Study On Selectivity Of Hydrocracking Catalysts

Hydrocracking is a kind of basic process in petroleum refining which is adopted to convert heavy oils into high quality middle distillates. Demands on middle-distillates selectivity have become increasingly severe; and a key problem to improve catalyst technology is how to achieve objectives for maximum middle-distillates selectivityTo assess the catalytic performance of hydrocracking catalysts, a new laboratory method using decane as the model compound was developed based on micro-reactor study; the data obtained from the tests can be used to compare the activities and selectivities with the pilot plant performance.A series of factors were systematically studied, which were related with the catalytic performances, in particular their impact on middle-distillate selectivity. Main conclusions were as follows:It was demonstrated that the higher strength acid sites showed a higher hydrocracking activity , which to a certain extent was in compliance with the "Brónsted" rules developed in liquid phase.It was put forward for the first time that the ratio of strong acid site and weak acid sites played an important role on catalytic performances. Increasing this ratio could enhance the middle-distillate selectivity.There probably exists a "synergistic effect" between the mixed two-type zeolites in hydroeraeking catalyst. The so called "synergistic effect" was caused by directional transfer of intermediate compounds from Y zeolite to H ~ zeolite, resulting in enhanced activity and isomerization reaction simultaneously.The impact of zeolite and amorphous silica-aluminum properties on the hydrocracking performance was studied, leading to the following outcomes:(1). With an increasing zeolite content, the catalytic activity increases, whereas the middle-distillate selectivity decreases. The content of Y zeolite in the catalyst aiming at producing middle-distillates should be 1500 20% or less.(2). At a constant zeolite content, reduction of zeolite cell could enhance the middle-distillate selectivity with decreased catalytic activity, but it caused only activity decline and little improvement in middle-distillates yield when the unit cell was below 24.35A(3). Proper amount of extra-frame alumina in hydrothermaldealuminated USY was advantageous to catalytic performance.(4). Reducing the diameter of zeolite crystals has little effect on the isomerization of hydrocracking catalyst, however it can change the ratio of different isomers in the products.(5). Secondary-pore volume plays an important role on hydrocracking catalytic performance. The bigger the secondary-pore volume, the more the isomerization of the catalyst.(6). The property of amorphous silica-alumina can also affect the catalytic performance. The maximum activity can be obtained by using amorphous silica-alumina containing 40% silica.To a certain extent, the larger the amount of metal in hydrocracking catalyst, the greater the hydrogenation function of the catalyst ,and the more the isomerization and selectivity for middle distillates. For a catalyst with 15 percentage of USY, the maximum amount of metal oxideswas 28Oo~30% (atNi/ (Ni±W) =0.5)to make a better middle-distillate hydrocracking catalyst, the carrier should have the properties characterized by a high acid strength, deeply clealuminated Y zeolite with high crystallinity, more mesopore volume, and a porper content of H fB.To prepare the zeolite that could meet the need of hydrocracking catalyst, the hydrothermal-acid treatment process was studied.It was found for the first time that two phases existed in hydrothermal treatment stage, i.e., the "initial stage " and the "elevation stage", dealun-iination kinetics equations given, respectively.After systematical study on the formation of secondary pore, a new secondary pore formation mechanism was proposed. The phenomenon regarding increasing in the total pore volume during zeolite hydrothermal treatment process can be explained by this mechanism.Pre-leachi...