Abstract:
Phosphine gas is generated by agitating a reaction mixture of a metal phosphide and water with agitation air in a reaction pot of a phosphine gas generator. The resulting phosphine gas is then diluted with dilution air to produce a fumigant phosphine gas which is directly delivered to a commodity for fumigation. The reaction pot does not have any rotating means such as agitators, rotors, or stirrers. The generator provides on-site generation of phosphine gas in a rapid manner improving the fumigation efficiency for a commodity, such as grain, preferably contained within a storage structures, such as a grain silo. The generator has a built in deactivation system for the unused metal phosphide and phosphine gas.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is broadly related to a generator and a corresponding method for fumigating a commodity with a phosphine gas. Phosphine gas is generated by mixing a metal phosphide and water upon agitation and dilution with air. 
     2. Description of Related Art 
     Fumigation of stored agricultural commodities such as grains with phosphine gas is the foremost method of preventing insect damage. Typically, fumigation is achieved by introducing pellets or tablets containing metal phosphide directly into the grain to be fumigated. The metal phosphide reacts with the ambient moisture in the air and grain, resulting in the generation of a phosphine gas and other inert gases. Forced air circulation devices are often used to assist in distribution of the phosphine gas throughout a storage structure, such as a grain silo. 
     A common fumigation problem encountered with prior practices is the inability to achieve a uniform concentration of phosphine gas within the storage structure quickly. It is known that for the most effective insect control, it is necessary to maintain the desired concentrations of phosphine for known periods. However, with prior methods the release of phosphine gas is slow and takes three to seven days or more depending on the ambient conditions. 
     Batch processes for the generation of phosphine gas have been proposed in the past. Such batch processes include a batch reactor for the hydrolysis of metal phosphides to obtain phosphine gas, the latter being stored in closed cylinders. Such containers can then be used on-site to deliver phosphine gas at a specific concentration throughout a selected period of fumigation. A significant drawback of this technique is that one must store the phosphine gas in pressure vessels with subsequent delivery of the vessels to the storage structure. This results in a need for expensive cylinders and poses handling hazards. Also, the phosphine gas must be transported to the fumigation site. 
     A very common fumigation problem encountered with prior practices is the use of CO 2 , N 2 , argon, and other similar gases which aim to dilute the phosphine gas concentration and to maintain low levels of phosphine gas until the phosphine gas reaches the storage structure for fumigation. At the same time, prior practices require an increased inventory and additional handling and hazards of pressurized cylinders. 
     Prior methods and devices for generating phosphine gas using metal phosphide have a number of additional problems. First, any unreacted metal phosphide in the reaction mixture would remain in the commodity after fumigation. Therefore, the metal phosphide had to be withdrawn in the form of a fine powder or collected in a bag. This handling of the unreacted phosphide metal poses increased health concerns to the operator. Second, the temperature of the reaction pot would increase during the reaction, since the reaction is exothermic. Hence, cooling jackets were needed around the reaction pot which increases the cost of generating the phosphine gas. 
     There is accordingly a need in the art for an improved apparatus and method allowing the safe, rapid production of phosphine gas for on-site generation. The desired apparatus and method would produce the desired phosphine gas concentration for fumigation while avoiding the need to produce phosphine gas at a remote site using intermediate containers of phosphine gas. Also, an apparatus that generated phosphine gas on a continuous basis without the addition of rotating parts like agitators, stirrers and rotors would be advantageous. Additionally, any improved apparatus or method would reduce or eliminate any hazards associated with the prior methods, including eliminated the usage of inert gases. 
     BRIEF SUMMARY OF INVENTION 
     The present invention is broadly related to a generator and a corresponding method for fumigating a commodity, such as grain, with a phosphine gas. The generator and corresponding method are used to generate phosphine gas from a metal phosphide, preferably aluminum phosphide, magnesium phosphide and other similar phosphides. The generator and corresponding method produce phosphine gas in a rapid manner to attain a uniform peak concentration of phosphine gas in a very short period of time. The generator enables on-site generation of phosphine gas directly to an adjacent storage structure or commodity so that phosphine gas is directly delivered to the commodity or into the storage structure without intermediate storage in containers. Preferably, the generator is used to fumigate silos, stored grain, transport containers, and ships. The present method is directed to a rapid generation of phosphine gas, which ensures that the required amount of phosphine gas is released in a short interval of time of less than two hours. Hence, the problems associated with previous generators, wherein the initial concentration of phosphine gas is low and later builds up and subsequently declines, does not arise in the present generator. 
