WO2006116809A1

WO2006116809A1 - An improved residual gas removal method

Info

Publication number: WO2006116809A1
Application number: PCT/AU2006/000570
Authority: WO
Inventors: Kenneth George Brash
Original assignee: AsiaWorld Shipping Services Pty Ltd
Current assignee: AsiaWorld Shipping Services Pty Ltd
Priority date: 2005-05-02
Filing date: 2006-05-02
Publication date: 2006-11-09
Anticipated expiration: 2007-11-02

Abstract

Residual gases can be extracted using a container internal pressure which is less than the ambient atmospheric pressure outside of the container (10), in order to draw residual gases out of the material goods. To achieve this, the operator can initiate a flow of residual gas out of the container (10) by actuating suction fan (20). The pressure in the container (10) can then fall by any fraction of atmospheric pressure until a pre-determined value is reached. After a period time in operation at the sub-atmospheric pressure, the suction fan (20) is switched off, and the pressure in the container (10) is maintained for a predetermined interval without any flow of flushing gas being passed through the container (10). This pre-determined interval is also known as a “balancing” or standing time. During the standing time, at least some desorption of adsorbed and absorbed residual gases occurs from the material goods. After the pre-determined interval is concluded, the flow of residual gas out of the container (10) is resumed. The pressure within the container (10) is not returned to atmospheric pressure levels, but is maintained at some sub-atmospheric level.

Description

An Improved Residual Gaa Removal Method

Field of the Invention

The present invention relates generally to a method for removing a residual gas from matter (eg. goods) placed in an enclosure. The invention will primarily be described with reference to its use in removing a residual gas from cargo such as timber dunnage, crates, pallets and other porous bulk materials, but the invention can have broader application to other bulk materials such as grain, foodstuffs, fruit and vegetables and the like.

Background to the Invention

Many types of goods and produce are .shipped all over the world in conventional shipping containers, or within a particular country by trains or trucks. Often the goods being transported are fumigated ^"prior to shipment, for example by passing a fumigant gas of some type into the shipping container, or by fumigating the goods to be carried under a tarpaulin or blanket prior to loading these goods into a truck. The fumigation is conducted with the aim of exterminating pests, parasites, insects or other vermin, for example borers, lice, ticks, fleas, fruit fly or termites, and also animal or plant pathogens. Containers or cargo shipments also often house timber dunnage, crates, pallets and other bulky items for packing around or supporting the goods being shipped.

Although the goods being transported can be Substantially vented after fumigation by known methods, there are often residual amounts of fumigant which are adsorbed onto, or absorbed into, the produce and the packing materials. These fumigants can slowly desorb during later storage or further shipment of the goods over the passage of time. Such deεorbed gases can pose an environmental exposure risk for persons who may come into contact with them, usually when access to the goods occurs prior to use. Current methods of ventilation of these goods are ineffective at fully eliminating trapped or desorbed gases, and are highly dangerous from an occupational health standpoint, since the gases used for effective fumigation are extremely toxic.

In other examples, recently painted, enamelled or laguered items such as furniture, vehicles or other articles can emit noxious smells or fumes over time. The gases or vapours generated can pose an environmental exposure risk and possibly overpower or poison persons who may access the goods after storage.

In some storage situations, certain agricultural goods (for example, rice, grain, seeds, corn cobs and stalks) can absorb furnigant when being subjected to fumigation to destroy pests such as parasites and insects etc. Subsequently, when stored in a warehouse, granary, silo or the like for a period of time, the fumigant can desαrb and become trapped in the interstices between the agricultural goods, posing an environmental and workplace hazard. Throughout the specification, when the term "residual gas" is used it refers to any measurable quantity of gases, fumes or vapours remaining, or generated, in an enclosed chamber, the chamber having been sealed for a period of time. When the term "conventional shipping container" is used it refers to the commonly used containers of varying lengths and heights (for example 40-foot length or 20-foot length containers) , well known in the cargo shipping and rail transportation industries. These containers normally are made of metal with corrugated side walls and an outwardly openable double door located at one end of the container for access thereinto.

Summary of the Invention In a first aspect the present invention provides a method for removing a residual gas that is associated with matter located in an enclosure, tϊie enclosure adapted for use with a gas extraction means, the method comprising the steps of.:

(a) extracting at least some of the residual gas present via the gas extraction means such that the gas pressure in the enclosure reaches a pressure that is less than atmospheric pressure outside the enclosure;

(b) maintaining the sub-atmospheric gas pressure for a pre-determined interval sufficient for at least some desorption from the matter of adsorbed and absorbed residual gases to occur,- and

(c) repeating the extraction step for a further predetermined interval at the or another sub-atmospheric gas pressure. Such a method allows a high degree of efficiency for the extraction of residual gases, which are initially absorbed into or adsorbed at the surface of matter that is placed inside the enclosure. As a result of repeating the step of extraction of residual gas whilst maintaining a sub-atmospheric gas pressure, the net tendency is for the residual gases to desorb with maximum yield and rate from small holes and/or pores in the matter. This means that the incidence of subsequent slow desorption of residual gases during transportation to, or storage in, other locations can be substantially reduced, thuB enhancing the personal safety of those who need to handle and/or use the matter.

In one embodiment, the method can further comprise repeating the abovementioned steps (b) and (c) in sequence, perhaps for a number of- times to maximise the yield of residual gas extracted.

In one embodiment, the method step of extracting at least some of the residual gas can involve selectively actuating the gas extraction means. In other embodiments, the gas extraction means can be operated continuously.

In one embodiment, the method can further comprise the steps of : providing a gas inlet means that is operatively coupled to the enclosure in use; and

- providing a flow of a flushing gas into the enclosure via the gas inlet means to flush at least some residual gas from the enclosure.

In one form of this, the flushing gas can be a gas which is recirculated from the gas extraction means and from which the residual gas has been wholly or partly captured and removed.

In this or another form, the flushing gas may be at least in part made up of atmospheric air.

