Patent Publication Number: US-2021188518-A1

Title: Membrane apparatus and method for use in shipping container

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method of, and apparatus for, controlling gas composition within a refrigerated container, such as to extend the life of perishable goods during transport within the refrigerated container. 
     BACKGROUND OF THE INVENTION 
     In order to prolong the storage life of perishable goods (such as fruit and vegetables) stored in sealed controlled atmosphere containers during transportation or storage it is generally important to control at least some environmental conditions within the container. This is because environmental parameters, for example temperature and gas composition within the container, affect the rate of respiration and deterioration of goods after harvest. 
     The conventional method of extending storage life of produce has been to refrigerate the sealed container and to reduce carbon dioxide levels (as carbon dioxide is generated by respiring produce), while maintaining a controlled atmosphere within that container (e.g. maintaining oxygen and nitrogen at desired levels). However, if the carbon dioxide concentration rises too high, then the perishable product may be damaged, resulting in even more rapid deterioration than might occur if no treatment was applied. Damage may also occur if the oxygen concentration is reduced too much. However, conversely higher concentrations of oxygen promote respiration which is also undesirable. 
     Consequently it is desirable to be able to adjust the composition of the atmosphere within the sealed chamber and apparatus for adjusting the atmosphere in the chamber has accordingly been developed. 
     Applicant&#39;s invention described in WO 20001023350 entitled ‘Apparatus for controlled venting of a chamber’ proposed a new approach of maintaining the controlled environment within a substantially sealed chamber containing respiring produce. The method is carried out without monitoring the carbon dioxide level in the sealed chamber and involved monitoring the oxygen level in the chamber and admitting ambient air into the sealed chamber when the oxygen level is detected to have fallen below an oxygen set point. Carbon dioxide is removed from the sealed chamber at a predetermined rate by way of a selected quantity of carbon dioxide absorbing material stored within the sealed container. The predetermined rate in the process is selected before the storage/journey such that the carbon dioxide concentration within the sealed chamber will not exceed a predetermined amount. 
     Other known methods for controlling the atmosphere within a sealed container utilise a permeable membrane within the sealed container which membrane is selective for removing certain gases while retaining others. That is, the membrane allows some gases to pass through, whilst excluding or minimising the passage of certain other gases. The selective membrane is installed in the sealed container as a liner layer which defines a buffer zone which can be opened to the ambient air outside the sealed container, or manipulated in other ways. Imposing a partial pressure difference across the membrane has the effect of selective removal of gases into the buffer zone. Such techniques advantageously avoid the need for carbon dioxide absorbing materials. Typically, with these methods, fresh ambient air is leaked into the container, such as through an air vent, to replenish the air removed via the membrane. There are a number of disadvantages with this approach including: high energy consumption to cool the fresh ambient air and a lack of control over the air composition of the internal environment within the container, particularly with respect to oxygen. 
     Applicant&#39;s invention described in WO 2014/066952 entitled ‘Improvements in control of gas composition within a container’ describes a method controlling the atmosphere within a substantially sealed container by removing carbon dioxide from the sealed chamber of a shipping container using a membrane system. In this publication, the Applicant proposed a method of controlling the atmosphere in which air from within the sealed chamber was passed through the membrane system to remove CO 2  whilst the air pressure inside the chamber was actively monitored. In response to a change in pressure, a controller would actuate an inlet valve on the shipping container to introduce external air into the container in an amount to result in the air within the sealed chamber having a set gas composition. Whilst this method provides advantages over the method described above, further improvements in energy efficiency and control over the internal composition of the air inside the container are desirable. 
     The present invention provides in various forms a new method of controlling the environment in a container, a new container apparatus, and apparatus for controlling the environment in the container. 
     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 
     In a first aspect of the invention, there is provided a method for controlling the atmosphere within a refrigerated shipping container containing respiring produce, the method including: 
     blending a first air stream withdrawn from an internal environment within the refrigerated shipping container with a second air stream obtained from an ambient environment external to the shipping container to form a blended air stream; 
     subjecting the blended air stream to a membrane separation process using a separation membrane having greater relative selectivity for CO 2  and O 2  than N 2  to provide an N 2 -rich gas stream; and returning the N 2 -rich gas stream to the internal environment. 
     The present invention thus advantageously provides a means for removing CO 2  and O 2  from the internal environment and offsetting this with the addition of N 2 . The blending of N 2  with the first air stream after the first air stream has been withdrawn from the internal environment and before the first stream to offset the removal of CO 2  and O 2  results in the internal environment having low concentrations of CO 2  and O 2  which reduces the rate of respiration and provides enhanced storage for the produce during transport. Furthermore, because N 2 -rich gas stream can be operated at a volumetric flow rate to offset the lost CO 2  and O 2 , the requirements to introduce fresh ambient air are minimised reducing the energy needed to maintain an internal refrigerated environment. 
     As discussed above, the membrane has a greater relative selectivity for CO 2  and O 2  than N 2 . This is intended to mean that the membrane possesses a selectivity which allows carbon dioxide gas and oxygen to permeate through the membrane at a higher rate than nitrogen. 
     In an embodiment, the N 2 -rich gas stream has a N 2  vol % that is greater than the N 2  vol % in the blended aft stream. 
     In an embodiment, the N 2 -rich gas stream has a N 2  vol % that is greater than the N 2  vol % of air (e.g. greater than 78 vol % on a dry air basis). In one or more forms, the N 2 -rich gas stream has a N 2  vol % of from about 80 vol % and up to about 98 vol %. Preferably, the N 2  vol % is from about 82 vol %. More preferably, the N 2  vol % is from about 84 vol %. Alternatively, or additionally, it is preferred that the N 2  vol % is up to about 96 vol %. More preferably, the N 2  vol % is up to about 94 vol %. 
     In an embodiment, the first air stream is a cooled CO 2 -rich air stream. By CO 2 -rich, it is meant that the first air stream has a CO 2  vol % that is higher than air (e.g. greater than about 0.04 vol % on a dry air basis). Typically, the cooled CO 2 -rich air stream has a CO 2  content of 2 to 10 vol %. The skilled person will appreciate that the first air stream contains N 2  (about 78 vol % on a dry air basis noting that N 2  does not participate in the respiration reaction) and a reduced amount of O 2  relative to dry air (e.g. less than 21 vol %) due to the consumption of O 2  during respiration. In one or more embodiments, the cooled CO 2 -rich air stream has an O 2  content of 2 to 10 vol %. 
     It will be appreciated that as the second air stream is obtained from the ambient environment, the second air stream will have a gas composition of air, e.g. dry air has a composition of about 78 vol % N 2 , about 21 vol % O 2 , about 0.04 vol % CO 2 , and a remainder of other gases. 
     An advantage of the various aspects generally described herein, is that the composition of the internal environment within the refrigerated container can be set, e.g. it is possible to balance the operation of the membrane, such as by controlling flows of the various gas streams and/or internal pressure of the container to maintain a desired container gas composition in terms of at least CO 2 , O 2 , and N 2 . 
     In an embodiment, the container air has a higher proportion of CO 2  than the N 2 -rich gas stream, such that the method is a method of reducing the CO 2  concentration within the refrigerated shipping container. 
     In an embodiment, the N 2 -rich gas stream is fed into the container at a rate sufficient to maintain a neutral or positive pressure difference between the internal environment and the external environment. An advantage of this arrangement is that it minimises or prevents ingress of ambient air into the container, which can be important as the container ages and the likelihood of air leakage into the container increases. However, it will be appreciated that in other embodiments, the N 2 -rich gas stream is fed into the container at a rate to provide a negative pressure difference between the internal environment and the external environment. 
     In an embodiment, the internal environment of the refrigerated container is maintained at a CO 2  concentration of from about 2 up to about 10 vol %. Preferably, the CO 2  concentration is from about 3 vol %. More preferably, the CO 2  concentration is from about 4 vol %. Most preferably, the CO 2  concentration is from about 5 vol %. Alternatively, or additionally, it is preferred that the CO 2  concentration is up to about 9 vol %. More preferably, the CO 2  concentration is up to about 8 vol %. 
     In an embodiment, the internal environment of the refrigerated container is maintained at an O 2  concentration of from about 2 up to about 10 vol %. Preferably, the O 2  concentration is from about 3 vol %. Alternatively, or additionally, the O 2  concentration is up to about 9 vol %. Preferably, the O 2  concentration is up to about 8 vol %. More preferably, the O 2  concentration is up to about 7 vol %. Most preferably, the O 2  concentration is up to about 6 vol %, 
     In an embodiment, the internal environment of the refrigerated container is maintained at a N 2  concentration of from about 80 up to about 95 vol %. Preferably, the N 2  concentration is from about 82 vol %. Alternatively, or additionally, the N 2  concentration is up to about 84 vol %. Preferably, the N 2  concentration is up to about 93 vol %. More preferably, the N 2  concentration is up to about 91 vol %. Most preferably, the N 2  concentration is up to about 89 vol %. 
     The skilled person will appreciate that the composition of the internal environment may be controlled or set by the flow rate of the N 2 -rich gas stream and/or a rate at which container air is leaked from the internal environment to the external environment. Thus, in an embodiment, the refrigerated shipping container includes a vent, and the method includes leaking a portion of the container air through the vent. 
     In one form of the above embodiment, the N 2 -rich gas stream is fed into the container and/or container air is leaked from the internal environment at a rate sufficient to maintain a CO 2  concentration within the internal environment of from about 2 up to about 10 vol %. Preferably, the CO 2  concentration is from about 3 vol %. More preferably, the CO 2  concentration is from about 4 vol %. Most preferably, the CO 2  concentration is from about 5 vol %. Alternatively, or additionally, it is preferred that the CO 2  concentration is up to about 9 vol %. More preferably, the CO 2  concentration is up to about 8 vol %. 
     In one form of the above embodiment, the N 2 -rich gas stream is fed into the container and/or container air is leaked from the internal environment at a rate sufficient to maintain an O 2  concentration within the internal environment of from about 2 up to about 10 vol %. Preferably, the O 2  concentration is from about 3 vol %. Alternatively, or additionally, the O 2  concentration is up to about 9 vol %. Preferably, the O 2  concentration is up to about 8 vol %. More preferably, the O 2  concentration is up to about 7 vol %. Most preferably, the O 2  concentration is up to about 6 vol %. 
     In an embodiment, the N 2 -rich gas stream is lean in CO 2  and O 2 . By lean in CO 2  and O 2 , it is meant that the N 2 -rich gas stream has a CO 2  and O 2  vol % that is lower than the vol % in the blended air stream. In a preferred form, the CO 2  and/or O 2  vol % that is lower than the vol % in air (e.g. less than 0.4 vol % CO 2  and/or less than 21 vol % O 2  on a dry air basis). 
     In an embodiment, the step of subjecting the blended air stream to the membrane separation process includes:
         contacting the blended air stream with the membrane to produce a CO 2 -, O 2 -rich permeate stream and a retentate stream that is the N 2 -rich gas stream; and   exhausting the CO 2 -, O 2 -rich permeate stream to the external environment.       

