Competitive Adsorption And Lubricity Of Components Of Biodiesel Based On Molecular Simulation

In order to effectively reduce harmful emissions from engines,it is urgent to improve the technology for the production of refined fuel.In current desulfurization process for refined diesel,a large number of polar substances containing oxygen,nitrogen and so on were removed.Therefore,the low sulfur diesel is in lack of lubricity.In this study,the fatty acid esters of methyl myrisate(C14),methyl palmitate(C16),methyl stearate(C18),methyl oleate(C18:1),methyl linoleate(C18:2)and methyl ricinoleate(C18:1 OH)were employed as the lubrication additives of the low sulfur diesel respectively.The lubrication test was carried out in the four ball friction and wear machine.Moreover,with the assistance of commercial software Materials Studio,the adsorption behavior of fuel blends with esters on the Fe surface was simulated.Effects of fatty acid ester concentration,carbon chain length,number of unsaturated C=C double bond and polar group-OH hydroxyl on the lubrication performance of low sulfur diesel and the mechanism of competitive adsorption of mixed fatty acid esters on Fe surface will be revealed.The major work is listed as follows:(1)Different fatty acid esters were added into low sulfur diesel with a dosing level of 1%,2%,3% and 5% respectively.The lubricity of fuel blends with esters was evaluated in a four ball friction and wear machine.The experimental results reveals that with the increase in the concentration of fatty acid ester,the low sulfur diesel blends presented better lubricity.The adsorption of n-tetradecane with various fractions of methyl myristate(C14)on the Fe surface was simulated in Materials Studio.It is revealed that with the increase of ester concentration,the adsorption energy and cohesive energy in mixed molecules were increased.Normally,the adsorption energy is much grater than the cohesive energy.Combining with the lubrication test results,it is revealed clearly that increasing the concentration of esters in low sulfur diesel significantly enhanced the adsorption strength of oil film.(2)According to the lubrication test results for low sulfur diesel with various esters,the carbon chain length of esters,the number of unsaturated C=C double bonds and the polar group-OH hydroxyl group can enhance the lubrication performance of the low sulfur diesel in various extents.A simulation of adsorption of n-tetradecane with 5% fatty acid esters(C14、C16、C18、C18:1、C18:2 and C18:1 OH)on Fe surface was carried out in Materials Studio.The adsorption energies in the mixed moleculesof n-tetradecane with 5% fatty acid esters are relatively close.However,the cohesive energy increases with the change of the carbon chain length,the degree of saturation and the polar group,which can significantly enhance the stability and compactness of oil film.(3)Adsorption of a single molecule for each of 6 fatty acid esters on Fe surface was simulated based on Monte Carlo method in Materials Studio.It is revealed that the adsorption energy for each of C14,C16 and C18 was dominated by their rigid adsorption energy,while the molecule deformation presented few influence on the total adsorption energy.With the increase in the number of C=C double bond,the rigid adsorption energy and molecule deformation energy of C18,C18:1 and C18: 2change in an opposite way,but the variation of the rigid adsorption energy is far greater than the molecule deformation energy.The adsorption energy of C18,C18:1and C18:2 are almost similar,which indicates that the C=C double bonds has little influence on the adsorption energy.The-OH hydroxyl group in C18:1OH greatly enhance its polarity,and it had higher rigid adsorption energy than the C18:1 and C18:2.So it has higher adsorption energy than other esters.(4)A simulation of competitive adsorption for esters mixture,including C14/C16/18,C18/C18:1/C18:2 and C18/C18:2/C18:1OH,was performed.The results show in the adsorption layer of C14/C16/C18 mixture,C14 took the major.Although the saturated esters with longer carbon chain have higher adsorption energy,the short chain ester was adsorbed preferentially.While in the adsorption layer of C18/C18:1/C18:2 mixture,C18 took a great account,because esters with more indeformable C=C can’t be easily adsorbed parallel to the Fe surface.In the adsorption layer of C18/C18:2/C18:1OH mixture,the majority was C18:1OH,since the polar hydroxyl group in the C18:1OH molecule sinks at one end and prefers to adsorb on Fe surface.