressors is provided with a number of recycle valves,
which are here schematically represented by recycle valves
130 and 131 in a recycle line that connects the compressor
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discharge, downstream of the respective coolers, to the low
pressure suction inlet.
The first refrigerant compressor 30 is driven by a
suitable motor, for example a gas turbine 35, which is
provided with a helper motor 36 for start-up, and the second
refrigerant compressor 31 is driven by a suitable motor, for
example a gas turbine 37 provided with a helper motor (not
shown). Alternatively, the compressors 30 and 31 may be
driven on a single shaft on a shared motor.
During normal operation after the main cryogenic heat
exchanger has been cooled down, a gaseous, preferably
methane-rich hydrocarbon feed stream is supplied at elevated
pressure through supply conduit 20 to the first tube side 29
of the main cryogenic heat exchanger 100 at its warm end 33.
The hydrocarbon feed stream passes through the first tube
side 29 where it is cooled, liquefied and optionally sub-
cooled, against a mixed refrigerant (MR) evaporating in the
shell side 110 forming spent refrigerant. The resulting
liquefied hydrocarbon stream is removed from the main
cryogenic heat exchanger 100 at its cold end 50 through
conduit 40. The flow of the hydrocarbon stream through the
system may be controlled, e.g. using rundown valve 44
provided in conduit 40.
Stream 40 may optionally be passed through a suitable end
flash system, wherein the pressure is brought down to storage
and/or transportation pressure. Finally, liquefied
hydrocarbon stream is passed as the product stream to storage
where it is stored as liquefied product, or optionally
directly to transportation.
During normal operation, and during cooling down of the
main cryogenic heat exchanger, spent refrigerant is removed
from the shell side 110 of the main cryogenic heat exchanger
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100 at its warm end 33 through conduit 25 and passed to
knock-out drum 56.
A refrigerant make-up adjustment conduit 65 also feeds
into knock-out drum 56 to optionally add refrigerant
inventory to the spent refrigerant stream. The adding of the
various refrigerant components is controlled by one or more
valves, typically one valve per component. Here, these valves
are schematically represented as valve 66.
The evaporated fraction 55 of the spent refrigerant,
which exits from the top of the knock out drum 56, is
compressed, in refrigerant compressors 30 and 31, to obtain a
compressed refrigerant stream, which is removed through
conduit 32. Other refrigerant compressor arrangements are
possible.
In between the two refrigerant compressors 30 and 31,
heat of compression is removed from the fluid passing through
conduit 38 in ambient cooler 23, which may comprise an air
cooler and/or a water cooler and/or any other type of ambient
cooler. Likewise, an intercooler (not shown) may be provided
between two successive compressor stages of a compressor.
The compressed refrigerant stream in conduit 32 is cooled
in air cooler 42 and partly condensed in one or more pre-cool
heat exchangers (shown are 43 and 41) against a pre-cool
refrigerant cycle that will be described in more detail later
herein below. The pre-cool heat exchangers 41, 43 may be
operating at mutually different pressures and/or be using
different refrigerant compositions.
The partly condensed refrigerant stream 39 is then passed
to and let into a liquid/vapour separator via an inlet
device, here depicted as separator vessel 45 and inlet device
46. In the separator vessel 45, the partly-condensed
refrigerant stream is separated into a, at this point liquid,
heavy refrigerant fraction (HMR) and a, at this point
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gaseous, light refrigerant fraction (LMR). These streams may
each be individually controlled by means of a JT valve or the
like, the first JT valve 58 for controlling the vapour
(light) refrigerant stream and a second JT valve 51 for
controlling the liquid (heavy) refrigerant stream.
The liquid heavy refrigerant fraction is removed from the
separator vessel 45 through conduit 47, and the gaseous light
refrigerant fraction is removed through conduit 48. The heavy
refrigerant fraction is sub-cooled in the second tube side 28
of the main cryogenic heat exchanger 100 to get a sub-cooled
heavy refrigerant stream 54. The sub-cooled heavy refrigerant
stream is removed from the main cryogenic heat exchanger 100
through conduit 54, and allowed to expand over an expansion
device comprising second JT valve 51. The expansion device
may further comprise a dynamic expander (not shown) in series
with the second JT valve 51, which does not have to be
operated during any cool down procedure of the main cryogenic
heat exchanger.