     In an embodiment, the first air stream and the second air stream are blended in a ratio of from about 5:1 to about 25:1. Preferably, the ratio is to about 20:1. More preferably, the ratio is to about 15:1. Alternatively, or additionally, it is preferred that the ratio is from about 7:1. More preferably, the ratio is from about 8:1. By way of example, in one form, the ratio is from about 8:1 to about 15:1. 
     In an embodiment, the second aft stream has a volumetric flowrate that is sufficient to provide a volumetric flow rate of the N 2 -rich gas stream that is from 80% and up to 120% of a volumetric flow rate of the first air stream. Preferably, the volumetric flow rate is from 85% of the volumetric flow rate of the first air stream. More preferably, the volumetric flow rate is from 90% of the volumetric flow rate of the first air stream. Even more preferably, the volumetric flow rate is from 95% of the volumetric flow rate of the first air stream. Most preferably, the volumetric flow rate is from or about 100% of the volumetric flow rate of the first air stream. Alternatively, or additionally, it is preferred that the volumetric flow rate is up to 115% of the volumetric flow rate of the first air stream. More preferably, the volumetric flow rate is up to 110% of the volumetric flow rate of the first air stream. Most preferably, the volumetric flow rate is up to 105% of the volumetric flow rate of the first air stream. By way of example, in one form, the volumetric flow rate of the N 2 -rich stream is from 100% and up to 110% of the volumetric flow rate of the first air stream. 
     In an embodiment, the membrane has a CO 2 :N 2  selectivity ratio of at least 5:1. Preferably, the membrane has a CO 2 :N 2  selectivity ratio of at least 7:1. More preferably, the membrane has a CO 2 :N 2  selectivity ratio of at least 10:1. Most preferably, the membrane has a CO 2 :N 2  selectivity ratio of at least 14:1. While there is no particular upper limit to the CO 2 :N 2  selectivity ratio, practically the membrane may have a CO 2 :N 2  selectivity ratio of up to 50:1. 
     In an embodiment, the membrane has an O 2 :N 2  selectivity ratio of at least 1.5:1. Preferably, the membrane has an O 2 :N 2  selectivity ratio of at least 2:1. Even more preferably, the membrane has an O 2 :N 2  selectivity ratio of at least 3:1. While there is no particular upper limit to the O 2 :N 2  selectivity ratio, practically the membrane may have an O 2 :N 2  selectivity ratio of up to 20:1. 
     In further forms of the above embodiment, the membrane has a selectivity which allows carbon dioxide to permeate through the membrane at a higher rate than oxygen. In one embodiment, the membrane has a CO 2 :O 2  selectivity ratio of at least 5:2. Preferably, the membrane has a CO 2 :O 2  selectivity ratio of at least 4:1. More preferably, the membrane has a CO 2 :O 2  selectivity ratio of at least 5:1. While there is no particular upper limit to the CO 2 :O 2  selectivity ratio, practically the membrane may have a CO 2 :O 2  selectivity ratio of up to 15:1. 
     In an embodiment, the refrigerated shipping container: (i) does not include a vent, or (ii) is operated with the vent set to a substantially dosed position, or (iii) is operated with the vent set to a position such that air flow through the vent is less than that required to replace the first air stream that is withdrawn from the internal environment. 
     In a second aspect of the invention, there is provided a refrigerated shipping container configured to be operated according to the method of the first aspect of the invention and/or embodiments thereof. 
     In a third aspect of the invention, there is provided a refrigerated shipping container configured to transport respiring produce, the refrigerated shipping container including: 
     a gas membrane separation module including:
         a first gas inlet open to an internal environment within the refrigerated shipping container configured to draw a first air stream from the internal environment;   a second gas inlet open to an ambient environment external to the refrigerated shipping container and configured to draw a second air stream from the ambient environment;   a membrane unit including:
           a membrane having greater relative selectivity for CO 2  and O 2  than N 2  and configured to provide an N 2  rich gas stream; and   an inlet to the membrane configured to receive a blended gas stream from the first gas inlet and the second gas inlet,   an outlet from the membrane open to the internal environment configured to return the N 2  rich gas stream to the internal environment.   
               

