Across the EU some 50 sites manufacturing or using hydrogen fluoride (HF) will come within the scope of the most stringent requirements of the recently adopted SEVESO II Directive. Many of these sites store (up to several hundred tonnes) and process HF in its most hazardous form, ie as anhydrous HF (AHF). AHF exhibits complex thermodynamics:
I) phase changes on release from pressurised containment, formation of aerosol etc
2) the formation of oligomers - up to (HF)6 which increase the cloud density 3) the dissociation of the oligomers - an endothermic process 4) the hydrolysis of AHF aerosol with water vapour - an exothermic process
Some dispersion models, which take account of the thermodynamics have been validated against experiments in desert conditions. However, the humidity was not sufficiently high for the hydrolysis reaction to generate any buoyancy in the cloud.
The same models predict that, at humidities greater than about 65% (at 15C) ie typical of European climates, the hydrolysis reaction generates sufficient heat for the cloud to become buoyant. The cloud Richardson number becomes negative and because the models have not been developed (or validated) to cope with the transition to buoyancy and any subsequent plume rise, the codes either stop calculating or predict ground level concentrations that are unvalidated. The buoyancy effects may have a major impact on the concentration levels at ground level. For example, if the thermodynamics are ignored, models predict hazard ranges for a 150 te release in low wind speeds and high humidity of well over 20 km. When the thermodynamics are included, models predict the onset of buoyancy at less than 500 m. If the buoyancy effects are sufficiently great the cloud may lift clear of the ground thereby providing further mitigation.
Compared to other substances that come within the scope of the SEVESO II Directive, the consequence and risk assessments for industrial releases of AHF contain considerable uncertainty. It is clear from the above that this uncertainty has major implications for the implementation of the SEVESO II requirements, particularly for land-use and emergency planning.
To resolve this uncertainty we will pursue the following objectives:
- To design and conduct reduced scale source term and dispersion trials at a site in France to resolve the current controversy over whether dispersing AHF clouds exhibit buoyancy and possible 'lift-off from the ground; and if so under what meteorological and source discharge conditions. Also to quantify the enhancement of the dispersion in clouds or plumes due to the thermodynamic effects of HF/humid air mixtures.
- To derive and validate (with the aid of wind tunnel and new thermodynamic data) a mathematical model of the HF dispersion process which takes account of the complex thermodynamics and the effect of the presence of hydrocarbons (re HF alkylation units) on the dispersion process.
- To generate quality information and models to enable duty holders and regulators to better understand the chemical and physical processes associated with industrial releases of AHF and thereby enable more realistic assessment of the risks to people and the environment. To generate a database in the REDIPHEM format for model development and validation purposes by other researchers and organisations.
- To assess the environmental impact of AHF releases on flora.
- To assess, for people who remain indoors following an industrial release of HF (as recommended in emergency plans), the mitigation due to the deposition of AHF on contact with buildings and their contents.
The results will be published in a special issue of the Journal of Hazardous Materials.
Industry are showing considerable interest in our proposal; we expect this to culminate in additional work (totally funded by industry) to assess the effectiveness of water spray mitigation strategies.
Funding SchemeCSC - Cost-sharing contracts
WA3 6AT Warrington
901 82 Umeå
901 83 Umeaa