Research On The Effect Of Asphaltenes And Resins On The Gelation Of Waxy Oils And Its Mechanism

The three-dimensional network structure formed by wax crystals is the fundamental cause of gelation of waxy oils.Asphaltenes and resins are the two most important polar components in crude oils.They can interact with wax molecules,participate in the wax crystallization process,and change the wax precipitation characteristics,thus affecting the gelation behavior of waxy oils.The current researches on the effect of asphaltenes and resins on the gelation of waxy oils are mainly limited to the rheological properties and wax crystal morphology.This paper continues to deeply analyze the influence of asphaltenes and resins on the microstructure(nanoscale)of wax crystals and the surface electrical properties of wax crystal surfaces by preparing waxy model oils containing asphaltenes/resins,and also studies the influence of polar solvent o-xylene content and the polydispersity of paraffin components to deepen the understanding of the gelation mechanism of waxy oils.Firstly,the effect of asphaltenes on the gelation of waxy oils and its mechanism are studied.From the macroscopic rheological aspects,asphaltenes can reduce the wax appearance temperature(WAT),wax precipitation amount,gelation temperature and yield stress of waxy oils.Because of the presence of asphaltenes,some of the wax crystals have more defects and exhibit non-crystalline properties,resulting in a decrease in crystallinity index and less than the wax precipitation amount.Based on the scaling theory,it is found that the gel structure of model oils changes from strong-link regime to weaklink regime with the increase of the wax precipitation amount,and the wax precipitation amount corresponding to the transition increases with the increase of asphaltene contents.From the mesoscopic wax crystal morphology,as asphaltene content increases,both wax crystal size and grain size show a decreasing trend.From the microscopic wax crystal structure,with the increase of asphaltene content,the lattice parameters a and b are almost unchanged,while the lattice parameter c and △n are significantly increased,indicating that the conformation of wax crystals gets more disordered,which results in the weakening of gel strength,corresponding to the decline of the measured yield stress of the waxy oil gel.The zeta potential on the surface of wax crystals also increases significantly,which indicates that with more negative charges adsorbed on the wax crystal surfaces,stronger electrostatic double layers have formed.The greater electrostatic repulsion among wax crystals would interfere with the growth and aggregation of wax crystals,thus holding up the gelation of waxy oils.The zeta potential on the surface of wax crystal is another main factor affecting the gelation.Next,the effect of resins on the gelation of waxy oils and its mechanism are studied.When the resin content is 0.0～5.0 wt%,the influence of resins on the WAT,wax precipitation amount,gelation temperature and yield stress of waxy oils is first reduced and then increased,and the minimum point is at 1.0 wt %.The lattice parameter c and △n shows a tendency to increase first and then decrease,indicating that the conformation disorder of wax crystals first increases and then decreases.With the increase of resin contents,the zeta potential on the surface of wax crystals increases significantly,and the electrostatic double layer results in a greater electrostatic repulsion among wax crystals.The results of gelation temperature and yield stress show non-monotonic changes,indicating that the electrical properties has less influence on gelation than wax crystal structure.By changing the content of polar solvent o-xylene,it is found that the increase of o-xylene content leads to the decrease of resin particle size and the increase of dispersion,the wax crystal morphology gradually changes from agglomerate into uniform distribution.The yield stress increases significantly,and the lattice parameter c and △n decrease,which are the main reasons for the increase of yield stress.Finally,the effect of asphaltenes on the gelation properties of model oils containing different polydispersity of n-alkanes and its mechanism are studied.As the polydispersity of n-alkanes increases,WAT and gelation temperature decrease first and then increase,the minimum values are both when carbon number distribution variance is 2.0,while the yield stress decreases monotonously.After asphaltenes are added,the decrease of gelation temperature and yield stress becomes larger,that is to say,the inhibition of asphaltenes on the gelation behavior of model oils with stronger polydispersity of n-alkanes is more significant.The particle size and average grain size of wax crystals show a tendency to decrease at the same time,and the morphology of wax crystals gradually changes from rodlike to needle-like.With the increase of polydispersity of n-alkanes,the increase of lattice parameter c and the decrease of yield stress are more and more large,which further indicates that the lattice parameter c,which represents the microstructure of wax crystals,is closely related to the structure strength of waxy oil gels.