CA1113387A - Low air flow fumigation method - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed is a method of treating an agricultural pro-duct, including the steps of placing the product within a storage container; providing a forced air supply to the container; introducing a gaseous chemical into the container, the particular chemical being selected for minimal sorption of the chemical by the product; and circulating the air and chemical mixture within the container through the product at a low rate of flow, utilizing the forced air supply, for a time sufficient to evenly distribute the chemical through-out the product volume. The rate at which the air is cir-culated is less than approximately .006 cfm/bu and is pref-erably maintained approximately between .0015 cfm/bu and .0008 cfm/bu. Hydrogen phosphide (phosphine) gas generated from aluminum phosphide is preferred as the chemical when the process is used for fumigation, but other gaseous fumi-gants exhibiting a low rate of sorption by agricultural products may also be used.

Description

33~7 BACKGROU~D OF THE INV~1~IO~ -.
This invention is related to methods for chemically treating stored quantities of agricultural products, such as grain. The invention îs particularly related to treatment methods wherein the circulation of air i5 utili~ed to dis- -tribute a gaseous che~.ical.
Agricultural products, such as grain, are frequently stored for a period of time, such as between harvestins and further processing of the products. This storage can last for considerable periods of time. Consequently, in order to maintain the quality of the stored product, certain procedures ~ ;
have been followed in the storage ind~stry to keep the product in good condition and prevent deterioration.
The tem?erature and humidity of the stored product, for example, must be maintained within certain limits to prevent spoilage. This conditioning has sometimes been accomplished by physically turning the grain within the storage facility.
Temperature and hu~,idity control has also been implemented by using large fans to aerate the product through a system of ; vents in the storage container, typically including an aeration manifold underneath the stored product and vents in the roof or upper structure (commonly known as the roverhead") of the storage facility. In this manner, external air may be . ~ . . . .. . . . . . . ................................. . . . forced upwardly or downwardly through the volume sf stored product.

In addition to the problems caused by excessive tem?era- -~ ture or humidity conditions, stored agricultural products are ; also susceptible to damage from various live pests, such as ~, ~

3~7 insects, which eat the product, lay eggs in it, etc. Conse-quently, as soon as such products are stored, measures must be taken to prevent the substantial degradation which can otherwise occur, Various liquid chemicals have been applied to stored agricultural products to kill such pests ~nd prevent their da~aging the stored product~ Such liquid fu~,igants are ap~lied to the top o the stored product, the che~.icals being intended to flow downwardly into the product and throughout-the volu~e of the p~oduct to reach infestations in alI loca-tions. Such liquid ap~licat;ons, however, are an expensive means of control and are difficult to apply so as to ensure a uniform distribution of the chemical throughout the stored product, which is necessary to eradicate an acceptably high percentage of the infestation.
Gaseous fu~igants have also been used for this purpose.
A gaseous fumigant ma~ be applied through a forced air venti-lation system to circulate the fu~igant through the store~
product and then out through the vent system. This method is known as the "one pass" system of fumigation. In an atte~?t to further improve the efficiency of the fumigation operation and the uniformity of gaseous fumigant distribution, a recir~
culating forced air fumigation method was developed in the art.
In this method, any existing vents in the storage container are sealed off from the external atmosphere. An air duct is . . ~, . .
attached to the container above the level of the product (the storage "overhead") and connected to the intake for a fan or blower which supplies air to an aeration system underneath the stored product. The gaseous fumigant is introduced î-nto the duct or into the container and the fan or blower is utilized , 38 ~
to force air and the gaseous fumigant through the stored pro-duct. The gaseous air and f~migant mixture is then routed to the intake of the blower by the air duct, and is recirculated through the product, the recirculation being continued for a period of time sufficient to achieve an evenly distributed concentration of f~migant throughout the vol~me of the stored product.
In practicing such circulating treatment methods, the total quantity of fumigating chemicals is typically released over a relative~y short period of time of approximately 10 to 40 minutes. Such short reIease ti~es necessitate the ùse in this technique o air flow ra~es which are relatively high in order to achieve a unifor~ distribution of the fu~igant. The ;~
distribution of the fu~igant is further affected by the che~i-cal properties of the particular fumigants utilizedO Such chemicals are subject to sorption by the stored product, i.e., the chemical may be absorbed into a grain or it may be adsorbed by the surface of the grain. Furthermore, some chemical fumigants will brea~ down into other compounds ater application. These factors tend to cause an unbalanced con-centration of fumigant, with the highest concentrations occurring at locations nearest the point of release of the fumigant. When such constraints are taken into consideration, the air flow rates required in circulating fumigation methods typically are between .01 cfm/bu ~cubic feet per minute per bushel of stored product) and .2 cfm/bu, which flow rates correspond to effecting one complete change of air ~hrough the stored product in between 2.5 and 50 minutes. Lower air flow rates have not been used, because it has been found that a - , :: , "-: "~ , , : ,: , .- ~ ~: .