The sub-cooled heavy refrigerant stream is, at reduced
pressure, introduced through conduit 52 and nozzle 53 into
the shell side 110 of the main cryogenic heat exchanger 100
at its mid-point 27. The heavy refrigerant stream is allowed
to evaporate in the shell side 110 at reduced pressure,
thereby cooling the fluids in the tube sides 29, 28 and 15.
The gaseous light refrigerant fraction removed from
separator vessel 45 through conduit 48 is passed to the third
tube side 15 in the main cryogenic heat exchanger 100 where
it is cooled, liquefied and sub-cooled to get a sub-cooled
light refrigerant stream 57. The sub-cooled light refrigerant
stream is removed from the main cryogenic heat exchanger 100
through conduit 57, and allowed to expand over an expansion
device comprising first JT valve 58. At reduced pressure it
is introduced through conduit 59 and nozzle 60 into the shell
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side 110 of the main cryogenic heat exchanger 100 at its cold
end 50. The light refrigerant stream is allowed to evaporate
in the shell side 110 at reduced pressure, thereby cooling
the fluids in the tube sides 29, 28 and 15.
Optionally (not shown), an optional side stream may be
drawn from the gaseous light refrigerant stream 48, which may
be cooled, liquefied and sub-cooled against one or more other
cold streams in one or more other heat exchangers other than
the main cryogenic heat exchanger 100. For instance, it may
be cooled, liquefied and sub-cooled against cold flash vapour
generated from stream 40 in an optional end flash system. The
optional sub-cooled side stream may be recombined with the
light refrigerant stream in conduit 57 or 59 in which case it
needs an auxiliary expander means such as an auxiliary first
JT valve. Reference is made to US Pat. 6,272,882 for a more
detailed description of such an option.
Pre-cool heat exchangers 41,43 are operated using a pre-
cooling refrigerant, which may be a mixed component
refrigerant or a single component refrigerant. For this
example, propane has been used. Evaporated propane is
compressed in pre-cool compressor 127 driven by a suitable
motor, such as a gas turbine 128. A pre-cooling refrigerant
compressor recycling valve 129 is provided as well, here
symbolically shown in a line connecting the first stage
compressor low pressure suction inlet with the intermediate
pressure level. However, a recycling line may optionally be
provided across all of or a selection of compression stages.
Compressed propane is then condensed in air cooler 130,
and the condensed compressed propane, at elevated pressure,
is then passed through conduits 135 and 136 to heat
exchangers 43 and 41 which are arranged in series with each
other. The condensed propane is allowed to expand to an
intermediate pressure over expansion valve 138, before
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entering into heat exchanger 43. There, the propane partly
evaporates against the heat from the multi-component
refrigerant in conduit 32, and the resulting evaporated
gaseous fraction is passed through conduit 141 to an
intermediate pressure inlet of the propane compressor 127.
The liquid fraction is passed through conduit 145 to the heat
exchanger 41. Before entering into the heat exchanger 41, the
propane is allowed to expand to a low pressure over expansion
valve 148. The evaporated propane is passed through conduit
150 to a suction inlet of the propane compressor 127.
As the person skilled in the art knows, knock-out drums
or the like may be provided in any conduit connecting to a
compressor suction to avoid feeding a non-gaseous phase to
the compressor. An economizer may also be provided.
In the present example, two pre-cooling heat exchangers
have been shown operating at two pressure levels. However,
any number of heat pre-cooling heat exchangers and
corresponding pressure levels may be employed.
The pre-cooling refrigerant cycle may also be used to
obtain hydrocarbon stream 20, for instance as follows. A
hydrocarbon feed, in the present example a natural gas feed,
is passed at elevated pressure through supply conduit 90. The
natural gas feed, which typically is a multi-component
mixture of methane and heavier constituents, is partially
condensed in at least one heat exchanger 93.