     The present invention overcomes the problems outlined above by providing a generator for the on-site generation of phosphine gas by directly delivering the phosphine gas to an adjacent commodity or storage structure as it is generated in a uniform manner. The generator preferably includes a mobile frame supporting a reaction pot. The reaction pot doesn&#39;t have any rotating parts like agitators, stirrers, and rotors and hence, there is no metal-to-metal friction, which leads to wear and tear. In the reaction pot, a metal phosphide and water are added to produce a reaction mixture which is agitated with air to produce a phosphine gas. The agitation of the reaction mixture is accomplished by passing air under pressure through the reaction pot. The air produces turbulence which agitates or stirs the reaction mixture. No additional inert gases are needed to carry the phosphine gas for fumigation. Additional air, preferably from a storage structure, is used to dilute the phosphine gas in the preferred range of 100 to 5000 ppm and this fumigant phosphine gas is directly delivered to the commodity to be fumigated. Since ambient air is preferably circulated within the reaction pot for agitation and dilution, the temperature within the reaction pot can easily be maintained at a temperature near 55° C. 
     The generator also preferably contains a deactivation system for removing any unused reaction mixture and/or phosphine gas. The deactivation system primarily includes a secondary reactor which is connected to various air compressors and blowers ensuring that the unused reaction mixture is free from any active metal phosphide. Any phosphine gas from the reaction pot or unreacted fumigant phosphine gas is also cleaned preferably using an absorption tank. This eliminates any handling hazards to persons involved in processing the commodity since the operator is not exposed to the unreacted metal phosphide. 
     Additionally, forced air circulation devices are used to evenly distribute the fumigant phosphine gas throughout the commodity to be fumigated. This greatly reduces the possibility of any localized increase in concentrate of phosphine gas which may lead to ignition or fire. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a flow diagram showing a preferred generator and method for making phosphine gas including a deactivation system containing a preferred secondary reactor and a preferred absorption tank; 
         FIG. 2  is a side view of a preferred reaction pot; 
         FIG. 3  is a top view of a preferred reaction pot; 
         FIG. 4  is a side view of a preferred secondary reactor; and 
         FIG. 5  is a top view of a preferred secondary reactor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the preferred generator  10  for generating a phosphine gas. The generator  10  contains a reaction pot  12  for generating the phosphine gas. The reaction pot  12  does not contain any rotating parts like agitators, stirrers, and rotors. Hence, there is no metal-to-metal friction which would lead to wear and tear of the reaction pot  12 . A supply of a metal phosphide is fed to the reaction pot  12  via a metal phosphide input  14 . The preferred metal phosphide is aluminum phosphide, magnesium phosphide and other similar phosphides. A supply of water is fed to the reaction pot via a reaction water input  16 . Water and the metal phosphide are mixed in the reaction pot  12  to form a reaction mixture. In a preferred embodiment, 1 kg. phosphine gas is generated using 2.2 kg of 77.5% aluminum phosphide mixed with 10 liters of water. A supply of agitation air from an agitation air compressor  18  is fed to the reaction pot  12  via an agitation air input  20 . As shown in the side view and the top view of the reaction pot  12 , see  FIG. 2  and  FIG. 3 , the agitation air input  20  provides a supply of agitation air to the reaction pot  12  via a tangential entry. In a preferred embodiment, a bottom portion of the reaction pot  12  is a conical shape where the supply of agitation air enters the reaction pot  12 . The reaction mixture containing the metal phosphide and water react to generate a phosphine gas upon agitation with the agitation air. Preferably, the agitation air fed to the reaction pot  12  is ambient air such that the phosphine gas generated does not increase beyond 55° C. The agitation air from the agitation air compressor  18  is preferably supplied at a turbulating pressure of 0.5 to 2 kg/cm 2 . The phosphine gas is then diluted by a supply of dilution air from a dilution blower  22  producing a fumigant phosphine gas. The dilution blower  22  is connected to the reaction pot  12  via a dilution air input  23 . As shown in  FIG. 2  and  FIG. 3 , the dilution air input  23  provides a supply of air to the reaction pot  12  via a tangential entry. Preferably, the dilution air is ambient air such that the fumigant phosphine gas does not exceed a temperature of 55° C. The fumigant phosphine gas in preferably diluted such that the amount of phosphine gas in the fumigant phosphine gas ranges from 100 to 5000 ppm. Preferably, the amount of dilution air added to the phosphine gas is such that the concentration of phosphine gas should not increase the auto ignition limit (1.8%) at STP. The fumigant phosphine gas is then directly delivered to a commodity  24  to be fumigated via fumigant line  26 . The term “directly delivered” should be construed to mean that the fumigant phosphine gas is fed directly to the commodity without the need for intermediate storage containers or additional dilution means, such as inert gases. The commodity  24  is preferably disposed within a storage structure, such as a silo, transport container or ship. In a preferred embodiment, any fumigant phosphine gas that does not react with the commodity exits the commodity as unreacted fumigant phosphine gas. The unreacted fumigant phosphine gas may be recycled to the commodity  24  by directing a portion of the unreacted fumigant phosphine gas to the dilution blower  22 . Alternatively, any unreacted fumigant phosphine gas may be cleaned as discussed below. 