3~7 less than totally effective kill o~ the pests will be obtained with lower air flows.
Relatively high capacity ducts and blowers must thus be provided in the traditional practice of this method, the ducts typically ranging between 12 and 36 înches in diameter and the blowers used reyuirîng 5 to 100 horsepower motors. Such large duc~s are relatively expensive and the size of the blowers necessitates a relatively large amount of energy use during such traditional fumigation operations.
The flo~ rate, as mentioned above, is normally expressed in terms of cuhic feet per min~te per b~shel of product (cfm/bu)~ In this ter~inology, agricul~ural products such as grain can be dried with a flo~ rate of approxima~ely .5 cfm~bu. (corresponding -to a 10 minute total air change), cooling and conditioning is accomplished with a flo~ rate of about ~-O05 cfm/bu. t2.5 to 10 minute air change~, and fu~-igation utilizing the recirculation or one pass forced air methods has traditionally been implemented with flow rates ranging approximately between .01 and .4 cfm/bu., commonly at .025 cfm/bu. ~corresponding to a 20 minute air change).
One particular fumigating chemical, aluminum phosphide, which may be described as a solid fumigant, is available in the form of tablets, pellets, or bagged powder. Gaseous hydrogen phosphide (phosphine) is generated from solid .
aluminum phosphide in the presence of atmospheric humidity.
Hydrogen phosphide has been used in the past as a fumigant in static applications, but those skilled in the art have maintained that it should not be used in forced air systems.
In its solid form it has been applied into grain as the grain :- : : . :: . :~

J ~ is m~ved from one container to another, it has been distribu- .
ted onto the top or top and bottom-o the grain within a con-~ainer, and it has been p.robed into the grain at various depthsO Each of these fumigation methods has relied on the penetrating ability of phosphine gas and the convectional cur~
rents within a storage facility to provide distribution throughout the stor!ed product. On occasion, aeration systems with high air flows have been used in an attempt to assist in the penetration of the phosphine. Whereas other fumigants 9~ !
will release and achieve a ~eak concentration of gas in approximately l0 to 40 minutes, however, alum.inum phosphide requires a much longer ~ime, fro~ 16 to 30 hours, to release :
the phosphine gas contained therein and the correlation be-tween this release time and the proper air flo~ xate has not heretofore been recognized.
Therefore, a need has developed for a treat~ent method for grain and other agricultural products which will provide an.even distribution of chemicals at air flows related to the release of the gas and a uniformly high rate of kill without the need for large and expensive air recirculating equipment or the need for an excessive amount of chemicals.

