Study On Vapor-Liquid Equilibria At High Temperatures And Pressures For Direct Coal Liquefaction System

The system involved in direct coal liquefaction is a three-phase state of solid, liquid, and gas phases under high temperature and high pressure. The substance of liquid phase is composed of a number of the hydrocarbons. The compositions and states of the reactants and products influence the process of direct coal liquefaction and the data of gas-liquid phase equilibrium at high pressures provides fundamental information for the development of direct coal liquefaction processing as well as the design of reaction plant. The second task of the fundamental research project on large-scale direct coal liquefaction, issued in the 973 program "Basic research on large scale direct coal liquefaction" in 2004, is to study the thermodynamic properties of the liquefaction products. One of the challenges in the analysis of the Coal hydrogenating process is due to the fact that the contents of both hydrogen and methane are very high and both of them are in the super-critical state. Besides, when small, spherical hydrogen and methane molecules are mixed with liquefaction oil, the huge difference between their molecule weights causes extreme asymmetry of thesystem, bring about the difficulty in analysis and modeling of the liquefaction process.This thesis focuses on the development of the mathematical model for theprocess of coal liquefaction by predicting the thermodynamic properties of the liquefaction products based on the fundamental variables including temperature, pressure, and composition. The traditional constitutive relations of mixed rules can not precisely describe the behaviors of liquid phase, particularly in the polar system or extremely asymmetric system. However, with the new EOS- G~E Gibbs free energy model, reasonable results are able to be obtained in the strong polar and strong asymmetrical system of coal liquefaction under high temperature and high pressure, by combining the model of activity coefficients and constitutive relations as well as adopting the concept of grouping model.A computational method to solve the flash model for a coal liquefaction system at high temperature and high pressure has been developed. The numerical code has two-cycle iteration, including the inner cycle ofβiteration and the outer cycle of K iteration, and the results of fast convergence can be achieved. Generally, after about 5 times of iteration (outer cycle), the program can reach the convergence precision. Based on the model of PSRK, the composition of 24 components of the gas-liquid phase and the gas-liquid equilibrium constant has been calculated . Also, the solubility of hydrogen in oil of coal liquefaction at high temperature and high pressure has been obtained. Further the solubility of hydrogen linearly increases with the rise of partial pressure of hydrogen as well as the temperature. But the linear fitting in the curve of hydrogen solubility-partial pressure departs from the origin of coordinate, which demonstrates that the solubility of hydrogen in coal liquefaction oil does not strictly comply with the Henry's law.