Economics-based NMPC strategies for the operation and control of a continuous catalytic distillation process

Process profitability is an yes or no criterion for the successful long-term operation of industrial processes. This article describes the use of dynamic online economic process optimization to improve the performance of chemical processes. Different model-predictive control techniques have progressively been applied to coupled multivariable control problems and in many cases, especially in the petrochemical industry, the reference values are adjusted infrequently by stationary optimization based upon a rigorous nonlinear stationary plant model (real-time optimization, RTO). In between these optimizations, however, the process may be operated suboptimally due to the presence of disturbances. Nonlinear dynamic model-based optimization has been proposed recently to combine optimal operation and feedback control. In this paper, a model of the complex dynamics of a pilot-scale continuous catalytic distillation process is used to explore the potential benefits of online economics optimizing control strategies. We compare the direct economic optimization scheme with a compromise scheme, the economics-oriented tracking controller. The outcome of this work indicates that by using direct economics optimizing NMPC the plant economics can be handled better while guaranteeing the product specifications which are formulated as explicit constraints.     

Application of the singular control test to periodic control problems of a continuous distillation process

The singular control test is applied to periodic control problems of a continuous distillation process in order to determine if the optimal periodic control is proper. It is also shown that the singular control test is easier to apply than the w-teat for the problems of concern. Some numerical calculations are carried out for seeking the optimal periodic controls.     

精馏过程动态机理模型建立

为了实现精馏过程的动态优化和先进控制,必须解决精确的动态数学模型的建立问题.本文介绍了精馏过程动态机理建模的方法,该模型的特点是考虑每块塔板温度、压力、汽相流量、液相流量、汽相浓度、液相浓度和持液量的动态特性,并且,利用正交配置方法离散模型方程,利用牛顿一拉夫逊方法解模型方程,研究结果表明该模型精度高,误差小于3%,适合于精馏过程的动态优化和先进控制策略研究.     

The level-crossing rate and above-level duration time of the intensity of a gaussian random process

The level-crossing rate of the intensity of a stationary, complex-valued gaussian random process is determined. The average time spent above a specified intensity is also obtained. Expressions for the joint product moments of the intensity and its time derivative are given, along with the joint PDF.     

Production/inventory systems with a stochastic production rate under a continuous review policy

A single production facility is dedicated to producing one product with completed units going directly into inventory. The unit production time is a random variable. The demand for the product is given by a Poisson process and is supplied directly from inventory when available, or is backordered until it is produced by the production facility. Relevant costs are a linear inventory holding cost, a linear backorder cost, and a fixed setup cost for initiating a production run. The objective is to find a control policy that minimizes the expected cost per time unit.The problem may be modeled as an M/G/1 queueing system, for which the optimal decision policy is a two-critical-number policy. Cost expressions are derived as functions of the policy parameters, and based on convexity properties of these cost expressions, an efficient search procedure is proposed for finding the optimal policy. Computational test results demonstrating the efficiency of the search procedure and the behavior of the optimal policy are presented.     

Aspen Dynamics simulation of a middle-vessel batch distillation process

Aspen Dynamics is a powerful dynamic simulator that is widely used to explore the dynamics and control of continuous processes around some steady-state design operating point. This paper explores its use to study the dynamics of a batch process. The example studied is a middle-vessel batch distillation system for separating a ternary mixture. The batch system is operated by adjusting the two reflux flowrates (one from the reflux drum to the top of the upper column and the second from the middle vessel to the top of the lower column). The liquid inventories in the three drums vary with time. This paper shows how this batch operation can be conveniently simulated by first using the steady-state simulator Aspen Plus to correctly size the equipment. Then the file is exported into Aspen Dynamics where a rigorous dynamic simulation can explore alternative control strategies by using the large library of control functions.     

Control of a continuous fermentation process

This paper presents the application to a real fermentation pilot plant of an original control technique called L/A control. Such a control takes into account implicitly the positivity of process variables and may be designed from classical linear methods. The underpinning idea is to release the positivity constraints through logarithmic and exponential transformations. Here simple algorithms are proposed and tested in simulation and then applied to a continuous bioreactor in order to control the substrate concentration. The performances are analysed, showing the advantage of this approach, which combines simplicity and good performance in the bioprocess control domain.     

Information macrodynamic modelling of a random process

The article introduces a new approach for dynamic modelling using the variation principle, applied to a functional on trajectories of random process, and its connection to the process’ information path functional. The synthesised dynamic model is found by the solution of the variation problem for this path functional that allows us to build a two-level information model with a random process at the microlevel and a dynamic process at the macrolevel. The mathematical formalism has been applied for the constructive solution to the problems of the object's identification, combined with optimal control's synthesis and process’ consolidation. The procedure is demonstrated in this article's examples and leads to the creation of a dynamic information hierarchical network. The developed computer-based methodology and software have been successfully implemented to such areas as technology, communications, computer science, intelligent processes, biology, economy and other non-physical and physical subjects for systems modelling, identification and control of a diversity of information interactions.     