SUM~RY OF THE INVE~TION

It is a general object of this invention to provide an improved method for the treatment of stored agricultural products. .
A mkthod of treating an agricultrual product stored in a container, according to this invention, comprises the steps of: :

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(a) introducing a qaseous chemical into the c~ntainer, the chemical being selected for minimal sorption of the chemical by the product, and (b~ circulating the contained air through the product at a low rate of flow~
In a more particular embodiment utilizing the one-pass technique, the method comprises the steps of:
(a) placing the product within a closed container, (b) providing an air duct into the container, (c) introducing a gaseous chemical int~ the container, the particular che~ical being selected for mini-mal sorption of the chemical by the product, and (d) forcing air and the cherical through the air duct at a very lo~ rate of flow, for a time sufficient to evenly distribute the cher,ical throughout the product.
In a more particular e~bodin,ent utilizing the recir-culating technique, the method comprises the steps of: ;
(a) placing the product within a closed containerr ~b) providing an air duct between the upper and lowerportions of the contaiher, (c) introducing a gaseous chemical into the container, the particular chemical being selected ~or minimal sorption oE the chemical by the product, and (d) forcing air through the air duct at a very low rate of flow, recirculating the air and chemical for a time sufficient to evenly distribu~e the chemical throughout the product.

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3~7 In a preferred embodiment, the method is utilized to fumigate a~ric~ltural products with a gaseous fumigant pro~ided in the form of phosphine gas derived fro~ aluminum phosphide.
In ~rder to achieve the optlmu~ benefits of this meth~d, the air in the container preferably is recirculated at a rate of flow which is less ~han approxi~ately .006 cfm/bu (1.5 hour air change). Optimum results have been obtained from the method when the rate of flo~ of the air is maintained between approximately .015 cf~/bu t6~5 hour air change) and .ooa8 cfm/bu (11 hour air change)~ ~he method has been successfully tested with an air flow low enough to effect a 3.5 day air chanye.
This method is particularly useful in fumigating farina-ceous products, such as flour and whole or processed grains.
Examples of the more important features of this invention have thus been broadly outlined in order that the detailed description thereof which follo~s may be better understood and so that the contributions which the invention provides to the art may be better appreciated. There are, of course, .
additional features of the invention which will be described herein and which will be included within the subject matter of the claims appended hereto.

BRIEF' DESCRIPTIO?`, OF THE DRAh~INGS

Additional objects, features, and advantages of the pre~
sent invention will become apparent by reference to the fol-lowing detailed description of the preferred embodlments thereof in connection with the accompanyiny drawing.~ In the drawing:

3~ 7 FIGURE 1 is a schematic view illustratiny a ~ypical st~rage container arrangement for agricultural products whic~
may be utilized to practice the method of this invention.

DESCRIPTIO~ OF THE PREFERRED EMBODIME~S

Now referring to Figure 1, a schematic outline of a grain storage system in which the method of this invention may be pr~cticed is illustrated. ln Figure~lr the agricultural pro-duct 10 is stored within a container 1~. A blower or fan 14 is connected to the lower portion of the container 12 through an air ~upply duct 16 and an aeration manifold 18~ ~hile a return air duct 20 is connected to the container 12 near the top of the container, and routes air fror, the cont~iner to the intakE blo~er 14 for recirculation through sup~ly duct 16 and the stored prod~ct 10.
In the kno~n method of treatment utilizing the one pass technique, the desired chemical is introduced into the pro-d~ct 10 by utilizing the blower 14 and eithe~ duct 16 or duct 20 to force air and the chemical either upwardly or down~
wardly, respectively, through the product and out vents to the atmosphere.
In practicing gaseous chemical treatment with the recir-culation technique, any vents to the atmosphere within the container 12 are first sealed. The recirculation systeF" c~n-sisting ~f the blower 14 and the associated supply and return ducts 16 and 20, is attached to the container to prov;de a recirculatin~ air path passing thro~gh the container and ~he stored product 10. The desired chemical for fumigatlon is then applied within the closed system.