At any stage when the pressure of residual gas in the container reaches a pre-determined value, or when a specific time has elapsed, the flow of flushing gas can be initiated and, in some embodiments, the gas pressure in the enclosure increased.

In one embodiment, the total pressure of gases within the enclosure can be monitored and controlled, for example using pressure gauges and manual valves, or an automated control system of some type.

In one embodiment, the concentration of residual gas in the enclosure can be monitored, for example by using a gas detection meter. Such a device allows an operator to know whether the concentration of residual gas is at safe levels so that the enclosure may be accessed by workers, or whether the gas extraction step has satisfactorily lowered the amount of residual gas present in the enclosure,

In one embodiment, the method further comprises the step of capturing and/or decomposing at least part of the residual gas extracted from the enclosure. In other embodiments, substantially all of the extracted residual gas ie captured and/or decomposed.

In one form, captured residual gas can be absorbed/adsorbed into/onto a capture means, for example a solid for absorption/adsorption of gases. The method can then further comprise the step of washing the capture means to remove captured residual gas. Alternatively, the method can comprise the step of decomposing the residual gas on the capture means by use of a chemical reagent. In another form, the residual gas can be reacted with a capture means to become decomposed, for example by a chemical reaction.

In yet another form, the residual gas can be captured in a solution. This method can then further comprise the step of treating the solution to decompose the captured residual gas, for example by a chemical reaction.

In one embodiment, a majority of the residual gas present in the enclosure can be extracted.

In a second aspect, the present invention provides a residual gas removal apparatus arranged to be operatively coupled to an enclosure for removing residual gas from inside the enclosure, the apparatus comprising: a gas extraction means for extracting gas from the enclosure; a gas inlet for introducing a flushing gaa into the enclosure and/or for returning extracted gas into the enclosure; and controlling means for controlling the flow of gases through at least one of the gas inlet and gas extraction means,- wherein the gas inlet and gas extraction means are arranged at or adjacent respective distal end regions of the enclosure to facilitate movement of gas from one end region to another in use. Such an apparatus permits a high degree of efficiency for the extraction of residual gases, which are initially associated with matter that is placed inside the enclosure. By positioning the gas inlet and gas outlet at spaced-apart locations, a cross-flow movement of gas within the enclosure can take place to avoid the incidence of free gases becoming trapped in pockets or small spaces between cargo articles or packing materials _r particularly where the enclosure has been very fully packed. By controlling the flow of gases through the enclosure, an operator can enable the gas pressure in the enclosure to reach a pressure that is less than atmospheric pressure outside the enclosure. This can result in deaorption of residual gases from the matter located in the enclosure.

When the term "end regions" is used it is envisaged that it can also refer to regions that are adjacent to opposing sides of a rounded, circular or spherical enclosure, and not just to regions near opposing walls of a square or rectangular enclosure.

In one embodiment, the gas extraction means and the gas inlet can be placed in fluid communication by a conduit that is locatable external of the enclosure in use. In one form, at least part of the conduit is arranged to be mounted to an exterior of the enclosure .

In one embodiment, the controlling means can comprise a baffle internally located in the conduit to slow the movement of gas therewithin. Other gas flow restriction devices are also possible, such as Venturis or orifice plates .

In a further embodiment, the controlling means can comprise a variable speed controller for the gas extraction means, for example where the gas extraction means is in the form of a variable speed fan. Such a controlling means can be additional to the use of a baffle or other flow restriction type controlling means located in the conduit.

In one embodiment, a gaa capture and/or decomposition means may be arranged in fluid communication with the gas extraction means for respective capture and/or decomposition of at least some residual gas extracted from the enclosure.

In one form of this, the gaa capture means comprises a solid for absorbing/adsorbing residual gas. For example, the solid can be present in an absorption/ adsorption bed to which at least part of the extracted residual gas attaches. One example solid is activated carbon. The captured gas can then be further treated to be decomposed, for example by a chemical reaction. In another embodiment, the gas decomposition means can comprise a solid for reacting with residual gas so that the gas is decomposed by a chemical reaction.

In another embodiment, the gas capture means can comprise a solution for capturing residual gas, for example an aqueous solution. The solution can then be further treated to decompose the captured residual gas, for_. example by a chemical reaction.

In one embodiment of this second aspect, the enclosure can be defined by a conventional shipping container. in a third aspect the present invention provides a method for removing a residual gas that is associated with matter located in an enclosure, the enclosure adapted for use with a gas heating means and a gas extraction means, the method comprising the steps of: (a) heating at least some of the residual gas present via the gas heating means such that the temperature in the enclosure reaches a temperature that is more than atmospheric temperature outside the enclosure;

(b) extracting at least some of the residual gas present via the gas extraction means;

(c) maintaining the or another above-atmospheric temperature for a pre-determined interval sufficient for at least some desorption from the matter of adsorbed and absorbed residual gases to occur; and (d) repeating the extraction step for a further predetermined interval ,

Such a method allows a high degree of efficiency for the extraction of residual gases, which are initially absorbed into or adsorbed at the surface of matter that is placed inside the enclosure. As a result of repeating the step of extraction of residual gas whilst maintaining a temperature in the enclosure that is higher than the local atmospheric temperature, the net tendency is for the residual gases to be warmed and to desorb with maximum yield and rate from small holes and/or pores in the matter. This means that the incidence of subsequent slow desorption of residual gases during transportation to, or storage in, other locations can be substantially reduced, thus enhancing the personal safety of those who need to handle and/or use the matter. In one embodiment, the method can further comprise repeating the abovementioned steps (c) and (d) in sequence, perhaps for a number of times to maximise the yield of residual gas extracted.

In one embodiment, the temperature of gases within the enclosure can be monitored and controlled.

In further embodiments, the steps of the method are otherwise as outlined for the first aspect.

