WITH RECIRCULATING GASEOUS FUMIGANT"

Technical Field

This invention concerns the fumigation of stored particulate commodities. More particularly, it concerns a method and an arrangement whereby commodities stored in a number of independent silos (also called "bins") at a storage facility can be fumigated simultaneously, using recirculation of a gas containing a gaseous fumigant supplied from a single source of fumigant.

Background to the Invention

Fumigation of a particulate commodity (for example, grain), which has been stored in a silo, by recirculating a gaseous fumigant through the silo is a fumigation technique that has been practised for some time. One such fumigation technique is described in the specification of US patent No 4,200,657 (to J S Cook).

A feature of all current fumigation systems using recirculation of a gaseous fumigant is that only one silo or bin can be fumigated in this manner from a single source of fumigant gas. Thus, in a storage facility having more than one silo, either (a) a separate recirculation system, with its own source of fumigant gas, is required for each silo or bin in the facility, or (b) as shown in Figure 1 of the accompanying drawings, the gas input ports of the silos or bins are connected to a single gas inlet manifold, the gas outlet ports of each silo or bin are connected to a single gas outlet manifold, fumigant gas from a single

source, mixed with a carrier gas (usually air), is recirculated from the outlet manifold to the inlet manifold, but the inlet port of each bin is provided with an on/off valve, and only the commodity stored in one bin in the multiple-bin facility is fumigated at any time. The obvious disadvantage of the former of these fumigation arrangements is that multiple sources of fumigant gas are required, together with multiple or portable recirculation fans and (particularly if the bins of the storage facility are leaky) multiple fumigant gas concentration sensors. The disadvantages of the latter system include (i) the time delay in achieving fumigation of all bins or silos in the facility, which can be serious when a fumigant requiring longer exposure periods to be effective (one such fumigant is phosphine) is used, and (ii) the potential for re-infestation of fumigated bins before all the silos or bins are fumigated, especially if there is an infestation of the grain or other particulate commodity in more than one of the bins in the storage facility before the fumigation process is started.

It is not feasible, in the arrangement shown in Figure 1 of the accompanying drawings (which is described in detail later in this specification), to simply open more than one of the on/off valves of the inlet ports of the bins or silos. Using the Figure 1 arrangement, balanced gas flow through more than one of the bins is very difficult to achieve. Each bin of the storage facility may be identical in size and construction, but different bin-fill ratios with different grains (or different grades of grains containing variable amounts of frass) produce different

back pressures, and the back pressures affect the magnitude of the gas flow from the common inlet manifold through the "opened" bins of the storage facility. Furthermore, there is a basic requirement that each bin needs to be gas tight to prevent the ingress of air on the negative side of the fan, with consequent reduction of the concentration of fumigant in the recirculating gas.

Disclosure of the Present Invention

It is an object of the present invention to provide an arrangement whereby any number of the storage bins or silos of a multiple-bin storage facility may be fumigated simultaneously, from a single source of gaseous fumigant, using recirculation of a gas containing the gaseous fumigant, without the requirement that the bins are all gas tight.

This objective is achieved by a modified form of the multiple-bin fumigation arrangement and technique that is described in the specification of International patent application No PCT/AU90/00268, and in the specification of Australian patent application No 38649/93. That arrangement and technique requires a single source of gaseous fumigant to be connected to a gas supply duct or manifold that is connected to the inlet port of each silo or bin. Each inlet port of a silo or bin is provided with an orifice plate that is adjusted so that the gas flow into each bin is essentially independent of the flow of gas from the manifold into each other bin of the facility.