In the present example, this heat exchanger operates at
approximately the same pressure level as pre-cooling heat
exchanger 43, using a side stream 137 of the pre-cooling
refrigerant drawn from conduit 135. Although not drawn in
Fig. 4, conduit 137 connects to conduit 137a. Prior to
entering into the heat exchanger 93, the pre-cooling
refrigerant is allowed to expand over valve 139 to
approximately intermediate pressure. The resulting evaporated
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gaseous fraction is passed through conduits 140a and 140 to
conduit 141 where it is recombined with the gaseous fraction
drawn from pre-cooling heat exchanger 43. The liquid fraction
of the pre-cooling refrigerant is drawn from the heat
exchanger 93 in conduit 151 and fed into heat exchanger 91
after expansion over valve 152 to approximately the low
pressure. The evaporated pre-cooling refrigerant is then led
to conduit 150 via conduits 153a and 153.
It is remarked that heat exchangers 43 and 93 and/or heat
exchangers 41 and 91 may be provided in the form of combined
heat exchangers comprising separate sides for the natural gas
and for the refrigerant in conduit 32.
The partly condensed feed 92 is introduced, e.g. via an
inlet device 94, into a gas/liquid separator 95 which may be
provided e.g. in the form of a scrub column or similar. In
the scrub column 95, the partly condensed feed is separated
to get a methane-enriched gaseous overhead stream 97 and a
liquid, methane-depleted bottom stream 115.
The gaseous overhead stream 97 is passed through conduit
97 via heat exchanger 91 to an overhead separator 102. In the
heat exchanger 100, the gaseous overhead stream is partly
condensed against the pre-cooling refrigerant in conduit 151,
and the partly condensed overhead stream is introduced into
the overhead separator 102 via inlet device 103. In the
overhead separator 102, the partly condensed overhead stream
is separated into a gaseous, stream 20 (which is
substantially depleted from 05+ components and/or relatively
rich in methane when compared to the feed stream) and a
liquid bottom stream 105. The gaseous stream 20 forms the
hydrocarbon feed at elevated pressure in conduit 20.
At least part of the liquid bottom stream 105 may be
introduced through conduit 105 and nozzle 106 into the scrub
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column 95 as reflux. The conduit 105 is provided with a flow
control valve (not shown) and/or a pump 108.
If there is less reflux required than there is liquid in
the partly condensed gaseous overhead stream 105, the surplus
may be passed on to conduit 20 over a bypass conduit (not
shown) and a flow control valve (not shown). In case too
little reflux is available, an external reflux medium,
suitably butane, may be added from an external source (not
shown), suitably into conduit 105.
The liquid, 03+-enriched bottom stream is removed from
the scrub column 95 via conduit 115. Here it may be withdrawn
from the process, sent to a fractionation train and/or
storage/transport and/or a reboiler in any fashion known to
the person skilled in the art.
Prior to its normal operation as described above, the
main cryogenic heat exchanger has to be cooled down to
operating temperature. The presently disclosed methods and
apparatuses achieve an automated cooling down of the main
cryogenic heat exchanger. This has been demonstrated in
accordance with the following.
Several temperatures, temperature rates of change, and
temperature differentials at various points in and around the
main cryogenic heat exchanger may be monitored by the
programmable controller during the cool down process. This
enables the programmable controller to determine the
evolution of the temperature profile over time. Figure 5
shows the points in and around the main cryogenic heat
exchanger 100 where in a test the temperature sensors (TR20;
TR25; TR33; TR40; TR47; TR48; TR52; TR54; TR57; TR59) and
differential temperature sensors (TDR2547; TDR2548; TDR2715;
TDR5254; TDR5759) were provided in addition to other
temperature and temperature differential sensors that will
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not be further discussed here as they were considered of less
relevance for the described automation.
The line-up in Fig. 5 corresponds to the line-up of Fig.
4, but the reference numbers have been omitted in the
interest of highlighting the reference numbers corresponding
to the various sensors that are shown. Temperature sensors
are marked by "TR" followed by a number that corresponds to
the reference number assigned to the component, stream or
line (conduit) where the sensor is provided. For temperature
differential sensors, the code TDR is used followed by two
two-digit numbers corresponding to the reference numbers
assigned to the components, streams or lines (conduits)
between which the differential sensor is provided. The
temperature sensors and differential temperature sensors
generate sensor signals that may be received by and monitored
by the programmable controller which may use one or more of
these as controlled variables.
