MXPA98001724A - Method and fumigation system of - Google Patents

Abstract

A method and system for fumigating a region with a non-flammable gas mixture including phosphine are disclosed. A sensor detects the concentration of the phosphine for the atmosphere of the region, and a flow controller controls the flow to the region in response to the detected concentration. A recycle passage removes a portion of the atmosphere from the region and returns the atmosphere to create a recycle flow through the region. In addition, a gas mixing system is provided to supply the non-flammable mixture by mixing the phosphine from a phosphine source and an inert substance from a source of the inert substance.

Description

"METHOD AND GAS FUMIGATION SYSTEM" BACKGROUND OF THE INVENTION The present invention relates to a method and system of gas fumigation. More particularly, the present invention relates to a method and system for fumigating a region with a nonflammable gaseous mixture containing phosphine.

DESCRIPTION OF THE RELATED TECHNIQUE Pests are a nuisance for many different types of businesses. In agricultural industries and related to food production, infestations of pests, such as rodents or insects, cause significant problems. Pests often contaminate crops or food products stored in silos, rendering them unsuitable or unattractive for use as food or food ingredients. In addition, pests feed on stored crops and food products, causing a significant loss of inventory. Although there are many different ways to control pests, the most common pest control processes depend on the use of chemical substances. Pest control methods using chemicals can be effective in distributing or limiting pest infestations, but often have significant inconsistencies. Most of the pesticide chemicals that are used today are toxic to humans. Therefore, operators must isolate themselves from the areas where chemical substances are being applied. In addition, crops or food products exposed to pesticide chemicals may contain potentially hazardous chemical residues, requiring additional processing to remove the residues. One type of chemical pesticide is phosphine (PH3). Phosphine is a gas capable of being used as a pesticide fumigant to eliminate pests and reduce pest infestations. This gas is advantageous because it does not leave a residue after being used as a fumigant. However, phosphine gas is extremely flammable in air unless it is mixed with other gases and diluted sufficiently. Due to this risk of flammability, pest control processes using relatively pure phosphine can be very dangerous. One type of phosphine gas fumigation process uses phosphine gas generated from metal phosphide, for example, aluminum phosphide or magnesium phosphide configured into pellets or granules and optionally placed in separate packages. When metal phosphide is exposed to water, such as moisture in the atmosphere, it reacts with water to generate phosphine gas. Fumigations of pests that depend on the generation of phosphine from metal phosphides have a number of disadvantages. Flammable concentrations of phosphine can accumulate and thus create a significant risk of fire or explosion. The phosphine generation of metal phosphides frequently depends on climatic conditions, such as humidity and temperature. Operators can be exposed to toxic and potentially dangerous phosphine levels because they must enter restricted areas for the deployment or recovery of unreacted metal phosphides and their packaging material. A significant number of operators must have personal protective equipment, a rescue gear and advanced training. In addition, the residual material of the metal phosphide reaction must be handled, packed, transported, treated and disposed of properly. The reaction of metal and water phosphides is difficult to control and often requires a significant amount of time to be terminated. The use of excessive amounts of metal phosphide to account for potential reaction inefficiencies sometimes generates excessive amounts of phosphine. Continuous generation of phosphine is difficult to terminate without exposing operators to toxic and potentially flammable amounts of phosphine. The difficulty in controlling phosphine generation often establishes a wide range of different phosphine concentrations. Even when large amounts of metal phosphides could achieve a pesticidal atmosphere in a short time interval, the use of excessive amounts of raw material and the cost of waste limits these approaches. Typically, the reaction time of the metal phosphides and the medium by which the phosphine gas is distributed prevent the rapid establishment of a pesticidal atmosphere. When the phosphine is mixed with other gases, the resulting gaseous mixture can be effective for fumigating pests. For example, U.S. Patent No. 4,889,708, which is incorporated by reference in its entirety, discloses a pressurized mixture of a diluent gas and phosphine gas in a container adapted to release the gas mixture during fumigation. However, there is a need for systems capable of managing the flow of gaseous mixtures during the fumigation of specific areas. In addition, there is a need for systems capable of mixing gases at the site during fumigation, to allow separate supply of the components of the mixtures. After the area is fumigated to destroy the pests, some measures must often be adapted to prevent pests from re-infesting the area. Current means to protect reinfestation products involve spraying liquid chemicals directly into the products. However, the use of these chemicals requires a period of quarantine while chemicals are degraded at low concentrations. During this quarantine period, which sometimes lasts a number of days, the product can not be used. In view of the foregoing, there is a need in the art to improve fumigation with phosphine gas.

