Field of the invention

[0001] The present invention relates to a system and method for conditioning a compressed airstream, in particular to supply a conditioned compressed airstream to a nitrogen selective membrane to act as a nitrogen generator. The invention further concerns a refrigerated container including such a system.

Background of the invention

[0002] Nitrogen is a gas in very plentiful supply (as a major constituent of the earth’s atmosphere) and is very important in a wide range of industrial applications. These include the food and beverage industries, laboratory applications, atmosphere control, pharmaceutical and chemical industries, textiles, heat treatment, laser cutting, electronics and many more.

[0003] Systems for producing nitrogen from ambient air are known, including membrane-based air separation systems. By supplying a suitable membrane unit with compressed air, such a system will generate a nitrogen stream, an oxygen-enriched stream passing separately out of the unit. A number of commercial membrane units are available on the market, for use in a variety of industry sectors.

[0004] In certain applications, it is important to provide a dry output gas stream, and particular products have been developed to meet this need. By way of example, in some commercial systems of this type, the input compressed air containing water vapour flows through a bundle of hollow polymer fibres, which allow water vapour to pass through. The air remains in the fibres to be discharged as dry output. A fraction of the dry air is redirected to the module in order to sweep the permeated water vapour as a gas to the outside of the fibre bundle module.

[0005] In such systems, operation of the membrane separation units relies on the difference in permeation rate of oxygen and nitrogen through the polymer membrane. The membrane permeates oxygen and water vapour faster than nitrogen, meaning that oxygen and water vapour permeate through the membrane to atmosphere, the nitrogen being left behind as the non-permeated gas within the hollow fibres, so to exit the module as a dry, pressurised stream.

[0006] A membrane air separation unit requires particular operating conditions, and it is common to provide, between the air compressor and the membrane separator module, one or more suitable filter units to remove any contaminants entrained in the airstream. In addition, a moisture separator unit and/or a refrigerated dryer unit can be included, to remove bulk liquids from the compressed air stream, and/or any water that has condensed out of the compressed air.

[0007] One important area of industrial application of controlled atmosphere delivery is in refrigerated shipping containers (“reefers”) and other enclosures used to store, collect or convey respiring produce. Providing a suitably controlled atmosphere by carefully regulating the amount of nitrogen, oxygen and carbon dioxide affords control of post-harvest shelf life and/or quality of such product, in combination with appropriate refrigeration to a desired optimum temperature for the particular content.

[0008] Controlling the atmosphere of a refrigerated container can involve the introduction of selected gases in order to maintain the desired atmosphere. Prior systems have used dedicated gas sources to this end, but this requires provision of these gas supplies in or in association with the container, not generally feasible in a typical commercial application. With this in mind, it is known to use membrane separator systems, of the general form described above, to condition an ambient air stream in order to provide the required gas mixture to the container.

[0009] For controlled atmosphere reefers, the desired equilibrium composition of the atmosphere can be achieved over time through intelligent control of the gas content. One example of such an approach is described in published International Patent Application No. WO2014066952 to Mitsubishi Australia Limited.

[0010] Effective atmosphere control can raise particular problems with older reefers, as these are more likely to exhibit relatively high leakage when operated under a positive or negative pressure differential. For older containers it is usually necessary to leak-test the container before use and/or to install a curtain to preclude or reduce leakage, both of which are relatively timely and costly additional steps. To avoid this, other methods of effecting the reefer atmosphere may be employed, such as using carbon dioxide scrubbers (bags of quicklime, calcium oxide, for example) in prescribed quantities to absorb carbon dioxide.

[0011] An alternative approach is to introduce, under controlled conditions, a nitrogen-rich gas into the reefer at a prescribed purity, in order to reduce the carbon dioxide concentration in the internal atmosphere and thus achieve a suitable balance of oxygen and carbon dioxide. An example of such a method is described in US Patent No. 5,457,963 to Carrier Corporation, in which heated, high pressure air is separated by a membrane into high purity nitrogen and oxygen/other gases, the nitrogen supplied to a refrigerated container in accordance with flow control based on oxygen and carbon dioxide sensor signals.

[0012] Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

Summary of the invention

[0013] In a first broad aspect, the invention provides a system for conditioning a compressed airstream to supply a conditioned compressed airstream to a nitrogen selective membrane, the system including an airstream inlet for receiving a compressed airstream from a compressor, an airstream outlet for supplying the conditioned compressed airstream to the nitrogen selective membrane, a separator to remove droplets and/or particulates from the compressed airstream, a first conduit extending from the airstream inlet to direct the compressed airstream to an inlet of the separator, and a second conduit extending from an outlet of the separator to an inlet of the nitrogen selective membrane to supply the compressed airstream to the nitrogen selective membrane, wherein a portion of the second conduit is in thermal communication with a portion of the first conduit to form a first heat exchanger to exchange heat between the compressed airstream passing through the portion of the first conduit and the portion of the second conduit. [0014] Preferably, the portion of the second conduit and the portion of the first conduit in thermal communication with each other extend for a length of at least 200mm.