The modification of the present invention is two-fold. Firstly, the outlets of each of the bins (each bin having an orifice plate at its inlet port) are connected to a second duct (an outlet manifold) which is joined, via a gas connection which contains a recirculating fan (also called a "blower"), to the inlet manifold. Secondly, the recirculation circuit thus established is provided with a valve (which has been termed a system valve) which is adjusted whenever a bin is added to or removed from the recirculation circuit, to ensure that the static pressure in the gas inlet or supply duct is maintained at a value which provides the required flow of fumigating gas through each operational silo or bin in the storage facility. That is, the setting of this system valve is adjusted to compensate for the perturbation of the static pressure in the gas supply duct when there is a change in the number of bins being fumigated, which occurs (a) when a silo or bin is "closed" (for example, when a bin is about to be emptied), and (b) when a previously "closed" bin in the storage facility is "opened" to the fumigating gas.

Thus, according to the present invention, there is provided a method of simultaneously fumigating particulate commodities stored in a plurality of silos or bins of a storage facility using a single source of gaseous fumigant, each silo or bin having a gas inlet port and a gas outlet port, said method comprising:

(a) connecting the inlet port of each silo or bin to a single gas supply duct;

(b) connecting the outlet port of each silo or bin to a single gas outlet duct;

(c) forming said gas supply duct, said gas outlet duct, a gas movement means (for example, a blower or fan), and said bins into a gas recirculation circuit, said bins being included in parallel in said recirculation circuit;

(d) providing a system control valve in said recirculation circuit downstream of said gas movement means, said system control valve being adapted to vary the static pressure within said gas supply duct to maintain said static pressure at a predetermined value, or within a predetermined range of values;

(e) providing each inlet port of each bin with an orifice plate having an adjustable aperture, and adjusting each adjustable aperture to control the flow of gas into the associated silo to maintain a substantially constant linear velocity of gas within the commodity and a substantially constant gas inlet pressure for the associated silo; and

( f) providing a source of gaseous fumigant operatively connected to said recirculation circuit and supplying gaseous fumigant thereto.

Also according to the present invention, there is provided a fumigation arrangement for the simultaneous fumigation of a plurality of silos or bins of a storage facility comprising a plurality of silos or bins, each silo or bin having a gas inlet port and a gas outlet port, said fumigation arrangement comprising:

(a) a single gas supply duct to which the gas inlet port of each silo or bin is connected;

(b) a single gas outlet duct to which the gas outlet port of each silo or bin is connected;

(c) a gas connection between said gas outlet duct and said gas supply duct, to form a gas recirculation circuit comprising said gas supply duct, said gas outlet duct, said gas connection and said silos or bins; said silos or bins being included in parallel in said recirculation circuit;

(d) gas movement means (for example, a blower or fan) included in said gas recirculation circuit;

(e) a system control valve in said recirculation circuit downstream of said gas movement means, said system control valve being adapted to vary the static pressure within said gas supply duct to maintain said static pressure at a predetermined value, or within a predetermined range of values;

(f) a respective orifice plate included in each of said gas inlet ports, each orifice plate having an adjustable aperture which is set to control the flow of gas into its associated silo to maintain a substantially constant linear velocity of gas within the commodity in, and a substantially constant gas inlet pressure for, the associated silo; and

(g) a single source of gaseous fumigant operatively connected to said gas recirculation circuit, and adapted to supply gaseous fumigant thereto.

Any suitable source of fumigant gas may be used with this method and arrangement (including a packaged source of phosphine of the type described in the specification of International patent application No PCT/AU93/00270) . If

the fumigant source is a cylinder of fumigant gas, the fumigation arrangement will preferably (unless a "one-shot" fumigation technique is contemplated) include fumigant gas injection apparatus which ensures that the concentration of fumigant in the recirculating gas is increased when it reaches or falls below a predetermined minimum value. The fumigant gas injection apparatus may comprise a fumigant gas sensor in the gas supply duct, the output signal of this sensor being monitored by a microprocessor which is programmed to cause additional fumigant gas to be supplied to the recirculation circuit whenever the concentration of fumigant gas falls below a pre-determined level. Alternatively, this apparatus may comprise a mechanism which provides a periodic injection into the recirculating gas of a quantity of fumigant, the frequency of this injection and the amount of fumigant added being based on the observed leakage rates of the bin.

