Described are thermal aerosol generators useful for creating aerosols of chemical compounds used in various types of crop treatments and associated methods.

BACKGROUND

Thermal aerosol generators ("thermofoggers") have long been used to create stable aerosols (fogs) of various types of chemical compounds used in treating crops, such apparatus generally operating at elevated temperatures, e.g., generally above 600°F (315.5°C). Many of the fogged chemicals decompose to some extent, at least at such elevated temperatures. Fogging at commercially useful rates at temperatures below about 600°F (315.5°C) often results in wet or unstable fogs.

One chemical compound used in treating stored potatoes to inhibit sprouting is chloroisopropyl carbamate ("CIPC").

The thermal aerosol generators of the type disclosed in U.S. Patent 6,322,002 to Forsythe, et al. are conventionally made of ordinary steel. Thermofoggers such as Leco devices, that have long been used in the United States and Swing Foggers of the type used in the U.K. also have barrels of ordinary steel.

In U.S. Patent 4,226,179 to Sheldon, the contents of the entirety of which are incorporated herein by this reference, it is asserted that CIPC tends to decompose at temperatures above about 250°F (103.3°C). Such techniques as disclosed in Sheldon, however, have not been known to be commercially demonstrated. Sheldon focused only on CIPC decomposition prompted by elevated temperature, but provided no evidence that such decomposition occurred. Contrary to the statements in Sheldon, it is generally accepted in the sprout inhibition industry that noticeable decomposition does not occur until the CIPC reaches its boiling point of about 450°F (214.4°C).

Decomposition of CIPC, to the extent it occurs, is generally undesirable because CIPC is lost and, further, decomposition products such as metachloroaniline ("MCA") may be detrimental to stored potatoes, so that deposit of such chemicals on a crop of food product is undesirable. MCA is a colorless liquid at room temperature with a boiling point of 230°C (446°F), which is essentially the same as that of CIPC. DISCLOSURE OF THE INVENTION

Described are methods and apparatus wherein aerosols of temperature-sensitive chemicals are successfully aerosolized at elevated temperatures in the substantial absence of metallic-type catalysts, especially iron-containing materials.

In a particular embodiment of the invention, a method for formation and delivery of chemical aerosols to a delivery site includes thermally creating an aerosol of the temperature-sensitive chemical in a substantially non-ferritic environment and delivering the temperature-sensitive chemical aerosol to a delivery site.

In another embodiment of the invention, a thermofogger for forming temperature-sensitive chemical aerosols includes an aerosol generating chamber substantially confined by a substantially non-ferritic surface.

In yet another embodiment, a method of forming a very dry, uniform aerosol of CIPC from molten CIPC fed to a thermofogger includes providing a thermofogger having a barrel interior coated with, e.g., a ceramic-type coating and creating the aerosol at a temperature of less than about 600°F (315.5°C).

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a representative thermofogger having a coated barrel interior.

FIG. 2 is a particular embodiment of a thermofogger with a free-standing liner which may be made of low-iron content or coated with a non-metallic coating.

FIG. 3 is another embodiment of a thermofogger with a ceramic tube.

MODE(S) FOR CARRYING OUT THE INVENTION Decomposition of CIPC in modest quantities at temperatures in excess of about 450°F (214.4°C) in the presence of iron has been experimentally determined. This temperature is approximately the boiling point of CIPC. It is also a temperature above which most thermofoggers are commercially operated. While suitable aerosols may be formed at temperatures of about 600°F (315.5°C), commercial applicators in the U.S. tend to operate thermofoggers at temperatures of about 650°F (325.5°C) and above in order to apply CIPC aerosols at a commercially acceptable rate. In the U.K., commercial applicators of CIPC typically fog at temperatures above 800°F (426.6°C). Thermal aerosol generators (thermofoggers) having aerosol chambers with interior surfaces composed of low-iron content materials or preferably non-iron containing materials have been invented. Generally, ferrous iron is considered more detrimental than ferric iron, however, the presence of either form of iron is preferably avoided. Other catalytic-type metals may also preferably be avoided.

Suitable aerosol chamber interiors may be the barrel itself, a liner insert or a coating upon the barrel or liner.