[0015] Preferably, the portion of the second conduit and the portion of the first conduit in thermal communication with each other extend for a length of up to 1500mm.

[0016] In a preferred form, the first conduit includes a second heat exchanger, preferably in thermal communication with an ambient environment.

[0017] Preferably, the second heat exchanger is located between the first heat exchanger and the inlet of the separator.

[0018] In a preferred form, the second heat exchanger is formed by a portion of the first conduit forming one or more coils.

[0019] The separator may be configured to remove water. In this aspect, the water remover may be a filtration device, an expansion cooler or a condenser, or a combination thereof. Alternatively or additionally, the separator may be a particulate remover.

[0020] Preferably, in order to provide said thermal communication therebetween, said portion of the first conduit and said portion of the second conduit are arranged in proximity to each other and an insulation sleeve at least partially encloses both portions.

[0021] The airstream inlet, defined above, may thus be the exhaust feedline for receiving a compressed airstream from a pump compressor. In a preferred form, the system includes a compressor to supply the compressed airstream to the airstream inlet. The compressor is preferably a positive displacement pump apparatus, such as a piston pump. The compressor may include a filter to remove particulates from ambient air drawn into the compressor prior to compression into the compressed airstream.

[0022] Preferably, the system includes a nitrogen selective membrane to receive the conditioned airstream from the airstream outlet.

[0023] Preferably, the relative humidity (RH) of the airstream exiting the airstream outlet is not more than 80%. [0024] In a preferred form, the airstream entering the separator is at least about 20°C (preferably at least about 25°C, preferably at least about 30°C, preferably at least about 35°C, preferably at least about 40°C) cooler than the airstream entering the airstream inlet.

[0025] Preferably, the airstream exiting the first heat exchanger is at least about 4°C hotter than the airstream exiting the separator.

[0026] In a second broad aspect, the invention provides a method of conditioning a compressed airstream for supply to a nitrogen selective membrane, the method including receiving a compressed airstream from a compressor, separating droplets and/or particulates from the compressed airstream, and exchanging heat between the compressed airstream before and after the separating step, to thereby reduce the relative humidity of the compressed airstream and provide a conditioned compressed airstream.

[0027] In a preferred form, the relative humidity (RH) of the conditioned compressed airstream is not more than 80%.

[0028] The method preferably includes cooling the compressed air received from the compressor by at least about 20°C (preferably at least about 25°C, preferably at least about 30°C, preferably at least about 35°C, preferably at least about 40°C) before being supplied to the separating step through the step of exchanging heat with the airstream following the separating step.

[0029] The method preferably includes heating the compressed airstream following the separating step by at least about 4°C through the step of exchanging heat with the airstream before the separating step.

[0030] In a preferred form, the method includes cooling the compressed airstream before being subjected to the separating step by heat exchange with the ambient environment.

[0031] The method may be conducted using the system as defined above with regard to the first broad aspect. [0032] In a further broad aspect, the invention provides a nitrogen generator unit for use with or in a refrigerated container including the system as defined above with regard to the first broad aspect, in combination with a nitrogen selective membrane module and an outlet to supply the output nitrogen to the interior of the refrigerated container.

[0033] In a further broad aspect, the invention provides a refrigerated container including the above-defined nitrogen generator unit.

[0034] The present invention thus relates to a membrane system and method for generating a high-concentrate nitrogen gas of a selected purity for use in a controlled atmosphere reefer, the gas specially conditioned for this purpose. The approach of the invention involves conditioning of the air using waste heat recovery from the drive pump in order to dry the gas (decrease its relative humidity) prior to delivering it to the membrane. Whilst prior art methods have contemplated heating an air stream before supplying to a separator membrane, this has generally been done by way of a heater (generally an electric heater) used to heat the compressed air to the membrane’s optimum operating temperature for the membrane being used in the system. To this purpose, a temperature sensor is used to control the energisation of an electric heater switch to maintain the temperature of the air being fed to the membrane.

[0035] The present invention takes a very different approach, owing to the fact that the power available in a reefer container is very limited indeed. In accordance with the invention, the only power needed to operate the system is that used to power the compressor pump, the heating of the compressed airstream delivered to the membrane being accomplished purely by internal heat exchange. The inventors have found through experiment that the relatively limited heating available in this way is sufficient to bring the relative humidity of the airstream to a suitable level.

