Study On Kinetics And Reactor For Methanol To Propylene Process

Methanol to propylene?MTP?process is an essential step to produce propylene from coal,natural gas and biomass.Since the first discovery,its kinetic study has been a hot topic for decades and plenty of models have been developed based on different mechanisms.However,these models were mostly established on the base of methanol conversion alone and did not take the effect of co-feeding olefin into account.On the other hand,MTP process has already been commercialized in China,but its propylene yield is far beyond satisfaction.An accurate kinetic model is crucial for process optimization by mathematical simulation.Accordingly,by employing HZSM-5 with Si/Al=200 as catalyst,this work mainly studied the catalyst,kinetics and reactor of MTP process.We first developed the kinetic models for methanol conversion,olefin interconversion and byproduct formation,respectively.Then we modelled and simulated the multi-stage fixed-bed reactor for MTP process under industrial operation conditions,analyzed its shortages and proposed an optimizated process.Finally,we prepared HZSM-5/cordierite monolithic catalyst and studied its potential in the application to MTP process by mathematical method.The main contents and results of this work are as follows:First of all,we studied the cracking routes of individual C₃^C₇ olefins.The results indicated that,heptene cracks exclusively into propylene and butene through monomolecular cracking,while propylene into butene and pentene through trimolecular cracking and butene into propylene and pentene through bimolecular cracking,respectively.Pentene and hexene with medium carbon number crack into other olefins through both monomolecular and bimolecular cracking.The apparent activation energy of monomolecular cracking is positive while that of bi-and trimolecular cracking is negative.Ethene is mainly produced from pentene and hexene cracking while propylene from the cracking of all C₄^C₇ olefins.Secondly,we studied the co-reaction of methanol and C₃^C₆ olefins.The products distribution showed methylation is the main reaction for methanol.Methanol dehydration rate is between the methylation rates of propylene and higher olefins.The methylation rate increases with temperature but decreases slightly with water concentration.When increasing methanol or olefin partial pressure,methylation rate increases at first but then remains almost constant.Thirdly,we investigated the characters of paraffin and aromatics formation.The mole ratio of paraffins to aromatics is higher than 3 and decreases with space time in both the conversion of olefin alone and the co-reaction of methanol and olefin.Compared to olefin transformation alone,methanol increases the yield of both paraffin and aromatics.Besides,increasing temperature decreases the yield of paraffins and aromatics.Fourthly,based on olefin methylation-cracking mechanism,we developed a comprehensive kinetic model for the co-reaction of methanol and olefins through three steps,which takes methanol dehydration,olefin methylation,olefin cracking and byproducts generation into account.It fits the experimental data fairly well under various operation conditions,which proved the rationality of methylation-cracking mechanism under investigated conditions.Fifthly,we simulated and optimized the six-stage fixed-bed MTP reactor by the hybride of Comsol and Matlab.The calculated propylene yield and space velocity are 62.01%and 0.75 g _MeOH/g_cat/h,respectively,both of which are close to the industrial outcome.Parameter analysis revealed the main reason to low performance is the severe intra-particle diffusion resistance and the overlong residence time through the six stages.An optimizated process is proposed in which only butene is recycled back into main reactor while other unwanted olefins are cracked in another reactor,resulting in a yield of C₂^C₃ olefins over 70%.Sixthly,we prepared the HZSM-5/cordierite monolithic catalyst by washcoating method.The load capacity can reach 25³5%with a thickness of0.05⁰.25mm after 3⁴ immersions of support with 31%water adsorption content in a slurry containing 20%ZSM-5 and 2%Ludox.Catalytic test showed the prepared monolithic catalyst produce much larger reaction rate in the conversion of methanol conversion alone and the co-reaction of methanol and butene,and 90%less paraffin and aromatics than the conventional extrudes in the co-reaction of methanol and butene.Finally,we simulated the industrial process by substituting the HZSM-5/cordierite catalyst for the randomly packed ZSM-5 pellets.At the same design criterion,the propylene yield and the space velocity increases significantly to 81.62%and 5.27 gMeOH/gcat/h,respectively,both of which are significantly higher than the reactor randomly packed with pelleted catalyst.