Note: Descriptions are shown in the official language in which they were submitted.
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MULTI-REFRIGERANT COOLING SYSTEM WITH PROVISIONS FOR
ADJUSTMENT OF REFRIGERANT COMPOSITION
Cross-Reference to Related Application
This application claims the benefit of United States Provisional Patent
Application Serial No. 61/058,947 filed June 5, 2008, which is hereby
incorporated by reference in its entirety.
Field of the Invention
The present invention is directed to a multi-refrigerant cooling system as
well as to a method for adjusting the composition of a refrigerant mixture of
a
multi-refrigerant cooling system.
Background
Multi-refrigerant cooling systems are previously known and operate with a
refrigerant mixture of two or more refrigerants having different condensation
temperatures. Thus, a mixture of refrigerants is circulated in the cooling
circuit of
the multi-refrigerant cooling system. Multi-refrigerant cooling systems are
used, in
particular, in industrial applications demanding very low temperatures. A
typical
application is the capture of carbon dioxide (CO2) from exhaust gases by
frosting
of CO2 ice.
Multi-refrigerant cooling systems are disclosed in for example
US 7,073,348 and US 2006/0277942. The disclosed systems operate according
to a cooling principle called integrated cascade.
In order to change the composition of the refrigerant mixture, a multi-
refrigerant cooling system is shut off, emptied of its refrigerant mixture and
refilled
with a refrigerant mixture of desired composition.
Summary
Objects of the present invention include the provision of a possibility for re-
use of refrigerants when an adjustment of the composition of a refrigerant
mixture
of a multi-refrigerant cooling system is performed; the provision of a
possibility of
keeping the amount of refrigerants that is wasted during an adjustment of the
composition of a refrigerant mixture of a multi-refrigerant cooling system to
a
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minimum; and the provision of a possibility to carry out an adjustment of the
composition of the refrigerant mixture during maintained operation of a multi-
refrigerant cooling system.
It is of economical as well as environmental interest to avoid or reduce
discharge of refrigerants from a multi-refrigerant cooling system in
connection
with adjustment of a composition of a refrigerant mixture. It is furthermore
of
operational as well as economical interest to allow adjustment of the
composition
of the refrigerant mixture during operation of the multi-refrigerant cooling
system.
The above-mentioned objects as well as further objects, which will become
apparent to a skilled person after studying the description below, are
achieved, in
a first aspect, by a multi-refrigerant cooling system comprising a cooling
circuit for
circulation of a refrigerant mixture comprising two or more refrigerants, the
cooling circuit comprising a compressor having an inlet and an outlet; one or
more separator(s) configured to separate and withdraw a respective refrigerant
fraction of the refrigerant mixture; and a client, the outlet of the
compressor being
connected to the client via the separator(s), wherein each separator is
connected
to a respective holding tank via a respective withdrawal conduit, each holding
tank being arranged to receive said respective refrigerant fraction from its
respective separator, wherein each holding tank further is connected to the
cooling circuit via a supply conduit, the supply conduit being configured to
supply
one or more refrigerant fraction(s) to the cooling circuit.
Thus, it is provided a multi-refrigerant cooling system allowing adjustment
of the composition of its refrigerant mixture to be performed under favourable
conditions. In particular, adjustment in view of changing client temperature
requirements and or changing environmental temperature can be addressed by
changing the composition of the refrigerant blend during operation of the
cooling
system.
As used herein, the term "client" relates to an item, along the cooling
circuit, which is to be cooled by the multi-refrigerant cooling system. Apart
from
what is described herein, the detailed layout of the cooling circuit, or its
working
principle, is not critical to the present invention.
The system may comprise a further holding tank connected via a further
withdrawal conduit to the cooling circuit at a position between the
separator(s)
and the client, the further holding tank being arranged to receive a
refrigerant
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fraction from the cooling circuit, wherein the further holding tank further is
connected to the cooling circuit via the supply conduit. Thus, also a
refrigerant
fraction remaining in the cooling circuit after separation, and subsequent
withdrawal to holding tank(s), of one or more other refrigerant fraction(s) by
the
separator(s), may be utilized when adjusting the composition of the
refrigerant
mixture.
The supply conduit may be connected to the cooling circuit at a position
between the client and the inlet of the compressor. A refrigerant fraction is
typically maintained in its holding tank at the pressure of its respective
separator
or slightly below. Since the separator(s) typically belong to the high
pressure side
of the cooling circuit, it is beneficial to supply such a refrigerant fraction
from its
holding tank via the supply conduit to the cooling circuit at a position
between the
client and the inlet of the compressor, i.e. on the low pressure side of the
cooling
circuit. Thus, it is possible to supply to the cooling circuit such a
refrigerant
fraction without, or with less need for, pumps or other pressure regulating
means.
