Modeling Of Catalytic Propylene Polymerization Reactor And Process

Polymerization mechanisms, systems and reactors are often complex, coupled and highly non-linear. It is difficult to assess, on-line, the state of the polymerization process and the quality of the polymer product such as the degree of polymerization and polymer molecular weight. The future development trend in this area is that the design, simulation and control of polymerization processes reply more and more on thorough mathematical modeling based on rigorous polymerization mechanisms and thermodynamic state equations. The level of ethylene industry, especially that of the polyolefin industry, is an important indicator of a country's level of industrialization. Modeling and optimization of olefin polymerization processes based on polymerization and process characteristics have to integrate polymer chemistry and polymerization kinetics, multi-scale polymerization reactor modeling, characterization of polymer structures and quality, large-scale model solution search and optimization calculations, process control, etc. It is an interdisciplinary field with many unsolved and challenging fundamental research topics and practical applications, relating to macromolecular chemistry, polymer reaction engineering, polymer material and process system engineering.The aim of this work is to develop a rigorous mathematical model for an industrial propylene polymerization process in order to significantly improve its performances. For that purpose, typical industrial slurry and gas-phase propylene polymerization processes composed of slurry and fluidized-bed reactors in series are analyzed based on a systematic evaluation of the current olefin polymerization technologies and its future development trend. A model is developed to calculate thermodynamic and physical properties of the propylene polymerization system. A method is proposed to validate the parameters of the model. A dynamic model is then built-up for an industrial Ziegler-Natta catalyzed propylene polymerization process composed of two slurry and two fluidized-bed reactors in series. Finally, the operating conditions of the reactors and dynamic characteristics of the process are analyzed; the grade change and process optimization are simulated. Most relevant results obtained in this work are summarized as follows.1. The equation of state of Perturbed-Chain Statistical Associating Fluid Theory is applied to predict thermodynamic properties and phase equilibria. The three pure-component parameters of PC-SAFT model, i.e., segment number m, segment diameter σ, and segment energy parameter ε/k_B, and binary interaction parameter k_ij are regressed based on the literature data for propylene-hydrogen-polypropylene system. The accuracy of PC-SAFT state equation using the regressed parameters obtained this work outperforms those of Peng-Robinson equation, S-L equation and PC-SAFT state equation using the literature parameters.2. The mechanism of Ziegler-Natta catalyzed propylene polymerization isanalyzed in a thorough manner. Simplified elementary reactions involved in the propylene polymerization that are suitable for subsequent modeling of industrial processes are then proposed. They include the activation of co-catalyst and monomer, initiation and propagation of chains, transfer of radicals to monomer and hydrogen, and deactivation of catalyst. The reasonable ranges of the kinetic constants of these elementary reactions and their activation energies are also determined. According to the molecular weights and molecular weights distribution of two different polypropylene grades, the number of active sites for the catalyst is found to be six. A sensitivity analysis shows that the monomer conversion is most sensitive to the chain propagation and deactivation constants and that the molecular weight is most sensitive to the chain transfer and propagation constants. This provides guidance for choosing appropriate model parameters for process modeling.3. The agitated slurry propylene polymerization reactor is analyzed from the point of view of fluid mixing. It is found that the principle of reactor optimization is to keep polypropylene particles suspended, enhance the dispersion and mass transfer of hydrogen, intensify the heat transfer by the vaporization of liquid propylene and in the jacket, and improve the dispersion of circulating propylene gas in the reactor. The power consumption, gas holdup and heat transfer in the gas-liquid agitated reactor are investigated experimentally. The results can be used to design and optimize this type of agitated reactor.Gas holdup: Power consumption under gassing condition: Heat transfer under gassing condition: 4. The power consumption, fluidization process and pressure fluctuation in an agitated fluidization bed with a frame impeller or a dual anchor impeller, cooperating with orifice gas distributor or half-cone-shaped cap gas distributor, are investigated. The effects of the impeller, gas distributor, powder diameter and superficial gas velocity on the fluidization process are studied and correlations between the critical fluidization gas velocity and pressure drop of the bed are obtained.As the rotation speed of the impeller increases, the transition point of the pressure drop vs. superficial gas velocity curve becomes smoother. There is no obvious relationship between power consumption and powder diameter, bed heightand gas distributor in the fluidization regime. At the same time, the pressure drop of the bed is independent of the type and the rotation speed of the impeller. The impeller in the agitated fluidized bed prevents the powder from agglomeration and slugging, stabilizes the fluidization and improves the fluidization quality.5. It is shown that for an agitated fluidization bed reactor, an optimized impeller should possess a suitable number of horizontal blades to restrain and break down air bubbles and several vertical blades to scrape the wall and prevent air bubbles from short circuit. The fluidization quality of the dual anchor impeller excels that of the frame impeller, and that of the half-cone-shaped cap gas distributor is better than that of the orifice gas distributor. The optimization policy, i.e., decreasing the polydispersity of the powder diameters and increasing the bed height and gas velocity, is proposed. It allows to convert the production of powder type polypropylene to that of small-sphere type polypropylene in the Hypol gas fluidized bed and to intensify the time-space yield load.6. A steady model of the Hypol polymerization process based on single active site reaction kinetics is developed with the yield of polypropylene of each of the reactors as a benchmark. Meanwhile, steady and dynamic models of the process based on multiple active sites reaction kinetics are developed with the molecular weight and its distribution as the targets. They are validated using plant data. Thereafter, the effects of the operating conditions of the process such as temperature, slurry level in the reactor, hydrogen molar fraction in the gas phase and its inlet flow rate on the process state variables such as polymerization yield and molecular weight, are investigated. The dynamic characteristics of the propylene polymerization process under pulse disturbance, process step action and measurement noise have offered quantitative support to the process optimization and control. The dynamic model simulates well grade transition processes. In addition, it is shown for the first time that the mass transfer performance of hydrogen influences the dynamic characteristics of the propylene polymerization process.7. Finally the steady and dynamic models are used to simulate new processes based on the existing one. The new processes are "a slurry reactor and a fluidized bed reactor in series" and "a slurry reactor and two fluidized bed reactors in series". Optimized operating conditions are then proposed. Under these conditions, the new processes can markedly produce much higher polymer yields than the existing process. Simulated results show that the "a slurry reactor & a fluidized bed reactor in series" process is most suitable for producing polypropylene with broad molecular weight distributions.