Separation Of Styrene From Pyrolysis Gasoline By Extractive Distillation

C8 aromatics fraction from pyrolysis gasoline contains about 35% to 50% of styrene. Recovering styrene from this fraction using a continuous extractive distillation technique is not only able to produce high-purity styrene, but also reduce the hydrogen consumption, the styrene polymerization, and the catalyst deactivation problems during hydrogenation unit. In addition, the fraction, obtained from hydrogenation of the extraction raffinate, contains quite low concentration of ethylbenzene concentration and is more suitable as the feedstock of an aromatic extraction unit.A separation process of styrene from C8 fraction by continuous extractive distillation has been investigated at 15 kPa (absolute), used sulfolane as solvent and sodium nitrite as polymerization inhibitor. Orthogonal array design method was applied to arrange the experiments. The affects of three factors, namely solvent ratio, reflux ratio and reboiler temperature, on the purity and extraction yield of styrene have been studied. The results show that: the effect order was solvent ratio> reboiler temperature > reflux ratio while styrene purity was set as the criterion, and the effect order was reboiler temperature > solvent ratio> reflux ratio while extraction yield was set as the criterion. A further optimization study shows that an increase in reflux ratio has no significantly effect on styrene purity but slightly increases the extraction yield. An increase in the solvent ratio can improve the purity and the extraction yield at the same time. The results of solvent stability experiments show that the sulfolane with a good chemical stability can be used repeatedly.The isobaric vapor-liquid equilibrium data have been obtained for two binary systems of the styrene + sulfolane and the o-xylene + sulfolane, using a vapor liquid-recycled equilibrium still. Tests are made for isobaric thermodynamic consistency of the measured data using Herrington method. Experimental data were correlated to five two-parameter models, namely van Laar, Margules, Wilson, NRTL and UNIQUAC. The maximum likelihood principle and Nelder and Mead's simplex algorithm were applied to estimate the model parameters. The result shows that the Wilson equation is the most suitable one to represent experimental data satisfactorily. UNIFAC equation is used to calculate the VLE data of o-xylene + styrene system, and the parameters of Wilson equation are also estimated for this system. Based on the results obtained for three binary systems above, VLE data of the corresponded ternary systems are predicted, and are in good agreetment with the experimental data. The obtained VLE data of o-xylene, styrene and sulfolane can be used for the calculation and design of the extractive distillation column.The flow rate and composition of product have been determined by solving the mass balance equations of the experimental data. A comparison of the experimental data and simulated data using ProII software shows that the Flory-Huggins activity coefficient model can be applied to the studied system suitablely. According to the design conditions, a designed-based computing has been carried out using FUG (Fenske– Underwood - Gilliland) simplied method. The simulated results by ProII software are in good agreetment with the results of the design.