Phosphine generator for producing phosphine-containing gas

Details Phosphine generator for producing phosphine-containing gas Phosphine generator for producing phosphine-containing gas

1999-07-12

2001-11-13

Knode, Marian C. (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Including solid reactant and means charging solids into, or...

C422S231000, C422S236000, C252S372000, C423S299000

Reexamination Certificate

active

06315965

ABSTRACT:

BACKGROUND OF THE INVENTION AND PRIOR ART
The present invention relates to a process for generating a mixture of phosphine and diluent gas or gases, wherein a hydrolysable metal phosphide selected from the group consisting of magnesium phosphide, aluminum phosphide and calcium phosphide is contacted with liquid water in a generating ;pace, whereby the metal phosphide is hydrolysed to release phosphine which is withdrawn from the generating space and, where applicable, diluted from the time of its generation to its reaching its locality of use with a diluent gas to a composition which is non-ignitable under the conditions of use. The invention also provides a novel generator suitable for carrying out the process.
Phosphine gas is a highly toxic and flammable gas used in large quantities in pest control, and in particular for the fumigation of agricultural bulk commodities, such as grain and grain products. Phosphine gas generation is also subject to some peculiarities giving rise to special problems which do not apply to the generation by hydrolysis of other gases, e.g. the well-known generation of acetylene gas by hydrolysis of calcium carbide as disclosed e.g. in British patent specifications 472 970 (Haworth), 776,070 (Union Carbide) and 291,997 (Haworth).
In the case of phosphine gas generation there has always been the problem that prior art hydrolysable technical grade metal phosphides contained impurities which on hydrolysis liberated autoignitable phosphine homologues, phosphine derivatives, organophosphines, diphosphines or polyphosphines. This circumstance has created a strong prejudice in the art against what the present application proposes in what follows.
Traditionally compositions containing hydrolysable metal phosphides, in particular aluminium, magnesium and calcium phosphides have been used for this purpose, applied either in sachets or other dispenser devices or as moulded bodies (pellets or tablets).
In either case, the traditional compositions have always been compounded with various additives to a) reduce the reactivity of the metal phosphide when exposed to water in vapour or liquid form and b) to depress their tendency to autoignite. (Rauscher et al U.S. Pat. No. 3,32,067, Friemel et al U.S. Pat. No. 3,372,088, Friemel et al U.S. Pat. No. 4,421,742 and U.S. Pat. No. 4,725,418, Kapp U.S. Pat. No. 4347241). In spite of these expedients, these prior art products remained dangerous substances, involving fire and explosion hazards which had never been fully overcome if the products are handled inexpertly and stringent safety precautions are neglected. The degree of safety also depends on the experience of the manufacturer and quality control. The traditional manner of using these products in bulk commodity fumigation is to introduce the compositions into the storage means (e.g. silos, shipholds) as such. In the case of pellets or tablets, these are usually introduced into the bulk commodity itself. This practice is nowadays criticised because of the resultant contamination of the bulk commodities with the residues of the decomposed tablets or pellets.
If prior art compositions are apportioned in sachets, bag-blankets, bag-chains or similar dispensers, the purpose is to divide the composition into small individual portions in order to reduce the hazards of large local accumulations of gas and heat build up and at the same time prevent direct contact of the compositions with the commodities. These devices must, after completion of the fumigation, be retrieved from the storage or like facility where the fumigation has taken place. This is often difficult and cumbersome. The spent devices must then be disposed of, a matter which nowadays may cause problems.
All these and other prior art fumigation means and their traditional methods of application suffer from the drawback that once the devices have been introduced into the silo or other storage space and once the fumigation has commenced, there is usually very little that can be done to influence or even monitor the further progress of the fumigation. In particular, if the composition should accidentally be deposited in a wet spot inside a grain store, this will neither be noticed in time, nor can the resultant dangerous situation be corrected. A fumigation of this type once commenced, can normally neither be stopped nor (usually) be decelerated or accelerated.
To overcome these shortcomings to some extent new processes have been developed wherein tablets and pellets or the aforesaid sachets, bag-blankets, bag chains or similar dispensers are distributed e.g. on the surface of the bulk commodity aid to then apply recirculation of the gas content of the silo, storage space or shiphold; see U.S. Pat. No. 4,200,657 (Cook), U.S. Pat. No. 4,651,463 and U.S. Pat. No. 4,756,117 (Friemel) and U.S. Pat. No. 4,853,241 and U.S. Pat. No. 4,729,298 (Dörnemann).
In those cases contamination, if any, is more localised and the spent dispensers are more readily retrieved, although these are still inaccessible whilst the process is in progress. The aforesaid climatic and humidity limitations still usually apply. The time taken for achieving a scheduled concentration of phosphine throughout the storage space still depends on the rate at which the metal phosphide composition is hydrolysed under prevailing circumstances. If the applied circulation is too slow or ceases, e.g. due to a power failure, undesirable concentrations of phosphine may accumulate.
It has been recognised that it would be highly advantageous if it were possible to transfer the generation of phosphine gas to a locality outside the fumigation space whereafter the gas could then be fed into the commodity or storage facility in a controlled manner. However, because of the conceived and real risks inherent in phosphine gas and phosphine-releasing compositions, very little real progress has been made in this regard.
Thus the use of bottled PH
3
, produced by one or other undisclosed industrial process, has been proposed in U.S. Pat. No. 4,889,708. Again, in order to prevent autoignition once the gas is released into air and the mixture of air and gas is used as a fumigant, it was considered necessary to bottle the PH
3
highly diluted with an inert carrier gas such as CO
2
or N
2
. According to U.S. Pat. No. 4,889,708, the PH
3
concentration in the bottled gas is to be 1.8 to 3% by weight. The storage and transport of this highly diluted phosphine gas involves considerable logistics problems, besides being very expensive. It also involves the grave risk that in the event of an accident on site, in transport or in storage or in the event of leaking bottles, e.g. due to defective or not properly closed valves, a gas cloud, albeit not readily flammable, is formed which is highly toxic and which, because it is heavier than air, can accumulate in low-lying areas or in cellars or the like.
U.S. Pat. No. 5,098,664 discloses a recent attempt to overcome the prejudice existing in the art against the generation of phosphine gas in an external generator apparatus, wherein relatively large concentrated batches of metal phosphide are hydrolysed by the passage therethrough of controlled amounts of water vapour dispersed in humid air, the air serving as a carrier gas. This proposal still suffers from certain potential shortcomings. That disclosure teaches interrupting the hydrolysis in the event of operational failures by displacing the humid air in the generator space by an inert fluid, (liquid or gas). The recirculation type of process has similarly been improved in accordance with European patent application 9 114 856.8 (Degesch GmbH; published after the priority date of the present application) in that the hydrolysis of the solid metal phosphide compositions takes place outside the space containing the commodities to be fumigated in a hydrolysis chamber through which the circulatory gas flow is passed. Again, in the event of problems necessitating the interruption of gas generation, inert gas is admitted into the hydrolysis chamber to displace the humid air. In both the afores

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