These and other features of the present invention will be exemplified in the following description of an embodiment of the present invention. In the following description, which is provided by way of example only, reference will be made to the accompanying drawings.

Brief Description of the Accompanying Drawings

Figure 1 illustrates, schematically, a known fumigation arrangement whereby multiple bins of a storage facility may be fumigated using a single source of fumigant gas and recirculation of a mixture of the fumigant gas and a carrier gas.

Figure 2 illustrates, also schematically, a multi-bin storage facility in which simultaneous fumigation of each bin of the facility is effected in accordance with a preferred form of the present invention.

Detailed Description of Illustrated Embodiments

In Figures 1 and 2, the same reference numerals have been used to identify common components of the illustrated fumigation arrangements. In each illustrated arrangement, three silos or bins 10, each having an inlet port 11 and an outlet port 12, are included, in parallel, in a gas recirculation circuit which also includes a gas supply duct or manifold 13, a gas outlet duct or manifold 14, a gas connecting duct 15 and a blower or fan 16. A supply of a fumigant gas (which is shown in the drawings as a cylinder 17 containing a pressurised mixture of carbon dioxide and phosphine, but which in practice may be any suitable source of a gaseous fumigant) is connected to the duct 13, to inject fumigant gas, as required, through a nozzle 18.

The silos or bins 10 each contain a quantity of a particulate commodity. The present invention was developed primarily for the fumigation of wheat grains, but grains of any other particulate foodstuff (for example, rice or edible pulses), or any other particulate commodity which is stored in silos or bins, may be fumigated using the present invention. Each inlet port of the bins 10 is supplied with an on/off valve 19.

In the Figure 1 arrangement, to fumigate the contents of the bins 10, the valve 19 in the gas inlet to one of the

silos or bins is opened and the valves 19 in the gas inlets to the other bins are closed. Fumigation of the bin having its associated valve 19 open is then effected using the recirculating fumigant technique. When fumigation of the contents of that bin is completed, the opened valve 19 is closed and one of the other valves 19 is opened. Fumigation of the contents of the bin which then has its associated valve 19 open is then effected. This procedure is repeated until the stored commodity in each bin of the storage facility has been fumigated.

As noted above, the fumigation of a number of silos, using the arrangement illustrated in Figure 1, takes considerable time when phosphine is the fumigant, and there is the problem of potential re-infestation of the fumigated contents of a bin while the commodity stored in another bin is being fumigated.

In the arrangement shown in Figure 2, a system valve 25 is included in the gas recirculation circuit. The system valve 25 is used to maintain the static pressure in the duct 13 at a predetermined value or within a predetermined range of values.

Normally a manometer 24 will be used to monitor the static pressure in the gas supply duct 13. Control of the setting of the system valve 25 to maintain the static pressure in the duct 13 at a required pressure may be effected by an operator who observes the manometer ( i ) whenever the fumigation of a silo ceases and that silo is removed from the recirculation circuit, and ( ii ) whenever a new silo is

added to the recirculation circuit. Alternatively, control of the setting of the system valve 25 may be effected automatically, using an electrical signal generated by the manometer and input to a microprocessor 26 which is programmed to generate a control signal to drive a motor (preferably a stepping motor) 27 which mechanically varies the setting of the system valve 25. Such servo-systems for maintenance of a predetermined pressure in a chamber, duct or the like are known per se.

Each bin 10 is provided with an orifice plate 20 at its inlet port. The aperture of each orifice plate 20 is adjusted so that gas flows through its associated silo at a required rate when the gas pressure in the gas supply duct 13 is at its required predetermined value (or is in the predetermined range of values). The commodities stored in any number of bins in the storage facility may then be fumigated, simultaneously, by opening the respective valves 19 and adjusting the system valve 25 to establish the required static pressure in the duct or manifold 13.