COMPENDIUM OF THE INVENTION Accordingly, the present invention is directed to a gas fumigation method and system that essentially avoids one or more of the limitations of the related art. In particular, the present invention is directed to the fumigation of gas with nonflammable gaseous mixtures containing phosphine and at least one inert gas. In order to achieve these and other advantages and in accordance with the objects of the invention, as they are attacked and extensively described herein, the invention includes a gas fumigation method comprising the steps of flowing the phosphine from a phosphine source. , to make an inert substance flow from a source of inert substance, to mix the phosphine and the inert substance to form a gaseous mixture, to control at least one flow of the phosphine from the phosphine source and the flow of the inert substance from the source of the inert substance so that the gaseous mixture is not flammable in air and passing the gaseous mixture into a region to fumigate the region. In another aspect, a system for fumigating a region with gas is provided. The system includes a source of phosphine, a source of inert substance, a mixer coupled to fluid with the phosphine source and a source of the inert substance to form a gas mixture, including the phosphine flowing from the phosphine source and the substance. inert flowing from the source of the inert substance, passing the gaseous mixture from the blender to the region during gas fumigation and a flow controller that controls at least one flow of the phosphine from the phosphine source to the blender and the flow of the inert substance from the source of the inert substance to the mixer, so that the gas mixture is not flammable in air. In a further aspect, a method for fumigating a product storage region is provided. The method includes the steps of removing a portion of the atmosphere from the region and returning the region back to the region to create a flow of recycling of the atmosphere through the region, making a gas mixture flow from a source from the gaseous mixture to the region, including the gaseous phosphine mixture and being non-flammable in air, detect the concentration of the phosphine for the atmosphere of the region and control the flow of the gas mixture to the region based on the detected concentration of the phosphine, in order to form a pesticidal concentration of phosphine in the region. In a further aspect of the present invention there is provided a system for fumigating a product storage region. The system includes a source of a gaseous mixture capable of fumigating the pests, including the gaseous phosphine mixture and being non-flammable in air, a sensor to detect the concentration of the phosphine for the atmosphere of the region, a flow controller to control the flow of the gaseous mixture from the source to the region based on the detected concentration of the phosphine to form a pesticide phosphine concentration in the region, and a recycle passage to remove a portion of the atmosphere from the region and return the portion back to the region to create a flow of recycling of the tósphere through the region. It should be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide a further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are included to provide further understanding of the invention and incorporated constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, Figure 1 is a schematic view of a gas fumigation system in accordance with a fashion of the invention, wherein the broken lines represent electrical interconnections, and the unbroken lines represent fluid couplings; and Figure 2 is a schematic view of the gas mixing system for use with the gas fumigation system of Figure 1.

