A Study On The Process Of Methanol To Olefins Reaction

In this paper, the rule of thermodynamics and kinetics during methanol to olefins (MTO) reaction was analyzed firstly and subsequently the behavior of coke deposition over SAPO-34 catalyst used for the catalytic conversion of methanol to olefins was investigated in an isothermal fixed bed integral reactor. The catalytic performance of SAPO-34 catalyst was evaluated in fluidized bed reactor with discontinuous operation and the experiment of process conditions was performed at the same time. And then, a model was developed, which coupled the kinetic model with three-phase bubble fluidized flow model, and used to simulate the performance of the transformation of methanol to olefins in a fluidized bed reactor. Moreover, the probability application of riser and moving bed reactor in MTO process was also investigated. Finally, the economic problem of MTO process was discussed simply.Methanol-to-olefins belonged to high exothermal reaction. The overall reaction heat was within 20~35KJ/mol and the adiabatic temperature rise was more than 200 degree if the feed was only methanol. The main reaction, namely the transformation of methanol to ethylene, propylene and butene, could proceed to reach very deep extent with almost no reversible process. The Gibbs energy and equilibrium constant for the formation of butene were bigger than that for the formation of ethylene and propylene, which proves the catalyst selected should have a very accurate small pore structure if we want more yield of ethylene and propylene. The equilibrium constant for the formation of dimethyl ether from methanol was near 2 and did not change much with the temperature. During the investigation of equilibrium relationship among the olefins, the equilibrium mole percent of ethylene remained increasing with the increasing temperature in the condition of constant dilute ratio. At the same condition, butene remained decreasing. However, the equilibrium mole percent of propylene, which showed the different evolution, increased firstly and then deceased with the increasing temperature. Propylene was the main products in relative lower temperature and on the contrary ethylene in higher temperature. The dilution to feed would favor the formation of ethylene because of the bigger expansion factor in the ethylene formation reaction. The kinetic of methanol conversion to olefins was studied in an isothermal fixed bed integral reactor. A reaction kinetic model including five lumps was obtained on the basis of Hydrocarbon Pool parallel reaction and parallel deactivation of catalyst. This model introduced an adsorption resistant item H to qualify the influence of water on each reaction.The parameters in model equations were calculated by fitting the data obtained at exit of reactor. This model showed a relative good ability for prediction. The average activity of catalyst showed a corresponding evolution with the increasing coke formation. Some water could be effective to reduce the deactivation of catalyst, but the contribution of water to the catalyst deactivation would be decrease in higher temperature (>475 degree) or bigger amounts of cumulated methanol. Methanol conversion did not drop instantaneously but instead it remained constant for some time before deactivation breakthrough. However, it did not mean that the catalyst was not deactivating during this period. The breakthrough time was the most important basis to determine the reaction time in fixed bed reactor. The influence of some kinds of operating factors on the breakthrough time was studied.The behaviors of coke deposition over SAPO-34 catalyst was investigated in an isothermal multiple fixed bed reactors in parallel. The coke deposition over SAPO-34 increased to 4wt% quickly within the beginning two minutes, and subsequently, showed a relative mild increase with the time on stream. Some water in feed showed obvious effect to reduce the coke formation in relative short time on stream and, however, showed no effect on the rate of coke formation in longer time on stream. The coke of coke deposition increased with the increasing methanol WHSV at the same time on stream. A coke distribution was found in the catalyst bed, which showed that the coke deposition was maximum at the reactor inlet and it attenuated along the reactor. This result corresponded to a deactivation mainly in parallel with the main reaction. An empirical model for the coke formation was obtained and made the coke content as a function of time on stream, temperature, WHSV, water/methanol in feed. The model obtained showed a very good linearity correlation. Besides, The selectivity of olefins reached maximum when the coke content was 5.7wt%.The results of MTO reaction in a fluidized bed reactor with discontinuous operation showed the good performance of SAPO-34 catalyst during methanol to olefins: over 99% methanol conversion, more than 80% ethylene and propylene, almost 90% ethylene plus propylene plus C4 alkenes. Increasing the reaction temperature could improve the methanol conversion and ethylene selectivity, but at the same time increase the amount of C1 components, such as methane, COx. A suitable amount of water favored the formation of ethylene and not the propylene, which resulted in an obvious increase of the ratio of ethylene to propylene. However, too much water did not favor the conversion of methanol. With the increasing cumulated amounts of methanol fed per catalyst, methanol conversion remained constant firstly and then decreased quickly when the cumulated amounts of methanol fed per catalyst reached 1.0. Besides, the selectivity of ethylene increased in the beginning and subsequently decreased with the increasing cumulated amounts of methanol fed per catalyst. However, the selectivity of propylene remained decreasing with the proceeding of reaction.A simulation using the bubble fluidized bed reactor model to predict the performance of MTO in an experimental fluidized bed reactor was performed. It is necessary to amend the traditional BFB model when considering the effect of water because of the back-mixing characteristic in fluidized bed reactor. Therefore, the water content formed in the reaction should be calculated according to the reactant concentration at the exit of bed. The values calculated by the model after amendment were identical to the experimental ones. The contact efficiency between gas phase and solid phase was reduced because of the existed back-mixing and bubble, which also reduce the methanol conversion in bubble flow in comparison with that in plug flow at the same height of reactor. The reaction resistance in emulsion and in cloud-wake gave a relative bigger influence on the methanol conversion, which could be reduced by adjusting such properties of catalyst as sophericity, average particle diameter. The MTO process wanting more propylene should select lower reaction temperature, lower line velocity, and higher catalyst activity. However, if we wanted more ethylene, higher temperature and higher line velocity could be chosen.The probability application of riser and moving bed reactor in MTO process, in which gas and solid phase was similarly in plug flow pattern and the catalyst could be regenerated after reaction continuously, was investigated. The production of common industrial riser reactor now was not enough by only one reactor to reach a methanol consumption scale in million tons grade. Therefore, multiple reactors operation was needed. But this operating mode caused a great many problems waited for future research. The moving bed reactor had lower ability to take out the reaction heat and also needed very complex mechanical device to control the moving of catalyst, which hampered the application of it as a MTO reactor. Therefore, the circulating fluidized bed with the catalyst continuous reaction-regeneration was the suitable reactor for MTO process.The economic of MTO process turned on the price ratio of coal or natural gas to oil. Today with very high oil price, MTO process had strong competitive in economic.