A phosphine distribution system

Abstract

A phosphine distribution system (1) comprises a phosphine gas supply sub-system (10) comprising a phosphine source (12) and a phosphine supply line (16); an air supply sub-system (20) comprising an air intake (22), a blower (24) for drawing air into the system (1) through the air intake (22) and an air supply line (26); and a mixer (30) connected to each of the phosphine supply line (16) and air supply line (26). The mixer (30) is operable to form a phosphine / air mixture having a selected phosphine concentration; a distribution line (40) extending from the mixer (30) for delivering the phosphine / air mixture for fumigation dosing when a selectively openable valve (25) is open; a diluent gas supply sub-system (50); and a control system (100) for controlling fumigation dosing including controlling dosing of phosphine into the phosphine supply line (16) to the mixer (30). The control system (100) demands supply of diluent gas from the diluent gas supply sub-system (50) to a selected portion of the phosphine distribution system (1) when a predetermined event occurs.

Description

A PHOSPHINE DISTRIBUTION SYSTEMTECHNICAL FIELD

[0001] The present invention relates to a phosphine distribution system for fumigation of agricultural crops, such as cereals, when stored.BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] Phosphine (PH3) is a gas used, amongst other applications, for fumigation of crops, in particular cereal crops, to rid them of insect and other pests. Phosphine is circulated through a silo or other storage container containing the crop or other material for the purpose of fumigation.

[0004] Phosphine is highly toxic and pyrophoric (i.e. spontaneously ignitable) above a certain threshold concentration (about 1 .6-1 .8 vol%) in air and thus its concentration is typically maintained well below that threshold to prevent any risk of combustion and injury or loss to personnel, in particular, or the crop or crop storage itself.

[0005] Two problems can derive from this compromise. First, the lower concentration of phosphine may not exterminate an acceptable proportion of pests. Second, some pests can become more resistant to phosphine fumigation. That is, at lower phosphine concentrations, some pests survive and breed with offspring that are more resistant to phosphine. This makes phosphine fumigation less effective.

[0006] Further, phosphine flammability or ignitability in air creates an obvious safety issue that could lead to further regulation of phosphine fumigation or, possibly, its abolition. This would have a negative consequence in removing a generally acceptable fumigation technique from use and creating a difficulty for crop producers in finding an alternative fumigation technique. Current phosphine distribution systems remain subject to either higher flammability or ignitability risk than desirable or, alternatively, less than desirable output concentrations of phosphine for effective fumigation.

[0007] It is against the above background that the present invention has been developed.SUMMARY OF INVENTION

[0008] In one aspect, the present invention provides a phosphine distribution system comprising: a phosphine gas supply sub-system comprising a phosphine source and a phosphine supply line; an air supply sub-system comprising an air intake, a blower for drawing air into the system through the air intake and an air supply line; a mixer connected to each of the phosphine supply line and air supply line, the mixer being operable to form a mixture of phosphine and air having a selected phosphine concentration; a distribution line extending from the mixer for delivering said mixture of phosphine and air for fumigation dosing when a selectively openable valve is open; a diluent gas supply sub-system; and a control system for controlling fumigation dosing including controlling dosing of phosphine into the phosphine supply line to the mixer; wherein the control system demands supply of diluent gas from the diluent gas supply sub-system to a selected portion of the phosphine distribution system when a predetermined event occurs.

[0009] The predetermined event may conveniently be selected from the group consisting of termination of fumigation dosing, detection of an ignition event in the phosphine distribution system, in particular in the distribution line, and purging of the selected portion of the phosphine distribution system. Purging of selected lines within the phosphine distribution system may be conducted according to a time schedule, for example through pulsing for removal of residual phosphine on conclusion of fumigation dosing or pulsing for preventing polymeric buildup in these lines.

[0010] The diluent gas is conveniently nitrogen though other inert gases, for example carbon dioxide, could be used in addition or as an alternative.

[0011] Optionally, the diluent gas supply sub-system may be connected to the mixer allowing the diluent gas to be mixed with phosphine and air. Diluent gas may therefore be a component of a phosphine gas mixture used for fumigation dosing, allowing further control over ignition risk.

