Research On The Reaction And Deactivation Pathways And Process Design Of Methanol Conversion To Propylene On ZSM-5 Zeolite Catalyst

Methanol to propylene process is an independent propylene production technology ensuring national energy security. The inherent vice of conventional fixed bed and fluidized bed MTP processes are becoming apparent in operation complexity and catalyst abrasion. Therefore, it is of enormous industrial and economical potential to develop advanced MTP technology with high propylene yield and low energy consumption.The dissertation was launched from the reaction and deactivation mechanism of MTP process. First, through the study on the regulation mechanism of the catalyst pellet sizes and reaction conditions on MTP reaction-deactivation pathways, the reaction route with high severity was put forward with separate processing of circulation alkanes and oxygenated compounds. Then the contradiction between high propylene yield and rapid coking deactivation of catalyst was resolved in accordance with the moving bed process where catalyst can be continuously regenerated. Besides, efficient recycling of MTP by-products was implemented according to the selective coking characteristics on the deactivated catalyst. Based on the above, a new three-step moving bed methanol to propylene (TMMTP) process was developed and the coke combustion kinetics of spent catalyst was investigated. The main achievements of this research are as follows:1. The thermodynamic calculation was performed on the methanol to propylene reaction system. The results show that the methylation reaction is exothermic and irreversible while olefins cracking reaction is endothermic and reversible. Under the condition of typical products distribution, the MTP reaction heat reaches 32.5 kJ/molMe and the adiabatic temperature rise is up to 411℃, with a third of the total released heat produced by dehydration of methanol etherification. In the equilibrium conversion of C2-C7 olefins, propylene has the highest concentration with the optimal temperature and partial pressure. The formation of alkanes and aromatic hydrocarbons is the basic reason for the reduction of total propylene yield.2. The MTP reaction mechanism was studied for selection of the optimum reaction conditions. First of all, the effect of particle size of zeolite catalyst on conversion and products distribution of MTP and olefins conversion reactions was investigated, and the regulation mechanism of MTP reaction pathways was proposed under the control of catalyst porous structure and space time. Afterwards, the reaction rate and products distribution in the conversion of methanol, olefins and olefin/methanol cofeeding were investigated, and the olefin methylation cracking reaction pathways were further deduced on SiC foam structured catalyst. Finally, the action mechanism of reaction conditions on MTP reaction performance was studied on the structured catalyst, and the orthogonal experiment was adopted to optimize the reaction conditions with P/E ratio as the optimization goal3. The coking and dealumination behavior of catalyst was studied for the investigation of deactivation rules in MTP process. First the study of reaction and deactivation performance was conducted in multi-period reaction-regeneration process. The results show that the irreversible dealumination reduced the acid strength and amount of the catalyst, leading to the fluctuation of catalyst single-pass lifetime and difference of products selectivity in each period. Thereafter, the distribution of coke in bed was separately investigated in conversion of methanol, butene and methanol/butane cofeeding, and the deactivation laws were studied under different reaction conditions when feeding methanol alone. According to the analysis and characterization of the porous structure of catalyst with different deactivation degrees, non-uniform coke caused by hydrogen pool reaction is distributed on the internal surface of micropores, while uniform coke is on the external surface, and the non-uniform coking rate is greater than that of uniform coking rate. In spite of the blockage of non-uniform coking in micropores, the acid site in the external surface of micropores is still reactive and catalytic for equilibrium transfer of olefins.4. According to the thermodynamic calculation results and research on reaction-deactivation pathways of MTP reaction, a novel TMMTP technology was put forward with methanol fed alone at high space velocity and olefins transformed individually. Based on experimental analysis, the feasibility and advancement were elaborated and a method to derive technical parameters was presented, which was further applied to a TMMTP process with the methanol treatment capacity at 1.8× 106 t/a. The results display that the propylene yield reaches 89.71c% and 2.55 tons of methanol will be needed for treatment of one ton of propylene. It suggests that the process can effectively reduce catalyst consumption, increase the plant capacity and save water and power.5. The combustion characteristics of coke on the two different catalysts separately from two-step and three-step technology were investigated by thermogravimetric analysis, and the corresponding power-law kinetics was further calculated.Two sorts of coke exist separately on the internal and external surface of micropores for catalyst in two-step technology, while there is only one kind for catalyst in three-step technology. For the two different technology, two-step and three-step, the oxygen partial pressure order for both the two coke in the former is higher than that in the latter and the carbon content order shows exactly the opposite. Besides, for combustion reaction of coke in three-step technology, though the reaction rate constant is significantly larger than that of coke in two-step technology, its activation energy is approximate to that of high temperature carbon combustion reaction. Moreover, the coke combustion behavior on ZSM-5 zeolite catalyst was also elucidated to be controlled by internal diffusion. As the diffusion rate decreases, the order of oxygen partial pressure rises and that of coke content decreases, increasing the coking area and the combustion reaction rate.