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Vacuum fumigation
Published April, 1959
Vacuum, Vol. VZ
VACUUM FUMIGATION W. Burns Brown, M.Sc.,
A.R.C.S., D.I.C., F.R.I.C.
Department of Scientific and Industrial Research, Pest Infestation Laboratmy, London Road, Slough, Bucks.*
Summary
Sommaire
SOME OF THE TECHNIQUESof fumigation employing reduced pressures are described. Recent investigations into the behaviour of the fumigant during treatments by these techniques and the effect of the technique upon the resistance of insects are reviewed. Features of some representative installations for vacuum fumigation are briefly described.
DESCRIPTION DE QUELQUES PROCI~DI~S de fumigation a pressions r~duites. On passe en revue les recherches r~centes faites sur le comportement du gaz pendant traitement par ces techniques et l'effet sur la r~sistance des insectes. Br~ve description d'installations pour la d~sinfection sous vide.
INTRODUCTION
erial is also usually related to the level of this 'c.t. product.' For this reason it is always desirable to employ a method of fumigation which will produce c.t. products throughout the treated material which are effective without being unnecessarily high. A great deal of fumigation is carried out at atmospheric pressure in chambeis, in sealed warehouses, in barges and ships' holds and under gas-proof sheets. In addition, many who undertake fumigation make use of special chambers, usually of steel, in which the pressure can be reduced substantially below atmospheric. The details of the technique are varied but all such methods are commonly described by the term 'vacuum fumigation.' In fumigation at atmospheric pressure, penetration of the gas to the centre of a bag or into a package may be quite slow and a relatively long period of treatment, say 24 hours or occasionally longer, may be necessary to ensure effective treatment throughout the material. Early tests showed that vacuum fumigation could be effective in much shorter periods, such as 1-4 hours. The time factor in itself is frequently unimportant, especially when dealing with a regular, large throughput of commodity, but
FUMIGATION PROVIDES a very important means for controlling infestation by insects and mites in a wide range of materials, for instance cereals and cereal products, pulses, oil seeds, animal feeding stuffs, cocoa, nuts and dried fruits; tobacco; natural fibres and clothing; hides and skins; timber and packaging materials; fresh fruits and growing plants. A recent additional use of fumigation has been the cold sterilisation of such materials as hospital equipment and certain foodstuffs, for example, the spices used in the preparation of meat products. Fumigants have the ability to penetrate into packages and through bulks of material and to destroy the pests at all stages of their development, whether freely exposed or hidden inside granular material. To be effective, a sufficient concentration of the fumigant must he maintained for a sufficient length of time. It has been found that the toxic effect is usually related to the integrated product of concentration and exposure time. Any deleterious effect on the mat* M.S. received April, 1957
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W. B U R N S B R O W N
by Courtesy of Degesch (Frankfurt/Main)
Fig. 1 View o f a large installation capable of f u m i g a t i n g 5oo tons of m i x e d c o m m o d i t i e s daily. I t comprises four c h a m b e r s , each of 50 m . 3 (I,75o ft. 3) capacity, a n d in addition one of 2o m. 3 (7oo ft.3), one of 6 m . 3 (21o ft. a) a n d one of 2 m . 3 (7o ft.3). See also F,g. 7. P'z~¢. 2 View o f a large installation c o m p r i s i n g c h a m b e r s of 70, 50, 20, IO, 4 a n d 2. 5 m. a (2,450, 1,75o, 7o0, 35o, I4o and 9 ° ft. 3) capacity. by Courtesy of Mallet S. A. (Paris)
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VACUUM FUMIGATION it is often claimed that the circumstances demand rapid treatment or at least make this a valued convenience. Thus, short period treatment in vacuum chambers is often adopted in quarantine stations where a wide range of products must be fumigated according to a variety of specifications. Vacuum fumigation methods have a more obvious application in the treatment of those materials which are particularly difficult to fumigate at atmospheric pressure, such materials being leaf tobacco in bales or hogsheads, boxes of compressed dates and multiple packages of manufactured at'tides. It is further claimed for vacuum fumigation methods that they are intrinsically safer, since the treatment takes place in a vessel at reduced pressure from which no leakage can occur and residual gas remaining at the end of the treatment can be swept away before the chamber is opened. However, if similar engineering skill and care is given to the design and construction of chambers operating at atmospheric pressure these too can be made to operate with equivalent safety.
EXISTING TECHNIQUES AND FUMIGANTS
Vacuum-Rdeased Technique. Early workers on vacuum fumigation attributed the effectiveness of the short exposure periods permissible in that method to the ease with which the fumigant penetrated into packages once the obstacle presented by the intergranular air had been largely removed, and they thought this effect could be enhanced by restoring the pressure to atmospheric immediately after introduction of the fumigant. It was assumed that the inrushing air would drive the fumigant into the package. This method was referred to as the 'dissipated vacuum' or 'vacuum released' technique. There has been some criticism of these terms. 1 Simultaneous Introduction of Air and Fumigant. In France and the French overseas territories a modification of this method was adopted which was widely used in vacuum fumigation installations there. This method resulted from researches in Algiers 2,3 and is best described as 'vacuum fumigation with simultaneous introduction of air and fumigant.' After
the initial reduction of pressure in the loaded chamber an air fumigant mixture is introduced of such composition that the desired close of fumigant is delivered in full by the time the pressure has risen to a value a little below atmospheric. Sustained-Vacuum Method. Other workers, for example in the United States and Canada, found that more effective treatment of the products they tested was obtained by maintaining the reduced pressure throughout the period of treatment. This method is usually referred to as the 'sustainedvacuum' method. It should be emphasized in this connection that the introduction of the dose of fumigant usually produces only a small rise in pressure. Means of stirring or circulating the air and fumigant within the chamber is sometimes provided. Air-Washing Process. With all these techniques it has usually been customary, at the end of the period of treatment, to apply what has been termed an 'air-washing process.' In this, the pressure in the chamber is alternately reduced to a low figure and restored to atmospheric by the introduction of fresh air. It has been assumed that this process is particularly effective in removing the fumigant before the chamber is opened. In describing these various processes much confusion can be avoided ff all low pressures are expressed in terms of absolute pressure and this practice will be followed here. Fumigants. The above methods of vacuum fumigation were developed to allow more efficient u s e of the older fumigants such as hydrogen cyanide and ethylene oxide or mixtures of ethylene oxide and carbon dioxide. These fumigants have now been largely replaced by methyl bromide in the disinfestation of many types of stored products both at reduced pressures and at atmospheric pressure although the use of ethylene oxide for sterilisation against moulds and bacteria is increasing. Mixtures of acrylonitrile and carbon tetrachloride have been recommended for the vacuum fumigation of tobacco and tobacco products. Carbon disulphide is usually considered unsuitable for use in vacuum chambers because of its extreme inflammability, although at one time mixtures with carbon dioxide were in use.
