Effect of Polymer Particle Size and Inlet Gas Temperature on Industrial Fluidized Bed Polyethylene Reactors

Static and dynamic bifurcation behaviors dominate the operation of fluidized bed catalytic reactors for the production of polyethylene (UNIPOL Process) and have important implications on the safe operating temperature and polyethylene production rate. The investigations show that the multiplicity of the steady state phenomenon covers a wide range of parameters together with the phenomenon of periodic oscillations with sharply changing amplitudes with a change of the chosen bifurcation parameter. In some cases, the periodic branches terminate through periodic limit point (PLPs), while in other cases it terminates homoclinically. A detailed parameteric investigation using two-parameter continuation diagrams for the loci of static and Hopf bifurcation points as well as one parameter bifurcation diagrams shows that it is possible to increase the productivity of the unit considerably without exceeding the constraints of the polymer melting point. Gas feed temperature, catalyst feed rate, and polymer particle size distribution are important operating parameters in polyethylene fluidized bed reactors. Gas velocity plays a significant role in keeping the fluidized bed bubbling in addition to the fact that it acts as a cooling media by removing excess heat generated from the polymerization reaction. The kinetic behavior of the catalyst and effect of reactor temperature on product properties require, in some cases, operating just below the softening point of the polymer which requires a suitable controller to avoid polymer melting.     

Dynamics and stability of ethylene polymerization in multizone circulating reactors

Multizone circulating bed reactors (MZCR) have the exclusive characteristics of producing polymers of different molecular weights in a single particle. Traditional fluidized bed reactors, on the other hand, can produce only one kind of molecular weight with relatively narrow distribution. A dynamic model for the MZCR is used to illustrate the basic dynamic behavior of the new reactor design used for polyethylene production. The model is used to study the copolymerization of ethylene with butene. Several parameter sensitivity analyses are performed to show the computer-simulated time responses for reactor temperature, number-average molecular weight, weight-average molecular weight, catalyst feed rate and the monomer/comonomer concentration along the reactor length. At certain operating conditions dynamic instability is observed and the results for the effect of cooling water temperature, catalyst feed rate, monomer and comonomer initial feed concentration on the reactor temperature and polymer molecular weight reveal that the system is very sensitive to disturbances in the heat exchanger coolant temperature. Also, at some operating conditions, the reactor temperature oscillates above the polymer melting temperature. Temperature runaway above polymer softening point is a serious problem which may cause polymer melting and hence reactor shutdown. The oscillatory behavior of the reactor temperature necessitates a suitable temperature control scheme to be installed.     

Fluidized‐bed reactor modeling for polyethylene production

Gas‐phase technology for polyethylene production has been widely used by industries around the world. A good model for the reactor fluid dynamics is essential to properly set the operating conditions of the fluidized‐bed reactor. The fluidized‐bed model developed in this work is based on a steady‐state model, incorporating interactions between separate bubble, emulsion gas phase, and emulsion solid polymer particles. The model is capable not only of computing temperature and concentration gradients for bubble and emulsion phases, calculating polymer particle mean diameter throughout the bed and polyethylene production rate, but also of pinpointing the appearance of hot spots and polymer meltdown. The model differs from conventional well‐mixed fluidized‐bed models by assuming that the particles segregate within the bed according to size and weight differences. The model was validated using literature and patent data, presenting good representation of the behavior of the fluidized‐bed reactor used in ethylene polymerization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 321–332, 2001     

气相法聚乙烯冷凝模式操作的模拟研究(Ⅰ) 反应器运行状态的分析

建立了气相法聚乙烯冷凝模式操作反应器的两相模型 .模型涉及气泡相和乳化相中的热量和质量守衡、乳相和泡相之间的热量传递和质量传递、乳相中的聚合反应以及乳相中粒子的停留时间分布等 .通过模型研究了常规操作和冷凝操作时操作变量和反应器运行状态变量之间的关系 .模型模拟结果与工业的常规操作和冷凝操作数据符合较好 .得到了冷凝操作时时空收率、低温区域、聚合物灰分等的变化规律以及催化剂特性对冷凝操作的影响规律 .提出了适合于冷凝操作的催化剂类型     

控制气相PE流化床高度对装置运行周期的影响

分析了聚合过程中控制流化床床高对聚合物粒径,催化剂聚合产率和装置运行周期的影响.增加流化床床高,有利于聚合物粉料对反应器上部扩大段的冲刷,有效减少了粉料堵塞循环回路和反应器结块的发生几率,提高了聚合反应器运行周期.     

OPTIMAL DESIGN FOR THE RESIDUAL OIL HYDRODEMETALLATION IN A FIXED BED REACTOR

A restricted diffusion model was used to investigate the optimal design for the residual oil hydrodemetallation in a fixed bed reactor. Based on the total lifetime activity, the optimal catalyst pore sizes and their corresponding optimal division locations in the bed were determined. The results indicate that the total amount of demetallation could be significantly improved by using a multi-layers reactor and the nonuniform activity catalysts. In addition, the influence of the Thiele modulus on the optimal designs were illustrated.     

