Environmental Engineering Reference
In-Depth Information
1. Objectives and Background
The authors have been studying Toxic Industrial Chemical (TIC) source emissions
models and dispersion models for several years. Hanna et al. (2008) applied linked
source emissions and dispersion models to three scenarios involving large chlorine
releases from railcar accidents, and concluded that a major source of uncertainty
in the estimation of health and environmental impacts was the source emission
uncertainty. In the current project, the source emissions model improvements have
been under study for 2 years, and the main objective is to suggest improvements
for the (TIC) source emissions models in the Hazard Prediction Assessment
Capability (HPAC) model system (DTRA, 2008). The study team is making use of
TIC emissions models suggested by the chemical industry (Hanna et al., 1996) and
detailed field experiments such as those involving two-phase jets (CCPS, 1999;
Witlox et al., 2007). The highest priority scenarios concern releases of many tons
of pressurized liquefied gases such as chlorine, anhydrous ammonia, and sulfur
dioxide. In most cases, these releases become dense due to their high molecular
weight, their cold temperature, and or/their imbedded liquid droplets. For most
scenarios, the worst case will be when all of the released material quickly ends up
in the gas phase or as a fine aerosol as it is transported downwind.
2. Source Emissions Models
Specific methodologies and the steps and equations for each category of TIC
source type are given in the detailed project report by Britter et al. (2009), plus
definitions and suggested input requirements and output formats, so that the steps
can be directly implemented in HPAC. Because of page limits in the current
paper, the equations are not listed here. The various modeling requirements have
been categorized as: fluid property information; release rate and exit thermodynamic
conditions; vessel response; flow after exit from the storage vessel; linkage or
hand-off to HPAC's transport and dispersion model, SCIPUFF (Sykes et al.,
2007); and liquid pool spreading and evaporation
A common source of fluid property information is required to ensure con-
sistent model usage and performance. The National Institute of Standards and
Technology (NIST) database is currently used in HPAC for many materials. Other
chemical data sources are also used by DTRA, such as the widely used DIPPR
database. Either NIST or DIPPR would be able to provide sufficient property data
for source-term models.
For most cases with a rupture of a container full of pressurized liquefied gas
the mass release rate will vary strongly with time. The optimum input for a
dispersion model is the release rate as a function of time with a resolution of a few
seconds. However, many dispersion models cannot use that much detail, and some
models can take only a constant release rate (although perhaps for a finite duration)
Search WWH ::




Custom Search