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W. BURNS BROWN
RECENT RESEARCH FINDINGS
ACTIVITIES
AND
Fumigant Concentrations A clearer understanding of the behaviour of the fumigant in these different processes has resulted from recent investigations, in which the variation of concentration of fumigant at selected points has been followed throughout the treatment by repeated gas-sampling and chemical analysis, a to Tests made on a variety of commodities confirm that penetration of fumigant is most rapid with the sustained-vacuum method. Differences between fumigation at atmospheric pressure and the various reduced pressure procedures were more pronounced with the older fumigants, hydrogen cyanide and ethylene oxidef1,8 Penetration of methyl bromide into coarsely granular products, such as wheat and groundnuts, was so rapid at atmospheric pressure that it was concluded 4#~ there could be little advantage in using vacuum methods unless there were some important biological effects associated with the use of reduced pressures. Important differences were obtained with this fumigant, however, in tests on products into which penetration is more difficult. 4 ~,to Judged by the concentration measurements made during the fumigation period the results by the 'released vacuum' method and by the method with simultaneous admission of air and fumigant showed little difference from those obtained by fumigation at atmospheric pressure. In the sustained-vacuum method, however, much more rapid penetration to the centres of packages was obtained resulting in a more efficient distribution of fumigant in the shortperiod treatment. It was also found ~ ,~,to that restoration of the pressure to atmospheric at a time when penetration had already taken place to a substantial degree, resulted in a sudden, large rise in concentration at the centre of the package. This is caused by contraction of the air-fumigant mixture which is within the package as the pressure rises. The phenomenon is not observed or is little pronounced and transient only, if the pressure is allowed to rise immediately after or during introintroduction of the fumigant. It was suggested that
78
this effect should be used to equalise the mean concentrations obtained at the surface and the centre of the package. However, all recent work has shown that the usual air-washing techniques are singularly ineffective in lowering the concentrations at the centre of a package of sorptive material and, as a result, the total integrated c.t. product for the fumigation and airing times together may be much higher at the centre than at the surface where the concentration rapidly falls to an ineffective level at the start of airing. Further investigation is necessary to determine the best technique for any particular commodity. It has been suggested ~ that for some commodities excessive c.t. products at the centres of packages could be avoided by removal of the residual adsorbed fumigant at reduced pressure. This could be effected by continuous evacuation of the chamber while allowing a controlled admission of air to sweep away the fumigant from the free space, the absolute pressure in the chamber being maintained at a few centimetres Hg. Alternatively, the production of high concentrations at the centre might be limited by restoring the pressure to atmospheric earlier in the process or by a careful choice of thc working pressure. Although the normal repeated airwashings have only a limited effcct on the conccntrations at the centres of packages the final restoration of the pressure to atmospheric has the effect of driving fumigant from the free space and surface layers of the package towards its centre, thus making it safer for persons to enter the chamber for unloading? ° The evidence would suggest, however, that little is to be gained by employing more than one reduction of pressure during this airing process. The efficiency of penetration in the sustained vacuum method is satisfactorily explained by the fact that thc rate of diffusion of a gas through air is inversely proportional to the total absolute pressure. Thus, diffusion at a pressure of 7.6 cm. Hg is ten times as fast as at 76 cm. Hg and there is little, if any, effect on this ratio when thc diffusion is occurring through the intergranular spaces of a mass of finely divided sorptive material, although the actual rates of diffusion will thcn be greatly
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W. B U R N S B R O W N
reduced 5. Sorption of a fumigant by a finely divided product freely exposed (e.g. in a thin layer) appears to be unaffected by the total pressure. 11 Sorption by freely exposed granular materials such as grains or pulses might be expected to be greater at lower pressures because of more rapid diffusion into the individual grains, but experimental data on wheat do not show this. 12
Exposure to low pressure alone, in the ranges of pressure and time normally employed in vacuum fumigation, may produce a significant mortality with some insects but with others, such as adult grain weevils, there is little obvious effect.13 In recent investigations with methyl bromide t4,t5 an attempt has been made to find out the extent to which the c.t. products required for a complete kill
by Courtesy of Science Service, Canada Dept. of Agriculture and the Editor of J. Scl. Fd. Agric. Fig. 5 Rectangular chamber of about 35 m. a (I,2OO ft. s) capacity operated by the Canadian Government.
Insect Mortality The effects of variation of pressure upon the response of insects to particular concentrations of fumigants have frequently been offered as an explanation of the effectiveness of vacuum fumigation methods.
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in fumigations at reduced pressure differ from those required at atmospheric pressure. These tests were made on insects exposed in an empty chamber. Factors which affect the comparison are the species and stage of insect, the temperature, the relative humidity, the absolute pressure and the period of
VACUUM FUMIGATION reduced pressure. In most cases the insects were more susceptible at reduced pressure than at atmospheric pressure, sometimes by a factor of between 2 and 3, but in other cases there was little or no difference. Thus in tests on all stages of the grain weevil in wheat grains, although the order of resistance of these stages differed in the different fumigation techniques, it was concluded that the c.t. product required for a high percentage kill (99.9) of all stages was little affected by the technique. There is some evidence, 12 clearest in tests on adult grain weevils with hydrogen cyanide, that the shape of the concentration-time curve affects the response of the insects to a particular c.t. product, and that the optimum effect is obtained when the concentration rises rapidly at the start of the fumigation. This effect would contribute to the efficiency of the sustained vacuum method in which penetration to the centres of packages is more rapid than in other methods. On the whole it would appear that it will seldom be possible to rely upon a marked increase in susceptibility to the fumigant resulting from the use of a particular technique of vacuum fumigation. Although this effect may sometimes be of considerable practical importance, it will usually be necessary to base the selection of fumigation technique first of all upon the achievement of an even distribution of c.t. products throughout the treated commodity within the fumigation and airing periods. The minimising of the time taken for a complete cycle of operations will usually be the second most important consideration, and, in practice, some compromise between these needs may have to be accepted. Assuming that the technique achieves an even distribution of c.t. products, then the shorter the fumigation time allowed, the larger the dose of fumigant which will be required. In general, the doses of fumigant required in vacuum fumigation, with the relatively short periods of fumigation which are customary, are greater than in fumigation at atmospheric pressure over the usual periods of 24 or 48 hours, though according to much of the older published evidence, the differences are less than might be expected.
