TRANSPORT CONTAINER WITH GAS SELECTIVE MEMBRANE EXHAUST

Abstract

Described herein is a method for operating a refrigerated shipping container containing respiring produce, the method including: passing a cooled CO.sub.2-rich air stream from an internal environment within the shipping container through a CO.sub.2 selective membrane of a membrane system to produce a cooled CO.sub.2-lean air stream and a CO.sub.2-rich permeate stream; retaining or returning the cooled CO.sub.2-lean air stream to the internal environment; and exhausting the CO.sub.2-rich permeate stream to an external environment outside of the shipping container; drawing external air or permitting external air to pass into the shipping container through an air vent with a pre-set fixed opening of the shipping container in a volume to at least balance a volume difference between the cooled CO.sub.2-rich air stream and the cooled CO.sub.2-lean air stream; wherein the membrane system is operated according to a pre-set mode, and the pre-set mode is independent of a measured gas concentration or pressure of the internal environment; and the pre-set fixed opening of the vent is selected based on one or more characteristics of the respiring produce and the pre-set mode of the membrane system.

Claims

1. A method for operating a refrigerated shipping container or other cooled enclosure containing respiring produce, the method including: passing a cooled CO.sub.2-rich air stream from an internal environment within the enclosure through a CO.sub.2 selective membrane of a membrane system to produce a cooled CO.sub.2-lean air stream and a CO.sub.2-rich permeate stream; retaining or returning the cooled CO.sub.2-lean air stream to the internal environment; exhausting the CO.sub.2-rich permeate stream to an external environment outside of the enclosure; and drawing external air or permitting external air to pass into the enclosure through an air vent with a pre-set fixed opening of the enclosure in a volume to at least balance a volume difference between the cooled CO.sub.2-rich air stream and the cooled CO.sub.2-lean air stream; wherein the membrane system is operated according to a pre-set mode, and the pre-set mode is independent of a measured gas concentration or pressure of the internal environment; and the pre-set fixed opening of the vent is selected based on one or more characteristics of the respiring produce and the pre-set mode of the membrane system.

2. The method of claim 1, wherein the vent is operated independently of a controller or control system.

3. The method of claim 1, wherein the pre-set mode is not adjusted, altered, or controlled in response to any measured variable of the internal environment.

4. The method of claim 1, wherein the method further includes initially selecting the pre-set mode according to one or more characteristics of the respiring produce.

5. The method of claim 1, wherein the pre-set mode is selected from the group consisting of: a pre-set constant gas throughput, a pre-set variable gas throughput, a pre-set constant electrical load, a pre-set variable electrical load, a pre-set constant pressure differential between an inlet and an outlet of the membrane system, a pre-set variable pressure differential between an inlet and an outlet of the membrane system, a pre-set constant pump speed on a pump associated with the retentate side of the membrane, or a pre-set variable pump speed on a pump associated with a retentate side of the membrane.

6. The method of claim 1, wherein the pre-set mode is additionally independent of one or more characteristics of the respiring produce.

7. The method of claim 6, wherein the pre-set mode is a fixed mode of operation.

8. The method of claim 7, wherein the pre-set mode is not adjusted, altered, or controlled.

9. The method of claim 8, wherein the fixed mode of operation is selected from: a pre-set constant gas throughput, a pre-set constant electrical load, a pre-set constant pressure differential between an inlet and an outlet of the membrane system, a pre-set constant pump speed on a pump associated with the retentate side of the membrane.