[0012] Diluent gas may be supplied to a line within the phosphine gas supply subsystem, such as the phosphine supply line; or the distribution line. However, diluent gas may be delivered to any location within the phosphine distribution system to remove phosphine prior to termination of fumigation dosing or to extinguish or inertise ignition of phosphine with air. Flow of diluent gas may also be directed to collect phosphine by suction, for example through venturi effect, where the phosphine gas supply sub-system is connected to the diluent gas supply sub-system. For example, the phosphine gas supply sub-system may be connected to a vacuum generator located in the diluent gas supply sub-system. A flow of diluent gas is directed through the vacuum generator to collect phosphine by suction for venting or collection.

[0013] Phosphine supply line pressure is regulated by a pressure regulation system. Conveniently, the phosphine gas supply sub-system includes a plurality of pressurised phosphine gas cylinders, preferably containing pure phosphine, with pressure being regulated down to a predetermined pressure for phosphine delivery through a phosphine supply manifold communicating with the phosphine supply line to the mixer.

[0014] The phosphine pressure regulation system conveniently comprises a plurality of pressure regulators or pressure regulation stages, which may include a manually adjustable pressure regulator as well as a pressure regulator automatically controlling the phosphine supply pressure to a configurable set point of the control system.

[0015] The phosphine supply line desirably includes a controller, forming part of the control system, for controlling a quantity of phosphine delivered to the mixer through the phosphine supply line. Conveniently, the controller is a mass flow controller which controls the ratio of phosphine flow: supply air flow rate. In this case, the mass flow controller set point is the desired delivery concentration of phosphine in the distribution line.

[0016] The selectively openable valve is preferably located in the phosphine supply line or in the distribution line though other locations are possible. The selectively openable valve is conveniently in an open state until a desired quantity, by mass or volume, of phosphine (in admixture with air) has been delivered through the phosphine supply line and distribution line or until a desired time for fumigation dosing has elapsed. The selectively openable valve may comprise a plurality of selectively openable valves, for example one selectively openable valve being located in the phosphine supply line and one selectively openable valve being located in the distribution line. Further phosphine flow control valves may be included if desired.

[0017] The selectively openable valve is desirably closed or maintained closed if diluent gas supply pressure falls below a first threshold pressure during fumigation dosing. The selectively openable valve is not opened at the beginning of fumigation dosing if the diluent gas supply is absent or the diluent gas supply pressure is below a second threshold pressure. The first and second threshold diluent gas supply pressure may be the same or different.

[0018] The selectively openable valve is also desirably closed or maintained closed if phosphine supply line pressure falls below a threshold pressure. Where this indicates a shortage of phosphine supply, a replacement phosphine supply may be deployed, for example by switching from a low pressure phosphine cylinder (i.e. with an insufficient level of phosphine) to a phosphine cylinder having pressure above the threshold pressure.

[0019] Conveniently, the diluent gas supply sub-system also includes a plurality of pressurised cylinders. While nitrogen is a preferred diluent gas, other non-toxic gases non-reactive with phosphine under phosphine distribution system operation conditions may be selected.

[0020] The phosphine supply line is desirably connected to the diluent gas supply line allowing purging of residual phosphine by the diluent gas. The resulting purge gas stream having a non-toxic concentration of phosphine may be vented to atmosphere. However, the purge gas stream could be captured or collected if required by regulation.

[0021] The diluent gas supply line conveniently includes a vacuum generator, for example a venturi, communicable with the phosphine supply line. By venturi effect caused by flow of higher pressure nitrogen through the vacuum generator, phosphine isdrawn into the higher pressure nitrogen flow allowing purging of phosphine from the phosphine gas supply sub-system. This has safety benefits when practised at the conclusion of fumigation dosing.

[0022] The distribution line may be connected to the diluent gas supply sub-system with the system desirably including a sensor for detecting ignition of a phosphine-air mixture within the distribution line. It will be appreciated that ignition sensors may be provided at other locations within the phosphine distribution system, in particular in locations where ignitable phosphine-air mixtures could form. In the event that ignition is detected by a suitable sensor (for example a UV sensor), the diluent gas supply delivers diluent gas to the distribution line to reduce phosphine concentration below the flammable range and inertise or extinguish the ignition.

[0023] The blower is operated during fumigation dosing from a first time prior to phosphine supply to the mixer to a second time following termination of fumigation dosing. On blower start up, the blower preferably ramps up to operating speed. On blower shutdown, the blower preferably ramps down to zero speed. To this end, the blower desirably includes a variable speed drive controlled by the control system.

[0024] The control system is conveniently communicable with a remote human machine interface allowing an operator to monitor and control fumigation dosing from a location spaced from the phosphine distribution system as described above. The spacing of operator from the phosphine distribution system is determined by safety issues relating to the toxicity and flammability of phosphine gas.