[0036] The present invention also avoids the need to modify the selective membrane or to use a more complex membrane unit than a conventional nitrogen separation membrane. For example, a drying function can be built into the membrane apparatus itself, however this will generally lead to a higher pressure drop, requiring more initial compression of the airstream in order to deliver equivalent pressure. This in turn will require more power from what is generally a very limited supply. [0037] As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

[0038] Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings

[0039] Figure 1 is a diagrammatic illustration of a refrigerated shipping container (“reefer”) with a controlled atmosphere system and a nitrogen generator, according to the present invention.

[0040] Figure 2 shows an embodiment of the system of the present invention.

[0041] Figure 3 is a graph illustrating performance of the system of Figure 2 in tests.

[0042] Figure 4 shows calculations demonstrating the ability of the system of Figure 2 to achieve the desired operational outcome.

Detailed description of the embodiments

[0043] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

[0044] Figure 1 shows reefer 2 containing respiring produce 4 in shipment. Reefer 2 is fitted with a programmed controller 6 and a nitrogen generator system 8.

[0045] Controller 6 includes suitable sensors (eg. an oxygen and/or a carbon dioxide sensor) to monitor respective gas concentrations within reefer 2 and to take prescribed steps to adjust that atmosphere in order to maintain a desired gas composition equilibrium, generally based on an oxygen set point selected for the particular produce 4. In particular, when the oxygen drops below the set point controller 6 draws in ambient air from outside reefer 2 through an inlet port (eg. by way of a fan) until the desired oxygen level is reached.

[0046] Nitrogen generator system 8, described in further detail below, is configured to draw in ambient air from the exterior of reefer 2 and deliver - under controlled conditions - high-concentrate nitrogen gas at a set flowrate to the interior by way of a membrane separator. This is done under ‘smart control’ by way of programmed controller 6, for example based on the measured carbon dioxide level. When the carbon dioxide concentration rises above a prescribed level nitrogen generator system 8 is activated to supply high purity nitrogen to the interior of reefer 2 to displace the carbon dioxide.

[0047] Alternatively or in addition, controller 6 can be used to effect a nitrogen purge of the atmosphere within reefer 2, for example as an initial step after loading produce 4 and closing up reefer 2, in order to more speedily achieve the desired oxygen set point than would otherwise be possible.

[0048] Reefer 2 also includes an integrated refrigeration unit 9, powered by a separate power supply to the power supply to CA system controller 6 and/or nitrogen generator system 8. For example, the programmed controller 6 may be a low power unit, powered by a battery, while the nitrogen generator system 8 may be configured to be powered by the power source of reefer refrigeration unit 9 (generally a three phase supply). In an alternative form, programmed controller 6 and nitrogen generator system 8 may be powered from a common power source, which may be particularly suitable if provided as an integrated atmosphere control unit.

[0049] Nitrogen generator system 8, as shown in Figure 2, is configured to supply a high-concentrate nitrogen gas of a selected purity to the interior of reefer 2. The primary components of system 8 are a pump 10, pump-filter line 14, 16, coalescing filter 20, filter-membrane line 22, 32, nitrogen separator membrane module 30, membrane outlet line 34, 36 and insulation sleeve 50. These components and their interaction is described below with reference to a prototype system built and tested by the inventors.

[0050] Pump 10 is the only powered component of system 8, used to compress air to feed it to membrane module 30. In the test system, pump 10 comprised a Thomas (Gardner Denver) piston pump, model 2680CWW40. As the skilled reader will appreciate, a wide range of different types and specifications of pump may be used, but it is important to be able to produce the relatively high pressures required to meet the operating conditions required for membrane module 30, and a suitable positive displacement pump unit is therefore preferred. Pump 10 may also include a heater element for use in low temperature environments, to avoid the pump operating outside its design operating conditions. Pump 10 may be mounted internally or externally of reefer 2, in accordance with operational requirements. Indeed, nitrogen generator system 8 may be provided as a single unit within a housing, the unit configured for mounting internally or externally. External mounting generally provides easier access for maintenance and repair, but exposure to ambient conditions (a potentially more harmful environment) may require compliance with stricter requirements. Conversely, internal mounting requires the design to take into account the impact on the reefer internal environment of the heating effect of and any air leakage from pump 10.