The perturbation to the flow of fumigant-containing gas that is caused by opening a hitherto closed valve 19 to enable the fumigation of the commodity stored in another silo 10, or by closing one of the opened valves 19 to delete one of the bins 10 from the fumigation process ( for example, when that bin is to be emptied of its commodity), is readily corrected by adjusting the setting of the system valve 25 to return the pressure in the gas supply duct 13 to its required value. When such an adjustment of the valve 25 has been made, the fumigant-containing gas will

again be passed simultaneously through the "opened" bins or silos 10, in parallel, at the respective required rates.

The addition of fumigant to the recirculating gas, to compensate for losses due to leakage and sorption by a commodity in the bins being fumigated, may be effected manually. For manual control of the addition of fumigant, earlier monitoring of the operation of the storage system is required, to establish, empirically, for the conditions under which the storage bins are being used, (i) when fumigant has to be added to the recirculating gas, and ( ii ) how much fumigant needs to be added, to maintain at least the required minimum concentration of fumigant in the recirculating gas.

Such periodic addition of fumigant to the recirculating gas may be effected automatically, using a known form of metering device, which is connected to a cylinder that contains the gaseous fumigant under pressure and which is operated by a timer-controlled solenoid.

However, the preferred arrangement for the addition of fumigant to the recirculating gas, to compensate for leakage and sorption, is shown in Figure 2. The concentration of fumigant in the gas in the gas supply duct 13 is monitored by a fumigant gas sensor 21. The output signal of the sensor 21 is connected to a microprocessor 22, which may be integrated with the microprocessor 26. The output of the microprocessor 22 is adapted to control the supply of gaseous fumigant from the cylinder 17 to the gas supply duct 13. Whenever the output signal from the

fumigant sensor 21 indicates that the concentration of phosphine (or other gaseous fumigant) in the duct 13 has fallen below a predetermined value, the microprocessor 22 causes the release of further fumigant from the cylinder 17 until at least the predetermined value of the fumigant concentration in the recirculating gas has been restored. Using this arrangement, the concentration of fumigant in the recirculating gas is automatically increased to maintain a required minimum value and compensate for different rates of gas loss from the bins 10.

The cylinder 17 may be mounted on scales 23. The scales 23 are used to monitor the quantity of liquefied gas remaining in the cylinder 17. If desired, a known form of apparatus which generates an alarm signal if the liquefied gas in the cylinder 17 should reach a low level may be included in the fumigation arrangement.

An advantage of the fumigation arrangement illustrated in Figure 2 and described above is that, by providing the same linear velocity of gas through each silo in the storage facility, and by providing the same inlet pressure to each silo, leaks in the tops of the silos or, alternatively, leaks in the bottoms or bases of the bins, have minimal effect. However, if the bins 10 are all sealed to a gas-tightness standard and there is little difference in the very low leakage rates of, or the distribution of leaks between, the bins, there is no difficulty in achieving a satisfactory distribution of fumigant in each bin of the system. Under these conditions, there will be a very low decay rate of the fumigant concentration in the

recirculating gas. The only significant loss of fumigant will be by sorption by a stored commodity in the bins. Thus the "one shot" fumigation technique may be practised, provided the single injection of fumigant gas into the recirculation circuit is such that the concentration of fumigant in the recirculating gas at the end of a specified period of fumigation is not lower than the minimum required concentration. Alternatively, and preferably, a "slow release" source of fumigant gas (such as one of the packaged formulations described in the specification of International patent application No PCT/AU93/00270) may be included in the recirculation circuit, in the knowledge that the generation of fresh fumigant will compensate for the low leakage from the bins and the sorption of the fumigant by the stored commodity, and thus a low (but acceptable) level of fumigant will be available for the entirety of the fumigation process.

