Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND APPARATUS FOR PRODUCING AN ARTIFICIAL ATMOS-
PHERE IN A STORAGE OR TRANSPORT CONTAINER
The invention relates to a method for producing an artificial atmosphere in a
storage or transport
container according to the preamble of claim 1. The invention further relates
to an apparatus for
carrying out such a method according to the preamble of claim 8.
Long known has been the practice of increasing the quality of perishable
products in transport or
storage containers over a long period of time through exposing the product to
an atmosphere, the
nitrogen content of which is up to 99%. Under such conditions the product is
put, as it were, into a
deep sleep and during this time undergoes no putrefaction and after-ripening.
Thus, under such
conditions products that would otherwise be spoiled within a few days can be
transported over
longer time periods and greater distances.
Known from EP 357 949 B1 is a device for producing an artificial atmosphere in
a transport con-
tainer, in which device the nitrogen is acquired from the ambient air by means
of a nitrogen-
generator unit and is continuously conveyed into the transport container. The
production of the
high nitrogen concentration takes place here by means of an apparatus for
separation of the es-
sential gas constituents of the ambient air, namely nitrogen and oxygen, into
a component that
contains approximately 99% nitrogen and a component that consists essentially
of oxygen (per-
meate).
By means of such gas-separation membrane, a continuous gas stream can be
generated. Also
known are apparatuses for discontinuous nitrogen generation, in particular
molecular sieves;
however, the apparatus-technology structure of these is of a considerably
higher level, due to the
requirement of a backflushing of the molecular sieve. Thus, for the purpose of
generating an arti-
ficial atmosphere, gas-separation membranes have essentially prevailed.
Known from EP 224 469 is a transportable cooling container in which, in
addition to the genera-
tion of an elevated nitrogen component in the transport container, a
humidification of the artificial
atmosphere is carried out. Thus, an optimal humidity level in the transport
container can be set in
accordance with the product to be transported. Such a system requires the
accompaniment of
water container, which must constantly be tested for germ formation and must
be disinfected from
time to time.
In general, gas-separation membranes react sensitively to moisture. The
impingement by moist
pressurized air has a negative effect on the service life of these membranes.
Thus, for their pro-
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tection gas-separation membranes are often protected by suitable devices, for
example cyclone
separators that separate the free water.
In gas-separation membranes known until now, an unavoidable dehumidification
of the supplied
pressurized air takes place during the gas separation. The water vapour
separated off is dis-
charged together with the permeate. However, this discharge of water vapour
negatively affects
the output of the nitrogen production.
Also known is the practice of connecting an energy-consuming heat apparatus
upstream to the
gas-separation membrane, in order to raise the temperature of the fed
pressurized air and
thereby lower the relative humidity, in order to protect the gas-separation
membrane from being
impinged upon by free water.
In addition to the upstream-connection of heat apparatuses, also known is the
practice of dehu-
midifying the pressurized air through upstream-connected absorption dryers.
However, these
consume a portion of the supplied pressurized air and thus lower the
pressurized-air feed supply
to the gas-separation membrane. Thus, the efficiency of the nitrogen
production is reduced.
The invention is based on the object of specifying a method and an apparatus
for producing an
artificial atmosphere in a storage or transport container whereby, with a high
efficiency of the sys-
tem, a setting of the moisture level in the transport or storage container is
possible without
requiring an additional fluid supply.
This object is achieved through the method specified in claim 1. An apparatus
according to the
invention is specified in claim 8.
The invention originates from a method for producing an artificial atmosphere
in a storage or
transport container, in which method the pressurized air supplied by an air
compressor is con-
veyed via a gas-separation membrane into the storage or transport container to
produce an
artificial atmosphere with increased nitrogen portion, the moisture content of
the supplied pressur-
ized air being reduced by means of a dehumidification apparatus.
According to the invention, the pressurized air fed to the gas-separation
membrane is dehumidi-
fied before its entrance into the gas-separation membrane by means of a
dehumidification
membrane.
The application, according to the invention, of a dehumidification membrane
reduces the humidity
of the pressurized air and thereby increases the nitrogen production. Through
the decreased
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moisture of the pressurized air fed to the gas separation membrane, the
service life and efficiency
of the latter can be increased.
In order to set the artificial atmosphere in the storage or transport
container to a desired moisture
level, the moisture removed from the pressurized air in the dehumidification
membrane is pref-
erably introduced into the storage or transport container in a controlled
manner.