In a fourth aspect the present invention provides a residual gas removal apparatus arranged to be operatively coupled to an enclosure for removing residual gas from inside the enclosure, the apparatus comprising; a gas extraction means for extracting gas from the enclosure; a gas heating means for heating gas extracted from or located within the enclosure;

- a gas inlet for introducing a flushing gas into the enclosure and/or for returning extracted gas into the enclosure; and a controlling means for controlling the gas heating means ; wherein the gas heating means is arranged at, adjacent to or within the enclosure to facilitate heating of gas in use.

Such an. apparatus permits a high degree of efficiency for the extraction of residual gases, which are initially associated with matter that is placed inside the enclosure. By controlling the temperature of gases within the enclosure to higher than the atmospheric temperature in the local area outside the enclosure, the desorption of residual gases from the matter located in the enclosure can be accelerated.

In one embodiment, the gas heating means can be placed in fluid communication with a conduit that is loeatable external of the enclosure in use. In other embodiments, the gas heating means can be located entirely within the enclosure .

In one form, the gas heating means may be an electric or a combustion heater.

At any stage when the temperature of residual gas in the container reaches a pre-determined value, or when a specific time has elapsed, the flow of flushing gas can be initiated and, in some embodiments, the gas temperature in the enclosure decreased.

In further embodiments, the apparatus is otherwise as outlined for the second aspect.

Brief Description of the Drawings

Notwithstanding any other forms which may fall within its scope, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure '1 shows- a right side view of a shipping container fitted with a residual gas removal apparatus in accordance with one embodiment of the invention; Figure IA shows an end view of the shipping container of Figure 1 when fitted with the residual gas removal apparatus ;

Figure 2 shows a schematic block flow diagram of one embodiment of a residual gas removal method when configured to remove residual gas and create a sub-atmospheric gas pressure, in accordance with one embodiment of the invention,-

Figure 3 shows the schematic block flow diagram of Figure 2 when configured to remove residual gas and to provide a return flushing gas flow into the enclosure; Figure 4 shows the schematic block flow diagram of ^' Figure 2 when not configured to remove residual gas from the enclosure, but only to vent the enclosure.

Detailed Description of Specific Embodiments Referring to the drawings, a residual gas removal apparatus is shown in Figures 1 and 1A which is arranged for removing residual gas from matter placed inside an enclosure. The enclosure is shown in the form of a conventional shipping container 10, although in other embodiments the enclosure can be in other forms, for example a silo, a shed, warehouse and rooms of any dimension capable of being depreasurised or heated as necessary. The enclosure can also be in the form of a purpose-built fumigation' chamber (for example made of concrete or bricks) or a converted cool room. Any structure that is made of appropriate materials that are capable of housing and withstanding depressurisation or heating (of the magnitude that will shortly be described) will be suitable . The container end wall -12 has a hole 14 which is fitted with protruding pipe 16 to which a gas extraction means is operatively coupled. The gas extraction means shown is in the form of a flexible hose pipe 18 which is in fluid communication with a suction fan 20, which is itself mounted onto a platform 22 which is located on surrounding ground 24. In other embodiments, a suction fan can be located on an adjacent service vehicle, for example. in the embodiment shown in Figures 1 and IA, the hole 14 and protruding pipe 16, via which gases are extracted from the container 10 in use, are located in the lower region of the end wall 12. This is typically where the heavier than air toxic gas molecules used in fumigation (for example methyl bromide) will naturally concentrate without recirculation.

The gas extraction means also includes a butterfly valve 26 fitted to the protruding pipe IS, the butterfly valve 26 being directly connected to the flexible hose pipe 18. The connection of such a valve at the outlet or another flow restriction device at a gas inlet to the container permits an operator to regulate the gag pressure in the interior- of the container 10, as will shortly be described.

In the embodiment, shown, the auction fan 20 is connected via a flexible pipe 28 to an absorption/adsorption means. in the . form of an absorption/adsorption bed 30. The bed 30 comprises a cylindrical housing 34 which is packed with activated carbon particles that are located in a cartridge. The extracted gases flow from the suction fan 20 via the flexible hose pipe 28 into a lower region of the housing 34, and exit via a neck 38 located at a hole 40 at the uppermost peaked cap 42 of the housing 34. In use, at least some of the extracted residual gas attaches at the surfaces of the activated carbon particles and in the pores of the carbon. Depending on the quantity of carbon and the rate of absorption/adsorption uptake, the toxic gas molecules found in the residual gaaes (for example methyl bromide) can be entirely stripped from the flow of gas that is extracted from the container 10 by the suction fan 20. In use, the remaining gases flow back to the container 10 via the hole 40, neck 38 and a flexible hose line 44 which is coupled by a hose coupling 46 to the neck 38.

One form of a gas inlet for returning extracted gas into the container 10 will now be described. The opposing end of the flexible hose line 44 is directly coupled to a T-piece conduit 47 by a connector 45. The remaining two arms of the T-piece are each fitted with a butterfly valve 48, 49. One such butterfly valve 48 is mounted to the end of a gas return line. The gas return line shown is in the form of a rigid gas return pipe 50 that is mounted on brackets 52 that are spaced along the exterior of both the end wall 12 and the roof 54 of the container 10. The other butterfly valve 49 is also connected to a further rigid pipe that is mounted to the end wall 12 of the container 10, and which extends vertically above the container 10 for use as a gas ventilation stack 56. Depending the selected position of the butterfly valves 48, 45, the apparatus in use can direct extracted gases that have passed through the absorption/adsorption bed 30, to flow to the gas return pipe 50 for return into the container 10 {if valve 49 is closed and valve 48 is open) , or to the gag ventilation stack 56 for expulsion to atmosphere (if valve 48 in closed and valve 49 is open) . If valves 48 and 49 are both open, then fresh air can enter into the container via the ventilation stack 56 and the gas return pipe 50 and be drawn into the container 10.

