Research On The Mechanism Of Micro-/Nano-composite Pour Point Depressants In Improving The Rheological Properties Of Waxy Oil

Owning the the high pour points and high viscosities at room temperatures,waxy crude oils bring huge problems in the economic and safety of pipeline transportation.Adding polymeric pour point depressants(PPDs)to waxy crude oil is a kind of efficiency oil transportation technology.In recent years,inspired by the excellent properties of polymer/inorganic micro-/nano-composites(not only keeping the original properties of polymers,but also possessing the specific properties of inorganic micro-/nano-particles),some scholars have dispersed the inorganic micro-/nano-particles into the polymeric PPDs to prepare the polymer/inorganic micro-/nano-composite PPDs to further improve the efficiency of PPDs.The preliminary study has found that the inorganic micro-/nano-particles could further improve the performance of polymeric PPDs,but the poor organic compatibility of inorganic particles restrains the theory and application development of composite PPDs.At the same time,it has been also found that the natural associated asphaltene particles(at micro/nano scale)in crude oil can cooperate with polymeric PPDs to improve the rheological properties of waxy oil at low temperatures,but the understanding of the relevant laws and mechanisms is seriously insufficient.Based on the above research status,in this paper,the modified inorganic micro/nanoparticles(organically modified nano-clay and organic-inorganic hybrid siloxane microspheres(including poly-methyl silsesquioxane microspheres and poly-amino-methylsilsesquioxane microspheres)),natural associated asphaltene particles and polymeric PPDs(POA and EVA)are chosen as the research objects to carry out the research on the mechanism of micro-/nano-composite pour point depressants in improving the rheological properties of waxy oil.The main research contents and conclusions are as follows:(1)Preparation and performance study of the organically modified micro-/nano-particlesUsing octadecyl trimethylammonium chloride(OTAC)as the intercalation agent,nanoclay particles were organically modified by cationic exchange reaction,and a series of organic-inorganic hybrid siloxane microspheres(including PMSQ and PAMSQ microspheres)with different particle sizes were prepared by hydrolysis-condensation method.The structure characteristics and oil dispersibility of the modified inorganic micro/nanoparticles were characterized.The effects of adding modified inorganic micro/nanoparticles alone on the rheological properties of waxy oil were studied with the aid of macro-rheological measurement,microscopic observation and thermal analysis.The results showed that,(a)OTAC can effectively insert into the interlayers of nano-clay,which enlarges the interlayer spacing of nano-clay and enhances its organic compatibility.Modified nano-clay can be dispersed in the oil phase in the form of loose flocculant particles with an average particle size of 30 microns.(b)The prepared PMSQ microspheres have centralized particle size distribution and good organic compatibility.The PMSQ microspheres can be dispersed in the oil phase as a single microsphere.After amino-modification,the morphology and size of PAMSQ microspheres almost remain unchanged,but with the increase of amino group content,the hydrophilicity and organic compatibility of the microspheres increase,which makes the microspheres tend to adhere and reunite in the oil phase.(c)Under static cooling conditions,PMSQ or PAMSQ microspheres occupy a certain space and weaken the interaction of wax crystals precipitated around the microspheres through spacial hinderance effect,which can inhibit the development of three-dimensional structure of wax crystals and improve the rheological properties of wax oil at low temperatures.However,under dynamic shear cooling conditions,the microspheres are difficult to weaken the precipitated wax crystals in motion to weaken the interaction between the wax crystals.On the contrary,the microspheres act as an additional dispersing phase and increase the concentration of dispersing phase in waxy oil,resulting in the slight deterioration of the rheological properties of waxy oil under dynamic cooling conditions.(2)Study on the synthesis and properties of organically modified nano-clay based composite PPDsTaking the comb-like POA PPDs and organically modified nano-clay as the research objects,the POA/clay composite PPDs were prepared by the solvent blending and melt blending method,respectively.The structure characteristics and oil phase dispersion of the composite PPDs were characterized.The rheological improvement rules and mechanism of the composite PPDs on waxy oil were studied by means of macrorheological measurement,microscopic observation and thermal analysis.The results showed that,(a)the POA PPDs molecules can insert or peel the modified nano-clay layer,further expanding the interlayer spacing of the modified nano-clay and hard to desorb or desorb.The composite PPDs prepared by solvent blending method can be dispersed in the oil phase to form the composite particles with an average particle size of 6 microns,while the melt blending method can promote the interaction between POA molecules and modified nano-clay,further peeling off and dispersing the clay layers,therefore,the dispersed particle size of the composite pour point depressant in the oil phase is reduced to 2 microns.(b)Composite particles dispersed in oil phase are the key factors to improve the molecular performance of PPDs: POA/clay composite particles can provide a large number of heterogeneous templates for the crystallization of wax molecules during cooling crystallization,promoting the crystallization of wax molecules and leading to the formation of larger,more compact and regular wax crystals or wax crystal aggregates.Large wax crystals reduce the interfacial area between crystals and liquids,and the compact and regular wax crystal structure is difficult to occlude the flowable liquid oil,which makes it difficult for waxy crude oil to form a strong three-dimensional network structure of wax crystals.