This paper presents a comparative study of simplified, ideal and inverted decoupling. The stability, robustness and implementation of the three decoupling methods are studied. The structured singular value (SSV) is used to carry out some comparisons. It is demonstrated that robust performance and robust stability of a nominally stable control system are equivalent for the three decoupling methods when the controllers are tuned to obtain identical nominal performance. A relation is derived between the presence of right-half plane (RHP) zeros of a process in series with its simplified decoupler and the instability of the ideal and inverted decouplers for the same process. This paper also describes a potential implementation problem related to the particular structure of the inverted decoupling. Finally, a recapitulative table of the main advantages and limitations of each decoupling method is presented.

Simplified, ideal or inverted decoupling?

In a conventional decoupling control scheme, process inputs (signals to valves or set points to lower level flow controllers) are produced as a time-weighted combination of process controller outputs. A lesser known technique, here called “inverted decoupling”, produces the process input signals by combining one controller output with the other process input signals. Where this technique is applicable, it has several advantages over conventional decoupling. These advantages as well as potential problems are addressed in this paper. Simulation results, related to distillation column applications, are also presented.

Inverted decoupling : a neglected technique

We propose a new process for the hydrotreatment of heavy oils in supercritical water (SCW). The discussion in this paper is composed of three parts:  (1) hydrogenation through water-gas shift reaction in supercritical water, (2) selective formation of carbon monoxide by partial oxidation in supercritical water and through combinations of these two, and (3) hydrogenation of hydrocarbons through their partial oxidation in supercritical water. In the experiments involving hydrogenation of dibenzothiophene, carbazole, and naphthalene, faster hydrogenation rates could be obtained in a CO−SCW atmosphere than in a H2−SCW atmosphere. Even in the case of a H2−CO2−SCW atmosphere, similarly faster reaction rates were obtained, which suggests that an intermediate species of the water-gas shift reaction is the actual reason for the high hydrogenation rates. Partial oxidation experiments were conducted for hexylbenzene and n-hexadecane. The selectivity of CO increased with increasing density of water, while CO2 was the...

Hydrogenation of Hydrocarbons through Partial Oxidation in Supercritical Water

Application of Wiener model predictive control (WMPC) to an industrial C2-splitter

Decouplers can be used to improve performances of multiloop control systems by compensating for process interactions, but they are sensitive to process changes and require fairly detailed process models, which are hard to obtain. These disadvantages of decouplers hinder their successful applications in industry. On the other hand, static decouplers can be designed from process steady-state gains, which are easier to obtain and can be tuned in the field. However, static decouplers can introduce undesirable effects on loop interactions at high frequencies for some processes. Here it is shown that static decouplers are better applied to the integral control modes for such processes. Because the integral mode is dominant at low frequencies and its magnitude decreases as frequency increases, the static decoupler implemented only on the integral mode will not disturb the frequency response of a control system at high frequencies. A simple antireset windup scheme, guaranteeing given control performances under process input saturations or failures, is provided. For online tuning of the proposed control system, a closed-loop identification method to find process steady-state gains is also presented. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Static decouplers for control of multivariable processes

Robustness analysis of process control systems. A case study of decoupling control in distillation

A novel extension of the Nyquist array methods to accommodate for system uncertainties is presented. The concept of robust diagonal dominance is introduced ; necessary and sufficient conditions for robust stability are developed and the issue of robust performance is addressed. Two examples are given to demonstrate the power of the method for analysis and design of robust feedback control systems.

Billy Manousiouthakis

Papers

Robustness analysis of process control systems. A case study of decoupling control in distillation

Robust Nyquist array methodology : a new theoretical framework for analysis and design of robust multivariable feedback systems

Design of chemical plants with improved dynamic operability in an environment of uncertainty

Computer-aided analysis and design of robust multivariable control systems for chemical processes

Robust controller design a hybrid approach