This has the advantage that the moisture required in the storage or transport
container can be
obtained from the dehumidification membrane, without requiring separate fluid
containers. Re-
duced thereby are, on the one hand, the equipment complexity and, on the other
hand, the
logistic complexity necessary to keep the otherwise-necessary fluid reservoir
filled during a trans-
port of long duration.
The moisture to be transferred into the storage or transport container can be
directly supplied to
the container; however, it can also be added to the gas stream leaving the gas-
separation mem-
brane.
In a first preferred embodiment form of the invention, a portion of the gas
stream leaving the gas-
separation membrane is guided by a shunt via the backflush section to the
dehumidification mem-
brane, before the gas stream is fed to the storage or transport container.
Through this means,
nitrogen is used as rinsing air for the dehumidification membrane and, in
common with the water
vapour separated by the dehumidification membrane, is fed to the atmosphere of
the transport or
storage container.
Through a suitable valve control, the portion of the nitrogen stream that is
guided over the dehu-
midification membrane can be adjusted, in order to thereby adjust the moisture
content of the
artificial atmosphere fed to the container as required.
In an alternative embodiment form, the permeate leaving the gas-separation
membrane is, at
least in part, guided via the backflush section to the dehumidification
membrane and only then
discharged to the ambient atmosphere. Through this measure, in contrast to the
absorption dryer,
no consumption of pressurized air takes place and the nitrogen production of
the gas-separation
membrane is correspondingly increased.
The above-mentioned alternatives can also be activated in an alternating
manner through a suit-
able valve control. Through this means, in particular in the initialization
phase of a transport or
storage container, the time required for adjusting the artificial atmosphere
to the desired state can
be considerably reduced.
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In a more far-reaching embodiment form, a cooling of the pressurized air
supplied to the dehu-
midification membrane can take place. Through this cooling, the membrane, in
the presence of a
cyclone separator and a subsequent warming, is protected from being acted on
by free water and
the associated negative consequences.
The apparatus according to the invention for carrying out the specified method
contains the se-
ries-connected arrangement of an air compressor, a dehumidification membrane,
and a gas-
separation membrane. The dehumidification membrane preferably contains a
backflush section,
via which either at least a portion of the permeate separated at the gas-
separation membrane or
at least a portion of the nitrogen exiting the gas-separation membrane is
conducted.
In an embodiment form in which the permeate is conducted through the backflush
section to the
dehumidification membrane, the permeate is subsequently discharged to the
environment. In an
alternative embodiment form, at least a portion of the nitrogen exiting the
gas-separation mem-
brane is conveyed to the dehumidification membrane via the backflush section
and then back into
the transport or storage container. The invention can contain a valve control
by means of which,
in an alternating manner, a flushing of the dehumidification membrane with
permeate or with ni-
trogen can be carried out. In the case of a flushing of the backflush section
of the
dehumidification membrane with nitrogen, a control valve can be connected
between the inlet and
outlet of the backflush section, by the aid of which valve the magnitude of
the partial stream con-
ducted through the backflush section to the dehumidification membrane can be
adjusted.
Between the pressurized-air generator and the dehumidification membrane, a
cooling apparatus
can be connected, in order to facilitate, through temperature lowering, a
removal of excess water.
In the following, the invention is explained in detail with reference to an
embodiment example. In
the drawings:
Fig. 1: shows a schematic diagram of the invention according to a first
embodiment form;
Fig. 2: shows a schematic diagram of the invention according to Fig. 1 with a
changed valve
position;
Fig. 3: shows a schematic diagram of a simplified embodiment form of the
invention.
Represented in Fig. 1 is an apparatus for producing an artificial atmosphere
in a transport con-
tainer 34. The container 34 can be a stationary storage space, but it can also
be a transportable
vessel (e.g. container), such as are used in air transport, on ocean ships, as
road-transport vehi-
cles, or as rail-associated vehicles. As a rule, such containers are
temperature-insulated and
essentially hermetically sealed. When they are used as food-transport or -
storage containers,
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they are often associated with a cooling system, in order to lower the
temperature in the interior of
the container.
In the method according to the invention, the atmosphere to be conveyed into
the container is
generated essentially from the ambient air. For this purpose, ambient air is
drawn in by a com-
pressor 3 driven by a drive 2. Optionally, a controllable suction device 1 in
the container with a
downstream-connected filter 35 can be provided in order to use a proportionate
amount of air
from the container. This proportionate amount of air, mixed with the ambient
air, can be fed to the
compressor 3. Especially in the starting phase of the initialization of a
container, the time needed
to adjust the atmosphere in the container 34 can thereby be shortened.