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Typically, the chemical in these methods may be applied over the upper surface of the agricultural product 10, although the fumigating or other treating material may be sup~
plied anywhere within the closed syste~, such as in the s~p~ly conduit 16 or the return conduit 20, as best suited to a par-ticular application. The blower 14 is then operated for an appropriate period of time to achieve an even distribution of the gaseous fumigant throughout the volume of the agric~ltural product 10. After the blower has run for a s~fficient period of time to achieve the desired even distribution, the blower is turned off and no f~rther air movement is necessary until it is desired to vent the contalner in order to aerate the agricultural product and remove the fu~igation gas.
Such forced air circulation methods of treat~ent have heretofore been generally kno~n in the art to persons practic-ing fumigation of grain and other agricu~tural products.
Prior to the present invention, howeverr the forced air treat-ment method has been accomplished with much higher air flows than are requ~ired ln practicing the present invention.
The blower 14, supply duct 16, and return duct 20 are sized schematically to represent the relatively large size of such components as are typically provided for drying, coolin~, conditioning, and treating the prod~ct 1~ with conventional--methods. Also illustrated in Figure 1 are a low flow return duct 22r a low ~low blower 24, and a low flow supply duct 26, which are relatively small in size and capacity and may be provided especially for practicing the method of this invention.
The method of this invention may be practiced by the one-pass ~echnique ~y applying the treating chemical into the ~ ; - 7n _ ~133~ 7 overhead area 28 of the container 12. with the container vented to the at~osphere, the gas is drawn downw2rd throug~
the product by blower 24 at a very low flow rate. The blo~er is then turned off and the vents are sealed. Alternativel~, the che~ical ~ay be introduced into the bottom of the con-tainer 12 and the blower 24 utilized to force the gas up~ard through the product 10, again at a very low flo~ rate.
The one-pass technique of this invention may also be i~ple~ented utilizing larger capacity circulation equip-entl such as blower 14, supply duct 16, and return d~ct 20~ When the lar~er, conventional equipment is used, the very lo~ air flo~s necessary to practice the present invention may be achieved by running the larger blower, such as blower 14, for from one to five minute periods spaced at intervals of three to four hours. ~hen the higher capacity systems are utilized, however, extreme caution must be observed since such systems operate with much higher pressures, requirins that all leaks in the syster, ~ust be sealed.
The method of this invention may similarly be practiced with the recirculating treat~ent technique, in which external vents of the container 12 are sealed and the blower 24 and ducts 22 and 26 are used to recirculate the chemical at a very low flo~ rate for a period sufficient to effect an even distribution. As with the onepass method, the recirculating technique may be practiced according to this invention util-.. . ....... . . . . _ . . .. . . . _ . _ _ izing conventional, higher capacity air flow systems, whenthe blo~er is cycled on for very short intervals and when suitable precautions, in view of the higher pressures, are implemented.

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~33~7 It is an outstanding feature of this invention to provid~
a circulatin~ chemical treatment method requiring much lo~er rates of air flow than in previously known methods. In the method practiced according to the prior art, for example, rates of flow of ~01 cfm~bu and higher are commonly used, so that a complete change of air within the product i5 achieved within 50 ~inutes or less. It has been found, however, that more effectiv~ resùlts with slow generating or slo~ly intro-duced low sorption chemicals can be obtained more efficientl~
~hile using rates of flo~ which are much lower in com~ination with the properly selected treating che~ical.
In the prior art methods, the rate of floh must be main-tained relatively high because it has been found that at lower flow rates, the treatins chemical is preferentially absorbed by the grain or other agric~ltural product closest to the point at which the gas is introduced to the storage container. Hydrocyanic acid, ethylene dibromide, and ethylen~
dlchloride, for example, chemicals which have com~only been used as fumigants for agricultural prod~cts, tend to be highly sorptive ~ith respect to agricultural products. This high sorption characteristic results in an unbalanced distribution of the fumigant over the volume of the product.
Consequently, in the prior art methods, in order to achieve an effective kill of pests throughout the stored product, large capacity air 10w equipment and an extended application time must be utilized, so that all portions of the stored product receive sufficient concentrations Qf the fumigant to achieve the desired kill, resulting in increased costs and additional difficulties in subsequently aerating . ~

