考虑间壁换热的间壁塔模型的模拟仿真研究

精馏是化工生产过程中应用最广泛的分离技术。间壁塔作为一个完全热耦合精馏塔,与传统两塔精馏序列相比,不仅可以减少设备投资,减少占地面积,还能节约精馏过程中的能耗。以环己烷-环庚烷-环辛烷的分离为研究对象,建立间壁塔和传统两精馏序列的模型,分析间壁塔的间壁传热对总能耗的影响,对新工艺(间壁塔分离工艺)和旧工艺(传统两塔精馏序列分离工艺)进行仿真和优化研究。通过优化计算,在满足产品纯度要求前提下,得到了两工艺的最优工艺操作条件。比较两种工艺的塔板数和能耗,间壁塔工艺在节约成本的同时节能17.9%。     

Decentralized control and identified-model predictive control of divided wall columns

As a thermal separation method, distillation is one of the most important separation technologies in the chemical industry. Given its importance, it is no surprise that increasing efforts have been made in reducing its energy inefficiencies. A great deal of research is focused in the design and optimization of the divided-wall column (DWC). Its applications are still reduced due to distrust of its controllability and robustness. Previous references have studied the decentralized control of DWC but still few papers deal about model predictive control (MPC) applied to DWC.In this work we present a decentralized control of both a divided-wall column along with its equivalent MPC schema, both approaches are compared. Instead of building a rigorous model or performing the step test to an existing plant, the MPC model is obtained by identification of a rigorous simulation. An ARX model is demonstrated to represent adequately the DWC column behavior. This approach might be very convenient if plant testing is not available.     

用分隔壁精馏塔分离松节油中蒎烯的模拟

提出采用分隔壁精馏塔代替常规精馏塔序列分离松节油以得到α-蒎烯和β-蒎烯的新工艺。为了选择适合松节油体系的分隔壁塔结构,采用Aspen软件分别模拟计算了DWC、DWC-SS和DWC-SR三种塔型,结果表明,与常规工艺相比DWC塔可节能26%,DWC-SS塔可节能16.6%,DWC-SR塔可节能12.5%。在此基础上考察了DWC塔进料位置、回流比、隔板位置、气液相分配比和中间产品出料位置对分离效果和热负荷的影响,得出气液相分配比是影响分隔壁塔操作稳定性的主要因素。     

Plant-wide design and control of acetic acid dehydration system via heterogeneous azeotropic distillation and divided wall distillation

Energy-saving plant-wide design and plant-wide control of an acetic acid dehydration system with the feed containing methyl acetate and p-xylene are investigated in the study. A heterogeneous azeotropic distillation using isobutyl acetate as an entrainer is designed to obtain high-purity acetic acid at the column bottom and to keep a small acetic acid loss through the top aqueous draw. The accumulation of p-xylene in the column is avoided by adding a side product stream. The mixture in the aqueous phase of decanter, containing mostly water, methyl acetate, and isobutyl acetate is separated using a divided wall distillation column. The whole acetic acid dehydration system includes a heterogeneous azeotropic distillation column and a divided wall distillation column.The control strategies using temperature loops are proposed for this acetic acid dehydration system. For the heterogeneous azeotropic distillation column, the requirements for acetic acid compositions in both the aqueous phase of the decanter and the column bottom can be satisfied by designing entrainer inventory temperature control and cascade temperature control simultaneously. The stages of controlled temperatures are chosen by singular value decomposition and closed-loop analysis methods based on the criteria of minimum entrainer makeup. For the divided wall distillation column, steady-state analysis methods are used for the selection of proper controlled and manipulated variables and the determination of their pairings. Dynamic simulation results demonstrate that the proposed plant-wide control strategy can maintain product purities and reject external disturbances in feed flow and composition changes as well as internal disturbances such as changes in liquid and vapor splits.     

催化精馏隔壁塔合成乙酸乙酯的模拟研究

提出应用催化精馏隔壁塔合成乙酸乙酯的新工艺流程,以催化精馏隔壁塔替代常规催化精馏塔及乙酸乙酯塔.利用Aspen Plus模拟软件,模拟催化精馏隔壁塔及常规工艺流程,比较分析2种工艺流程塔内的液相组成分布以及温度分布,并分析液相分配比对催化精馏隔壁塔的影响.证明催化精馏隔壁塔既可节能25.7%,又可降低设备投资费用和操作费用.     

