U.S. patent application number 12/742674 was filed with the patent office on 2010-11-04 for method and apparatus for cooling a process stream.
Invention is credited to Jeroen Van De Rijt.
Application Number | 20100275645 12/742674 |
Document ID | / |
Family ID | 39446108 |
Filed Date | 2010-11-04 |
United States Patent
Application |
20100275645 |
Kind Code |
A1 |
Van De Rijt; Jeroen |
November 4, 2010 |
METHOD AND APPARATUS FOR COOLING A PROCESS STREAM
Abstract
The invention provides a method of cooling a process stream, the
method comprising at least the steps of: (a) heat exchanging a
first coolant supply stream in a first cooling circuit against a
process stream at a first process temperature to produce a first
coolant return stream and a cooled process stream; (b) passing the
first coolant return stream to a first coolant return tank to
provide warmed first coolant; (c) withdrawing a portion of the
warmed first coolant from the first coolant return tank as a warmed
first coolant stream; (d) heat exchanging a cooled second coolant
stream in a second cooling circuit against the warmed first coolant
stream to produce a cooled first coolant stream, (e) passing the
cooled first coolant stream to a first coolant supply tank to
provide cooled first coolant; (f) withdrawing a portion of the
cooled first coolant from the first coolant supply tank as the
first coolant supply stream; wherein the rate of flow of the warmed
first coolant stream in step (c) is controlled in response to the
cooling duty available from the second cooling circuit and the flow
from the supply to the return buffer is controlled by the required
cooling of the process stream and wherein the difference between
the minimum and maximum cooling duty of the process stream over a
time period is larger than the difference between the minimum and
maximum cooling duty of the second cooling circuit over said time
period.
Inventors: |
Van De Rijt; Jeroen;
(Amsterdam, NL) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
39446108 |
Appl. No.: |
12/742674 |
Filed: |
November 14, 2008 |
PCT Filed: |
November 14, 2008 |
PCT NO: |
PCT/EP2008/065562 |
371 Date: |
July 26, 2010 |
Current U.S.
Class: |
62/613 |
Current CPC
Class: |
F25B 25/005 20130101;
F25B 2700/04 20130101; F25D 17/02 20130101 |
Class at
Publication: |
62/613 |
International
Class: |
F25J 1/02 20060101
F25J001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 15, 2007 |
EP |
07120806.0 |
Claims
1. A method of cooling a process stream, the method comprising at
least the steps of: (a) heat exchanging a first coolant supply
stream in a first cooling circuit against a process stream at a
first process temperature to produce a first coolant return stream
and a cooled process stream; (b) passing the first coolant return
stream to a first coolant return tank to provide warmed first
coolant; (c) withdrawing a portion of the warmed first coolant from
the first coolant return tank as a warmed first coolant stream; (d)
heat exchanging a cooled second coolant stream in a second cooling
circuit against the warmed first coolant stream to produce a cooled
first coolant stream, (e) passing the cooled first coolant stream
to a first coolant supply tank to provide cooled first coolant; and
(f) withdrawing a portion of the cooled first coolant from the
first coolant supply tank as the first coolant supply stream;
wherein the rate of flow of the warmed first coolant stream in step
(c) is controlled in response to the cooling duty available from
the second cooling circuit and the flow from the supply to the
return buffer is controlled by the required cooling of the process
stream and wherein the difference between the minimum and maximum
cooling duty of the process stream over a time period is larger
than the difference between the minimum and maximum cooling duty of
the second cooling circuit over said time period.
2. A method according to claim 1 wherein the second cooling circuit
is any chiller system, such as compression refrigeration systems
with refrigerants such as propane, ammonia, R-134a or absorption
chilling systems based on Lithium Bromide.
3. A method according to claim 2 wherein the first coolant
comprises water.
4. A method according to claim 3, wherein the first coolant is a
refrigerant, and wherein the second coolant is cooling water or
ambient air.
5. A method according to claim 4 wherein the sum of the amount of
warmed first coolant in the first coolant return tank and the
amount of cooled first coolant in the first coolant supply tank is
held at a constant value.
6. A method according to claim 5 wherein steps (a) to (g) are
repeated over at least one day and night cycle.