Therefore, the system may be provided with holding tank(s) that is/are
maintained at a pressure between the pressure in the cooling circuit where the
respective refrigerant fraction is separated and the pressure in the cooling
circuit
where the supply conduit is connected.
Each withdrawal conduit may further be connected to a flare. If a
withdrawn fraction is not of desirable purity, this fraction may be removed
from
the cooling circuit, rather than stored and/or reused. Therefore, a separated
and
withdrawn refrigerant fraction of the refrigerant mixture may be discarded.
This
may conveniently be achieved by passing of the separated and withdrawn
refrigerant fraction to an outlet flare.
As an example, the client may be a carbon dioxide frosting vessel, i.e. a
vessel in which gaseous carbon dioxide is captured as carbon dioxide ice at
low
temperature. Accordingly, the present invention additionally relates to the
use of
a multi-refrigerant cooling system as described above for cooling of a carbon
dioxide frosting vessel.
The system may be configured to be controlled by a control system. A
control device with associated control signalling infrastructure keep track of
the
quantities in each holding tank via, for example pressure sensors, and the
control
device may further keep track of the percentages of each refrigerant in the
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system via a multi-component detector. The control device may also determine
the rate and length of opening of different control valves, according to what
adjustment to the refrigerant mixture is needed.
In a second aspect, certain objects of the present invention are
accomplished by a method for adjusting the composition of a refrigerant
mixture
of a multi-refrigerant cooling system, said method comprising the following
steps:
a) withdrawing from the multi-refrigerant cooling system one or more
fraction(s) of the refrigerant mixture, said fractions being of different
refrigerant
compositions;
b) supplying to the multi-refrigerant cooling system a refrigerant stream;
so that the composition of the refrigerant mixture of the multi-refrigerant
cooling system is adjusted to a new composition, said new composition being
different from the composition of the refrigerant stream; and
c) maintaining during steps a) and b) the refrigerant mixture of the multi-
refrigerant cooling system in an amount allowing operation of the multi-
refrigerant
cooling system;
thereby allowing adjustment of the composition of the refrigerant mixture
during operation of the multi-refrigerant cooling system.
It is of economical as well as environmental interest to avoid or reduce
discharge of refrigerants from the multi-refrigerant cooling system in
connection
with adjustment of the composition of a refrigerant mixture. Thus, one or
more,
preferably all, of the fraction(s) withdrawn in step a) may be individually
stored.
Storing the withdrawn fractions individually facilitates further treatment,
such as
recovery or recycling, thereof. Any fraction not stored may be discarded,
e.g.,
flared off. A withdrawn fraction may typically be discarded, rather than
stored, if
this fraction is not of desirable purity.
In order not only to avoid or reduce discharge of refrigerants but also to
achieve a desirable closing of the operation of a multi-refrigerant cooling
process,
the refrigerant stream supplied in step b) may comprise, preferably consist
of,
one or more of the stored fraction(s). Fraction(s) withdrawn from the multi-
refrigerant cooling system may thus be returned thereto, albeit in amounts
and/or
proportions rendering the composition of the refrigerant mixture of the multi-
refrigerant cooling system to change. Make-up refrigerants, not withdrawn from
the multi-refrigerant cooling system but typically supplied substantially pure
or in
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mixtures of a set composition, may conveniently be supplied to the multi-
refrigerant cooling system as part of the refrigerant stream of step b).
Typically, operation of a multi-refrigerant cooling system inherently
involves separation of its refrigerant mixture into different fractions
corresponding
at large to the respective refrigerants of the refrigerant mixture. Thus,
beneficially
the number of fractions withdrawn in step a) may be equal to or less than,
preferably equal to, the number of refrigerants in the refrigerant mixture of
the
multi-refrigerant cooling system.
Occasionally, it may not be useful to store any withdrawn fraction of the
refrigerant mixture, i.e., the fraction(s) withdrawn in step a) may be
discarded. As
mentioned above, such fraction(s) may be flared off. In such a situation, the
number of fractions withdrawn in step a) may suitably be one. Make-up
refrigerants, not withdrawn from the multi-refrigerant cooling system but
typically
supplied substantially pure or in mixtures of a set composition, may however
be
supplied to the multi-refrigerant cooling system as part of the refrigerant
stream of
step b).
The stored fraction(s) may each be maintained at a pressure between the
pressure of the refrigerant mixture at the position of the multi-refrigerant
cooling
system where in step a) the respective fraction is withdrawn and the pressure
of
the refrigerant mixture at the position of the multi-refrigerant cooling where
step
b) is performed. Alternatively or additionally, the fraction(s) withdrawn in
step a)
may each be withdrawn at a position of the multi-refrigerant cooling system
where the refrigerant mixture is present at a higher pressure than at the
position
of the multi-refrigerant cooling system where step b) is performed. As a
favourable consequence of considering pressure aspects as suggested here,
withdrawal of fraction(s) of refrigerant mixture, storing of such fraction(s)
and/or
supply of the refrigerant stream can be performed without, or with less need
for,
pumps or other pressure regulating means.
