Study On Diffusion Behavior Of Hydrocarbon Molecules In Molecular Sieves For Oil Refining By Zero Length Column Method

Zeolite catalysts are widely used in the process of catalytic cracking and hydrocracking in petroleum processing.The diffusive performance of hydrocarbon molecules in zeolite is very important to its catalytic conversion process,which not only affects whether the reaction goes on wheels,but also affects the service life of the catalyst.At present,the research methods of diffusion performance are more complicated and time-consuming,and lack of direct structure-activity relationship with diffusion performance and molecular structure of hydrocarbons in molecular sieve.In this paper,zero length column method is used to study the diffusion results of normal,cyclic and aromatic adsorbates in ZSM-5,and the diffusion results of ZSM-5 and Y-type zeolites commonly used in oil refining are compared.O-xylene,p-xylene,ethylbenzene,n-octane,toluene,benzene and 1,3,5-diisopropylbenzene were selected as adsorbents,and the diffusion behaviors of them in ZSM-5 and SBA-15 zeolites were studied.Besides,the effects of structure and acidity of ZSM-5 on diffusion were studied.The diffusion results of different adsorbates in ZSM-5 were studied by model expansion method.It was found that the diffusion time constant decreased sharply with the increase of n-alkane carbon number and then tended to be flat.The diffusion results of cyclohexane were two orders of magnitude smaller than that of n-hexane,and the diffusion results of benzene were between them.The results show that the critical diameter of adsorbate is not the only factor that affects the diffusive performance,but also the molecular size and the corresponding steric resistance are to be considered.For the pore size of Y zeolite is larger,its diffusivity was larger than that of ZSM-5,which shows that pore size is an important factor affecting the diffusivity of different zeolites.The diffusion coefficients of the six kinds of adsorbates in ZSM-5 and SBA-15 zeolites were calculated by model expansion method.It was found that the diffusion coefficients of all the adsorbates in SBA-15 zeolite were about three orders of magnitude higher than those in ZSM-5,indicating that the structural characteristics of onedimensional straight channels of SBA-15 zeolite promotes the diffusion of hydrocarbons.Under the control of kinetics,the diffusion coefficient of p-xylene in ZSM-5 zeolite is 2.7times that of o-xylene,while the results of two kinds of adsorbate molecules in SBA-15 zeolite are similar,which shows that the special shape selection effect of ZSM-5 limits the diffusion of o-xylene in ZSM-5 molecular sieve channel.The diffusive properties of benzene in microporous ZSM-5 zeolites with different Si/Al ratios show that the difference of molecular sieve pore structure and its surface properties are the main reasons for the different diffusive properties.While ensuring the volume of micropores,the increase of mesopores will be beneficial to the diffusion of benzene in the channels.