7. An apparatus for cooling a process stream, such as a stream
derived from natural gas, the apparatus at least comprising: a
first cooling circuit comprising a first heat exchanger, a first
coolant return tank, a second heat exchanger and a first coolant
supply tank, said first heat exchanger having a first inlet which
is connected to a process stream line, a first outlet which is
connected to a cooled process stream line, a second inlet which is
connected to the outlet of the first coolant supply tank and a
second outlet which is connected to the inlet of the first coolant
return tank, said second heat exchanger having a first inlet which
is connected to the outlet of the first coolant return tank and a
first outlet which is connected to the inlet of the first coolant
supply tank; and a second cooling circuit comprising the second
heat exchanger and a cooling system, said second heat exchanger
having a second inlet connected to the outlet of the cooling system
and a second outlet connected to the inlet of the cooling system,
wherein the cooling system of the second cooling circuit comprises
a compressor, a condenser, and an expansion device, said compressor
having an inlet connected to the second outlet of second heat
exchanger, an outlet connected to an inlet of the condenser, said
condenser having an outlet connected to an inlet of the expansion
device and said expansion device having an outlet connected to the
second inlet of the second heat exchanger and wherein the first
inlet of first heat exchanger is connected to the outlet of an air
cooler.
8. An apparatus according to claim 7 wherein a first pump is
provided between the outlet of first coolant return tank and the
first inlet of second heat exchanger and a second pump is provided
between the outlet of first coolant supply tank and the second
inlet of the first heat exchanger, wherein a temperature sensor is
provided in stream line at the outlet of heat exchanger to detect
the temperature in process stream line, said temperature sensor
being connected to a first processor which determines the net
supply of first coolant via the first pump to first heat exchanger
and wherein a temperature sensor is provided between the outlet of
second heat exchanger and the inlet of first coolant supply tank,
said sensor being connected to a second processor which determines
the supply of first coolant via the second pump to second heat
exchanger and wherein a level sensor is provided in first coolant
supply tank, said level sensor being connected to a third processor
which determines the duty of the cooling system.
Description
[0001] The invention relates to a method of cooling a process
stream, such as a liquid or gaseous stream in a process plant such
as a refinery, Liquefying Natural Gas plant or chemical plant,
particularly a process stream derived from the treatment of natural
gas, and an apparatus for use in such a method.
[0002] The invention is aimed at providing a method and apparatus
which can be designed to have a lower capacity compared to
conventional units, while still providing comparable cooling
performance. Such a reduction in design capacity provides
significant savings in capital expenditure, allowing the use of
smaller equipments, such as heat exchangers, compressors and
condensers.
[0003] The invention provides method of cooling a process stream,
the method comprising at least the steps of:
(a) heat exchanging a first coolant supply stream in a first
cooling circuit against a process stream at a first process
temperature to produce a first coolant return stream and a cooled
process stream; (b) passing the first coolant return stream to a
first coolant return tank to provide warmed first coolant; (c)
withdrawing a portion of the warmed first coolant from the first
coolant return tank as a warmed first coolant stream; (d) heat
exchanging a cooled second coolant stream in a second cooling
circuit against the warmed first coolant stream to produce a cooled
first coolant stream, (e) passing the cooled first coolant stream
to a first coolant supply tank to provide cooled first coolant; (f)
withdrawing a portion of the cooled first coolant from the first
coolant supply tank as the first coolant supply stream; wherein the
rate of flow of the warmed first coolant stream in step (c) is
controlled in response to the cooling duty available from the
second cooling circuit and the flow from the supply to the return
buffer is controlled by the required cooling of the process stream
and wherein the difference between the minimum and maximum cooling
duty of the process stream over a time period is larger than the
difference between the minimum and maximum cooling duty of the
second cooling circuit over said time period.
[0004] Conventionally, a first cooling circuit which is heat
exchanged against a process stream is operated to provide a
particular cooling duty, corresponding to the peak capacity
required of the cooling circuit said peak capacity being set by the
process stream. This is achieved by providing a corresponding heat
rejection to a second cooling circuit, which is for instance a
closed refrigeration cycle.
[0005] However, the amount of heat which can be rejected to the
second cooling circuit can change as a result of external
conditions, such as ambient temperature. Additionally, the required
heat rejection from the process can change as a result of the same
external conditions. Indeed, the temperature variance between day
and night in some climates may be more than 10.degree. C.,
sometimes more than 20.degree. C., and even more than 30.degree.
C.