The rigid gas return pipe 50 is arranged to extend along the length of the container 10 and to terminate at a hole 58 which is located in the roof 54 of the container 10 to provide access to the container interior 59. The gas return pipe 50 is arranged with a curved bend section S2 which is coupled to the roof hole 58. As shown, the roof hole 58 and the end wall hole 14 are generally spaced apart at distal end regions of the container 10 in order to improve the cross-flow movement of gas within the container. This cross-flow avoids the incidence of freed residual gases not being extracted from the interior 59 of the container 10, where the freed residual gases are those gases that have been freed over time due to temperature or pressure variations within the container, and then become trapped in pockets or other small spaces between articles of cargo and packing materials . In the embodiment shown in Figures 1 and IA, a circulation fan housing 64 is positioned at the end wall 12 of the container 10 , and a cirpulation fan 66 is mounted therewithin. The housing 64 protrudes from the end wall 12, and the circulation fan 66 is arranged to face inwardly toward the interior 59 of the container 10. The circulation fan 66 can assist the general movement of gases within the container interior 59, and may reduce the incidence of freed residual gases being trapped in pockets within the cargo.

In further embodiments_/ the circulation fan 66 may not be required, as the positive flow of gas through the container 10 by recirculation out of the end wall hole 14 for return back into the container via the roof hole 58 may be sufficiently strong for sufficient movement of residual gas to occur. The gas return pipe 50 is arranged with an internally located baffle 70 which in use slows the movement of gas within the pipe 50. In other embodiments, other types of flow restriction can be used, such as a venturi, orifice plate etc. The location of such a flow restriction device permits regulation of the gas pressure in the interior of the container 10, and enables an operator to guide the gas pressure in the container 10 to a pressure that is less than the ambient atmospheric pressure outside of the container 10. This can be achieved by ensuring that the outlet butterfly valve 26 is fully open, but that the flow of gases being returned to the container 10 is restricted.

In other embodiments, the auction fan can be a variable speed fan and so can be regulated by an operator

(or an operating system) to similarly guide and control the gas pressure in the container 10 to a pre-determined level. Such an apparatus can be used in conjunction with the baffle 70 or venturi, orifice plate etc,

As will be explained in more detail in the forthcoming description and experimental results, reducing the gas pressure to below the ambient atmospheric pressure outside the enclosure can result in desorption of residual gages from the matter located in the enclosure.

In the embodiment shown, the butterfly valves 48 and 49 can both be arranged in the open (or partially open) position to deliver a flow of at least some atmospheric air as a flushing gas into the container 10, via the gas ventilation stack 56 and into the gas return pipe 50, in situations where the pressure of the container interior needs to be raised back up to atmospheric pressure, or to cool the gas temperature in the container by introducing atmospheric air.

In still other embodiments, there is no necessity for the atmospheric air inlet to be located at or in fluid communication with the gas return pipe 50, and the same pressure raising and temperature lowering effect can be achieved by the use of a gas inlet port and valve which is located directly on a side wall or an end wall of the container, for example.

The shipping container 10 can also be fitted with pressure monitoring means in the form of a pressure gauge for monitoring the total pressure of gaaeB within the container 10. Typically the gauge can be mounted to a further hole in a wall of the container 10, with a pressure sensor device connected thereto that is located within the container 10. The gauge can also be located in a pipe, for example the pipe 50.

The apparatus can also include a controlling means such as an electronic controlling system, for controlling the flow of gases through at least one of the gas inlet butterfly valve 48 and gas extraction butterfly valve 26 in response to the monitored pressure within the container. For example, the valves 26, 48 can be independently operated so that the monitored pressure in the container 10 can be allowed to fall below atmospheric pressure (such as by actuating the suction fan 20, whilst the gas inlet valve 48 remains partially open, or even closed) , or maintained at a pre-determined pressure value. Such pressure control can permit the pressure in the container to be reduced below ambient atmospheric pressure levels so that gases absorbed in goods or packing that are located in the enclosure can be forced out of the pores etc S of the material/ or from the interstices between the materials/ and efficiently extracted from the enclosure with minimal risk to persons who may need to access ' the container during later unpacking of its contents.

When residual gases are extracted from the 0 container, other types of gas capture and/or decomposition means may be arranged for capture and/or decomposition of at least some of the residual gas to prevent venting to air

(which may still be very hazardous to nearby workers, or even illegal, depending on the gaa) instead of the 5 absorption/adsorption bed 30 that has been previously described. The residual gas, which is extracted and absorbed/adsorbed or even decomposed, can also be of a different type than alkyl halides (such as methyl bromide) , for example, phosphine, sulfural fluoride or 0 carbon dioxide.

In one embodiment, a gas decomposition means can comprise a solid for reacting with residual gas so that the gas is decomposed by a chemical reaction. For example, in the case of .sulfural fluoride, passing the gas through an 5 absorption cartridge containing calcium carbonate causes the gas to be converted to form various sulfur salts which again can be safely disposed of,

In another embodiment, a gas capture means can comprise a solution for capturing residual gaa, for example 0 an aqueous solution. For example, in the case of phosphine, passing the gas through an absorption cartridge of wet carbon causes the gas to be converted to form phosphoric acid on the outside surface of the carbon.

This weak ^' acid can be subsequently washed away from the carbon.

If carbon dioxide is used as a fumigant to suffocate aerobic pests etc, this gas may simply be removed from the container by bubbling into a vessel or cartridge containing water to form carbonic acid, and subsequently discarded. In those cases where residual gases are passed over an absorption/adsorption means to physically collect them, depending on the nature of the residual gases, other absorbing or adsorbing materials may be equally suitable for this purpose (eg - zeolites, activated eartlh materials . etc) . In some embodiments the absorption/adsorption bed can be periodically washed to remove the absorbed/adsorbed gases and regenerated for reuse, and the residual gas decomposed by a chemical reaction.. In one form, an activated carbon bed with absorbed/adsorbed methyl bromide fumigant can be washed with a solution of sodium thiosulphate to chemically decompose the methyl bromide and to yield one or more benign salts, such as sodium bromide and sodium methylthiosulphate.

Any of the gas capture and/or decomposition means described can be located on a service vehicle which in use is located adjacent to the container 10 and. which can receive the flow of gas from the suction fan 20. Such an arrangement ensures that at ^' all times the extracted residual gases are quickly removed from the gas stream, which in turn ensures that a highly safe operating environment can be maintained with a low incidence of occupational health risk to operators.

