No abstract

Six widely‐used dense gas dispersion models (ALOHA, HGSYSTEM, SLAB, SCIPUFF, PHAST, and TRACE) were used to calculated downwind chlorine gas concentrations following three railcar accidents. The accidents, where as much as 60 tons of chlorine were released, were located at Festus, MO (release from a ruptured 1‐in. line while offloading), and Macdona, TX, and Graniteville, SC (release from a large hole due to an accident). Public sources such as National Transportation Safety Board (NTSB) reports were used to develop input data. Source emissions rates were refined based on source modeling with PHAST and TRACE and derivations using fundamental thermodynamic equations. No chlorine removal mechanisms, such as photolysis, chemical reactions, or deposition were accounted for. Given the same source emissions rates, the models' simulations of 10‐min averaged cloud centerline concentration, at downwind distances ranging from 0.1 to 10 km, agree with each other within plus and minus a factor of two most of the time. For a very large release (Graniteville), the 2,000, 400, and 20 ppm contours are predicted to extend downwind about 1.3, 3.1, and 14 km, respectively, from the source. There is also agreement among the models simulations of the plume widths and heights to the 2,000, 400, and 20 ppm contours. A major conclusion of the study is that estimation of the source or release term is important for reliable results, since the calculated chlorine concentrations are approximately proportional to the mass release rate. It must also be stressed that observed concentrations would be expected to be less than those calculated here, because removal by dry deposition at the surface and by chemical reactions in the plume have not been accounted for. © 2008 American Institute of Chemical Engineers Process Saf Prog, 2008

Several models are currently available to model the discharge and dispersion of toxic or flammable materials to the environment. A few of the Gaussian dispersion modeling tools allow the representation of the complex environment within a manufacturing plant or urban area in determining the impact of continuous releases from a plant. For atmospheric dispersion of dense gases, a correction is made for the presence of the buildings and other complexity by using a surface roughness parameter, which is only a crude approximation. A need exists to obtain realistic estimates of plume dispersion in a complex environment, particularly accounting for buildings/obstructions at a plant and the associated turbulence. With the advance of computational technology, and greater availability of computing power, computational fluid dynamics (CFD) tools are becoming more available for solving a wide range of problems. A CFD model, called FLACS (flame acceleration simulator), developed originally for explosion modeling, has been upgraded for atmospheric dispersion modeling. CFD tools such as FLACS can now be confidently used to understand the impact of releases in a plant environment consisting of buildings, structures, and pipes, and accounting for all complex fluid flow behavior in the atmosphere and predicting toxicity and fire/explosion impacts. With its porosity concept representing geometry details smaller than the grid, FLACS can satisfactorily represent geometry even when using a coarse‐grid resolution to limit the simulation time. The performance of FLACS has recently been evaluated using a wide range of field data sets for sulfur dioxide (Prairie Grass), carbon dioxide (Kit Fox), and ethylene (EMU), for example. In this paper, details about the improvements made to FLACS, model validation exercises, and results from the modeling of releases from an industrial facility are presented. © 2005 American Institute of Chemical Engineers Process Saf Prog, 2005

The nature and role of the shear layer, which occurs at the level of the average building height in urban canopies, are poorly understood. Velocity data are analyzed to determine the characteristics of the shear layer of the urban canopy, defined as the broad, linear segment of the mean velocity profile in a region of high shear. Particle image velocimetry measurements in a water tunnel were undertaken to resolve velocity profiles for urban canopies of two geometries typical of Los Angeles, California, and New York City, New York, for which the aspect ratios (average building height-to-width ratio) H /w b are 1 and 3, respectively. The shear layers evolve with distance differently: For H /w b ϭ 1 the urban canopy shear layer extends quickly from above the building height to ground level, whereas for H /w b ϭ 3 the urban canopy shear layer remains elevated at the vicinity of the building height, only reaching to a depth of z /H ϳ 0.5 far downstream. Profiles of the mean velocity gradient also differ from each other for urban canopies associated with H /w b of 1 or 3. Values of shear dU/dz increase toward ground level for an urban canopy associated with H /w b ϭ 1. For an urban canopy associated with H /w b ϭ 3, localized peaks of shear dU/dz exist at the building height and at ground level, with values of shear decreasing to zero at building midheight and far above the building height. A consequence of the different forms of the shear layers of the two urban canopies is that the ground-level dispersion coefficient is likely to be greater for urban canopies associated with H /w b ϭ 1 than for those associated with H /w b ϭ 3 because of an increased ventilation and exchange mechanism for cities such as Los Angeles relative to cities such as New York City that possess urban canyons.