U.S. patent application number 13/581128 was filed with the patent office on 2012-12-13 for cryogenic cooling method using a gas-solid diphasic flow of co2.
Invention is credited to Thierry Dubreuil, Mohammed Youbi-Idrissi.
Application Number | 20120312505 13/581128 |
Document ID | / |
Family ID | 42543064 |
Filed Date | 2012-12-13 |
United States Patent
Application |
20120312505 |
Kind Code |
A1 |
Youbi-Idrissi; Mohammed ; et
al. |
December 13, 2012 |
Cryogenic Cooling Method Using a Gas-Solid Diphasic Flow of CO2
Abstract
The invention relates to a method implementing liquid CO.sub.2
as a cryogenic fluid for transferring negative calories to
products, said method being of a so-called indirect injection type
wherein the liquid CO.sub.2 is sent into a heat exchanger system
where same evaporates, the method being characterized in that,
prior to reaching the exchanger system, the liquid CO.sub.2
undergoes an expansion operation, at a pressure selected to obtain
a gas/solid mixture at the output of the expansion operation.
Inventors: |
Youbi-Idrissi; Mohammed;
(Massy, FR) ; Dubreuil; Thierry; (Boissets,
FR) |
Family ID: |
42543064 |
Appl. No.: |
13/581128 |
Filed: |
January 27, 2011 |
PCT Filed: |
January 27, 2011 |
PCT NO: |
PCT/FR11/50159 |
371 Date: |
August 24, 2012 |
Current U.S.
Class: |
165/104.21 |
Current CPC
Class: |
F25B 2309/06 20130101;
F25D 3/10 20130101 |
Class at
Publication: |
165/104.21 |
International
Class: |
F28D 15/00 20060101
F28D015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2010 |
FR |
1051344 |
Claims
1-6. (canceled)
7. A process employing liquid CO.sub.2 as cryogenic fluid, for
transferring cold to products, process of the "indirect injection"
type where liquid CO.sub.2 is conveyed into a heat exchanger system
where it evaporates, the transfer of cold to the products involving
an exchange between the air surrounding the products and the cold
walls of the heat exchanger, promoted by the involvement of
ventilation means associated with the heat exchanger system, the
process being characterized in that, before reaching the exchanger
system, the liquid CO.sub.2 has been subjected to an operation for
reducing in pressure to a pressure chosen in order to obtain, at
the outlet of the pressure-reducing operation, a solid/gas
mixture.
8. The process of claim 7, wherein before reaching the
pressure-reducing operation, the liquid CO.sub.2 exchanges heat
with the cold gases obtained at the outlet of the heat exchanger
system, in a means which makes possible such a heat exchange.
9. The process of claim 8, wherein the exchange surface of the
exchanger system is given dimensions so as to carry out, in the
exchanger, only a partial melting of the entering gas/solid
mixture, the complete melting of the mixture then taking place in
said means which makes possible heat exchange.
10. The process of claim 8, wherein said means which makes possible
heat exchange is a plate exchanger.
11. A plant for the transfer of cold to products using liquid
CO.sub.2, the plant employing a process of the "indirect injection"
type and comprising: a heat exchanger system capable of passing
liquid CO.sub.2 in transit therein; and ventilation means
associated with the heat exchanger system, capable of bringing the
air surrounding the products into contact with the cold walls of a
heat exchanger system, the plant being characterized in that it
comprises a pressure-reducing system, positioned upstream of the
exchanger system, thus capable of reducing the liquid CO.sub.2 in
pressure, before it arrives in the exchanger system, to a pressure
chosen in order to obtain a solid/gas mixture at the outlet of the
pressure, reducing operation.
12. The plant of claim 11, wherein it additionally comprises a
subcooling system, for example a plate exchanger, positioned in the
plant according to the following arrangement: the subcooling system
is positioned between the source of liquid CO.sub.2 and the
pressure-reducing system, in order to make it possible for the
liquid CO.sub.2 to be able to pass in transit by a first pathway of
this subcooling system, before reaching the pressure-reducing
system; said arrangement is furthermore such that it makes it
possible for the cold gases extracted from the heat exchanger
system to pass in transit by a second pathway of the subcooling
system.