Foggers having aerosol-contacting surfaces constructed of stainless steel, nickel-containing alloys and other metallic alloys having very low-iron content, preferably no iron content, as well as ceramics and glass, are useful in minimizing the decomposition at commercial fogging temperatures of chemicals applied to crops as aerosols. Such foggers are particularly useful in creating aerosols of CIPC, the predominant chemical used to inhibit sprouting of stored tubers.

Existing thermofoggers may be retrofitted with barrel liners of a low-iron or no-iron content material, or coated with a low-iron or no-iron, high temperature-tolerant material. Plasma sprayed metals, ceramics and glasses are such useful coating materials. Liners and coatings must be able to endure the elevated temperatures which are common within the thermofogger barrel (aerosolization zone). Elevated temperatures useful in creating stable, dry aerosols may range typically from about 600°F (315.5°C) to over 1000°F (537.7°C). Also, such liners or coatings must be substantially immune from damage by combustion gasses, since many foggers have hydrocarbon gas (propane, etc.) burners to provide the energy levels required to obtain dry, stable aerosols. For example, CIPC exhibits minimal decomposition at temperatures below about 450°F (214.4°C) even in the presence of moderate quantities of iron. However, even minimal decomposition may be undesirable because of the type of decomposition products formed. CIPC throughput is very low at temperatures below 700°F (371.1°C), and especially at temperatures below 650°F (325.5°C), consistent with creation of good quality aerosols. Thus, lowering the aerosolization temperature is not a currently commercially acceptable way to avoid CIPC decomposition in conventional thermofoggers.

Furthermore, the barrels, liners or coatings must be generally oxidation resistant at elevated temperatures. Many thermofoggers operate with high-pressure air to increase the desired energy level within aerosolization nozzles utilized in the aerosolization chamber. Also, the barrels, liners or coating must be generally corrosion resistant since many of the chemicals aerosolized are corrosive at aerosolization temperatures. Stainless steel, INCONEL®, and similar metals of low iron content, as well as ceramics and temperature-tolerant glasses, have the desired characteristics to perform as effective liners or coatings on barrel constructions

Adherent liners and, in particular, coatings including those that have a coefficient of thermal expansion sufficiently close to that of the barrel material are used, so that delamination of the liner or coating does not occur. Delamination of coatings could be especially troublesome should bits of the coating, for example, become entrained with the aerosol, resulting in foreign, undesirable material being deposited upon the crops being treated. Also, delamination of a coating or liner would expose the aerosol formed to the underlying barrel wall, which for existing thermofoggers would often be of high iron content and could cause undesirable chemical decomposition.

Liners, including coated liners may be free-standing {i.e., not adhered to the interior of the barrel surface), in which instance a close match of coefficients of expression may not be necessary. Such liners then form the aerosolization chamber and isolate the aerosol from the thermofogger barrel. Liners of such a construction may be advantageously used to retrofit existing thermofoggers.

Ceramic liners and coatings {e.g., CERAKOTE™, DURACOAT™, TEFLON®, GU -KOTE™, Moly resin, etc.) especially including coated liners are particularly useful inasmuch as ceramics are generally of very low or no iron content, have high temperature tolerance, and are generally oxidation, corrosion and erosion resistant. Ceramics generally have a very low coefficient of thermal expansion. Ordinary, steel barrels have a higher coefficient of expansion than most ceramics or temperature-resistant glasses, such as PYREX®. Thus, for a thermofogger having a liner or coating of a material having a low coefficient of expansion, it is generally desirable to bring the fogger up to operating temperatures slowly. Thermofoggers having barrels composed of a low-iron or no-iron material may be more expensive than coated or lined barrels, but eliminate concerns regarding delamination, slow warm-up, and the like. The terms "non-ferrite" or "non-ferritic" are used herein to mean metals having no significant iron content whether in the ferrous or ferric state.

Thermofoggers of the instant invention may be utilized with a wide variety of treatment chemicals, such as CIPC, substituted naphthalenes, lower and higher alcohols, aromatic oils, including clove oil, mint oil, carvone, and the like.

The barrels of such foggers may have single or multiple ports for injection of chemicals to be fogged. Multiple injection sites within the aerosolization chamber of a thermofogger permit injection of one or more chemicals simultaneously or sequentially at different temperature gradients within the aerosolization chamber. Also, additional ports may be utilized for introducing air or other fluids to assist in aerosolization of the particular chemical being used.