At the top of the main cryogenic heat exchanger 100,
temperatures in conduits 57 and 59, upstream and downstream
of the first JT valve 58, were monitored using temperature
sensors TR57 and TR59. The difference between these
temperatures was also monitored, which may be used to
determine the actual JT effect over the first JT valve.
The difference between the shell temperature at mid-point
27 was measured and the temperature in tube side 15 at mid-
point 27 was determined (TDR2715). In addition, the shell
temperature near the warm end 33 was measured using TR33, as
well as the temperature of the spent refrigerant drawn from
the heat exchanger in conduit 25 (TR25).
The inlet temperature of the heavy liquid refrigerant
fraction may be measured using TR47, inlet temperature of the
hydrocarbon stream immediately upstream of the main cryogenic
heat exchanger 100 may be measured using TR20, and the
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temperature of the hydrocarbon rundown stream immediately
downstream of the main cryogenic heat exchanger 100 may be
measured using TR40.
All temperature measurements stabilize and are reliable
when there is forward flow. Thus, the measurements can be
unreliable at times, for instance when stagnant gas goes back
to the temperature sensor at the beginning of cool down. The
monitoring depends on the initial conditions, pressure
conditions for example. The temperature that indicates the
end of the cool down is the hydrocarbon product rundown line
temperature TR40. However, this measurement may not be
reliable at the beginning of cool down when the hydrocarbon
flow is extremely low. Therefore, at the beginning of cool
down another temperature, suitably the LMR temperature TR59
downstream of the first JT valve 58, may be monitored
instead. However at the end of cool down the reference
temperature will be TR40.
Several pressures and pressure differentials, in various
points in the line-up, may be monitored by the programmable
controller during the cool down process. The most relevant
pressure sensors (PR32; PR54; PR55; PR57; PR150) are
indicated in Figure 5, using PR followed by a number that
corresponds to the reference number assigned to the component
or line (conduit) where the sensor is provided. The most
important pressures to be monitored include the pre-cool
compressor suction pressure PR150 in conduit 150, the mixed
refrigerant compressor 30 suction pressure (PR55) in conduit
55; and the mixed refrigerant compressor discharge pressure
PR32 in conduit 32.
These pressure sensors generate sensor signals that may
be received by and monitored by the programmable controller
which may use one or more of these as controlled variables.
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The pressure in the line-up after a long shut down can
affect the cooling procedure, especially if the line-up has
been in full recycle for days. Small changes, while having a
high pressure, may have big consequences in the overall
cooling of the main cryogenic heat exchanger 100.
Additionally, PR57 and PR54 (LMR and HMR tube pressure
upstream of the first (58) and second (51) JT valves,
respectively) may be monitored before cool down. Any valve
manipulation may have faster dynamics if these pressures are
too high, so as initial condition the system should have a
pressure level that is lower than a predetermined initial
maximum pressure value (in the test we used 20 barg).
Flow rates may be calculated for the LMR and HMR streams,
in order to be used as a controlled variable or at least a
variable to be monitored. Such calculations may be based on
the differential in pressure and the nominal valve opening of
the first (58) or second (51) JT valve, respectively. For
this, measurements of the pressures before the first and
second JT valves on both LMR and HMR circuits (PR57 and PR54,
respectively) and the suction pressure (PR55) of the
refrigerant circuit before going to the compressors may be
used.
The standard deviation of flow measurements for small JT
valve openings may be quite large, which could lead to errors
if used as monitored variable. A linear model of the LMR and
HMR flows has been calculated as the Least Squares Linear
model from all measurements with high valve openings. Based
on this model, the estimated flows will be given by:
FLmR = KLmR-X584(PR57 - FR55); and
FHMR = RHMR*X51 4(PR54 - PR55)
wherein FLmR (FHMR) represents the flow rate in the LMR
conduit 48 (HMR conduit 47); X58 (X51) represents the amount
of opening of the first (second) JT valve 58, resp. 51; and
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KLMR (KHMR) represents the least squares linear model
constant corresponding to the slope. A linear least squares
model has been found to satisfy the desired accuracy.
However, other types of functions may be employed instead. In
particular, a quadratic function could be estimated for the
HMR, while for the LMR flow a characteristic shape resembling
a square root function has been found.
