Reinforcement Of Cracking And Sulfur Transfer Reactions In The Catalytic Cracking Process

FCC catalyst with better cracking ability and efficient sulfur-reducing additives are the current research focus.To reinforce cracking capbilities,?-Al₂O₃ and USY were modified to modulate their surface acid sites by increasing the ratio of Br?nsted/Lewis acid.Besides,nanostructured MgAl₂O₄ spinel was prepared to improve SO2 sorption.Sulfur-reducing additives supported on ?-Al₂O₃ or MgAl₂O₄ were prepared and the effects of acidity and basicity of additives on desulfurization and coking were tested.?-Al₂O₃ with various surface acidity were prepared using pseudo-boehmite via a sol-gel method.The results show that Br?nsted acid sites form when hydroxyl groups connected with six-coordinated Al atoms are partially substituted by F atoms.NH₄BF₄ modified ?-Al₂O₃ has rich Br?nsted acid site?BAS?but reduced Lewis acid site?LAS?.Besides,modified ?-Al₂O₃ possess good cracking capbility for n-decane.N-decane cracking occurs on LAS via the monomolecular protolytic cracking mechanism.Whereas,it is prone to the classical bimolecular mechanism on the BAS of ?-Al₂O₃.The FCC catalyst prepared with modified ?-Al₂O₃ as matrix has higher VGO conversion than that with usual ?-Al₂O₃ by 14.44%.In contrast,the yields of gasoline and diesel respectively increase by 6.59% and 3.65% while coke yield decreases by 3.05%?Modified ?-Al₂O₃ with rich BAS has better cracking ability but less coke formation in heavy oil conversion.It is also found that H₃BO₃ modified ?-Al₂O₃ has reduced LAS.Therefore,we further studied the modification of USY,pseudo-boehmite,and FCC catalyst with H₃BO₃ to reduce their Lewis acidity.The results show that Al atoms on USY framework are partially removed which makes the pore more clear after modification.Lewis acid on modified USY decreases drastically,leading to the increase of BAS/LAS ratio from 0.64 to 1.55.The conversion of n-dodecane increases by 8.92%?It shows that n-dodecane cracks through the classical protolytic cracking mechanism on USY.Similarly,modified FCC catalyst has lower LAS but an increase of B/L value form 0.55 to 1.17.Modified FCC catalyst improves the VGO cracking and produces more gasoline and diesel but less LPG and coke,showing better performances in hydrocarbon cracking and inhibiting coke formation.Nanostructured MgAl₂O₄ spinel was also prepared in the presence of glucose.The results show that nano-MgAl₂O₄ has higher surface area(324 m²·g^-1),centralized pore distribution?3.3 nm?,and good thermal stability.Nano-MgAl₂O₄ has higher SO2 pickup than usual MgAl₂O₄ by 51.58%,showing a pseudo-second order kinetic model.As a sulfur transfer additive,Fe/MgAl₂O₄ shows the highest SO2 pickup up to 5742 ?mol·g-1,lower initial reductive temperature at 530 °C,and highest reduction efficiency to 81%.Finally,sulfur-reducing additives using nano-MgAl₂O₄ and modified ?-Al₂O₃ as supporter were prepared and tested in VGO conversion.The results show that Lewis acid amount and the acidity strength increase with the introduction of ZnO.The sulfur contents of FCC gasoline decrease when adding additives into base catalyst.It is ascribed to the synergistic effect of acidity and basicity of Zn-Mg-Al ternary additive.However,excessive basic sites lead to higher coking.Besides,FCC catalyst are responsible for the in-situ desulfurization in VGO conversion via consecutive reactions,converting macromolecular sulfur species into smaller ones and finally cracking them into H₂ S.LAS is in favor of the selective adsorption and cracking of aromatic sulfur species while BAS helps macromolecular sulfur species cracking.The more LAS exists on additives,the higher desulfurization of aromtic sulfur species and coke yield are observed.In addition,it is found that both BAS and LAS contribute to hydrocarbon cracking.Light olefins are prone to polycondensation on BAS at high concentration levels of BAS and olefins,showing higher coke yield and CHT value.However,LAS selectively absorb heavy aromatics with potential dehydration reactions,showing higher yields of coke and H₂.LAS are more likely to lead to coke formation.