The operation of the residual gas removal apparatus will now be described in detail. When a shipment of goods is received from another location, where the goods have previously been subjected to fumigation, it is often unknown whether the goods have been substantially aired or are even free of fumigant contamination prior to being shipped. Such residual cases may have partly desorbed from the goods during transportation and as evidence of this, it is not unusual to have a residual gas concentration of 20- 30 parts per million of methyl bromide in a sealed, shipping container, for example, although levels of 1000 parts per million are not unknown.

Xn the example of fresh produce or perishable goods, simply allowing these goods to be naturally ventilated over a number of days to allow the release of residual fuinigant is not an option, because the goods may become spoiled or oxidised. In the case of fine, granular materials such as ride or seeds, natural ventilation may not satisfactorily remove the residual fumigant at all . If residual gases are present, then a residual gaa removal procedure can be followed. The operator (dressed in safety ventilation, equipment) can load the contaminated material into the container 10 via the openable end entry doors 72. These doors 72 are then sealed to substantially prevent atmospheric leakage during the subsequent steps of extraction, desorption and flushing of residual gases. Once the end doors are closed, a conventional shipping container is also generally airtight and can provide a suitable enclosure for the present method.

In operation, it is desired to extract residual gases at a container internal pressure which is less than the ambient atmospheric pressure outside of the container, in order to draw residual gases out of the material goods and any . interstices between the goods. To achieve this, referring to Figure 2, the operator can keep the butterfly valve 48 closed initially and the butterfly valve 49 open whilst initiating a flow of residual gas out of the container 10 by actuating suction fan 20. The gas contents of the container 10 are extracted via hole 14, pipe lβ, butterfly valve 26 and flexible hose pipe IS to the fan 20, and eventually to the gas ventilation stack 56. The pressure in the container can then fall by any fraction of atmospheric pressure until a pre-determined value is reached (Pl) , which is measured by a pressure gauge and sensor, for example. As a ready visual aid for an unskilled worker using an enclosure such as a conventional shipping container, the pressure can be lowered until the container side walls start to internally buckle and implode to a minor extent. Creaking sounds can be heard when this occurs .

At a desired point in time, the butterfly valve 49 is closed and the butterfly valve 48 is opened, (or partly opened) so that extracted gases are recirσulated from the fan 20 via absorption/adsorption bed 30 and gas return pipe 50 back into the container 10. In this regard, reference should be made to Figure 3. The recirculated (or ^x flushing' ) gas has had all or at least part of the extracted residual gas removed therefrom during passage through the absorption/adsorption bed 30. Once the flushing gas flow is established, the pressure inside the container can be stabilised at a level (P2) that is still below ambient atmospheric pressure outside of the container 10. A flow of flushing gas, which enters the container 10 via the gas return pipe 50 and hole 58, flushes the freed residual gases into the suction fan 20 and the absorption/adsorption bed 30. In some examples P2 can be relatively higher than Pl or even the same as P1.

After a period of time in operation at the sub- atmospheric pressure, the suction fan 20 is switched off, and the low pressure (P2) in^' the container 10 is maintained for a predetermined interval without any flow of flushing gas being passed through the container. This predetermined interval is also known as a "balancing" or standing time: In some instances the container 10 can also be entirely isolated by closing butterfly valves 26 and 48, although this may not^' be necessary. During the standing time, at least some desorption of adsorbed and absorbed residual gases occurs from the material goods, when the net tendency is for the residual gases to desorb from small holes and/or pores in the goods (or from gaps or interstices between the goods) and be drawn into the low- pressure environment, of the interior 59 of the container 10.

After the pre-determined interval is concluded, the butterfly valve 26 is opened and the suction fan 20 ia then switched on, and the flow of residual gas out of the container 10 is resumed. The flow of flushing gas being passed back into the container can also be. resumed if the butterfly valve 48 is also re-opened (as shown in Figure 3) . This flow can carry out the residual gases that were desorbed during the balancing or standing time interval, and carry these gases for at least partial capture in the absorption/adsorption bed 30. In this extraction step, the pressure within the container 10 is not returned to atmospheric pressure levels, but is maintained at some sub- atmospheric level. If required, the pressure in the container can be maintained at one level (around P2) , or can even be further reduced by first closing the butterfly valve 48 and then opening the butterfly valve 49 during operation of the suction fan 20 (as shown in Figure 2) to reduce the pressure down to Pl, or even below.

Depending on the goods being treated, the above- mentioned stepwise or pulsing procedure of an extraction interval followed by a "balancing" interval can be repeated multiple times to achieve maximum desorption of residual gases (all intervals being conducted at various sub- atmospheric pressures) ♦ For example, the steps . of extraction and balancing could readily be performed around two to ten times. The selected length of the extraction and balancing time intervals used during each repetition can be varied, as can the pressures Pl and P2.

In some embodiments, in order to move from a configuration where a flow of flushing gaa enters the container 10 to flush the freed residual gases out, and the predetermined interval where no flow of flushing gas occurs, it is not necessary to actuate the suction fan 20 between on and off, but instead the suction, fan can. be operated continuously, and may instead be taken "off-line" by simply closing the butterfly valves 26 and 48 and opening another gae entry valve (or bypass) into the fan 20 as well as opening valve 49 so that air is sucked into the fan 20 and vented to atmosphere, without actually needing to turn, the fan off.

After a specific number, of repetitions of the aforementioned procedure, when a majority pf the residual gas in the chamber has been removed, the gas pressure in the enclosure can be increased back to atmospheric levels by opening up butterfly valve 49 and allowing air to be drawn into the interior 59 of the container 10 via ventilation stack 56 and gas return line 50 (as shown in Figure 4) . Having regard to the climatic conditions and the altitude at which the residual gas removal method is to be performed; as well as to the nature of the goods being treated and the type of residual gas to be removed, an, operator may also be able achieve an effective removal of residual fumigant by varying the temperature in the enclosure rather than the pressure. In some situations, depressurising a particular material may be preferable to heating it, particularly if the material is perishable. On the other hand, if a suitable ' enclosure is not available for depreesurisation, heating may be the preferred option. Also, if the residual gases to be removed are known to be particularly volatile, a more effective release from the goods being treated may be able to be achieved through heating. A combination of heating and depressurisation is also possible.

