Measurement and Modelling of Thermodynamic Equilibrium and Kinetics of Solid-liquid Phase Transition for Short-chain Alkane Mixtures
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Wax formation especially in oil and gas industry has been one of the major issues in hydrocarbon processing and transport. The wax consists of hydrocarbon mixtures in solid phase and is do dominted by alkanes. The phase equilibrium and kinetic of wax complex mixtures depend on the components composing the wax and their equilibrium behaviour. There have been numerous studies on hydrocarbon mixtures to evaluate their phase equilibrium and kinetic behaviour through experiments and modelling. However, there is still some room where the knowledge of solid-liquid equilibrium of the wax is still limited compared to the large research area of wax precipitation. The present work main objective is to improve understanding of short chain alkanes contribution to wax formation by investigation of phase equilibrium for short-chain alkane mixtures (five binary mixtures and one ternary mixture). The research provides new data references of solid-liquid equilibrium, revealing the solid-liquid phase diagram of the mixtures and also investigate the solid phase mixture behaviour. Furthermore, the work also has objective to establish understanding of wax crystallization kinetic by investigation of kinetics parameter which influence the crystallization of alkanes, i.e., nC5H12 to nC9H20. In addition, it evaluates selected thermodynamic model and provide recommendation of suitable thermodynamic model for wax formation prediction. Investigation of solid-liquid phase equilibrium is performed by the means of Differential Scanning Calorimeter (DSC) operated at a low temperature. The equilibrium temperature is determined from the heat-peak evaluation of the measured heat-flow during the heating process and applying the shape-factor correction to the measured temperature. The thermodynamic model evaluation is performed by the evaluation of thermodynamic model accuracy on solid-liquid phase equilibrium temperature compared to the measured data from the solid-liquid equilibrium reference. The thermodynamic model prediction is performed by the simplified PCSAFT, Peng Robinson EoS and activity model – Regular solution theory. The kinetic of crystallisation short chain alkane is also investigated by using the Differential Scanning Calorimeter. The kinetic parameters of crystallisation process are determined through the evaluation of the heat flow and the degree of crystallinity in order to estimate the surface free energy that contributes to the energy barrier for nucleation. The present works successfully provides the data of solid-liquid equilibrium temperature of five binary mixtures of short-chain alkanes (nC5H12-nC7H16; nC6H14-nC8H18; nC7H16-nC9H20; nC9H20- nC11H24; nC11H24-nC13H28) and also equilibrium measurement on the ternary mixture (nC5H12- nC11H24-nC13H28). The phase equilibrium investigation reveals a eutectic system for all mixtures with partial miscibility in the solid phase in nC9H20-nC11H24 and nC11H24-nC13H28 mixtures. The thermodynamic model evaluation confirms that simplified-PCSAFT is able to provide more accurate prediction compared to the other two models, i.e., PR-EoS and RST. Overall, this works provides the foundation for further works for complex wax evaluation on pressurised system, which represents the real operating conditions in phase equilibrium and kinetic behaviour. Furthermore, it is important to perform advanced observation of wax composition, which is a valuable reference for multi solid phase thermodynamic model evaluation. In addition, the wax deposition must be studied further, which is essential for the hydrocarbon transport and liquefaction processes.