Liners or coatings of low-iron or no-iron may be readily used as well as in the construction of a complete barrel. Metallic liners and coatings may be advantageous in more closely matching the coefficient of thermal expansion of the underlying barrel than ceramics or glasses. Close matches may diminish occurrences of delamination.

Coatings and liners may be very thin. Thicknesses of as little as 0.010 inch (0.0254 cm) may be sufficient although thicker coatings and linings may be longer lasting. Generally, coatings and liners of about 0.10 inch (0.254 cm) to 0.25 inch (0.635 cm) are useful, although they may have a thickness of about 0.25 inch (0.635 cm) and greater, which can be readily made and insertable. As indicated, a coated tubular liner may be formed, coated, and then advantageously inserted within a thermofogger barrel, so long as its interior forms the aerosolization chamber. The coated tubular liner may be in contact with the thermofogger barrel or free-standing with some space between the external surface of the liner and the interior of the thermofogger barrel.

Liners and coatings may be installed or applied to barrels at an elevated temperature to ensure a tight fit for liners at operating conditions and to minimize the effects of any differentials in coefficients of thermal expansion between the coating and the barrel. Such a technique of "shrink-fitting" further ensures a tight fit for liners.

Although organic coatings, such as polymers, generally do not have high temperature tolerances, certain organic coatings may be loaded with inorganic fillers and applied to the interior of a fogger barrel. The barrel and coating may then be heated to very high temperatures to evaporate and/or bum off the organic material to deposit a high temperature-tolerant inorganic material (such as a metal, ceramic, or glass) as a substantially continuous coating on the barrel or liner interior surface. Thus, any suitable method of applying a high-temperature, corrosion-resistant, non-reactive, non-oxidizing coating to a barrel interior may be utilized.

The foggers of the instant invention having non-reactive aerosolization chamber surfaces are particularly useful when CIPC is the chemical being thermofogged, whether the CIPC is introduced as a molten liquid or as a solution. Aerosolization of molten CIPC is generally preferred in the U.S., since CIPC solutions have solvents which detrimentally decompose, as explained hereinafter.

Various metals may be used to construct thermofogger barrels, liners, etc., other than iron-containing metals. Such metals include, for example, aluminum, nickel, titanium, chromium, and various alloys thereof. While aluminum (Al) has a melting point of 660°C (approximately 1210°F), various alloys of aluminum have higher melting points, such as aluminum/antimony alloys (e.g., 90% Al/10% Sb has a melting point of 750°C (approximately 1382°F), while Al 80%/20% Sb has a melting point of 840°C (approximately 1532°F)). Various alloys of the various identified metals have even much higher melting points than aluminum and its alloys.

FIG. 1 represents, in outline form, an embodiment of a thermofogger 10 having a standard combustion chamber 1 1 into which a fuel 12, typically propane, and air 13 are introduced into a burner 14. The very hot combustion gases flow into an aerosolization chamber 15 where they contact a liquid sprout inhibitor 16, for example, to create an aerosol. The aerosolization chamber 15 is formed by a barrel 17 having a non-catalytic, especially low-iron or non-iron, coating 18 adhered to the interior of the barrel.

FIG. 2 illustrates, in outline form, an embodiment of a thermofogger 10 similar to that shown in FIG. 1 , except that in FIG. 2, a free-standing, tubular liner 19 is utilized to form an aerosolization chamber 15 within the tubular liner 19. The tubular liner 19 may be secured by a flange 20 inserted between the combustion chamber 1 1 and the barrel 17, thus providing a cantilevered structure for the tubular liner 19, which in such construction, the tubular liner 19 need not be otherwise secured to the barrel 17. Alternatively, the tubular liner 19 could be secured by a flange at the discharge end of the barrel 17, secured at both ends or secured tightly to the interior, cylindrical surface of the barrel 17. A cantilevered structure, such as that shown in FIG. 2, makes the tubular liner 19 substantially independent of the barrel 17 so that a close match of coefficients of expansion is not necessary, provided that the tubular liner 19 has sufficient physical strength and integrity at the temperatures occurring in the aerosolization chamber 15 to be structurally enduring over many years of use.