[0006] At low ambient temperature the second cooling circuit can
provide greater cooling to the first cooling circuit, however the
required cooling of the process by the first circuit is less. In
the method of the present invention, the rate of flow of the warmed
first coolant stream from return to supply tank is controlled in
response to the cooling duty available from the second cooling
circuit; whereas the flow of the cooled first coolant from the
supply to the return buffer is controlled by the required cooling
of the process. When more heat can be rejected to the second
cooling circuit, the flow rate of the warmed first coolant stream
can be increased, so that more warmed first coolant is heat
exchanged against cooled second coolant in step (d), providing more
cooled first coolant which can be accumulated in the cooled first
coolant buffer supply tank. When more heat needs to be rejected
from the process, the cooled 1.sup.st coolant flow from supply to
return buffer is increased.
[0007] When ambient temperatures increase, the amount of cooling
provided by the second cooling circuit to the first cooling circuit
decreases and the rate of flow of the warmed first coolant stream
can be correspondingly reduced. Should the cooling duty placed on
the first cooling circuit by the process stream increase, the flow
rate of the first coolant supply stream can be increased in order
to ensure that the cooled process stream is provided at a
controlled, preferably constant, temperature.
[0008] Increasing the flow rate of the first coolant supply stream,
for example due to an increased cooling requirement of the process
stream, may consume more cooled first coolant than is being
supplied by the cooled first coolant stream (by heat exchange with
the second cooling circuit). However, the accumulated cooled first
coolant in the cooled first coolant buffer supply tank allows the
effective operation of the cooling apparatus to be maintained.
[0009] The method of the present invention therefore enables a
reduction in the capacity of the second cooling circuit compared to
a conventional circuit which is designed to meet the peak load
placed on the apparatus by the process stream. Instead of designing
an apparatus for peak conditions e.g. maximum ambient temperatures
(a capacity which is only partially used at lower ambient
conditions), the first and second cooling circuits can be designed
for a lower capacity. Any deficit of cooled first coolant (produced
by heat exchange with the second cooling circuit) required to cool
the process stream can be met by cooled first coolant from the
cooled first coolant buffer supply tank. During lower ambient
temperatures, the cooling capacity of the second cooling circuit
can be higher than required by the first cooling circuit to cool
the process stream, and so excess cooled first coolant can be
produced and stored in the cooled first coolant supply tank.
[0010] Preferably the method is operated such that the sum of the
amount of warmed first coolant in the first coolant return tank and
the amount of cooled first coolant in the first coolant supply tank
is held at a constant value. The relative amounts of first coolant
can, of course, vary between the first coolant supply and return
tanks.
[0011] The present invention therefore permits a cooling apparatus
to be provided with a lower capacity than a conventional unit. For
instance a capacity reduction to about 60% of the conventional
capacity needed can be provided for the second cooling circuit,
leading to large reductions in capital expenditure, even despite
the additional buffer tanks and circulation pumps needed.
[0012] Preferably, the method of the invention is a continuous
method for at least 24 hours and contains at least one day and
night cycle i.e. steps (a) to (g) are repeated over at least one
day and night cycle. More preferably, the method is a continuous
method with a duration of one week, preferably one month, more
preferably 6 months, and even more preferably one year.
[0013] The process stream cooled in heat exchange step (a) may be
any liquid or gaseous stream. For example, the process stream
cooled in heat exchange step (a) may be a liquid or gaseous stream
derived from the treatment of natural gas. Preferably, the process
stream is an amine stream from an acid gas treatment unit or a
sweet or sour natural gas stream.
[0014] The process stream is provided at a first process
temperature, which is for example in the range of 20 to 65.degree.
C. The process stream can be provided by pre-cooling a warm process
stream with an air cooler. The effectiveness of an air cooler will
vary depending upon ambient conditions, such as ambient
temperature. This is because the air cooler is designed to provide
a particular cooling duty under maximum ambient temperatures. At
lower than maximum temperatures, the process stream will exit the
air cooler with a lower temperature than that provided under peak
conditions. Consequently, the cooling duty placed on the first
cooling circuit by the process stream will be less. The flow rate
of the first coolant supply stream can thus be reduced, lowering
the consumption of cooled first coolant from the cooled first
coolant supply tank. Cooled first coolant can therefore be
accumulated in the cooled first coolant supply tank, for use when
the cooling duty on the first cooling circuit increases.
[0015] By providing a buffer supply tank and a buffer return tank,
the present invention allows the flow rate of the first coolant
supply stream, which cools the process stream, to be varied
independently from the flow rate of the warmed first coolant
stream, which ejects heat to the second cooling circuit.