DESCRIPTION OF THE PREFERRED MODALITIES Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. When possible, the same reference numerals are used in the drawings and in the description to refer to the same or similar parts. In accordance with the invention, a gas fumigation system is provided. As shown in Figure 1, the fumigation system includes a source 10 of a fumigant gas mixture, a plurality of regions 44a, 44b and 44c, a recycle passage 42 for recycling the atmosphere through regions 44a to 44c , and a supply line 40 for supplying the gaseous mixture to the recycle passage 42 and the regions 44a-44c in order to fumigate the regions 44a-44c. Regions 44a-44c are any type of partially enclosed or closed areas in which gas fumigation is desired. Normally, the walls of regions 44a-44c allow some escape of gas from regions 44a-44c, however, the invention can also be used for gas-tight enclosures. In one embodiment of the invention, the regions 44a-44c are storage areas such as silos for food products or agricultural crops such as grains. In another embodiment of the invention, the regions 44a-44c are areas in a vehicle, such as a compartment in a truck or railroad car. The present invention can be practiced to fumigate a number of different types of regions 44a-44c, and any number of different types of products stored in each of regions 44a-44c. In a preferred embodiment of the invention, the gaseous mixture supplying the source 10 includes phosphine mixed with one or more inert gases, such as carbon dioxide and / or nitrogen, which are not flammable in air. Preferably the gas mixture supplied by the source 10 includes a sufficient amount of inert gas to dilute the phosphine sufficiently to be such that the mixture itself is not flammable in air. The use of a non-flammable gas mixture is significantly safer than gas fumigation processes where high concentrations of the flammable phosphine are used. In addition, when carbon dioxide is one of the inert gases mixed in the mixture, the carbon dioxide acts synergistically with the phosphine in the mixture to improve the fumigation of pests and limit the amount of phosphine required. An appropriate mixture supplied by the source includes phosphine in an amount of less than about 3.1 volume percent, with the remainder of the mixture being inert gas, such as carbon dioxide and / or nitrogen. Another suitable mixture includes phosphine in an amount of less than about 5 volume percent, with the remainder being an inert gas. In addition, the nonflammable gaseous mixture in source 10 may be any of the gas mixtures disclosed in the aforementioned US Patent No. 4,889,708. In one embodiment of the invention, the source 10 is a pressure vessel although it contains the non-flammable gas mixture. This source is advantageous when the gas mixture is combined in an off-site location and transported to where the fumigation is to be carried out. With this arrangement, the potentially dangerous generation of phosphine is restricted to an installation that has sufficient safety measures to handle the flammable concentrations of phosphine. The use of a pressure vessel containing the gas mixture also does not require transport of flammable phosphine concentrations. In another embodiment, the source 10 of the gas mixture includes a gas mixing system 20 shown in Figure 2. The gas mixing system 20 includes a source 22 of the inert substance, a source 24 of phosphine and a mixer 26. of gas coupled to fluid with sources 22 and 24, in order to mix the inert substance flowing from the source 22 of inert substance with the phosphine flowing from the phosphine source 24. The source 22 of inert substance is preferably a pressure vessel containing an inert substance such as carbon dioxide, nitrogen or a mixture of carbon dioxide and nitrogen. Alternatively, source 22 of inert substance is an inert substance generator capable of generating carbon dioxide or nitrogen in a chemical reaction. When the inert substance in the source 22 of inert substance is initially in a liquid form, such as when the inert substance is pressurized in a pressure vessel, a vaporizer (not shown) is preferably placed between the source 22 of the inert substance and the mixer 26 in order to heat the inert substance, and in this way change its phase from a liquid to a gas.

The phosphine source 24 is preferably a pressure vessel that contains essentially pure phosphine. Alternatively, the phosphine source 24 is a phosphine generator capable of generating phosphine, for example, from the reaction of metal phosphide and water. When the phosphine source 24 is a phosphine generator, a non-filled pressure vessel (not shown) is also preferably placed in selective fluid communication with the source 24 to store the excess phosphine produced in the generator. The gas mixer 26 is a static or dynamic gaseous mixer capable of inducing a turbulent flow for mixing. For example, a gas mixer 26 can be a "T" shaped connector, a section of pipe having internal reflectors, or a length of pipe having a chain segment in its flow path to cause mixing of the pipe. inert substance flowing from the source 22 of the inert substance and the phosphine flowing from the phosphine source 24. As shown in Figure 2, a flow meter 28 of the inert substance and a flow valve 30 of the inert substance are placed between the source 22 of the inert substance and the gas mixer 26, to measure the flow rate of the inert substance flowing from the source 22 and to vary the flow rate, respectively. Similarly, a phosphine flow meter 32 and a phosphine flow valve 34 are placed between the phosphine source 24 and the gas mixer 26, to measure the flow rate of the inert substance flowing from the source 24. and to vary the flow regime, respectively. In addition, a gas sensor 36 is provided to detect the concentration of the phosphine and / or the concentration of the inert substance in the gas mixture flowing from the gas mixer 26. Optionally, the gas sensor 36 also measures the flow rate of the gaseous mixture produced in the gas mixing system. A controller 38, which includes one or more microprocessors, preferably controls the operation of both the total gas fumigation system shown in Figure 1 and the gas mixing system 20 shown in Figure 2. During the control of the mixer system 20 gas, the controller 38 receives signals from the flow meter 28 of the inert substance, the flow valve 30 of the inert substance, the phosphine flow meter 32, the phosphine flow valve 34 and the gas sensor 36, and sends control signals to the flow valve 30 of the inert substance and to the flow valve of phosphine 34 to control the flow of the inert substance and / or the phosphine flowing into the gas mixer 26.