3~`7 the s~ored product to remove residual concentrations of the fumigating chemicals. A further incident of the hi~her rates of flow is increased leakage of the fumigating gas under the attendant higher pressure differentials, resulting in still additional amounts of yaseous fumigant which are re-quired in prior art methods, along with associated hazards.
It has been fo~nd by the present inventors, however, that by selecting a gaseous chemical which is sorptive to only a very small extent by agricultural prod~cts, or not at all, and b~ slowly releasing such a chemical, the rate of flo~ of the circulating air in the treatment may be drastically reduced while achievins improved results at a lower cost of materials and power. In practicing a fumigation method according to the present invention, for example, flow rates are utilized which are less than approximately .006 cfm/b~. Preferably, flo~
rates between .0015 cfm~bu and .0008 cfm/bu are utilized and have been found to provide excellent distribution of the gas-eous fumigant. These flow rates correspond to a complete air change through the stored product in fror 6.5 to 11 hours~
In the preferred embodiment of the method according to this invention, inhibited phosphine gas is utili~ed as the gaseous fumigant. The phosphine fumigant may be obtained from aluminum phosphide, which is available in the form of tablets, pellets, or bags. 'rhe aluminum phosphide may be applied at various points within the air recirculation system, as best 5Ui ted to a particular application.
Although this method has been practiced with~the use of aluminum phosphide to provide phosphine gas as a fumigating ~aterial, it will be understood by those skilled in the art .- ' . ' ,~.

33~`7 that other gaseous fumi~ants may also be utilized in this method, provided that the fumigant is selected to be one which is relatively non-sorptive with respect to agricultural pro-ducts. ~urthermore, the invention may also be practiced with other lo~ sorption chemicals to achieve treatments other than fumigation, such as, for example, deodorizing.
The features and advantages of this invention may be more fully explained and illustrated through several examples, . . . ~.
EXA~.PLE I
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, Three identical steel welded grain tanks, approximately 40 feet tall and 114 feet in diameter, were loaded and leveled to about 36 feet of depth with 320,000 bushels of long grain rough rice. All tanks were tightly sealed. A two horsepower blower with a 6 inch intake and 4-3/8 inch exhaust was used on the latter jtwo tan~s. A 6 inch sheet metal return pipe was erected from the top of each tank to the fan intake. A short 6 foot section of 4 inch flexible hose was connected from the fan exhaust to the central collection system in the botto~, of the tank. A .00125 cfm/bu rate of air flow resulted, with about an 8 hour air change within the rice mass.
- As a control comparison, the first tank was ~umigated ^
following PhostoxinR (a trademarked brand of aluminu~ phos-phide) label recommendations, without air circulation. Two cases of PhostoxinR tablets (a~proximately 40 tablets per 1,000 bushels of rice) were scattered evenly o~er the rice surface. The fumigation was considered complete and the test concluded after 500 hours (21 days).

In the first tank, high ~verhead concentrations ;n excess of ~400 ppm were recorded. The gas required 5 days to penetrate to the bottom of the tank with sublethal concentra-tions of 10-15 ppm. After 21 days, the bottom concentration had not exceeded 20 ppm, aithough a minimum concentration of 50 ppm would have been preferred.
The second tank was treated using the air recirculation sys~e~,. Two cases of PhostoxinR ~approximatély 40 tablets per 1,000 bushels) were scattèred eve~ly over the rice surface.
After about 3 hours, when overhead concentrations of fumigant had reached 490 ppm, the blower was turned on and, except for a short interruption, was run continuously for about 13 hours.
After 10 hours of inactivity, the fan was turned on again for about 8 hours, running a total of abo~t 21 hours fo~ the test.
The blower was discontinued at this time and the test was concluded after 201 hours (8.3 days). A complete distribu-tion of 450 ppm was obtained throughout the tan~ within hours after the application. Even and lethal concentrations were maintained, res~lting in complete pest control within a 5.5 to 8 day exposure.
The third tank was treated using air recirculation with one case of Phostoxin tablets (approximately 20 tablets per 1,000 bushels). The tablets were pulverized and blown into the space above the rice from one position. After 1-1~2 hours of exposure, with-a--650 ppm reading over the rice, the fan was turned on and run continuously ~or an additional 18-1/2 hoursO The analysis was discontinued and the fumigation con-sidered complete after 135 hours (5.67 days). With half the ~ previous dosage, complete distribution of ~he gas was attained , ` ';
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within an 3 hour period. Total control was ~chieved in 5~67 days.
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EXAMPLE II