[0016] The first cooling circuit is preferably a closed
recirculating cooling circuit. Similarly, it is preferred that the
second cooling circuit is a closed recirculating cooling
circuit.
[0017] The first cooling circuit utilizes a first coolant.
Preferably, the first coolant comprises water. More preferably, the
first coolant consists essentially of water. In such cases, the
first coolant may also contain standard water additives, such as
antifoams, antiscalants, biocides and corrosion inhibitors. The
first coolant supply stream would thus be a chilled water supply
stream. The first coolant return stream would therefore be a warmed
water return stream.
[0018] In the second cooling circuit is preferably a heat pump,
more preferably a chiller system of the compression or absorption
type. The heated second coolant stream produced in step (d) may be
passed to a cooling system to regenerate the cooled second coolant
stream. If the second coolant circuit is an absorption cycle, the
second coolant may comprise water with a lithium bromide absorbent.
If the second cooling circuit is a compression cycle, the cooling
system may comprise a compressor, a condenser and an expansion
device. In the latter case, the second coolant comprises a
refrigerant, for example propane, ammonia, R-134a or any other
commercially available refrigerant.
[0019] It is preferred that heat exchange step (a) is carried out
in a first heat exchanger. It is further preferred that heat
exchange step (d) is carried out in a second heat exchanger. The
heat exchange may be achieved through direct contact of the process
stream and the first coolant supply stream in step (a) or the
warmed first coolant stream and the cooled second coolant stream in
step (d). Alternatively, indirect heat exchange may be used in
steps (a) and (d), and this is preferred. Indirect heat exchange
may be carried out in a shell and tube heat exchanger, an EM baffle
heat exchanger, a plate and frame heat exchanger or a fin tube heat
exchanger.
[0020] In a further embodiment, the present invention provides an
apparatus for cooling a process stream, said apparatus at least
comprising:
[0021] a first cooling circuit comprising a first heat exchanger, a
first coolant return tank, a second heat exchanger and a first
coolant supply tank,
[0022] said first heat exchanger having a first inlet which is
connected to a process stream line, a first outlet which is
connected to a cooled process stream line, a second inlet which is
connected to the outlet of the first coolant supply tank and a
second outlet which is connected to the inlet of the first coolant
return tank,
[0023] said second heat exchanger having a first inlet which is
connected to the outlet of the first coolant return tank, and a
first outlet which is connected to the inlet of the first coolant
supply tank; and
[0024] a second cooling circuit comprising the second heat
exchanger and a cooling system,
[0025] said second heat exchanger having a second inlet connected
to the outlet of the cooling system and a second outlet connected
to the inlet of the cooling system.
[0026] It is preferred that the cooling system of the second
cooling circuit comprises a compressor, a condenser, and an
expansion device. The compressor can have an inlet connected to the
second outlet of second heat exchanger and an outlet connected to
an inlet of the condenser. The condenser can have an outlet
connected to an inlet of the expansion device, with the expansion
device having an outlet connected to the second inlet of the second
heat exchanger.
[0027] Furthermore, an air cooler may be provided upstream of the
first heat exchanger in the process stream, for example by
connecting the first inlet of the first heat exchanger to the
outlet of the air cooler.
[0028] In a further aspect of the invention, a first pump can be
provided between the outlet of first coolant return tank and the
first inlet of second heat exchanger. Furthermore, a second pump
can be provided between the outlet of first coolant supply tank and
the second inlet of the first heat exchanger. These pumps can
control the flow rate of the first coolant supply stream and the
warmed first coolant stream. The flow can also be controlled using
control valves. It will be apparent that the first pump (or a
further pump) could be placed in the cooled first coolant stream
rather than the warmed first coolant stream. Similarly, the second
pump (or a further pump) could also be placed in the first coolant
return stream rather than the first coolant supply stream.
[0029] In another aspect, the coolant flow through the first heat
exchanger (process/first coolant) can be adjusted to maintain a
constant controlled process outlet temperature ex first heat
exchanger. Depending on the process flow rate, physical properties
and specifically its temperature (downstream the air cooler) at the
inlet of the first heat exchanger, the required heat transfer rate
(to maintain a controlled process temperature ex first heat
exchanger) in the first heat exchanger varies. The actual heat duty
can be manipulated in different ways. Preferably, the temperature
in the process stream exiting the first heat exchanger is measured
by a sensor and said temperature sensor is connected to a processor
which determines the cooled first coolant flow to the first heat
exchanger and thereby manipulating the heat duty in first heat
exchanger.). The person skilled in the art may readily find
alternative control schemes.