The air compressed in the compressor is now fed to a heat exchanger 4, in
which the tempera-
ture of the pressurized air emerging from the compressor is cooled. For
further cooling of the
pressurized-air temperature, the pressurized air is guided through an air
cooler 5, in which the
pressurized air is cooled by means of a cooling fan 6 in order to generate a
cooled pressurized-
air stream. Alternatively, the cool air stream can be produced through an air-
current generator
coupled to the drive of the compressor or to the compressor itself.
In the presence of a cooling system in the container 34, an air cooler 5a can
also be used to
lower the temperature of the pressurized air. In this case, the pressurized-
air stream is guided
through the blower-free air cooler in the container.
After the compression to, for example, 7.5 bar, the compressed air contains a
large amount of
water. Through cooling of the air, the storage capacity of the air can be
reduced, forming free wa-
ter that can be separated in a cyclone separator.
Thus, following the cooler 5 is a water separator 7 for removing free water
from the pressurized
air. The discharging of the separated water takes place through the opening of
a water-removal
device (e.g, by means of a magnetic valve) through the pressure of the
pressurized air.
There follows a pressurized-air filter 8 for cleaning the pressurized air, in
particular of oil residues.
The water and/or oil separated here can likewise be removed
electrically/electronically.
The pressurized air thus cleaned is guided in reverse flow through the heat
exchanger 4, in the
process of which the air is again heated somewhat, in order to keep the
content of free water por-
tions low in the following dehumidification membrane.
In order to regulate the pressurized-air temperature before entrance into the
following stages, a
bypass valve 9 can be provided, which controls through more or fewer openings
and closings the
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portion of the pressurized air fed back through the heat exchanger 4 and thus
adjusts the tem-
perature of the air supplied to the dehumidification membrane.
The pressurized air is now fed to a dehumidification membrane (10), known in
itself, for dehumidi-
fication of the pressurized air. The dehumidification membrane displays a
pressurized-air inlet or
a pressurized-air outlet, as well as a backflush section with an inlet 37 and
an outlet 36, from
which the separated moisture can be discharged.
An applicable dehumidification membrane is formed by a cylinder filled with
hollow fibres,
whereby the water-vapour molecules can be diffused through the hollow fibres
and discharged
via the so-called backflush section.
The pressurized-air outlet of the dehumidification membrane leads to the input
of the gas-
separation membrane 11, which is likewise constructed in a manner known in
itself. This displays
an inlet and two outlets. The pressurized air fed to the inlet is divided into
a nearly oxygen-free
nitrogen stream (approximately 30% of the introduced pressurized air) and an
oxygen-rich air
stream (permeate). The nitrogen gas stream is conveyed into the container 34
via the outlet 39.
In the embodiment form represented in Fig. 1, the oxygen discharged from the
permeate outlet 38
of the gas-separation membrane 11 is released to the environment via a
humidification valve 15,
which is electrically actuable as a 3/2-port directional control valve, and
via the backflush section
of the dehumidification membrane 10 as well as the humidification valve 16,
which is likewise
electrically actuable as a 3/2-port directional control valve. In order to set
the required flow rate of
the permeate through the dehumidification section of the dehumidification
membrane 10, a per-
meate valve 14 designed as a controllable throttle is provided, which valve
discharges a portion
of the permeate to the environment directly after emergence from the gas-
separation membrane
11.
The nitrogen stream exiting the gas-separation membrane 11 at the outlet 39 is
fed, via an op-
tional pressure accumulator 12 with built-in check valve for storage of
nitrogen, and via an
electrically controllable nitrogen control valve 13 to the container 34. The
control valve 13 serves
to adjust the nitrogen stream or rather the nitrogen purity, in that the
adjustment of the volume
flow effected thereby adjusts the relationship between the volume of the
nitrogen stream and the
volume of the permeate. The control valve 13 can also be manually operated.
The connecting line between the gas-separation membrane 11 and the container
34 contains a
pressure gauge 21 for monitoring the system pressure, an oxygen gauge 22 for
monitoring the
nitrogen production of tt~e system, as well as moisture/temperature gauge 23
for monitoring the
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moisture of the nitrogen stream to the container 34. Corresponding sensors 24,
25 for oxygen
and moisture/temperature, respectively, are arranged directly in the container
34, in order to ob-
serve the atmospheric conditions at the stored product. Additionally, C02
sensors 26 and CZH4
sensors 27 as well additional sensors for additional gases or conditions (e.g.
temperature) can be
assigned to the storage space.
