Design of Gas Distribution Systems for cylinder based low volume phosphine applications to bulk grain

Abstract

The aim of the project was to develop a robust practical method of obtaining adequate concentrations of phosphine in all parts of farm and commercial floor-stored grain bulks. This has been achieved by a system based on the production of a peripheral positive pressure to balance the pressures caused by natural forces which cause ingress of air.

Trials were conducted on bulks of feed wheat of different shapes and different tonnages up to 370 tonnes and up to 4.5 m deep in a relatively well sealed bay. Cylinderised formulations of phosphine were used and dosed either via an automated dosing system or via a simple flowmeter. Initial trials utilised re-circulation using the in-floor ventilation system but this was abandoned since it produced an inconsistent distribution of phosphine and could not be used on unventilated bulks.

The principal of re-circulation by sucking dosed air from the centre of the bulk and delivering it to ducts that can be easily inserted around the periphery was then established. This has the effect of producing positive pressures around the edge of the bulk to prevent the ingress of air. A stepwise series of trials using this system when completely under a covering sheet showed that the optimum distance between delivery ducts for this particular store was about 5.25 m and an optimum economic dosing rate was established.

The design of the system was aided by Computational Fluid Dynamics (CFD) modelling which, though limited by uncertainty about the location of leaks in practice, proved useful in understanding the flows under different system designs and experimental conditions. It could give predictions of the percentage of the grain below a given concentration, the amount of phosphine required to replace losses and lateral and terminal flows through input ducts. It also showed that a modest 1 m s-1 wind did not affect to the efficiency of the system.

An understanding of the forces acting on the grain was obtained in trials by logging pressures and temperatures in the grain simultaneously with ambient conditions. This showed that gas density differences due to diurnal temperature changes and normal barometric pressure fluctuations caused gaseous exchange in the bulk. The high gas loss during high winds was due to relatively large but short-term atmospheric pressure fluctuations. The effect of varying the re-circulation rate was studied to provide an optimum for this store. In practice, the system cannot cope with short periods of strong wind but their effect can be overcome by extending the exposure period according to the duration of these conditions.

The system was trialed under commercial conditions in the same store and, while a uniform phosphine concentration was not possible, the variation was low and the system could guarantee the required minimum. This can easily be increased, according to the pest species or phosphine resistant strain present, by increasing the dosing rate.

These results form the basis for an efficient method of dosing phosphine in order to prevent the development of resistance and to control existing resistant populations. It is anticipated that larger bulks can be treated using multiples of the re-circulation system. It is expected that there will be an economy of scale possible in the efficiency of the system but further development is required to optimise the multiple system for different bulk shapes, grain depth, tonnages and economy of materials, labour and phosphine usage.

SUMMARY.

Introduction.

There is a potentially serious problem with resistance to the fumigant phosphine which has been used for over 30 years to fumigate bulk grain and bagged and packaged commodities in stacks. The resistance occurs in thirteen species of stored product insect pests and is widespread in many countries. Resistant strains have been known to exist in the UK for over 20 years but there is no recent survey data to give an accurate estimate of the incidence of these. However, there is likely to be sufficient genetic variation present within the grain trade to allow selection for resistance to occur in sub-standard fumigations. Resistant strains are known to require higher phosphine concentrations and longer exposure periods than for control of normal strains.

There is a decrease in the reliance on the admixture of organo-phosphorus insecticides for disinfestation of and protection of bulk grain due to health concerns and some have been withdrawn from the market. It is predicted that the use of phosphine will increase dramatically in both farm and commercial grain stores whenever cooling and drying strategies fail. At the same time, we are facing a reduction in use of the fumigant methyl bromide due to its implication in damage to the stratospheric ozone layer. In both developed and developing countries this will, inevitably, mean an increase in the use of phosphine, more low standard fumigations and more resistance. In addition, the often fortuitous counter-selection pressure provided by methyl bromide will not be available. Experts on fumigation, internationally, have warned of the reliance on phosphine, the only remaining grain fumigant, in the face of a growing resistance problem. However, dosages for the control of resistant strains using phosphine can generally be achieved with the use of new methods.