[0030] In yet another aspect, a temperature sensor is provided in
the cooled chilled water ex second heat exchanger (chilled water
vs. refrigerant), said temperature being connected to a processor
and said processor manipulating the chilled water flow through the
second heat exchanger to maintain a constant chilled water supply
temperature.
[0031] The duty of the refrigerant cycle can be manipulated in
order to match the required cooling. For example level control on
the chilled water supply tank can manipulate the duty (e.g.
refrigerant compressor duty control system) of the refrigerant
cycle, so varying the refrigerant supply to the refrigerant/chilled
water heat exchanger.
[0032] The described control schemes maintain constant and
controlled temperatures in the supply and the return tanks.
Alternative control schemes may readily be found but always aiming
at restoring the buffer of cooled first coolant in the supply tank
when the duty of the second cooling circuit exceeds the cooling
duty of the process.
[0033] Hereinafter the invention will be further illustrated by the
following non-limiting drawings.
[0034] FIG. 1 schematically shows a process scheme in accordance
with an embodiment of the present invention.
[0035] FIG. 2 shows a plot of the levels of the cooled and warmed
primary coolant buffer tanks over time when carrying out the method
of the present invention.
[0036] For the purpose of this description, a single reference
number will be assigned to a line as well as a stream carried in
that line. Same reference numbers refer to similar components.
[0037] FIG. 1 is a schematic diagram showing an apparatus of the
present invention 1 comprising a process cascade 2, a first cooling
circuit 10 and a second cooling circuit 60. First cooling circuit
10 is used to cool process stream 6. Process stream 6 is a liquid
or gaseous stream preferably in a refinery, natural gas treatment
plant, liquefied natural gas, Gas-to-Liquids (also known as Shell
Middle Distillate Synthesis) or chemical process plant, for example
an amine stream from an acid gas treatment unit or a sweet or sour
natural gas stream.
[0038] The present invention is directed towards the cooling of the
process stream. Process stream 6 is provided at a first process
temperature, which is preferably in the range of 20 to 65.degree.
C. Process stream 6 is passed to first heat exchanger 20 via first
inlet 7 where it is heat exchanged against a first coolant supply
stream 52 to provide a cooled process stream 9 at first outlet 8.
Cooled process stream 9 is produced at a second process
temperature, which is preferably in the range of 0 to 35.degree.
C., preferably 25.degree. C. Cooled process stream 9 can then be
passed on to other units for further processing. For instance, when
process stream 6 is an amine stream from an acid gas treatment
unit, it can be passed to a regeneration column after cooling.
[0039] First cooling circuit 10 is used to provide the cooling duty
necessary to cool process stream 6. First cooling circuit 10 is
preferably a closed loop cooling circuit. First cooling circuit 10
comprises a first coolant which preferably comprises water, more
preferably the first coolant consists essentially of water, even
more preferably for low temperatures a mixture of water and glycol
or other anti-freeze agent. In this case, the coolant water stream
may also contain standard closed loop water additives, such as
antiscalants, biocides and corrosion inhibitors.
[0040] First coolant supply stream 52, which is preferably a
chilled water stream, is provided at a first coolant supply
temperature, which can be in the range of 5 to 30.degree. C. First
coolant supply stream 52 is passed to first heat exchanger 20 via
second inlet 53, where it cools process stream 6 and is itself
warmed to produce a first coolant return stream 22 at second outlet
21. First coolant return stream 22 has a first coolant return
temperature, which can be in the range of 10 to 45.degree. C.
[0041] First coolant return stream 22 is passed to a first coolant
return tank 30 via inlet 23. First coolant return tank 30 provides
a storage buffer for the warmed first coolant. Return tank 30 may
be provided with insulation, in order to minimise heat ingress from
surroundings. Return tank 30 operates to buffer the warmed first
coolant between the first and second heat exchangers. The warmed
first coolant is drawn from return tank 30 via outlet 32 as warmed
first coolant stream 32.
[0042] When the cooling duty placed on the first cooling circuit 10
by process stream 6 is greater than the heat which can be ejected
to second cooling circuit 60, the rate of flow of first coolant
return stream 22 will be more than the rate of flow of warmed first
coolant stream 32, and the level of warmed first coolant in return
tank 30 will rise.
