Linear matrix inequality-based robust model predictive control for time-delayed systems

This work addresses the solution to the problem of robust model predictive control (MPC) of systems with model uncertainty. The case of zone control of multi-variable stable systems with multiple time delays is considered. The usual approach of dealing with this kind of problem is through the inclusion of non-linear cost constraint in the control problem. The control action is then obtained at each sampling time as the solution to a non-linear programming (NLP) problem that for high-order systems can be computationally expensive. Here, the robust MPC problem is formulated as a linear matrix inequality problem that can be solved in real time with a fraction of the computer effort. The proposed approach is compared with the conventional robust MPC and tested through the simulation of a reactor system of the process industry.

Development and Evaluation of a Pseudoreference Pt//Ag/AgCl Electrode for Electrochemical Systems

The use of standard reference electrodes, such as Ag/AgCl or saturated calomel electrodes, in potentiometric and amperometric studies involving miniaturized electrochemical systems, or those operating under positive hydraulic pressure, is often impractical. Placement of the reference electrode in the direct vicinity of the working electrode is often prohibited by the dimensions or layout of the electrochemical cell, while the alternative strategy of locating the reference electrode in a separate compartment often leads to electrolyte leakage and contamination of the system. In the present study, we have investigated the functionality of a pseudoreference electrode comprising a platinum wire, one end of which was maintained in intimate contact with the internal solution of an Ag/AgCl reference electrode while the other was connected, via a BNC connector, to a platinum probe located within the electrochemical cell. Linear and cyclic voltammetric studies, involving both aqueous and nonaqueous electrolytes, were carried out using the pseudoreference electrode and an electrochemical cup-type cell with three electrodes or an electrochemical flow reactor. In all cases, the functionality of the Pt//Ag/AgCl system was similar to that of a conventional Ag/AgCl reference electrode. Variations in the electrolyte did not alter the potential or voltammetric profile recorded when using the pseudoreference system, although peak currents were generally improved and potential values shifted by approximately +350 mV in comparison with the Ag/AgCl electrode, therefore, the system pseudoreference can be applied in any electrochemical system due to the constant potential difference. It is concluded that the pseudoreference electrode can be used with advantage to obtain potentiometric and amperometric measurements in both simple and complex electrochemical systems.

Transesterification of Palm Oil Catalyzed by Pseudomonas fluorescens Lipase in a Packed-Bed Reactor

Transesterification of palm oil with ethanol catalyzed by Pseudomonas fluorescens lipase immobilized on epoxy-polysiloxane-polyvinyl alcohol composite (epoxy-SiO2-PVA) was performed in a continuous packed-bed reactor (PBR). Two strategies were used for improving the miscibility of the substrates: the addition of the organic solvent tert-butanol and the surfactant Triton X-100. Results were compared to those obtained in a solventless reactor, which displayed a biphasic system that passed through the reactor. Using this system, the ethyl ester yield of 61.6 +/- 1.2% was obtained at steady state. Both Triton X-100 and tert-butanol systems were found to be suitable to promote the miscibility of the starting materials; however, the use of Triton X-100 reduced the yield to levels lower than 20%, because of the enzyme desorption from the support surface, as confirmed by scanning electron microscopy analysis. The best performance was found for the reactor running in the presence of tert-butanol which resulted in a stable operating system and an average yield of 87.6 +/- 2.5%. This strategy also gave high biocatalyst operational stability, revealing a half-life of 48 days and an inactivation constant of 0.6 X 10(-3) h(-1).

Closed-loop stable model predictive control of integrating systems with dead time

Model predictive control (MPC) applications in the process industry usually deal with process systems that show time delays (dead times) between the system inputs and outputs. Also, in many industrial applications of MPC, integrating outputs resulting from liquid level control or recycle streams need to be considered as controlled outputs. Conventional MPC packages can be applied to time-delay systems but stability of the closed loop system will depend on the tuning parameters of the controller and cannot be guaranteed even in the nominal case. In this work, a state space model based on the analytical step response model is extended to the case of integrating time systems with time delays. This model is applied to the development of two versions of a nominally stable MPC, which is designed to the practical scenario in which one has targets for some of the inputs and/or outputs that may be unreachable and zone control (or interval tracking) for the remaining outputs. The controller is tested through simulation of a multivariable industrial reactor system. (C) 2012 Elsevier Ltd. All rights reserved.

Liquid-liquid equilibria for systems composed of rice bran oil and alcohol-rich solvents: Application to extraction and deacidification of oil

The liquid-liquid equilibria of systems composed of rice bran oil, free fatty acids, ethanol and water were investigated at temperatures ranging from 10 to 60 degrees C. The results of the present study indicated that the mutual solubility of the compounds decreased with an increase in the water content of the solvent and a decrease in the temperature of the solution. The experimental data set was correlated by applying the UNIQUAC model. The average variance between the experimental and calculated compositions was 0.35%, indicating that the model can accurately predict behavior of the compounds at different temperatures and degrees of hydration. The adjustment of interaction parameters enables both the simulation of liquid-liquid extractors for deacidification of vegetable oil and the prediction of phase compositions for the oil and alcohol-rich phases that are generated during cooling of the stream exiting the extractor (when using ethanol as the solvent). (C) 2012 Elsevier Ltd. All rights reserved.

OPTIMIZATION AND CONTROL OF A CONTINUOUS POLYMERIZATION REACTOR

This work studies the optimization and control of a styrene polymerization reactor. The proposed strategy deals with the case where, because of market conditions and equipment deterioration, the optimal operating point of the continuous reactor is modified significantly along the operation time and the control system has to search for this optimum point, besides keeping the reactor system stable at any possible point. The approach considered here consists of three layers: the Real Time Optimization (RTO), the Model Predictive Control (MPC) and a Target Calculation (TC) that coordinates the communication between the two other layers and guarantees the stability of the whole structure. The proposed algorithm is simulated with the phenomenological model of a styrene polymerization reactor, which has been widely used as a benchmark for process control. The complete optimization structure for the styrene process including disturbances rejection is developed. The simulation results show the robustness of the proposed strategy and the capability to deal with disturbances while the economic objective is optimized.