ENGINEERING OF VACUUM FUMIGATION PLANT Full advantage can be taken of the greater speed of fumigation possible in a vacuum chamber only if the installation allows rapid loading and unloading of goods and efficient control of all operations. In many of the larger, commercial installations the goods are loaded on to low trolleys or trucks which can be quickly pushed into, or out of the chamber, see Figs. 1-4. Where the trucks run on rails, moveable sections of rail must be provided for bridging the gaps at the chamber doors. Alternatively, the goods can be handled on trays or pallets, see Fig. 5. Vacuum fumigation chambers are frequently housed in a building which has been specially designed as a disinfestation or quarantine station, complete with control and machinery rooms, laboratory and offices. Fig. 6 shows the layout for a small station containing one large and one small chamber. These have doors at both er/ds and are erected through a wall which divides the main hall into a 'dirty' side where the infested goods are received and loaded into the chambers, and a "clean' side where the fumigated goods are taken from the chambers and despatched. This segregation of infested from fumigated goods is most desirable and, particularly where flying insects may be encountered~ the dividing wall should be extended to the roof to provide a complete insect barrier. The layout in Fig. 6 provides for delivery and collection of goods by road, and for fumigation on trolleys running on a rail system. The fumigation controls are grouped in a control room adjacent to the rooms housing pumps and other equipment. Figs. 7 and 8 show parts of the control rooms in two large installations with recording instruments and various visual aids to correct operation. A variety of automatic devices can be incorporated into the installation to prevent faulty operation or damage to the pumps, and to safeguard the operators. Of course, the smaller types of vacuum fumigation plant such as might be required in a factory for occasional treatment of either raw materials or
81
W. B U R N S B R O W N
finished products, can be compactly arranged in a relatively small space, see Fig. 9. Small vacuum fumigation units have sometimes been mounted on a lorry (Fig. 10) or trailer.
dished either inwards (Fig. l) or outwards (Figs. 2 and 3) but in rectangular chambers a fiat door is usually designed with suitable r e i n f o r c e m e n t (Figs. 4 and 9). T h e door may lze hinged at the side (Figs. 1, 2, 9 and 10), or it may be hinged at the top and counterbalanced (Fig. 3). Alternatively, it may slide to thc side (Fig. 4), or swing to the side by means of a davit (Fig. 5), or lift upwards (Fig. 11). Where hinges are used they usually allow some freedom of movement so that
Chamber Design Chambers are almost invariably of welded steel construction. A cylindrical chamber is cheaper to construct than a rectangular chamber of equal capacity, but the latter is sometimes preferred for
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by Courtesyof Degesch (l~rankfurt[Main) Fig. 6 D e s i g n for a disinfestation station c o m p r i s i n g one c h a m b e r of 5° m . "~(i,75 o ft. •) and one c h a m b e r of 5 m-:~ (x75 ft. ~) capacity, s h o w i n g t h e loading and u n l o a d i n g facilities, a control room, r o o m s h o u s i n g the v a c u u m p u m p s and other apparatus, offices and store rooms.
cased goods because of better utilisation of the space. T h e shell is usually reinforced by welding rings or girders on to it. Fig 11 shows an unusual chamber of reinforced concrete construction designed to take full-size railway wagons. D o o r s may be provided at one or at both ends of the chamber. In cylindrical chambers they are usually
82
the door may be drawn tightly on all sides against a soft rubber gasket. In the chambers illustrated in Figs. 2, 3 and 10, a hollow tube is used as gasket, and the seal between chamber and door is effected by pumping air or water into this tube. Many doors are fitted with special mechanisms for rapid closing.
VACUUM FUMIGATION
Fig. 7 The control room for the installation illustrated in Fig. I.
Fig. 8 The control panel of an installation consisting of four chambers.
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W. B U R N S B R O W N
by Courtesy of Degesch (Frankfurt/Main)
Fig. 9 A compact installation with a chamber o f 5 m. 3 (I75 ft. a) capacity. Fig. io A fumigation unit with a chamber of 5 m.3 (I75 if.a) capacity m o u n t e d on a lorry. by Courtesy of Mallet S. A. (Paris) .... ]
84
VACUUM FUMIGATION
Pumps and Pumping The pumps may be of a rotary or a reciprocating type. The plants illustrated in Figs. 2 and 3 employ rotary vacuum pumps whereas the plants shown in Figs. 1 and 4 are provided with large single-stage, slow-speed piston pumps. The performance is usually such that the pressure in the chamber can be reduced from atmospheric to the normal working level in less than ten minutes. Views differ as to the amount of pressure reduction required in practice. In some plants the pressure normally employed is around 12.5 cm. Hg absolute. More frequently a pressure of about 5 cm. Hg is used and some workers vary the reduced pressure according to the type of commodity treated. Pressures as low as 1 cm. Hg have been suggested for some finely divided productslL Circulation. In some plants, operating on the sustained-vacuum technique, pipe connections are arranged so that after the initial evacuation the pump can be used to circulate the residual air through the chamber during the introduction of the fumigant and for a period of 5 to 15 minutes afterwards. It has been stated is that the increase in efficiency of distribution of fumigant resulting from this circulation permits a useful reduction in dose. When this type of circulation is not employed it has been found advantageous to introduce the vaporised fumigant at multiple gas-inlets arranged, for example, along the four longitudinal angles of a rectangular chamber. In a small chamber use of a stirring fan, mounted on a shaft passing through a vacuum-tight bearing, is sufficient to ensure even distribution of fumigant through the free space of the chamber. Fumigant Introduction. The apparatus provided for the introduction of fumigant varies according to the choice of fumigant and the method of vacuum fumigation employed. For liquid fumigants such as methyl bromide, ethylene oxide or hydrogen cyanide a heated vaporiser with water or steam jacket is usually provided. In some German plants where the system of circulating at reduced pressure is employed, the pressure at the outlet side of the pump is adjusted by means of a regulating valve to about one atmosphere. The heat produced at this
point can be utilised for vaporisation of fumigant. Thus, when the absorbed form of hydrogen cyanide fumigant is used, this warm air stream is passed through the generator which incorporates a can-opening device and it is claimed that vaporisation is complete within a few minutes.
by Courtery of Mallet S. A. (Paris) Fig. I I A large c h a m b e r of reinforced concrete construction designed to take railway wagons.
