Study On The Relationship Between Structure And Performance Of FCC Flue Gas Sulfur Transfer Catalysts

With the rapid development of the industrialization process, environment pollutionbecomes increasingly serious; especially for sulfur oxides which have become one of theenvironmentally-unfriendly pollutants. As to the refineries, after reacting with the water vapor,the sulfer oxides in the flue gas increase the corrosion of the regenerator and other equipments.Meanwhile, sulfur oxides emited to the will cause acid rain which undoubtedly exerts adestructive impact on humanity and ecosystem. With the advent of highly sulfer-containedraw material in FCC and legislation which rigidly restricts the discharge of the pollutants, theproblem about how to lower the SO₂content in the flue gases has been concerned recently. Inthe aspect of the catalysts, sulfur transfer catalysts with2%³%MgAl2O4has been becomingthe cheapest and most effective means. However, some adverse properties, such as thealkalinty for the spinel, the high cost of the active components and the weak reducing capacitymake it to be less active for the reaction extent, distribution of the products and economicefficiency.Therefore, it will effectively lower the discharge of SO₂to add a moderate amount ofsulfer-transfer in the main catalysts, meanwhile, which will not influence the properties of themain catalysts. This work mainly focused on two aspects. The first is to enhance the oxidationof the sulfur transfer catalysts in order to accelerate the SO₂oxidation to SO₃, and explore thecomposition and performance of the new sulfur transfer catalysts. The second is to improvethe reduction performance of sulfur transfer catalysts, and analyze the relationship betweenthe different reducing atmosphere and the reduction product.In this work, sulfur transfer catalysts with different active components prepared withacid-gel method were discussed firstly. Then the element Mn in the spinel was evaluated ascatalysts under conditions similar to FCCU regenerator. A series of M/MnAl mixed oxideprecursor was prepared by acid-gel method. Sulfur transfer catalysts mixed with oxides was prepared after calcination, and the composition and performance of this new style sulfurtransfer catalyst was preliminary explored. In addition, the impact of carbon monoxide on theoxidation and adsorption was studied. The results showed that the copper is good activecomponents in sulfur transfer catalysts. Catalytic activity does not decrease but reductiveability is not obvious when carbon monoxide exists.To increase the desulfurization of sulfur transfer catalysts, it should be taken into accountthe consequence of high efficiency of the active component on the desulfurizationperformance and specific surface areas of the sulfur transfer catalyst. In order to increase thespecific surface area, the preparation methods and conditions were optimized. Hence,enhancing the contact of sulfur dioxide and surface activity, and the adsorption performanceand reduction property were improved. The results indicated that SO₂emissions can bereduced when they have higher specific surface area and suitable pore volume.In order to study the relationship between the product and surface structure in the sulfatespecies, in the combined-type pulse reduction device, pulse reduction reaction of sulfurtransfer catalyst, reduction products with in-situ test and the analysis of the connectionbetween reducing atmosphere and reduction product were carried. The reaction mechanismA. Bhattacharyya proposed was optimized based on the previous research, and a new reactionmechanism was deduced. Considering the signal strength and the reduction temperature ofH₂S and SO₂, the reducing capacity of the three gases was compared.An appropriate adsorption model was established by using the Canonical Ensemble MonteCarlo method, and several characters of SO₂, SO₃on different metal surfaces have beensimulated including adsorption site, adsorption micro-configuration and the energy structure.As can be seen from the simulated results, single component of SO₂adsorbed on MgO（200）crystal surface has better adsorption properties, meanwhile SO₃molecules mainly adsorbesnear the crystal surface of the aluminum atoms. Sulfur dioxide on MgO（200） crystal surfacehas a great influence on the adsorption site of sulphur trioxide, which makes the spacesbetween SO₃and MgO（200） crystal surface shorter.