Study On Catalytic Cracking Of C₄、C₅ Olefins From MTO

Ethylene and propylene are important chemical industry materials. Ethylene is mainly produced from petrochemical cracking, while most propylene is from steam cracking and fluidized catalytic cracking as byproduct. These traditional processes cannot meet the ever-increasing demand of light olefins and oil resource is limited, so non-oil based routes for the production of ethylene and propylene have attracted much attention. Methanol-to-olefins (MTO) process can effectively convert methanol to light olefins. Even though the process has very high selectivities of ethylene and propylene, it also produces much C4 and C5 olefins. This paper focused on effect of SiO2/Al2O3 ratio, P modification, Ni modification and Fe modification on the structure, basicity, acidity and catalytic performance in C4、C5 olefins cracking of HZSM-5 catalyst. The kinetic model of C4、C5 olefins cracking in isothermal integral fixed bed was investigated, and the adiabatic and continuous heat-exchange fixed bed reactor simulation of C4、C5 olefins cracking was studied.Effects of SiO2/Al2O3 ratio and P modification on catalytic performance were studied. HZSM-5 zeolites with different SiO2/Al2O3 ratios were modified by phosphorus. The selectivity of propylene was obviously enhanced over P modified HZSM-5 zeolite with SiO2/Al2O3 ratio of 50. HZSM-5 zeolite with SiO2/Al2O3 ratio of 50 was modified with different P loadings through incipient-wetness impregnation. Results indicated that the increase of P loadings decreased Br(?)nsted acid amount, and weakened acid strength. Thus, P modification decreased butene initial conversion, depressed hydrogen transfer reaction and enhanced the cracking of octyl carbenium ion intermediates to propylene, increasing propylene selectivity. Reaction stability increased with P loadings. When P loadings increased to 3%, butene conversion kept stable.Effects of Ni and Fe on catalytic performance of P modified HZSM-5 were investigated. Different amounts of Ni were impregnated into 3P-50HZ catalyst. Ni interacted with previous added P, broke part Al-O-P bonds which formed during P modification, and increased the content of tetrahedral framework aluminum. As indicated by Py-IR, the addition of Ni could not only increase Br(?)nsted acidity, but also form new Lewis acid sites, and then improved catalyst activity in the cracking of butene. The increment of catalyst acidity enhanced pentene cracking and increased ethylene and propylene selectivity, but this was also beneficial to hydrogen transfer reaction and made the cracking of octyl carbenium ion intermediates to ethylene. Therefore, propylene selectivity slightly decreased when Ni loading was high. The addition of Ni decreased the reaction stability of P modified HZSM-5 catalyst.The impregnation of Fe into P modified HZSM-5 catalysts decreased the content of condensed polyphosphates and aluminum phosphate, increased the content of tetrahedral framework aluminum, strengthened Br(?)nsted acidity, improved catalyst activity, and increased ethylene and propylene selectivity in butene catalytic cracking. Reaction stability and propylene selectivity of Fe and P modified HZSM-5 were better than those of Ni and P modified HZSM-5. It was found that 2Fe/4P-50HZ catalyst owned best stable catalytic performance, and propylene selectivity could reach to 60% in the cracking of butene. In the coupling cracking of butene and pentene with 2Fe/4P-50HZ used as catalyst, the selectivity of propylene and that of ethylene could reach to 65% and 15%, respectively.Effects of reaction conditions on catalytic performance in the coupling cracking of butene and pentene were investigated. The impact of reaction temperature on reactant conversion and products selectivity was largest. As reaction temperature increased, butene conversion decreased due to the suppression of bimolecular cracking, while pentene conversion increased due to the enhancement of monomolecular cracking. The increment of reaction temperature was beneficial to C6+hydrocarbon cracking and inhibited hydrogen transfer reactions, enhancing ethylene and propylene selectivity. Butene and pentene conversion decreased with the increment of WHSV. As RSH (the weight ratio of steam to olefins in feed) rose, butene conversion significantly declined due to the decreasing partial pressure.The kinetic model of C4、C5 olefins catalytic cracking over 2Fe/4P-50HZ catalyst was studied under reaction conditions:temperature 490-610℃, WHSV 3-15h-1, RSH 0.18-0.9. A six-lump kinetic model was proposed. The model considered dimerization, cracking and hydrogen transfer reactions. Marquardt algorithm was utilized for the estimation of kinetic parameters. Statistical test result and the comparison between experimental values and calculated values indicated that the kinetic model was a good fit and acceptable.Based on six-lump kinetic model, one-dimensional pseudo-homogenous C4、C5 olefins catalytic cracking fixed bed reactor mathematical models were developed on adiabatic and tubular reactors, lump concentration distribution and axial temperature profile were calculated, and effects of operation condition on reaction results were investigated. As indicated by simulation results, catalyst bed temperature increased and C6+ hydrocarbon was main product around reactor inlet, and as catalyst bed height increased, C6+ hydrocarbon and pentene cracked to propylene and ethylene, decreasing catalyst bed temperature. In the tubular fixed bed reactor, the heat transfer of flue gas counterflow kept the catalyst bed temperature in a suitable range for the production of propylene, so propylene selectivity was better than in the adiabatic reactor.