The control of the supply of fumigant using a microprocessor, as illustrated in Figure 2, is particularly useful when there are larger - and possibly different - leakages from the bins 10. If there are holes in the tops and bottoms of the bins, leakage of gas from the lower holes and ingress of outside air through the higher holes can occur, with a consequential rapid dilution of the concentration of the gaseous fumigant. In this situation, the sensor 21 and its associated microprocessor 22 will ensure that a predetermined concentration of fumigant gas is maintained in the gas supplied to the bins 10 at all times.

It will be appreciated that the pressure conditions within the recirculation circuit are such that if the bottoms of the bins 10 and the walls of the bins 10 are essentially sealed and free of leaks, diluent air will not be drawn into the recirculation circuit through holes in the tops of the bins, even with different sized holes in the various bins. Similarly, if the tops of the bins and the walls of the bins are essentially sealed, diluent air will not be drawn into the recirculation circuit through holes in the bottoms of the bins.

Although reference has been made, above, to the SIROFLO fumigation process, which is described in the specification of Australian patent No 640,699 (granted on the Australian patent application derived from International patent application No PCT/AU90/00268) , and to the phosphine sources described in the specification of International patent application No PCT/AU93/00270, it is not necessary that the fumigant gas is phosphine. Any suitable gaseous fumigant - including carbonyl sulphide and cyanogen - may be used in the present invention.

Trials of the present invention are in progress at the time of writing this specification. Those trials are being conducted at the Black Mountain site of the Commonwealth Scientific and Industrial Research Organisation, in the Australian Capital Territory, Australia, where a storage facility comprising three silos, each of 50 tonnes storage capacity, has been established. The top of each of these silos is provided with a removable circular manhole of diameter 148 mm. The bottom or base of each silo is

provided with a removable circular manhole of diameter 100 mm. As each of these silos is constructed to be gas tight, the manholes are being used to simulate leaks from the tops and bottoms of the silos. The silos or bins have been configured as shown in Figure 2, except that the test facility does not contain the microprocessor 22 and its associated control of the gas output from the cylinder 17, and thus the injection of fumigant gas (phosphine) into the gas supply duct has been effected manually and (more recently) using a timer-controlled metering device.

These trials, which have been in progress for 10 months, have shown that the present invention is effective in maintaining an efficacious fumigant concentration in the bins, irrespective of whether one, two or three bins have been included in the gas recirculation circuit. In particular, the trials have shown that

(a) with leaks in the tops (only) of the bins (up to and including the removal of all three manhole covers), the decay rate of phosphine is exponentially related to the total area of the "leaks", varying from y = 32.979e" ^0178x (where y is the leakage rate and x is the total area of the leaks) with no leak, to y = 185.9e ^"27163x with a 148 mm diameter top leak and all three bins in the recirculation circuit, to y = 3203e ^{"5 1911x} when all three bins are in the recirculation circuit and all three top manhole covers are removed;

(b) with leaks in the bases (only) of the silos, the effect 'of the leaks is similar to, but less significant than, the effect of leaks in the tops of

the silos, ranging from y = 41.359e" ^0,2759x with a 100 mm diameter leak in the base of one bin only, to y = 51.529e ^"03361x with a 100 mm diameter base leak and all three bins in the recirculation circuit; (c) with openings in both the tops and bases (bottoms) of the silos, the decay rate of phosphine is similar to that observed with top leaks only; and (d) in all configurations of the multiple-bin storage facility, it is possible to maintain an efficacious minimum concentration of phosphine in the fumigation system by a regular periodic injection of phosphine, and the intervals between the additions of phosphine are such that the multiple-bin fumigation with recirculation of fumigant-containing gas is very economical, less fumigant gas being required than the quantity of gaseous fumigant that would be required for a flow-through system, using the "SIROFLO" technique that is described in the specification of Australian patent No 640,699.

It should be noted that although exemplary embodiments of the present invention have been illustrated in this specification, and described above, variations to, and modifications of, those embodiments may be made without departing from the present inventive concept.