Free Radicals Investigation On The Thermal Cracking Of Residues And Their Sara Fractions

How to reduce the yield of coke in the process of the heavy oil converted into light oil is the key to solve the energy demands of today's economic development.Therefore,it is necessary to understanding the chemical composition and reaction characteristics of residual oil.Residue's composition is complex,but studies have shown that it can be divided into four fractions,called SARA fractions,which have similar components of structure separately.Asphaltene and Maltene can be divided from AQ vacuum residue and QD vacuum residue by using n-heptane extraction,then they are investigated by the variation of radical concentration in the process of heating to 400?,420?,440? and 460?.We can find the fact that the concentration of free radicals detected in the Asphaltene is about 10 times higher than the original oil.This paper also compares the differences of the Residue and Asphaltene coking rates and investigates the relationship between the coking rates and the concentration of their own free radicals.As we can find the Asphaltenes in two kinds of residual oils are the precursors of coking,of which the vast majority convert into coke during the selected range of temperature.The reaction of two kinds of residues has some differences.The percent of AQ Asphaltene would be increasing as time goes on,which has larger radical concentration and whose coking rate is rather low.QD residue,however,tends to product coke where radical concentration was increased.Adding different proportions of tetralin(THN)into Asphaltene could significantly reduce the free radical concentration of products which has less heavy components.Under 460?,because of the insufficient amount of solvent,the changes of radical concentrations occur apparent differences as hydrogen donor in different rates has similar effects on the feedstock under the rest of temperatures.After hydrogenation reaction,QD Aspaltene would be detected higher radical concentration than AQ,thus it can be concluded that more free radicals in former are difficult to be stabilized by hydrogenation.It has verified that the structure of residues and Maltenes is near to each other,but the Asphaltenes have noticeable differences by the means of 13C-NMR analysis.For example,there are more number of aromatic carbons in QD Asphaltene.Although Asphaltene is one of most important factors to coke formation,the free radical concentration and change rule of the Residue are all close to the Maltene,because of the low content of Asphaltene in the residual oil,often less than 5%.The other three fractions:saturation,aromatic and resin are separated with different solvents through silica gel adsorption column and their radicals are measured during their heating process.The concentration of radicals detected can be used to quantify the differences in heavy material content and the coking trends between the residues and their SARA fractions.Elemental and NMR analysis provided powerful evidences for the structure diversion of the fractions.Meanwhile,divergent fractions are mixed together in a ratio of 1:1 and the interactions between the different components are studied.It was found that this kind of interactions can promote or inhibit the coke produced.