[0025] Conveniently, the phosphine distribution system is transportable allowing transport between desired locations to allow fumigation dosing at the desired locations. The phosphine distribution system components, in particular the phosphine gas supply sub-system, the air supply sub-system and the diluent gas supply sub-system may conveniently be integrated with a mobile skid allowing transport to required locations for fumigation dosing. The mobile skid may be mounted within a truck or other suitable vehicle. The phosphine distribution system could be integrated with a mobile trailer.

[0026] In another aspect, the present invention provides a phosphine fumigation system comprising: a container having a volume for holding material to be fumigated by dosing of phosphine in a gas mixture in a fumigation operation, said container beingcommunicable through an inlet connected to the distribution line of a mobile phosphine distribution system as described above, wherein phosphine concentration in the gas mixture - which includes at least phosphine and air and may include diluent gas - delivered through the inlet to the container by the mobile phosphine distribution system is controlled below a threshold level.

[0027] The threshold phosphine level in air should be controlled below the lower explosive limit which is 1.6-1.8% phosphine in air. However, a substantially lower phosphine level than this may provide acceptable fumigation performance and therefore set as the threshold level. For example, the phosphine level may be controlled within a selected range, preferably 6000-7000 ppm in air, though this may be varied as required to address issues such as pest resistance.

[0028] The fumigation dosing is preferably conducted to a schedule related to resistance of pests subject to the fumigation dosing.

[0029] Fumigation may be conducted by dead end injection (also known as single pass) or recirculation mode. In dead end injection, the container is pressurised with a gas mixture comprising phosphine gas and air until fumigation dosing is complete. The gas mixture may also include diluent gas.

[0030] Where recirculation is required, the container may be connected to the air supply line through a return air suction line collecting air exhausted from the container.

[0031] The container is conveniently a crop storage, for example a bulk silo for storage of grain. It will be understood that the container may hold any crop or material susceptible to phosphine fumigation. The container may be of any practical size, shape or volume known in the crop storage art.

[0032] The phosphine distribution system as described has a number of advantages through enabling safe and efficient operation. Safety is enhanced by ability to control a diluent gas supply to remove phosphine from the phosphine gas supply sub-system on termination of fumigation dosing. Safety is further enhanced by ability to extinguish ignition events caused by excessive phosphine concentration in air. Efficiency is enhanced by more precise control of phosphine fumigant dosing minimising wastage and addressing issues of pest resistance allowing improved stewardship over phosphine fumigation.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Further features of the phosphine distribution system of the present invention are more fully described in the following description of a non-limiting embodiment thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawing in which:

[0034] The Figure is a block diagram of a phosphine distribution system according to one embodiment of the present invention.DESCRIPTION OF PREFERRED EMBODIMENTS

[0035] Referring to Figure 1 , there is shown a block diagram of a phosphine distribution system 1 for use in fumigating a grain silo 80 with phosphine (PH3) for destruction of insect and other pests. In other embodiments, the phosphine distribution system 1 could be used for other fumigation applications.

[0036] Phosphine distribution system 1 comprises a phosphine gas supply sub-system 10 comprising a phosphine source 12 and a phosphine supply line 16, 19; and an air supply sub-system 20 comprising an air intake 22, a blower 24 for drawing air into the phosphine distribution system 1 through the air intake 22 and an air supply line 26.

[0037] The phosphine supply line comprises phosphine manifold 16 and phosphine delivery line 19 as well as selectively openable valve 25 which controls phosphine flow towards mixer 30 which is connected to both the phosphine supply line 16, 19 and air supply line 26. Selectively openable valve 25 is conveniently a flow control valve, conveniently solenoid operated.

[0038] The mixer or mixing element 30 is operable to form a mixture of phosphine and air having a selected phosphine concentration for fumigation dosing. A distribution line 40 extends from the mixer 30 for delivering a mixture of phosphine and air when required for fumigation dosing of the grain in silo 80.

[0039] Phosphine distribution system 1 further includes a diluent gas supply subsystem 50 communicable with at least the phosphine supply manifold 16 and the phosphine supply line 19; and a control system 100 for controlling fumigation dosingincluding controlling dosing of phosphine into the phosphine supply line 19 to the mixer 30 and operation of the selectively openable valve 25. The control system 100 demands supply of diluent gas to at least one of the phosphine supply line 16 and the distribution line 40 when a predetermined event occurs.

