Temperature control of industrial gas phase polyethylene reactors

The performance of linear and nonlinear temperature control schemes is assessed for an open-loop unstable gas-phase polyethylene reactor (GPPER), based on speed, damping, robustness and the ability to maintain closed-loop stability in different operating regimes. An existing industrial GPPER model is improved by modelling the temperature states in the external heat exchanger using linear and nonlinear driving force models with varying numbers of heat transfer stages. Differences in heat exchanger models do not produce gain mismatch but do result in phase mismatch. It is shown that the nonlinear error trajectory controller (ETC) exhibits significantly superior responses in terms of speed, damping and robustness compared with an optimally-tuned PID controller. Therefore, substantial benefits could be realized using nonlinear controllers because they can provide good disturbance rejection capabilities and ensure closed-loop stability over a wide range of operating conditions. An approach is presented for tuning ETCs for minimum-phase processes of arbitrary relative degree.     

Optimal polymer grade transitions for fluidized bed reactors

Industrial polymerization plants experience frequent changes of products, driven by end-use properties to meet various market requirements. Efficient grade transition policies are essential to save time and materials. In this study, the gas-phase catalytic polymerization is modeled in a fluidized bed reactor by a single-phase model, and dynamic optimization is implemented to determine optimal operating sequences for grade changes. Two optimization formulations, a single-stage and a multi-stage formulation, are introduced and compared. The superiority of the multi-stage formulation is demonstrated owing to a better control on each stage during the transition and a further reduction of off-grade time. Subsequently, an on-line optimal control framework is established by incorporating shrinking horizon nonlinear model predictive control with an expanding horizon weighted least-square estimator for process states and unknown parameters. The results of a case study indicate the designed framework is able to handle process uncertainty, while reducing the transition time.     

Control induced instabilities in fluidized bed spray granulation

This contribution is concerned with the stability problems occurring during the operation of continuous fluidized bed spray granulation processes with external sieve mill cycle. These processes are in general operated by a mass controller, which guarantees that the overall mass of particles in the granulation chamber stays in well-defined bounds. It is well-known that, depending on the milling diameter, instabilities may occur, which result in a nonlinear limit cycle of the particle size distribution. To overcome this problem two approaches have been proposed in the literature, constrain the admissible parameter space to exclude regions of instability and design additional stabilizing control loops. In the present contribution, the cause of this instability phenomenon will be studied. It will be shown that the instability is not inherent to the process, which turns out to be open-loop stable over the studied parameter range, but due to the mass controller. More specifically, it will be demonstrated that the zero dynamics of the granulation process become unstable for certain parameter ranges resulting in closed-loop unstable process behavior. To point out, that this behavior does not depend on the specific mass controller design procedure, three prototypical mass controllers of practical relevance are designed and analyzed.     

丙烯间歇液相本体聚合模拟

从聚合反应动力学出发,对丙烯间歇液相本体聚合工业装置建立数学模型,采用PR状态方程进行汽液平衡计算。链增长速率常数k_p、链转移速率常数K_t由质均分子量为20万及30万2个品牌的生产条件及产品性质去拟合得到。应用拟合出的参数,对质均分子量为40万的产品的生产工艺条件进行模拟,模拟结果与实际相比为:聚合物的产量误差0.66%,质均分子量误差2.07%,模拟结果与实际较为吻合。并对反应过程中变量随时间变化情况进行分析,得出聚合物质量、氢气浓度、液相体积等随时间的变化规律,对实际生产工艺的开发与改进具有指导意义。     

间歇液相本体丙烯聚合过程的模拟与分析

在丙烯聚合反应宏观动力学的基础上,针对间歇液相本体丙烯聚合工艺的特点,将丙烯聚合过程分解为升温阶段和恒温阶段建立聚合反应器模型。采用MathCAD软件解算模型,计算出的结果与实际生产数据偏差小于±5%。从反应放热和系统热负荷两个方面模拟分析装置的反应过程,反应前期(<50℃)反应放热最少,当反应进行到67 min～70 min时,反应放热达到最大值,在反应前期控制温度在20 min之内达到50℃,有利于催化剂活性的发挥,降低反应放热峰值;在50℃～75℃升温阶段,反应放热集中,以缓和的速度升温,能够减少热负荷波动的频率和幅度,实现平稳生产控制,降低单位聚丙烯能耗。     

CFD模拟与流化床气体分布板的结构设计

针对流化床工业设备,简要阐述了加强气体分布板结构设计的重要性。在此基础上,结合对计算流体力学(computational fluid dynamics,CFD)理论及相关计算软件的介绍,重点阐明了FLUENT软件的计算特点及气固流化系统的流场模型,同时还列举了若干基于FLUENT、FIRE、PHOENICS、CFX等CFD软件的流化过程模拟及气体分布板结构优化的实例,并指出了该类研究的几点不足之处,以明确下一步的研发重点。     

Control of an industrial polymerization reactor using flatness

The aim of this paper is to report the design and use of a controller for the world's largest polypropylene reactor. This is the first industrial process-controller to use the so-called flatness property of the system, which is presented here in a concise and application oriented manner. Industrial results are given and the control strategy is presented in the context of today's fast and competitive market of polymers.     

