Recycle Compensator Research Articles

Control of recycle system using extended state observer concept

Control of recycle process particularly with the design of recycle compensator accompanied by the controller, arises with some issues. As sufficient evidence shows the necessity of recycle compensation for the desired performance of recycle process but the model-based recycle compensator design happens to be problem-dependent and non-realizable, which makes cancellation of recycle effect problematic. In such a scenario model-based controller design accompanied by recycle compensator has to accommodate both uncertainty and uncompensated recycle effect, which becomes a hard task. To overcome these control design issues in recycle system, formulation of control law in two possible ways with the estimates obtained from the Extended State Observer’s (ESO’s) design has been proposed. The reason for two possible ways of proposed control law formulation depends on the way recycle effect gets cancelled which based upon the order of the ESO’s design selected. Selecting the appropriate order for the ESO’s design and tuning parameters required for the proposed formulation, considered to be available from model knowledge of recycle system represented in a specific recycle structural form. However, this paper focuses only on the way the control law could be formulated for recycle system based upon the availability of recycle path data and has not considered providing an optimal selection of ESO’s order as well as tuning parameters. In simulation results, the performance of the proposed formulation under different strategies have been provided using the performance index of Integral Square Error (ISE) under nominal, perturbed, and external disturbance cases.

Application of Taiwo’s Recycle Compensator to an Heat Integrated Ammonia Production Plant

Designing fixed-bed catalytic reactor to have heat integration is a common practice in the process industries. However, such design usually poses control challenges as a result of unusual dynamic behavior of the overall process, such as oscillations and instability. It is shown in this paper that the application of Taiwo’s recycle compensator eliminates the undesirable dynamics and leads to a better performance. Thus facilitating the design of more effective control system for heat integrated processes.

On Robust Control System Design for Plants with Recycle

A direct procedure for deriving the component transfer functions in plants with recycle when mechanistic procedure is used in the mathematical modeling is demonstrated. Multi-loop feedback controllers are then designed for the plants with recycle incorporating recycle compensator and without recycle compensator using internal model control (IMC) parameterization. In general, it is found that controllers parameterized using the recycle compensated plant model result in closed-loop systems with better nominal and robust performance characteristics when implemented on the uncompensated plant model than when its controller is parameterized using the uncompensated plant model. Furthermore, recycle compensated plants result in overall best closed-loop systems when their feedback controllers are parameterized using the compensated plant model.

On the robust tuning of controllers with recycle compensators

The control of processes with recycle normally considers a recycle compensator and a main controller. This work analyses the use of a H∞ approach to design the main controller and compares its performance with IMC and PI controllers. Stability conditions are derived for assessing the robustness of the main controller in presence of uncertainty in the forward and recycle model. These robust stability conditions along with practical issues, like time delay in the recycle model and integral action, are all considered in the controller design. Several simulations examples compare the behavior of the different controllers and highlight the positive features of the H∞ design approach.

Control of a Process with Recycle: Robustness of a Recycle Compensator

Several studies on the dynamics of processes with recycle are reported in the literature. A complete summary of the effects of the recycle, along with a detailed analysis of the disturbances behavior when matter or energy is recycled, is presented in the first part of the paper. The control of processes with recycle is then discussed, including the benefits of adding a recycle compensator to a controller. Representing the system equations with Bode plots shows that the performance of a PI (proportional and integral) controller with a recycle compensator is usually better than the use of a PI controller alone. A systematic analysis of the sensitivity of the model demonstrates that this is true, even for important model errors in the recycle path. This work suggests that the negative result of recycle addition on the process dynamics can be overcome when the recycle effects are anticipated even though the recycle path model is not error free.

Recycle Effect Index: A Measure to Aid in Control System Design for Recycle Processes

Systems with material and/or energy recycles are quite common in the chemical industry. Modeling and DCS-level controller design are quite challenging for such processes because separation of the overall process response into a direct (forward-path) response and an indirect (recycle-path) response is often difficult. Even when this is possible, one needs to know whether a feedback-only control structure is sufficient or an advanced control structure that employs a recycle compensator is necessary. Using control loop performance assessment concepts, this paper proposes a quantitative measurethe recycle effect index (REI)to help determine whether a simple feedback control system can provide the desired quality of control.