Fumigation of grain is used when there is an insect infestation detected and this commonly happens just prior to or when the grain is sold. The presence of such infestation can cause grain to be down-graded with loss of value and can cause trading problems and extra transportation costs. The problem is most acute in grain presented for export when total eradication is the aim. Phosphine fumigation is now being carried out prophylactically before grain is cooled and when insect numbers are low in order to help prevent a serious infestation from developing.

While a high standard of phosphine fumigation is possible in bins and silos, this can be difficult to achieve in floor-stored bulks which are inherently difficult to seal effectively. The general absence of any pre-sealing before the grain enters the store puts at risk the ability to attain the minimum exposure period required for full efficacy. The use of solid metal phosphide formulations in tablet and sachet form probed into the grain or, at worst, only into the surface layer cannot guarantee good gas distribution. This practice can also give erroneous estimates of free phosphine residues due to the unfortunate sampling of grain adjacent to a partially decomposed formulation. The biggest drawback of these formulations is that they give up their phosphine in only 2-4 days, according to temperature, and leakage of gas results in low concentrations or even complete loss of gas and an inadequate exposure period.

Previous projects financed by the H-GCA have shown the considerable advantages of using a cylinderised formulation of phosphine in carbon dioxide as a diluent. While cylinderised formulations are not currently cleared as pesticides in the UK, they are cleared in other parts of the world and are starting to appear in Europe. It is only a matter of time before they are available for commercial fumigators to use in the UK under harmonisation of registrations in the EU. Their main advantages are that any concentrations in the grain can be achieved by manipulating the flow of dosing gas and that gas can be introduced continuously in order to ensure sufficient fumigant during the entire exposure period.

They can also be used under the control of an automated dosing system for economy. They are safer to use and do not produce any solid phosphide residues and so, even at high dosage rates, the free phosphine residues are well below permitted limits. In terms of the actual kilogram cost of phosphine, they are more expensive than solid phosphide formulations. However, with careful sealing and the use of appropriate methods they have been shown to be competitive on cost since it is possible to use less phosphine per tonne of grain treated.

There is a pressing need to be able to consistently carry out phosphine fumigations with the goal of achieving total control of infestation in all life stages. This goal is at the core of any pro-active strategy to combat resistance. The alternative is to design fumigations which permit the survival of some insects in order to maintain genes for susceptibility to phosphine in a population. This classic resistance management strategy may be appropriate for some crop growing systems where an 'economic threshold' of pest incidence is possible. It is considered that this technically difficult strategy would not be acceptable to a grain trade seeking the highest control standards.

Unfortunately, some current fumigations are carried out to enable grain to be marketed and this may not necessarily require the total eradication of infestation. The control of the more susceptible active insect stages means that a survival consisting of eggs and pupae can be difficult to detect. While this approach can be attractive commercially, it is precisely this philosophy which can produce a selection pressure for resistance, when genes for its expression are present in a pest population. Even in a good fumigation there will be pockets of infestation, particularly at the bulk periphery, where ingress of fresh air produces dilution of phosphine and allows survival. This survival can cause immediate commercial problems and, if selection for resistance occurs, longer term consequences for the reliability of phosphine when it is used with current dosage schedules.

Objectives.

The overall brief for the current project was to produce a new dosing system and generate new recommendations and guide-lines for the practical application of phosphine for insect disinfestation of bulk grain in the UK. To achieve this end, a set of sub-objectives was identified to understand the problem and to progress, by practical trials, towards the goal. Firstly, to develop a design for a practical re-circulation system to achieve more even gas distribution in treatments of farm and commercial grain bulks. Secondly, to apply pressure logging methods to measure pressure distribution in grain bulks during fumigation in order to understand the physics of air re-circulation. Finally, to test the use of re-circulation to limit leakage at the bulk boundaries by balancing flow rates and external air pressures.

The project aims to ensure a general improvement of standards by the use of cylinderised formulations, the mixing of phosphine in the bulk by air re-circulation particularly targeted to protecting vulnerable areas in the grain. The system should be robust and controllable. It is expected that the careful application of this fumigation system will prevent the development of phosphine resistance. It can also be used to control resistant insects, where resistance tests have identified them, by the careful implementation of an appropriate dosage schedule.

The multi-disciplinary research approach.