     In an embodiment of the second or third aspects, the refrigerated shipping container further includes: 
     gas circulation means configured to:
         draw the first aft stream through the first inlet,   draw the second gas stream through the second inlet,   contact the blended aft stream with the membrane, and   return the N 2 -rich gas stream to the internal environment.       

     In an embodiment of the second or third aspects, the refrigerated shipping container: (i) does not include a vent, or (ii) includes a vent set to a substantially closed position, or (iii) includes a vent configured to an open position such that air flow through the vent is less than that required to replace the first air stream that is withdrawn from the internal environment. 
     In a fourth aspect of the invention, there is provided a gas membrane separation module, the gas membrane separation module including: 
     a mount for installing the gas membrane separation module into or onto a refrigerated shipping container; 
     a first gas inlet open to an internal environment within the refrigerated shipping container configured to draw a first air stream from the internal environment; 
     a second gas inlet open to an ambient environment external to the refrigerated shipping container and configured to draw a second air stream from the ambient environment; 
     a membrane unit including:
         a membrane having greater relative selectivity for CO 2  and O 2  than N 2  and configured to provide an N 2 -rich gas stream; and   an inlet to the membrane configured to receive a blended gas stream formed from the first air stream and the second air stream,   an outlet from the membrane open to the internal environment configured to return the N 2 -rich gas stream to the internal environment.       

