Study On The Catalytic Removal Of Trace Olefins From Aromatics

Aromatic hydrocarbons are important raw materials among which the BTX aromatics (benzene, toluene, xylene) are called basic organic raw materials, playing an extremely important role in in petro-chemical products. While aromatic streams contain undesirable olefin impurities which are affect the product quality and also deleterious to the followed technological processes and applications of aromatics. To meet aromatic purity standards, trace quantities of olefins must be removed before aromatic streams are sent to the petrochemical processes.Hydrogenation treating and clay treatment have been widely used for removing trace olefins from aromatics. Hydrogenation treating could remove olefins from a single narrow fractions, hydrogenation but it is difficult to fully take into account to wide distillate product like aromatics, and leads to loss of aromatics. Clay treatment is simple to operate, while the particular clay used has a very limited lifetime and could not be regenerated. However, the frequently emptied and reloaded clay tower resulted in high labor and financial consumption and gave rise to environmental problems.This research firstly introduced the alkylation reaction and catalytic method to remove the trace olefins in aromatics. According to a large number of literature and the research on the mechanism of de-olefins, the reaction between aromatics and olefins are belong to the scope of the alkylation reaction which with carbonium ion mechanism. Then on this basis, two different types of catalysts to remove the olefins were investigated and the research was as follows:(1) Zeolite catalyst:Different types of zeolites such asβ,ZSM-5 MCM-22,USY are used as the support and the results showed the catalyst with MCM-22 had the best performance. The MCM-22 zeolite was modified by ion exchange or impregnation of additives and the whole synthesis process such as the calcination temperature and iron exchange times and contents of additives were also optimized. The experimental results revealed that calcination temperature of catalysts at 550℃had the best catalytic activity and the optimum ion-exchange times was 2 times; ion exchange did not change the lattice structure of MCM-22 zeolite;while catalytic activity and lifetime was greatly improved after the loading of the lanthanum oxide and zinc oxide and the best content of them was to 8% and 6% respectively. (2) Modified clay catalyst:Three types of activated clay modified with different active components were studied to remove the olefins in aromatics. Experiments showed that lin-an clay modified with Lewis acid A showed a lifetime 3.5 times as long as the active clay, the calcination temperature was 150℃and the optimal loading amount of Lewis acid A was 10%; the lifetime of modified E-zhou active clay with the 15% loading of Lewis acid A was 3.3 times as long as the granule bentonite used in Jin-ling refinery. BET,XRD and TG-DTG and pyridine-FTIR were used to investigated the prosperities of the catalyst. Results showed that the catalytic activity can be correlated with the acid properties and the proportion of the acid site. The catalytic activity was increased with increasing surface acidity especially Lewis acidity while the surface area of the catalyst was reduced after the modification.The catalysts which with good catalytic activity and prolonged lifetime were conducted in the 1 kilogram,10 kilogram sidestream experiments and the industrial test. Both sidestream experiments and industrial tests showed that the newly catalyst, with a better performance to remove the olefins, a prolonged lifetime (3-6times as long as the commercial used clay), and also a good distribution of refined product.The results indicted the possibility of removal of the trace olefins in aromatics by catalytic reactions. It can make up the rapid inactivation and limited adsorption capacity of industrial clay, and also avoid the aromatic loss, hydrogen consumption and highly cost in the hydrogenation treating process, so remove the trace olefins in aromatic with catalytic method find a up a new way in aromatic refining, has important theoretical significance and great value for industrial applications.