Fumigation modelling of hopper-bottom railcars

Abstract: Bulk railcars are a common method of moving commodities in the United States.
Allowances are given for the practice of treating railcars with fumigates during transit because the routes are limited access and not on public roads. The phosphine concentrations at the top varied with time with phosphine spiking over 1600 ppm and gradually settling to over 300 ppm at the end of the eight days. Total gas dosage was estimated as concentration × time (CT) over the eight days as 115,000 and 125,000 ppm × hr at the top of each railcar. These tests found significant phosphine penetration into the bulk at 2 m depth with ~ 380 ppm after two days and going down to ~ 260 ppm after eight days with the high phosphine treatment. Bioassays of both phosphine susceptible and resistant, Rhyzopertha dominica (F.), lesser grain borer, and Tribolium castaneum (Herbst), red flour beetle, were included at both the surface (0 cm), 25 cm and 60 cm below the surface. All insects, at all locations, were dead after eight days. The railcar and the fumigation treatments were additionally modelled with a CFD simulation approach. The simulation models were shown to provide estimates of the phosphine concentration and distribution which matched well the observed data, validating the computational fluid dynamic (CFD) approach as an efficient tool for future planning and analysis of similar fumigations.