Study Of The Selective Hydrogenation Catalyst For The Removal Of Diolefins In FCC Gasoline

For the FCC naphtha, olefins concentrate in the light cracked naphtha (LCN) fraction and sulfur in the heavy cracked naphtha (HCN) fraction. It can minimize the octane number loss in the process of hydrodesulfurization of FCC gasoline after splitting. This is the key for the technology of selective hydrodesulfurization of FCC gasoline in China. In this paper, a kind of presulfurized catalyst was prepared, which could promote the transfer of light sulfur to heavy sulfur while removing diolefins. After that, light cracked naphtha with low sulfur was obtained.A series of presulfurized Ni-Mo/Al2O3 catalysts for selective hydrogenation of FCC gasoline were prepared by step impregnation method. The preparation conditions were investigated, which could influence the activity and selectivity of catalyst. The catalysts were characterized by adsorption of N2, XRF, XRD, Py-FTIR and HRTEM.Experimental results showed that when optimum preparation conditions were selected (the mole ratio of complex to ammonium tetrathiomolybdate was 2.4, the calcination temperature of support was 550°C and no oxygen was 400°C, the content of Ni was 8.2% and Mo was 5.6%), the activity and selectivity of catalyst were relatively superior (the conversion of light sulfur was 66.0%, the diolefins convertion reached 61.2%, the selectivity of monoolefins reached 99.1%). The characterization results showed that there existed obvious differences between the physicochemical properties of final catalyst CA-1 and that of reference catalyst HR845.The process conditions of catalyst CA-1 were investigated and the optimal conditions were under a reaction temperature of 160°C, a reaction pressure of 2.2MPa, a LHSV of 4h-1 and a H2/Oil volume ratio of 20. After that, the stability of catalyst (CA-1) was measured and compared with HR845. The results suggested that the selectivity of monoolefins for CA-1 was similar to that for HR845 and the initial activity of CA-1 was a little lower than HR845, while the rate of activity descending more quickly than HR845. The stability of catalyst (CA-1) was not ideal. It was hoped that the activity, selectivity and stability of catalyst could be improved by changing the physicochemical properties of catalyst.Furthermore, the reaction mechanism of thiol and olefin was studied using model compounds. The results indicated that the reaction of thiol and olefin happened on the active sites of hydrogenation on catalyst surface. Hydrodesulfurization of thiol firstly produced adsorbed hydrogen sulfide, then the adsorbed hydrogen sulfide reacted with adsorbed olefin and formed macromolecular thiol or thioether. The whole process didn't produce hydrogen sulfide. It was presumed that the reaction mechanism of thiol and diolefin was similar to that of thiol and monoolefin .