(c)Enhancing the organic modification degree of the nano-clay can increase the organic compatibility of clay,thus further improving the performance of POA/clay composite PPDs and increasing the time-efficiency of composite PPDs.(d)Compared to the solvent blending method,the composite pour point depressant prepared by melt blending method promotes the formation of wax crystals or wax crystal aggregates with larger size and more compact structure,and has better effect on improving the rheological properties of wax oil,so melt blending method possesses more advantages.(3)Study on the synthesis and properties of the organic-inorganic hybrid siloxane microspheres based composite PPDsTwo kinds of the composite PPDs,EVA/PMSQ and EVA/PAMSQ,were prepared by melt blending method.The rheological properties of wax-oil system were studied by macrorheological measurement,microscopic observation and thermal analysis.The adsorption behavior of EVA PPDs on the surface of microspheres was investigated by adsorption experiments and molecular simulation.Then,the mechanism of the rheological property improvement of waxy oil with the composite PPDs based on organic-inorganic hybrid siloxane microspheres were investigated and revealed.The results showed that,(a)in oil phase,the EVA molecules could adsorb and concentrate on the surface of PMSQ microspheres,forming EVA/PMSQ composite particles.EVA/PMSQ composite particles are evenly dispersed in the oil phase,changing the dispersion state of the original EVA molecules,and can be used as the templates for wax molecular crystallization during cooling process,so that wax molecules can be concentrated on its surface to nearby precipitate and grow,eventually leading to the formation of wax crystal aggregates with larger size and more compact structure.(b)At a settled EVA concentration,the adsorption capacity of EVA molecules on the PMSQ microspheres rises to the highest level when the dosage of PMSQ microspheres increases to 2.5 ppm,associating with the best performance of composite PPDs.(c)Compared to the EVA/PMSQ composite PPDs,EVA/PAMSQ composite PPDs have stronger nucleation template effect on the crystallization of wax molecules,and thus possess a better rheology improving performance.(d)On the premise of good dispersion of PAMSQ microspheres,the size and structure compactness of wax crystals precipitated increased with the increase of surface polarity of PAMSQ microspheres,and the rheological property improving performance of composite PPDs became better.(4)Theory and application of the natural associated asphaltene particles based composite PPDsBased on the preparation of synthetic waxy oil consisting of solvent oil,paraffin wax and asphaltene and the polymeric PPDs,the influences of paraffin wax properities,PPDs properties and asphaltene properties on the synergistic performance of PPDs and asphaltenes were studied by means of macrorheological measurement,microscopic observation,thermal analysis and molecular simulation.And a composite PPD based on the naturally associated asphaltene particles has been developed.The results showed that,(a)Adding the neat polymeric PPDs or neat asphaltenes can moderately improve the rheological properties of waxy oils at low temperatures,but adding the polymeric PPDs and asphaltenes together can apparently improve the rheological properties of waxy oils at low temperatures,meaning that the synergistic improving efficiency exists between polymeric PPDs and asphaltenes.The synergistic performance of PPDs and asphaltenes is gradually inhibited by increasing the wax content of the waxy oil.(b)Increasing the content of VA polar group in EVA pour point depressant properly can enhance the polar interaction between EVA molecule and asphaltene in oil phase,thereby significantly improving the synergistic effect of EVA pour point depressant and asphaltene rubber particles.The increase of saturated alkyl chain in asphaltene structure and the decrease of polarity of asphaltene rubber particles are beneficial to the decrease of dispersed particle size of asphaltene in oil phase,which makes the increasing solid-liquid interface and enhances the interaction between asphaltene,EVA and wax,leading to the more compact precipitated wax crystal aggregates.(c)The rheological properties of wax oil system improved gradually with the increase of EVA concentration(in the range of 0-1000 ppm)or asphaltene content(in the range of 0-3 wt%),but the synergistic effects of EVA and asphaltene were improved first and then worse,and reached the optimal at the concentration of EVA of 100 ppm or asphaltene content of 0.75-1.5 wt%.(d)Polymer PPD molecules can be selectively adsorbed on the surface of associated asphaltene particles through polar interaction to form polymer PPD/asphaltene composite particles,which can improve the dispersion of PPD molecules in the oil phase.In the cooling process of waxy oil system,the composite particles can be used as heterogeneous templates for wax molecular crystallization precipitation,which promotes the formation of spherical wax-like flocculants with larger size and more compact structure.In this way,the interface area between wax crystals and oil phases is greatly reduced,and the interaction between wax crystalline flocculants is weakened.On the other hand,the solvation layer thickness of wax crystals is effectively reduced,which makes it difficult for wax crystals to construct a three-dimensional network structure and greatly improves the low-temperature fluidity of wax oil.(e)According to the theory of natural associated asphaltene particles based composite PPDs,a composite PPD based on the natural associated asphaltene particles was developed.Compared to other composite PPDs based on the modified inorganic particles,the composite PPDs possess a much better rheological improving performance for waxy oils.Based on the above research,the theoretical system of micro/nano-composite PPDs to improve the rheological properties of waxy oil was initially established,which contributes to the development and application of new and efficient waxy crude oil pour point depressant.