Thermal Aspects during Depressurization of Process Systems

Abstract

The present work investigates the internal heat transfer mechanisms in a gas- and liquid filled pressurized vessel during depressurization and aims to improve the models for determining minimum temperatures in depressurization scenarios without a fire. Basic aspects concerning depressurization systems in oil- and gas processing plant are also presented.Relevant heat transfer theory have been thoroughly presented and discussed. The most important heat transfer mechanisms include free- and forced convection and nucleate boiling in single- and multicomponent fluids. Numerical calculations of flow patterns inside both horizontal and vertical vessels have been performed with the finite element analysis and solver software Comsol Multiphysics. This analysis was done using different evaporation rates in the interface between vapour and liquid. Whether the flow pattern is dominated by forced- or free convection is shown to be dependent on the evaporation rate. When the evaporation rate is small, the flow has a circular pattern which is characteristic for free convection flow. For higher evaporation rates the flow pattern is dominated by a bulk motion out of the vessel.Depressurization simulations have been performed with the computer code NEW*S for different initial conditions and fluid compositions. The results have been compared with experimental data. Differences between the simulations and the experimental data have been presented and thoroughly discussed. Based on the comparison between simulations and experimental data an analysis of the different heat transfer mechanisms has been performed. Heat transfer coefficients calculated with available published heat transfer correlations have been compared with the heat transfer coefficients in NEW*S. For the heat transfer between vessel and vapour and between liquid and vapour the heat transfer coefficients from NEW*S are in agreement with other correlations. For the heat transfer between vessel and liquid on the other hand, large differences in both numerical values and in the shape of the curves were observed. Hence a new model for the heat transfer between the vessel wall and the liquid phase has been developed in NEW*S. In this model Mostinski s correlation for nucleate boiling heat transfer in single component fluids and Palen and Yangs s correlation for nucleate boiling heat transfer in multicomponent mixtures have been used.