[0043] When the cooling duty placed on the first cooling circuit 10
by process stream 6 is less than the heat which can be rejected to
second cooling circuit 60, the rate of flow of first coolant return
stream 22 will be less than the rate of flow of warmed first
coolant stream 32, and the level of warmed first coolant in return
tank 30 will fall.
[0044] After exiting return tank 30, warmed first coolant stream 32
is passed to first inlet 33 of second heat exchanger 40, where it
is cooled against cooled second coolant stream 72 to regenerate
cooled first coolant stream 42 via outlet 41. Cooled first coolant
stream 42 is produced at the first coolant supply temperature,
which can be in the range of 5 to 30.degree. C., as discussed
above.
[0045] Cooled first coolant stream 42 is passed to first coolant
supply tank 50 via inlet 43. Supply tank 50 provides a storage
buffer for the cooled first coolant. It is preferred that supply
tank 50 is provided with insulation, in order to minimise heat
ingress from the surroundings. Supply tank 50 operates to buffer
the cooled first coolant between the second and first heat
exchangers. The cooled first coolant is drawn from supply tank 50
via outlet 51 as first coolant supply stream 52.
[0046] When the cooling duty placed on first cooling circuit 10 by
process stream 6 is greater than the heat which can be ejected to
second cooling circuit 60, the rate of flow of cooled first coolant
stream 42 will be less than the rate of flow of first coolant
supply stream 52, and the level of cooled first coolant in supply
tank 50 will fall.
[0047] When the cooling duty placed on first cooling circuit 10 by
process stream 6 is less than the heat which can be ejected to
cooling circuit 60, the rate of flow of cooled first coolant stream
42 will be greater than the rate of flow of first coolant supply
stream 52, and the level of cooled first coolant in supply tank 50
will rise.
[0048] From the foregoing discussion, it will be apparent that the
flow rate of warmed first coolant stream 32 should be equal to the
flow rate of cooled first coolant stream 42 in order to provide a
uniform flow through second heat exchanger 40. Similarly, the flow
rate of first coolant supply stream 52 should be equal to the flow
rate of first coolant return stream 22 in order to provide a
uniform flow through first heat exchanger 20.
[0049] However, there is no requirement in the present invention
that the flow rate of cooled first coolant supply stream 52 (or
first coolant return stream 22) should be the same as warmed first
coolant stream 32 (or cooled first coolant stream 42). Indeed, the
method and apparatus of the present invention can provide unequal
flows of streams 52 and 22 compared to streams 32 and 42.
Consequently, the levels of warmed and cooled first coolant in
return tank 30 and supply tank 50 respectively can be varied in
response to changes in the cooling duty required by process stream
6 and the heat which can be ejected to second cooling circuit
60.
[0050] By buffering the cooled first coolant in supply tank 50 and
warmed first coolant in return tank 30, it is possible to
compensate for changes in ambient temperature. The ambient
temperature can effect both the cooling duty which can be provided
by second cooling circuit 60 (and hence the amount of heat which
can be ejected by the first coolant circuit) and the temperature of
process stream 6 (and hence the cooling duty required of first
refrigerant circuit 20).
[0051] For instance, at low ambient temperatures, such as at night,
the second cooling circuit 60 can generate more cold. This is
because lower temperatures can be achieved in the second cooling
system, for example in the condenser of the discharge of the second
coolant compressor, when the ambient temperature is lower. The
generation of more cold in second cooling circuit 60 allows more
heat to be removed from the first cooling circuit 10. Under these
conditions, the cooling capacity of the of the second cooling
circuit 60 can be higher than required by process stream 6, and
cooled first coolant can be accumulated in supply buffer 50,
because the rate at which warmed first coolant stream 32 is cooled
in second heat exchanger 40 can be increased. The excess cooled
first coolant in supply buffer 50 can be stored for use when the
cooling duty on the first cooling circuit 10 increases, for
instance at high ambient temperatures.
[0052] Instead of designing the second cooling circuit to generate
all the cold required by first cooling circuit 10 under peak
conditions (e.g. during operation at maximum ambient temperatures)
i.e. a peak capacity which is only partially used during lower
ambient temperatures, second cooling circuit 60 can be designed for
a lower capacity in accordance with the present invention.