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A steel walled flat storage buil~ing 90 ~eet by 360 feet long with 40 foo~ high walls contained four 90 feet by 90 feet bins with a capacity of 1,100,000 bushels. Each bin was loaded to a leveled depth o 36 feetl containing 7~0lO00 : -~
bushels of rough rice. 80 tablets per 1,000 bushels of alumi-nu~ phosphide were blown into the overhead ends of the build-ing through a 2-1/4" diameter tube connected to a 1-1/2 horse~
power high speed fan. A fan was connected to a lower aerating manifold under each binO The air flow rate was calculated to be approxi~ately .0015 cfm/bu~(6 hour total air change). A
natural infestation of rice weevils and lesser grain borers was controlled and the rice was shipped out in about four months free of any living infestation. Complete distribution :
was att.ained wi:thin 6 hours, with concentrations well within .
: the tolerances set for good controls. :~
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. EXAMPLE III

Two identical grain storage tanks made of corrugated -: steel~ 72 feet in diamater, with a Sl foot eave and a 72 fo~t.. _ .:
peak, having a rated capacity of 198,000 bushels, were loaded with 196,350 bushels of No. 2 yell.ow milo. A 6 inch pvc pipe was installed ln the roof 2 feet to 3 feet over the eave line, and was routed down the outside wall to within 5 feet of the `7 ground. A manifold of pvc pipe and flexible tubing connected . .
the return pipe to a two horsepower blower, with the supply from the blower connected to the aeration system in the botto~
:~
~f the tank under the grain. Air pressure cal'culations indi-cated an approximate air flow rate of .0025 cfm/bu (3.5 hour air change). Both tanks were fumigated ,with identical dosages of approximately 80 tablets per 1,000 bushels, using 2 cases ~14,400 t~blets) of PhostoxinR tablets per tank.
` In the first tank ~he f~ll dosage was broadcast ~ver the grain surface on the side opposite from the return air line.
The fan system was turned on soon after all the tablets were introduced and was allowed to run for 12 hours. 7 hours later it was reactivated for an additional 5-1/2 hoursl bringing the total fan time to 17-1/2 hours, or until 24 hours of fum;ga-tion exposure had occurred. An even and complete distrlbution was attained in 3.5 hours'and complete res~lts were attained.
In the second tank 9600 tablets ~2/3 of the dosage) were broadcast into the overhead area and the remainin~ 4800 tab-lets were thrown into the four aeration ducts in the bottom of ~, the tank. There was no air flow utilized. Erratic concentra-tions of fumigant were recorded at various locations from'the top to the bottom of the tank. 2.5 days were required for the gas to penetrate to the middle of the tank~

The advantages of this invention may be summarized by comparing various treatment parameters for a typical tight steel storage tank with a 200,000 bushel capacity. Aeration and condition~ng may be accomplished in such a tank with a ~ , ~L3L33~
flow rate of .1 cfm/bu, requiring a 20 horsepower blower sup-plying a 48 i~ch diameter d~ct to achieve 20,000 cfm. Tradi- -tional recirculating fumigation methods typically wo~ld utilize a flow rate o .025 cfr~/bu, which co~ld be supplied by a 3-5 horsepower blower and a 36 inch duct. In comparison, the fumigation method of the present invention may be prac-ticed in ~uch a tank with a flow rate of .001 cfm/bu, requir-ing only a 1/3 horsepower blower supplying a 4.5 inch dia-meter duct. The method may be effectively practiced at even lower flow rates o oO008 cfm/bu, wlth even smaller equipment.
The advantages of the invention may be further evidenced by a comparison of dosage rates required for the application of aluminum phosphide. Label instructions recommend 180 tab-lets per 1000 bushels, while general usage in the trade has involved 40 to 80 tablets per lO00 bushelsr leading to effec-tive controls in 8 to 10 and 8 to Zl days, respectively.
Utilizing the method of the present invention, however, no more than 20-40 tablets per lO00 bushels need be used to obtain effective controls in 5.5 to 6 days.
- Although typical émbodiments of the present invention have been illustrated and discussed herein, further modifica-tions and alternative embodiments of the method of this inven-tion will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be con~
strued as illustrative only and is provided for the purpose , . . . . , , . , . , . . . . , . . , . ....... .. , ~, . .. , _ .. . . .. . . . .
of teaching those skilled in the art the manner and technique of practicing the method of this invention. It is to be understood that the for~s of the invention shown and described herein are considered the presently preferred embodiments, ' `

