Study On Separation Technology Of Ethylene Feedstock Components

In the process of cracking to produce ethylene,the hydrocarbon composition of the feedstock has a greater impact on its yield.Naphtha is the main raw material for ethylene production.Due to the low yield of naphtha,adding some straight-run diesel can solve the problem of insufficient ethylene raw materials.The n-alkanes separated from naphtha are the best raw materials for cracking ethylene.At the same time,after removing the aromatics in naphtha and straight-run diesel,it can not only increase the ethylene yield,but also reduce the coking of aromatics in the furnace tube,reduce the operating cost and extend the life of the device.Therefore,it is of great significance to study the separation of n-alkanes and the removal of aromatics from ethylene feedstock..In this thesis,the separation of n-alkanes from naphtha by urea/thiourea complexation and the removal of aromatics from naphtha and diesel by solvent extraction were studied.In the simulated oil complex separation of n-alkanes,the results show that: the amount of urea:n-octane is 5:1(mass ratio),the amount of thiourea: n-octane is 2:1,the amount of 95%ethanol is 80 m L(relative to 60 m L raw material oil),reaction temperature of-10℃,reaction time of 1h,the separation effect of n-alkanes is the best,the n-octane content in the complex increases from 25.02% in the raw material to 57.16%,and the n-octane yield is 84.57%.At the same time,it was found that the higher the carbon number of n-alkanes,the easier it is to be complexed by urea.When the carbon number is less than 7,it is not easy to be complexed.Lowering the temperature can improve the separation effect of n-alkanes with low-carbon number.In the experiment of removing aromatics from naphtha by solvent extraction,the mixture containing 10.6% toluene was used as a simulated oil for solvent extraction.The results showed that the best dearomatization effect was achieved when dimethyl sulfoxide was used as the solvent.Under the conditions of solvent to oil ratio of 1:1,extraction temperature of50℃,extraction time and standing time of 1h,the solubility and selectivity of aromatic hydrocarbons reached the maximum,the extraction rate of toluene reached 83.21%,and the content of toluene in the raffinate oil was 4.89%,the toluene content in the extracted oil is increased to 51.04%.When the number of cross-flow extraction stages is six,the toluene content in the raffinate phase only contains 0.73%.Under the optimal conditions,the eight-stage cross-flow extraction of straight-run naphtha containing 10.47% aromatics,the aromatics content in the raffinate phase was reduced to 0.92%,and the dearomatization effect was significant.In the experiment of solvent extraction of aromatic hydrocarbons in the straight-run diesel,the results show that the dearomatization effect is better when N-methylpyrrolidone(NMP)and furfural are used as extractants.Under the same conditions of extraction time of30 minutes and standing for 20 minutes at constant temperature,in the system with NMP as solvent,when the ratio of solvent to oil is 2:1 and the extraction temperature is 30°C,the aromatics content in dearomatized diesel will be reduced from 25.22% to 11.73%,the aromatics extraction rate reached 73.38%.The aromatics content in the raffinate oil after five-stage cross-current extraction was 0.98%,and the aromatic content after five-stage counter-current extraction was 2.88%.At the same time,it is found that the dearomatization effect of the five-stage counter-current flow is equivalent to the three-stage cross-flow effect.In the system with furfural as the solvent,the optimal reaction conditions are the ratio of solvent to oil 2:1 and the extraction temperature of 80℃.The aromatics content in the raffinate oil after the five-stage cross-current extraction is 1.55%,and the aromatic hydrocarbon content after the five-stage counter-current extraction is 3.45%.Under the same extraction method and number of extraction stages,the dearomatization effect of using furfural as the extractant is not as good as that of using NMP as the extractant.