State estimation and inferential control for a reactive batch distillation column

An optimal reflux ratio profile is obtained for a reactive batch distillation system utilizing the capacity factor as the objective function in a nonlinear optimization problem. Then, an Artificial Neural Network (ANN) estimator system, which utilizes the use of several ANN estimators, is designed to predict the product composition values of the distillation column from temperature measurements inferentially. The network used is an Elman network with two hidden layers. The designed estimator system is used in the feedback inferential control algorithm, where the estimated compositions and the reflux ratio information are given as inputs to the controller to see the performance of the ANN. In the control law, a scheduling policy is used and the optimal reflux ratio profile is considered as pre-defined set-points. It is found that, it is possible to control the compositions in this dynamically complex system by using the designed ANN estimator system with error refinement whenever necessary.     

Experimental investigation of conventional control strategies for a heterogeneous azeotropic distillation column

In this work, a laboratory scale sieve plate distillation column was constructed to investigate the conventional control strategies of an isopropyl alcohol (IPA), cyclohexane (CyH) and water (H₂O) heterogeneous azeotropic distillation column. Steady state process analysis showed that the optimal operation point should be located at a critical reflux, a transition point at which the distillation path switches from a route that passes through IPA+H₂O azeotrope to one that passes through IPA+CyH azeotrope. At this critical reflux, a high purity IPA product can be obtained with minimum energy consumption and maximum product recovery. However, the steady state is extremely sensitive to feed disturbances. A good control strategy must be able to maintain a steady column temperature profile that shows a plateau near 70°C to ensure passage around IPA+CyH azeotrope. In this study, an inverse double loop control strategy is recommended. Through experimental testing, the proposed control strategy was demonstrated to keep the product IPA purity at the desired high-purity level under all feed disturbance changes while other conventional control strategies fail.     

Adaptive dynamic matrix control of a distillation column with closed-loop online identification

This paper presents an online identification technique where a process is identified in terms of pseudo impulse response coefficients and subsequently used to update convolution type models to accommodate process-model mismatch. As an example, dynamic matrix control has been applied adaptively to control the top product composition of a distillation column for both servo and regulatory problems. The algorithm automatically detects a large step-like disturbance requiring fresh identification of the process and subsequently adapts the controller to the new model. Simulation studies using an analytical dynamic full order model of a distillation column demonstrated the usefulness of the adaptation scheme. Experimentation on a pilot scale distillation unit vindicated the simulation results.     

Drag reduction in transitional linearized channel flow using distributed control

This work focuses on feedback control of incompressible transitional Newtonian channel flow described by the twodimensional linearized Navier-Stokes equations. The control objective is to use distributed feedback to achieve stabilization of the parabolic velocity profile, for values of the Reynolds number for which this profile is unstable, and therefore to reduce the frictional drag exerted on the lower channel wall compared to the open-loop values. The control system uses measurements of shear stresses on the lower channel wall and the control actuation is assumed to be in the form of electromagnetic Lorentz forces applied to the flow near the bottom wall. Galerkin's method is initially used to derive a high-order discretization of the linearized flow field that captures the flow instability and accounts for the effect of control actuation on all the modes. Then, a low-order approximation of the linearized flow field is derived and used for the synthesis of a linear output feedback controller that enforces stability in the high-order closed-loop system. The controller is applied to a simulated transitional linearized channel flow and is shown to stabilize the flow field at the parabolic profile and significantly reduce the drag on the lower channel wall.     

软测量技术在DeltaV DCS上的实现和应用

针对丁二烯生产装置精馏塔塔顶控制回路存在的问题,建立了软测量模型,并设计了串级推断控制回路。利用从集散控制系统采集的大量现场数据,运用基于多元线性回归方法的软测量建模技术,建立了塔顶中丁二烯产品纯度的软测量模型,实现了产品质量闭环控制。通过DeltaV DCS系统实现控制回路的改造。实际使用证明,该软测量模型具有良好的特性,较高的估计精度和实时性;塔的控制更加平稳,节约了能耗,具有较好的经济效益。     

