Claudio Scali

Brief Identification and control of open-loop unstable processes by relay methods

This paper describes a reliable automatic PID tuning method for open-loop unstable processes. Identification with low order models is performed by means of two relay tests, one with an additional delay, which does not require a priori knowledge about the process, with the only necessary condition being that the process be gain stabilisable. This paper provides an overview of the method, states conditions that need to be satisfied for its successful implementation, and demonstrates its application on a number of examples.

Control of the quality of polymer products in continuous reactors: comparison of performance of state estimators with and without updating of parameters

Abstract In the paper the control of the product quality in polymerization reactors is analysed in the presence of persistent perturbations (unmodeled disturbances, modeling errors), as met in industrial reactors. The free radical polymerization of methyl-methacrylate in a continuous stirred tank reactor is studied. It is shown that state estimators (Extended Kalman Filters) with constant parameters cannot give offset free performance. Criteria for the selection of a set of parameters to be updated as additional states in the filter and to evaluate their effectiveness in opposing the action of realistic perturbations are given by an analysis of the linearized model of the system. Performance of different types of estimators, including one and two-time scale filters, with and without updating of parameters, is analysed by simulation on the full order process and the predictions made by previous analysis are confirmed. In the most common case of presence of perturbations affecting the energy balance and the concentration of initiator in the reactor, offset free control of the molecular weight of the product can be achieved by means of a filter which is based only on measurements of temperature and conversion and makes an update of two parameters. In the case that also the kinetic model of the polymerization reaction is affected by errors, a two-time scale filter, which makes use also of Molecular Weight values and updates three parameters, becomes necessary to obtain offset free performance.

Performance of control systems based on recycle compensators in integrated plants

Abstract The paper illustrates the effects of recycle streams on the controllability of integrated plants and the improvement of performance made possible by a direct compensation of the recycle. A procedure for the decomposition of the global process in a part representing the process without recycle and in a part representing the recycle is presented. This is the key issue for evaluating the relevance of the recycle and to reduce the control system to two blocks: a regulator, depending on the process without recycle, plus a recycle compensator. The design of this control scheme is much easier with respect to a specific regulator computed for the global process and allows to achieve relevant improvement of performance with respect to a standard PI regulator. This is clearly illustrated for a SISO reference case, where process parameters are changed to create situations of particular evidence, and have been confirmed in the application to the two MIMO benchmarks, proposed in literature: the two reactors in series (W.H. Ray, Advanced Process Control, McGraw–Hill, New York, 1981, pp. 219–224), and the two reactors plus three distillation columns (M.L. Luyben, W.L. Luyben, Design and control of a complex process involving two reaction steps, three distillation columns and two recycle streams, Ind. Eng. Chem. Res. 34 (1995) 3885–3898). For a full evaluation of the effectiveness of the proposed control system, the effect of the uncertainty, of the approximated structure of the compensator, as well as possible improvements by including partial adaptive features in the control system, are also taken into consideration.

Improved identification and autotuning of PI controllers for MIMO processes by relay techniques

Abstract A technique for on-line identification and tuning is proposed to be used in the framework of a MIMO autotuning procedure. The proposed technique does not suffer from the risks of instability and the lack in performance of common tuning techniques in MIMO autotuning. Identification is accomplished through an extension of the well known ATV autotune identification method and requires only few additional tests in order to obtain some more knowledge about the process. The resulting model, which describes with good precision the process in a region of frequencies around the critical point, is then used for tuning: the integral time is found as a function of the model time constants and delay, while the gain is computed in order to give a desired value of the closed-loop resonance peak. Examples of application show that advantages over other proposed techniques can be retained for processes having different dynamic characteristics.

Optimal operation strategies to control the molecular weight distribution of polymer products

Abstract A mathematical model for bulk and suspension polymerization of methylmethacrylate has been extended to solution polymerization and verified experimentally. On the basis of the model two operating strategies are analysed for suspension and concentrated solution polymerization. For suspension polymerization, a batch operation under isothermal followed by adiabatic conditions can be adopted in industrial reactors. This gives a good approximation to the optimal temperature profile which makes it possible to eliminate the bimodality in the molecule weight distribution curves. For concentrated solutions carried out in a tubular reactor, the solvent concentration can play a key role in limiting the increase in molecular weight during the gel effect and thus in controlling the molecular weight distribution.

