| As engine emission standards became stricter,the sulfur content in diesel fuel was getting lower and lower.Vehicle diesel was desulfurized to meet the national sulfur quality requirements(less than 10 ppm)of ultra-low sulfur diesel(ULSD).Surfactants such as lubricants,anti-wear agents and stabilizers were added to the desulfurized diesel to maintain ULSD quality.The presence of surfactants improved the stability of the water-fuel emulsion and increased the difficulty of water-fuel separation.After being sheared by the pump blades,the emulsified droplets(3-45 ?m)flowed with diesel into the high-pressure common rail system,causing damages such as the corrosion of the engine internal components,the blockage of the fuel injection nozzle(10-15?m)and engine broken down.Therefore,the removal of emulsified water in diesel by coalescence mechanism was an urgent problem to be solved.Fluorinated resins were commonly used coating polymers for oil-water separation.It had good chemical resistance,pollution resistance and excellent water-fuel separation performance.Among them,fluorinated polyurethanes FC1(X-[CH2CH2O]nCH2CH2O-Y-NH-COO-CH2CF2CF2CF2CF3)with special wettability had a better separation effect on surfactant-containing diesel emulsions.It had very good application prospects,but its coalescence mechanism was still unclear.Therefore,in this paper,by studying the interfacial rheological properties of lipophilic surfactants on the water-diesel interface,and the solid-air/solid-liquid surface/interface properties of fluorinated polyurethanes FC1(solid surface free energy,solid-liquid interfacial free energy,adhesion work),to explore its special wettability and its coalescence mechanism.The main results achieved in this paper are as follows:(1)In the single surfactant system,the interfacial rheological property of surfactant at the water-diesel interface was related to its emulsion stability.In the static state,the concentration of different lipophilic surfactants(PETO-B,high-purity dimer acid,high-purity trimer acid,monoolein),the molecule weight,the number of polar groups,the length of alkane chain and the branch chains affected the water-diesel interfacial tension.In the water-diesel separation experiment,the separation efficiencies of stainless steel felt for diesel emulsion containing 300 ppm monoolein and 300 ppm Pentaerythritol oleate(PETO-B)were 17.5%and 91.8%,respectively.The elastic modulus of monoolein at the water-diesel interface was 15.5 mN/m,which was greater than that of PETO-B 4.21 mN/m.Monoolein tightly arranged on the water-fuel interface with a high elastic modulus,making water-diesel separation difficult.PETO-B loosely arranged on the interface with a low elastic modulus,making the oil/water separation easy.Therefore,the greater the elastic modulus of the surfactant,the higher the stability of the emulsion,which was consistent with the results of the oil-water separation efficiency.(2)In the binary surfactant system,monoolein and PETO-B with different mass ratios were prepared to study their contribution to the composite interfacial film.At a low mass ratio of monoolein(1:3),its relaxation process was restricted by steric hindrance provided by PETO-B.And its elastic modulus tended to be a constant with the increasing concentration.As the ratio of monoolein increased(1:1,3:1),the elastic modulus of the binary system increased with increasing concentration.Therefore,the interfacial viscoelastic modulus of the binary system surfactant was mainly derived from the contribution of monoolein.The addition of another surfactant with a complex structure will cause the two to compete for the adsorption site on the oil-water interface,loosening the arrangement of the tighten one.Therefore,the arrangement of surfactant on the oil-water interface in a binary surfactant system reduced the elastic modulus of the surfactant film and facilitated the oil/water separation.(3)The solid surface properties of the four fluorocarbon polymers were measured to analyze their water-diesel separation performance.FC1 with special wettability had high oil-water separation efficiency for surfactant-containing diesel emulsion.By the contact angle measurement and the five theories of Acid-Base,Kitazaki-Hata,Owens-Wendt,Kaelble-Uy and Wu,we calculated the solid surface energy,the solid-liquid interface free energy,and the adhesion work of the four kinds of fluorocarbon materials PTFE,PVDF,FC1,FC2.We can infer that the outermost layer of FC1 was fluorine arrangement,and the intramolecular polar groups had a certain influence on the outermost arrangement.The separation efficiencies of PTFE,PVDF,FC2 for surfactant-containing diesel emulsion were 34.4%,25.4%,and 26.5%.Only FC1 had a higher separation efficiency of 91.2%.Therefore,the results proved that the fluorine arranged in the outermost layer of FCl was not the key factor in adjusting coalescence separation performance.(4)The rolling angle,adhesion energy,and time-response of under-oil water contact angle measurement showed that the branched fluorocarbon chain of FC1 had fluidity.On the inclined stage or in the surfactant-containing diesel emuslion,the fluorocarbon chain was prone to incline to a certain extent,exposing the polar group-NH-COO-,and increasing the hydrophilicity of FC1.Then,the coalescence mechanism of FC1 was analyzed through the tilted model:in the surfactant-containing diesel fuel,the adsorption of the surfactant on the FC1 surface enhanced the influence of the intramolecular polar groups on the branched fluorocarbon chain,so that the branched fluorocarbon chain tilted to a certain extent,changing the wettability of the solid surface.The special property of FC1 was conducive to the capture of emulsified water,increased the residence time of water droplets on it,increased the opportunity of droplets'collision and coalescence,and improved the separation efficiency.At the meanwhile,it destroyed the high-elastic structure of the surfactant film,promoted the demulsification of the colliding emulsified droplets.In summary,FC1 with special wettability had good application prospects for surfactant-containing diesel emulsions. |
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