(657f) Determination and Modeling of the Limiting Oxygen Concentration of Pesticide Dusts with Highly Corrosive Combustion Products

Within the production of agricultural pesticides various different kinds of dusts occur which can be considered as explosive due to their chemical and physical characteristics. In various steps of the production process hybrid mixtures (e. g. dust in acetone) are present. For most of the considered dusts with high contents of halogens there are no sufficient data available in order to determine the explosion hazardous potentials of the different process steps. In this work safety characteristics, especially the limiting oxygen concentration, of 2,4- Dichlorophenoxyacetic acid- dimethylamine (2,4D- DMA) and different urea herbicides (Chlortoluron, Fluometuron) have been determined experimentally and theoretically.

To characterise the occurring dusts also not only in terms of their chemical composition extensive research on the particle structure and surface as well as on the particle size distribution were carried out. The particles show a crystal structure with no porosities or inclusions. The parameters of the particle size distribution were approximated by a RRSB distribution. All dusts show values around 1 for the scattering parameter n. Besides the particle size distribution also the burning behaviour of the dusts was investigated. For all dusts no autonomous burning could be observed.

After research of the dusts under a microscope and the analysis of the particle size distribution and burning behaviour, experiments in the ISO 20l apparatus of the company FireX were carried out. All observed dusts show significant values of pressure rise and maximum explosion pressure. The maximum explosion overpressures are situated around 8bar and the measured minimum ignition energies lie between 8 and 16mJ.

The measured values for all analyzed dusts where also compared to results of different models for calculation of safety relevant characteristics.

A main goal of the present work was to determine the limiting oxygen concentration of the chosen dusts. In the course of the research the combustion products of the dusts turned out to be highly corrosive, which can be explained by the chemical composition of the dusts and the burning conditions in the ISO 20l apparatus. Some combustion products caused smaller damage on the 20l apparatus, due to corrosion effects. A reliable method to measure the limiting oxygen concentration had to be found as alternative for the 20l apparatus.

By modification of the Hartmann apparatus, which is used for screening purposes, a method was to determine the limiting oxygen concentration (LOC) was investigated. A goal of the chosen method was also to reduce the amount of sample needed for the analysis. The method was first checked by several dusts with well known values of limiting oxygen concentration. All values for the LOC of the investigated dusts, could also be found by this alternative method in consideration of the defined failure limit. The needed sample amount for the analyses is significantly lower compared to the ISO 20l apparatus.

All results of the test dusts were also compared to results of several different models for the determination of the limiting oxygen concentration. The models showed good agreement with the values found in experiment, so that the method could be considered as reliable under the chosen experimental conditions.

As a next step all pesticide dusts where investigated with this alternative method. Compared to all other safety relevant characteristics determined in the course of this work, also the limiting oxygen concentrations of the dusts showed comparable behaviour, regarding their chemical composition. All results of the experiments where compared to results of mathematical models and showed good agreements. The determined limiting oxygen concentrations where also comparable to other known dusts with quite similar chemical composition.

The used method offers the possibility to determine the limiting oxygen concentration of dusts with highly corrosive or hazardous combustions products by limiting the amount of sample needed and simplifying of the cleaning process of the equipment due to the assembly of the used Hartmann apparatus.