A typical hazard analysis of a chemical process installation involves several hazardous materials/streams. Rigorous thermodynamics is a standard feature of models used in process plant design and process simulation. However, models used in hazard and risk assessment frequently make simplistic assumptions. The "pseudo component" approach is often used and the mixture is assumed to have a constant composition throughout the model and to behave like a single component. The actual compositions and properties of the two phases can be very different, affecting the accuracy of the release and dispersion calculations. The current paper discusses more rigorous multi-component modelling for multi-component two-phase flashing releases of hazardous chemicals into the atmosphere. A new multi-component property system has been implemented in the consequence modelling package PHAST and applied to discharge and dispersion models. The new multi-component property system allows usage of cubic equations of state (Soave-Redlich Kwong and Peng-Robinson), which are most favoured by the industry for mixture calculations. These allow modelling of non-polar and slightly polar mixtures for consequence-analysis purposes. The phase-equilibrium calculations of a multi-component stream produce results for vapour composition, liquid composition, pressure, temperature, enthalpy, entropy and density. Currently the property system allows the following types of flash calculations: isothermal, isentropic, isenthalpic, constant energy and constant vapour fraction. The property and phase equilibrium calculations have been verified and validated by means of comparison against a commercial process simulation package. The discharge model calculates the expansion from the stagnation conditions to the orifice conditions (isentropic expansion for the release from a vessel, conservation of energy for expansion along a pipe), and the expansion from the orifice conditions to the ambient conditions (conservation of energy). These expansion calculations have been extended to allow usage of the above multi-component property system. It is demonstrated that the new multi-component calculations may significantly affect the vapour and liquid composition of the released pollutant. The dispersion model includes a thermodynamic model for mixing of a non-reactive multi-component pollutant with moist air, which includes possible water-vapour transfer and/or heat transfer from the substrate to the cloud. Conservation of enthalpy is assumed during the mixing with air. Two methods have been included, i.e. the case for which all components are assumed to form a single aerosol and a case for which separate aerosols form. The new multi-component version of the dispersion model has been tested and verified thoroughly. It is demonstrated that the new multi-component calculations may significantly affect the predicted vapour and liquid composition of the dispersing cloud, and also the predicted temperature, concentration and amount of rainout.
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