Immediately prior to executing the automated cooling
down, the main cryogenic heat exchanger 100 was first pre-
cooled, under manual control, to a temperature between about
-25 C and about -35 C. Other tasks that have been completed
at this stage, for the time being manually but these could
also be automated and incorporated in the module structure as
presently disclosed, include:
- level control in any in-line NGL (natural gas liquid,
typically consisting of molecules having mass comparable to
propane and higher) extraction column (e.g. scrub column);
- temperature control of stream 20;
- depressurisation of the refrigerant circuit, notably tube-
sides 15, 28;
- defrosting of gas/cold gas mixture controls, used to cool
the refrigerant circuit tubes to the temperature of between
about -25 C and about -35 C.
Further cooling down of the main cryogenic heat exchanger
to the operating temperature of below about -155 C, here to
an operating temperature of about -160 C, was achieved using
the automated cooling down method and apparatus. The further
cooling down may hereinafter be referred to as the "final
cool down".
So, step (i) as described above may comprise obtaining
one or more refrigerant temperature indications comprising at
least one of a refrigerant temperature indication of the
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liquid, heavy refrigerant fraction (HMR) and/or the gaseous,
light refrigerant fraction (LMR)
- at a suction side of the JT valve 14;
- at a discharge side of the JT valve 14;
- at an entry side of the cryogenic heat exchanger 1;
- at a point inside the cryogenic heat exchanger 1;
- at a discharge side of the cryogenic heat exchanger 1.
So, in view of the embodiment described above with
reference to Fig.'s 4 and 5, according to an embodiment the
refrigerant recirculation circuit to recirculate spent
refrigerant back to the cryogenic heat exchanger comprises a
plurality of compression stages with each compression stage
comprising a compressor recycle valve (130, 131) and the
adjustment step (507) comprises adjusting and closing the
plurality of recycle valves (509a, 509b).
So, in view of the embodiment described above with
reference to Fig.'s 4 - 6, according to an embodiment
downstream of the cooler 42 and upstream of the first JT
valve a liquid/vapour separator 45 is provided in the
refrigerant recirculation circuit, to receive a partly
condensed refrigerant and separate the partly-condensed
refrigerant stream into a liquid heavy refrigerant fraction
(HMR) and a gaseous light refrigerant fraction (LMR) and to
discharge the liquid heavy refrigerant fraction via a liquid
outlet and the gaseous light refrigerant fraction via a gas
outlet, which fractions are passed to the cryogenic heat
exchanger, wherein the first JT valve is arranged to control
passage of one of these fractions, preferably the light
refrigerant fraction and wherein a second JT valve is arrange
to control passage of the other of these fractions,
preferably the heavy refrigerant fraction.
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Next, with reference to Fig. 6 a block diagram for
automatically cooling down the cryogenic heat exchanger of
Fig. 4 or Fig. 5 will be described.
Similar to the embodiment described above with reference
to Fig. 3, after a start signal is generated in step 501, a
comparison step 502 is performed which comprises:
(i) receiving one or more refrigerant temperature
indications, providing an indication of the temperature of
the refrigerant,
(ii) comparing the one or more refrigerant temperature
indications with one or more associated predetermined
threshold values, and
(iii) based on the outcome of the comparison under (ii),
selecting one of an automated warm cooling down procedure 503
of the cryogenic heat exchanger or an automated cold cooling
down procedure 504 of the cryogenic heat exchanger.
The warm cooling down procedure 503 will not be described
in more detail here. Reference is made to W02009/098278 in
which a detailed description of the warm cooling down
procedure is provided.
The cold cooling down procedure 504 starts with defining
the initial conditions in action 505 much in the same way as
described above. Examples of critical initial conditions are:
- presence of an excess of heavy components in the
hydrocarbon feed (e.g. in line 20) if the hydrocarbon flow is
manipulated (generally a maximum of 0.08 mol% of 05+ is
tolerated);
- first and second JT valves (58, 51) not sufficiently
closed (in the test a value of more than 1 % open was used);
- pressure in refrigeration circuit (LMR and HMR) is lower
than the compressor 31 discharge;
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- one or more of refrigerant compressors 30, 31, and pre-cool
refrigerant compressor 127 is not on-line and running (as
e.g. monitored by compressor speed);
- suction and discharge valves on these compressors are not
open;
- refrigerant pressure at the compressor 31 discharge is too
high (the test used a maximum of 20 barg);
- pre-cooling refrigerant compressor 127 suction pressure
outside of a predetermined pressure window (suitably a window
around approximately 0.5 barg);
- any IGV valve present is not sufficiently closed.
