Simulated Cracking Of Naphtha And Study On Speciation Of Sulfides In Naphtha And C₅ Distillate

With the growth of domestic demand for ethylene and increasingly heavy and inferior crude oil,the proportion of heavy distillate used as cracking raw material is becoming bigger and bigger.As a result,the problems of environmental safety and production quality and equipment corrosion caused by sulfur impurities in heavy distillate occured undoubtedly,which would seriously affect the pyrolysis process of raw materials of ethylene.Acted as some kind of the main impurities in cracking raw materials,sulfer compounds with different forms have different properties and composition,which determined the amount of enrichment and allocation in different product as well as the degree of pollution to the environment and corrosion of equipment.Moreover,the influences of sulfides on production varied with various species.Therefore,it is significant to study on conversion and distribution of sulfur with different forms in the pyrolysis process of ethylene cracking materials and its downstream product.In order to broaden the cracking raw material source,reduce the production cost and improve market competitive power,petrochemical company of Mao Ming mainly used naphtha and hydrocracking tail oil to make ethylene.Due to the poor properties of ethylene cracking material resulted from inferior change of crude oil,it came to a series of problems such as furnace tube coking severely and equipment corrosion and poor product quality.Based on the problem that sulfur concentration of C₅ distillate in the downstream product of ethylene plant exceeded the standard value,a series of cracking experiments with heavy naphtha as the research object were carried out in laboratory simulated cracking unit designed by ourselves.The distribution of sulfur compounds in the cracking raw material of naphtha and the pyrolysis product of C₅ distillate were studied and discussed to provide basis for solving the issue that the C₅ distillate was unqualified because of high sulfur content.Some of the principal work and conclusions are listed below.According to the characters of ethylene process,the process parameters including the residence time,inlet and outlet temperature of cracking furnace and the heat trans-fer area required in the process of rapid cooling as well as the applicability of the raw materials were taken into consideration before a set of laboratory simulated cracking atmospheric pressure experiment device with simple structure,low cost and feasibility for a variety of raw materials was designed and optimized.Trial results showed that the device not only could effectively avoid blockage,poor condensation,serious cooking problem and so on when naphtha or tail oil was cracked,but also could carry out burning experiments to study the coking regulation of raw materials.A series of simulated pyrolysis experiment of heavy naphtha were carried out on laboratory simulated cracking plant in different conditions by changing pyrolysis temperature,quality ratio of w/o or residence time.Analysis and calculated methods were established for cracking gas component in the pyrolysis products.By comparing the yields of ethylene,propylene and butadiene in different conditions,the optimal conditions of heavy naphtha cracking with cracking temperature of 860⁰C,quality ratio of water and oil of 0.79 and the residence time of 0.32s were obtained.Sulfur contents of thiol compounds,disulfides,sulfoether compounds and thiophenic compounds and total sulfur contents in heavy naphtha and its cracking product of C₅ distillate were determined by using potentiometric titration instrument and sulfur and nitrogen analyzer,respectively.Speciation of sulfides of pretreated naphtha and C₅ distillate were identified by using gas chromatography with mass spectrometry detection.The results showed that the total sulfur in heavy naphtha had a high content and speciation of sulfides was very complex.The contents of various sulfides were listed in the order of thiophenic sulfur>sulfoether sulfur>thiol sulfur>disulfides.Among of them,the thiophenic sulfur accounted for 60%and its main existing forms were methyl thiophene and dimethyl thiophene;the sulfoeher sulfur was approximately 31%whose possible forms were methyl ethyl ether,methyl tert-butyl ether,etc.;thiol sulfur mainly contained isomers of three to six carbon atoms and its mass percentage just accounted for 5.09%;10.33%of total sulfur in naphtha would enter into C₅ distillate.And sulfur compounds in C₅ distillate were mainly thiophene and dimethyl thiophene,whose quality percentage accounted for as high as 84%.