Where the method of vacuum fumigation with simultaneous introduction of air and fumigant is employed, a counter-balanced gas holder is provided to hold the vaporised fumigant at atmospheric pressure. From this it is withdrawn and mixed in the correct proportion with air in a special valve at the entry to the chamber.
85
W. BURNS
The dose of liquid fumigant is measured by weight during delivery from the cylinder or by application through a calibrated glass vessel.
Ventilation and Heating In most installations the vacuum pump is used to remove fumigant remaining in the chamber at the end of the treatment. Usually the pressure is alternately reduced to a low figure, about 5-10 cm. Hg and restored to atmospheric by the introduction of fresh air as described earlier in this paper. In some of the larger German plants, however, after raising the pressure to atmospheric, the fumigant is swept out at this pressure by a high pressure blower, and repeated reductions of pressure are given only in cases where it is thought that efficient removal of gas is particularly important or difficult. Many chambers in the colder countries are provided with some means of heating. This should assist satisfactory distribution of the fumigant. Any rise in temperature is also likely to increase the susceptibility of the insects present, but, of course, many types of package would have to remain in the chamber for a considerable time before such heating could affect the inner layers. It may be more practical to arrange for the commodity to be brought to a suitable temperature before it is loaded into the chamber. CONCLUSIONS Although vacuum fumigation methods have been in use for more than forty years and considerable engineering skill has been applied to the design of such installations, there is still a confusing variety of views on the respective merits of the different operating procedures. Recent investigations have shown how variations of technique can effect the behaviour of the fumigant and the expected efficiency of the treatment. In many respects the picture is more complex than was visualised by earlier workers, and further experimental work will be required to establish clearly which techniques
BROWN
are best suited to the treatment of particular commodities. The basic requirements for a vacuum fumigation plant are, however, likely to remain, as now: One or more chambers of a design which allows rapid loading and unloading and closure of the doors; vacuum pumps for reducing the pressure of a chamber to 2-3 cm. Hg within 10 minutes; means for measuring and vaporising the dose of fumigant, and instruments for indicating or recording pressures in the chambers. It is advantageous, in the case of larger chambers, to provide means for stirring or circulating the air and fumigant in the chamber during, and for a period after, the introduction of the fumigant, and in temperate climates some heating of the chamber is probably worthwhile. Vacuum fumigation methods are most worthy of consideration in stations undertaking disinfestation of a wide variety of commodities for the purposes of quarantine or in connection with processing plants where rapidity of treatment is considered to be of paramount importance. Vacuum fumigation is unlikely to compete economically with fumigation at atmospheric pressure for the treatment of very large bulks of commodity, particularly of those stored in circumstances which allow fumigation without any additional handling.
This paper is published by permission of the Department of Scientific and Industrial Research. Grateful acknowledgment is made to Deutsche. Gesellschaft ffir ScMdlingsbekampfung m.b.H, (Degesch), Frankfurt (Main), to Mallet S.A., Paris and to the Canadian Department of Agriculture, Science Service, for permission to publish the various pictures accompanying this article. All these organisations have also been kind enough to provide information on the design and operation of vacuum fumigation installations. Certain features of some of the plant referred to are subject to Patent regulations.
For References see opposite page.
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VACUUM FUMIGATION
References 1 PAGE, A. B. P., BLACKITH, R. E., BURNS BROWN, W. and HE~rSER, S. G., Chem. & Industr., (I953), 353 'Descriptive Terms for Vacuum Fumigation.' 2 LEPIGRE, A. L., Chim. & Industr., 32, (June 1934) 'D&infection sous Vide Partiel. Rapport sur les Recherches Entreprises fi l'Insectarium d'Alger (Choix d'une Technique de vide).' z LEPIGRE, A. L., Documents Phytosanitaires, No. 9, (Serie Entomologie), 1949. 'La Ddsinsectisation par Fumigation avec Vide Prdalable.' 4 BURNSBROWN, W. and HEUSER,S. G.,ft. Sci. FoodAgric., 4, (I953), 48. 'Behaviour of Fumigants during Vacuum Fumigation I. Penetration of Methyl Bromide into Boxes of Dates.' BURNSBROWN,W. and HEUSER,S. G.,J. Sci. FoodAgric., 4, (1953), 378. 'Behaviour of Fumigants during Vacuum Fumigation II. Penetration of Methyl Bromide into Bagged Wheat Feed, and Related Diffusion Experiments.' BURNSBROWN,W. and HEUSER,S. G.,J. Sci. FoodAgric., 7, (1956), 595. 'Behaviour of Fumigants during Vacuum Fumigation III. Penetration of Methyl Bromide into Bagged Whalemeat Meal.' NAHAL, A. M. EL, ft. Sci. Food Agric., 4, (I953), 517. 'Fumigation of Agricultural Products VI. Penetration and Sorption of Hydrogen Cyanide in Wheat Fumigated at Reduced Pressure.' NAHAL, A. M. E L , J . Sci. FoodAgric., 5, (I954), 205. 'Fumigation of Agricultural Products VII. Penetration and Sorption of Ethylene Oxide in Wheat Fumigated at Reduced Pressures.'
9 NAHAL, A. M. EL, ft. Sci. Food Agric., 5, (I954), 369. 'Fumigation of Agricultural Products VIII. Penetration and Sorption of Methyl Bromide in Wheat Fumigated at Reduced Pressures.' 10 MUNRO, H. A. U. and KING, J. E., ft. Sci. Food Agric., 5, (I954), 619. ' T h e Behaviour of Methyl Bromide in the Vacuum Fumigation of Jute Bags.' 11 HEUSER, S. G., Paper in preparation. 12 PAGE, A. B. P. and BLACKIXH,R. E . , J . Sci. Food Agric., 5, (I954), 373. 'Fumigation of Agricultural Products IX. Sorption of Fumigants at Reduced Pressures.' 1:5 NAHAL,A. M. EL, Bull. Ent. Res., 44, (1953), 651. 'Responses of Pests to Fumigation IV. The Responses of Calandra spp. to Reduced Pressures.' 1~ Pest Infestation Research. London, 1953), 29.
(H.M.
Stationery Office,
15 Pest Infestation Research. London, 1954), 33.
(H.M.
Stationery Office,
1G NAI-tAL,A. M. EL, Bull. Ent. Res., 44, (I953), 641. 'Responses of Pests to Fumigation III. The Fumigation of W]aeat Containing Calandra spp. (Curculionidae) with Three Fumigants, under Reduced Pressure.' ~ Gouvlzs, P. LE, Private communication. ,8 COTTON,R. T., Pests of Stored Grain and Grain Products. (Burgess Publishing Co., 1956), Ch. 12.
