Kinetics Of Methanol And Olefins Transformation Over SAPO-34 And ZSM-5 Catalyst

Methanol to Olefins (MTO) and Methanol to Propylene (MTP) are two hotspots of coal chemical technology in recent years, which are becoming important industrial processes in the production of bulk chemicals from coal. MTO and MTP, belonging to a process that transforms methanol into light olefins, contain three parts:direct methanol transformation into light olefins, methylation of methanol with olefins, and olefins interconversion. However, MTO/MTP differs greatly in the reaction mechanism and kinetic behavior in case of different catalysts. MTO is a direct conversion process, with methanol transforming to ethylene, propylene and butene rapidly through parallel reaction steps. The olefins interconversion is slow and can be ignored in the MTO kinetic modeling. On the contrary, MTP is an complex interconversion process. The production of propylene comes from methylation and olefins interconversion over ZSM-5 catalyst, whereas direct methanol conversion into propylene is not important, and can be ignored. Hence specific reactor type and operating condition should be chosen to be suitable for MTO or MTP processes. Ethylene and propylene can be produced in the MTO process by one step in turbulent or fast fluidized bed reactor, whereas fixed bed reactor is applied in the MTP process, and huge amount of olefins are recycled and separated in order to maximum the propylene production. Parallel studies of the reaction mechanism and kinetics of MTO/MTP process would give a comprehensive understanding of methanol transformation in the SAPO-34 and ZSM-5 catalysts.Kinetics is very necessary for the reactor and process development. Though there are many reports on kinetics of MTO and MTP in the literatures, the results are diverse because of the differences in catalyst acidity, pore structure, particle size, active ingredient content and experimental conditions used in the experiments. Thereby the applicability of the existing kinetics to the industrial process has not been confirmed. At present, the kinetics for industrial catalyst and conditions are rarely reported, so it is necessary to study the kinetics in the condition as close to industrial process as possible. The results can thus be directly applied on the improvement of the existing industrial process.For this reason, kinetic studies over industrial SAPO-34 and ZSM-5 catalyst under conditions close to industrial conditions were carried out in this paper. The content includes following aspects:1. MTO kinetic was studied over SAPO-34 catalyst in a micro fixed bed reactor under high methanol flux. The effect of space velocity, temperature and water content on MTO reaction was studied. The kinetic data for the main reactions was obtained under temperatures of 450℃, 475℃, and 490℃, methanol weight hour space velocity (WHSV) between 30 and 955 gMeOH·(g cat)-1·h-1, and water/methanol mole ratio in the feed of 0,2 and 4. The experimental data shows that methanol is converted into ethylene, propylene and butene rapidly in parallel steps in the MTO process. The rate of each step is of first order in the methanol concentration. The selectivity of ethylene increases with increasing temperature. Water content in the reaction medium attenuates reaction rate. Based on the experimental data, a four lumped and sixed lumped kinetic model were established and the kinetic parameters were obtained.2. The deactivation kinetic of SAPO-34 catalyst was studied in a micro fluidized bed reactor. Temperature and coke distribution in the micro fluidized bed reactor are uniform, which is suitable for the study of coke and deactivation behavior. Therefore, in this paper a micro fixed bed reactor was adopted to study the fast methanol reaction kinetics under high methanol flux, and a micro fluidized bed reactor was adopted to study the relative slow deactivation kinetics. The deactivation of MTO reaction is caused by deposition of coke in the catalyst, and is parallel to the main reaction. The deactivation rate is related to temperature, water content and space velocity. Under the temperature range of 450 to 490℃, the correlation between catalyst activity, product selectivity and coke content can be divided into two parts:stable deactivation period and fast deactivation period. When the catalyst coke content is below 7.8 wt%, with increase in the catalyst coke content the catalyst activity decreases slowly, the selectivity of light olefins increases and that of paraffins and C4 products decrease, whereas the selectivity of methane remains unchanged. When the catalyst coke content is above 7.8 