Fumigation of particulate commodities

Details Fumigation of particulate commodities Fumigation of particulate commodities

1996-06-25

1998-05-26

Thornton, Krisanne

Chemical apparatus and process disinfecting, deodorizing, preser

Process disinfecting, preserving, deodorizing, or sterilizing

Using disinfecting or sterilizing substance

43125, 422 3, 422 28, 422 40, 422292, A61L 900

Patent

active

057560463

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention concerns the fumigation of stored particulate commodities that are susceptible to attack and spoilage by insect pests. It is particularly useful in the controlled fumigation of particulate foodstuff (for example, grain) that is stored in a vertical silo which is vented at the top. However, this invention is not limited in its application to foodstuffs, or to commodities stored in silos.


BACKGROUND OF THE INVENTION

A technique for the effective and efficient fumigation of grain, using low concentrations of phosphine as the fumigant gas, is described in the specification of International patent application No PCT/AU90/00268, which is WIPO Publication No 91/00017. That technique requires the establishment of a slight over-pressure of the fumigant-containing gas in the grain, to the extent that the fumigant-containing gas flows through the grain mass at a constant linear velocity and leaves the top surface of the grain in the silo at a velocity in the range of from 0.5.times.10.sup.-4 to 2.times.10.sup.-4 meters per second. The concentration of phosphine in the gas passing through the grain is in the range of from 4 to 200 micrograms per liter.
While that technique works well, further work in connection with its commercial adoption has shown that a problem arises when grain in a vertical silo is being fumigated and the temperature within the silo is different from the ambient air temperature outside the silo. When the temperature of the grain is greater than the outside ambient air temperature, the density of the air within the silo is less than the density of the air outside, and there is a tendency for the air inside the silo to rise. This phenomenon has been termed the "chimney effect". The difference in density between the air inside the silo and the air outside the silo produces a difference between the pressure gradient with height inside the silo and the pressure gradient with height outside the silo. Hence, in a top vented silo, the air pressure in the grain at the base of the silo will be different from the air pressure outside the silo at the level of the base of the silo.
In this situation, if the bottom of the silo is not completely sealed (in commercial silos, this is usually the case, for even notionally sealed bases contain cracks and crevices which permit the ingress of air), the chimney effect causes an airflow through a top vented silo. This flow of air through the silo will dilute the concentration of fumigant within at least part of the grain mass or other foodstuff stored in the silo, and may prevent effective fumigation of the stored product.
The same chimney effect occurs in other forms of storage structures which are vented at their tops and not completely sealed at their bases. Thus, for convenience, in the remainder of this specification (including the claims), the term "silo" will include within its scope any form of storage structure in which a particulate commodity may be stored.


DISCLOSURE OF THE PRESENT INVENTION

The prime object of the present invention is to provide a technique which compensates for the chimney effect, and prevents an undesirable dilution of fumigant during the fumigation of the charge of a top vented silo.
This objective is achieved by maintaining (and preferably continuously adjusting) the flow rate of the fumigant-containing gas so that, even when the chimney effect is present, there is sufficient fumigant gas present to ensure continuous fumigation of the stored product. It has been found that the required adjustment of the flow rate depends upon the difference between the temperature of the stored product and the ambient temperature outside the silo. Thus the necessary modification of the flow rate of the fumigant-containing gas to compensate for the chimney effect, by maintaining a small positive pressure of a predetermined concentration of fumigant within the grain mass or other stored product, can be determined from measurements or estimates of the temperatures within and outside the silo.
According to the present i

REFERENCES:
patent: 4966755 (1990-10-01), Smith
patent: 5417921 (1995-05-01), Dove et al.
Hunter, "Pressure Difference Across an Aerated Seed Bulk for Some Common Duct and Store Cross-Sections", J. Agric. Engng. Res., 28:437-450 (1983).

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