10. The method of claim 1, wherein the membrane system is operated independently of a controller or control system.

11. The method of claim 1, wherein the cooled enclosure is not operated under controlled atmosphere conditions.

12. A refrigerated shipping container or other cooled enclosure for containing respiring produce, including a control system programmed to carry out the following: pass a cooled CO.sub.2-rich air stream from an internal environment within the enclosure through a CO.sub.2 selective membrane of a membrane system to produce a cooled CO.sub.2-lean air stream and a CO.sub.2-rich permeate stream; retain or return the cooled CO.sub.2-lean air stream to the internal environment; and exhaust the CO.sub.2-rich permeate stream to an external environment outside of the enclosure; the control system further programmed to draw external air or permit external air to pass into the enclosure through an air vent with a pre-set fixed opening of the enclosure in a volume to at least balance a volume difference between the cooled CO.sub.2-rich air stream and the cooled CO.sub.2-lean air stream; wherein the control system operates the membrane system according to a pre-set mode, the pre-set mode being independent of a measured gas concentration or pressure of the internal environment; and wherein the pre-set fixed opening of the vent is selected based on one or more characteristics of the respiring produce and the pre-set mode of the membrane system.

13. A refrigerated shipping container or other cooled enclosure configured to transport or store respiring produce, including: a membrane system including: a gas inlet open to an internal environment within the enclosure; a first gas outlet open to the internal environment within the enclosure; a second gas outlet open to an external environment outside the enclosure; a CO.sub.2 selective membrane configured to: receive a cooled CO.sub.2-rich air stream from the internal environment via the gas inlet, and to separate at least a portion of CO.sub.2 from the cooled CO.sub.2-rich air stream to form a cooled CO.sub.2-lean air stream on a first side of the CO.sub.2 selective membrane and a CO.sub.2-rich permeate stream on a second side of the CO.sub.2 selective membrane, and discharge the cooled CO.sub.2-lean air stream through the first gas outlet and the CO.sub.2-rich permeate stream through the second gas outlet, and gas circulation apparatus configured to pass the cooled CO.sub.2-rich air from the internal environment through the CO.sub.2 selective membrane; wherein the membrane system is configured to be operated according to a pre-set mode, and the pre-set mode is independent of a measured gas concentration or pressure of the internal environment; and wherein the enclosure includes an air vent with an opening, the air vent configured to be opened to a selected pre-set fixed opening based on one or more characteristics of the respiring produce and the pre-set mode of the membrane system.

14. The enclosure of claim 13, wherein the pre-set mode is operated independently of a controller or control system.

15. The enclosure of claim 13, wherein the pre-set mode is independent of any measured variables of the internal environment.

16. The enclosure of claim 13, wherein the vent is operated independently of a controller or control system.

17. The enclosure of claim 13, wherein the enclosure does not include a controlled atmosphere control system.

18. A CO.sub.2 selective gas membrane module when used in a refrigerated shipping container or other cooled enclosure that does not include a controlled atmosphere control system, the membrane module including: a mount for installing the membrane module into the enclosure; a gas inlet configured to be open to an internal environment within the enclosure; a first gas outlet configured to be open to the internal environment within the enclosure; a second gas outlet configured to be open to an external environment outside the enclosure; a CO.sub.2 selective membrane configured to: receive a cooled CO.sub.2-rich air stream from the internal environment via the gas inlet, and to separate at least a portion of CO.sub.2 from the cooled CO.sub.2-rich air stream to form a cooled CO.sub.2-lean air stream on a first side of the CO.sub.2 selective membrane and a CO.sub.2-rich permeate stream on a second side of the CO.sub.2 selective membrane, and discharge the cooled CO.sub.2-lean air stream through the first gas outlet and the CO.sub.2-rich permeate stream through the second gas outlet; and gas circulation apparatus configured to pass the cooled CO.sub.2-rich air from the internal environment through the CO.sub.2 selective membrane; wherein the membrane system is configured to be operated according to a pre-set mode and the pre-set mode is independent of a measured gas concentration or pressure of an internal environment of the enclosure.

19. A method of installing a membrane module according to claim 18 in a refrigerated shipping container or other cooled enclosure that does not include a controlled atmosphere control system.

20. A method of modifying a controlled atmosphere refrigerated shipping container or other controlled atmosphere cooled enclosure to provide a cooled enclosure according to claim 12, the method including removing the controlled atmosphere control system from the enclosure.