Description
[0001] The present invention relates to the field of the processes
using CO.sub.2 as cryogenic fluid in processes for the cooling,
deep-freezing and case-hardening of products, in particular
foodstuffs, but also as source of cold in refrigerated trucks which
transport fresh and/or deep-frozen (thus heat-sensitive)
products.
[0002] In such processes and applications, the CO.sub.2 is most
often intended to be used in direct injection, with temperatures
for regulating the products to be cooled which typically vary
between 0 and -20.degree. C., in the case of refrigerated
transport, and between -40.degree. C. and -70.degree. C., in
cooling cells and other cooling tunnels.
[0003] While the use of CO.sub.2 in direct injection exhibits
unquestionable advantages, in particular the absence of thermal
barrier and consequently the guarantee of maximum thermal
efficiency, on the other hand it exhibits disadvantages, among
which may be mentioned: [0004] the question of safety, it requires
the installation of devices making it possible to prevent the risk
of asphyxia (alarm systems, extraction systems, CO.sub.2 sensors),
with the costs and constraints which this involves; [0005] from the
thermodynamic viewpoint: the heat of the extraction gases, in
particular those at -40.degree. C./-70.degree. C., are difficult to
recover in value as, after they have come into direct contact with
the products to be cooled, they become contaminated by the presence
of traces of moisture, of particles of products, and the like.
[0006] However, there are also numerous applications where CO.sub.2
is used in indirect injection in an open loop, in particular in
applications for refrigerated transportation but also in
deep-freezing tunnels; where a heat exchanger is fed with liquid
CO.sub.2 which, when evaporating in this exchanger, extracts the
heat from the medium to be cooled and thus produces the desired
cold (the transfer of the cold to the products involves an exchange
with the internal air of the tunnel or the truck via ventilation
means associated with each exchanger). Use is thus made here of a
Liquid/Vapor phase change which, from the viewpoint of the
thermodynamic properties of the CO.sub.2, is "restricted" to a
theoretical pressure of 5.18 bar corresponding to the pressure of
the triple point of this fluid. In other words, the temperature at
which the phase change takes place is found to be limited and, in
all cases, it is strictly greater than -56.6.degree. C. The
demonstration is thus made of the fact that the use of CO.sub.2 in
indirect injection does not make it possible to achieve very low
temperature levels, in contrast to what is made possible by liquid
nitrogen, for example.
[0007] The present invention hopes to provide novel conditions for
the use of CO.sub.2 as source of cold in such indirect injection
applications.
[0008] As will be seen in more detail below, the invention provides
for the installation of a gas-solid two-phase flow.
[0009] The invention relates to a process employing liquid CO.sub.2
as cryogenic fluid, making it possible to transfer cold to
products, process of the "indirect injection" type where liquid
CO.sub.2 is conveyed into a heat exchanger system where it
evaporates, the transfer of cold to the products involving an
exchange between the air surrounding the products and the cold
walls of the heat exchanger, promoted by the involvement of
ventilation means associated with the heat exchanger system, the
process being characterized in that, before reaching the exchanger
system, the liquid CO.sub.2 has been subjected to an operation for
reducing in pressure to a pressure chosen in order to obtain, at
the outlet of the pressure-reducing operation, a solid/gas
mixture.
[0010] According to a preferred embodiment of the invention, before
reaching the pressure-reducing operation, the liquid CO.sub.2 has
been heat-exchanged with the cold gases obtained at the outlet of
the heat exchanger system (resulting from the melting carried out
in the heat exchanger system).
[0011] This heat exchange between the liquid CO.sub.2 and the cold
gases obtained at the outlet of the heat exchanger system is, for
example, carried out in a plate exchanger.
[0012] The following will thus have been understood, on reading the
above: [0013] there is sent, into the exchanger of this indirect
injection process, not, as according to the prior art, liquid
CO.sub.2 but a fluid resulting from a reduction in pressure, in
which there is a part of solid (this is a gas/solid two-phase
liquid); [0014] and the advantageous embodiment of the invention
explained above, where, before being sent to the pressure-reducing
valve, the liquid exchanges with the gas phase extracted from the
heat exchanger system (which is a way of subcooling this liquid),
offers a higher thermal efficiency since the solid fraction in the
liquid which has been subcooled and then reduced in pressure is
then greater.