For the transport or storage of certain products, a COZ feed into the
container 28 can be provided;
this takes place through the conveying of a desired COZ volume flow from a C02
supply bottle
into the container 34 via a pressure-reduction valve 32 to reduce the supply-
container pressure to
an operating pressure measured at the pressure gauge 28 and via a controllable
shut-off valve
33.
Sensors, valves, and switching elements connected in common through control
lines 41 are con-
trolled and monitored through a system control 100 for a quick and efficient
setup of the artificial
atmosphere in the container. The collected data can be displayed or
transmitted via data commu-
nication to a control center, which, if necessary, can carry out via data
retransmission a
parameter change of the atmosphere to be adjusted.
In the operating position of the apparatus represented in Fig. 2, valves 15
and 16 are switched
over with respect to the switch position of Fig. 1. Through this, a partial
stream of the nitrogen
stream exiting the gas-separation membrane 11 from the outlet 39 is guided
through the nitrogen
valve 17 formed as a controllable throttle and the humidification valve 15,
through the dehumidifi-
cation section of the dehumidification membrane 11, and back through the
humidification valve
16, so that this partial stream is mixed again with the nitrogen stream to be
fed to the container
34.
In this operating position, the partial stream led through the
dehumidification membrane is humidi-
fied with the moisture separated in the latter, so that through this means the
moisture level in the
container 34 can be adjusted.
The nitrogen valve 17 serves to set the required nitrogen flow rate in
collaboration with the nitro-
gen control valve 13. In the switch position of the humidification valve 15
shown in Fig. 2, the
permeate flow to the valve 15 is blocked, so that the permeate is discharged
directly to the envi-
ronment via the permeate valve 14.
The operating position of valves 15 and 16 can be adjusted in a temporally-
controlled manner
according to the degree of the desired humidity in the container 34. A large
number of different
operating parameters can be adjusted through the setting of the operating
durations of the switch-
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ing positions of valves 15 and 16, through the appropriate adjusting of the
nitrogen control valve
13 and the permeate valve 14, or through the regulating of the nitrogen valve
17 according to re-
quirement.
With the aid of a suitably programmed system control 100, temporal variations
of the operating
parameters can also be set, for example oscillating gas-portion values of
nitrogen, oxygen, or
COZ in the atmosphere of the container 34. The control 100 can also be used to
regulate the
temperature in the container, since the cooling unit is coupled to the control
of the atmosphere.
Fig. 3 shows a simplified embodiment form of the invention in which the
humidification valves 15
and 16 are eliminated. In this embodiment form, the permeate leaving the gas-
separation mem-
brane at the connector 38 is released directly to the environment. By means of
an electrically-
operable nitrogen bypass valve 18, a specific nitrogen flow rate in the
nitrogen partial stream
flowing through the dehumidification section of the dehumidification membrane
10 and an ad-
justment of the moisture level of the atmosphere supplied to the container 34
can be achieved.
The method according to the invention and the apparatus according to the
invention accelerate
the formation of a nitrogen atmosphere, improve the nitrogen yield, and lower
the energy use and
system costs. During the operation of the system according to the invention, a
fixed pressure is
maintained in the storage space. After the pressure is first built up, an
amount of atmosphere
equalling the amount of nitrogen supplied in each case is correspondingly
released from the con-
tainer 34, so that a slight overpressure constantly prevails
The application of a suction device 1 increases the efficiency of the system
through the fact that
an oxygen-poor atmosphere mixture is formed from the atmosphere in the
container and the am-
bient air. This formed atmosphere is compressed and separated in the gas-
separation membrane
11. Through this means, the desired oxygen-poor atmosphere is formed in the
container in a
short time.
REFERENCE NUMBERS
1 Suction device
2 Drive
3 Compressor
4 Heat exchanger
Air cooler
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5a Alternative air cooler
6 Cooling fan
7 Water separator
8 Pressurized-air filter
9 Bypass valve
Dehumidification membrane
11 Gas-separation membrane
12 Pressure accumulator
13 Nitrogen control valve
14 Permeate valve
Humidification valve
16 Humidification valve
17 Nitrogen valve
18 Nitrogen bypass valve
21 Pressure gauge
22 Oxygen gauge
23 Moisture/temperature
gauge
24 Oxygen gauge
Moisture/temperature
gauge
26 COz gauge
27 Ethylene gauge
28 Pressure gauge
29 Filter
31 COZ container
32 Pressure reduction
valve
33 Blocking valve
34 Storage and transport
container
Filter
36 Outlet
37 Inlet
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38 Permeate
outlet
39 Gas outlet
40 Shunt
41 Control
line
100 System control