精馏塔多通道对角矩阵解耦控制仿真

在化工生产中,许多单元操作的控制涉及至少2个被控变量,因此产生至少2个以上的控制回路。在该类具有多个控制回路的多变量控制系统中,同一被控对象上的多个控制回路,在实现其各自的控制目标时,将发生相互作用和相互影响。精馏塔是化工等行业中广泛使用的高耗能分离设备,为满足工艺要求和节能,需将塔顶和塔底产品流控制在设计值,即双成分控制。通常,利用回流量和加热量作操纵变量分别控制塔顶和塔底产品的组成。不过,回流量的变化也会影响塔底产品的组成,加热量的改变也会影响塔顶产品的组成。当采用温度作为反映产品组成的间接指标时,塔顶和塔底温度控制回路具有强烈的耦合作用,会导致系统不稳定。一个简单有效的方法就是解出耦合,因此,本文先介绍了一种基于对角矩阵的温度解耦控制方案,并在MATLAB/Simulink平台进行了控制仿真。结果显示:与两个独立PID回路控制相比,对角矩阵解耦控制不仅能达到更优的控制效果,还具有极强的对象特性鲁棒性。     

Design and implementation of totally-self checking SHA-1 and SHA-256 hash functions’ architectures

Many cryptographic primitives that are used in cryptographic schemes and security protocols such as SET, PKI, IPSec and VPN's utilize hash functions - a special family of cryptographic algorithms. Hardware implementations of cryptographic hash functions provide high performance and increased security. However, potential faults during their normal operation cause significant problems in the authentication procedure. Hence, the on-time detection of errors is of great importance, especially when they are used in security-critical applications, such as military or space. In this paper, two Totally Self-Checking (TSC) designs are introduced for the two most-widely used hash functions: SHA-1 and SHA-256. To the best of authors’ knowledge, there is no previously published work presenting TSC hashing cores. The achieved fault coverage is 100% in the case of odd erroneous bits. The same coverage is achieved for even erroneous bits, if they are appropriately spread. Additionally, experimental results in terms of frequency, area, throughput, and power consumption are provided. Compared to the corresponding Duplicated with Checking (DWC) architectures, the proposed TSC-based designs are more efficient in terms of area, throughput/area, and power consumption. Specifically, the introduced TSC SHA-1 and SHA-256 cores are more efficient by 16.1% and 20.8% in terms of area and by 17.7% and 23.3% in terms of throughput/area, respectively. Also, compared to the corresponding DWC architectures, the proposed TSC-based designs are on average almost 20% more efficient in terms of power consumption.     

基于运行剖面的列控系统测试用例生成研究

为了提高列控系统(列车运行控制系统,简称列控,是保证列车安全、快速运行的系统)测试的可靠性,生成合理的测试用例,需要设计相应的运行剖面来仿真实际使用的列控系统的情况,并且基于得到的运行剖面来生成相应的测试用例。因此,通过对列控中心系统的输入输出特点以及功能需求的研究与总结,提出了通过构建任务剖面、系统模式剖面、环境剖面来最终完成列控系统运行剖面构建的方法,并在最终得到的运行剖面基础上,应用优先级度量机制和程序插桩方法对粒子群算法进行合理改进,最终实现了测试用例的自动生成。实验结果证明,通过合理地构建运行剖面,可以保证测试功能的完整性以及测试数据的合理性,改进后的粒子群算法能够进一步提高测试的时间效率。     

基于PLC的工件表面检测与分拣控制系统设计

本文主要介绍了由视觉传感器、传送带及机械手构成的分拣系统和可编程序控制器与位置控制单元构成的控制系统,实现对工件表面的检测及对次品工件的剔除操作。视觉传感器实施工件检测,配置位置控制单元的参数实现机械手各轴的精确定位,运用脉冲指令控制传送带的运动节拍,调用移位寄存器指令实现检测与剔除信号的衔接,编写梯形图控制程序,实现了机械手对次品的抓取及剔除操作。经过实践证明,该控制系统结构简单,编程简便,性价比高,具有应用价值。     

Online tuning of a steady state crude distillation unit model for real time applications

The steady state simulators, used for on-line performance prediction and for on-line optimization in crude distillation units are often sensitive to small variations in the feed composition, which is specified in terms of a True Boiling Point (TBP) vs volume percent distilled curve. The exact feed TBP is often not available during the plant operation. Also stratification of raw crude oil into layers in the large tank farm sections cause severe operating problems in terms of the stability of the column. If feed TBP can be predicted online, necessary feedforward action can considerably reduce the operating problems. A model has been developed for backcalculation of feed TBP using measured plant parameters. A heat balance is performed around an envelope encompassing the rectifying section of the fractionator and is followed by the calculation of Equilibrium Flash Vaporization (EFV) temperatures at six different locations of the column which are correlated with corresponding feed TBP temperatures. The second part of model tuning consists of calculating model parameters in the form of point efficiencies so as to minimize the discrepancy between the simulator predicted and measured column parameters which arises out of modelling approximations such as assumption of phase equilibria at each stage and use of imperfect thermodynamics correlations. The simulator results, after tuning, were found to match the plant measurements within two percent in all the cases investigated. The simulator output was used to predict various product properties using a Property Prediction package and these were also found to match well with those of laboratory measurements. Both the backcalculation of feed TBP and the efficiency tuning need to be implemented on-line for inferential control and supervisory optimization.     