A multi-disciplinary approach was taken to solve this complex problem. This involved the use of a model to predict the effect of different systems to show the way forward. This expertise was provided by the Silsoe Research Institute, BBSRC. Expertise on the physics of air movement, pressure measurement and the design of an engineered solution was supplied by ADAS Ltd., Silsoe. Chemical expertise on the application and measurement of phosphine in the grain and the environment was essential. Finally, biological expertise on the problem of phosphine resistance, toxicity of phosphine to insects and the use of a bioassay to demonstrate effectiveness was also necessary. This overall expertise in phosphine fumigation was supplied by CSL who also provided store and grain facilities and managed the project.

Modelling.

A complex computational model was designed using the principle of computational fluid dynamics to understand the gas flow in a bulk under controlled conditions. It could model the gas flows and predict gas velocities, pressures and temperatures throughout a bulk. The dimensions of the store, the covering sheet and the re-circulation inlets and outlets were reproduced in the model. The shape of the bulk, whether level, heaped or front-sloping, could be incorporated. The effect of wind on phosphine concentration was modelled. The model worked on wheat at 13 % m.c. but it is capable of working with any grain type if changes in parameters are programmed.

The plastic sealing sheet covering the grain was modelled as a thin impervious membrane. The model assumed likely leakage paths to occur at all edges of the covering sheet and at all edges of the bulk rear retaining wall. This proved useful though had the obvious limitation that, in practice, leakage points are unknown.

Tests in the store with a leak detector showed that the sheet edges were well sealed but it is possible that the phosphine concentrations were low in some locations due to other leaks in the vicinity, maybe from the floor/wall joint.

The findings from the model were very interesting and mainly applicable to the system in practice. The pressure distribution created by the re-circulation system itself leads to gas exchange between the bulk and the environment. The volume of grain below a defined concentration and the shape of this part of the bulk in relation to leaks could be predicted. It suggested that reducing the re-circulation rate reduces the leakage rate but not the affected grain volume. It could also estimate the loss of concentration of phosphine in air returning to the re-circulation fan. The significance of this, in practice, is that reducing the re-circulation rate will reduce the amount of phosphine to be replaced, a desired economy. The model predicted that the effect of a modest wind on leakage rates, when the system was running, were negligible. Also, assuming a well-sealed store, rapid changes in barometric pressure are more damaging to the phosphine concentration than is wind pressure. After adverse weather, phosphine concentrations throughout the bulk will recover most quickly by using a high re-circulation rate. Hence there is scope, certainly in multiple systems in large bulks, for linking the fan speed to an anemometer so that it will run faster for a set period after high wind ceases.

The experimental store and the trials.

A floor-ventilated bay in the Central Science Laboratory (CSL) Storage Research Unit at York capable of holding 400 tonnes of wheat and measuring 13.5 m by 10 m was used for all the trials. It was typical of many farm floor-stores, having a poured concrete plenum chamber of the ventilation system as one side. The other side was of sealed corrugated metal sheeting and the front and rear of the store consisted of wooden bulkheads. The wood was sealed with polyethylene film on the exterior and the lateral ventilation ducts were sealed in the normal commercial manner. In addition, the inside of bulkheads was sealed and the sheet extended 1 m over the bay floor before loading the grain as recommended in the H-GCA's 1999 Grain Storage Guide. A covering sheet was sealed with spray adhesive to the sheets at the front and rear on the bulk and it was pushed well into the grain along the sides, creating a slight leakage area. Different shapes of bulk, grain depth and different tonnages of feed wheat were examined.

In all trials, the grain and ambient temperatures were monitored and logged and the phosphine concentration in the grain was measured automatically by gas chromatography in a mobile laboratory located in front of the store.

Air pressure was measured with an electronic micro-manometer which could monitor up to 6 points inside and outside the bulk. In addition, a sealed reference chamber was located under the covering sheet. Ambient pressure measurements were made using a static pressure probe to minimise the dynamic effects of wind.

Phosphine was dosed in initial trials from cylinders containing the gas dissolved in liquid carbon dioxide (ECO2FUMER) and dosed by an automated system which measured the concentration in the grain and dosed as required. Later, the four main trials were dosed from cylinders containing a mixture of phosphine and compressed nitrogen (FRISINR). The first two of these were dosed by the automated dosing system and the last two by using a simple needle valve and flowmeter.