Thermofoggers operated at temperatures of up to 800°F (426.6°C) aerosolization temperature may be constructed of aluminum barrels, such as when a thermofogger structure, as shown in FIG. 3, is utilized. A thermofogger 10 of this type can include a combustion chamber 1 1 and an aerosolization chamber 15 formed within a high temperature-tolerant tube 21. The high temperature- tolerant tube 21 can be a flanged ceramic insert with a short tube length sufficient to enclose an aerosolization nozzle 22 and to contain the aerosol as it is immediately generated from the aerosolization nozzle 22. The outer surface of the high temperature-tolerant tube 21 may be spaced from the interior surface of the barrel 17 of the thermofogger 10.

As the aerosol emanates from the insert tube, it may be contacted with a cooling gas 23 (e.g., air) introduced between the high temperature-tolerant tube 21 and the interior of the barrel 17 to "freeze" the minute liquid aerosol particles into an aerosol of minute solid particles. The structure of the thermofogger 10 of FIG. 3 can be utilized advantageously with barrels of any construction material, including ordinary steel, inasmuch as the aerosol is generated within a high temperature-tolerant, non-reactive tube and then the aerosol of liquid particles is converted promptly to a non-reactive aerosol of solid particles before it contacts the barrel 17 of the thermofogger 10.

An aerosolization nozzle 22 is one which may preferably include high-pressure, hot air 24 introduced into nozzle jets (not shown) to contact the ejected hot, liquid CIPC from the aerosolization nozzle 22 to form a good aerosol of minute liquid particles. The CIPC in molten form, for example, is fed to the aerosolization nozzle 22 at temperatures well above 105°F (22.7°C) (the melting point of solid CIPC), and preferably above 150°F (47.7°C) with temperatures up to 250°F (103.3°C) and above being especially useful. The hot pressurized air introduced into the aerosolization nozzle 22 may be at temperatures of above 150°F (47.7°C) and preferably above 250°F (103.3°F), at pressures above about 20 psig and preferably above about 30 psig. The use of hot, high-pressure air can assist in aerosolizing CIPC, whether the CIPC is in a molten state or in solution. The use of high-pressure, hot air in an aerosolization nozzle provides additional aerosolization energy and permits, if desired, creation of effective aerosols at lower temperatures than required in the absence of the hot, high-pressure air without lowering the CIPC throughput rate or the quality of the aerosol.

The specification and drawings of U.S. Patent 5,935,660, especially FIGs. 8 and 9 and the description thereof, the contents of the entirety of which are incorporated herein by this reference, relate to useful aerosolization nozzles.

In the United States, the use of the processes described in U.S. Patent 5,935,660 has become predominant in converting molten CIPC into effective aerosols. However, in other countries, such as in the U.K., CIPC is provided as a solution with the principal solvents being methanol, isopropyl alcohol and dichloromethene. Also, the thermofoggers in the U.K. are generally run at hotter temperatures than in the U.S., with temperatures typically above 800°F (426.6°C). Thus, thermofoggers of the type disclosed herein may also be useful in the U.K., since the presence of iron or other catalytic metals may contribute to decomposition of such solvents. The decomposition products of alcohols may contain formaldehyde, formic acid, and various other acids, aldehydes and ketones while dichloromethane may form phosgene (COCl ₂ ), all of which are very undesirable chemicals to be in contact with humans, animals or food crops or products. The presence of aluminum has been implicated as a contributing factor in the decomposition of dichloromethane. Thus, while aluminum-type alloys may be acceptable for use with molten CIPC or CIPC in an alcohol solution, it should be avoided when CIPC is present in dichloromethane. As indicated elsewhere herein, non-metallic contact surfaces for thermofoggers are generally preferred with excellent results obtained from thermofogger barrels coated with a ceramic material.

EXAMPLE

A thermofogger of the type schematically illustrated in FIG. 2 had a commercially available proprietary ceramic coating identified as CERAKOTE™ applied to a tubular liner forming the aerosolization chamber and the exit cone. CERAKOTE™ is available from, e.g., NIC Industries of White City, OR, US. The coating, identified as CERAKOTE™ V series satin silver high temperature ceramic coating, is designed for protection of metal surfaces against high temperature heat oxidation. This is a single component, oven-cured system, which upon exposure to high temperatures in use, continues to flex and harden.

The satin silver V- 1 19 has about 34% solids by wt., a viscosity of 13 seconds in a Zahn cup #2 and VOCs' of 0.0% per gal. The cured coating has heat stability at temperatures of 1200°F (631°C).