Examples of non-critical initial conditions are:
- TDR5759 too small (a typical minimum value recommended in
case of a coil wound heat exchanger from Air Products &
Chemicals Inc is 25 C);
- one or more of refrigerant compressor recycle valves (e.g.
130, 131) are not sufficiently open (the test used less than
99 % open);
- discharge pressure of compressor 31 below a pre-determined
minimum value (the test used 18 barg).
When the initial conditions are defined and possible
warnings are resolved, actions 506 and 511 are triggered.
In initial opening action 506 the first JT valve 58 for
controlling the vapour (light) refrigerant stream and a
second JT valve 51 for controlling the liquid (heavy)
refrigerant stream are set at an initial opening (506a),
wherein the initial opening according to the automated warm
cooling down procedure 503 differs from the initial opening
step of the first JT valve 14 according to the automated cold
cooling down procedure 504. Preferably, the initial openings
of the JT valves are greater in the automated warm cooling
down procedure 503 than in the automated cold cooling down
procedure 504.
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Action 506b, following action 506a initiates a waiting
time as described above.
The initial opening step may comprise imposing an initial
opening of the first and second JT valve, wherein the initial
opening step of the first and second JT valves (51, 58)
according to the automated warm cooling down procedure
differs from the initial opening step of the first and second
JT valves (51, 58) according to the automated cold cooling
down procedure.
In particular, the initial opening of the first and
second JT valves is greater in the automated warm cooling
down procedure than in the automated cold cooling down
procedure.
The initial opening step may further comprise performing
a TROC step comprising adjusting the opening of the first and
second JT valves 51, 58 based on a determined temperature
rate of change (TROC) of the refrigerant over the first and
second JT valves 51, 58 in accordance with an adjustment
scheme, wherein the automated warm cooling down procedure 503
and the automated cold cooling down procedure 504 comprise
different adjustment schemes.
In particular, the adjustment scheme of the cold cooling
down procedure may comprise waiting a predetermined time
interval between imposing an initial opening of the first and
second JT valves 51, 58 and initiating adjusting the opening
of the first and second JT valves 51, 58 based on a monitored
temperature rate of change (TROC) of the refrigerant over the
first and second JT valves 51, 58.
Next, after action 506 has been completed the automated
cold cooling down procedure comprises performing an
adjustment step (507) comprising simultaneously
- adjusting and closing recycle valve (509) and
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- further adjusting the first and second JT valves (508a,
508b).
As described above, there may be a plurality of
compression stages with each compression stage comprising a
compressor recycle valve (130, 131) and action 509 may thus
comprise adjusting and closing a plurality of recycle valves
130, 131.
The make-up adjustment is controlled in action 512 which
is performed parallel to action 507 and manipulates the make-
up to:
= Increase the compressor 31 discharge pressure along a
ramp towards a target operating pressure (in the test, 30
barg);
= Move the refrigerant composition towards a target
composition, which may be an end target for normal operation
of the main cryogenic heat exchanger 100 or an intermediate
target.
The refrigerant target composition may change during the
cool down procedure. It may change gradually or step wise
upon a controlled variable reaching a predetermined value.
For instance, it may change once the temperature TR57 goes
below a predetermined value of -135 C or -140 C.
Parallel to actions 506 and 507, action 511 is executed
which adjusts one or more of the pre-cool refrigerant
compressor recycle valve(s), here in the form of the first
stage recycle valve 129 that controls recycle stream through
the first compression stage of compressor 127. The module
objective is to maintain a suction pressure on the pre-cool
refrigerant suction pressure (in conduit 150 of Fig. 4)
within a pre-determined range, e.g. 0.25 - 0.50 barg, but
without reducing the surge deviation too close to the control
line. The low pressure will assure that the temperature of
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the hydrocarbon feed gas going into the main cryogenic heat
exchanger 100 (e.g. via conduit 20) has a reasonable value.