Initial trials aimed to attain a low but effective concentration of phosphine for economy. Re-circulation was achieved by using a low volume centrifugal fan outside the covering sheet and utilising the in-floor ventilation ducts, initially sucking from a central floor duct and delivering to ducts at the ends of the bulk. This was refined for a second trial so that sucking was from the central floor duct with delivery to the surface in the four corners. While these systems were a considerable improvement over existing practice, they did not give the required degree of mixing and left some areas with low concentrations. They suffered the obvious limitation that they could only be employed on bulks placed over a ventilation system. The last initial trial employed a more powerful fan placed under the covering sheet. This was connected to four 1 m long perforated ducts, two sucking at the bulk centre and two delivering centrally to both ends of the bulk. This system was suitable for all floor-stored bulks. The results from this trial were much more encouraging with good concentrations in most locations. This concept was taken forward to design a suitable commercial system and to optimise the delivery locations to reliably achieve a slight negative pressure at the bulk centre and a slight positive pressure at the periphery. The positive pressure would tend to protect this area from the pressure effects of wind and diurnal temperature fluctuations.

Following the initial trials, a step-wise approach was taken to achieve the design and the optimum spacing of the peripheral delivery ducts. Three systems were tested (trials 1-3) with experimental manipulations to the re-circulation rate and the dosing rate. A final trial 4 was carried out as it would be in commercial practice with these system parameters held constant.

In trials 1 and 2, a new re-circulation system was designed with a manual speed- controlled low power axial fume cupboard-type fan housed in a specially fabricated polypropylene fan box with six outlets to be used or sealed as required. This rested on an integral inlet shaft sunk into the grain by using a vacuum cleaner. Pipes ran from the fan house and were connected by flexible polypropylene pipe to polypropylene perforated outlet shafts put into the grain periphery in the same way. In these and subsequent trials the dosing gas was changed to the mixture of phosphine in nitrogen.

Trial 1 employed four outlet shafts near to the corners of a 250 tonne bulk in a heaped profile with the peak towards the back wall where the grain was 3 m deep. The pipe for dosing phosphine was positioned in the fan inlet shaft. The trial was run for nine days and during this period the fan speed was adjusted to give total flows ranging from 3.5 to 6 cubic metres per minute. Air pressures within the bulk were initially mapped with the fan running at full speed to discover the three-dimensional pattern of re-circulation flows and to identify regions at risk from inward leakage of air. Even with the re-circulation at full speed an even distribution of gas was never achieved and areas of low peripheral pressure were linked to low concentrations. Clearly, a leak at these locations would allow air to be drawn in.

Trial 2 utilised the same system but with two additional delivery ducts inserted in the centre of the longer sides of the bulk to eliminate the negative pressures noted in trial 1. The first two days of the 9-day trial were very windy and low concentrations were seen throughout the bulk but, after the wind reduced, the concentrations quickly recovered throughout. The distribution of phosphine was much more even than in trial 1. However, areas of negative pressure were now seen at the centres of the front and back walls due to the total flow being shared between six rather than four delivery ducts. This indicated a need for two more delivery ducts at these locations. This was made possible by a more 'user-friendly' re-design of the system to be used in trials 3 and 4.

The fan box of the new system had eight inlets as well as eight outlets of a similar design to the outlets in the previous design. All the inlets and outlets were connected to 1.5 m perforated sucking or delivery ducts as before. These were of a smaller diameter (0.1 m ID) than the ones used in trials 1 and 2 since the flow through each was less and they could be more easily inserted and removed from the grain. A total of eight peripheral delivery ducts could be served, one near each corner and one at the mid-point of each side. The fan box, itself, was re-designed to hold two fans positioned to give opposite flow directions and both were connected to a manual speed controller. The fans were separated by a diaphragm to form separate inlet and outlet chambers. Thus, the direction of flow could be reversed in order to take phosphine to any 'dead spots' in the bulk by-passed by circulation paths. The 250 tonne wheat bulk was re-shaped so the grain depth was 2.75 m. deep at the back with a plateau over more than half of the bay sloping to 1.25 m at the front. The eight sucking ducts were positioned in a circle with one in the centre and the fan box to one side This system was easier to use and is expected to be attractive commercially.

