Gustavo Alberto Neumann

Parameter estimation for LLDPE gas-phase reactor models

Product development and advanced control applications require models with good predictive capability. However, in some cases it is not possible to obtain good quality phenomenological models due to the lack of data or the presence of important unmeasured effects. The use of empirical models requires less investment in modeling, but implies the need for larger amounts of experimental data to generate models with good predictive capability. In this work, nonlinear phenomenological and empirical models were compared with respect to their capability to predict the melt index and polymer yield of a low-density polyethylene production process consisting of two fluidized bed reactors connected in series. To adjust the phenomenological model, the optimization algorithms based on the flexible polyhedron method of Nelder and Mead showed the best efficiency. To adjust the empirical model, the PLS model was more appropriate for polymer yield, and the melt index needed more nonlinearity like the QPLS models. In the comparison between these two types of models better results were obtained for the empirical models.

Why Some APC Applications Lose Performance Over Time And How You Can Avoid It

Abstract For a long time, companies in the Chemical Process Industry have turned to Advanced Process Control (APC) as a means to increase profitability and production capacity. Among the most widely used APC techniques is Model Predictive Control (MPC). Many service and software vendors have become knowledgeable in executing projects and delivering MPC applications throughout a diverse range of processes. The focus of this work is on what happens after project conclusion, when the project team is dismissed and the application is handed over to the customer operations. It has been observed in practice that without proper maintenance, APC applications will tend to progressively lose their ability to capture benefits. This work will present how we developed a program aimed at sustaining the value of our APC applications. The program reflects our experience in supporting APC applications from multiple vendors installed in 11 different polymerization lines in seven different sites. It will be presented which steps were taken in building such program starting with the cause-effect analysis, where we identify the causes for application malfunction, going through the identification of stakeholders, key performance indicators and assignment of roles and responsibilities. Although the outlined program was built based on the characteristics of the plants and APC applications found in the polymers business, it should be equally applicable to other industries. It is also interesting that the program does not depend on the choice of APC vendor.

COMPARSION BETWEEN PHENOMENOLOGICAL AND EMPIRICAL MODELS FOR POLYMERIZATION PROCESSES CONTROL

Abstract In this work, linear, quadratic, and nonlinear empirical models were built and compared with a dynamic nonlinear phenomenological model with respect to the capability of predicting the melt index and polymer yield rate of a low density polyethylene production process. Based on steady-state gains and on known first and second order time constants of the process, the empirical models were generated using PLS, QPLS, and BTPLS methods in order to predict the system dynamics. As the quadratic model provided more reliable predictions, it was used as melt index virtual analyzer of an advanced control strategy for an industrial plant, improving the controller action and the polymer quality by reducing significantly the process variability.

MULTIVARIABLE CONTROL STRATEGY BASED ON BIFURCATION ANALYSIS OF AN INDUSTRIAL GAS-PHASE POLYMERIZATION REACTOR

Abstract In an industrial gas-phase polyethylene reactor, the safe operating range of temperature is rather narrow. Even within this temperature range, temperature excursions must be avoided because they can result in low catalyst productivity and significant changes in product properties. In previous work, using a first-principles model, including the recycle stream and the heat exchange system, a PID temperature controller with robust performance was designed via optimization in the frequency domain for different operating points. For the reactor total pressure, ethylene partial pressure, H2/C2 and C4/C2 molar ratios, PI controllers were designed. In the PID temperature controller, if the manipulated variable (cooling water valve opening) saturates then the reactor operates without a feedback temperature controller, leading to oscillatory behaviour and limit cycles. It has been demonstrated that the manipulated variable saturation and the nonlinear dynamic behaviour are removed when auxiliary manipulated variables, obtained by bifurcation analysis, are used in a multivariable control strategy for the reactor temperature control. In this work, two control structures are compared to define the most suitable proposal for implementation in an industrial reactor and the impact of these control structures in the reactor production and in the polymer melt index are analyzed. The first control structure considers the control problem using the designed PID controller for the reactor temperature and includes a switching strategy with a PI controller for the auxiliary manipulated variables. The second control structure considers the control problem also using the designed PID controller for the reactor temperature, however including a MPC controller for the auxiliary manipulated variables.

Comportamentos dinâmicos em um reator industrial de polimerização em fase gasosa

(invited paper): In gas-phase ethylene polymerization reactors, tight temperature control is of utmost importance to ensure that the temperature in the reaction zone is kept above the dew point of reactants, yet below the melt point of the polymer. Another important point is that if the temperature was under open-loop conditions, these reactors are prone to instability and limit cycles, however can be easily stabilized with an appropriate temperature controller. In this work, a detailed study of the dynamic behavior of an industrial fluidized-bed polymerization reactor is carried out, with the location of bifurcation points and system stabilization by PID controller designed via optimization in the frequency domain.

Dynamic behaviour and control of an industrial fluidised-bed polymerisation reactor

Abstract In gas-phase reactors for ethylene polymerisation, tight temperature control is of utmost importance to ensure that the temperature in the reaction zone is kept above the dew point of reactants, yet below the melt point of the polymer. Another important point is that if the temperature was under open-loop conditions, these reactors are prone to instability and limit cycles, however they can be easily stabilised with an appropriate temperature controller. In this work, a detailed study of the dynamic behaviour of an industrial fluidised-bed polymerisation reactor is carried out, with the location of bifurcation points and system stabilisation by controllers designed via optimisation in the frequency domain.