[0053] At high ambient temperatures, such as during the day, the
second cooling circuit 60 will be operating closer to, or at its
maximum capacity, which is still lower than the peak capacity of
first heat exchanger 20. Any deficit of cooled first coolant
produced by heat exchange against the second cooling circuit during
high ambient temperature conditions is met by the buffered cooled
first coolant from first coolant supply tank 50.
[0054] During lower ambient conditions, the cooling capacity of the
second cooling circuit 60 is higher than actually required by
process stream 6, so that additional cooled first coolant can be
generated in second heat exchanger 40 to restore the level in first
coolant supply tank 50.
[0055] As discussed above, the first coolant increases in
temperature due to the cold ejected by the heat exchange between
first coolant supply stream 52 and process stream 4. Second cooling
circuit 60 provides cooling to the first coolant in first cooling
circuit 10.
[0056] Second cooling circuit 60 is preferably a closed
recirculating cooling circuit. Second cooling circuit 60 comprises
second heat exchanger 40 and a cooling system 70. Cooling system 70
generates the cooled second coolant stream 72 which is fed to
second inlet 73 of second heat exchanger 40. In second heat
exchanger 40, cooled second coolant stream 72 is heat exchanged
against warmed first coolant stream 32 to produce a heated second
coolant stream 46 at the second outlet 45 of second heat exchanger
40 and cooled first coolant stream 42. Heated second coolant stream
46 is then returned to cooling system 70 via inlet 47.
[0057] In the embodiment shown in FIG. 1, the second cooling
circuit is a compression cycle. More particularly, the compression
cycle is a refrigeration cycle in which the second coolant is
propane. In such a case, cooling system 70 will comprise a
compressor, a condenser and an expansion device (not shown). The
compressor has an inlet which is connected to the second outlet 45
of second heat exchanger 40. The compressor also has an outlet
which is connected to the inlet of a condenser. The condenser has
an outlet which is connected to the inlet of an expansion device,
such as an expansion valve. The expansion device has an outlet
which is connected to the second inlet 73 of second heat exchanger
40. The operation of such refrigeration systems is well known and
will not be discussed in greater detail here.
[0058] A further advantage of the present invention is that by
buffering the cooled first coolant in supply tank 50 and warmed
first coolant in return tank 30, it is possible to compensate for
changes in the temperature of process stream 6.
[0059] In the embodiment shown is FIG. 1, an air cooler 80 is
provided upstream of first heat exchanger 20 in process cascade 2.
Process stream 6 is produced by air cooler 80 at outlet 5 and
passed to first inlet 7 of first heat exchanger 20. Air cooler 80
is supplied by warm process stream 3 via inlet 4. Air cooler 80
cools warm process stream 3, preferably to a temperature in the
range of 40 to 65.degree. C. However, the temperature to which warm
process stream 3 is cooled will depend on the ambient air
temperature drawn through air cooler 80.
[0060] For instance, at lower ambient temperatures, such as during
the night, the first process temperature of process stream 6,
downstream of air cooler 80 will be lower than during high ambient
temperature. This is because air cooler 80 can provide greater
cooling to warm process stream 3 because the surrounding air is
cooler than during the day. Thus, if a constant second process
temperature for cooled process stream 9 is to be maintained
downstream of first heat exchanger 20, the cooling duty required of
first cooling circuit 10 during low ambient temperatures is less.
Under such low ambient temperatures, the flow rate of first coolant
supply stream 52 can be reduced, because the cooling duty placed on
first cooling circuit 10 by process stream 6 is less. A reduction
in the flow rate of first coolant supply stream 52 allows cooled
first coolant to be accumulated in supply buffer tank 50, because
the flow rate of first coolant supply stream 52 will be less than
the flow rate of cooled first coolant stream 42.
[0061] At higher ambient temperatures, such as during the day, the
first process temperature of process stream 6 will be higher than
at night, because air cooler 80 will not be able to provide as much
cooling to warm process stream 3. A higher cooling duty will
therefore be placed upon first cooling circuit 10. In order to
provide more cold to process stream 6 in order to maintain the
second process temperature downstream of first heat exchanger 20 at
a constant temperature, the flow rate of first coolant supply
stream 52 can be increased. When this flow rate becomes greater
than the flow rate of warmed first coolant stream 32 into second
heat exchanger 40, the cooled first coolant in supply buffer tank
50 will be consumed and the level of first coolant in the tank will
drop.