Preferably, the controller 38 controls the flow of the phosphine and the flow of the inert substance before mixing in the gas mixture 26, to ensure that the phosphine is sufficiently diluted with the inert substance to create a non-flammable mixture. In addition, the controller 38 optionally controls the mixing of the inert substance and the phosphine based on the detected conditions of the gas fumigation system, such as the phosphine concentrations for the atmosphere in regions 44a-44c, and the temperatures in the regions 44a-44c, as will be explained below. In one embodiment of the invention, the gas mixing system 20 shown in Figure 2 is mounted on a wheeled chassis such as a trailer in a vehicle such as a car, a boat or a rail car so that the system 20 Gas mixer can be transported to different locations and connected with regions that require fumigation. Alternatively, the gas mixer system 20 is mounted on a vehicle and supplies the gas mixture to a region placed on the vehicle itself. As shown in Figure 1, the recycle passage 42 includes a common feed passage 46, a common exhaust or exhaust passage 48, the branching inlet passages 50a, 50b and 50c in direct fluid communication with the respective regions 44a-44c, and the branching output passages 52a, 52b and 52c also in direct fluid communication with the respective regions 44a-44c. A blower 54 is placed in the recycle passage 42 between the common discharge passage 48 and the common feed passage 46. The controller 38 regulates the speed of the blower 54 to control the flow through the recycle passage 42 and the regions 44a-44c. Preferably, the controller 38 operates the blower continuously during fumigation to ensure proper distribution in the gas mixture through the regions 44a-44c. During the operation of the blower 54, the branch outlet passages 52a-52c remove a portion of the atmosphere in the respective regions 44a-44c and feed this atmosphere to the common discharge passage 48. After passing through the common discharge passage 48, the common feed passage 46 distributes the atmosphere to each of the branching input passages 50a-50c which then feed the atmosphere to the respective regions 44a-44c. The flow in the recycle passage causes flow through regions 44a-44c and mixing of the atmosphere from each of regions 44a-44c. This provides fumigation of - 1 essentially all the space in the regions 44a-44c, and conserves the amount of the gas mixture used during the fumigation. The conservation of the gas mixture is beneficial because it reduces the amount of phosphine that could be released into the environment. The valves 62a, 62b and 62c are provided in the respective branch inlet passages 50a-50c to control the flow of the atmosphere to each of the individual regions 44a-44c. The valves 62a-62c are either manual flow control valves or automatic valves controlled by the controller 38. When one or more of the regions 44a-44c is empty and does not require fumigation, the corresponding valves 62a-62c are closed to prevent the flow to these regions and preserve the gaseous mixture. As shown in Figure 1, a recycle flow feed line 56 is coupled between the supply line 40 and the common feed passage 46 and the supply lines 58a, 58b and 58c of the region are coupled between the line 40 of supply and branch passages 50a-50c. The recycle flow feed line 56 passes the gaseous mixture from the supply line 40 to the common feed passage 46, and the feed lines 58a-58c of the region pass the gas mixture from the supply line 40 to the consumers. respective branching input passages 50a-50c. This configuration allows the introduction of the gaseous mixture into the recycle passage and in each of the regions 44a-44c, during the fumigation of each of the regions 44a-44c. A valve 58 is placed in the recycle flow feed line 56 to regulate the flow of the gas mixture through the recycle flow feed line 56. Similarly, valves 60a, 60b and 60c are placed in the feed lines 58a-58c of the respective region to regulate the flow of the gas mixture through each of the feed lines 58a-58c of the region. Preferably, the valves 58 and 60a-60c are automatic flow control valves controlled by the controller 38, in response to the various detected connections of the system, as will be explained below. Preferably, a structure is provided to discharge the atmosphere from the regions 44a-44c into the ambient air. As shown in Figure 1, a valve 64, optionally controlled by the controller 38 selectively places the common discharge passage 48 in flow communication with a ventilation duct 66. The ventilation duct 66 remains in flow communication with the air environment to the outside of the system and preferably includes a scrubber or filter 68 to remove potentially hazardous gases such as phosphine. A socket 70 of ambient air is also provided for introducing the ambient air into the recycle passage 42 and regions 44a-44c. A valve 72, placed in the common discharge passage 48 is movable between a first position, in which the valve 72 allows the flow of recycle from the atmosphere through the common discharge passage 48, and blocks the flow through the takes 70, and a second position, in which the valve 72 blocks the flow of recycle through the common discharge passage 48 and allows the flow of ambient air from the air intake 70 to a portion of the common discharge passage 48 between the valve 72 and the blower 54. Preferably, the controller 38 controls the movement of the valve 72 between the first and second positions in order to selectively introduce ambient air to the regions 44a-44c. To dilute the potentially harmful gases in the regions 44a-44c quickly, for example, when the operators enter the regions 44a-44c, the controller 38 places both the air inlet 70 and the ventilation duct 66 in flow communication, with the common discharge passage 48. Several sensors are provided to detect system conditions during a fumigation procedure, and to provide feedback signals to the controller 38 based on the detected conditions. As shown in Figure 1, the phosphine sensors 74a, 74b and 74c are provided to detect the concentrations of the phosphine in the respective regions 44a-44c, and to send corresponding signals to the controller 38. The sensors 76a, 76b and 76c of phosphine are also provided to detect the phosphine concentrations for the atmosphere flowing in the respective branch outlet passages 52a-52c and to send corresponding signals to the controller 38. In addition, the phosphine sensors 78 and 80 are provided to detect the phosphine concentrations in the passage 42 d recycle at the inlet and outlet ends of the blower 54, and to send the corresponding signals to the controller 38. The controller 38 controls the valves 30, 34, 58, 60a-60c and 62a-62c, and the blower 54, in response to the phosphine concentrations detected by the phosphine sensors 74a-74c, 76a-76c, 78 and 80. This allows the controller 38 to regulate the phosphine concentration for each of the regions 44a-44c during a fumigation procedure. Preferably, the controller 38 administers the phosphine concentration in the regions 44a-44c to preserve the gas mixture, while effectively fumigating the pests in the regions 44a-44c..