[0051] The air feed to pump 10 is drawn from inlet feedline 12 in flow connection with the atmosphere outside reefer 2, while the compressed air from pump 10 is passed via exhaust feedline 14 and pipe coil portion 16 (discussed further below) to coalescing filter 20. Coalescing filter 20 is a modular inline air filter adapted to remove condensate (ie. liquid droplets) and particulates from the airstream. In the test system, filter 20 was provided by an SMC AF30 unit, but the skilled reader will appreciate that other suitable filter modules may be employed. This type of filter unit has a limited operating temperature range (for example, the operating conditions specified for the AF30 include a working temperature of -5 to 60 °C), and pipe coil portion 16 is disposed to act as a heat exchanger (heat sink) to ensure that the temperature of the compressed air at pump exhaust 14 drops sufficiently to fall within that temperature range. In the test system, the length of feedline from pump 10 to filter 20 was 3310mm. As will be understood, other heat sink arrangements may be used in place of pipe coil portion 16, such as a radiator element with heat dissipator fins. The heat sink may be arranged partly or wholly in contact with the air outside reefer 2 if desired.

[0052] The air outlet from filter 20 is passed via a filter outlet feedline 22 to an inlet feedline 32 to nitrogen separator membrane module 30, which in the test system was provided by a GENERON™ Model 330. This particular unit is 978mm in length, with an outer diameter of 87mm and an overall weight of 2.7 kg (including the aluminium casing). This type of membrane module 30 generally comprises a bundle of hollow, semipermeable, polymer fibres configured to separate compressed air into streams of enriched nitrogen and enriched oxygen, the separation driven by the difference in permeation rate of oxygen and nitrogen through the polymer membrane. In particular, the membrane permeates oxygen and water vapour significantly faster than nitrogen, resulting in the oxygen and water vapour permeate through the membrane to atmosphere, while the retentate nitrogen remains within the hollow fibres to exiting the module as a dry stream at a pressure a little below the feed air pressure. The nitrogen flow and purity from membrane module 30 are determined by the membrane characteristics (such as chemical nature of the membrane, thickness, permeability, surface area, etc.) and environmental factors of the airstream (such as pressure, relative humidity, temperature, etc.). Under design operation conditions the GENERON Model 330 is able to generate greater than 95% v/v nitrogen purity at a pressure from around 7 bar.

[0053] The oxygen permeate stream passes via oxygen outlet line 34 to an atmospheric exhaust port 36, while the nitrogen retentate stream passes via nitrogen outlet feedline 40, 42 to outlet port 44 which connects to the interior 5 of reefer 2, thus providing a high-concentration nitrogen feed to the container atmosphere, controllable by control of operation of pump 10. With pump 10 running at constant power a constant nitrogen feed flow can be provided whenever the system is powered, but it will be understood that the power supplied to pump 10 may be varied if desired under control of programmed controller 6 to vary the nitrogen flow rate.

[0054] In a membrane separation module the airstream separation relies on membrane performance and to provide a substantially consistent nitrogen enriched conditioned airstream to the container it is therefore important to ensure that the system operates within membrane specifications and tolerances. For example, such modules are high surface area devices, and must be protected from contamination by particulate matter and organic materials (oils, solvent vapours, etc.) which may be entrained in the feed air stream. Filter 20 serves to remove any liquid droplets and particulate matter from the inlet feed, and additional components may be included if required to ensure that the system operates at optimum performance and reliability (such as one or more carbon filters or traps). In the test system, the feedlines and associated fittings were of 10mm dia. copper tubing stainless steel, although the skilled reader will appreciate that suitable alternatives may be used (in particular, of a material selected to minimise the risk of introduction of particulate matter into the airstreams), and connections were sealed with suitable tape or thread sealant.

[0055] Further, it is important that liquids do not reach membrane module 30. For the GENERON unit used in the test system, for example, relative humidity is limited to 80%. With this in mind the inventors have developed a novel approach to minimising this risk, based on the realisation that only a small temperature increase is sufficient to provide the desired operating conditions. To this end, as shown in Figure 2, a portion 15 of the feedline connecting to pump exhaust feedline 14 is arranged in heat exchange with a portion 23 of the feedline connecting to filter outlet line 22, by running these portions 15, 23 in parallel in close proximity within an insulation sleeve 50.

[0056] In the test system, the length of feedline from filter outlet 22 to membrane module inlet 32 was 1450mm, with sleeve 50 provided by an 800mm length K-FLEX INSUL-TUBE™ of 15mm wall thickness. This is flexible, durable, non-fibrous closed cell elastomeric tube of nitrile butadiene rubber, protected with an antimicrobial agent to resist mould, fungal and bacterial growth. It will be understood that any suitable alternative heat exchange assembly may be employed to enable the heat exchange between feedline portions 15 and 23.