[0062] From the foregoing discussion, it is evident that the effect
of ambient temperature on the cooling duty available from second
cooling circuit 60 and the effect of ambient temperature on the
first temperature of process stream 6 operate in combination. In
particular, at lower ambient temperatures, second cooling circuit
60 can provide a higher cooling duty, allowing greater heat
rejection in the first cooling circuit 10, increasing the level of
cooled first coolant in supply buffer tank 50. At the same time,
the cooling duty placed upon the first cooling circuit by process
stream 6 is reduced because air cooler 80 can provide greater
cooling, reducing the first process temperature of process stream
6, allowing more cooled first coolant to be accumulated in supply
buffer tank 50.
[0063] These effects allow the level of cooled first coolant in
supply tank 50 to be increased at lower ambient temperatures, such
that when the temperatures increase during the day, and the cooling
duty available from second cooling circuit 60 decreases, and the
cooling duty placed on the first cooling circuit 10 by process
stream 6 increases, a reserve of cooled first coolant is available
in supply buffer tank 50 to meet the increased cooling demands.
[0064] In this way, the second cooling circuit of the present
invention can be provided with a lower capacity than conventional
units, which results in corresponding savings in capital
expenditure, while still meeting the same operational requirements.
This will be explained in greater detail in the following
non-limiting Example.
EXAMPLE 1
[0065] A closed loop chilled water circuit is provided as the first
cooling circuit. The first coolant supply stream (chilled water)
has a temperature of 25.degree. C., and the first coolant return
stream (warmed water) has a temperature of 40.degree. C. A propane
compression refrigeration cycle is provided as the second cooling
circuit.
[0066] Following the method of the invention, an apparatus was
constructed with a capacity of 58% of the equipment required for
conventional peak capacity operation. In particular, a 70 MW
chilled water/refrigerant heat exchanger (the second heat
exchanger) and refrigerant system with a capacity of 70 MW heat
rejection from circuit 1, with an additional 40 000 m.sup.3 chilled
water supply and return tank capacity was found to provide
equivalent performance including peak heat rejection from process
up to 120 MW. Without the invention the second circuit
(refrigerant, including compressors) would have been designed for
120 MW. The compressor and the rest of the propane cooling cycle
(second cooling circuit) was so reduced to 58% of the equivalent
equipment capacity required for conventional operation.
[0067] FIG. 2 is a plot showing the variation in the levels of the
chilled water buffer supply tank 50 and the warmed water buffer
return tank 30 over time for the apparatus of the Example. In this
experiment, the maximum level of chilled water in chilled water
supply tank 50 was set at 90% and the minimum level of warmed water
buffer return tank 30 was set at 10%.
[0068] From FIG. 2, it is apparent that the troughs in the level of
buffer supply tank 50, which occur when the difference between the
flow rate of chilled water supply stream 52 and warmed water return
stream 32 are at a maximum, are mirrored by peaks in the level of
warmed coolant in buffer return tank 30. This situation occurred at
peak ambient temperatures during the day, when the cooling duty
placed upon first cooling circuit 10 is at a maximum.
[0069] During lower ambient conditions at night, the level of
chilled water in buffer supply tank 50 was restored as the cooling
duty placed on first cooling circuit 10 decreased, and more cooling
was provided by second cooling circuit 60, while less cooling was
required by process stream 6.
[0070] The day and night cycles shown in the centre of the plot of
FIG. 2 occurred when extremely day-time ambient temperatures were
experienced. The chilled water level in the buffer supply tank 50
was unable to return to a level of 90% over a single day/night
cycle. Such a situation arose when the cooling duty placed upon the
first cooling circuit over a 24 hour period was greater than the
available heat rejection to the second cooling circuit. However,
the apparatus of the invention still functioned effectively because
additional chilled water was stored in buffer supply tank 50.
During the hottest day, the buffer supply tank level was reduced to
a minimum level of approximately 35%, and recovered to a level of
approximately 75% that night. Three subsequently hot days reduced
the level of chilled water in the buffer supply tank to
approximately 45%. However, the apparatus recovered to equilibrium
levels of chilled water and warmed water thereafter when the
ambient conditions return to a normal cycle.
[0071] The person skilled in the art will readily understand that
many modifications may be made without departing from the scope of
the invention. For example, alternative constructions can be
provided for the second coolant system utilising an absorption
system rather than compression system described with reference to
FIG. 1.
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