For example, controller 38 maintains a phosphine concentration in regions 44a-44c from about 10 parts per million to about 700 parts per million or from about 20 parts per million to about 250 parts per million., During fumigation of the pest , the controller 38 establishes and maintains a predetermined phosphine concentration in the regions 44a-44c for a period of time sufficient to eliminate any of the pests, and then the controller 38 maintains the phosphine concentration at a level below the predetermined level. In order to reduce the possibility of the entry of pests into the regions. Also, the controller 38 increases the phosphine concentrations when the pests may have entered the regions 44a-44c, or when they become more active in the regions 44a-44c, for example when the food products or crops are placed in the regions 44a-44c. Preferably, the controller 38 adjusts the phosphine concentration in each of the regions 44a-44c, based on the conditions related to the possibility of pest activity in the regions 44a-44c. The controller 38 preferably retains the amount of the gas mixture used in regions 44a-44c, in order to provide optimal pest control at a minimal cost and with reduced risk of releasing the potentially toxic levels of the gases into the environment . As shown in Figure 1, the temperature sensors 82a, 82b and 82c are provided in the respective regions 44a-44c. The temperature sensor 82a-82c detects the temperature in the regions 44a-44c and provides signals corresponding to the controller 38, to allow control of the phosphine concentrations in the regions 44a-44c, based on this detected temperature. At cooler temperatures, certain of the pests become inactive or dormant and at relatively higher temperatures, some of the regions to be sprayed experience a chimney effect where extreme temperature gradients heat the gases and force them rapidly towards above. In addition, pest spraying with phosphine is more effective at higher temperatures. The controller 38 preferably adjusts the phosphine concentrations in regions 44a-44c to compensate for the effects of temperature fluctuations. The gas fumigation system also preferably includes phosphine detectors 84a, 84b and 84c, an electronic data recording apparatus 86 and an audible and visual warning alarm 88. The phosphine sensors 84a-84c are located outside the regions 44a-44c and the recycle passage 42 to detect any escape of the phosphine from the system into the ambient air surrounding the system. The phosphine sensors 84a-84c provide a "boundary line" that monitors the area surrounding the system and could be any type of gas sensor, such as a laser beam sensor or a point sensor. The controller 38 receives signals from the phosphine sensors 84a-84c and stores the data related to the phosphine concentrations detected in the data recording apparatus 90. If the concentration of phosphine detected by the sensors 84a-84c reaches an unsafe level, the controller 38 preferably activates the alarm 88 to alert people in the area about the increased levels of phosphine. In addition, the controller 32 preferably closes the valves 30, 34 58, 60a-60c and 62a-62c and deactivates the blower 52 to prevent further escape of phosphine from the system. Preferably a communication device 90 is connected to the controller 38 to allow remote monitoring and control of the gas fumigation system through another corresponding communication device (not shown). The communication device 90 is any type of device capable of sending and receiving data so that the controller 38 can be monitored and adjusted. For example, the communication device 90 can be a conventional telephone modem, a wireless telephone modem, a radio or any other type of communication device that allows the exchange of bidirectional information. The methods of fumigating pests with the structure shown in Figures 1 and 2 will be discussed below. Although the invention is described in association with this structure, the method of the invention, in its broadest sense, could be implemented with another structure. During a pest fumigation procedure, the controller 38 initiates operation of the blower 54 to establish flow in the recycle passage 42. The flow in the recycle passage 42 attracts the atmosphere from the regions 44a-44c, through the branching exit passages 52a-52c and brings the atmosphere back to the regions 44a-44c through the passages 50a-50c of branching entry. When the source 10 of gas mixture in a pressure vessel containing the gas mixture, a valve in the container is opened to allow the flow of the gas mixture from the source 10. When the source 10 of the gas mixture includes the system 20 gas mixer shown in Figure 2, the flow of the inert substance, such as carbon dioxide and / or nitrogen, and the flow of phosphine is initiated from