丁二烯气相聚合过程中的单体吸附效应和本征动力学模型

采用负载型稀土催化剂 ,在搅拌床反应器内进行丁二烯气相聚合动力学的研究 .考察了单体在聚合物小粒子中的吸附效应和单体压力与温度对聚合速率影响 ,在此基础上 ,提出了本体系的本征动力学模型     

Reactor Modeling of Gas‐Phase Polymerization of Ethylene

A model is developed for evaluating the performance of industrial‐scale gas‐phase polyethylene production reactors. This model is able to predict the properties of the produced polymer for both linear low‐density and high‐density polyethylene grades. A pseudo‐homogeneous state was assumed in the fluidized bed reactor based on negligible heat and mass transfer resistances between the bubble and emulsion phases. The nonideal flow pattern in the fluidized bed reactor was described by the tanks‐in‐series model based on the information obtained in the literature. The kinetic model used in this work allows to predict the properties of the produced polymer. The presented model was compared with the actual data in terms of melt index and density and it was shown that there is a good agreement between the actual and calculated properties of the polymer. New correlations were developed to predict the melt index and density of polyethylene based on the operating conditions of the reactor and composition of the reactants in feed.     

Modeling of a multizone gas‐phase polyethylene reactor with a cluster‐based approach

A circulating fluidized reactor of polyethylene was modeled with the proper hydrodynamics for a riser and downer and combined with a kinetic model based on the moment equations. The hydrodynamic model was able to predict the profiles of the following parameters through the riser and downer: cluster velocity, bed porosity, concentration of potential active sites, active sites, gas‐phase components, molecular weights, and reactor temperature. It was shown that one could control the monomer consumption and molecular weight, which are crucial in the reactor behavior and production properties, respectively, by setting different operating hydrodynamic conditions, such as the gas velocity in the riser and the solid circulation rate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

双峰聚乙烯生产技术的优化

分析了双峰聚乙烯装置运行中存在的主要问题,根据双峰工艺技术特点,从流化床参数控制、产品牌号切换、催化剂切换、在线色谱分析等方面采取了优化措施。严格控制流化床的乙烯分压和乙烯分压的增长速率、1-丁烯的加入速率、流化密度、流化速率、气相反应器入口温度以及反应温度变化率,并对在线色谱进行改造,增加了一个旋风分离器。通过优化,提高了流化床参数控制的稳定性,避免了块料的产生,实现了长周期运行的目标。     

多段流化床技术用于多相催化与纳米材料合成过程

简要回顾了清华大学十余年来开发多段流化床技术与工业应用历程,包括利用冷态实验确定多段流化床的操作域,不同段中催化剂密相高度随气速变化以及与外置溢流管尺寸的关系,并且将其运用于硝基苯气相加氢制备苯胺、间苯二腈氨氧化、甲醇合成、乙炔法制备氯乙烯以及化学气相沉积法制备碳纳米管过程,并实现了工业化应用。多段流化床结构灵活,可以根据不同过程的工艺要求（如对变温或变气氛及高转化率与长催化剂寿命的要求等）进行调节。相关技术开发拓展了流化床的应用领域。     

基于催化剂颗粒停留时间分布的双峰聚乙烯牌号切换优化

针对环管-流化床串联的Borstar双峰聚乙烯工艺,建立了包括催化剂切换在内的双反应器串联牌号切换模型,使产品牌号的切换时间最短、过渡料数量最少。同时,推导了双反应器串联工艺中,存在催化剂切换时各反应器内聚合物的瞬时与累积性能指标（熔融指数与密度）的模型。研究表明,对于涉及催化剂切换的牌号切换,最优策略是采用催化剂分步进料方式,综合考虑新旧催化剂对操作参数的不同要求及各反应器内残留原催化剂比重对最终产品性能的影响,从而达到明显缩短切换时间、有效抑制操作变量急剧变化、平缓产品性质变化轨迹的优化目的。     

流化床反应器中气相丙烯聚合反应的动力学和预测控制(英文)

A two-phase dynamic model, describing gas phase propylene polymerization in a fluidized bed reactor, was used to explore the dynamic behavior and process control of the polypropylene production rate and reactor temperature. The open loop analysis revealed the nonlinear behavior of the polypropylene fluidized bed reactor, jus- tifying the use of an advanced control algorithm for efficient control of the process variables. In this case, a central- ized model predictive control （MPC） technique was implemented to control the polypropylene production rate and reactor temperature by manipulating the catalyst feed rate and cooling water flow rate respectively. The corre- sponding MPC controller was able to track changes in the setpoint smoothly for the reactor temperature and pro- duction rate while the setpoint tracking of the conventional proportional-integral （PI） controller was oscillatory with overshoots and obvious interaction between the reactor temperature and production rate loops. The MPC was able to produce controller moves which not only were well within the specified input constraints for both control vari- ables, but also non-aggressive and sufficiently smooth for practical implementations. Furthermore, the closed loop dynamic simulations indicated that the speed of rejecting the process disturbances for the MPC controller were also acceotable for both controlled variables.     

