Design Of Zeolites And Hydrocracking Catalyst For Maximizing Middle Distillates

Petroleum resource shortage and recent strict environmental regulations have brought more attention to effective utilization of petroleum products and clean fuel processes and technologies. Hydrocracking is a kind of effective catalytic process for heavy oil conversion and the only technology that can produce clean fuels to meet the Euro IV emission standard. The development and utilization of hydrocracking catalysts for maximizing high quality middle distillates is one of the hot topics in petrochemical industry. The key issue for improving heavy oil conversion ability, activity and middle distillate selectivity of catalysts is to develop the hydrocracking catalysts with suitable acidity, hierachical pore structure zeolites and higher hydrogenation activity components. Based on the latest research results of zeolite modification, hydrogenation active phase characterization and mesoporous material synthesis, the target of this thesis is to achieve a tailored design for hydrocracking catalysts producing maximum middle distillates. Modified large pore Y zeolite and novel catalytic material mesoporous Al-SBA-15/Y zeolite composites were first prepared. Then heavy oil hydrocracking performance of both zeolites and the influences of an acid support and the W-Ni hydrogenation active components on catalyst surface properties and catalytic performance were studied. The W-Ni hydrogenation active phases of catalysts and the influential factors on heavy oil hydrocracking performance and middle distillate selectivity of hydrocracking catalysts were also investigated. Structure and properties of zeolites and catalysts were characterized by XRD, N2 adsorption, TEM, FT-IR, and TG-DTG. The main results are as follows:The effects of chemical and hydrothermal treatment methods on Y zeolite framework structure, surface acidity and pore structure during large pore Y zeolite modification procedures were studied. The forming mechanism and forming rules of secondary pores in Y zeolites were discussed. The modified products were then characterized by XRD, N2 adsorption, TEM and FT-IR techniques. Modified large pore Y zeolites with total pore volume of 0.688mL/g and mesopore volume of 0.548mL/g were prepared by novel chemical-hydrothermal treatment methods. Heavy oil hydrocracking performance of the catalyst using modified large pore Y zeolites as support was evaluated by using Daqing VGO (Vaccum Gas Oil) as feedstocks in a pilot hydrogenation unit at a conversion of 75v%. The hydrocracking performance evaluation results revealed that the catalyst using modified large pore Y zeolite prepared by novel method achieved a middle distillate selectivity of 80.82% and a middle distillate yield of 64.86wt%, which showed that the catalyst had more excellent catalytic performance than that using modified Y zeolite prepared by traditional method.Mesoporous Al-SBA-15/Yzeolite composites were prepared by a two-step method in moderate acid media and their forming mechanism was discussed. Examined were their acidity stability under different hydrothermal treatment processes, acid catalysis reaction performance using probe molecules of different size, and the catalytic performance of the catalyst using as-prepared materials for hydrocracking heavy oil. The forming mechanism of Al-SBA-15/Yzeolite composites was investigated. Mesoporous Al-SBA-15/Yzeolite composites were characterized by N2 adsorption, XRD and TEM techniques. The catalytic cracking reaction results revealed that Y/Al-SBA-15 composites possessed high catalytic activities in the cracking of both small (cumene) and large molecules (TIPB), which might be related to some large pores and strong acidic sites in the Y/Al-SBA-15 composites. More importantly, for the hydrocracking of vacuum gas oils, the lower BMCI value of the tail oil showed that Y/Al-SBA-15 composite catalysts have significantly superior performance on the conversion of large hydrocarbon molecules.Hydrocracking catalysts for maximizing middle distillates were prepared using modified large pore Y zeolite or mesoporous Al-SBA-15/ Y composites as main acid components,γ-Al2O3 and amorphous silica-alumina as carrier, and Ni-W as catalytic active phases. The effects of acid components, Ni/( Ni+W ) atomic ratio and promoters on catalyst surface properties and hydrogenation activity were examined and characterized. Characterization results of hydrocracking catalysts for maximizing middle distillates showed that the catalyst acidity types were mainly L acid sites with few B acid sites by Py-IR, and more weak acid sites and less strong acid sites at 150-400℃by NH3-TPD. The proportion of catalyst pore size of 2-4 nm was more than 85%, and the pore size exhibited bimodal distribution at 2-4 nm and 4-10 nm by N2 adsorption. W-Ni hydrogenation active components showed excellent dispersion on the hydrocracking catalyst by XRD and SEM-EDS. Hydrogenation active phases were W-Ni sulfide species, which were the reduction phases at 400 - 850oC (especially 400 - 700oC) by O2-TG characterization.The catalyst performance was evaluated by both 1800hr activity and stability test and 1000hr nitrogen tolerance test using Daqing VGO as feedstocks in the pilot hydrogenation unit at a conversion of 75v%. The deactivation rate of the catalyst was 0.013℃/day, and middle distillate selectivity was over 82% and 78% respectively, showing the catalyst's good activity and stability, as well as good nitrogen tolerance performance and heavy oil catalytic performance.