21. A method for reducing refrigeration energy requirements in operation of a refrigerated shipping container or other cooled enclosure containing respiring produce, the method including: a membrane exhaust activity involving drawing and/or driving air from the interior of the enclosure through a CO.sub.2 selective membrane of a membrane system installed in the enclosure, and exhausting the resulting CO.sub.2-rich downstream air stream to the exterior of the enclosure; and an atmosphere replenishment activity involving causing or permitting ambient air from the exterior of the enclosure to pass into the enclosure to balance the volume of air lost through the membrane; wherein the method is conducted in the absence of controlled atmosphere operation, such that neither the membrane exhaust activity nor the atmosphere replenishment activity is conducted in accordance with the monitoring of a gas concentration or pressure in the interior of the enclosure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0090] FIG. 1 is a photograph of a refrigeration panel of a refrigerated shipping container.

[0091] FIG. 2 is a schematic of a membrane separation system for installation into a refrigerated shipping container.

[0092] FIG. 3 is a schematic illustrating one embodiment of the membrane separation system.

[0093] FIG. 4 is a graph showing modelled results of equilibrium CO.sub.2 concentration within a shipping container as a function of membrane surface area for bananas and avocados for different respiration rates.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0094] The invention relates to a method and/or apparatus for storing and/or transporting respiring produce in an unsealed refrigerated shipping container or other cooled enclosure without an actively controlled atmosphere. Respiring produce produces CO.sub.2 which needs to be removed from the internal environment of the refrigerated shipping container to preserve the freshness of the respiring produce. Such respiring produce typically includes fruit, vegetables, plants, seedlings, plant materials, and the like.

[0095] It will be appreciated that the methodology and system is applied without a ‘controlled atmosphere’ regime. A controlled atmosphere regime is associated with a substantially sealed reefer, and is one in which one or more conditions of the internal atmosphere are monitored, and operation of the membrane system is controlled (such as via a controller or control system) to maintain the one or more monitored conditions at a set point or within a set point range. An example of a controlled atmosphere regime is the monitoring of CO.sub.2 concentration within a sealed reefer, and controlling the operation of the membrane system to maintain the CO.sub.2 concentration within the reefer within an acceptable concentration range. In contrast with this, the invention of the method resides, in part, in removing CO.sub.2 from the internal environment of the reefer while minimising the loss of cooled air to the external environment, and the introduction of external air, and hence heat energy into the internal environment of the reefer. As a consequence, the load drawn by a refrigeration system to cool the air is reduced. This method is performed in the absence of a controlled atmosphere regime.

[0096] This method finds particular application in unsealed reefers, such as those that have an external vent. A refrigeration panel 100 of a reefer is illustrated in FIG. 1. The standard refrigeration panel 100 includes an air vent 102 which has two openings that define an inlet and an outlet (not shown). A rotatable vent cover 104 is located over the air vent 102, and this rotatable vent cover 104 can be rotated to open, close, or adjust the size of the inlet and outlet in the air vent 102 for the purpose of fresh air exchange. In FIG. 1, the rotatable vent cover 104 is shown in the closed position. However, the rotatable vent cover 104 includes two openings 106A and 106B which correspond with the openings (not shown) in the air vent 102. The refrigeration panel also includes a refrigeration system 108 for cooling air within the reefer.

[0097] The vent cover 104 includes gradations 110 which relate the size of the inlet and outlet openings to a corresponding fresh air exchange rate during standard operation. Larger inlet and outlet openings provide for a greater fresh air exchange rate. The fresh air exchange (and thus the size of the inlets and outlets) is dependent on the respiration rate of the respiring product. That is, respiring products that have a high respiration rate require a greater fresh air exchange rate than respiring products with a low respiration rate. At this point, it is important to note that if the reefer is intended for climate controlled operation, then the reefer is sealed by removing the rotatable vent cover 104, and installing a climate controller and valves over the vent openings to seal the vent. As a result, a sealed climate controlled reefer does not include a permanently open vent.