Empirical modelling and control of processes with recycle: some insights via case studies

Issues related to the identification and control of systems with recycle of material and/or energy are investigated via simple case studies. We aim for high performance control using recycle compensators that can be easily implemented on industrial DCS systems. The modelling strategy centers around system identification and/or decomposing the overall process dynamics into a direct effect component and a recycle component. When fundamental process models are available, such decomposition is relatively straightforward. In their absence, the models required for controller design may only be gained via plant tests and subsequent system identification. This study will examine the issues related to the empirical identification of integrated systems such as the proper use of available sensor information and appropriate implementation of the recycle compensator. Though only two case studies are presented here, the principles and conclusions are broadly applicable to industrial chemical process systems.

Seasonal Model Based Control of Processes with Recycle Dynamics

Plants with recycle systems have complex dynamics and can present challenging control problems. In this paper, the dynamics of recycle systems were examined by analyzing a two-stage heating process with recycle. It was found that, when a positive feedback arrangement is used to represent the linear dynamics of systems with recycle, the recycle-path time constant and recycle fraction must be carefully included. Also, the overall recycle dynamics can be represented by seasonal time-series models. The overall transfer function for the recycle system might be nonrational, thus complicating controller design. Three methods of applying model-based predictive control to recycle processes were compared. The first method was a Taylor series expansion used to linearize the process model, the second method used a seasonal time-series model to represent the process, and the third method was a recycle compensator that removed the effect of recycle dynamics. Although all three methods gave good control, practical considerations determine the method that should be used.

Performance of control systems based on recycle compensators in integrated plants

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.

Control of systems with recycle by means of compensators

The paper analyses the effects of the presence of recycle streams on the performance of the control system of chemical processes and the possibility of success of compensators. Control schemes based on a regulator designed for the process without recycle and augmented by a recycle compensator are compared with specific controllers designed for the global process; potential benefits, in terms of superiority of performance and simpler design, are clearly illustrated for a SISO reference case. Application to two multivariable processes proposed in literature, the two reactors in series ( Ray, 1981) and the two reactors plus three distillation columns ( Luyben and Luyben, 1995) shows that also in these more realistic cases main advantages of the compensator are retained.

Issues in the Control of Systems with Recycle

A specific design of the controller for the process with recycle is compared with the adoption of a recycle compensator added to a controller designed for the process without recycle. In the SISO case the main effect of the recycle is to slow down the response of the system. The adoption of a compensator is required to obtain a fast suppression of disturbances. In the MIMO case interaction can be eliminated by a compensator added to a simple diagonal controller. Therefore when the compensator is physically realizable and a good knowledge of the process dynamics is possible, its use gives advantages over the specific design and can eliminate the negative effects of the recycle in the stage of design of the control system.

Robust control system design for plants with recycle

A general recycle compensator, applicable to multivariable plants, is derived. For the single-variable case, the internal model control method is used to design robust controllers for the compensated plant leading to an overall better system than for the case of the uncompensated plant while, for the multivariable case, diagonal proportional plus integral controllers ensuring performance robustness are obtained by minimizing a performance index and considering the robustness measure as a constraint in the design. Again, the overall dynamic characteristics of the system resulting from the compensated plant are more favourable than those of the system resulting from the uncompensated plant.

Performance of different regulators for plants with recycle

Performance of simple PI and model based (IMC) regulators for plants with recycle, are compared in control schemes with and without a recycle compensator. By considering different dynamic characteristic of the process, it is analysed in which cases the adoption of a compensator is necessary to eliminate completely the effect of recycle streams, both for set point tracking and for disturbance suppression. The robustness of the compensator in the presence of parametric uncertainty in the recycle model is also examined.

The design of robust control systems for plants with recycle

A combination of Zakian's design criteria for vague infinite-dimensional plants and the author's previous work, which specifies a recycle compensator for plants with recycle, is exploited to design a controller for a plant with recycle when the parameters of the process in the recycle path are subject to variations. The method gives a closed-loop system with guaranteed performance for large variations in process parameters.