Study Of The Prediction Model For Solid-Liquid Phase Equilibrium Of Normal-Alkane Mixtures

The research on the thermodynamic model of phase equilibrium is of great value to the separation process of the chemical industry,while the research on the model of solid-liquid phase equilibrium(SLE)relatively falls behind,staying in the empirical or semi-empirical way.The problem of solid-liquid phase equilibrium of the n-alkane system is the essence of oil wax appearance.Prediction and prevention of oil wax appearance should be considered in the design of oil storage and transportation system.However,most of the existing solid-liquid equilibrium models of n-alkanes use prior estimated parameters,lacking experimental data correction.Therefore,the following research work is carried out in this paper:(1)Describe the solid phase behavior of pure,binary and multicomponent mixtures of n-alkanes.The solid phase of n-alkanes can be divided into two main types:rotator solid phase and ordered solid phase.To establish the prediction model easily,the binary phase diagram is simplified.The correlations of physical properties of pure n-alkanes,the experimental data of binary system phase equilibrium and the experimental data of multicomponent system phase equilibrium are listed.(2)Establish the thermodynamic framework of phase equilibrium of liquid phase,rotator solid phase and ordered solid phase and the calculation method of fugacity ratio of pure components is given.The regression method of parameters in activity coefficient model is also built.Based on the experimental data of binary system phase equilibrium,the non-ideality of the binary mixtures of the rotator phase and ordered phase is analyzed by using the Redlich-Kister expansion.It is found that the nonideality of the rotator phase was not obvious and the non-ideality of ordered solid is related to the mismatch parameter of carbon numbers.Based on the above findings,an empirical correlation of binary interaction energy of NRTL equation is proposed,and the value of the undetermined coefficient of empirical correlation is obtained by regression of binary phase equilibrium data,thus the predictive NRTL model is established.(3)A numerical algorithm is developed to calculate the flash,cloud and melting point temperatures of the two-phase system.The results show that the proposed model is more accurate and can reflect the changing trend of phase diagrams of different systems.An algorithm is developed to solve the problem of multiphase flash with an unknown number of phases.The inner cycle of the algorithm calculates the number of phases and the concentration of each phase component under the condition of a given equilibrium constant.The outer cycle judges whether each phase given by the inner cycle is stable and iterates the phase equilibrium constant.By combining the model and the algorithm,the cloud point temperature,solid precipitation mass and component concentration can be predicted accurately,and the potential multi-solid solutions phenomenon can be predicted.The predicted amount of solid solution is related to the carbon number distribution range of feed,and the concentration of each solid solution is close to normal distribution.To conclude,a predictive NRTL model based on the phase equilibrium data of nalkanes and the non-ideality relationship of ordered phase is established,and a multiphase flash algorithm with unknown phase number is developed.The model and algorithm can be used to predict the solid-liquid equilibrium of n-alkane mixture and the multi-solid solutions phenomenon.