(32a) Study on the Coke Distribution on Catalyst for MTO Fluidized Bed Reactor-Regenerator System

The methanol to olefins (MTO) process provides an alternative approach to produce light olefins from nonoil resources. In industry, the fluidized bed reactor-regenerator system is applied for MTO process, where the SAPO-34 catalyst particles are circulated between the reactor and regenerator. During the circulation, coke deposits on the catalysts in the fluidized bed reactor and is burned off in the fluidized bed regenerator, which lead to that coke content on catalysts demonstrates a certain distribution in the system. For MTO process, the nonlinear relation of reaction rates with coke content on catalysts indicates that the catalytic performance of the reactor is highly dependent upon the coke distribution of catalysts in the reactor.

In our recent work, a coke distribution model of MTO process was developed based on our new lumped kinetic model. The kinetic model for SAPO-34 catalyst, based on the dual-cycle reaction mechanism, was established from our laboratory fluidized bed experiments. In this kinetic model, a deactivation function was proposed to account for the deactivation process of SAPO-34 catalysts. Then, three dimensional (3D) partial differential equations of coke distribution based on population balance model was developed, and analytical coke distribution expressions of steady state were subsequently obtained under the perfectly mixed assumption of the catalysts in the beds. These expressions indicates that the probability density functions of coke distributions of the reactor and regenerator are determined by three factors, i.e. catalyst residence time, coke deposition (or burning) rate and coke distribution of catalyst inflow. The first two factors could be further merged into a dimensionless number, when the rate of coke deposition (or burning) could be expressed in first order relation of coke content.

Compared with age distribution approach, which has been used for MTO process, coke distribution approach shows the physical picture in a clearer way. Most interested properties of MTO process, such as product selections, MeOH conversion and average coke content, could be directly estimated from the profile of coke distribution. Especially, when encountering a wide coke distribution of catalyst inflow, or investigating space-dependent coke distributions of a fluidized bed, the coke distribution approach possesses a distinct advantage.

The simulations based on the coke distribution model of three MTO reactor-regenerator systems, i.e. pilot-scale, demo-scale and commercial-scale with reactor diameters of 0.261, 1.25 and 10.75 m respectively, were performed. The simulated results were in good agreement with the operation data. The effect of coke distribution on MTO reaction rates, as well as the influence on product selections, was also investigated. The use of coke distribution could be helpful in optimizing the operation conditions and the reactor design of the reactor-regenerator system.