[0040] The phosphine source is a pressurised supply of phosphine 12 available, for example at 500 psig pressure at 20°C. Conveniently, the pressurised supply of phosphine comprises one to three cylinders of pure phosphine communicable with phosphine supply manifold 16. In other embodiments, phosphine may be supplied in combination with a diluent gas such as carbon dioxide or nitrogen. Phosphine pressure when supplied to phosphine manifold 16 is regulated by pressure regulation system 14, in this embodiment a two-stage regulator system as further described below. The phosphine cylinders are located on a load cell so that phosphine weight, as well as pressure, can be measured and used as a sensed input to the control system 100.

[0041] In preferred embodiments, the phosphine pressure regulation system 14 includes two stages of regulation for reducing the phosphine pressure from cylinder pressure (typically 500 psig) to operating pressure (15-30 psig). The first stage of regulation involves manual adjustment of pressure, for example to 150 psig, by an operator of the phosphine distribution system 1. The second stage of regulation involves a pneumatically controlled pressure regulator that automatically controls phosphine delivery pressure through a phosphine delivery valve (not shown) to a configurable set point in controller 100 as described below.

[0042] Phosphine supply sub-system 10 further includes a mass flow controller 18 forming part of control system 100 for controlling, by mass, a quantity of phosphine delivered to the mixer 30 through the phosphine supply line 19.

[0043] Mass flow controller 18 controls the ratio of phosphine flow: supply air flow rate delivered from the phosphine supply sub-system 10 and its phosphine manifold 16 to phosphine supply line 19 when selectively openable valve 25 is open. In this embodiment, the mass flow controller 18 set point is the desired delivery mass of phosphine to be delivered through the phosphine supply line 19 and distribution line 40. The desired delivery mass of phosphine for a fumigation operation is determined based on parameters such as target pest; and pest resistance as well as safety. Other factors may be taken into account as known in the phosphine fumigation art.

[0044] Whilst a mass flow controller 18 is convenient, other forms of controller that allow metered quantities of phosphine to be delivered to mixer 30 may be used in alternative embodiments.

[0045] The diluent gas supply sub-system 50 comprises a pressurised supply of diluent gas, in this embodiment nitrogen, in particular high purity nitrogen or pure nitrogen. It will be understood that diluent gases other than nitrogen could be used, for example carbon dioxide, as long as the selected diluent gas is not reactive with phosphine under the operating conditions. The pressurised supply of nitrogen is stored in a plurality of cylinders, desirably two cylinders. Nitrogen pressure is regulated by pressure regulation system 52 of conventional type. The nitrogen pressure within the diluent gas supply sub-system 50, can be measured and used as a sensed input to the control system 100.

[0046] The diluent gas supply sub-system 50 is connected through flow control valve 55 and first nitrogen line 52 to phosphine supply manifold 16 and through flow control valve 55 and second nitrogen line 54 to phosphine supply line 19.

[0047] The diluent gas supply sub-system 50 also includes a line 56 including a vacuum generator 60 which communicates with the phosphine manifold 16 through first nitrogen line 52. Vacuum generator 60 is conveniently a venturi.

[0048] The phosphine distribution system 1 cannot be operated to supply a phosphine- air mixture for fumigation of grain silo 80 unless pressure of nitrogen available from the diluent gas supply 50 is above a threshold pressure, for example 60 psig. The available nitrogen pressure is sensed and provided as an input to controller 100.

[0049] First nitrogen line 52 and second nitrogen line 54 each deliver nitrogen under predetermined conditions for dilution purging of phosphine as described below. Nitrogen flow through first and second nitrogen lines 52 and 54 is controlled by nitrogen flow control valve 55 which is conveniently solenoid operated and controlled by controller 100 as described below.

[0050] Blower 24 is connected to air intake line 22 connected to atmosphere and an air supply line 26 connected to mixer 30. Air intake line 22 may also communicate with a return air suction line 82 returning gas from grain silo 80 in a specific mode of operation as described below. Blower 24, driven by an electric motor with a variable speed drive, induces an air flow, by suction, at a controlled rate into the air intake line 22 and furtherinto air inlet line 26. The air intake line 22 desirably includes an air filter (not shown) to remove particulates prior to introduction of air to the blower 24. Blower 24 may be provided with a low flow set point below which phosphine cannot be delivered through selectively openable valve 25 to the phosphine supply line 19. Phosphine also cannot be delivered through selectively openable valve 25 unless the blower 24 is on and operating.