wt%, the catalyst activity and selectivity of light olefins decrease rapidly as the coke content increases, while the selectivities of paraffins, C4, C5 and methane increase. Based on the deactivation characteristic, a subsection deactivation model was proposed to account for the deactivation characteristic of MTO reaction. The proposed model is capable of reflecting the change in the catalyst activity and product selectivity with catalyst coke content.3. The retained coke species in the working SPAO-34 catalyst were analyzed by dissolution-extraction procedure. The results show that the coke species consist of soluble and insoluble parts. Methylbenzenes and methylnaphthalenes are the most abundant soluble species at lower temperatures (400℃,425℃ and 450℃), while poly-cyclic aromatics (methylnaphthalenes, phenanthrene and pyrene) become the dominant species at 475℃ at the expense of methylbenzenes. Methylnaphthalenes might function as another active reaction center at higher temperature. Therefore, the catalyst deactivated under lower temperature can regain activity by raising temperature. Furthermore, the regeneration kinetic of deactivated SAPO-34 catalyst was studied by TG method. Experiment results indicate that the combustion rate of SAPO-34 catalyst is a first order reaction with respect to the coke conversion. Compared to methylbenzenes and methylnaphthalenes, the poly-cyclic aromatics with more than three rings are more stable, resulting in a higher energy barrier.4. The olefins interconversion is a main reaction in the MTP process. The kinetics of C2-C7 olefins interconversion were studied in a micro fixed bed reactor by feeding different linear olefins. The effect of partial pressure,11.84-101.325 kPa; temperature,400-490℃ and space time 0.0046-0.34 h·(g cat)-1·(g feed)-1 were investigated. The kinetic data was obtained and the reaction routes for C2-C7 olefins interconversion were proposed. The experiment results indicate that oligomerization and β-cracking are the main reactions in the olefins interconversion. The reactions of C3-C5 olefins proceed mainly through oligomerization and β-cracking steps, whereas for C6 and C7 olefins, monomolecular cracking reaction plays an important role in the reaction medium. The activity of olefin depends on the carbon length, the longer is the carbon length, the higher activity the olefin has, and therefore ethylene shows the lowest activity. A kinetic model taking into account of oligomerization, cracking and aromatization of olefins was established. The kinetic parameters were obtained on the basis of the experimental results. The kinetic results show that the calculated apparent activation energy is negative for oligomerization reactions and positive for cracking reactions. The adsorption effect included in the kinetic model can well represent aforementioned phenomenon. Furthermore, the result confirms that activation energies of C2-C7 olefins interconversion obtained from alcohols feed are disguised by the alcohol dehydration steps, and therefore it is difficult to obtain the intrinsic kinetics by alcohols feed.5. A pilot MTO plant with methanol feed of 300 t·a-1 was simulated by combining MTO kinetics, dynamic two phase model (DTP) and particle residence time distribution model. The calculated result is in accordance with the experimental data. The reactor performances of turbulent fluidized bed and multi-fluidized bed in series were detailed studied through model simulation. The modeling results show that the product selectivity is closely related to the catalyst residence time. Suitable catalyst residence time and distribution determine the suitable coke content and coke distribution, which in turn affect the reactor capacity and light olefin selectivity. The multi-fluidized bed in series operating mode can reduce the particle back-mixing and narrow the coke distribution, and therefore improve the catalyst lifetime and light olefin selectivity, decrease the load of regenerator. More importantly, grading utilization can be applied according to the characteristics of different age catalysts. The proposed three reactors connected in series mode with temperature rising sequence can fully utilize residual activity of heavily deactivated catalyst in reactor 1 by use of dense phase fluidized bed, higher catalyst activity in reactor 2 by use of turbulent bed and pre-coking in reactor 3 by use of fast fluidized bed. The simulated result show that when compared to single stage fluidized bed, three reactors connected in series mode make the catalyst production capacity increased by 31.1%, and the light olefin selectivity increased by 1%, to 79.36%.