Temperature Oscillation Calorimetry: Robustness analysis of different algorithms for the evaluation of the heat transfer coefficient

Abstract Applicability of different temperature oscillation calorimetry algorithms, previously proposed in the literature, for the evaluation of the heat transfer coefficient during the course of the reaction, is analysed by referring to the experimental data from a methyl methacrylate polymerisation reactor. Results are coincident in the nominal case (no errors on data), while differences appear in the elaboration of real data. The effect of uncertainty in the experimental data has been simulated in terms of error on the amplitude and on the phase of reactor and jacket temperature profiles. This approach allows to indicate algorithms having the best robustness properties and to give an explanation of phase errors in terms of a lumped parameter which accounts for different phenomena, not included in the basic hypotheses of the method. Based on this observation, a procedure to eliminate this source of errors from experimental data is proposed for the more general algorithm presented to handle oscillating temperature profiles generated by different techniques.

Implementation, parameters calibration and field validation of a Closed Loop Performance Monitoring system

Abstract The paper illustrates the main features of a performance monitoring system which supervises more than 1200 loops of refinery plants. The system analyses data recorded by the DCS during routine operations and originates verdicts about the performance of control loops; also, indications of low performance causes and different strategies to adopt (retuning, valve maintenance, upstream actions) are given. First, the architecture of the system is illustrated, and characteristics of modules which accomplish different tasks of data acquisition and transfer, system configuration and priority assignment, performance analysis and verdicts emission, database query and operator support, are described. A synthesis of main techniques adopted in the system for stiction detection, process identification and performance evaluation, is also reported. The procedure for calibration of threshold values of key parameters, in order to match verdicts issued automatically by the system with operators evaluations and to avoid false alarms, is also discussed. Examples of results achieved from field validation are then presented, with illustration of loop performance before and after actions suggested by the monitoring system.

Parameter estimation in Extended Kalman Filters for quality control in polymerization reactors

The issue of parameter estimation in Extended Kalman Filters is addressed in this paper. It is shown that the use of parameters as additional system states is appropriate whenever there are systematic errors in the model of the process. Such is the case of the filters which are used to estimate quality properties of polymers which are not measured on-line in polymerization reactors. Criteria for the choice of the parameters to be updated are suggested. Moreover, in order to realize the possibility of obtaining offset free response, the effect of errors on both updated and not updated parameters, both on the observable and on the unobservable system states is analysed. Applications to the polymerization of methylmethacrylate (MMA) in a continuous stirred tank reactor confirm the theoretical results.

Optimization of batch polymerization reactors:: Modelling and experimental results for suspension polymerization of MethylMethAcrylate

Abstract A model for the simulation of suspension polymerization of MMA has been developed, which allows to accounts for the effect of diffusive phenomena on reaction rate and to evaluate molecular weight distribution under non-isothermal conditions. The model has been validated by experimental runs carried out in a laboratory reactor. Obtained results show that an increase of the reaction temperature during the gel effect time can have a favourable influence on the polymer quality. The control of temperature profiles in the reactor is indicated as an interesting operating strategy to be adopted in industrial units.

Analysis of the molecular weight distribution in free radical polymerization: Modelling of the MWD from the analysis of experimental GPC curves

In this paper different techniques for the deconvolution of experimental gel permeation chromatography (GPC) curves of polymers obtained from batch reactors are examined with the aim of improving the modelling of the polymerization process. The deconvolution of the GPC curves gives insight into the effects of diffusive phenomena on the values of the average molecular weights (M n and M w ) and the molecular weight distribution (MWD) of the polymer. Two methods, one based on a two-peak deconvolution and the other one based on a multi-peak deconvolution, are compared. The different characteristics and relative merits of the two methods are illustrated using several sets of experimental data. In conclusion, the first method is less accurate but allows a separation of the two contributions given by the chemical and diffusive control of the MWD; the second method has a sound theoretical background (confirms the prediction of the free volume theory) and is potentially much superior but the results prove to be more sensitive to the quality of the experimental part.