87
Vacuum, Vol. VZ
VACUUM FUMIGATION W. Burns Brown, M.Sc.,
A.R.C.S., D.I.C., F.R.I.C.
Department of Scientific and Industrial Research, Pest Infestation Laboratmy, London Road, Slough, Bucks.*
Summary
Sommaire
SOME OF THE TECHNIQUESof fumigation employing reduced pressures are described. Recent investigations into the behaviour of the fumigant during treatments by these techniques and the effect of the technique upon the resistance of insects are reviewed. Features of some representative installations for vacuum fumigation are briefly described.
DESCRIPTION DE QUELQUES PROCI~DI~S de fumigation a pressions r~duites. On passe en revue les recherches r~centes faites sur le comportement du gaz pendant traitement par ces techniques et l'effet sur la r~sistance des insectes. Br~ve description d'installations pour la d~sinfection sous vide.
INTRODUCTION
erial is also usually related to the level of this 'c.t. product.' For this reason it is always desirable to employ a method of fumigation which will produce c.t. products throughout the treated material which are effective without being unnecessarily high. A great deal of fumigation is carried out at atmospheric pressure in chambeis, in sealed warehouses, in barges and ships' holds and under gas-proof sheets. In addition, many who undertake fumigation make use of special chambers, usually of steel, in which the pressure can be reduced substantially below atmospheric. The details of the technique are varied but all such methods are commonly described by the term 'vacuum fumigation.' In fumigation at atmospheric pressure, penetration of the gas to the centre of a bag or into a package may be quite slow and a relatively long period of treatment, say 24 hours or occasionally longer, may be necessary to ensure effective treatment throughout the material. Early tests showed that vacuum fumigation could be effective in much shorter periods, such as 1-4 hours. The time factor in itself is frequently unimportant, especially when dealing with a regular, large throughput of commodity, but
FUMIGATION PROVIDES a very important means for controlling infestation by insects and mites in a wide range of materials, for instance cereals and cereal products, pulses, oil seeds, animal feeding stuffs, cocoa, nuts and dried fruits; tobacco; natural fibres and clothing; hides and skins; timber and packaging materials; fresh fruits and growing plants. A recent additional use of fumigation has been the cold sterilisation of such materials as hospital equipment and certain foodstuffs, for example, the spices used in the preparation of meat products. Fumigants have the ability to penetrate into packages and through bulks of material and to destroy the pests at all stages of their development, whether freely exposed or hidden inside granular material. To be effective, a sufficient concentration of the fumigant must he maintained for a sufficient length of time. It has been found that the toxic effect is usually related to the integrated product of concentration and exposure time. Any deleterious effect on the mat* M.S. received April, 1957
75
W. B U R N S B R O W N
by Courtesy of Degesch (Frankfurt/Main)
Fig. 1 View o f a large installation capable of f u m i g a t i n g 5oo tons of m i x e d c o m m o d i t i e s daily. I t comprises four c h a m b e r s , each of 50 m . 3 (I,75o ft. 3) capacity, a n d in addition one of 2o m. 3 (7oo ft.3), one of 6 m . 3 (21o ft. a) a n d one of 2 m . 3 (7o ft.3). See also F,g. 7. P'z~¢. 2 View o f a large installation c o m p r i s i n g c h a m b e r s of 70, 50, 20, IO, 4 a n d 2. 5 m. a (2,450, 1,75o, 7o0, 35o, I4o and 9 ° ft. 3) capacity. by Courtesy of Mallet S. A. (Paris)
76
VACUUM FUMIGATION it is often claimed that the circumstances demand rapid treatment or at least make this a valued convenience. Thus, short period treatment in vacuum chambers is often adopted in quarantine stations where a wide range of products must be fumigated according to a variety of specifications. Vacuum fumigation methods have a more obvious application in the treatment of those materials which are particularly difficult to fumigate at atmospheric pressure, such materials being leaf tobacco in bales or hogsheads, boxes of compressed dates and multiple packages of manufactured at'tides. It is further claimed for vacuum fumigation methods that they are intrinsically safer, since the treatment takes place in a vessel at reduced pressure from which no leakage can occur and residual gas remaining at the end of the treatment can be swept away before the chamber is opened. However, if similar engineering skill and care is given to the design and construction of chambers operating at atmospheric pressure these too can be made to operate with equivalent safety.
EXISTING TECHNIQUES AND FUMIGANTS
Vacuum-Rdeased Technique. Early workers on vacuum fumigation attributed the effectiveness of the short exposure periods permissible in that method to the ease with which the fumigant penetrated into packages once the obstacle presented by the intergranular air had been largely removed, and they thought this effect could be enhanced by restoring the pressure to atmospheric immediately after introduction of the fumigant. It was assumed that the inrushing air would drive the fumigant into the package. This method was referred to as the 'dissipated vacuum' or 'vacuum released' technique. There has been some criticism of these terms. 1 Simultaneous Introduction of Air and Fumigant. In France and the French overseas territories a modification of this method was adopted which was widely used in vacuum fumigation installations there. This method resulted from researches in Algiers 2,3 and is best described as 'vacuum fumigation with simultaneous introduction of air and fumigant.' After
the initial reduction of pressure in the loaded chamber an air fumigant mixture is introduced of such composition that the desired close of fumigant is delivered in full by the time the pressure has risen to a value a little below atmospheric. Sustained-Vacuum Method. Other workers, for example in the United States and Canada, found that more effective treatment of the products they tested was obtained by maintaining the reduced pressure throughout the period of treatment. This method is usually referred to as the 'sustainedvacuum' method. It should be emphasized in this connection that the introduction of the dose of fumigant usually produces only a small rise in pressure. Means of stirring or circulating the air and fumigant within the chamber is sometimes provided. Air-Washing Process. With all these techniques it has usually been customary, at the end of the period of treatment, to apply what has been termed an 'air-washing process.' In this, the pressure in the chamber is alternately reduced to a low figure and restored to atmospheric by the introduction of fresh air. It has been assumed that this process is particularly effective in removing the fumigant before the chamber is opened. In describing these various processes much confusion can be avoided ff all low pressures are expressed in terms of absolute pressure and this practice will be followed here. Fumigants. The above methods of vacuum fumigation were developed to allow more efficient u s e of the older fumigants such as hydrogen cyanide and ethylene oxide or mixtures of ethylene oxide and carbon dioxide. These fumigants have now been largely replaced by methyl bromide in the disinfestation of many types of stored products both at reduced pressures and at atmospheric pressure although the use of ethylene oxide for sterilisation against moulds and bacteria is increasing. Mixtures of acrylonitrile and carbon tetrachloride have been recommended for the vacuum fumigation of tobacco and tobacco products. Carbon disulphide is usually considered unsuitable for use in vacuum chambers because of its extreme inflammability, although at one time mixtures with carbon dioxide were in use.
