Methanol To Propylene Process In A Fixed Bed Reactor: Experimental And Modeling Study

Methanol to propylene (MTP) technology, which enables thetransformation of coal or natural gas to propylene via methanolsynthesis, is an alternative route to produce petrochemicals and gasolinefrom coal other than crude oil with great potential. The operatingcondition and process intensification of MTP process in a fixed bedreactor over ZSM-5catalysts have been investigated in this work. In atypical MTP process, several olefin-containing streams are sent back tothe main synthesis loop as the additional propylene sources. Therefore,the coupled reaction of C4/C5olefin with methanol, and the multi-stageMTP process with olefins co-feeding were studied in a fixed bed reactor.Considering the intra-particle mass transfer restriction in industrialcatalyst pellets, a monolith catalyst and reactor has been proposed andmodelled for the MTP process.The main contents and results in this work are as follows:First of all, effects of operational conditions on methanol conversionto propylene were investigated in an isothermal fixed bed reactor, suchas reaction temperature, space time, methanol partial pressure andwater/methanol molar ratio. The results indicated that methanolconversion and propylene selectivity increased with increase in reactiontemperature, the methanol space time or methanol partial pressure andso did selectivities of alkanes and aromatics. Increasing water content inthe feed mixture is conducive to forming propylene, but harmful to themethanol conversion.Secondly, the coupled reaction of butene and pentene with methanolhave been investigated, respectively. Both two co-reaction systemsobtained similar results that increasing reaction temperature increasesmethanol conversion and light olefins selectivity, but decreases the formation of C5-C7olefins,alkans and aromatics. The formation ofalkanes and aromatics is inhibited and propylene/ethylene ratioenhanced by lower methanol space time. Compared with the reaction ofmethanol alone, co-feeding butene or pentene with methanol resulted inmuch less alkanes and aromatics. The selectivity of propylene increaseswith increasing methanol/olefin molar ratio.Thirdly, the three-stage methanol to propylene reaction process hasbeen simulated experimentally based on a high silicon ZSM-5catalyst.The catalyst life time was long up to650hours, and the selectivity ofpropylene was above75%in all three stages.Fourthly, a reaction-diffusion model for MTP reaction system hasbeen proposed and validated by comparing with the experimental resultsobtained in a berty type reactor. The calculated intraparticle diffusionefficiency factors are in good agreement with the experimental results,which indicates that the proposed model is appropriated for MTPreaction system and qualified for further reactor design and industrialsimulation.Fifthly, a multi-stage adiabatic fixed-bed reactor has been simulatedby a one-dimensional heterogeneous reactor model. The effects ofreactor inlet temperature, methanol space velocity and feed compositionon methanol conversion, reactor temperature and product distributionwere calculated and analyzed, to provide a theoretical basis for furtherscale-up, optimization and intensification of MTP process.Finally, a monolith reactor has been proposed for MTP reaction andcalculated based on a two-dimensional adiabatic heterogeneous reactormodel including the interactions of mass and heat transfer and chemicalreactions between the gas and catalyst phases and inside the catalystphase. Concentration and temperature profiles in both radial and axialdirection were calculated with special focus to influence of monolithchannel geometries on methanol conversion, propylene and by-productselectivity. Hints for monolithic catalyst design were derived. Thecalculation results showed that the monolith catalyst intensifies thereactor efficiency significantly. Compared with the randomly packed catalyst, for a complete methanol conversion, the monolithic catalystamount is80%less than the randomly packed reactor, and propyleneselectivity is increased by7%.