Modelling blowdown of pressure vessels containing two-phase hydrocarbons mixtures with the partial phase equilibrium approach

This paper describes the development and validation of a new unsteady model, VBsim, for the simulation of the rapid depressurization or blowdown of pressure vessels containing two-phase (vapour and liquid) hydrocarbons mixtures with non-ideal behaviour. Accounting for non-equilibrium effects between the constituent fluid phases, i.e. temperature differences between phases, the model calculates the time variation of fluid pressure, temperature and composition inside the vessel adopting the equations of state for non-ideal gas. It is based on a split two fluids model (vapour and liquid) and it considers internal heat and mass transfer processes, as well as heat transfer with the vessel wall and the external environment. In order to account for the mass exchanged between the gas and the liquid phase, in conditions away from the thermodynamic equilibrium, an innovative version of the partial phase equilibrium (PPE) approach has been introduced. A validation of the proposed model has been performed by comparing its results with experimental and calculated data from literature showing a good agreement.