| Styrene is an important basic raw material for the organic chemical industry. There is 4-6%of styrene in the pyrolysis gasoline from the process of ethylene by steam cracking the raw materials such as naphta,diesel and liquefied natural gas. Styrene cannot be easily removed from the pyrolysis gasoline by traditional distillation because of the closeness of the boiling points of styrene(nbp,145.14℃) and other C8 aromatic hydrocarbons such as ethyl benzene(nbp,136.19℃) and o-xylene(nbp, 144.41℃).Extractive distillation is an important process for separating the azeotropic or close boiling point mixture.There were some studies on the solvents for recovery of styrene from pyrolysis gasoline by extractive distillation.But the suitable solvents selected by the different researchers were different too.In this research,the candidate solvents were selected and evaluated by property restriction method.And then the results were examined by the vapor-liquid equilibrium experiment.The results showed that the most suitable single extractant was sulfolane,and the optimum mixed solvent was the mixture of n-methyl-2-pyrrolidone(NMP,50%) and sulfolane(50%) which was better than either of the two pure solvents in selectivity.The separation process was simulated by Aspen plus.And influence factors such as ratio of solvent to raw solution,reflux ratio of the distillation column,temperature and feeding stages of extractant and raw solution were investigated.When conditions were as following: mass fraction of o-xylene in raw solution was 0.5,top pressure of column was 0.2bar, mass fraction of o-xylene in distillate was≥99.0%,the mixed solvent of sulfolane(0.5) and NMP(0.5) was used as extractant,the optimum parameters for extractive distillation of o-xylene and styrene could be described as:for the extractive distillation column,number of theoretical plates was140,feeding plates for solvent and raw solution were 8 and 68 respectively,ratio of solvent was 10,reflux ratio was 7,and for the solvent recovery column,number of theoretical plates was 100,raw solution feeding plate was 60,reflux ratio was 3.5.The simulation results would provide basic information for the industrial experiment and design.The disadvantage of extractive distillation is the large amount of solvent,which causes high cost of equipments and high energy consuming in the recovery of solvent. Salt-containing distillation can increase the relative volatility of compounds to be separated.But there are some difficulties in piping and recovery of salt,which limit the application of this technology.Extractive distillation with salt is the synthesis of the traditional extractive distillation and salt-containing distillation.It has all advantages of the two processes and eliminates the disadvantages of them.The technology attracted many researchers' attentions and has successfully been used to separate the polar systems as alcohol-water,ester-water.But so far it has only been used in a few non-polar systems such as C4 hydrocarbons,benzene-heptane,and there is no report of separating o-xylene and styrene by extractive distillation with salt.In this research,vapor-liquid equilibrium of o-xylene-styrene was investigated in the presence of sulfolane dissolving different inorganic salts.It was found that sulfolane with addition of salt could improve the relative volatility of o-xylene-styrene, and potassium thiocyanate(KCNS) had the significant advantage over other salts added to the basic solvent.And factors of batch extractive distillation with salt for o-xylene-styrene as concentration of KCNS in sulfolane,ratio of solvent,reflux ratio were studied.The results showed that sulfolane with addition of 3%water and 0.03g·mL-1 KCNS was the most suitable extractant for the separation of o-xylene-styrene by extractive distillation with salt.This work not only was a good try of a new method to separate the mixture of o-xylene and styrene,but also enlarged the application of extractive distillation with salt in non-polar systems.
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