[0051] In the embodiment shown, the mixer 30, which may also be described as a mixing element, is a static in-line mixer. In other embodiments, the mixer 30 could be a venturi. Other forms of gas mixer could be employed in other embodiments. Both phosphine supply line 19 and air inlet line 26 are connected to mixer 30.

[0052] The phosphine distribution system 1 , as described above, is conveniently comprised within a mobile process skid which may be mounted within a truck for moving between fumigation locations. The skid is schematically shown in the Figure. The skid may include a phosphine sensor (PSS) for monitoring phosphine levels and allowing the controller 100 to trigger a warning if phosphine level is above a safe threshold level.

[0053] Phosphine-air mixture is delivered for fumigation of grain silo 80 through phosphine-air outlet line 70 when the phosphine distribution system 1 is operating. The phosphine concentration of the air-phosphine mixture may, for example, be 6000- 7000ppm (e.g. 6400ppm), well below the lower explosive limit (LEL). Outlet line 70 is conveniently connected to the base of grain silo 80 in a manner known in the fumigation art. For example, the outlet line 70 may be a flexible hose connected to a SIROFLO or like distribution system.

[0054] Operation of phosphine distribution system 1 is controlled by a control system 100 comprising a electronic control unit being a programmable logic controller (PLC, more correctly a safety PLC because safe operation of phosphine distribution system 1 requires a number of safety instrumented functions) which can be included within a cabinet. More specifically, control system 100 controls operation of the flow control valves 25 and 55 as well as phosphine supply pressure through the automatic regulator described and operation of the mass flow controller 18.

[0055] The electronic control unit of control system 100 is, in the embodiment shown, wirelessly communicable with a remote human machine interface (HMI) 110. This allows instruction of control system 100 by an operator of phosphine distribution system1 with the operator being at a determined safe spacing (e.g. around 20m) for systems involving phosphine fumigation. The HMI 110 may be included within a tablet, laptop, smart phone or other electronic communications device and could be a touchscreen. HMI 110 may include an emergency stop for phosphine distribution system 1 .

[0056] Control system 100 is desirably configured with password-controlled access levels. In one embodiment, the access levels may include, in order of seniority, “Operator”, “Engineering” and “Administration” access levels in one embodiment. ““Operator” level allows operation and monitoring of the system along with viewing, acknowledging and clearing alarms. “Engineering” access level also allows optimisation of system settings. The “Administration” access level allows system changes such as control software changes. Phosphine distribution system 1 may be provided with mechanically activated emergency stops and the control system 100 configured to initiate ‘abort’ sequences to shut the system down under hazardous conditions. These are not password protected to ensure that operators can quickly shut phosphine distribution system 1 down in case of an emergency situation.

[0057] Control system 100 also logs performance of the phosphine distribution system 1 recording parameters such as phosphine dosed through mass flow controller 18, fumigation time (with selectively openable valve 25 open), mode of blower 24 operation (single pass or recirculation), start time, termination time and so on.

[0058] Phosphine distribution system 1 may be used for dead end (or single pass) fumigation or recirculation. In the dead-end injection mode, fresh air from air intake line 32 is mixed with phosphine from phosphine line 20 in mixing element 60. In recirculation mode, air from grain silo 80 is returned to the air intake line 22 through return air suction line 182 without intake of air from atmosphere (ATM).

[0059] A general description of how the phosphine distribution system 1 is operated follows.

[0060] The cylinders of the phosphine gas supply sub-system 10 and diluent gas supply sub-system 50 are connected. Blower 24 is turned on by controller 100 and ramps up, under control of its variable speed drive, to operating speed and the required air flow for fumigation of silo 80. Nitrogen pressure from diluent gas supply system 50 is sensed and provided above threshold pressure, selectively openable valve 25 is opened and phosphine is dosed through phosphine manifold 16 and mass flow controller 18 to thephosphine supply line and mixer 30 where it is mixed with air delivered by blower 24. The operator sets the mass flow controller 18 set point so that the required concentration of phosphine is delivered in admixture with air to distribution line 40, line 70 and silo 80.

[0061] Fumigation is continued for a selected time or a selected volume of phosphine as controlled by controller 100.

