Study On Synthesis, Characterization And Performance Of The Catalyst For Removing Trace Olefins From Aromatics

Aromatic hydrocarbon, an important raw material in petrochemical processes and refineries, can be obtained from reforming and cracking process. However, these aromatic streams always contain undesirable trace olefins which are harmful to the following technological processes and the applications of aromatics. The commercial clay was widely used to remove these olefins in mostly refineries with the drawbacks of limited lifetime and pollution. Pressure from legislative and environmental bodies together with a growing awareness within the chemical industry has led to a search for new eco-friendly products and processes to replace polluting reactions. In this paper, the clay was modified and increased its running time. And the high activity zeolite catalyst had been developed. The catalyst were characterized by BET, XRD, FT-IR, TG-DTG and NMR in order to find out the reaction mechanism and influencing factors of the reaction of removing trace olefins from aromatics. In addition, the deactivation of catalyst was studied. The main and important results are described as follows:This research firstly modified the clay with the Lewis acid. The optimal preparation condition for modified clay was that the modified clay was dried at393K for5h to remove the solvent and the moisture, baked in a muffle furnace at423K for2h. The Lewis acid amount of catalyst was2times than that of clay after modifying by15%ZnC12. The catalytic activity of this sample was also2times than that of the clay. The catalyst showed high catalytic activity when treating different aromatics. The conversion of olefins were50%,70%and80%, when the bromine index of three aromatics was about1200,650and400mgBr/100g. The industrial experiment was conducted in Sinopec Zhenhai Refining and Chemical Company. The result presented that the effective running time of the catalyst was5times than the clay used in industrial unit. The distribution of the aromatics was not changed after treating by the modified clay. The reuse experiment found the modified clay could not be recycled.Next, the zeolite was modified in order to develop a high catalytic activity and reproducible catalyst. According to the experiment, the optimal composition for USY zeolite catalyst was zeolite:Al2O3=75%:25%. The optimal preparation condition for the catalyst was=550℃. The modified process decreased the amount of B acid and increased the amount of L acid. The catalyst modified by0.2mol/L citric acid could keep the conversion of olefins over90%for6hours. The catalyst still remained6hours when the conversion of olefins above90%after once repeated experiments.Dealumination of USY zeolite proceeds in a manner of citric acid treatment. The Al removed from the framework of zeolite was existed as the form of AlO+species. The AlO+species can supply the Lewis acid sites in the zeolite. The positive charge sited on the extra framework AlO+was the main reason for the decrease in the number of B acid sites (70%decrease) and the increases in Lewis acid sites. An approach to the quantification of the property of acid sites has been possible. The relation between acid properties of zeolite and the ratio of Si/Al was determined.The amount of L acid in MCM-22zeolite was3times than the parent one after kneading with ZnCl2. The catalytic activity increased50%. The catalytic activity of MCM-22zeolite decreased, Because of decreasing of the amount of B acid in MCM-22zeolite, the catalytic activity was reduced after the citric acid treatment. The B acid site was one of the most important factors. More B acid site could lead high activity and rapid deactivation. The L acid site influences the stability of removing olefins reaction. The key point of high activity was suitable acid in zeolite.Finally, the mechanism of the reaction was explored using the model oil. It was proved that the reaction of removing trace olefins from aromatics was the typical alkylation reaction. The alkyl aromatics and polycyclic aromatic which were generated during the side reaction were the main reason for the deactivation of catalyst. The carbon deposits were mainly distributed in the pore of zeolite. The carbon deposits blocked inside the pores or on the surface of zeolites were responsible for their deactivation. The catalyst would be deactivation completely when the bigger and heavier polycyclic aromatic was generated.