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altho~gh various changes might be made in the configurations, sizes, and arrangements of the parts and steps utilized, as will be recognized by those skilled in the art without depart- -~
ing from the scope of the invention. Equivalent steps, for.
exa~ple, miqht be substituted for those illustrated and des-cribed herein, and certain features of the invention might b~
utiliæed independently of the use of other features, all as will be apparent to one skilled in the art after receivîng the benefit obtained through reading the foregoing description of the invention.

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Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of treating an agricultural product stored in a container, comprising the steps of:
introducing a gaseous chemical into the container, the chemical being selected for minimal sorption of the chemical by the product; and circulating the chemical and air through the product at a very low rate of flow.

2. The method of claim 1, wherein the gaseous chemical used is phosphine gas.

3. The method of claim 2, wherein the phosphine gas is obtained from aluminum phosphide.

4. The method of claim 1, wherein the chemical and air is circulated at a rate of flow less than approximately .006 cfm/bu.

5. The method of claim 4, wherein the rate of flow is main-tained between approximately .0015 cfm/bu and .0008 cfm/bu.

6. The method of claim 4, wherein the rate of flow is main-tained sufficiently low to effect a complete change of of air within the product in approximately 3.5 days.

7. The method o claim 1, wherein the agricultural product treated is a farinaceous product.

8. The method of claim 7, wherein the product fumigated is a grain.

9. The method of clalm 7, wherein the product treated is a processed grain.

10. A method of treating an agricultural product, comprising the steps of:
placing the product within a closed container;
providing an air duct into the container;
introducing a gaseous chemical into the container, the particular chemical being selected for minimal sorption of the chemical by the product; and forcing the air through the air duct at a very low rate of flow, for a time sufficient to evenly distribute the chemical throughout the product.

11. The method of claim 10, wherein the gaseous chemical used is phosphine gas obtained from aluminum phosphide.

12. The method of claim 10, wherein the rate of flow of the circulated air is maintained below approximately .006 cfm/bu.

13. The method of claim 12, wherein the rate of flow is maintained approximately between .0015 cfm/bu and .0008 cfm/bu.

14. The method of claim 12, wherein the rate of flow is maintained sufficiently low to effect a complete change of air within the product in approximately 3.5 days.

15. A method of treating an agricultural product, comprising the steps of:
placing the product within a closed container;
providing an air duct between upper and lower portions of the container;

introducing a gaseous chemical into the container, the particular chemical being selected for minimal sorption of the chemical by the product; and forcing air through the air duct at a very low rate of flow, recirculating the air and chemical for a time sufficient to evenly distribute the chemical throughout the product.

16. The method of claim 15, wherein the rate of flow of the recirculated air is maintained below approximately .006 cfm/bu.

17. The method of claim 16, wherein the rate of flow is maintained approximately between .0015 cfm/bu and .000 cfm/bu.

18. The method of claim 15, wherein the air is recirculated for a time sufficient to accomplish between one and ten complete changes of the air in the container.

19. The method of claim 16, wherein the rate of flow is maintained sufficiently low to effect a complete change of air within the product in approximately 3.5 days.

20. The method of claim 15, wherein the agricultural product is fumigated and the chemical used is phosphine gas obtained from aluminum phosphide.