77
W. BURNS BROWN
RECENT RESEARCH FINDINGS
ACTIVITIES
AND
Fumigant Concentrations A clearer understanding of the behaviour of the fumigant in these different processes has resulted from recent investigations, in which the variation of concentration of fumigant at selected points has been followed throughout the treatment by repeated gas-sampling and chemical analysis, a to Tests made on a variety of commodities confirm that penetration of fumigant is most rapid with the sustained-vacuum method. Differences between fumigation at atmospheric pressure and the various reduced pressure procedures were more pronounced with the older fumigants, hydrogen cyanide and ethylene oxidef1,8 Penetration of methyl bromide into coarsely granular products, such as wheat and groundnuts, was so rapid at atmospheric pressure that it was concluded 4#~ there could be little advantage in using vacuum methods unless there were some important biological effects associated with the use of reduced pressures. Important differences were obtained with this fumigant, however, in tests on products into which penetration is more difficult. 4 ~,to Judged by the concentration measurements made during the fumigation period the results by the 'released vacuum' method and by the method with simultaneous admission of air and fumigant showed little difference from those obtained by fumigation at atmospheric pressure. In the sustained-vacuum method, however, much more rapid penetration to the centres of packages was obtained resulting in a more efficient distribution of fumigant in the shortperiod treatment. It was also found ~ ,~,to that restoration of the pressure to atmospheric at a time when penetration had already taken place to a substantial degree, resulted in a sudden, large rise in concentration at the centre of the package. This is caused by contraction of the air-fumigant mixture which is within the package as the pressure rises. The phenomenon is not observed or is little pronounced and transient only, if the pressure is allowed to rise immediately after or during introintroduction of the fumigant. It was suggested that
78
this effect should be used to equalise the mean concentrations obtained at the surface and the centre of the package. However, all recent work has shown that the usual air-washing techniques are singularly ineffective in lowering the concentrations at the centre of a package of sorptive material and, as a result, the total integrated c.t. product for the fumigation and airing times together may be much higher at the centre than at the surface where the concentration rapidly falls to an ineffective level at the start of airing. Further investigation is necessary to determine the best technique for any particular commodity. It has been suggested ~ that for some commodities excessive c.t. products at the centres of packages could be avoided by removal of the residual adsorbed fumigant at reduced pressure. This could be effected by continuous evacuation of the chamber while allowing a controlled admission of air to sweep away the fumigant from the free space, the absolute pressure in the chamber being maintained at a few centimetres Hg. Alternatively, the production of high concentrations at the centre might be limited by restoring the pressure to atmospheric earlier in the process or by a careful choice of thc working pressure. Although the normal repeated airwashings have only a limited effcct on the conccntrations at the centres of packages the final restoration of the pressure to atmospheric has the effect of driving fumigant from the free space and surface layers of the package towards its centre, thus making it safer for persons to enter the chamber for unloading? ° The evidence would suggest, however, that little is to be gained by employing more than one reduction of pressure during this airing process. The efficiency of penetration in the sustained vacuum method is satisfactorily explained by the fact that thc rate of diffusion of a gas through air is inversely proportional to the total absolute pressure. Thus, diffusion at a pressure of 7.6 cm. Hg is ten times as fast as at 76 cm. Hg and there is little, if any, effect on this ratio when thc diffusion is occurring through the intergranular spaces of a mass of finely divided sorptive material, although the actual rates of diffusion will thcn be greatly
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W. B U R N S B R O W N
reduced 5. Sorption of a fumigant by a finely divided product freely exposed (e.g. in a thin layer) appears to be unaffected by the total pressure. 11 Sorption by freely exposed granular materials such as grains or pulses might be expected to be greater at lower pressures because of more rapid diffusion into the individual grains, but experimental data on wheat do not show this. 12
Exposure to low pressure alone, in the ranges of pressure and time normally employed in vacuum fumigation, may produce a significant mortality with some insects but with others, such as adult grain weevils, there is little obvious effect.13 In recent investigations with methyl bromide t4,t5 an attempt has been made to find out the extent to which the c.t. products required for a complete kill
by Courtesy of Science Service, Canada Dept. of Agriculture and the Editor of J. Scl. Fd. Agric. Fig. 5 Rectangular chamber of about 35 m. a (I,2OO ft. s) capacity operated by the Canadian Government.
Insect Mortality The effects of variation of pressure upon the response of insects to particular concentrations of fumigants have frequently been offered as an explanation of the effectiveness of vacuum fumigation methods.
80
in fumigations at reduced pressure differ from those required at atmospheric pressure. These tests were made on insects exposed in an empty chamber. Factors which affect the comparison are the species and stage of insect, the temperature, the relative humidity, the absolute pressure and the period of
VACUUM FUMIGATION reduced pressure. In most cases the insects were more susceptible at reduced pressure than at atmospheric pressure, sometimes by a factor of between 2 and 3, but in other cases there was little or no difference. Thus in tests on all stages of the grain weevil in wheat grains, although the order of resistance of these stages differed in the different fumigation techniques, it was concluded that the c.t. product required for a high percentage kill (99.9) of all stages was little affected by the technique. There is some evidence, 12 clearest in tests on adult grain weevils with hydrogen cyanide, that the shape of the concentration-time curve affects the response of the insects to a particular c.t. product, and that the optimum effect is obtained when the concentration rises rapidly at the start of the fumigation. This effect would contribute to the efficiency of the sustained vacuum method in which penetration to the centres of packages is more rapid than in other methods. On the whole it would appear that it will seldom be possible to rely upon a marked increase in susceptibility to the fumigant resulting from the use of a particular technique of vacuum fumigation. Although this effect may sometimes be of considerable practical importance, it will usually be necessary to base the selection of fumigation technique first of all upon the achievement of an even distribution of c.t. products throughout the treated commodity within the fumigation and airing periods. The minimising of the time taken for a complete cycle of operations will usually be the second most important consideration, and, in practice, some compromise between these needs may have to be accepted. Assuming that the technique achieves an even distribution of c.t. products, then the shorter the fumigation time allowed, the larger the dose of fumigant which will be required. In general, the doses of fumigant required in vacuum fumigation, with the relatively short periods of fumigation which are customary, are greater than in fumigation at atmospheric pressure over the usual periods of 24 or 48 hours, though according to much of the older published evidence, the differences are less than might be expected.
