Study On Synthesis, Properties And Mechanism Of Hydrotalcite-derived Sulfur-transfer Additives For FCC Flue Gas Desufurization

The fluid catalytic cracking (FCC) process, a petroleum refining process applied commercially on a very large scale, is one of the major source of sulfur oxides emissions, especially when crude oil has become heavier and more sour in the aggregate. In order to meet the increasing stringent environmental regulation, the utilization of sulfur-transfer catalysts has been the best choice in FCC units for the least expensive and most convenient alternative. Because of their excellent performance, Ce/MgAl hydrotalcite-derived mixed oxides have become the most popular basic system for the sulfur-transfer catalyst in the FCC process. On the basis of the mechanism of sulfur transfer in FCC unit, the influence of the synergistic effect of CeO2 and MgO, chemical composition and the structural and morphological properties on the sulfur transfer performance was investigated in this thesis. The method involving separate nucleation and aging steps (SNAS) was successfully applied in the preparation of the Ce/MgAl hydrotalcite derived mixed oxides and the excellent SOX pick-up performance was embodied. It's a new art and craft with high efficiency and low energy consumption for the preparation of sulfur transfer catalyst in FCC units. The details are shown below:1. The influence of the crystal size of CeO2 and MgO on the SOX pick-up activity of CeO2/MgO physical mixed oxides was proclaimed. The comparison of the SOx pick-up activity of CeO2/MgO mixed oxides with different crystal size showed that:The excellent SOx pick-up activity of CeO2/MgO mixed oxides roots in the synergistic effect between catalytic oxidation of CeO2 and large absorption capacity of MgO; During the initial reaction period, the apparent SOx pick-up rate depends on the SO2 catalytic oxidation rate decreasing with the increase of CeO2 crystal size; The maximum SOx oxidation and adsorption capacity, i.e. the effective availability of MgO, decreases with the increase of MgO crystal size.2. The relation between chemical compositions (Mg/Al ratio and CeO2 content) and the SOx pick-up activity was indicated. The Ce/MgAl hydrotalcite derived mixed oxides were prepared by the SNAS method, and samples with different Mg/Al ratios and CeO2 contents were attained by different feed ratios. The comparison of their SOx pick-up activity showed that:The decreasing of Mg/Al ratio increases effective availability of MgO, but decreases the theoretical maximum SOx pick-up capacity; By comparison, the mixed oxides with Mg/Al ratio 3 is superior to others, with almost 100% effective availability of MgO and the largest maximum SOx pick-up capacity; The rate of SOx pick-up increases with the CeO2 content increasing, but increases little while the CeO2 content over 8%. 3. The effect of the pore structure of Ce/MgAl hydrotalcite derived mixed oxides on the SOx pick-up activity was revealed. The Ce/MgAl hydrotalcite derived mixed oxides with different pore structure were prepared by the SNAS method and different aging time of the hydrotalcite precursors. The comparison of their SOx pick-up activity showed that:As a result of volume expansion by sulfation, the blockage of pores decreases the accessibility of the active sites (-Mg-O-structure) to SO3; The Ce/MgAl mixed oxides with the largest pore volume and the least pores less than 10 nm presented the largest SOx pick-up capacity.4. The SOx pick-up performance of the Ce/MgAl hydrotalcite derived mixed oxides by the three methods (impregnation, conventional coprecipitation and SNAS) were investigated and compared. The Ce/MgAl hydrotalcite derived mixed oxides by SNAS method possess the most reasonable pore structure (the largest pore volume, the largest average pore size and the least pores less than 10 nm) so that both the highest rate and the largest capacity of SOx pick-up were exhibited.5. A new reliable and anticorrosive evaluation device for SOx pick-up performance of the sulfur transfer catalyst was designed and exploited on the basis of thermogravimetric instrument. It can provide different respective reaction atmosphere by multiple sets of gas regulation and hybrid systems, and switch reaction atmosphere by multiplexing valve. The environment of the regenerator and the riser reactor of FCC were simulated, and the SOx pick-up performance of sulfur transfer catalyst was evaluated successfully.