In one form the shipping container 10 already shown in the drawings can also be fitted with temperature monitoring means in the form of a thermocouple and temperature gauge for monitoring the total temperature of gases within the container 10. Typically the thermocouple and temperature gauge can be mounted to a further hole in a wall of the container 10, with the thermocouple located within the container 10. The apparatus can also include a controlling means auch as an electronic controlling system, for controlling the operation of the heater and/or the flow of gases through at least one of the gas inlet butterfly valve 48 and gas extraction butterfly valve 26 in response to the monitored temperature within the container.

The heater 74 which is fully located within the container 10 can first be actuated to produce heat energy so that the monitored temperature in the container 10 rises' above ambient atmospheric temperature. The heater 74 can be turned on and off by the operator or the temperature control system. Gases that are absorbed in goods or packing located in the enclosure will tend to gasify and be desorbed from, the pores and interstices etc of the those goods, allowing extraction of such gases from the enclosure with minimal risk to persons who may need to access the container during later unpacking of its contents.

The operation of the residual gas removal apparatus at elevated temperatures is similar to the operation already described when using elevated pressure conditions. Referring again to Figures 1 and 3, the operator actuates the heater 74 and opens the butterfly valve 48 whilst initiating a flow of residual gas out of the container 10 by actuating suction fan 20. The gas contents of the container 10 are extracted via hole 14, pipe IS, butterfly valve 26 and flexible hose pipe 18. to the fan 20. Extracted gases are then recirculated from the fan 20 via absorption/adsorption bed 30 and gas return pipe 50 back into the container 10. Again, the recirculated (or ⁴ flushing') gas has had all or at least, part of the extracted residual gas removed therefrom during passage through the absorption/adsorption bed 30.

Once the flushing gas flow is established, the temperature inside the container can be stabilised at a level that is above the ambient atmospheric temperature outside of the container 10 by controlling the flow of recirculated gas and the operation of the heater 74, A flow of flushing gas, which enters the container 10 via the gas return pipe 50 and hole 58, flushes the freed residual gases into the suction fan 20 and the absorption/adsorption bed 30. The temperature in the container can be raised, to any practical level until a pre-determined value is reached, as measured by the temperature gauge and sensor.

After a period of time in operation at ^"the selected above-ambient atmospheric temperature, the suction fan 20 is switched off, and perhaps the heater is also switched off, or turned down. The temperature in the container 10 is then maintained for a predetermined interval without any flow of flushing gas being passed through the container. This pre-determined interval is also known as a "balancing" or standing time. During the standing time, at least some desorption of adsorbed and absorbed residual gases occurs from the material goods, when the net tendency is for the residual gases to volatilise and desorb from small holes and/or pores in the goods as a result of the higher- temperature environment of the interior 59 of the container 10, ^•

After the pre-determined interval is concluded, the suction fan 20 is then switched on, and the flow of residual gas out of the container 10 is resumed. The flow of flushing gas being passed back into the container is also resumed (as shown in, Figure 3) . This flow can carry out the residual gases that were- desorbed during the "balancing" or standing time interval, and carry these gases for at least partial capture in the absorption/adsorption bed 30. The temperature within the container 10 is not reduced to the outside ambient atmospheric temperature levels, but is maintained at some level above the local atmospheric temperature. if required, the temperature in the container can be maintained at this one level, or can even be further increased by actuating the heater 74 and also restricting the recirculation flow of gases during operation of the suction fan 20 by partially closing butterfly valve 48.

After a specific number of repetitions of this procedure, when a majority of the residual gas in the chamber has been removed, the gas temperature in the enclosure, can be lowered down again to ambient atmospheric levels by deactivating the heater 74 and opening up butterfly valve 49 and allowing air, to be drawn into the interior 59 of the container 10 via ventilation stack 56 and gas return line SO (as shown in Figure 4) .

At any stage of the operation of the apparatus, when the temperature of residual gas in the container 10 reaches a pre-determined value, or when a specific time has elapsed, a flow of flushing gas can be initiated via the ventilation stack 56, and the gas temperature in the enclosure decreased.

As a typical example, the temperature in a shipping container 10 on a cool day may be around 20^°C. An increase in the internal temperature in the container to around 30- 40^°C has been shown to result in a significant increase in the removal of residual gases when using three repeated stepwise or pulsing procedures each comprising a 10 minute extraction interval followed by a 5 minute "balancing" or standing time interval.

In further embodiments, the gas heating means can be present in other forms, and in other locations rather than inside the container 10. For example, electric, radiative or conductive heaters can be placed in fluid communication with the gas return pipe 50 that is external of the container 10. Waste heat in the form of piped hot water, oil or steam can be arranged in proximity to the container or to the gas return pipe 50, for example, to convey heat energy into the residual gas removal apparatus. Now that preferred embodiments of the present invention have been described in some detail it would be apparent to those skilled in the art that the residual gas extraction apparatus has at least the following advantages over the admitted prior art:

1. The apparatus .and method ia adapted for use with a conventional shipping container or any type of enclosed space, and is relatively convenient and uncomplicated to operate;

2. The apparatus and method is relatively effective in removing residual gases from goods, and can be configured to operate at a number of pressures and temperatures to maximise gas extraction; and

3. The apparatus and method is "environmentally friendly" in that any collected residual gases can be safely expelled or captured rather than be discharged into a workplace or populated environment consequently minimising the risk of gas exposure to persons who operate the apparatus, as well as to those persons who need to access the container itself during its later unpacking.