[0015] Other characteristics and advantages of the present
invention will thus become more clearly apparent in the following
description, given by way of illustration but without implied
limitation, made in connection with the appended drawings, for
which:
[0016] FIG. 1 is a partial diagrammatic representation of an
embodiment of the invention;
[0017] FIG. 2 presents enthalpy difference curves which make it
possible to visualize the difference in enthalpy between points 2
and 3 of FIG. 1, including the latent and sensible heats, for two
pressure levels, 5.18 bar (triple point pressure) and 1 bar;
[0018] FIG. 3 is a partial diagrammatic representation of an
advantageous embodiment of the invention employing subcooling of
the liquid CO.sub.2 before it arrives in the pressure-reducing
valve.
[0019] FIG. 1 makes it possible to visualize, in a simple and clear
way, the progress of the liquid CO.sub.2 in a process in accordance
with the invention. If necessary, but without in any way being
obligatory, reference may be made, to better follow that which
follows, to a Mollier diagram, a diagram well known to a person
skilled in the art, but which the applicant company has chosen not
to display here for reasons of readability.
[0020] As may be read in FIG. 1, the liquid CO.sub.2 (point 1)
withdrawn from the storage tank, for example under standard
conditions of 20 bar/-20.degree. C. type (or also 45.degree. C./8
bar type, depending on the country concerned), is reduced in
pressure to a pressure below that of the triple point, for example
below 5.18 bar (point 2), before reaching the exchanger system.
[0021] The exchanger system is employed in an "indirect injection"
process: for example in an operation for cooling, deep-freezing or
case-hardening products, in particular foodstuffs (the exchanger
system is then, for example, present inside a cryogenic cell or
tunnel), or in a refrigerated truck transporting perishable
heat-sensitive products.
[0022] There is thus obtained, at point 2, a gas/solid two-phase
mixture, the solid fraction of which varies as a function of the
pressure at point 2. By way of illustration, it is typically 52% at
5.18 bar/-56.6.degree. C. and 47% at 1 bar/-80.degree. C.
[0023] This two-phase mixture is then circulated inside the
exchanger system, where the mixture gives up its latent heat of
fusion in addition to a portion of its sensible heat. The design of
the exchanger and in particular its exchange surface, and also the
CO.sub.2 flow rate, will define the refrigerating capacity
delivered and also the outlet temperature of the gas at point
3.
[0024] FIG. 2 exhibits enthalpy difference curves, making it
possible to visualize the enthalpy difference between the points 2
and 3 of FIG. 1, including the latent and sensible heats, for two
pressure levels after reducing the liquid CO.sub.2 in pressure,
5.18 bar (i.e., the triple point pressure) and 1 bar.
[0025] This FIG. 2 clearly shows the available energy (expressed as
enthalpy variation) present in one kilogram of CO.sub.2 when the
latter is reduced in pressure from 20 bar to 5.18 bar, representing
the limit of the liquid/vapor phase change (bottom curve in the
figure), or else from 20 bar to 1 bar (top curve in the figure),
making it possible to obtain, in accordance with the invention, a
solid/gas two-phase mixture. It is noted that, in both cases, the
enthalpy variation increases in proportion as the outlet
temperature of the gas also increases, and the fact that this
enthalpy variation increases in proportion as the pressure after
reducing in pressure decreases. Hence the indisputable energy
advantage of what is provided by the present invention by the use
of a solid/gas fluid instead of a liquid/gas fluid as according to
the prior art.
[0026] Nevertheless, it should be mentioned that, for some food
cryogenic applications, for example for certain products in
deep-freezing applications in tunnels, the cryogenic temperature
effect is keenly desired. Thus, in such applications, gases at such
a high temperature can be obtained with difficulty at the exchanger
outlet since the temperature of the air surrounding the products
which is desired in such processes typically has to reach
-60.degree. C. to -80.degree. C.
[0027] For such requirements, just as for other applications, it
will then be very particularly advantageous to employ the
advantageous embodiment of the invention which is illustrated in
FIG. 3 below.