     In an embodiment, there is provided the gas membrane separation module when used in a refrigerated shipping container. 
     In a fifth aspect of the invention, there is provided a method including installing the gas separation module of the fourth aspect of the invention and/or embodiments thereof into or onto a refrigerated shipping container. 
     It will be appreciated that in one or more forms of the invention, the membrane system includes a retentate side gas circulation system. The retentate side gas circulation system may include one or more pumps, such as one or more pumps located upstream of the CO 2  selective membrane and/or one or more pumps located downstream of the CO 2  selective membrane. While variable drive speed pumps may be used, it is preferred that each pump is operated at a single speed. Given this, it is further preferable that each pump is a single speed pump. 
     In an embodiment, the membrane system includes a single pump retentate side gas circulation system for passing the cooled CO 2 -rich air stream to the CO 2  selective membrane and returning the cooled CO 2 -lean air stream to the internal environment. In one form of this embodiment, the single pump is located upstream of the CO 2  selective membrane and the step of passing the cooled CO 2 -rich air through the CO 2  selective membrane includes: providing air to the CO 2  selective membrane under positive pressure. In an alternative form of this embodiment, the single pump is located downstream of the CO 2  selective membrane and the step of passing the cooled CO 2 -rich air stream through the CO 2  selective membrane includes drawing air through the CO 2  selective membrane under negative pressure. 
     It will be appreciated that in one or more forms of the invention, the membrane system includes a permeate side gas circulation system, also commonly referred to as a sweep gas circulation system. In such embodiments, the CO 2 -rich permeate stream is a CO 2 -rich sweep stream. The permeate side gas circulation system may include one or more sweep pumps, such as one or more sweep pumps located upstream of an inlet to a permeate side of the CO 2  selective membrane and/or one or more sweep pumps located downstream of an outlet to the permeate side of the CO 2  selective membrane. Although it is preferred that the permeate side gas circulation system includes the one or more pumps downstream of the outlet. While variable drive speed sweep pumps may be used, it is preferred that each sweep pump is operated at a single speed. Given this, it is further preferable that each sweep pump is a single speed pump. In alternative forms of the invention, the permeate side gas circulation system is a vacuum system, e.g. the system does not make use of a sweep gas circulation system, but is instead run under negative pressure. 
     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 
         FIG. 1  is a photograph of a refrigeration panel of a refrigerated shipping container. 
         FIG. 2  is a schematic of a membrane separation system for installation into a refrigerated shipping container. 
         FIG. 3  is a schematic illustrating one embodiment of the membrane separation system. 
         FIG. 4  is a schematic of a gas flow balance through a membrane filtration unit. 
         FIG. 5  is a schematic showing operation of the system under (a) a neutral pressure differential between the internal and external environments, and (b) positive pressure differential between the internal and external environments. 
         FIG. 6  is a schematic showing a membrane separation system of the invention installed in a container. 
         FIG. 7  is a graph showing flows (Q SLPM) over time measured using the system shown in  FIG. 6  in two modes—the first mode (denoted ‘standard operation’) does not involve the addition of ambient air and the second mode (denoted ‘hybrid operation’) operated according to the invention. 
         FIG. 8  is a graph showing pressure (kPa) at various points of the system shown in  FIG. 6  over time in the same two operation modes as described for  FIG. 7 . 
         FIG. 9  is a graph comparing CO 2  removal over time across operation of the system shown in  FIG. 6  in the first versus the second mode as described for  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The invention relates to a method of, and apparatus for, controlling gas composition within a refrigerated container, such as to extend the life of perishable goods during transport within the refrigerated container. hi particular, the inventors have devised a way to enhance the energy efficiency and storage capabilities of a refrigerated transport container by: blending a first air stream withdrawn from an internal environment within the refrigerated shipping container with a second air stream obtained from an ambient environment external to the shipping container to form a blended air stream, passing this blended air stream to a separation membrane having greater relative selectivity for CO 2  and O 2  than N 2  to provide an N 2 -rich gas stream, and then returning N 2  rich gas stream to the internal environment. 
     Respiring produce produces CO 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. 
     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  106 A and  106 B 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. 
     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. 
     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 typically 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. 
     The gas exchange process generally results in cool CO 2 -rich, O 2 -lean air from within the reefer being exchanged for air at ambient temperature and composition. This is advantageous in that CO 2  is removed from the system. However, there are several issues associated with introducing fresh air into the system. Firstly, 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. Secondly, introducing fresh air replenishes the oxygen lost to respiration. Replenishing this lost oxygen maintains the rate of respiration and thus also contributes to the degradation of the respiring produce. 
     The inventors have included a membrane separation system according to aspects of the present invention 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 drawing a blended air flow obtained from (i) the internal environment of the reefer (e.g. ostensibly a CO 2 -rich air stream), and (ii) ambient air from an external environment outside the reefer. This blended air flow is subjected to a membrane separation process where a portion of the CO 2  and O 2  is removed to provide an N 2 -rich retentate stream that is fed 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 2  and O 2  that passes across the membrane from the retentate side of the membrane  208  to the permeate side of the membrane is 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. 
     The membrane system  200  can be operated to reduce or completely offset the volume of gas that would otherwise need to be introduced through the vent in response to the removal of CO 2  and O 2  by the membrane separation system  200 . 
     The cooled CO 2 -rich air within the reefer is blended with ambient air and cycled through the membrane system (such as at a pre-set rate determined based on a characteristic of the respiring produce) to remove a portion of the CO 2  and O 2  from the blended air. Sufficient ambient air can be added such that the portion of CO 2  and O 2  removed from the cooled CO 2 -rich air is effectively offset by additional N 2  that is introduced with the blended air. In this way, the N 2  rich stream is fed back into the reefer at the same flow rate as the cooled CO 2 -rich air stream that is withdrawn from the reefer. The result is that the internal environment within the reefer has a N 2  vol % that is greater than ambient air (e.g. greater than 78 vol % on a dry air basis) and an O 2  vol % that is lower than ambient air (e.g. lower than 21 vol % on a dry air basis). The reduced O 2  content means that respiration is inhibited (which lowers the rate of CO 2  production), and thus lowers the electrical load required by the membrane separation and refrigeration systems. 
