Study On The Reaction Process Of Methanol To Propylene (MTP)

Propylene is one of the most important raw materials for production of polypropylene, acrylonitrile and acrylic acid. Conventionally, propylene is obtained from steam cracking and fluid catalytic cracking (FCC) units using petroleum as feedstock, with its cost governed by oil pricing. Due to the shortage of petroleum resource in the future and the increasing market demand for propylene, developing new technology for producing propylene independent of petroleum route is required. Methanol to propylene propylene (MTP), which enables the transformation of coal or natural gas to propylene via methanol synthesis, draws more and more attention. In this dissertation, thermodynamics, reaction pathway and reaction kinetics of MTP process over H-ZSM-5 catalyst were studied. On the basis of the reaction kinetics, a multi tubular fixed bed MTP reactor has been proposed and modelled. The main work and results in the dissertation are as follows:(1) Thermodynamic analysis of MTP reaction system has been investigated. The results indicated that:the formation of propylene from per mole methanol will release about 30 kJ heat; alkene methylation is exothermic whereas alkene cracking is endothermic; alkene methylation is virtually irreversible while alkene cracking is limited by thermodynamic equilibrium. The equilibrium compositions of C2-C6 olefins are affected by temperature and pressure. For each methanol partial pressure in methanol conversion to olefins, there exists an optimum temperature at which the composition of propylene is maximal among the final equilibrium composition of C2-C6 olefins. The maximum propylene composition is close to 40 wt%.(2) The effects of operation conditions on methanol conversion to olefins have been investigated in an isothermal fixed bed reactor. It is found that olefin distribution was affected evidently by methanol conversion. At low methanol conversions, decreasing methanol partial pressure increases C3=-C5=selecitivity at the expense of C6=-C7=. Increasing water/methanol ratio or decreasing temperature increases C2=-C3= selecitivity at the expense of C4=-C7=. At complete methanol conversion, selectivity of C3-～C7=is hardly affected by operation condition. The dependence of product distribution on methanol conversion derives from the shift of the dominant reaction pathway from olefin methylation-cracking to oligomerization-cracking. Product distribution at low methanol conversions may be determined by the reaction between olefin methylation and cracking, which is affected by operation conditions evidently. At complete methanol conversion product distribution is determined by oligomerization and cracking of C4=-C7=likely, with selectivities of C3=～C7= affected by operation condition slightly.(3) The dominant reaction pathway for olefin formation in MTP process was investigated by comparing the experimental results from two kinds of feeding:alkene only, mixed alkene and methanol. The results show that alkene methylation with methanol is dominant for the case of methanol and individual C3-C6 alkenes co-feeding. C2= is almost un-reactive. C7=cracks to propene and butenes immediately whether co-fed with methanol or not, and C6-cracks to propene readily when reacted alone. Methylation-cracking has been verified as the main reaction pathway of a typical MTP process in which recycling of C2= and C4=-C6= to the reactor inlet is required. A reaction scheme has been presented including a cycle composed of a consecutive methylation from C4=through C5= to C6=and further to C7=, theβ-scission of hexene and heptene for propene, and the a-scission of hexene for ethylene as well. Based on the dominant reaction pathway in MTP, a reaction process with C4-C6 olefins co-fed with methanol rather than ethylene has been proposed. High propylene selectivity can be obtained by the methylation-cracking reaction of C4-C6 olefins with methanol.(4) The yields of aromatics, paraffins at high methanol space time (>10 gcath/mol) and at methanol partial pressure of 20 kPa in co-reaction of methanol and butene, pentene or hexene were investigated at 460℃. For each feed, the CH2 mass ratio of alkenes to methanol was 4. It was found that at constant space time each yield of aromatics, individual C2-C4 paraffins and ethylene did not rely on the feed composition. Only slight dependence of mass fraction on the feed composition was observed for propylene, C4, C5 and C6, respectively. These results indicate that in co-reaction of methanol and butene, pentene or hexene, a mixture of C3=-C6= with steady distribution is first formed through the methylation of C4=-C6= followed by the cracking of C6=and C7= rapidly. Ethylene is formed subsequently through oligomerization and cracking of the C3--C6= mixture. Aromatics are suggested to be formed through aromatization of C7 and Cg olefins along with the formation of paraffins through hydrogen transfer reactions.(5) A MTP kinetic model has been proposed based on the dominant reaction pathway in MTP. The kinetic parameters were estimated by experimental results of methanol co-reacted with individual C3-C6 olefin in an isotherm fixed bed reactor. The proposed kinetic model describes MTP reaction by three consecutive steps with 16 reactions:olefin methylation followed by cracking, olefin inter-conversion to approach thermodynamic equilibrium and the formation of aromatics plus paraffins through hydrogen transfer reactions. Each of C2-C7 olefins, C7-C9 aromatics and C1-C4 paraffins has been characterized separately. The kinetic model fits the experimental results generally well.(6) A multi tubular fixed bed reactor for MTP process has been simulated based on the MTP kinetic model according to the two-dimensional pseudo-homogeneous reactor model. The results indicated that propylene selectivity is increased with increasing water content, but the hot point temperature of reactor is lowered. Water to methanol ratio of 2～3 is suitable for MTP process. Increasing methanol space velocity will lower methanol conversion and increase propylene selectivity, but has little effect on the hot point temperature of the reactor. Methanol velocity of 0.05～0.1 mol/(gcath) is suitable for MTP process. Increasing the temperature of the molten salt will increase propylene selectivity, and the suitable temperature of the molten salt for MTP is about 420-440℃. Increasing the inner diameter of the reactor tubes will lead to increasing hot point temperature, and the proper inner diameter should less than 0.040 m to avoid too high hot point temperature in the reactor.