As an example, the multi-refrigerant cooling system may cool a carbon
dioxide frosting vessel, i.e. a vessel in which gaseous carbon dioxide is
captured
as carbon dioxide ice at low temperature.
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Brief Description of the Drawings
Figure 1 is a schematic illustration of a multi-refrigerant cooling system
according to an embodiment of the invention.
Figure 2 is a schematic illustration of another multi-refrigerant cooling
system according to an embodiment of the invention.
Detailed Description
In Figure 1 is depicted a multi-refrigerant cooling system comprising a
cooling circuit 100 for circulation of a refrigerant mixture and arrangements
200
for adjusting the composition of the refrigerant mixture.
The cooling circuit 100 comprises a compressor 101, refrigerant
separators 102 and 103 as well as a client 104 which is to be cooled by the
multi-
refrigerant cooling system. The outlet of the compressor 101 is connected to
the
client 104 via the separators 102 and 103. The cooling circuit 100 further
comprises expansion means 105, 106 and 107 for different fractions of the
refrigerant mixture as well as heat exchangers (condensers/evaporators) 108
and
109, all laid-out for operation of the cooling circuit according to a cooling
principle
called integrated cascade. The illustrated cooling circuit is designed for a
refrigerant mixture of three components. It is emphasized that the detailed
layout
of the cooling circuit, or its working principle, is not critical to the
present
invention.
The arrangements 200 for adjusting the composition of the refrigerant
mixture comprise holding tanks 201, 202 and 203 for each of the refrigerants
making up the refrigerant mixture being circulated in the cooling circuit 100.
The
holding tanks are connected to the cooling circuit 100 via respective valves
204,
205 and 206 and a supply conduit 207. The supply conduit 207 is connected to
the cooling circuit 100 at a position between the client 104 and the inlet of
the
compressor 101, i.e. on the low pressure side of the cooling circuit. A flare
208 is
connected via a valve 209 to the cooling circuit 100.
Not shown in Figure 1 is a control device with associated control signalling
infrastructure which keeps track of the quantities in each holding tank 201,
202
and 203 via pressure sensors, and of the percentages of each refrigerant in
the
refrigerant mixture of the cooling circuit 100 via a multi-component detector,
and
controls valves 204, 205, 206 and 209.
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To adjust the composition of the refrigerant mixture of the cooling circuit
100, the cooling circuit is emptied of a portion of the refrigerant mixture
and re-
filled with suitable amounts of one or more refrigerants in order to influence
the
composition of the mixture. Thus, the valve 209 is temporarily opened to allow
a
portion of the refrigerant mixture to be vented from the cooling circuit 100
to the
flare 208. One or more of the valves 204, 205 and 206 are temporarily opened
to
allow refrigerant(s) from holding tanks 201, 202 and/or 203 to be supplied to
the
cooling circuit 100 via the supply conduit 207. The control device (not shown)
determines the rate and length of opening of the valves according to what
adjustment is to be made and outputs corresponding signals to one or more of
the valves 204, 205, 206 and 209 as may be required.
In Figure 2 is depicted another multi-refrigerant cooling system comprising
a cooling circuit 100 for circulation of a refrigerant mixture and
arrangements 200
for adjusting the composition of the refrigerant mixture.
The cooling circuit 100 in Figure 2 is similar to the cooling circuit 100 of
Figure 1. It is, however, again emphasized that the detailed layout of the
cooling
circuit, or its working principle, is not critical to the present invention.
The arrangements 200 for adjusting the composition of the refrigerant
mixture comprise holding tanks 201 and 202, each connected to a respective
separator 102 or 103 via respective withdrawal conduits 210 and 211 and
respective valves 212 and 213. A further holding tank 203 is connected via a
withdrawal conduit 214 and a valve 215 to the cooling circuit 100 at a
position
between the separator 103 and the client 104. Each holding tank 201, 202 and
203 is thus arranged to receive a respective refrigerant fraction from the
cooling
circuit 100. The holding tanks are connected to the cooling circuit 100 via
respective valves 204, 205 and 206 and a supply conduit 207. The supply
conduit
207 is connected to the cooling circuit 100 at a position between the client
104
and the inlet of the compressor 101, i.e. on the low pressure side of the
cooling
circuit. A flare 208 is connected via respective valves 216, 217 and 218 to
the
withdrawal conduits 210, 211 and 214.