[0062] On termination of fumigation (a first predetermined event), the selectively openable valve 25 is closed. Blower 24 speed is ramped down, under control of its variable speed drive, to zero. Flow control valve 55 is opened and nitrogen is driven - as motive fluid - through line 56 and vacuum generator 60 while communicated - through first nitrogen line 52 - with phosphine manifold 16. As phosphine manifold 16 pressure is lower than nitrogen pressure in lines 52 and 56, phosphine is drawn into vacuum generator 60 at a low concentration allowing safe venting to atmosphere ATM. Such operation of the selectively openable valve 25 and the diluent gas supply subsystem 50 may also be conducted on start-up or if there is a requirement for changeover of the phosphine cylinders.

[0063] During operation of phosphine distribution system 1 for recirculation fumigation, phosphine concentration (PCR) in line 182 is monitored and the mass flow controller 18 operation may be corrected accordingly. Operation of the selectively openable valve 25 may also be controlled in line with monitored PCR so that recirculated phosphine does not interfere with precise dosing of phosphine in admixture with air to lines 40, 70 and silo 80.

[0064] An ignition sensor (not shown) is located in distribution line 70. A suitable ignition sensor, for example a UV sensor, may detect an ignition event (second ignition event). If an ignition event is detected, control system 100 opens nitrogen flow control valve 55 and causes a nitrogen flow through lines 52 and 54 reducing phosphine concentration below the ignition limit, that is the LEL limit. Such an event would likely indicate a malfunction of the phosphine gas supply sub-system 10 and / or mass flow controller 18. Detection of ignition would also trigger a shutdown of the phosphine distribution system 1 .

[0065] As described above, the phosphine sensor (PSS) mounted on the skid monitors phosphine concentration. If this exceeds a threshold level, the control system 100 willcommand shutdown of phosphine distribution system 1. This would also be a predetermined event causing a nitrogen purge of phosphine through vacuum generator 60 when the phosphine delivery valve is closed.

[0066] Those skilled in the phosphine fumigation art will appreciate that the phosphine distribution system of the invention described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, formulations and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[0067] Any manufacturer’s instructions, descriptions, product specifications, and product sheets for any products mentioned herein or in any document incorporated by reference herein, are hereby incorporated herein by reference, and may be employed in the practice of the invention.

[0068] The invention described herein may include one or more range of values (e.g. pressure, concentration and temperature etc). A range of values will be understood to include all values within the range, including the values defining the range, and values adjacent to the range which lead to the same or substantially the same outcome as the values immediately adjacent to that value which defines the boundary to the range.

[0069] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

The claims defining the invention are as follows:1 . A phosphine distribution system comprising: a phosphine gas supply sub-system comprising a phosphine source and a phosphine supply line; an air supply sub-system comprising an air intake, a blower for drawing air into the system through the air intake and an air supply line; a mixer connected to each of the phosphine supply line and air supply line, the mixer being operable to form a mixture of phosphine and air having a selected phosphine concentration; a distribution line extending from the mixer for delivering said mixture of phosphine and air for fumigation dosing when a selectively openable valve is open; a diluent gas supply sub-system; and a control system for controlling fumigation dosing including controlling dosing of phosphine into the phosphine supply line to the mixer; wherein the control system demands supply of diluent gas from the diluent gas supply sub-system to a selected portion of the phosphine distribution system when a predetermined event occurs.

2. The phosphine distribution system of claim 1 , wherein said predetermined event is selected from the group consisting of termination of fumigation dosing, detection of an ignition event in the phosphine distribution system and purging of the selected portion of the phosphine distribution system.

3. The phosphine distribution system of claim 1 or 2, wherein said predetermined event is an ignition event in the distribution line.

4. The phosphine distribution system of claim 2, wherein said selected portion of the phosphine distribution system is included in the phosphine gas supply subsystem.

5. The phosphine distribution system of any one of the preceding claims, wherein the diluent gas is selected from the group consisting of nitrogen and carbon dioxide.

6. The phosphine distribution system of any one of the preceding claims, wherein said diluent gas supply system is connected to the mixer allowing the diluent gas to be mixed with phosphine and air.

7. The phosphine distribution system of any one of the preceding claims, wherein diluent gas is supplied to the phosphine supply line or the distribution line of the gas supply sub-system.

8. The phosphine distribution system of any one of the preceding claims, wherein the phosphine gas supply sub-system is connected to a vacuum generator located in the diluent gas supply sub-system, a flow of diluent gas being directed through the vacuum generator to collect phosphine by suction.

