Study On Mechanism And Kinetics Of Methanol To Olefins On SAPO Catalysts

Methanol to olefins is currently the most promising technology for the synthesis of olefin products from non-oil routes.The focus of its technological research is on highly selective and highly active catalysts.Due to its small pore size,low acidity,and strong hydrothermal stability,SAPO-34 catalysts exhibit excellent high conversion and olefin selectivity in the catalytic MTO reaction.This paper aims at the catalytic MTO reaction of SAPO-34 catalyst,and carries out thermodynamic calculation and analysis,lumped kinetic model study,reaction condition inspection and catalyst carbon deposition to realize the optimization of process conditions and selective regulation of product distribution..In this paper,the thermodynamic calculation and analysis of the MTO reaction are first performed,and the thermodynamic data of each reaction of MTO is obtained.According to the calculation and analysis of the reaction equilibrium constants,it is shown that the reaction to produce MTO main olefin products ethylene,propylene,butene and the reaction to produce by-product methane are irreversible reactions,thermodynamic equilibrium is not the limiting factor of these reactions.The MTO reaction was divided into five aggregates and a reaction kinetics model was established.Para?eters were regressed by experimental data.Reaction rate constants,activation energy and pre-exponential factors at different temperatures were obtained,and probabilistic and statistical verifications were performed.According to the results of thermodynamics and kinetics,the MTO reaction catalyzed by SAPO-34 under conditions of 350?450?,methanol velocity of 0.41?5.01 h-1 and methanol partial pressure of 10.4?50.3 kPa was investigated and analyzed.The reaction conditions were discussed.Effect on product selectivity.Methane,ethylene and propylene belong to the kinetic control process.The increase of temperature will increase the reaction rate constant.The reaction rate of ethylene increases with temperature,the rate constant increases the fastest,and high temperature favors ethylene selectivity.According to different production purposes,different reaction temperatures can be selected.For the purpose of producing more ethylene,the reaction temperature is selected at 450?,and the purpose of producing more propylene is to select 425°C.for the reaction temperature.The effects of space velocity and methanol partial pressure are mainly concentration effects,and the concentration of reactants is increased when the rate constant is constant High space velocity and high partial pressure of methanol are good for the production of ethylene,but will reduce the methanol conversion rate.The maximum yield of diene can be selected as the targe twiha space velocity of 2.55 h₁ and a methanol partial pressure of 20.0 kPa.The carbon deposition behavior in the MTO reaction catalyzed by SAPO-34 was investigated.Catalyst coke deposits can alter the product distribution and thus produce an increase in ethylene selectivity as the reaction time increases.The effect of coking on the SAPO-34 catalyst was found by NH3-TPD characterization.The main effect was to cover the strong acid center,which changed the acidity of the molecular sieve.The BET specific surface area was measured by physisorption and it was found that when the carbon deposit reaches a certain level,a sudden decrease in the specific surface area is caused because the catalyst pores are caused by carbon deposition blocking the pores.A SAPO-34 molecular sieve with a silica-alumina ratio of 0.05-0.2 was synthesized,and the acidity decreased as the silica-alumina ratio decreased.By evaluating different silica-alumina ratio molecular sieves,it was found that low acidity favors ethylene selectivity.The coke deposition of SAPO-34 catalyst with different reaction temperatures and reaction times was obtained by TGA.The amount of coke deposition increased with the increase of reaction time and reaction temperature,and the rule of reaction between carbon deposition and reaction time and reaction temperature was similar to the power function relationship.Based on the data,a function model of coke deposition and reaction time and temperature was regressed.