基于Aspen Plus软件的循环流化床烟气脱硫模型

本文研究和模拟循环流化床烟气脱硫的流程和模型.以微元分析SO2的传质为基础,建立循环流化床烟气脱硫的数学模型,模型用双膜理论分析脱硫反应对SO2传质过程的增强影响,并采用惯性碰撞理论解释浆滴的形成过程.借助Aspen Plus过程模拟平台,用FORTRAN语言编写基于该模型的用户单元模块,模拟循环流化床烟气脱硫工艺,分析Ca/S、增湿水量、塔内颗粒物浓度、水滴粒径等参数对脱硫的影响,模拟计算结果和实验数据的对比显示模型能如实反映实际的趋势.本文为应用循环流化床烟气脱硫技术提供参考.     

间歇液相本体丙烯聚合过程中产品性质的模拟与分析

在25 kt/a间歇液相本体聚丙烯工业装置反应器机理模型的基础上,建立间歇丙烯聚合过程中产品分子量及分子量分布、熔融指数等性质的数学模型.采用BWRS状态方程计算氢在氢.丙烯系统气-液平衡常数,以分析生产中聚丙烯分子量及分子量分布、聚丙烯熔融指数等参数的变化情况及其影响因素.结果表明:氢在液相中的浓度是影响聚丙烯瞬时分子量的主要因素,主要受氧.丙烯系统气液平衡常数和液相固含率影响,氢在液相中的浓度与温度成正比,与液相固含率成反比.聚丙烯熔融指数模型由重均分子量关联得到,二者呈反比关系,熔融指数的变化与重均分子量变化相对应.瞬时分子量和产量是聚丙烯重均分子量和熔融指数的主要影响因素.     

Computer simulation of a cold fluidized bed

The theory of two-phase flow provides a set of partial differential equations describing the dynamics of a fluidized bed. A code, developed by Systems, Science, and Software,¹ to solve this system of equations is used to simulate a facility at Alexandria, Virginia.³ Calculated values of pressure drop are compared with experiment.     

循环流化床过程仿真的数学模型

传统的循环流化床仿真的数学模型都采用反应动力学的方法。本文采用了CSTR的平衡反应算法,对循环流化床建立仿真的数学模型,已用于100MW循环流化床的实际工业装置仿真培训器中,算法比较稳定,模拟了循环流化床的开停车、事故处理和正常操作,动态趋势和实际工业装置相符。与设计值相比,模拟的稳态误差在1%以内,动态误差在5%以内。     

气-固加压流化床压力脉动信号的分析

对气-固流化床特别是气-固加压流化床压力脉动信号进行深入分析有助于更好地了解该类反应器内的流动行为。为了研究压力脉动信号与加压气-固流化床内流动行为的关系,以FCC催化剂为实验物料,采用密相段内径300 mm、高度3800 mm、扩大段内径600mm、高度800mm的加压流化床,对其压力脉动信号进行快速傅里叶变换(FFT)和小波分析,获得了代表气泡行为的特征频率(D5-D7);分析此频段能量特征值随表观气速的变化规律,得出了流型转变速度u_c,并考察压力对u_c的影响。结果表明:压力波动能量集中在低频,随着气速的增加能量特征值先增加后减小,压力升高u_c减小。     

环氧丙烷聚合反应过程的动态模拟

为了深入了解环氧丙烷聚合工艺并为国内生产厂家提供生产操作的理论依据。本文针对聚醚装置反应工段,根据聚合釜的特点,首先,通过研究环氧丙烷聚合反应机理,利用链节动力学的方法推导出了反应动力学方程。其次,建立环氧丙烷聚合反应的数学模型,实现聚合反应过程的实时动态模拟。最后,以模拟程序的计算结果,讨论操作条件对平均分子量和双键度的影响。对实际生产具有一定的指导意义。     