Not shown in Figure 2 is a control device with associated control signalling
infrastructure which keeps track of the quantities in each holding tank 201,
202
and 203 via pressure sensors, and of the percentages of each refrigerant in
the
refrigerant mixture of the cooling circuit 100 as well as of the percentages
of each
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refrigerant in the refrigerant fractions in each holding tank 201, 202 and 203
via a
multi-component detector, and controls valves 204, 205, 206, 212, 213, 215,
216,
217 and 218. The control device may include, for example, a general-purpose
computer, application specific computing device or other programmable
controller
that receives input signals indicative of these system parameters, processes
the
input signals using stored instructions, and provides output signals to the
various
control valves to operate the system in the manner described herein.
To adjust the composition of the refrigerant mixture of the cooling circuit
100, the cooling circuit is emptied of a portion of the refrigerant mixture
and re-
filled with suitable amounts of one or more refrigerant fractions in order to
influence the composition of the mixture. Thus, one or more of the valves 212,
213 and 215 are temporarily opened to allow a portion of the respective
refrigerant fractions to pass to the respective holding tanks 201, 202 or 203,
or is
one or more of the valves 216, 217 and 218 temporarily opened to allow a
portion
of the respective refrigerant fraction to be vented from the cooling circuit
100 to
the flare 208. One or more of the valves 204, 205 and 206 are temporarily
opened to allow refrigerant fractions(s) from holding tanks 201, 202 and/or
203 to
be supplied to the cooling circuit 100 via the supply conduit 207. The control
device (not shown) determines the rate and length of opening of the valves
according to what adjustment is to be made and outputs corresponding signals
to
one or more of the valves 204, 205, 206, 212, 213, 215, 216, 217 and 218 as
may be required.
The multi-refrigerant cooling system of Figure 2 may alternatively be
described as follows.
The multi-refrigerant cooling system (MRC) comprises a cooling circuit 100
for circulation of a refrigerant mixture comprising two or more refrigerant
fractions. The MRC operates by blending two or more refrigerants with
different
condensation temperatures in one process. The cooling circuit 100 comprises a
compressor 101, a client 104, one or more separator(s) 102, 103 located
between the compressor 101 and the client 104 in the circuit 100. Each
separator
102, 103 is configured to being able to, in addition to separate, also
withdraw a
particular refrigerant fraction through a piping connector 210, 211 from the
lower
part of each separator of the refrigerant mixture, where the piping
connector(s)
210, 211 is/are connected to a particular holding tank 201, 202 that holds one
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particular refrigerant. The holding tank(s) 201, 202 is/are arranged to
receive a
particular fraction from its respective separator 102, 103. Each of the piping
connectors 210, 211 is equipped with two sets of control valves 212, 216; 213,
217, wherein one set 212, 213 regulates flow into the holding tank and one set
216, 217 regulates flow to an outlet flare 208. Each holding tank 201, 202 is
further and separately connected through a pipe wherein each said pipe is
joined
to the cooling circuit through common header pipe 207. Each of the pipes is
fitted
with a control valve 204, 205 which is configured to regulate the supply of
one or
more particular refrigerant fraction(s) from one or more of the holding
tank(s) 201,
202 to the cooling circuit 100. The refrigerant with the lowest condensation
temperature does not have its own separator and exists in a pure state after
the
last separator 103, and is connected to its holding tank 203 through a piping
connection 214. This piping connection 214 is equipped with one set of control
valves 215, 218, wherein one 215 regulates flow into the holding tank 203 and
one 218 regulates flow to an outlet flare 208. The holding tank 203 is also
separately connected through a pipe to the cooling circuit through common
header pipe 207. This pipe is also fitted with a control valve 206 which is
configured to regulate the supply of the particular refrigerant fraction from
the
holding tank 203 to the cooling circuit 100.
To lower the quantity of a particular refrigerant in the MRC system, the
respective control valve 212, 213, 215 is opened to allow the desired
refrigerant
to exit the MRC and to either enter its respective holding tank 201, 202, 203
or to
be vented to the flare by opening one or more of the control valves 216, 217,
218.
To increase the quantity of a particular refrigerant in the MRC system, the
respective control valve 204, 205, 206 is opened, to allow the respective
refrigerant to exit its respective holding tank 201, 202, 203 and to enter the
process flow, preferably on the low pressure side. These refrigerant transfers
may be achieved by using the differential pressures only, without the need for
pumping.
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While there have been described what are presently considered to be
preferred embodiments, it will be understood by those skilled in the art that
other
modifications can be made within the spirit of the invention. The above
descriptions of embodiments are not intended to be exhaustive or limiting in
scope. It should be understood that the invention is not limited to the
embodiments described above, but rather should be interpreted within the full
meaning and scope of the appended claims.
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