Dosing was via a simple needle valve and flowmeter since it was decided that for a single dosing point this would be simple and adequately economical. The dosing pipe delivered phosphine 1 m below the grain surface in the centre of the sucking ducts. The trial was run over a period of 26 days on order to access the effect of changes to the dosing rate and the re-circulation rate and direction. The concentrations were monitored for about 3 days at each flow pattern. Pressures within the bulk were monitored at the full fan speed as in trials 1 and 2.

The system worked well in the normal re-circulation direction with no areas of negative peripheral pressure seen. Positive pressure extended to the full depth of the bulk at the rear even though they were over 1 m. above the floor. This was due to a flow through the ends of the delivery ducts which could overcome any leaks in or near the floor. Phosphine concentrations were planned to be in excess of 0.1 g per cubic metre, a concentration well above the toxic threshold for insect pests.

During the first five days, the wind was far too strong for any fumigation of this type to be successful. After this, a period followed to the end of the trial when winds were no more than moderate. The target concentration was reached everywhere with the exception of one position though this was corrected when the flowrate was increased to an optimum for this bulk of about 1.83 cubic metres per minute. A reversal in the flow direction resulted in a less even phosphine distribution due to air being sucked from the edges. However, the flow direction was reversed for 7 days with changes of flowrate during this time. It is likely that only a 30-60 minutes period of flow reversal per day would be beneficial in reaching 'dead-spots' and this could easily be automated.

The project required that the final system be tested under commercial conditions on a larger tonnage of commercially-owned grain using a store of different dimensions to the CSL store. This was not possible since the cylinderised formulations are not yet approved as pesticides in the UK. Obtaining trials clearance was too expensive for the project budget and so the CSL store was used but the fumigation was run as in commercial practice without experimental manipulations. The grain tonnage was increased to 370 tonnes with a peak behind the bulk centre. Exactly the same system was used as in trial 3 except that one sucking duct was located in the grain peak to ensure that it was treated adequately. The fan box was located in front of the peak. The optimum running parameters for the store and tonnage were used: 0.06 g phosphine dosed per minute via a flowmeter and a slightly higher recirculation rate of 2.9 cubic metres per minute.

The weather during the trial was exceptionally calm and a good distribution of phosphine rapidly resulted. After running the trial for 12 days, the flow of phosphine was considerably reduced in order to mimic the effect of a period of high wind. The phosphine concentration reduced but on return to near the original dosing rate it rapidly increased. Once again, a good positive pressure distribution was noted around the bulk periphery. The effectiveness of the treatment was assessed by the inclusion of a bioassay of a common pest, the Rust-red grain beetle, Cryptolestes ferrugineus. Caged naturally tolerant pupae of a phosphine-susceptible strain were inserted at all grain depths at two locations previously shown to have the lowest concentration, including the mid-point between two peripheral input ducts. The dosing gas was switched off after 15 days and the insect samples retrieved and emergence of adults beetles compared to untreated controls. All the fumigated pupae were killed showing that a typical infestation of this pest throughout the bulk would have been eradicated.

Grain samples taken after the fumigation at representative locations were analysed and found to contain free phosphine residues well below the permitted limit. Indeed, these would have been somewhat pessimistic since some of the grain had been fumigated in other trials. The residue would be expected to reduce drastically upon further airing.

It is important to note that these were experimental fumigations and so the minimum concentration target in the grain was in the normal toxic range, though chosen in order to conserve a limited supply of dosing gas. The system readily allows the target concentration to be increased in order to control more naturally tolerant pest species or resistant strains by simply increasing the phosphine dosing rate appropriately. The use of phosphine in trial 3 was approximately 4 g per tonne but this was required in order to achieve a much longer exposure period than would be normally be necessary. The use in trial 4 was approximately 2 g per tonne and gives a more realistic estimate of what would be used in practice over normal exposure periods for tolerant species. This amount is much less than normally used for solid phosphide formulations due to the savings from the prevention of unequal distribution and leakage. The cylinderised formulations are expected to be slightly more expensive in terms of cost per gram of phosphine. The overall cost of fumigant would not be excessive and, in any case, is a small proportion of the total cost of a fumigation which is mainly made up of labour charges.

Key results, conclusions and implications.

1.

A practical method of re-circulation of phosphine in bulk grain has been developed which is robust enough to cope with most of the leakage and physical forces acting on a bulk. It can be used in both ventilated and unventilated storages. A paper describing was presented in October 2000 at the International Conference on Controlled Atmospheres and Fumigation in Stored Products held in Fresno, USA where it was well received.

