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UBC Theses and Dissertations

Phase-splitting prediction in isothermal flash calculations Romero, Alejandro Antonio


The development of processes that operate at conditions where multiple fluid-phase equilibria may occur demands fast and reliable simulation algorithms. The success of these algorithms depends on the correct prediction of the number and compositions of the phases present at a given temperature, pressure and overall fluid composition. The most commonly used routine for this purpose is the isothermal (single-stage) flash calculation. A robust and efficient method to predict phase-splitting previous to performing an isothermal flash is implemented in this work. It is based on the thermodynamic stability analysis of the source phase using a Gibbs energy tangent-plane criterion. Depending on the outcome of the phase-split tests, the system may be declared stable as a single phase and thus no further calculations are needed, or unstable in which case a potential two- or three-phase solution can be obtained. Only in this last instance would the corresponding flash calculation follow, depending on the nature and quantity of the phases found: a vapour-liquid (VL) or a liquid- liquid (LL) flash if two phases are predicted, or a multiphase flash (VLL) when three phases are expected. In addition to readily recognizing the number and type of phases present, the stability tests provide excellent initial composition estimates for the flash which assure fast convergence to the stable solution. Even though the algorithms developed can be used with any suitable model to calculate equilibrium properties, cubic equations of state are used throughout in this work as a single model to describe all fluid phases.

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