[0098] During the unsealed storage and/or transport of respiring produce, the respiring produce consumes oxygen and produces carbon dioxide. If the oxygen levels and carbon dioxide levels fall outside of a particular range, the quality of the respiring produce can rapidly deteriorate. To address this, and as alluded to above, the rotatable vent cover 104 is adjusted (by rotation) so as to provide inlet and outlet openings of a suitable size to permit an appropriate rate of gas exchange between the outside environment and the internal environment within the reefer to maintain suitable oxygen and carbon dioxide levels. The required rate of gas exchange is determined from the respiration rate of the respiring produce (being dependent on the type of respiring produce), and the appropriately sized opening in the air vent 102 is selected (e.g. by way of a lookup table) to provide the required rate of gas exchange.

[0099] The gas exchange process generally results in cool CO.sub.2-rich, O.sub.2-lean air from within the reefer being exchanged for air at ambient temperature and composition. This is advantageous in that CO.sub.2 is removed from the system, and fresh O.sub.2 is introduced into the system. However, introducing air at ambient temperature introduces heat energy into the system, and raises the internal temperature with the reefer. Increasing the temperature has a deleterious effect on the respiring produce. Thus, the refrigeration system 108 must remove this additional energy that has been introduced into the reefer.

[0100] The inventors have included a membrane separation system into the refrigeration panel 100 of the reefer. FIG. 2 provides a schematic of a membrane separation system 200. The membrane separation system 200 includes a top bracket 202 and a bottom bracket 204 for mounting the system 200 inside a reefer. The system 200 further includes a circulation system that includes at least a lumen pump 206 for circulating CO.sub.2-rich air from within the reefer, to the retentate side of the membrane 208 for CO.sub.2 removal, and then back into the reefer. The system 200 also includes a sweep pump assembly 210 for providing a stream of sweep gas (ambient air) on the permeate side of the membrane 208 such that the CO.sub.2 that passes from the retentate side of the membrane 208 to the permeate side of the membrane and is then entrained in the sweep gas. As part of installing the membrane separation system 200, blank panel 112 of the refrigeration panel 100 is removed and replaced with a membrane scrubber panel (see item 312 of FIG. 3) which includes an air inlet 314 and air outlet 316 for the sweep gas.

[0101] The inclusion of the membrane system 200 into the reefer reduces the volume of gas exchange through the vent to attenuate a reduction in O.sub.2 concentration and an increase in CO.sub.2 concentration in the cooled air within the reefer due to the respiration of the respiring produce. The cooled CO.sub.2-rich air within the reefer is cycled through the membrane system at a pre-set rate (determined based on a characteristic of the respiring produce) to remove a portion of the CO.sub.2 with the cooled air, the CO.sub.2-lean cooled air is then returned to the internal environment of the reefer. The actual process of gas exchange involves the CO.sub.2 transferring from the CO.sub.2-rich air from a retentate side of the membrane across the gas exchange membrane and into a sweep gas stream on the permeate side of the membrane in which the CO.sub.2 is entrained and subsequently exhausted outside the reefer. The sweep gas stream is essentially an air stream, which air is taken from outside the reefer. Because the CO.sub.2 rich gas is low in oxygen, and the sweep gas is relatively high in oxygen (i.e. containing about 21% oxygen) a partial pressure differential for oxygen exists across the membrane. As a result, and although the membrane is selective for CO.sub.2, in some cases (depending on the type of membrane) oxygen migrates from the sweep gas on the ‘permeate’ side of the membrane across the membrane to the ‘retentate’ side of the membrane where it is entrained in the now CO.sub.2-lean cooled air. This helps to increase the oxygen concentration within the reefer.

[0102] Additional oxygen is introduced into the reefer via the conventional gas exchange process that is associated with the vent. However, as the CO.sub.2 is being removed from and O.sub.2 is being introduced into the cooled air within the reefer via the membrane separation system, a lower rate of gas exchange through the vent is required. This means that less cool air is lost to the external environment via the vent and consequently less warm air is introduced into the internal environment of the reefer. Given this, the air within the reefer has a lower cooling requirement which reduces the load on the refrigeration system (e.g. the compressor of the refrigeration system). In practice, as a lower rate of gas exchange between the outside environment and the internal environment within the reefer is required, the vent cover can be rotated to reduce the size of the inlet and outlet openings in the vent (again determined by, for example, using look-up table designed for use with the system of the present invention).