     Notwithstanding the above, the amount of ambient air blended with the CO 2 -rich air to form the blended air stream can be controlled such that: (i) the flow rate of the N 2 -rich air returned to the reefer is lower than the flow rate of the cooled CO 2 -rich air withdrawn from the reefer, in which case some external air will leak into the reefer such as through a vent—this operating strategy may be useful if an increase in the O 2  vol % is required; (ii) the flow rate of the N 2 -rich air returned to the reefer is the same as the flow rate of the cooled CO 2 -rich air withdrawn from the reefer, in which case effectively no air exchange will occur between the internal environment of the reefer and the ambient air outside the reefer—this is likely to be the most energy efficient manner of operating the reefer; or (iii) the flow rate of the N 2  rich air returned to the reefer is higher than the flow rate of the cooled CO 2 -rich air withdrawn from the reefer, in which case internal air may leak out of the reefer such as through a vent—this will prevent outside air from leaking into the reefer. 
     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  301 , a lumen inlet  303  for receiving a blended gas formed from a first gas taken from the internal environment of a shipping container via inlet  304  and a second gas drawn from an ambient environment external to the shipping container via inlet  305 , a T-connector  306  for merging the first and second air flows obtained from inlets  304  and  305 , and a lumen outlet  307  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 . 
     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. 
     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 oxygen from the internal environment of the reefer. This allows the use of no vent, a closed vent, or a smaller vent opening which reduces the leakage of fresh air into the reefer from the external environment, and thus minimises the loss of cool air and the introduction of heat energy from ambient fresh air. The reduced oxygen content within the reefer also inhibits respiration which may additionally reduce the energy required to operate the reefer. 
     In operation, lumen pump  308  draws (i) a first air stream (e.g. cooled CO 2 -rich, O 2 -lean gas) from the internal environment of a reefer, and (ii) a second air stream from ambient air from outside the reefer. The first air stream and second air stream are blended to form a blended stream. The first air stream and second air stream may be blended in a number of different ways.  FIG. 3  illustrates the use of a T-connector  306 , however the skilled person will appreciate that other pipe connections may be used, such as a Y connector upstream of the lumen pump  308 . Alternatively, other means known to those skilled in the art for merging or blending gas streams may be used. In such cases, the respective feedlines to inlets  304 ,  305  may include valves or other methods of flow restriction controllable or settable to provide the desired ratio of the first air stream to the second air stream. In some embodiments, feedlines to inlets  304  and/or  305  further comprise a restrictor (or resistor) to control the flow rate of the respective gas streams, such as restrictors  601  and  602  as shown in  FIG. 6 . The skilled addressee will appreciate that the first air stream may be blended with the second air stream in other ways and that such blending may occur downstream of lumen pump  308  but upstream of the hollow fibre membrane filtration unit  301  or at the point of entry to the hollow fibre membrane filtration unit  301 . 
     The lumen pump  308  pushes this blended gas stream, under positive pressure, through the hollow fibre membrane filtration unit  301  via lumen inlet  304 . Inside the membrane scrubbing unit  302 , the blended gas is forced through lumens of a hollow fibre membrane separation unit. The membrane lumens are formed from a membrane material having greater relative selectivity for CO 2  and O 2  than N 2 , which results in the selective transfer of CO 2  and O 2  across the lumen wall from a retentate side of the lumen to a permeate side of the lumen. This results in a N 2 -rich gas stream (typically lean in CO 2  and O 2 ) on the retentate side of the lumen. The N 2 -rich 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 gas through the hollow fibre membrane filtration unit  301 . 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 the blended gas through the hollow fibre membrane filtration unit  301  under negative pressure. 
     The sweep gas assembly provides a sweep gas (e.g. ambient air drawn from outside of the reefer) to the permeate side of the hollow fibre membrane filtration unit  301 . 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 hollow fibre membrane filtration unit  301  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 selectively remove CO 2  and O 2  that has filtered across the membrane lumens from the blended gas on the retentate side of the lumen resulting in a CO 2 - and O 2 -rich sweep gas. The CO 2 - and O 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. 
     It will be appreciated that a variety of different membranes may be used in the membrane gas scrubber. In some embodiments, the membrane comprises a material selected from one or more of the group consisting of polydimethylsiloxane (PDMS), cellulose acetate, polyethersulfone, poly(benzoxazole-co-imide), poly(phthalazinone ether sulfone ketone) (PPESK), a polyimide (eg matrimid, 6FDA-p-PDA, etc.), polyetheretherketone (PEEK) and polysolfone. 
     The thickness of the membrane layer will vary depending at least in part on the membrane material selected. In some embodiments, the minimum thickness of the membrane may be at least about 0.01 μm, 0.05 μm or about 0.1 μm. The maximum thickness of the membrane may be not more than about 70 μm, 50 μm or 35 μm. The membrane thickness may be from any of these minimum values to any of these maximum values, for example, from about 0.01 μm to about 70 μm or about 0.1 to about 35 μm. 
     The total surface area of the membrane will also vary depending on the material selected, its thickness and the rate of CO 2  removal required. In some embodiments, the minimum total surface area of the membrane may be at least about 0.01 m 2  or about 0.02 m 2 . The maximum total surface area of the membrane may be not more than about 100 m 2 , 50 m 2 , 20 m 2 , 15 m 2  or 11 m 2 . Membranes with larger total surface area may not be suitable due to space constraints imposed by the container. The total surface area of the membrane may be from any of these minimum values to any of these maximum values, for example, from about 0.01 m 2  to about 100 m 2  or about 0.02 m 2  to about 15 m 2 . 
     Membranes contemplated include an overall permeability for CO 2  of about 3000 Barrer. These membranes may have a thickness of about 35 μm to 45 μm. Preferred membranes have about 3100 Barrers of permeability for CO 2  and may be about 40 μm in thickness. 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 2 , at about between 4 and 5, and a CO 2 /N 2  selectivity of between about 10 and 11. Other membranes, including non-silicon membranes, may also be used. In other embodiments, the invention uses cellulose acetate as the membrane material, which has an overall permeability for CO 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. 
       FIG. 4  is a schematic providing a flow balance through a membrane filtration unit according to an embodiment of the invention, such as the membrane filtration unit  301  of  FIG. 3 . In  FIG. 3  Q represents the mass flow rate of various gas streams as follows: Qc is container feed flow, Qa is ambient air feed flow, Qf is total feed flow, Qt is module transflow, Qb is the air sweep inlet flow (which is zero when operating in vacuum mode), Qx is the air sweep exhaust flow, and Qe is the total effective container flow. The following equations hold: Qf=Qc+Qa, Qr=Qf−Qt, Qx=Qb+Qt, and Qe=Qc−Qr. 