2.

This method of re-circulation involves suction from perforated ducts in the centre of the bulk and delivery to similar ducts at the edge. This has the effect of creating positive pressure around the edge of the bulk which prevents the ingress of air. The ducts are easily inserted and removed from the grain.

3.

The system can be dosed using either FRISINR (phosphine in compressed gaseous nitrogen) or ECO2FUMER (phosphine in liquid carbon dioxide). A simple needle valve and flowmeter can be used but the gas can be used more economically using the CSL automated dosing system. It is important that the dosing gas supply does not run out and a suitable amount of manifolded cylinders will be required for the whole treatment unless they are changed manually during the treatment.

4.

It was not possible to fully meet the objective of maintaining a completely uniform concentration of phosphine within a bulk since the method does not rely on a total atmosphere replacement. However, the system can readily distribute and maintain concentrations within a relatively small range. It can produce a minimum concentration which is effective against all stages of both susceptible and resistant strains of common insect pest species at temperatures for which data exists. It is predicted that the dosing system can produce the required concentrations and exposure periods to control even the most resistant populations at low temperatures.

5.

The engineering objective to minimise leakage under a range of climatic conditions, apart from very strong winds, has been achieved.

6. Adequate concentrations are not maintained during periods of high winds with this system but they are re-established within a short time once the winds have died down. An extension to the fumigation would be required in order to replace periods of low concentration.

7.

Re-circulation of phosphine within the grain bulk is required to produce adequate concentrations of phosphine in all parts of the bulk.. There is an optimum re-circulation rate. A minimum is required to produce the necessary pressures in the grain but too high a rate requires more dosing gas and is less economic.

8.

The project has provided an understanding of the leakage mechanisms:

a. The high rate of gas loss observed during high winds is caused by short term fluctuations in atmospheric pressure. During calm periods the pressure was seen to cycle through as much as 20 Pa (typically 8-12 Pa) within a period of 4-5 minutes. In windy periods the pressure could cycle through 60 Pa.

b. Gas density difference due to diurnal temperature changes causes gaseous exchange.

c. Normal barometric pressure fluctuations cause gaseous exchange.

d. Diffusion of gases is not important.

9.

The system design features for the effective system are:

a. The pressure at the centre of the bulk should be negative. This is provided by a fan(s) located so they do not influence the pressure at the periphery.

b. The pressure at the periphery of the bulk should be positive and a spacing of inlet ducts of about 5.25 mis indicated for this store. It will be different for other stores; more in larger stores and less in smaller ones.

c. Perforated ducts need to be sunk into the grain at the bulk centre and at the periphery.

d. The fan should be capable of producing a moderate flowrate through the grain of at least 3 cubic metres per minute in stores of the type tested.

10. A good level of sealing as a pre-requisite for any phosphine fumigation system to be effective. The lining of storage walls and bulkheads before loading grain is a good method of achieving this and is recommended in the H-GCA's 1999 Grain Storage Guide. A covering sealing sheet should be pushed into the surface margins of the bulk and should enclose the re-circulation system.

11.

It is recommended that the fumigation parameters be determined from a knowledge of the resistance status of the pest population by a rapid test method capable of giving results in a working day and using doses for the control of all stages of resistant strains, if necessary.

12.

If phosphine is to continue to provide reliable control of insect pests in bulk grain there needs to be a wider appreciation in the trade of the long-term consequences of sub-standard treatments. Phosphine fumigation has been carried out in the past in difficult circumstances and accepting that control of mobile stages (larvae and adults) only will allow the grain to be traded. While this appears to be sound commercial practice, it does risk the development of resistance and future control problems. The topic of phosphine fumigation is considered to be a prime candidate for an H-GCA technology transfer exercise which should encourage uptake of the system resulting from this project.

13.

he current project has demonstrated that we can do much better when dosing phosphine though the solution provided by this system is technically more difficult than current practice. An informed customer, working with a fumigation contractor who is trained to use this technique, will be needed to implement the pre-sealing required to achieve the necessary standard of treatment.

14. The re-circulation system will use less phosphine than the SIROFLOR flow-through system to achieve the same concentration.

15. A major implication from the project is that further development of the system is required for larger bulks.