Antonio Carlos Brandão de Araújo

Optimal Operation of an Industrial PVC Dryer

This article describes the design of a control structure architecture for an industrial polyvinyl chloride (PVC) dryer, currently operating at Braskem Company (Marechal Deodoro, Alagoas, Brazil). The underlying motivation is to search for a control configuration that leads to optimal economic operation, while promptly rejecting disturbances at lower layers in the control hierarchy. We start by optimizing a nonlinear model of the process with respect to reducing energy consumption as a criterion of optimization for different disturbance scenarios. The results show that it is optimal to control the temperature level of the utilities serving the dryer and the outlet PVC moisture contents at their respective upper bounds. In addition, the flow of air to the dryer should be fixed at its optimum nominal setpoint despite disturbances. Application of this strategy results in a reduction of about 16% in energy consumption with respect to the current dryer operation policy and a 22% increase in throughput under nominal operation. In addition, almost perfect indirect control of the outlet PVC moisture was achieved by tightly controlling a temperature difference in the dryer. The proposed decentralized control configuration gives good dynamic performance for the outlet PVC moisture content with maximum settling time of about 1.8 h for the more difficult disturbance of increasing the inlet slurry moisture content by 40% and magnitude of overshoot of ca. 5% w.r.t. the optimum setpoint for an increase of 20% in PVC feed flow rate to the dryer.

A new correlation for hazardous area classification based on experiments and CFD predictions

A correlation developed by the authors for prediction of hazardous area extension due to fugitive emissions is described in this work. The correlation is based on computational fluid dynamic (CFD) simulation data. The CFD simulation setups were obtained from a Computational Design of Experiments. The CFD model has been validated experimentally. The transport properties, orifice size, temperature, pressure, gas molar mass, and lower explosive limit (LEL) were varied in a range of practical interest using the statistical Technique Latin hypercube to spread the simulation setups. The effect of each variable on hazardous area extension was obtained from the CFD results leading to an analytical correlation for practical use in estimating the extension of the hazardous area. The results showed that the extension of hazardous area in still ambient is merely a function of the leaking gas molar flow rate and gas volume fraction at LEL. The results from the correlation described in this work were compared to results from correlations existing in the literature.

Time scale separation and the link between open-loop and closed-loop dynamics

This paper aims at combining two different approaches ([1] and [2]) into a method for control structure design for plants with large recycle. The self-optimizing approach ([1]) identifies the variables that must be controlled to achieve acceptable economic operation of the plant, but it gives no information on how fast these variables need to be controlled and how to design the control system. A detailed controllability and dynamic analysis is generally needed for this. One alternative is the singular perturbation framework proposed in [2] where one identifies potential controlled and manipulated variables on different time scales. The combined approaches has successfully been applied to a reactor-separator process with recycle and purge.

Optimized Control Structure for a Wastewater Treatment Benchmark

Abstract In this paper, we define and implement the design of an optimized control structure for the activated sludge process given as COST/IWA benchmark simulation model No.1. Emphasis is given to the identification of controlled variables that contribute to minimize economic costs while the effluent requirements are met. This is achieved considering the self-optimizing procedure as reference method for the controlled variables selection. The proposed optimal control strategy consists of multivariable PID loops which manipulate the airflow rate in the aerobic basins, the nitrate and sludge recirculation flows and the waste sludge flow proportionally to the influent flow such that the overall cost function is minimized. Dynamic simulations validate the resulting optimized controller structure, showing that minimal costs can be achieved.

Entropic-Model-Based PI Controller

Abstract Contributions of entropic modelling to the performance of reactive process control have been investigated. The modelling has been developed based on mass, energy and entropy balances and thermodynamics relations, resulting in a model for calculating the entropy production rate. Using the conventional optimization technique, a minimum for the entropy production rate has been found when a given relationship between the temperatures of the inlet stream and of the reaction is satisfied for a particular residence time in the reactor. A new class of nonlinear controller has been developed by means of introducing entropic models into classical PI algorithms designed by reference system synthesis. The results indicate that such a controller yields a superior performance when compared to classical feedback control strategies.

APPLICATION OF PLANTWIDE CONTROL TO LARGE SCALE SYSTEMS. PART I - SELF-OPTIMIZING CONTROL OF THE HDA PROCESS

Abstract This paper describes the application of self-optimizing control to a large scale process, the HDA plant. The idea is to select controlled variables which when kept constant lead to minimum economic loss. In order to avoid the combinatorial problem common to the selection of outputs/measurements for such large plants, applications of singular value decomposition (SVD) based methods are used which, although not guaranteeing optimality, give a consistent and practical way for selection. A controllability analysis is carried out to compare the dynamic performance of the selected sets of controlled variables and the conclusion is that the expected performances of the proposed control structures are essentially the same.

CONTROLLABILITY OF PROCESSES WITH LARGE GAINS

Abstract There is some disagreement in the literature on whether large plant gains are a problem or not when it comes to input-output controllability. In this paper, the effect of two kinds of input errors is studied and controllability requirements are derived. First, input disturbances are studied. These may pose a problem if the plant gain is large at high frequencies. Second, we study the nonlinear effect of limited input resolution which causes limit cycle behavior similar to that found with relay feedback. The magnitude of these limit cycles depends on the high-frequency process gain, but is independent of the controller tuning. They can be reduced by pulse modulating the input signal, but this may cause excessive input movement. In summary, large gains at frequencies corresponding to the closed-loop bandwidth may cause control problems, but large steady-state gains are not by themselves a problem.