the respective sources 22 and 24. If the source 22 of the inert subtance is a generator of inert substance, the inert substance generator generates the inert substance, preferably through a fumigation process. Similarly, if the phosphine source 24 is a phosphine generator, the phosphine generator generates the phosphine through fumigation. Optionally, the source 22 of the inert substance includes a pressure vessel containing the inert substance, and / or the source 24 of phosphine includes a pressure vessel containing the phosphine. The controller 38 controls the flow valve 30 of the inert substance and the flow valve of phosphine to mix the non-flammable mixture of the phosphine and the inert substance, in the mixer 26. The controller 38 receives the signals from the flow meter 28 of the inert substance, the phosphine flow meter 32 and the gas sensor 36 to adjust the flow through the valves 30 and 34 based on the flow rate of the inert substance, the flow rate of the phosphine, the concentration of the phosphine and / or the inert substance in the mixture and optionally the flow rate of the mixture. The information provided by the flow meters 28 and 32 and the gas sensor 36 allows the controller 38 to constantly monitor and manage the relative concentrations of the inert substance and the phosphine in the gas mixture to ensure that the mixture is not flammable. In addition, the gas mixing system 20 is capable of adjusting the relative proportions of the phosphine and the inert substance, during fumigation. Even though the gas mixer system 20 preferably mixes the phosphine and the inert substance through the fumigation process, the controller 38 optionally also controls the valves 30 and 34 to allow the flow of the inert subtance only instead of the gas mixture. . This type of control is sometimes preferred at the beginning of a pest fumigation procedure to purge the atmosphere of regions 44a-44c with the inert substance, before flowing the gas mixture to regions 44a-44c. When the gas mixture flows through the supply line 40 from the source 10, the controller 38 controls the valves 58 and 60a-60c to allow the flow of the gas mixture to the recycle passage 42, and the regions 44a-44c . Initially, the gas mixture is diluted when combined with the atmosphere present in regions 44a-44c, and recycle passage 42 before the initiation of gas fumigation. Over time, more gas mixture flows into recycle passage 42 and regions 44a-44c, and eventually the concentration of the phosphine in regions 44a-44c increases to a pesticidal level. The flow in the recycle passage 42 provides for the mixing of the atmospheres of each of the regions 44a-44c. This recycle stream maintains a relatively uniform phosphine concentration throughout each of regions 44a-44c, during fumigation. In addition, the recycle stream reduces the amount of gas mixture required during fumigation, because the gaseous mixture initially introduced into recycle passage 42 and regions 44a-44c passes continuously through regions 44a-44c. As the gas flows from the gas supply 10, the controller 38 receives the input from the phosphine sensors 74a-74b and 76a-76c to determine the concentration of phosphine in the atmosphere of the regions 44a-44c. The controller 38 also receives input from the phosphine sensors 78 and 80 and the temperature sensors 82a-82c, in order to determine the concentration of phosphine in the recycle stream and the temperature in the regions 44a-44c. Based on the concentrations and temperatures of phosphine detected, the controller 38 controls the valves 30, 34, 58, 60a-60c and 62a-62c and the blower 54, in order to regulate the concentration of phosphine in each of the regions 44a- 44c. The controller 38 optionally controls the valves 30, 34, 58, 60a-60c and 62a-62c in a number of different ways in order to modify the flow. For example, the controller 38 controls the valves 30, 34, 58, 60a-60c and 62a-62c to provide a continuous, variable and continuous flow of the gaseous mixture or pulsation to each of the regions 44a-44c. In the embodiment shown in Figure 1, the controller 38 varies the rate of a continuously operating blower 54 in order to modify the flow through the recycle passage 42. However, the flow through the recycle passage 42 can be varied in other ways and by other means. For example, the flow in recycle passage 42 could be driven by running blower 54 intermittently. Further, the blower 54 could have a shunt that selectively places the inlet of the blower 54 in flow communication with the outlet of the blower 54, a throttle opening could be provided in the recycle passage 42, or multiple blowers could be provided. Optionally, the controller 38 monitors the operation of the blower 54 and interrupts the flow of the gas mixture in the event of failure of the blower 54.

