| Emulsions are one of the most widely used systems in colloid chemistry.Therefore,studies on the emulsions have always been a subject of great interest.Demulsification of oil emulsions with high efficiency is an important process in oil field extraction and desalination/dehydration in refineries.The emulsification and demulsification usually accompanied by the processes such as flocculation,coalescence,formation of multiple emulsions and phase transitions because of its complexity.Specialized and complicated techniques and instruments are required to track and record it.Moreover,this process changes very quikly.Therefore,scientific research progresses in this area are relatively slow.For above reasons,Whether macroscopic nature or microscopic phenomena,it needs more further understanding.The computer simulation method can effectively compensate for the lack of experimental research.It not only can clearly and directly observe the conformational arrangement of all molecules in system,but also can obtain the action mechanism of molecules at the molecular/mesoscopic level.Besides,the dynamic evolution process of the system is tracked and recorded in real time to obtain its dynamic nature and law of motions.The microscopic process of demulsification is generally divided into three steps:the demulsifier diffuses from the bulk phase to the oil-water interface,the demulsifier adsorbs at the oil-water interface,and the droplets flocculate and coalesce.In this paper,a simple and highly efficient method-dissipative particle dynamics simulation(DPD)was used to study the following block polyether:block polyether with different branch lengths,different number of branches and different EO/PO block ratios.The displacement behavior of block polyether is explored by investigating the concentration distribution of the interfacial groups and the conformation of the interfacial molecules.And the demulsification performance of block polyether is explored by studying the diffusion rate,flocculation coalescence rate constant in the emulsion system.And the microscopic dynamic evolution process of demulsification was tracked.Studies show that,in the polyether systems with different branch lengths(D5a,D5b,and D5c:increasing in length),the quantitative description of the interface molecules by the concentration distribution function defines the order of the interactions between the various substances and the water molecules:PO groups<asphaltene molecules<EO groups.And the order proves that the EO groups of block polyether are the substitution of asphaltenes at oil-water interface.The adsorption modes of D5a,D5b and D5c are:"attached" adsorption,"jellyfish-like" adsorption and "seaweed-like" adsorption,respectively.The interface space occupied by the D5b and D5c is lager,indicating that they have better exclusion effect.Combining the displacement action and exlusion action,the order of replacement efficiency of block polyether at the oil-water interface is obtained:D5a<D5c?D5b.Among them,the efficiency of D5b and D5c is basically the same.In the demulsification process,due to the fluffy adsorption mode,the D5b and D5c polyether molecules can make droplets coalesce more efficiently than D5a.While one D5c molecule can be stably adsorbed at the interfaces of two droplets at the same time due to the excessively long chain,which in turn slows the diffusion of D5c and the aggregation of droplets to some extent.Therefore,the order of coalescence rate constant of block polyether is:D5a<D5c<D5b.As a result,for the size of the simulation system in this paper,the demulsification efficiency of D5b is optimal.The same method was used to analyze the block polyether system with different branched chains(D1,D5,and D9:representing linear,5-branched,and 9-branched,respectively)and different EO/PO ratios(D5(1/3),D5(3/1):EO/PO ratio is 1/3 and 3/1,respectively).The researches show that the interface displacement efficiency of D5 and D9 is higher than D1,while the former two have no big difference.In the process of demulsification,although the "jellyfish-like" adsorption mode of D9 has advantage in adsorbing other droplets then coalescing the droplets,the larger molecular volume makes the diffusion rate of the D9 not as fast as that of the D5.The order of the coalescence rate constants is:D1<D9?D5.Therefore,the demulsification performance of D5 polyether is optimal,but the performance of D9 polyether is not much different.For the different block ratios system,the D5(1/3)molecule has higher interface displacement efficiency than the D5(3/1)molecule.The tighter"attached" adsorption mode of D5(3/1)results in a lower diffusion rate,so the rate constant for coalescence of D5(1/3)is higher than that of D5(3/1).Therefore,by comprehensive comparison,the demulsification performance of D5(1/3)is slightly better than D5(3/1).In addition,two ways of making droplets coalesce that may exist in all emulsion systems are clearly and concretely revealed for the first time through the tracking record of the evolution process.In the case of where the two droplets are connected by the "dumb-bell"pattern of the asphaltene aggregates,the block polyethers adsorb to the vicinity of the asphaltene aggregates connecting the two droplets,then the aggregates are gradully destroyed by the interaction of the EO groups of block polyethers with the heteroatom group of the asphaltenes.Finally,the droplets can not exist stably and then coalesce;In the case of that the droplets disperse in the bulk,one or more of the branches of block polyether adsorbing on the surface of the droplets are free in the bulk.Then the EO groups of free branches take the opportunity to adsorb the heteroatoms of asphaltenes or the water of the lacuna on the surface of other droplets followed by that the droplets are fully connected by block polyether and then coalescence together.The above analysis of the performance of block polyether can provide reference for the design and optimization of demulsifiers. |
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