     When Qa=0, the membrane transflow will result in a decrease of container pressure, leading to makeup air flowing into the container from the ambient atmosphere (e.g. through a vent). Increasing Qa (e.g. Qa&gt;0) to the module lumen flow (e.g. providing a blended flow) alters the overall effective flow from the container to ambient atmosphere, and thus reduces the makeup flow required through the vent to stabilize the container pressure. Also, as discussed previously, adding Qa means that Qr will be nitrogen enriched relative to ambient atmosphere due to the higher permeability of the membrane for CO 2  and O 2  relative to N 2 . 
       FIG. 5A  and  FIG. 5B  provide further illustrative examples of arrangements according to aspects and/or embodiments of the invention. 
       FIG. 5A  provides an example in which the exchanger operation results in an outflow of CO 2  enriched gas from the container Qx using a vacuum sweep (e.g. no sweep gas, Qs is set to zero). In this mode, the operation of the exchanger results in an outflow of CO 2  enriched gas (Qx). Ambient atmosphere is mixed with the CO 2  enriched gas from the container Qc upstream of the membrane exchanger to compensate for the membrane exchanger outflow. In this example the container pressure is balanced such that there is no differential pressure between the internal environment within the container and the ambient environment. In this way, all makeup air is introduced via Qa to the container. When operated in this way, once equilibrium has been reached, Pc is equal to Pa. 
       FIG. 5B  provides an example in which the exchanger operation results in an outflow of CO 2  enriched gas from the container Qx using a vacuum sweep (e.g. no sweep gas, Qs is set to zero). Operation of the membrane exchanger in this example provides an N 2 -rich flow to the container (Qe). This N 2 -rich flow results in the container pressure being greater than the ambient pressure, causing egress of CO 2 -rich air from the internal environment to the external environment via any container leaks that may exist (Ql). When operated in this way, once equilibrium has been reached, Qe equals the sum of Ql and Qc, and Pc is greater than than Pa. 
       FIG. 6  provides an example of a membrane system of the invention configured within a test container. The schematic shows pressures (denoted P) and flows (denoted Q) at various points before and after the membrane system when the system was in use. In  FIG. 6  Q represents the mass flow rate of various gas streams as follows: Qc is container feed flow, Qfi is the initial container feed flow, Qfx is the air flow for the N 2  enriched airstream, Qa is ambient air feed flow, Qmix is the feed flow of the combined airstream from Qa and Qc, Qs.x is the airflow for the CO 2  enriched airstream, Qx is the air sweep exhaust flow, and Qr is the outlet flow of from the membrane system. In  FIG. 6  P represents the pressure at various points of the system as follows: Pa is the pressure of the ambient air outside the container, Pc is the pressure of the air within the container, dPa is the pressure of the ambient air stream prior to entering the membrane system, dPc is the pressure of the container air stream prior to entering the membrane system, Pmix is the pressure of the combined air stream prior to entering the membrane system, Px is the pressure of the air sweep exhaust stream, and Pr is the pressure of the N 2  enriched air stream exiting the membrane system. This system can be operated in a similar manner to the operation of the system described in  FIG. 3 . Results from tests of this system are shown in  FIGS. 7-9 . These Figures show results from operation of the system in two modes, the first mode was operated for a first period (from about 200-1200 seconds) with only recirculated air from within the container (ie ‘standard operation’, wherein Qa was about zero) and the second mode was operated for a second period (from about 1400 seconds to the end of the experiment) where ambient airflow from outside the container is added (ie ‘hybrid operation, in a manner in accordance with the present invention, and similar to that described with reference to  FIG. 5B  above).  FIG. 7  shows the flow results,  FIG. 8  shows the pressure results and  FIG. 9  shows a comparison of measures of CO 2  removal from each of the two modes of operation over time by comparing the flow rate of the CO 2 -rich exhaust stream (the subscripts ‘s’ and ‘h’ used with reference to the plotted points referring to ‘standard operation’ and ‘hybrid operation’, respectively). These results indicate that operating the membrane system according to the invention provides a comparable CO 2  removal rate (within experimental error) and maintains desirable pressures within the container. 
     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. 
     EXAMPLES 
     The below examples report steady state modeling results with the container under different pressure conditions, and with the desired oxygen concentration within the container set to 5 vol %. 
     The model is designed to simulate the operation of a shipping container transporting respiring produce, and calculate the equilibrium conditions for gas concentrations and container pressure given user specified inputs. 
     The overall method is to perform a total mass flow balance of all component gasses of the system being simulated to within a user specified tolerance (typically less than 1 μL/min imbalance). 
     The inputs into the model include elements relating to the shipping container, produce, and the gas exchange equipment. While most of these inputs will be known the skilled addressee, e.g. temperatures, pressures, produce respiration rates, valve and flow settings etc., the following information is provided below regarding modeling the following forms of leakage: (i) hydrodynamic leakage, (ii) diffusion leakage, and (iii) pumped leakage. 
     (i) Hydrodynamic leakage is the leakage measured in a standard leak test for an unpowered container. The hydrodynamic leakage represents the sum of all leaks in all areas of the container under a specific static pressure elevation. In operation the container evaporator fans produce a region of low pressure (upstream of the fans) and high pressure (downstream of the fans), while the pressure in the container near the doors is considered to be unchanged. For this reason, the simulation considers the container as three zones, where the leakage is calculated by the pressure difference between each zone and the external ambient pressure. The division is achieved by a weighting assigned to each zone. 
     (ii) Diffusion leakage is the leakage due to gas partial pressure differences. This is assumed to operate over the entire surface of the shipping container, and is determined by Fick&#39;s law. 
     (iii) Pumped leakage simulates the effect of a constant flow rate being pumped into or out of the shipping container. 
     Examples 1 to 3 report results with the refrigerated container running under negative pressure relative to the ambient environment with differential pressures of −70.5 Pa, −587 Pa, and −48.4 Pa corresponding to respiration rates of 6, 8, and 10 SLPM respectively. In each of these case there is an inflow to the container from the external environment through leakage, e.g. via the vents. 
     Examples 4 to 6 report results with the container running under positive pressure relative to the ambient environment with differential pressures of 26.5 Pa, 18.1 Pa, and 13.1 Pa corresponding to respiration rates of 6, 8, and 10 SLPM respectively. In each of these case there is an outflow from the container from the internal environment through leakage, e.g. via the vents. 
     Note: All flows reported in Examples 1 to 6 are in the sense of into the shipping container (negative flows are outflows). 
     Example 1 
       