9. The phosphine distribution system of any one of the preceding claims, wherein phosphine supply line pressure is regulated by a pressure regulation system.

10. The phosphine distribution system of claim 9, wherein said pressure regulation system comprises a plurality of pressure regulators.

11. The phosphine distribution system of claim 10, wherein said plurality of pressure regulators comprises a manually adjustable pressure regulator and a pressure regulator automatically controlling the phosphine supply pressure to a configurable set point of the control system.

12. The phosphine distribution system of any one of the preceding claims, wherein said phosphine supply line includes a controller, forming part of the control system, for controlling a quantity of phosphine delivered to the mixer through the phosphine supply line.

13. The phosphine distribution system of claim 12, wherein said controller is a mass flow controller which controls the ratio of phosphine flow: supply air flow rate.

14. The phosphine distribution system of claim 13, wherein the mass flow controller set point is the desired delivery concentration of phosphine in the distribution line.

15. The phosphine distribution system of any one of the preceding claims, wherein said selectively openable valve is located in the phosphine supply line or in the distribution line.

16. The phosphine distribution system of any one of the preceding claims, wherein said selectively openable valve is in an open state until a desired quantity, by mass or volume, of phosphine has been delivered through the phosphine supply line and distribution line or until a desired time for fumigation dosing has elapsed.

17. The phosphine distribution system of any one of the preceding claims, wherein said selectively openable valve is closed or maintained closed if diluent gas supply pressure falls below a first threshold pressure during fumigation dosing.

18. The phosphine distribution system of any one of the preceding claims, wherein said selectively openable valve is not opened at the beginning of fumigation dosing if the diluent gas supply is absent or the diluent gas supply pressure is below a second threshold pressure.

19. The phosphine distribution system of any one of the preceding claims, wherein said selectively openable valve is closed or maintained closed if phosphine supply line pressure falls below a threshold pressure.

20. The phosphine distribution system of any one of the preceding claims, wherein the phosphine supply line is connected to the diluent gas supply line allowing purging of residual phosphine by the diluent gas.

21. The phosphine distribution system of claim 20, wherein the resulting purge gas stream having a non-toxic concentration of phosphine is vented to atmosphere.

22. The phosphine distribution system of claim 13 or 14, wherein said distribution line includes a sensor for detecting ignition of a phosphine / air mixture.

23. The phosphine distribution system of any one of the preceding claims, wherein the blower is operated during fumigation dosing from a first time prior to phosphine supply to the mixer to a second time following termination of fumigation dosing.

24. The phosphine distribution system of claim 23, wherein the blower ramps up to operating speed; and, on blower shutdown, the blower ramps down to zero speed25. The phosphine distribution system of any one of the preceding claims, being transportable between selected locations to allow fumigation dosing at said selected locations.

26. The phosphine distribution system of claim 25, wherein said phosphine gas supply system, said air supply sub-system and said diluent gas supply subsystem are integrated with a mobile skid.

27. A phosphine fumigation system comprising: a container having a volume for holding material to be fumigated by dosing of phosphine in a gas mixture in a fumigation operation, said container being communicable through an inlet connected to the distribution line of a phosphine distribution system as claimed in any one of the preceding claims, wherein phosphine concentration in the gas mixture including at least phosphine and air delivered through the inlet to the container by the phosphine distribution system is controlled below a threshold phosphine level during fumigation dosing.

28. The phosphine fumigation system of claim 27, wherein the threshold phosphine level in air is controlled substantially below 1.6-1.8% phosphine in air.

29. The phosphine fumigation system of claim 28, wherein phosphine concentration is controlled in the range 6000-7000 ppm phosphine in air.

30. The phosphine fumigation system of any one of claims 27 to 29, wherein fumigation dosing is conducted to a schedule related to resistance of pests subject to the fumigation dosing.

31. The phosphine fumigation system of any one of claims 27 to 30, wherein fumigation dosing is conducted by dead end injection in which the container is pressurised with a gas mixture comprising phosphine and air until fumigation dosing is complete.

32. The phosphine fumigation system of any one of claims 27 to 30, wherein fumigation dosing is conducted in a recirculation mode in which the container is connected to the air supply line through a return air suction line collecting air exhausted from the container.

33. The phosphine fumigation system of any one of claims 27 to 32, wherein the gas mixture also includes diluent gas.

34. The phosphine fumigation system of any one of claims 27 to 33, wherein the container is a crop storage, optionally a bulk silo for storage of grain.

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