Therefore, the temperature in conduit 20 itself does not need
to be monitored or used as condition for control in this
module.
Additionally, the pre-cooling refrigerant compressor 127
discharge temperature (in conduit 135) was not monitored,
since the automated cool down procedure as used in the test
did not offer a capability to manipulate any variable that
could be used to improve the situation of a high discharge
temperature of the pre-cooling refrigerant compressor 127.
However, this may be implemented without departing from the
scope of the invention.
There may be built in some overriding boundaries, for one
or more of the monitored variables. Crossing of one of these
boundaries (i.e. exceeding a pre-determined maximum and/or
minimum value) by one or more of the monitored variables may
result in issuance of a warning signal to alert an operator,
or pausing the cooling down, or abortion of the cooling down,
or a combination of these.
Typical examples of such overriding boundaries are:
- a pre-determined maximum temperature rate of change
(e.g. 28 C/hour as specified for an Air Products cryogenic
heat exchanger) on any selected temperature, suitably one or
more of a temperature of the hydrocarbon product at a
location in tube side 29 and/or in the discharge conduit 40;
the spent refrigerant temperature (e.g. in bottom warm end of
the shell side 33 or in conduit 25); the refrigerant
temperature at the discharge side of the first JT valve 58 or
the second JT valve 51, or at the suction side thereof; any
shell side temperature in the heat exchanger 1;
- a pre-determined maximum spatial temperature gradient,
reflecting a specified maximum temperature difference between
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two spatially separated points in or around the heat
exchanger (e.g. a maximum temperature difference of 28 C),
suitably the temperature difference TDR2547 between the light
refrigerant upstream of main cryogenic heat exchanger 100 and
the spent refrigerant (also possible: TDR3347, not shown);
the temperature difference TDR2548 between the heavy
refrigerant upstream of main cryogenic heat exchanger 100 and
the spent refrigerant (also possible: TDR3348, not shown);
TDR2715; and TDR5759;
- a predetermined maximum content (0.08 mol%) of heavy
components in the hydrocarbon feed stream that would freeze
in the main cryogenic heat exchanger 100;
- suction and discharge valves on the refrigerant
compressors closed;
- a maximum specified top shell pressure (5 barg) at the
cold end of the main cryogenic heat exchanger;
- detection of a trip;
- existence of communication errors in the control
system.
Clearly, other overriding boundaries may be used, e.g. in
case of other types of cryogenic heat exchangers being used.
Although not implemented in the test, it has been
contemplated to further embed the above block diagrams (Fig.
6 or a similar one for another line-up or heat exchanger) in
a larger structure comprising other, preceding or subsequent
actions, or both. An example is shown in Fig. 7.
Fig. 7 shows an example with some post cool-down tasks.
These may, for instance, be intermediate tasks that need to
be completed before an automatic process control system for
normal operation can take over the control. For instance,
module 401 manipulates the run down valve 44, with the goal
to ramp up the flow through conduit 20 and 40 and the
hydrocarbon tube side 29.
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Other modules could therefore be in parallel to module
401. As an example, module 402 has been depicted, but also
included could be a module for ramping up any fractionation
section that may be provided downstream of any NLG extraction
column to receive and further fractionate the extracted NLG
liquids. The person of skill in the art would be able to work
out which manipulated and controlled variables could be used,
depending on the type of line-up and equipment used.
The apparatuses and methods described herein may be
applied to cryogenic heat exchangers whenever a cryogenic
heat exchanger needs to be cooled down before operation. This
could for instance be initial cooling down, or cooling down
after a maintenance operation or after a trip: the reason why
the heat exchanger was warmer than operation temperature is
not material to the application of the subject matter
described herein.
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. The
invention has been described with particularity, including
providing target values for certain controlled variables.
However, it will be apparent to the person skilled in the art
that these values were chosen in connection to the specific
line up and equipment used for the test. Such details may
need to be optimized when the invention is to be carried out
on another line-up using other equipment, and therefore
should not be considered as limiting the scope of the present
invention.