4. The apparatus and method reduces the incidence of subsequent slow desorption of residual gases during transportation to, or storage in, other locations, thus enhancing the personal safety of those who need to handle and/or use the goods later.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. For example, the gas return pipe does not need to be rigid, nor does it need to be mounted on top of the container but rather may be loosely placed along the side, or lower side wall/base edge, of the container. The gas capture apparatus does not need to be located on a platform adjacent to the shipping container, but can be located on a mobile cart or other vehicle. The invention need not be restricted to removal of residual fumigants such as methyl bromide, but can extend to the removal of any residual adsorbed gaseous substance which is undesirable or hazardous to human, or animal health. The invention need not be restricted to the specific constructional features described, and may for example use a different type or size of shipping container, or another type of enclosure. Other types of valves, fans etc are also within the scope of the invention.

All such variations and modifications are to be considered within the scope of the present invention the nature of which is to be determined from the foregoing description.

Experimental examples

Blocks of pinewood were cut and the timber samples ware conditioned for two weeks at 25±2°C and 60% relative humidity in a constant temperature room. The blocks were then placed into a fumigation chamber to achieve, a volumetric loading ratio of 20-0%v/v.

Methyl bromide was then injected into the chamber and its concentration monitored by means of gas chromatography. The initial dose was 48 g/m³ of methyl bromide, which is the current Australian Quarantine Inspection Service (AQIS) recommended standard dose for wood fumigation. The concentration of methyl bromide multiplied by the time of exposure of the timber exceeded 400 g h/m³ which AQIS indicates will give a 100% kill of most insect pests, at all development stages (see Table 1 for different pest types) .

After 48 hours of fumigation, the blocks were considered to be fully fumigated, and to have absorbed/adsorbed some quantity of fumigant. At this point, the top of chamber was opened and timber blocks were aired in a fumehood for desorption studies .

In the course of normal aeration of the timber blocks, it was found that 80%, 95%, 95% and 97% of the absorbed methylbromide was released from the timber within 4, 8, 24 and 48 hours respectively. It was rioted that desorption rate was a function of aeration time, however the procedure of natural methyl bromide desorption can take a long time (2 days to get only 97% release) .

Some desorption experiments were then conducted at a negative pressure, as will now be outlined. The blocks were placed in the same chamber, but this time no fumigant was injected. Gases were extracted from the chamber to a certain measured pressure level. Then the blocks were allowed to . stand for a period of time so that adsorbed/absorbed methyl bromide desorbed to some extent at that pressure level (a "balance" interval) . The experiment was then repeated a number of times. The pressure of the gases in the container was monitored.

Experiment 1

A suction was- placed on the container until the pressure in the container reached 550 mm Hg (normal atmospheric pressure 760τnm Hg) . This pressure was maintained by suction for 30 minutes and then the suction was turned off and the pressure in the container held at around that level for a 30 minute "balance interval" to permit desorption to occur- Suction was then introduced to maintain a sub-atmospheric pressure in the container of about 550 mm Hg and to withdraw some of the methyl bromide gas present therewithin. This suction was maintained for 30 minutes and then the suction was turned off and the pressure in the container held at around that level for a 30 minute "balance interval" to permit desorption to occur, and so on. The 1 hour process of a suction interval and a "balance interval" was repeated 4 times in all. It was found that 96.5% of the methyl bromide had been released after this 4 hour procedure (the equivalent result was only achieved after a 24 hour "normal" or natural aeration in a fume cupboard) .

Experiment 2

A suction was placed on the container until the pressure in the container reached 550 mm Hg (normal atmospheric pressure 760nm Hg) , This pressure was maintained by suction for 40 minutes and then the suction was turned off and the pressure in the container held at around that level for 20 minutes "balance interval" to permit desorption to occur. Suction was then introduced to maintain a sub-atmospheric pressure in the container of about 550 mm Hg and to withdraw some of the methyl bromide gas present therewithin. This suction was maintained for 40 minutes and then the suction was turned off and the pressure in the container held at around that level for a 20 minute "balance interval" to permit desorption to occur, and so on. The 1 hour prcϊcess of a suction interval and a "balance interval" was repeated 4 times in all. It was found that 98.5% of the methyl bromide had been released after this 4 hour procedure (the equivalent result was not achieved even after 2 days of "normal" or natural aeration in a fume cupboard) .

Experiment 3 The same experimental conditions as for Experiment 1 were conducted, except that the full 1 hour procedure was repeated 6 times. It was found that 99.5% of the methyl bromide had been released after this 6 hour procedure (the equivalent result was not achieved even after 2 days of "normal" or natural aeration in a fume cupboard) .

These experimental results indicated that desorption of methyl bromide from fumigated timber blocks into air was influenced by using negative gas pressures and the time of air exchanges. Total desorption of residual gas can be increased by using a greater total elapsed time of suction relative to the standing time (or "balance interval") for residual gas desorption (compare Experiments 1 and 2) , although both intervals appear to be required. Furthermore, the total desorption of residual gas can be increased by using both a longer total elapsed time of auction and a longer total elapsed standing time (compare Experiments 1 and 3) . Based on these results it was concluded that natural methyl bromide desorption times can be significantly reduced by using this technique compared with the usual 48 hour standing time. By extracting gasea from a chamber in which contaminated materials are placed, where the extraction is arranged to apply a negative suction pressure in the chamber followed by an interval of maintaining that pressure without suction, desorption times can be reduced by a factor of 5 or 10. Repetition of the inventive procedure was found to produce even better results.

Claims

The claims defining the invention ate as follows:

1. A method for removing a residual gas that is associated with matter located in an enclosure, the enclosure adapted for use with a gas extraction means, the method comprising the steps of:

(b) maintaining the sub-atmospheric gas pressure for a pre-determined interval sufficient for at least some desorption from the matter of adsorbed and absorbed residual gases to occur; and (c) repeating the extraction step for a further predetermined interval at the or another sub-atmospheric gas pressure.

2. A method as claimed in claim 1 further comprising repeating the steps (b) arid (c) in sequence.

3. A method- as claimed in claim 1 or claim 2 wherein the step of extracting at least some of the residual gas involves selectively actuating the gas extraction means.