[0028] This advantageous form is targeted at being able to recover
in value as much as possible of the heat still present in the gases
extracted at the outlet of the exchanger system.
[0029] Let us examine the embodiment of FIG. 3.
[0030] In this figure, the presence of an additional means is
noted; it is a means which makes it possible to carry out exchange
of heat, to be specific a subcooler, for example composed, as is
the case here, of a plate exchanger, the operation of which means
will be explained here: [0031] the liquid CO.sub.2 (point 1)
withdrawn from the storage tank, for example under standard
conditions already mentioned above in the context of FIG. 1,
passes, before reaching the pressure-reducing valve, through a
plate exchanger where it exchanges heat with the gases resulting
from the exchanger system (point 4), exchange system present in the
tunnel, or the truck, and the like; [0032] it is thus seen that the
liquid CO.sub.2 coming from the storage tank (point 1) and the
gases extracted from the heat exchanger system (point 4) circulate
countercurrentwise in the plate exchanger, which makes possible the
subcooling of the stream of liquid CO.sub.2 before the latter
reaches the pressure-reducing station (point 2); [0033] between
points 1 and 2, the liquid thus remains at a substantially constant
pressure but is subjected to cooling; [0034] at the outlet of the
pressure-reducing station (point 3), the solid/gas mixture obtained
is directed towards the heat exchanger system; [0035] the gases
extracted from the heat exchanger system (point 4), once they have
passed through the subcooler, are discharged (point 5); [0036] the
production of cold thus takes place in the exchanger system between
points 3 (after reducing in pressure) and 4 (exchanger outlet).
[0037] As indicated above, the temperature at this point 4 will be
dictated by the technical constraints of the application which uses
the cold, which makes it possible to result in a higher or lower
level.
[0038] Two examples of conditions and compositions of phases at the
various points 1, 2, 3, 4 and 5 of FIG. 3 are described in detail
below.
FIRST EXAMPLE
TABLE-US-00001 [0039] T P h Point (.degree. C.) (bar) (kJ/kg)
Thermodynamic state 1 -20 20 40.8 Liquid/vapor equilibrium 2 -34 20
13.1 Subcooled liquid 3 -80 1 13.1 Gas/solid mixture 4 -60 1 323.5
Superheated gas 5 -25 1 351.2 Superheated gas
SECOND EXAMPLE
TABLE-US-00002 [0040] T P h Point (.degree. C.) (bar) (kJ/kg)
Thermodynamic state 1 -20 20 40.8 Liquid/vapor equilibrium 2 -52 20
-22.4 Subcooled liquid 3 -80 1 -22.4 Gas/solid mixture 4 -80 1
288.0 Gas/solid mixture 5 -25 1 351.2 Superheated gas
[0041] This second example illustrates a case where, if the
application which uses the cold requires a temperature of the
medium to be cooled which is as cold as possible, it is possible to
envisage making partial use of the heat of fusion in the exchanger
system (between points 3 and 4), the complete melting of the
mixture and the superheating thereof then taking place in the
subcooler with recovery of the heat.
[0042] In other words, by varying the exchange surface of the
exchanger, it is possible to carry out partial melting in the
exchanger, a solid/gas mixture thus exiting at point 4; there then
takes place, in the exchanger, a change of state of melting, which
takes place at the constant temperature for a pure fluid, such as
CO.sub.2 (in the form illustrated here, it is not the temperature
which changes but the fraction by weight of the solid, which
decreases as it goes along in order to be converted into
vapor).
[0043] As will have been understood on reading all the explanations
given above, the process according to the invention in its form of
FIG. 3 makes it possible: [0044] to increase the refrigerating
capacity of the exchanger of the indirect injection system since
the subcooling of the liquid CO.sub.2 makes it possible to gain up
to 12% of available energy; [0045] to improve the heat exchange as
a subcooled fluid, once reduced in pressure, gives rise to a
greater solid fraction, which is beneficial for the transfer
coefficient.
[0046] If the application requires a temperature of the medium
which is as cold as possible, it is possible to envisage making
partial use of the heat of fusion in the process (between points 3
and 4), the complete melting of the mixture and its superheating
then taking place in the subcooler with recovery of the heat.
* * * * *