流程产品结构优化

针对一个流程产品结构数学模型的特点,开发了求解这类问题的遗传算法程序,并以此为基础研究了遗传算法在求解此类多参数、多变量及多峰值优化问题时的进化特性,包括遗传操作方法、遗传操作概率和其它遗传特性。通过试验,确定遗传算法操作条件,并在该条件下求得了问题的解,表明遗传算法求解该类问题的有效性。提出的方法及所得结果对流程企业制定生产计划具有一定的指导意义。     

Optimal predictor for a singular random process

An optimal predictor is developed for a singular random process generated by a known system driven by a white noise sequence. The properties of the predictor are compared with those of the optimal predictor for a nonsingular random process.     

精馏过程动态仿真建模

精馏过程的模型化与仿真在化工操作和工艺设计中具有重要的意义。对精馏塔进行动态数学模型的建立与仿真,不仅可以研究精馏过程在不同工况下的变化情况,而且还可以用于精馏塔的优化控制,进而提高精馏过程的生产效益。本文针对精馏塔操作过程,建立了基于平衡级假设和非平衡级假设的精馏过程动态机理数学模型,并对平衡级假设的模型进行了动态模拟。该模型从机理分析入手,进行合理的简化,模型的计算时间大大的缩短,从而使模型具有比较广泛的实用性。该模型采用的动态数学模型为METSH(质量平衡方程、相平衡方程、塔板效率方程、摩尔分数归一化方程、能量平衡方程)方程,通过计算METSH方程,可以模拟出精馏塔内温度、汽相流量、液相流量、汽相组分以及液相组分的变化趋势。通过仿真结果可以看到,该模型比较准确的预测了精馏塔中各个操作参数的动态趋势,与实际情况基本一致,其稳态结果与实际情况也基本吻合。该模型对于仿真培训及精馏过程的控制分析具有较高的理论研究意义和实际应用价值。     

The sequential alternative search as a continuous Markov random walk

The course of the process is analyzed for the choice by an individual of alternatives from a few alternatives, among which he has paired preferences, possibly intransitive and probabilistic. It is supposed that the individual carries out the choice with the aid of the process of the sequential exhaustive search of alternatives, on which a number of conditions (axioms) are imposed. It is shown that in this case the process is modeled by the continuous Markov random walk in the set of alternatives; the intensities of transitions are equal to the comparative utility function; the choice result will be probabilistic and will correspond to the established probability distribution in the random walk. It is shown that the choice result will be the Nash equilibrium in the game, which is set up on the paired preferences of the individual.     

Optimal selection of sensors for state estimation in a reactive distillation process

The success of state estimation in a high dimensional system like multicomponent reactive distillation depends on the rigorous evaluation of the observability and appropriate selection of measurements that adequately characterize the process behavior. In this work, the dynamic state sensitive measurement information extracted from the nonlinear reactive distillation process is employed to configure the gramian covariance matrices which are then subjected to various scalar quantification measures to find the degree of observability in order to select the temperature sensors for state estimation in the process. These optimally configured process measurements are then incorporated in a process model based composition estimation scheme. The validity of the sensors that are selected by the gramian quantification measures are further ascertained through the evaluation of the estimator performance for various nonoptimal measurement combinations. The results on application to a metathesis reactive distillation column exhibit the usefulness of the empirical observability gramian based sensor selection strategy for inferential state estimation of reactive distillation process.     

Neural-network process modeling of a continuous manufacturing operation

Neural-network techniques for the development of models of critical parameters in continuous forest products manufacturing processes are described. Predictive models of strength parameters in particleboard manufacturing were developed utilizing both backpropagation and counterpropagation neural network techniques. The modeled strength parameters were modulus of rupture and internal bond. The backpropagation neural network model did not provide sufficient accuracy in predicting the values of the strength parameters. Counterpropagation was successful at predicting modulus of rupture within ± 10% and internal bond within ± 15%. The trained counterpropagation network can be used to improve process control and reduce the amount of substandard and scrap board produced. Efforts are underway to refine the counterpropagation network and further improve its predictive capability, as well as to evaluate alternative neural network paradigms.     

Optimization of a fertilizer spreading process

An optimization method for accurate fertilizer application by centrifugal spreader is described. The cost functional is a distance between a prescribed dose and the distributed dose. The distributed dose is computed using a simplified spread pattern and the unknowns of the problem are three time-dependent parameters of the spreader: the average (distribution) center, the average (distribution) angle and the mass flow rate. In order to take into account the mechanical limits of the device, constraints are introduced in the form of bounds on the parameter functions and their time derivatives. Numerical experiments show that application errors can be significantly reduced for parallel tracks within a main field body.     

Triangular decoupling for a binary distillation column under pressure control

The possibility of attaining complete decoupling of a distillation column under pressure control is studied using the state space model. It is shown that under the situation where the distillation column is under pressure control by the manipulation of the condenser duty, the complete decoupling using constant state feedback may not be attained, while the triangular decoupling can be attained with eight measurement variables.     

煤焦油蒸馏过程的改进

我国煤焦油资源丰富，但工艺落后，煤焦油分离水平不高，导致后续分离过程负荷大，环境污染程度高。为提高煤焦油分离水平，改善馏分混杂情况，减小环境污染，本文针对原流程的不足，提出2种煤焦油蒸馏改进流程，并以PRO／II为工具，对原流程及改进流程进行了模拟。模拟结果表明，改进流程大大提高了酚、萘的回收率，减轻了馏分混杂程度。从而说明，本文提出的改进流程是可行的。