     Once the commodity  24  has been fumigated, any unused reaction mixture and phosphine gas may be directed to a secondary reactor  30  for deactivation. Any unused reaction mixture and phosphine gas remaining in the reaction pot  12  are preferably discharged from the reaction pot  12  via an unused reaction mixture line  28  to the secondary reactor  30  at an unused reaction mixture inlet  29 . The secondary reactor  30  contains an air sparger  31  which is connected to a residue compressor  32  via a sparger inlet  33 . The secondary reactor  30  is also connected to a unreacted gas blower  34  which provides air and unreacted fumigant phosphine gas to the secondary reactor  30  via an unreacted gas inlet  35 . Additionally, a supply of cleaning water is supplied to the secondary reactor  30  via at least one cleaning water inlet  37 . The cleaning water may be used to scrub the unused reaction mixture and unreacted fumigant phosphine gas. The secondary reactor  30  produces a drainable residue that is free from any active metal phosphide. The drainable residue from the secondary reactor  30  is removed via residue outlet  36 . The phosphine gas remaining in the secondary reactor  30  is fed to an absorption tank  38  via phosphine gas absorption line  40 . Likewise, any unreacted fumigant phosphine gas may be cleaned by directing the unreacted fumigant phosphine gas from the commodity  24  to the residual collection tank  30  via the unreacted gas blower  34  and unreacted gas inlet  35 . The secondary reactor  30  and absorption tank  38  provide any environmentally friendly means by which to clean the unused reaction mixture and phosphine gas after the commodity is fumigated. In a preferred embodiment, the deactivation process takes about 180 minutes. It should be understood that one of ordinary skill in the art may utilize other similar means for removing and/or cleaning the unused reaction mixture and phosphine gas from the generator  10 . 
     Additionally, forced air circulation devices are preferably used to evenly distribute the fumigant phosphine gas throughout the commodity  24 . This greatly reduces the possibility of any localized increase in concentrate of phosphine gas which may lead to ignition or fire. 
     The generator  10  produces phosphine gas having a uniform peak concentration in a short interval time, preferably less than two hours. This generation produces a maximum amount of phosphine gas and the reaction time of the phosphine gas with the commodity is very slow. The amount of phosphine gas released to the environment is negligible, preferably between 5-10 ppm. Phosphine gas can be generated in amount of from 0.56 gms to 10,000 Kg or higher. Multiple reaction pots  12  can be used to generate larger quantities of phosphine gas. Since, there is through passage for phosphine gas and the generator is completely closed, there is no leakage of the fumigant gas. There is no usage of inert gases to carry phosphine gas to the commodity or storage structure to be fumigated. One treatment of a grain silo is sufficient to completely kill the insects in the commodity. Also, the preferred generator has a battery back up and hence continues safe operation in the event of power failure. Preferably, the generator is a mobile unit. 
     EXAMPLE 
     An aluminum phosphide formulation, preferably containing 77.5% of the active ingredient, is used for phosphine gas generation for fumigation. The aluminum phosphide formulation contains 90% aluminum phosphide technical material, which contains a total of 86.2% active aluminum phosphide. The aluminum phosphide contains 10% paraffin wax having a melting point of 60-62° C. Other inert products like paraffin oil, at a different melting point, and petroleum jelly can also be used for dilution of the aluminum phosphide technical material. The aluminum phosphide formulation is in a granular form, hence safe to handle. There is no need to add additional diluents and/or ammonia to the formulation to reduce the active aluminum phosphide to make it safe for use as a fumigant 
     Either the aluminum phosphide formulation is added to water or water is added to the aluminum phosphide formulation to generate phosphine gas in the reaction pot. A first source or air under pressure is used to stir the water and aluminum phosphide. A second source of air is used to dilute the phosphine gas generated from the reactor and to carry the resulting fumigant phosphine gas to the commodity for fumigation. 
     Although the present invention has been disclosed in terms of a preferred embodiment, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention as defined by the following claims.