ENGINEERING OF VACUUM FUMIGATION PLANT Full advantage can be taken of the greater speed of fumigation possible in a vacuum chamber only if the installation allows rapid loading and unloading of goods and efficient control of all operations. In many of the larger, commercial installations the goods are loaded on to low trolleys or trucks which can be quickly pushed into, or out of the chamber, see Figs. 1-4. Where the trucks run on rails, moveable sections of rail must be provided for bridging the gaps at the chamber doors. Alternatively, the goods can be handled on trays or pallets, see Fig. 5. Vacuum fumigation chambers are frequently housed in a building which has been specially designed as a disinfestation or quarantine station, complete with control and machinery rooms, laboratory and offices. Fig. 6 shows the layout for a small station containing one large and one small chamber. These have doors at both er/ds and are erected through a wall which divides the main hall into a 'dirty' side where the infested goods are received and loaded into the chambers, and a "clean' side where the fumigated goods are taken from the chambers and despatched. This segregation of infested from fumigated goods is most desirable and, particularly where flying insects may be encountered~ the dividing wall should be extended to the roof to provide a complete insect barrier. The layout in Fig. 6 provides for delivery and collection of goods by road, and for fumigation on trolleys running on a rail system. The fumigation controls are grouped in a control room adjacent to the rooms housing pumps and other equipment. Figs. 7 and 8 show parts of the control rooms in two large installations with recording instruments and various visual aids to correct operation. A variety of automatic devices can be incorporated into the installation to prevent faulty operation or damage to the pumps, and to safeguard the operators. Of course, the smaller types of vacuum fumigation plant such as might be required in a factory for occasional treatment of either raw materials or
81
W. B U R N S B R O W N
finished products, can be compactly arranged in a relatively small space, see Fig. 9. Small vacuum fumigation units have sometimes been mounted on a lorry (Fig. 10) or trailer.
dished either inwards (Fig. l) or outwards (Figs. 2 and 3) but in rectangular chambers a fiat door is usually designed with suitable r e i n f o r c e m e n t (Figs. 4 and 9). T h e door may lze hinged at the side (Figs. 1, 2, 9 and 10), or it may be hinged at the top and counterbalanced (Fig. 3). Alternatively, it may slide to thc side (Fig. 4), or swing to the side by means of a davit (Fig. 5), or lift upwards (Fig. 11). Where hinges are used they usually allow some freedom of movement so that
Chamber Design Chambers are almost invariably of welded steel construction. A cylindrical chamber is cheaper to construct than a rectangular chamber of equal capacity, but the latter is sometimes preferred for
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by Courtesyof Degesch (l~rankfurt[Main) Fig. 6 D e s i g n for a disinfestation station c o m p r i s i n g one c h a m b e r of 5° m . "~(i,75 o ft. •) and one c h a m b e r of 5 m-:~ (x75 ft. ~) capacity, s h o w i n g t h e loading and u n l o a d i n g facilities, a control room, r o o m s h o u s i n g the v a c u u m p u m p s and other apparatus, offices and store rooms.
cased goods because of better utilisation of the space. T h e shell is usually reinforced by welding rings or girders on to it. Fig 11 shows an unusual chamber of reinforced concrete construction designed to take full-size railway wagons. D o o r s may be provided at one or at both ends of the chamber. In cylindrical chambers they are usually
82
the door may be drawn tightly on all sides against a soft rubber gasket. In the chambers illustrated in Figs. 2, 3 and 10, a hollow tube is used as gasket, and the seal between chamber and door is effected by pumping air or water into this tube. Many doors are fitted with special mechanisms for rapid closing.
VACUUM FUMIGATION
Fig. 7 The control room for the installation illustrated in Fig. I.
Fig. 8 The control panel of an installation consisting of four chambers.
83
W. B U R N S B R O W N
by Courtesy of Degesch (Frankfurt/Main)
Fig. 9 A compact installation with a chamber o f 5 m. 3 (I75 ft. a) capacity. Fig. io A fumigation unit with a chamber of 5 m.3 (I75 if.a) capacity m o u n t e d on a lorry. by Courtesy of Mallet S. A. (Paris) .... ]
84
VACUUM FUMIGATION
Pumps and Pumping The pumps may be of a rotary or a reciprocating type. The plants illustrated in Figs. 2 and 3 employ rotary vacuum pumps whereas the plants shown in Figs. 1 and 4 are provided with large single-stage, slow-speed piston pumps. The performance is usually such that the pressure in the chamber can be reduced from atmospheric to the normal working level in less than ten minutes. Views differ as to the amount of pressure reduction required in practice. In some plants the pressure normally employed is around 12.5 cm. Hg absolute. More frequently a pressure of about 5 cm. Hg is used and some workers vary the reduced pressure according to the type of commodity treated. Pressures as low as 1 cm. Hg have been suggested for some finely divided productslL Circulation. In some plants, operating on the sustained-vacuum technique, pipe connections are arranged so that after the initial evacuation the pump can be used to circulate the residual air through the chamber during the introduction of the fumigant and for a period of 5 to 15 minutes afterwards. It has been stated is that the increase in efficiency of distribution of fumigant resulting from this circulation permits a useful reduction in dose. When this type of circulation is not employed it has been found advantageous to introduce the vaporised fumigant at multiple gas-inlets arranged, for example, along the four longitudinal angles of a rectangular chamber. In a small chamber use of a stirring fan, mounted on a shaft passing through a vacuum-tight bearing, is sufficient to ensure even distribution of fumigant through the free space of the chamber. Fumigant Introduction. The apparatus provided for the introduction of fumigant varies according to the choice of fumigant and the method of vacuum fumigation employed. For liquid fumigants such as methyl bromide, ethylene oxide or hydrogen cyanide a heated vaporiser with water or steam jacket is usually provided. In some German plants where the system of circulating at reduced pressure is employed, the pressure at the outlet side of the pump is adjusted by means of a regulating valve to about one atmosphere. The heat produced at this
point can be utilised for vaporisation of fumigant. Thus, when the absorbed form of hydrogen cyanide fumigant is used, this warm air stream is passed through the generator which incorporates a can-opening device and it is claimed that vaporisation is complete within a few minutes.
by Courtery of Mallet S. A. (Paris) Fig. I I A large c h a m b e r of reinforced concrete construction designed to take railway wagons.
Where the method of vacuum fumigation with simultaneous introduction of air and fumigant is employed, a counter-balanced gas holder is provided to hold the vaporised fumigant at atmospheric pressure. From this it is withdrawn and mixed in the correct proportion with air in a special valve at the entry to the chamber.