[0103] A process flow diagram illustrating one embodiment of the membrane separation system 300 is provided in FIG. 3. The system 300 includes: a membrane scrubbing unit 302 including a hollow fibre membrane filtration unit, a lumen inlet 304 for receiving gas from the internal environment of a shipping container, and a lumen outlet 306 for returning filtered gas to the internal environment of the shipping container; a lumen pump 308 for circulating gas from the internal environment of the shipping container and through a retentate side of the membrane scrubbing unit 302. The system 300 also includes a sweep gas assembly that includes: a sweep pump 310 for circulating sweep gas though a permeate side of the membrane scrubbing unit 302 via sweep gas inlet 318 and sweep gas outlet 320, wherein the sweep pump 310 is in gas communication with a scrubber panel assembly 312 having an ambient air inlet port 314 and an exhaust port 316.

[0104] This membrane separation system 300 is installed in a reefer as discussed in relation to FIG. 2. The operation of the system 300 of FIG. 3 is briefly described below.

[0105] During shipping and/or storage of refrigerated respiring produce, the respiring produce consumes oxygen and produces carbon dioxide. The skilled person will appreciate that the degree of refrigeration and the rates of oxygen consumption and carbon dioxide production depend on one or more characteristics of the respiring produce. As previously discussed, to minimise degradation of the respiring produce, the oxygen and carbon dioxide concentrations should be maintained at appropriate levels. In a standard reefer, the vent cover (e.g. item 104 of FIG. 1) is rotated to a particular sized opening to permit fresh air exchange at an appropriate rate to maintain the oxygen and carbon dioxide concentration at an appropriate level. However, in a reefer system including the membrane separation system 300, the membrane separation system 300 removes a portion of the carbon dioxide and may additionally introduce oxygen into the internal environment of the reefer. This allows a smaller vent opening to be selected which reduces the rate of fresh air exchange, and thus minimises the loss of cool air and the introduction of heat energy from ambient fresh air.

[0106] In operation, lumen pump 308 draws cooled CO.sub.2-rich, O.sub.2-lean gas from the internal environment of a reefer. The lumen pump 308 pushes this gas, under positive pressure, through the membrane scrubbing unit 302 via lumen inlet 304. Inside the membrane scrubbing unit 302, the gas is forced through lumens of a hollow fibre membrane separation unit. The membrane lumens are formed from a CO.sub.2 gas selective membrane material, which results in the selective transfer of CO.sub.2 across the lumen wall from a retentate side of the lumen to a permeate side of the lumen. For reasons that will be further outlined below, O.sub.2 may be transferred from the permeate side to the retentate side of the lumens. This results in a cooled CO.sub.2-lean gas stream (which may include additional O.sub.2) on the retentate side of the lumen. The cooled CO.sub.2-lean gas is then returned to the internal environment of the reefer via lumen outlet 306. In this embodiment, the downstream ends of the lumens are exposed directly to the lumen outlet 306 (e.g. there is no pump on the downstream side to draw the cooled air through the membrane system 300). Notwithstanding the above, the skilled addressee will appreciate that the membrane system may include an additional pump downstream of the lumen outlet 306 for drawing air through the membrane scrubbing unit 302. In another form, the membrane separation system 300 does not include a lumen pump upstream of the lumen inlet 304, and instead includes a lumen pump downstream of the lumen outlet 306 to draw gas from the internal environment through the membrane scrubbing unit 302 under negative pressure.

