Influence Of Configuration On Diffusion Behavior Of Molecules In Micropores

High efficiency catalytic conversion of heavy oil has become one of the challenges faced by the oil refining industry.The diffusion resistance of heavy oil in the catalyst channel is more serious in the processing,which limits the efficient conversion of heavy oil.At present,the researchers take heavy oil,narrow fraction or the model compound as the object,investigate the diffusion behaviors in the porous membrane or the catalyst channel,and take the heavy oil molecules as spherical or spheroid to build the restricted diffusion model,so it is difficult to obtain accurate results of the diffusion behaviors.Few studies have considered the influence of configuration on the diffusion study towards petroleum and its fractions.In this paper,compounds of four configurations,spherical,dislike,rod like and flexible model compounds were used to investigate the diffusion behavior in porous polycarbonate membranes with aperture of 1000 nm,50 nm and 15 nm by diaphragm pool,including fullerenes,porphyrins,polycyclic aromatic hydrocarbons and thiophene.At the same time,the effect of solvent was also investigated as taking toluene and 1,2,3,4-tetrahydronaphthalene as solvent with quiet different viscosity.It is found that the molecular size and pore size are two main factors that affect the diffusion.Comparing the experimental results of model compounds under the same pore size,the degree of diffusion resistance is influenced by the relative mass and molecular configuration,the decrease of the effective diffusion coefficient of the different model compounds is different in the decrease of the pore size.For compounds with similar molecular configurations,the degree of diffusion resistance increases with the increase of relative molecular mass.The selection and determination of the characteristic size of the model compound is the foundation for the construction of the restricted diffusion model.In this paper,the free diffusion coefficient obtained by the diaphragm diffusion cell combined with the Stokes-Einstein equation is used to calculate the equivalent sphere size of the model compound.The optimal structure of the compound is constructed by Gaussian,from which Van der Waals equivalent sphere size,the critical size and the maximum molecular size are obtained.On this basis,four characteristic sizes are used to build the hindered diffusion model.It is found that the fitting effect of the equivalent sphere diameter obtained by the diaphragm diffusion cell is the best characteristic size.Among the three calculated sizes,the maximum size is better in the construction of the dislike molecule hindered diffusion model,the Van der Waals equivalent sphere size is better in the construction of rod like molecules.Comparing the following results F???=?1-??^4.63 for dislike molecular,F???=?1-??^6.13 for rod like molecular and F???=?1-??^5.99 for flexible large molecules.It shows that the degree of diffusion of molecules in the same size follows the order:rod like molecules>flexible macromolecules>dislike molecules>spherical molecules.When the compound deviates from the sphere,the greater the deviate degree is,the greater the degree of diffusion is hindered.By comparison,it shows that hindered degree of sulfide in the residue fractions is slightly larger than that of model sulfide.Comparing the diffusion results between two solvents,it is found that there is a significant difference in the numerical value of the diffusion coefficient due to the difference of viscosity,but the hindrance factor is very close,indicating that the effect of the solvent on the hindered diffusion is few.