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Modelling conditions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Af 
                 103446.362897 
                 [cm2] 
               
               
                   
                 ValveDC 
                 0.078038 
                 [fraction] 
               
               
                   
                 Pc 
                 101254.5 
                 [Pa] 
               
               
                   
                 dPc 
                 −70.5 
                 [Pa] 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: Af is the calculated area of the gas exchange module; ValveDC is the air exchange valve duty cycle at equilibrium; Pc is the pressure within the container at equilibrium, and dPc is the differential pressure between the container and atmosphere. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Steady state results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 O 2   
                 N 2   
                 CO 2   
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 fc: 
                 0.05 
                 0.88115 
                 0.06885 
                   
               
               
                   
                 Qr: 
                 −2 
                 0 
                 2 
                 0 
               
               
                   
                 Qv1: 
                 1.57302 
                 5.953392 
                 0 
                 7.526412 
               
               
                   
                 Qv2: 
                 −0.225962 
                 −3.982127 
                 −0.311149 
                 −4.519238 
               
               
                   
                 Qe: 
                 −0.540038 
                 −5.569547 
                 −1.597626 
                 −7.707211 
               
               
                   
                 Qll: 
                 0.768077 
                 2.906932 
                 0 
                 3.675009 
               
               
                   
                 Qlh: 
                 −0.066250 
                 −1.167516 
                 −0.091225 
                 −1.324991 
               
               
                   
                 Qlz: 
                 0.491154 
                 1.858865 
                 0 
                 2.350018 
               
               
                   
                 Qt: 
                 0.000001 
                 −0.000001 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: fc is the steady state gas composition within the container; Qr is the respiration flows; Qv1 and Qv2 are the flow rates through inlet and outlet valves on the vent; Qe is the gas exchanger scrubber flows; Qll, Qlh, and Qlz are estimates of low pressure leakage, high pressure leakage, and zero pressure leakage flows from zones within the refrigerated container; and Qt is the total flow. Flow values in SLPM. 
     Example 2 
       
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Modelling conditions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Af 
                 103446.362897 
                 [cm2] 
               
               
                   
                 ValveDC 
                 0.128744 
                 [fraction] 
               
               
                   
                 Pc 
                 101266.3 
                 [Pa] 
               
               
                   
                 dPc 
                 −58.7 
                 [Pa] 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: Af is the calculated area of the gas exchange module; ValveDC is the air exchange valve duty cycle at equilibrium; Pc is the pressure within the container at equilibrium, and dPc is the differential pressure between the container and atmosphere. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Steady state results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 O 2   
                 N 2   
                 CO 2   
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 fc: 
                 0.05 
                 0.873701 
                 0.076233 
                   
               
               
                   
                 Qr: 
                 −2.66667 
                 0 
                 2.666667 
                 0 
               
               
                   
                 Qv1: 
                 2.525018 
                 9.556409 
                 0 
                 12.08143 
               
               
                   
                 Qv2: 
                 −0.39939 
                 −6.97898 
                 −0.60947 
                 −7.98784 
               
               
                   
                 Qe: 
                 −0.51933 
                 −5.54961 
                 −1.94116 
                 −8.01011 
               
               
                   
                 Qll: 
                 0.727138 
                 2.751991 
                 0 
                 3.479129 
               
               
                   
                 Qlh: 
                 −0.07604 
                 −1.32879 
                 −0.11604 
                 −1.52087 
               
               
                   
                 Qlz: 
                 0.409276 
                 1.548982 
                 0 
                 1.958258 
               
               
                   
                 Qt: 
                 0.000001 
                 0 
                 −0.000001 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: fc is the steady state gas composition within the container; Qr is the respiration flows; Qv1 and Qv2 are the flow rates through inlet and outlet valves on the vent; Qe is the gas exchanger scrubber flows; Qll, Qlh, and Qlz are estimates of low pressure leakage, high pressure leakage, and zero pressure leakage flows from zones within the refrigerated container; and Qt is the total flow. Flow values in SLPM. 
     Example 3 
       
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Modelling conditions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Af 
                 103446.362897 
                 [cm2] 
               
               
                   
                 ValveDC 
                 0.192444 
                 [fraction] 
               
               
                   
                 Pc 
                 101276.6 
                 [Pa] 
               
               
                   
                 dPc 
                 −48.4 
                 [Pa] 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: Af is the calculated area of the gas exchange module; ValveDC is the air exchange valve duty cycle at equilibrium; Pc is the pressure within the container at equilibrium, and dPc is the differential pressure between the container and atmosphere. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Steady state results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 O 2   
                 N 2   
                 CO 2   
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 fc: 
                 0.05 
                 0.863392 
                 0.086608 
                   
               
               
                 Qr: 
                 −3.33333 
                 0 
                 3.333333 
                 0 
               
               
                 Qv1: 
                 3.679147 
                 13.92443 
                 0 
                 17.60357 
               
               
                 Qv2: 
                 −0.63009 
                 −10.8803 
                 −1.09142 
                 −12.6018 
               
               
                 Qe: 
                 −0.6588 
                 −5.47126 
                 −2.09519 
                 −8.22524 
               
               
                 Qll: 
                 0.690926 
                 2.614939 
                 0 
                 3.305865 
               
               
                 Qlh: 
                 −0.08471 
                 −1.4627 
                 −0.14673 
                 −1.69414 
               
               
                 Qlz: 
                 0.336852 
                 1.274879 
                 0 
                 1.61173 
               
               
                 Qt: 
                 −1E−06 
                 0.000001 
                 0 
               
               
                   
               
            
           
         
       
     
     Note: In the table above: fc is the steady state gas composition within the container; Qr is the respiration flows; Qv1 and Qv2 are the flow rates through inlet and outlet valves on the vent; Qe is the gas exchanger scrubber flows; Qll, Qlh, and Qlz are estimates of low pressure leakage, high pressure leakage, and zero pressure leakage flows from zones within the refrigerated container; and Qt is the total flow. Flow values in SLPM. 
     Example 4 
       
     
       
         
           
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 Modelling conditions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Af 
                 103446.362897 
                 [cm2] 
               
               
                   
                 ValveDC 
                 0.036992 
                 [.] 
               
