| The pyrolysis gasoline from the ethylene treating unit contains about 4% 6% styrene.It isdifficult to separate styrene from the pyrolysis gasoline by conventional distillation for therelative volatility of o xylene(light key component) to styrene(heavy key component) is only1.04. The extractive distillation is commonly used to separate styrene from the pyrolysis gasolinein industry.The candidate solvents were first selected and evaluated by property restrictionmethod ,then verified by ternary vapor liquid equilibrium experiment. At last, N methyl pyrrolewas selected as the suitable solvent.The VLE datas of the three binary systems of o xylene– styrene , o xylene N methylpyrrole and styrene N methyl pyrrole were obtained at 20kPa by using the modified Othmer'sbalance equipment. These datas were correlated respectively by the NRTL , Wilson , UNIQUACequations. Then the VLE datas of the correspond ternary systerm were predicted based on thecorrelated results above. It is shown that the prediction by Wilson model was the most close tothe experiment datas.The distillation of the binary systerm o xylene styrene and extractive distillation withN methyl pyrrole as solvent were simulated respectively at 20kPa. The results showed that theseparation effect of extractive distillation was much better than the former. And the number oftheoretical trays, the feeding stages, solvent ratio and reflux ratio were investigated to obatain thesuitable operating conditions. The results were as follows: 135 for the number of theoreticaltrays,7 and 73 for the feeding tray of solvent and feedstock respectively, 10 for solvent ratio and9 for reflux ratio. Under this operating condition,the relative mass fraction of styrene to o xyleneat bottom of the tower were 99.346%, and the recovery of styrene was 99.24%. The suitableoperating conditions for the solvent recovery column were: the number of theoretical trays was40, the feeding tray of solvent was 20, the reflux ratio was 3.5. |
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