基于EMMS模型的聚乙烯流化床流体力学模拟

目前,针对气-固流化床的数值模拟多采用欧拉-欧拉双流体模型(TFM)。而传统的TFM基于均匀拟流体假设,不能准确描述流化床内流态化的非均匀性及介观尺度的性质,如颗粒团聚。本文采用基于能量最小多尺度方法(EMMS)的双流体模型(EFM)来研究聚乙烯流化床流体动力学,模拟结果证明EFM较好把握了流化床内的非均匀性以及介观尺度性质,更能准确描述聚乙烯流化床反应器内部的流体动力学。EFM模型的应用可以为聚乙烯生产过程中流化床结构改进以及"热点"的避免提供更有说服力的模拟验证。     

基于静态平衡的循环流化床锅炉床温控制技术的研究

针对循环流化床锅炉燃烧控制系统存在的分布参数、非线性强、时变性强、多变量紧密耦合,造成锅炉燃烧控制系统尤其是床温控制系统无法稳定运行的问题,提出了基于静态平衡的燃烧控制系统实现床温控制方案,通过仿真试验,验证了控制策略的可行性。该方法很好地实现了床温的稳定控制,而床温的稳定控制,有利于减少各控制量相互之间的耦合,达到解耦的目的,也为主蒸汽压力、燃烧控制系统的投运创造了条件。     

循环流化床锅炉燃烧过程建模研究

针对具有多维非线性和纯滞后特性的循环流化床锅炉燃烧过程,采用基于PLS学习算法和OLS学习算法的径向基函数(RBF)神经网络进行建模研究。首先通过循环流化床锅炉仿真平台产生用于建模实验的网络训练数据和泛化数据,然后分别采用OLS算法和PLS算法进行网络训练和泛化研究,最后讨论了影响建模结果的算法参数及其选取方法,重点讨论了PLS算法的4个网络参数的影响和选取。与基于小波网络的建模实验比较,对具有复杂特性的循环流化床锅炉燃烧过程,采用RBF网络建模在保证建模精度的同时,算法参数的选取也较为方便易行。     

循环流化床锅炉效率相关参数的建模研究

循环流化床锅炉具有污染物排放少、燃料适应性广、负荷调节能力强等优点,近年来在电力、供热等行业中得到广泛应用。然而目前大部分循环流化床锅炉均存在自动投入率低,操作依赖人工经验的特点,造成这一状况的一个重要原因是缺乏合理的数学模型。首先对工艺流程进行分析,选取对锅炉效率影响最大的7个参数作为建模对象:过量空气系数、床温、排烟温差、飞灰含碳量、一次风机电流、二次风机电流、引风机电流。每个参数有各自不同的特点,对不同的统计模型的适用性也不尽相同。为了达到最佳建模效果,分别应用多元线性回归、多元逐步回归、偏最小二乘回归及BP神经网络对这些参数进行建模。实例研究表明,过量空气系数和二次风机电流适合采用偏最小二乘回归法建模;床温、排烟温差、一次风机电流和引风机电流适合采用多元线性回归法建模;飞灰含碳量采用BP网络模型对其预测效果相对较好。本文所建的模型对循环流化床锅炉的节能分析和进一步的操作优化研究具有一定的实际意义。     

气固鼓泡流化床的流动特性数值模拟

为了模拟研究均匀布风的气固鼓泡流化床的流动特性,建立双流体力学模型,利用计算流体力学软件(FLUENT)并通过软件接口插入自编的计算气泡频率、气泡速度、气泡长度程序模块,成功模拟出鼓泡流化床的颗粒循环及气泡的特殊流动状态。计算的气泡频率、气泡速度、气泡长度与实验结果相符。     

模糊控制在生物流化床污水处理中应用研究

介绍了生物流化床新型污水处理技术,分析了生物流化床污水生物处理工艺难于控制的原因,以及模糊控制系统的结构和特点。根据生物流化床污水处理工艺特点,提出用模糊控制的方法实现生物流化床污水处理的自动控制。实验结果表明,将模糊控制应用于生物流化床污水处理技术可以在出水稳定达标的前提下达到节能降耗、避免过量曝气的目标。     

Nonparametric identification and adaptive control of an anaerobic fluidized bed digester

In this paper, a new adaptive and robust control algorithm that is able to successfully deal with unpredictable internal changes (unmodeled dynamics) and external disturbances (changes in input) of the processes in an anaerobic digestion bioreactor is presented. The adaptive controller is based on a nonparametric statistical approach of the process identification. The regulation is done by optimally adapting the input liquid flow rate of the wastewater to designated changes in the output flow rate of the biogas (methane and carbon dioxide) resulting from the biological reaction. The fundamental advantage of this approach is its freedom from any a priori modeling assumptions about uncertain dynamic components. Experimental results, obtained using a pilot-scale 150 l fluidized bed reactor for the treatment of industrial wine distillery liquid wastes, demonstrates the usefulness of this approach in controlling biological processes.