               
                   
                 Pc 
                 101351.5 
                 [Pa] 
               
               
                   
                 dPc 
                 26.5 
                 [Pa] 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: Af is the calculated area of the gas exchange module; ValveDC is the air exchange valve duty cycle at equilibrium; Pc is the pressure within the container at equilibrium, and dPc is the differential pressure between the container and atmosphere. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Steady state results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 O 2   
                 N 2   
                 CO 2   
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 fc: 
                 0.05 
                 0.877812 
                 0.072188 
                   
               
               
                   
                 Qr: 
                 −2 
                 0 
                 2 
                 0 
               
               
                   
                 Qv1: 
                 0.558064 
                 2.112097 
                 0 
                 2.670161 
               
               
                   
                 Qv2: 
                 −0.1596 
                 −2.80193 
                 −0.23042 
                 −3.19195 
               
               
                   
                 Qe: 
                 1.362551 
                 2.418939 
                 −1.49349 
                 2.288005 
               
               
                   
                 Qll: 
                 0.430215 
                 1.628231 
                 0 
                 2.058446 
               
               
                   
                 Qlh: 
                 −0.14708 
                 −2.58213 
                 −0.21234 
                 −2.94155 
               
               
                   
                 Qlz: 
                 −0.04416 
                 −0.7752 
                 −0.06375 
                 −0.88311 
               
               
                   
                 Qt: 
                 −0.000001 
                 0 
                 0.000001 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: fc is the steady state gas composition within the container; Qr is the respiration flows; Qv1 and Qv2 are the flow rates through inlet and outlet valves on the vent; Qe is the gas exchanger scrubber flows; Qll, Qlh, and Qlz are estimates of low pressure leakage, high pressure leakage, and zero pressure leakage flows from zones within the refrigerated container; and Qt is the total flow. Flow values in SLPM. 
     Example 5 
       
     
       
         
           
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 Modelling conditions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Af 
                 103446.362897 
                 [cm2] 
               
               
                   
                 ValveDC 
                 0.087553 
                 [fraction] 
               
               
                   
                 Pc 
                 101343.1 
                 [Pa] 
               
               
                   
                 dPc 
                 18.1 
                 [Pa] 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: Af is the calculated area of the gas exchange module; ValveDC is the air exchange valve duty cycle at equilibrium; Pc is the pressure within the container at equilibrium, and dPc is the differential pressure between the container and atmosphere. 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Steady state results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 O 2   
                 N 2   
                 CO 2   
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 fc: 
                 0.05 
                 0.866307 
                 0.083693 
                   
               
               
                   
                 Qr: 
                 −2.666667 
                 0 
                 2.666667 
                 0 
               
               
                   
                 Qv1: 
                 1.364879 
                 5.165641 
                 0 
                 6.53052 
               
               
                   
                 Qv2: 
                 −0.368658 
                 −6.387419 
                 −0.61709 
                 −7.37316 
               
               
                   
                 Qe: 
                 1.381335 
                 2.433526 
                 −1.76454 
                 2.050319 
               
               
                   
                 Qll: 
                 0.459399 
                 1.738682 
                 0 
                 2.198081 
               
               
                   
                 Qlh: 
                 −0.140096 
                 −2.427321 
                 −0.2345 
                 −2.80192 
               
               
                   
                 Qlz: 
                 −0.030192 
                 −0.523109 
                 −0.05054 
                 −0.60384 
               
               
                   
                 Qt: 
                 0 
                 −0.000001 
                 0.000001 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: fc is the steady state gas composition within the container; Qr is the respiration flows; Qv1 and Qv2 are the flow rates through inlet and outlet valves on the vent; Qe is the gas exchanger scrubber flows; Qll, Qlh, and Qlz are estimates of low pressure leakage, high pressure leakage, and zero pressure leakage flows from zones within the refrigerated container; and Qt is the total flow. Flow values in SLPM. 
     Example 6 
       
     
       
         
           
               
             
               
                 TABLE 11 
               
               
                   
               
               
                 Modelling conditions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Af 
                 103446.362897 
                 [cm2] 
               
               
                   
                 ValveDC 
                 0.138788 
                 [fraction] 
               
               
                   
                 Pc 
                 101338.1 
                 [Pa] 
               
               
                   
                 dPc 
                 13.1 
                 [Pa] 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: Af is the calculated area of the gas exchange module; ValveDC is the air exchange valve duty cycle at equilibrium; Pc is the pressure within the container at equilibrium, and dPc is the differential pressure between the container and atmosphere. 
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 Steady state results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 O 2   
                 N 2   
                 CO 2   
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 fc: 
                 0.05 
                 0.859337 
                 0.090662 
                   
               
               
                   
                 Qr: 
                 −3.333333 
                 0 
                 3.333333 
                 0 
               
               
                   
                 Qv1: 
                 2.204618 
                 8.343792 
                 0 
                 10.54841 
               
               
                   
                 Qv2: 
                 −0.575548 
                 −9.891789 
                 −1.04361 
                 −11.5109 
               
               
                   
                 Qe: 
                 1.384939 
                 2.452461 
                 −2.00384 
                 1.833559 
               
               
                   
                 Qll: 
                 0.476989 
                 1.805255 
                 0 
                 2.282245 
               
               
                   
                 Qlh: 
                 −0.135888 
                 −2.335469 
                 −0.2464 
                 −2.71776 
               
               
                   
                 Qlz: 
                 −0.021776 
                 −0.374251 
                 −0.03948 
                 −0.43551 
               
               
                   
                 Qt: 
                 0.000001 
                   
                 −0.000001 
               
               
                   
                   
               
            
           
         
       
     
     Note: In the table above: fc is the steady state gas composition within the container; Qr is the respiration flows; Qv1 and Qv2 are the flow rates through inlet and outlet valves on the vent; Qe is the gas exchanger scrubber flows; Qll, Qlh, and Qlz are estimates of low pressure leakage, high pressure leakage, and zero pressure leakage flows from zones within the refrigerated container; and Qt is the total flow. Flow values in SLPM.