The Study Of Preparation, Structure And Performance Of Micro-fiber Structured Catalysts Based On F-T Synthesized Wax Hydrocracking

Hydrocracking of F-T wax is an important process for LTFT synthisis to increase middle distillate selectivity and improve the oil quality. The F-T wax is prone to cracking deeply for the large particles of metal supported catalysts due to the restriction of interal heat transfer and diffusion factors. The carriers are degenerated due to the high calcination temperature during the catalyst preparation which cause the reduction of catalyst activity, when the catalysts are prepared by microfiber wrapped technology. In this paper, the preparation technology and process of microfiber structured catalysts for F-T synthesized wax hydrocracking process was improved. The microfiber foam, Ni and SS microfiber which were used to load the active metal by impregnation method were designed and prepared for preparation of three kinds of catalysts. The surface structure and properties of the catalysts were characterized via XRD, SEM and NH3-TPD. The relationship between the catalyst reactivity and micro-fiber structure, surface crystal form of zeolites were investigated. The influence of the reaction temperature, pressure, silica/alumina ratio and the amount of active components of zeolites on conversion and product selectivity are studied, therefore, the catalytic activity of new structured microfiber catalysts were investigated via hydrocracking reactions of F-T synthesized wax. The main contents and conclusions are as follows:The forth chapter, based on NiW bimetallic supported catalysts, studied the influence of adding ZSM-5 zeolite in amorphous silica-alumina carrier on reaction activity, catalyst acdity and acid strength, and appropriate amont of zeolites were determined. On this basis, the Pt loaded catalyst with additional 25% ZSM-5 zeolite in carriers were prepared (Pt/SAZ). Micro-fiber wrapped catalyst with 25% ZSM-5 zeolites (Pt/ZSM-5/Ni-fiber) and micro foam with growing ZSM-5 zeolite catalyst (Pt/ZSM-5/Ni-foam) were prepared as well. Therein, Pt/SAZ catalyst showed stronger acidity and the activity of hydrocraking reaction was higher, but the gas selectivity was also higher due to the limitation of poor heat transfer characteristics. The activity of Pt/ZSM-5/Ni-foam catalyst was similar to Pt/ZSM-5/Ni-fiber catalyst. The reaction activity of Pt/ZSM-5/Ni-fiber catalyst decreased due to the degeneration of catalyst under high temperature and the deactivity of Pt/ZSM-5/Ni-foam catalyst caused by high porosity foam structure.The fifth chapter focused on the preparation of ZSM-5 zeolite growth on the Ni microfiber catalyst (Pt/ZSM-5/NiMF) via microfiber surface dip-coating and secondary hydrothermal growth method which based on the substrate of sintered nickel microfiber. NH3-TPD curves showed that the higher silica/alumina ratio of ZSM-5 zeolite led to the lower acidity of the catalyst while better crystal of ZSM-5 zeolite formed on the microfibers. The hydrocracking activity results showed that Pt/ZSM-5/NiMF catalytic activity was stronger than that of conventional metal supported catalyst as the reaction temperature for reaching the same conversion rate was about 30-40℃ lower. This chapter also investigated the influence of reaction temperature, pressure, silica-alumina ratio of the zeolites on the reaction conversion and product selectivity to find the optimal parameters of process.The sixth chapter studied the preparation of the catalyst (Pt/ZSM-5/SSMF) with ZSM-5 zeolite growth on the SS microfiber based on last chapter. XRD showed the successful growth of ZSM-5 zeolite on the stainless steel microfibers. Pore structure analysis indicated that the catalyst had smaller pore volume and average pore diameter. The experiments of hydrocracking reaction with different silica/alumina ratio found that the reaction conversion rate was 95% and the selectivity of liquid product and gasoline were up to 78% and 64% with the condition of silica/alumina wt% ratio at 200, the reaction temperature of 280℃, a pressure of 3.5MPa and SV of 1h-1. The reaction activity of the catalyst was lower with increasing pressure and SV. The SEM characterization illustrated that the particulate debris on crystal surface appeared when the growth amount of ZSM-5 had reached 30 wt%, which indicated that the growth of the zeolites should not be too high. The hydrocracking reaction results of F-T synthesized wax showed that the conversion ratio could reach 98.7% when the growth amount of ZSM-5 zeolite was 25 wt%. This chapter also conducted the four lumping kinetic model of hydrocracking reaction of F-T synthesized wax with Pt/ZSM-5/SSMF catalyst classifying reaction materials and products into four lumps of C20+, C10-C20, C5-C9 and C1-C4. The residual analysis and statistical test showed that the model was accurate. Optimal simulation proved that higher SV and higher temperature might increase the yield of diesel, while higher temperature and lower SV help increasing the yield of gasoline.The seventh Chapter investigated the heat transfer characteristics of catalyst bed within the fixed bed under gas flowing state. The temperatue distribution of fixed bed with different process paramters was obtained for the catalyst of stainless steel microfiber with growing ZSM-5 zeolites. The partial differential equations was established which could describe the temperature distribution with different radial position and bed height. Heat transfer mathematical model was solved via orthogonal collocation method. The effective thermal conductivity was obtained and the data was 0.6-0.9W/(m·K), meanwhile, the wall heat transfer coefficient was 34.6-47.3W/(m2·K). The results showed that the new structured microfiber catalyst had better heat transfer characteristics in comparsion with traditional metal loaded catalyst. Then dynamic heat transfer experiments were conducted to detect the difference between realistic reaction temperature and the setting temperature based on new structured microfiber catalysts and conventional particulate catalyst. The results showed that the two micro-fiber structured catalysts had better heat transfer performance.Finally, this paper summarizes the work and prospects.