The coating system is stated by the manufacturer to be proprietary, however, organic systems loaded with ceramic components are known in the art, which can be applied, cured at elevated temperatures to bum off resin components, and deposited as an integral ceramic coating of the type achieved by the CERAKOTE™ system.

The CERAKOTE™ was applied in accordance with the CERAKOTE™ Application Guide, as follows:

In the instant example, a sheet of thin sheet metal was cleaned to remove oils and other residues, then abraded to remove rust and other impurities. The metal was then heated in an oven at 450°F (214.4°C) to further remove any residual oils and other impurities.

The CERAKOTE™ V series system was thoroughly mixed and sprayed upon the one surface of the metal sheet. An application film thickness of about 1.0 mil was formed. The coated sheet was air dried for about 20 minutes, then heated in an oven for about 20 minutes at about 175°F (61.6°C) to allow excess solvents from the coating to out-gas. The oven temperature was then raised up to about 700°F (371.1 °C) and the coated sheet was cured for about 60 minutes.

The cured, coated sheet was allowed to cool, then formed into a cylindrical tube for insertion into a thermofogger. A fully cured CERAKOTE™ coating of approximately one millimeter thickness substantially covered the inner surface of the thermofogger aerosolization barrel's inner surface and the inner surface of the exit nose cone.

The coated thermofogger was tested under the following operating conditions: The coated sheet metal tube was inserted into a thermofogger generally fitting the description of the device described and illustrated in U.S. Patent 6,322,002 to Forsythe et al., the contents of the entirety of which are incorporated herein by this reference, except that the aerosol nozzle was directed away from the combustion chamber. The coated tube was fitted so that it formed the aerosolization chamber of the thermofogger.

Molten CIPC was fed to the nozzle at a rate of about 30 lbs/hour at a temperature of about 200°F (93.3°C); the air temperature (combustion gases) before the nozzle was about 500°F (260°C). The high pressure air fed to the nozzle was at a pressure of 40 psig. The fog temperature, after the nozzle in the aerosolization chamber, was about 446°F (212.2°C).

An especially fine, uniform, very dry fog was obtained. This fog, judged empirically against typical fogs obtained from commercial thermofoggers, was deemed superior, which was surprising, given the relatively low aerosolization temperature. The ability to obtain a superior dry fog at a temperature of 500°F (260°C) is a significant achievement at the CIPC throughput involved.

CIPC evaporation and decomposition, although not measured, would have been minimal based upon evidence from commercial applications where CIPC decomposition was generally observed to increase proportionally with increasing fogging temperatures. Also, at such a low temperature, CIPC evaporation would have been significantly less.

Also, the fact that the barrel was coated with a ceramic coating, thereby precluding contact of the CIPC aerosol with an iron-containing substrate would have further minimized CIPC decomposition. Other ceramic-type coatings may be substituted for the CERAKOTE™ coating to achieve similar results.

In a further embodiment of the instant invention, CIPC aerosols formed in a low-iron or no-iron surfaced thermofogger are conducted to a tuber storage facility in a duct which is made with low-iron or non-iron materials.

A preferred example of such a duct is a flexible, silicone-coated, fiberglass with temperature tolerances of from about -65°F (-53.9°C) to about 550°F (270°C). One such product is available from Rubber-Cal, Inc., Santa Ana, California, designated GS. Aerosols exiting thermofoggers operated even at temperatures of 600°F (315.5°C) and above generally are at temperatures less than 500°F (260°C).

In the particular embodiment of the thermofogger of FIG. 3, the aerosolization nozzle is located closer to the combustion zone than in current thermofoggers utilized commercially. In U.S. Patent 6,322,002 to Forsythe et al., a flame arresting status is described, wherein the aerosol emanating from the illustration of the nozzle there is directed towards the combustion gas flow, which was determined at that time to give the best operation.

However, in the thermofogger of FIG. 3, the aerosolization nozzle 22 directs the ejected aerosolized material away from the combustion zone. Directing the ejected CIPC away from the combustion zone minimizes the charge of "backfire," i.e., ignition of the CIPC aerosol, even without a flame arresting status.

The thermofogger of FIG. 3, when operated at about 500°F (260°C) at a commercially acceptable throughput of CIPC, produced an exceptionally dry, stable aerosol, as indicated in the previously described example.