When the phosphine is exhausted from the system, for example, due to leakage from the walls of the regions 44a-44c, the controller 38 continues the supply of the gas mixture and adjusts the flow of the gas mixture to compensate for the loss. Preferably, controller 38 maintains a predetermined concentration of phosphine in regions 44a-44c for a period of time sufficient to kill any of the pests in regions 44a-44c. Then, the controller 38 maintains a lower concentration of the phosphine in the regions 44a-44c, to ensure that the pests do not try to re-infest the regions 44a-44c. The controller 38 preferably increases the concentration of the phosphine in regions 44a-44c, when the pest infestations are more likely to occur. For example, controller 38 preferably increases the concentration of phosphine in regions 44a-44c after a product, such as a grain, is added to the region. This increase in phosphine concentration fumigates any of the pests in the added product. During fumigation, controller 38 preferably monitors phosphine concentration sensors 84a-84c to detect increased concentrations of phosphine escaping from regions 44a-44c or other areas of the gas fumigation system. The recording apparatus 86 maintains a record of the concentrations detected by the sensors 84a-84c to allow monitoring of the system. When sensors 84a-84c detect potentially unsafe phosphine concentrations, controller 38 activates alarm 88 to provide a warning to people in the area, and controller 38 closes valves 30, 34, 58, 60a-60c and 62a- 62c to prevent further escape of the system. If desired, the fumigation process can be monitored from a remote location through the communication device 90. If desired, adjustments are made to the fumigation process, the communication device 90 allows adjustment of the process from a distant location. Upon completion of the fumigation process, the controller 38 preferably adjusts the valve 64 and the valve 72 to discharge the atmosphere from the regions 44a-44c through the vent duct 66 and to introduce fresh ambient air through the outlet 70 of air. After a sufficient period of time, this decreases the amount of the gas mixture in regions 44a-44c so that people can enter regions 44a-44c. Because the mixing system and the gas fumigation system regulate and control the gas flows, the present invention allows the controlled dosing of the specific amounts of phosphine in each of the regions 44a-44c, and for the start-up and relatively instantaneous paralysis of gas flows. Controlling the flow of the gas mixture to regions 44a-44c instead of generating phosphine in the regions reduces the possibility of the residue remaining in regions 44a-44c and eliminates the need for a high moisture supply or external water to regions 44a-44c. The present invention flows non-flammable gas mixtures to regions 44a-44c, instead of allowing flammable concentrations of phosphine to be introduced or accumulated in regions 44a-44c. Therefore, the present invention is safer than the other phosphine fumigation methods and systems. In particular, the present invention has a significant number of advantages compared to pest fumigation approaches where phosphine is generated from metal phosphides without providing control of gas flow or control of gas mixing. The risk of fire is extremely low because the gaseous mixture is not flammable and is initially diluted when the mixture is combined with the atmosphere in regions 44a-44c. The release and control of the gas mixture is relatively independent of weather conditions. The exposure of a potential operator to high concentrations of phosphine is minimized to the extent that the non-flammable gas mixture is contained in the fluid handling equipment. further, operators do not have to enter regions 44a-44c and carry out tasks such as the removal and disposal of residual material. This reduces the number of operators required to carry out the fumigation. The present invention provides direct control and measurement of phosphine release and termination of release upon request. The flow control in recycle passage 42 can be used to provide a desired concentration of phosphine in regions 44a-44c. The method and system of the present invention can establish a uniform pesticidal atmosphere at a rate that depends on the geometry of the regions 44a-44c and the rate of recycle flow. Instead of relying on liquid chemical sub-tastes, the method and system of the present invention can maintain a relatively low concentration of phosphine in regions 44a-44c, over a period of time to reduce infestations by mobile pests. The products in regions 44a-44c are available for use as soon as the phosphine is vented from regions 44a-44c without requiring a significant quarantine period. It will be apparent to those skilled in the art that various modifications and variations may be made in the structure of the present invention, without deviating from the scope of the spirit of the invention. For example, the present invention could be used to handle the flow of flammable gases other than phosphine, or to provide means for fumigating a single region, rather than a plurality of regions. In addition, the gas mixing system of Figure 2 could be used in combination with a gas fumigation system different from that shown in Figure 1. In view of the foregoing, it is intended that the invention cover the modifications and variations of this invention. invention that are provided as they fall within the scope of the following claims and their equivalents.