85
W. BURNS
The dose of liquid fumigant is measured by weight during delivery from the cylinder or by application through a calibrated glass vessel.
Ventilation and Heating In most installations the vacuum pump is used to remove fumigant remaining in the chamber at the end of the treatment. Usually the pressure is alternately reduced to a low figure, about 5-10 cm. Hg and restored to atmospheric by the introduction of fresh air as described earlier in this paper. In some of the larger German plants, however, after raising the pressure to atmospheric, the fumigant is swept out at this pressure by a high pressure blower, and repeated reductions of pressure are given only in cases where it is thought that efficient removal of gas is particularly important or difficult. Many chambers in the colder countries are provided with some means of heating. This should assist satisfactory distribution of the fumigant. Any rise in temperature is also likely to increase the susceptibility of the insects present, but, of course, many types of package would have to remain in the chamber for a considerable time before such heating could affect the inner layers. It may be more practical to arrange for the commodity to be brought to a suitable temperature before it is loaded into the chamber. CONCLUSIONS Although vacuum fumigation methods have been in use for more than forty years and considerable engineering skill has been applied to the design of such installations, there is still a confusing variety of views on the respective merits of the different operating procedures. Recent investigations have shown how variations of technique can effect the behaviour of the fumigant and the expected efficiency of the treatment. In many respects the picture is more complex than was visualised by earlier workers, and further experimental work will be required to establish clearly which techniques
BROWN
are best suited to the treatment of particular commodities. The basic requirements for a vacuum fumigation plant are, however, likely to remain, as now: One or more chambers of a design which allows rapid loading and unloading and closure of the doors; vacuum pumps for reducing the pressure of a chamber to 2-3 cm. Hg within 10 minutes; means for measuring and vaporising the dose of fumigant, and instruments for indicating or recording pressures in the chambers. It is advantageous, in the case of larger chambers, to provide means for stirring or circulating the air and fumigant in the chamber during, and for a period after, the introduction of the fumigant, and in temperate climates some heating of the chamber is probably worthwhile. Vacuum fumigation methods are most worthy of consideration in stations undertaking disinfestation of a wide variety of commodities for the purposes of quarantine or in connection with processing plants where rapidity of treatment is considered to be of paramount importance. Vacuum fumigation is unlikely to compete economically with fumigation at atmospheric pressure for the treatment of very large bulks of commodity, particularly of those stored in circumstances which allow fumigation without any additional handling.
This paper is published by permission of the Department of Scientific and Industrial Research. Grateful acknowledgment is made to Deutsche. Gesellschaft ffir ScMdlingsbekampfung m.b.H, (Degesch), Frankfurt (Main), to Mallet S.A., Paris and to the Canadian Department of Agriculture, Science Service, for permission to publish the various pictures accompanying this article. All these organisations have also been kind enough to provide information on the design and operation of vacuum fumigation installations. Certain features of some of the plant referred to are subject to Patent regulations.
For References see opposite page.
86
VACUUM FUMIGATION
References 1 PAGE, A. B. P., BLACKITH, R. E., BURNS BROWN, W. and HE~rSER, S. G., Chem. & Industr., (I953), 353 'Descriptive Terms for Vacuum Fumigation.' 2 LEPIGRE, A. L., Chim. & Industr., 32, (June 1934) 'D&infection sous Vide Partiel. Rapport sur les Recherches Entreprises fi l'Insectarium d'Alger (Choix d'une Technique de vide).' z LEPIGRE, A. L., Documents Phytosanitaires, No. 9, (Serie Entomologie), 1949. 'La Ddsinsectisation par Fumigation avec Vide Prdalable.' 4 BURNSBROWN, W. and HEUSER,S. G.,ft. Sci. FoodAgric., 4, (I953), 48. 'Behaviour of Fumigants during Vacuum Fumigation I. Penetration of Methyl Bromide into Boxes of Dates.' BURNSBROWN,W. and HEUSER,S. G.,J. Sci. FoodAgric., 4, (1953), 378. 'Behaviour of Fumigants during Vacuum Fumigation II. Penetration of Methyl Bromide into Bagged Wheat Feed, and Related Diffusion Experiments.' BURNSBROWN,W. and HEUSER,S. G.,J. Sci. FoodAgric., 7, (1956), 595. 'Behaviour of Fumigants during Vacuum Fumigation III. Penetration of Methyl Bromide into Bagged Whalemeat Meal.' NAHAL, A. M. EL, ft. Sci. Food Agric., 4, (I953), 517. 'Fumigation of Agricultural Products VI. Penetration and Sorption of Hydrogen Cyanide in Wheat Fumigated at Reduced Pressure.' NAHAL, A. M. E L , J . Sci. FoodAgric., 5, (I954), 205. 'Fumigation of Agricultural Products VII. Penetration and Sorption of Ethylene Oxide in Wheat Fumigated at Reduced Pressures.'
9 NAHAL, A. M. EL, ft. Sci. Food Agric., 5, (I954), 369. 'Fumigation of Agricultural Products VIII. Penetration and Sorption of Methyl Bromide in Wheat Fumigated at Reduced Pressures.' 10 MUNRO, H. A. U. and KING, J. E., ft. Sci. Food Agric., 5, (I954), 619. ' T h e Behaviour of Methyl Bromide in the Vacuum Fumigation of Jute Bags.' 11 HEUSER, S. G., Paper in preparation. 12 PAGE, A. B. P. and BLACKIXH,R. E . , J . Sci. Food Agric., 5, (I954), 373. 'Fumigation of Agricultural Products IX. Sorption of Fumigants at Reduced Pressures.' 1:5 NAHAL,A. M. EL, Bull. Ent. Res., 44, (1953), 651. 'Responses of Pests to Fumigation IV. The Responses of Calandra spp. to Reduced Pressures.' 1~ Pest Infestation Research. London, 1953), 29.
(H.M.
Stationery Office,
15 Pest Infestation Research. London, 1954), 33.
(H.M.
Stationery Office,
1G NAI-tAL,A. M. EL, Bull. Ent. Res., 44, (I953), 641. 'Responses of Pests to Fumigation III. The Fumigation of W]aeat Containing Calandra spp. (Curculionidae) with Three Fumigants, under Reduced Pressure.' ~ Gouvlzs, P. LE, Private communication. ,8 COTTON,R. T., Pests of Stored Grain and Grain Products. (Burgess Publishing Co., 1956), Ch. 12.
87