[0107] The sweep gas assembly provides a sweep gas (e.g. ambient air drawn from outside of the reefer) to the permeate side of the membrane scrubbing unit 302. During operation, sweep gas pump 310 applies a negative pressure to the sweep gas assembly to draw ambient air from outside the reefer via inlet port 314 and into the membrane scrubbing unit 302 via sweep gas inlet 318. The sweep gas is drawn through the sweep gas inlet 318 and along the permeate side of the membrane lumens to entrain and remove CO.sub.2 that has filtered across the membrane lumens from the cooled CO.sub.2-rich, O.sub.2-lean gas on the retentate side of the lumen resulting in a CO.sub.2-rich sweep gas. As the sweep gas has a relatively higher O.sub.2 concentration than the gas on the retentate side of the membrane, an O.sub.2 partial pressure gradient exists which drives a portion of the O.sub.2 in the sweep gas through the membrane and into the retentate gas. The CO.sub.2-rich sweep gas is then drawn through sweep gas outlet 320, through sweep gas pump 310, and then discharged under positive pressure through exhaust port 316 to an environment outside the reefer.

[0108] It will be appreciated that a variety of different membranes may be used in the membrane gas scrubber.

[0109] In an embodiment, the membrane has a selectivity which allows carbon dioxide gas to permeate through the membrane element at a higher rate than oxygen and nitrogen. Preferably the membrane has a CO.sub.2:O.sub.2 selectivity ratio of at least 5:2. More preferably, the membrane has a CO.sub.2:O.sub.2 selectivity ratio of at least 4:1. Even more preferably, the membrane has a CO.sub.2:O.sub.2 selectivity ratio of at least 5:1. While there is no particular upper limit to the CO.sub.2:O.sub.2 selectivity ratio, it is desirable that some O.sub.2 is able to transfer across the membrane. Thus, in practice it is preferred that the membrane has a CO.sub.2:O.sub.2 selectivity ratio of up to 15:1. Additionally or alternatively, it is preferred that the membrane has a CO.sub.2:N.sub.2 selectivity ratio of at least 5:1. More preferably, the membrane has a CO.sub.2:N.sub.2 selectivity ratio of at least 7:1. Even more preferably, the membrane has a CO.sub.2:N.sub.2 selectivity ratio of at least 14:1. While there is no particular upper limit to the CO.sub.2:N.sub.2 selectivity ratio, practically the membrane may have a CO.sub.2:N.sub.2 selectivity ratio of up to 50:1.

[0110] Membranes contemplated include an overall permeability for CO.sub.2 of about 3000 Barrer and comprise a thickness of about 35 μm to 45 μm. Preferred membranes have about 3100 Barrers of permeability for CO.sub.2 and 40 μm in thickness. Therefore the permeability per unit thickness for a suitable membrane is about 78 Barrers/μm. This is a very high permeability. However, other membrane materials are contemplated to be useful. One type of suitable membrane for use with preferred embodiments of the present invention is manufactured from Polydimethylsiloxane (PDMS), which has moderate selectivity to CO.sub.2, at about between 4 and 5, and a CO.sub.2/N.sub.2 selectivity of between about 10 and 11. Other membranes, including non-silicon membranes, may also be used. Still further, the invention contemplates the use of cellulose acetate, which has an overall permeability for CO.sub.2 of 6.3 Barrer. This is a large difference, but gas transfer can be improved by altering the thickness of the membrane or by increasing the total surface area of the membrane.

[0111] 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.

EXAMPLE

Example 1

[0112] The method of the invention was evaluated for storage of onion (dry), melon, apple (Fuji), potato, sweet corn, and grape produce using a PDMS gas exchange membrane (GEM) system. Table 2 summarises the results below, which indicate that the method and systems of the present invention provide substantive energy savings.

TABLE-US-00001 Standard Heat Load Vent Vent at 25° C. Setting Set Setting Ambient with GEM Heat Load Energy % Commodity Temp (CMH) (Watts) (CMH) (Watts)*.sup.1 saving Saving Onion (dry) 0° C. 50 418 4 135 283 68% Melon 5° C. 50 335 4 128 207 62% Apple (Fuji) 0° C. 50 418 4 135 283 68% Potato 7° C. 26 157 8 147 10  6% Sweet Corn 0° C. 77 644 29 340 304 47% Grape 0° C. 26 218 2 119 99 45% *.sup.1Including Load of GEM System