4. A method as claimed in any one of the preceding claims further comprising the steps of:

- providing a gas inlet means that is operatively coupled to the enclosure in use_? and - providing a flow of a flushing gas into the enclosure via the gas inlet means to flush at least some residual gas from the enclosure.

5. A method as claimed in claim 4 wherein the flushing gas is a gas which is recirculated from the gas extraction means and from which the residual gas has been wholly or partly captured and removed.

6. A method as claimed in claim 4 or claim 5 wherein the flushing gas is at least in part made up of atmospheric air.

7. A method as claimed in any one of the preceding claims wherein the total pressure of gases within the enclosure is monitored and controlled.

8. A method as claimed in any one of the preceding claims further comprising the step of monitoring the concentration of residual gas in the enclosure.

9. A method as claimed in any one of the preceding claims further comprising the step of at least one of (i) capturing and (ii) decomposing at least part of the residual gas extracted from the enclosure.

10. A method as claimed in claim 9 wherein substantially all of the extracted residual gas is captured or decomposed.

11. A method as claimed in claim 9 or claim 10 wherein captured residual gas is absorbed/adsorbed into/onto a capture means.

12. A method as claimed in claim 11 wherein the capture means comprises a solid for absorption/adsorption of gases.

13. A method as claimed in claim 11 or claim 12 further comprising the step of one of (i) washing the capture means to remove captured residual gas and (ii) decomposing the residual gas on the capture means.

14. A method as claimed in claim 9 or claim 10 wherein the residual gas reacts with a capture means to become decomposed.

15. A method as claimed in claim 9 or claim 10 wherein the residual gas is captured in a solution.

16. A method as claimed in, claim 15 further comprising the step of treating the solution to decompose the captured residual gas.

17. A method as claimed in any one of the .preceding claims wherein a majority of the residual gas present in the enclosure is extracted.

IS . Residual gas removal apparatus arranged to be operatively coupled to an enclosure for removing residual gas from inside the enclosure, the apparatus comprising: a gas extraction means for extracting gas from the enclosure; - a gas inlet for introducing a flushing gas into the enclosure and/or for returning extracted gas into the enclosure; and a. controlling means for controlling the flow of gases through at least one of the gaa inlet and gas ^• extraction means; wherein the gas inlet and gas extraction means are arranged at or adjacent respective distal end regions of the enclosure to facilitate movement of gas from one end region to another in use-

19. Apparatus as claimed in claim IS wherein the gas extraction means and the gas inlet are placed in fluid communication by a conduit that is locatable external of the enclosure in use.

20. Apparatus as claimed in claim 19 wherein at least part of the conduit is arranged to be mounted to an exterior of the enclosure.

21. Apparatus as claimed in claim 19 or claim 20 wherein the controlling means comprises a baffle internally located in the conduit to slow the movement of gas therewithin.

22. Apparatus as claimed in any one of claims 18 to 21 wherein the or a further controlling means comprises a variable speed controller for the gas extraction means.

23. Apparatus as claimed in claim 22 wherein the gas extraction meana is a variable speed fan.

24. Apparatus as claimed in any one of claims 18 to 23 wherein at least one of (i) a gas capture means and (ii) a gas decomposition means is arranged in fluid communication with the -gas extraction means for respective capture or decomposition of at least some residual gas extracted from the enclosure.

25. Apparatus as claimed in claim 24 wherein the gas capture means comprises a solid for absorbing/adsorbing residual gas.

26. Apparatus as claimed in claim 25 wherein the solid is present in an absorption/adsorption bed to which at least part of the extracted residual gas attaches.

27. Apparatus as claimed in claim 25 or claim 26 wherein the solid includes activated carbon.

28. Apparatus as claimed in claim 24 wherein the gas decomposition means comprises a solid for reacting with residual gas.

29. Apparatus as claimed in claim 24 wherein the gas capture means -comprises a solution for capturing residual gaa .

30. Apparatus as claimed in claim 29 wherein the solution is an aqueous solution.

31. Apparatus as claimed in any one of claims 18 to 30 wherein the enclosure is defined by a conventional shipping container.

32. A method for removing a residual gas that is associated with matter located in an enclosure, the enclosure adapted for use with a gas heating means and a gas extraction means, the method comprising the steps of:

(a) heating at least some of the residual gas present via the gas heating means such that the temperature in the enclosure reaches a temperature that is more than atmospheric temperature outside the enclosure; £b) extracting at least some of the residual gas present via the gas extraction means;

(c) maintaining the or another above-atmospheric temperature for a pre-determined interval sufficient for at least some desorption from the matter of adsorbed and absorbed residual gases to occur; and

(d) repeating the extraction step for a further predetermined interval .

33. A method as claimed in claim 32 further comprising repeating the steps (c) and (d) in sequence.

34. A method as claimed in Claim 32 or Claim 33 wherein the temperature of gases within the enclosure is monitored and controlled.

35. A method as claimed in Claim 33 or Claim 34 wherein the steps of the method are as otherwise defined in any one of Claims 2 to 17.

36. Residual gas removal apparatus arranged to be operatively coupled to an enclosure for removing residual gas from inside the enclosure, the apparatus comprising: a gas extraction means for extracting gas from the enclosure; a gas heating means for heating gas extracted from or located within the enclosure;

- a gas inlet for introducing a flushing gas into the enclosure and/or for returning extracted gas into the enclosure; and a controlling means for controlling the gas heating means; wherein the gas heating means is arranged at, adjacent to or within the enclosure to facilitate heating of gas in use.

37. Apparatus as claimed in claim 36 wherein the gas heating means is placed in fluid communication with a conduit that is locatable external of the enclosure in use.

38. Apparatus as claimed in claim 36 or claim 37 wherein the gas heating means is an electric or a combustion heater.

39. Apparatus as claimed in any one of claims 36 to 38 wherein the apparatus is as otherwise defined in any one of claims 18 to 31.