气相法聚乙烯工艺冷凝态操作模式的稳定性和动态行为

气相法聚乙烯工艺冷凝态操作模式由于显著提高了循环气移热能力和反应器时空产率,已成为流化床乙烯聚合工艺的主流操作模式。建立了气相法聚乙烯工艺冷凝态操作模式的数学模型,包括流化床反应器模型,多级换热器模型和反应温度、压力以及循环气组成的控制模型。基于此,采用流程模拟方法,计算了系统在反应器温度采用闭环控制时的稳态解;根据系统对小扰动的动态响应特点,定性判断了反应器温度采用开环控制和闭环控制时聚合反应系统的稳定性;考察了系统对1-己烯分压和催化剂进料速率的阶跃响应特性。结果表明,反应器温度采用闭环控制时,聚合反应系统在所考察操作条件下均是稳定的,而采用开环控制时,解曲线被分叉点分割为稳定区域和不稳定区域。反应器温度对1-己烯分压阶跃变化的动态响应表明聚合反应系统存在长、短周期两类振荡,表明冷凝态操作模式下乙烯聚合反应过程是一个多控制回路耦合的复杂过程。     

A diffusion‐reaction model for α‐chymotrypsin carrying uniform thermosensitive gel beads

In this study, the kinetic behavior of α‐chymotrypsin‐immobilized, uniform poly(isopropylacrylamide) gel beads was investigated. The kinetic study was performed by using a continuous reactor operated at steady‐state conditions. In the experiments, substrate feed concentration, residence time, and reactor temperature were changed. The results were explained by a diffusion–reaction model developed for steady‐state conditions. The effectiveness factor and Thiele modulus values of the thermosensitive enzyme–gel system were estimated at different temperatures by using an iterative procedure based on fourth order Runge–Kutta algorithm. The results indicated that the overall hydrolysis rate was controlled by the substrate diffusion through the gel matrix. A bending point was detected for the Thiele modulus at the lower critical solution temperature (LCST) of the thermosensitive gel. The effective diffusion coefficient of substrate and effectiveness factor decreased suddenly at LCST. The mass transfer process within the thermosensitive carrier could be described in detail by the proposed model. The results of our numerical procedure were also compared with an analytical approximate solution available in the literature. The consistency between two different model was reasonably good. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1025–1034, 1999     

Two-phase modeling of a gas phase polyethylene fluidized bed reactor

A two-phase model is proposed for describing the behavior of a fluidized bed reactor used for polyethylene production. In the proposed model, the bed is divided into several sequential sections where flow of the gas is considered to be plug flow through the bubbles and perfectly mixed through the emulsion phase. Polymerization reactions occur not only in the emulsion phase but also in the bubble phase. Voidages of the emulsion and bubble phases are estimated from the dynamic two phase structure hydrodynamic model. The kinetic model employed in this study is based on the moment equations. The hydrodynamic and kinetic models are combined in order to develop a comprehensive model for gas-phase polyethylene reactor. The results of the model are compared with the experimental data in terms of molecular weight distribution and polydispersity of the produced polymer. A good agreement is observed between the model predictions and actual plant data. It has been shown that about 20% of the polymer is produced inside the bubble phase and as such cannot be neglected in modeling such reactors.     

环管反应器中聚丙烯颗粒内部的质量与热量传递模型

针对环管反应器中Ziegler-Natta催化剂催化的聚丙烯颗粒增长过程,采用多层模型建立了聚丙烯颗粒内部的质量与热量传递预测模型.通过模型模拟得到聚丙烯颗粒内部的丙烯单体浓度梯度和温度梯度.此外,通过模型分析了聚丙烯颗粒的增长规律.结果表明,环管反应器中的聚丙烯颗粒内部的单体浓度梯度明显,该浓度梯度值随扩散系数的增大而减小,随催化剂初始粒径减小而减小;相比于浓度梯度而言,聚丙烯颗粒内部的温度梯度并不明显,温度值随聚合进行而增加,温度梯度则随催化剂初始粒径的增加而增加;不同大小的催化剂颗粒增长得到的聚丙烯颗粒的增长倍数不同.     

煤基高流动性LLDPE DNDA-8320的工业化生产

总结了煤基气相流化床聚乙烯装置从线型低密度聚乙烯(LLDPE)DFDA-7042转产LLDPE DNDA-8320的工业化过程,重点在于反应器和挤压造粒机的工况变化,出现的故障和生产难点,并对DNDA-8320的性能和应用进行了评价.结果表明:转产实施时,反应器流化松密度和床重下降,催化剂活性从21740 g/g降至1...     

Catalytic reactions in transport reactors

By assuming plug flow of both gas and solids phases, and simple kinetics, a model of the transport reactor has been prepared which accounts for the unsteady state behaviour of the catalyst particles. Computational results are presented to show the influence of the major variables. As in a steady state analysis, the Thiele modulus serves to indicate the catalyst effectiveness.