Optimal design of thin-walled columns for buckling under loadings controlled by displacements

In this paper the problem of the optimal design of thin-walled tubular columns under loadings controlled by displacement is investigated. A radius of cross-sectional circular profile varying along the axis of the shell-column as well as a wall thickness, which lead to the maximal axial displacement caused by compression before the structure buckles are sought. Both global (buckling of a column) and local (wall buckling of a shell) stability of a structure are taken into account. The geometry of the structure is approximated by the convex Bézier polynomial. The results are obtained using numerical optimization method, namely the simulated annealing method.     

全液压型位置控制在热轧工艺中的应用

位置控制是轧制工艺中一项重要控制内容,其控制的好坏直接影响到成材如厚度、宽度等的尺寸精度要求。为了提高位置控制的精度,一种新型的全液压型位置控制技术开始投入应用,它从控制方法和执行机构两方面入手,尽可能地杜绝位置控制过程中的不利因素。实际应用结果表明,产品的控制精度有了明显提高,在类似的生产控制领域中,有一定的推广应用价值。     

Control strategies for reactive batch distillation

A batch reactor may be combined directly with a distillation column by distilling off the light component product in order to increase the reactor temperature or to improve the product yield of an equilibrium reaction. The same amount of the light product should be removed as the amount being formed by the reaction at any time. A linearized model has been developed which describes the process behaviour satisfactorily for control analysis purposes. The controllability of a combined batch reactor/batch distillation column is found to depend strongly on the operating conditions and on the time during the run. In general, controlling only the reactor temperature (one-point bottom control) is difficult since the set-point has to be specified below a maximum value in order to avoid break-through of an intermediate component in the ditillate. This maximum value may be difficult to know a priori. For the example considered in this study, control of both reactor temperature and distillate composition (two-point control) is also found to be difficult due to large interactions in the column. As with one-point bottom control, the reactor temperature has to be specified below a maximum value. However, energy can be saved since the heat duty can be decreased with time. Controlling the temperature on a tray in the column (one-point column control) is found to give good performance for the given process with no loss of reactant and a high reactor temperature, although no direct control of the reactor temperature is obtained.     

Dynamic version of steady state optimizing control of a distillation column by trial method

The best optimizing control procedure using an automatic optimizing controller, based on a simple pattern-search type, steepest ascent method is presented for a distillation column. The control system is composed of a conventional feedback loop by which the composition of overhead product is maintained constant, and an optimizing control unit which holds the partial derivatives of the objective function with respect to control variables to zero. The objective function used in this study is the profit rate obtained from the distillation process, and the two control variables; feed flow rate and energy supplied to the plant, are used.     

Decentralized and centralized model predictive control to reduce the bullwhip effect in supply chain management

Mitigating the bullwhip effect is one of crucial problems in supply chain management. In this research, centralized and decentralized model predictive control strategies are applied to control inventory positions and to reduce the bullwhip effect in a benchmark four-echelon supply chain. The supply chain under consideration is described by discrete dynamic models characterized by balance equations on product and information flows with an ordering policy serving as the control schemes. In the decentralized control strategy, a MPC-EPSAC (Extended Prediction Self-Adaptive Control) approach is used to predict the changes in the inventory position levels. A closed-form solution of an optimal ordering decision for each echelon is obtained by locally minimizing a cost function, which consists of the errors between predicted inventory position levels and their setpoints, and a weighting function that penalizes orders. The single model predictive controller used in centralized control strategy optimizes globally and finds an optimal ordering policy for each echelon. The controller relies on a linear discrete-time state-space model to predict system outputs. But the predictions are approached by either of two multi-step predictors depending on whether the states of the controller model are directly observed or not. The objective function takes a quadratic form and thus the resulting optimization problem can be solved via standard quadratic programming method. The comparisons on performances of the two MPC strategies are illustrated with a numerical example.