Study Of SO_x Transfer Additives Applied In FCC Units

In FCC flue gas, there usually exist SOx, NOx and catalyst fines. SOx emission is mainly related to sulfur content in crude oil. About5-15%of the total sulfur fed is left on the catalyst as part of coke and is converted into SOx that are released to the atmosphere during coke burn-off in the regenerator, causing serious environmental problems. Among the measures for controlling SOx emission, the use of SOx removal catalysts requires almost no additional capital investments except for the cost of catalyst materials, is an economical and practical way to reduce SOx emission and more applicable in FCC units.There are usually about2-3%V2O5and14%CeO2in commercial SOx transfer additives, which increases both the toxicity and cost. One of our researches is to propose a new-type of MnMgAlFe mixed oxide SOx transfer additive. A series of hydrotalcite-like compounds were prepared by coprecipitation method and mixed oxides were obtained after calcination. This novel SOx transfer additive with effective De-SOx ability and regenerability can decrease toxicity and cost. The results are summarized as follows:1. Compared with MgA1-based SOx transfer additives, MnMgAlFe mixed oxides possess better SOx oxidative adsorption capacity and regenerability. Fe20.3plays a dual role, as an oxidizing and a reducing catalyst. The SOx adsorption capacity, adsorption rate and regenerability enhanced with Fe2O3content; MnOx could promote SO2oxidative adsorption and could also adsorb SOx itself. The SOx oxidative adsorption rate and capacity increased with MnOx content; The total amount of adsorbed SO2increased with an increase in Mg/Al molar ratio and at the ratio of3, the adsorbed SO2amount reached the largest, which may due to the beneficial Mg(Al)O structure for SOx adsorption.2. Addcording to the research, the best composition of MnMgAlFe catalyst for SOx abatement is Mg/A1=3(molar ratio), with5%Fe2O3and10%MnOx. After eight adsorption-reduction cycles, the catalytic performance was stable, suggesting good regenerability and durability of MnMgA1Fe mixed oxides derived from hydrotalcite-like compounds.In order to improve structural properties of mixed oxides, a series of CuMgAlCe mixed oxides were prepared through CTAB-assisted method. The results are summarized as follows: 1. Addition of CTAB increased the specific surface area and pore size of mixed oxides. CTAB/metal molar ratio of0.1reached the highest specific area142.2m2/g and higher ratio would decrease the specific surface area. The pore size increased with CTAB/metal ratio, because the much heat and gas from CTAB decomposition could destroy the pore walls and then increased the pore size.2. Surfactant (CTAB) to metal molar ratio had a significant effect on the catalytic activity. CuMgAlCe-0.1mixed oxide presented the best SO2adsorption rate and capacity. This behavior is mainly due to more adsorption sites provided by high specific surface area, which facilitates SO2diffusion and contact with active components.This work will provide some valuable experimental data as well as a theoretical base for the production and application of SOx transfer additive in industry scale.