[0057] Also illustrated in Figure 2 are a number of sensors included in the test system as shown, including gas sensors S (to measure oxygen concentration), flowmeters F (F1-F3, in this case Honeywell Zephyr™ 50 and 100 SLPM digital airflow units, to measure the rate of input feed as well as the permeate and retentate outflow from membrane module 30) and temperature sensors T (to monitor the temperature at different points in the airfeed). As will be understood, such sensor devices S, F, T are not essential for a production unit, however some or all of the sensor devices S, F, T may be included (in suitable forms) in a production unit, in order to afford operational monitoring and diagnostics. In addition, the membrane module exhaust pressure may be monitored by way of a restrictor pressure gauge fitted to the arm of the T-junction at membrane outlet 40. In the test system, an absolute pressure sensor with a 0-100psi (6.9 bar) range was used to monitor membrane module exhaust pressure.

[0058] Figure 2 is annotated with air temperatures (°C) at different points in the feedlines, as measured in a trial of the test system (ambient temperature 16.1°C). In particular it will be noted that compressed air exits pump 10 at 69.6°C, cools through heat exchange with the surrounding atmosphere through pipe coil portion 16 to reach filter 20 at 23.1°C. The air exits filter 20 at 20.8°C, meaning there is a risk of condensation of any water vapour entrained in the airstream if this were provided directly to membrane module 30. The heat exchange between feedline portions 15 and 23 has the effect of raising the temperature reaching module inlet feed 32 to 30.2°C, which the inventors have determined fully mitigates this risk.

[0059] Feed air temperature affects the performance of the system, so the relatively modest temperature rise provided by this arrangement is found to be ideal for this application. As feed air temperature rises, the membrane permeability increases, requiring an increased feed air flow rate to maintain output product flow (assuming constant air pressure and nitrogen purity). In addition, feed air temperature has a direct effect on the membrane fibres, high temperatures shortening the life of the membrane module. The heat exchanger arrangement of the present invention acts therefore as an air conditioner, controlling the feed air temperature to an optimum level.

[0060] It will be appreciated that this particular approach avoids the need to provide a powered solution, such as a process heater in membrane module inlet feed 32 (ie. an electric heater arranged to super-heat the airstream) or a condenser to cool the airflow and remove humidity that way (ie. cooling plus separation). The power available in a conventional reefer is limited, and the novel solution of the present invention avoids the need to draw any current beyond that required to power pump 10.

[0061] The graph of Figure 3 represents the performance of the heat exchanger of the test system at different ambient temperatures Tc, dT representing the temperature drop from pump 10 to filter 20, demonstrating that even at high ambient temperatures the temperature drop is sufficient to cool the airflow sufficiently for supply to filter 20. [0062] Figure 4 represents the temperature increase dT calculated to be required to reduce the relative humidity of saturated air at filter outlet 22 to 80% at various inlet air temperatures Ta, 80% RH determined by the inventors to be an acceptable humidity level of the membrane unit feed air to avoid risk of droplet formation. Tdp refers to the corresponding dew point temperature. As can be seen, even with an air inlet temperature of 80°C, a temperature increase of only 5.4°C is sufficient to provide the desired conditions at membrane module inlet 32. The heat exchanger arrangement provided by sleeve 50 was demonstrated to provide a temperature increase of at least 7°C, thus quite sufficient to achieve the desired result.

[0063] Modifications and other additions beyond those set forth herein will be apparent in light of the benefit of the teachings presented in the foregoing descriptions and the associated drawings. It is therefore to be understood that the systems and methods described herein are not to be limited to the specific implementations disclosed and that modifications and other implementations are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense and not for purposes of limitation.

FEATURE LIST (FIGS. 1 AND 2)

2 reefer

4 respiring produce

5 internal atmosphere

6 programmed controller

8 nitrogen generation system

9 refrigeration unit

10 pump

12 pump inlet feedline

14 pump exhaust feedline

15 heat exchange portion

16 pipe coil section

20 filter

22 filter outlet feedline

23 heat exchange portion

30 nitrogen membrane separator module

32 module inlet feedline

34 oxygen outlet line (GENERON permeate outlet)

36 oxygen exhaust port (to atmosphere)

40 membrane module outlet (GENERON retentate outlet)

42 nitrogen outlet line

44 nitrogen exhaust port (to reefer interior)

50 insulation sleeve

T temperature sensor (connected to Advantech ADAM multichannel thermistor data acquisition unit)

F flowmeter (Honeywell Zephyr™ 50 [F2, F3] and 100 [F1] SLPM)

S gas sampling (sampling chamber, connected to Servomex gas analyser)