Claims (10)

R E I V I N D I C A C I O N E S:

1. A gas fumigation method comprising the steps of: flowing phosphine from a phosphine source; to make an inert substance flow from the source of the inert substance; mix the phosphine and the inert substance to form a gaseous mixture; controlling at least one flow of the phosphine from the phosphine source and the flow of the inert substance from the source of the inert substance so that the gas mixture is not flammable in air; and passing the gaseous mixture to a region to fumigate the region.

2. The gas fumigation method of claim 1, wherein the source of phosphine is a pressure vessel containing phosphine gas, the source of inert substance is a pressure vessel containing the inert substance, and the inert substance. it is selected from the group consisting of carbon dioxide, nitrogen and a mixture of carbon dioxide and nitrogen.

3. The gas fumigation method of claim 1, further comprising the steps of detecting the concentration of the phosphine for the atmosphere of the region, or detecting the temperature of the region, and regulating at least one flow of the mixture gaseous, the flow of the phosphine from the phosphine source and the flow of the inert substance from the source of the inert substance based on the detection.

4. The gas fumigation method of claim 1, further comprising the steps of removing a portion of the atmosphere from the region, returning the portion back to the region in order to create a recycle stream from the atmosphere and combine the gaseous mixture with the recycle stream.

5. A system for fumigating a region with gas, comprising: a source of phosphine; a source of the inert substance; a fluid-coupled mixer with the phosphine source and the source of the inert substance to form a gaseous mixture including phosphine flowing from the phosphine source and the inert substance flowing from the source of the inert substance, passing the gaseous mixture from the mixer to the region during gas fumigation; and a flow controller that controls at least one flow of the phosphine from the phosphine source to the mixer and the flow of the inert substance from the source of inert substance to the mixer so that the gas mixture is non-flammable in air. The gas fumigation system according to claim 5, further comprising a sensor for detecting at least one concentration of the phosphine in the gas mixture and the concentration of the inert substance in the gas mixture, and the controller for Flow receives the input from the sensor to control the flow based on the detection. 7. The gas fumigation system according to claim 5, further comprising a sensor for detecting at least a concentration of the phosphine in the atmosphere of the region and the temperature of the region, the flow controller receives the input from the sensor to regulate at least one flow of the gaseous mixture, the flow of the phosphine from the phosphine mixture and the flow of the inert substance from the source of inert substance based thereon. The gas fumigation system according to claim 5, further comprising a recycle passage to remove a portion of the region's atmosphere and return the portion back to the region in order to create a recycle stream for the atmosphere, the recycle passage remains in fluid communication with the mixer so that the gas mixture is combined with the recycle stream. The method of claim 8, wherein the step of controlling the flow includes at least one of controlling the combined flow of the gaseous mixture and the recycle stream from the atmosphere, and controlling the flow of the gas mixture before combine the gaseous mixture with the recycle stream of the atmosphere. 10. A system for fumigating a product storage region comprising: a source of a gaseous mixture capable of fumigating pests, the gas mixture includes phosphine and is non-flammable in air; a sensor to detect the concentration of phosphine for the atmosphere of the region; a flow controller for controlling the flow of the gas mixture from the source to the region, based on the detected concentration of the phosphine in order to form a pesticidal concentration of phosphine in the region; and a recycle passage to remove a portion of the atmosphere from the region, and return the portion back to the region in order to create a recycle stream into the atmosphere throughout the region.