Study On Phase Equilibrium Of Extraction Aromatics Using N-formylmorphoiline And Its Simulation

As one of the new and excellent aromatics extractants, N-formylmorphoiline(NFM) was investigated through measuring experimentally the liquid-liquid equilibrium (LLE) data for aromatics extraction from hydrocarbon (heptane, cyclohexane, methylcyclohexane), and its extraction capability was studied thoroughly. According to the conditions of phase equilibrium, the mathematical model was established to correlate liquid-liquid equilibrium data and the parameters of the model were obtained by optimization method. Then the applications of the two thermodynamic equations in the extraction systems were carried out by calculation of phase equilibrium, critical points and simulation of extraction process. The main conclusions are as follows.1,Measurement of LLE and consistency of experimental data. The LLE data were measured for three binary systems (NFM + heptane or cyclohexane or methylcyclohexane) and six ternary systems (NFM+hydrocarbon (heptane, cyclohexane, methylcyclohexane) +aromatics (benzene, toluene)) using custom-built liquid-liquid equilibrium cell. It was found that the mutual solubilities increased in the higher temperature for the three binary systems. The mutual solubilities for NFM with different hydrocarbons satisfied the following relationship: cyclohexane > methylcyclohexane > heptane. The LLE diagrams obtained showed that they were type I and there exsited the large immiscibility regions, ensuring NFM to meet the demand of extraction solvent. The reliability of experimentally measured tie-line data was correlated by the Othmer-Tobias and the Bachman equations, and it was shown that the correlation coefficients had satisfactory degree of consistency.2,The extraction capabilities of NFM. The extraction capabilities of NFM were studied systematically. Distribution coefficients, separation factors and selectivity were evaluated for the immiscibility region, and their connections with temperature and the concentration of aromatic hydrocarbon were studied respectively. The results showed that NFM had excellent extraction capabilities. The values of distribution coefficient, separation factor and selectivity for benzene in these systems are 0.99, 12.4 and 5.2 respectively; and 0.85, 9.6 and 3.4 are also obtained for toluene. It was also found that with temperature increasing, the distribution coefficients of aromatic hydrocarbon increased, while the separation factors and the selectivity decreased. In the two phase coexistence area, as the concentration of aromatic hydrocarbon increased, distribution coefficients became higher and selectivity lower.3,The correlation of thermodynamic equations and the LLE calculation. Based on phase stability conditions, the objective function was established to correlate the LLE data. Because the solution of the global optimization is difficult to be obtained for the non-linear equations, several measurements had been employed, such as the minimization of Gibbs free energy of multi-component systems, feasible initial guess for a new minimization, conjugate Gradient method. The parameters of the two thermodynamics equations (NRTL and UNIQUAC) were guaranteed to be near the solution of global optimization. The correct solutions were obtained by the equality of chemical potentials, materiel conservation, and the Broyden method. It was found that NRTL and UNIQUAC used for LLE could provide a good prediction, and the former model was more suitable for the studied systems. In hydrocarbon-NFM binary systems, the root mean square deviation (RMSD) values of NRTL are in the range of 0.0104-0.0817, and those of UNIQUAC are between 0.0182-0.7105. In the ternary systems, the RMSD values are 0.0110-0.0937 for NRTL, 0.0133-0.1471 for UNIQUAC, showing that all RMSD values are lower than 1.37 prescribed by the literature.4,Critical points of the equilibrium. Based on the parameters of two thermodynamics equations and thermodynamics principle, the critical points of the binary and ternary systems were studied. Non-linear equations for calculating the critical points are given using the NRTL and the UNIQUAC models, respectively. The critical points can be determined by solving these non-linear equations through Broyden method. These results can be applied in practical extraction process to operate in the conditions far from critical points. 5,Simulation of extraction process using thermodynamics equations. Simulation of extraction process using NFM as solvent to separate aromatics from hydrocarbon was carried out by adopting classical extraction tower model and Isothermal Sum Rates method. The two thermodynamics equations(NRTL and UNIQUAC) were used to calculate equilibrium constants of components. The optimization results obtained by the two equations show that aromatics can be separated effectively by NFM in extraction tower, and the relative temperature and solvent ratio are favorable on the condition of liquid-liquid phase separation. In the given conditions, in Heptane-NFM solvents system, when theoretical tray is 8, solvent ratio is 2.875, the recovery of aromatics can reach 95% at 353K. In the case of Cyclohexane-NFM, the recovery of aromatics is higher than 96% at 298K, in the conditions of theoretical tray=8, solvent ratio=2.875; As for Methylcyclohexane-NFM, the recovery of aromatics is as high as 92% at 353K, when theoretical tray is 15, solvent ratio is 2.875.