A previously developed mathematical model for ethylene polymerization in a fluidized bed reactor was used to investigate the process static and dynamic behavior. The static analysis determined optimal operating conditions at which the monomer conversion can be increased to 25% per pass. However, this optimal operating point was shown to be unstable, and thus any changes in the plant operation may lead to temperature runaway, degrading the reactor performance. In addition, the reactor temperature must be kept within a narrow range between the gas dew point and the polymer melting point. For these reasons, two control algorithms, that is, proportional-integral (PI) and nonlinear model predictive control (NLMPC), were tested for the stabilization of the process during load changes. The PI closed-loop dynamic simulations indicated that using the reactor feed temperature solely as a manipulated variable was not sufficient to stabilize the reactor temperature. However, good PI feedback performance was obtained when the feed temperature and the superficial velocity were used to control the reactor temperature and the monomer concentration. This control structure was selected arbitrarily whereas the control structure recommended by standard methods such as RGA and SVD performed poorly. On the other hand, the NLMPC out-performed the PI control in terms of minimum tuning and controller design efforts.

In this study, the performance of chromium oxide catalysts supported on Al 2 O 3 was evaluated. The effects of the percent loading of Cr on Al 2 O 3 and the pretreatment conditions on the activity and the selectivity of the catalysts in the oxidative dehydrogenation of i-C 4 H 10 to i-C 4 H 8 were also tested. Different types of Al 2 O 3 were tested as supports for chromium oxide, and the activity and selectivity of these samples were correlated with the alumina properties. The obtained results show that the optimum loading of Cr on Al 2 O 3 lies between 5 and 15%. Also, it has been observed that the calcination temperature affects the conversion at lower reaction temperatures, whereas its influence on product selectivity is limited by O 2 availability in the reaction mixture. The catalyst samples were characterized by FTIR and XPS measurements. The XPS analysis confirmed the presence of Cr 6+ and Cr 3+ on the surface of the catalyst. Cr 3+ was dominant in the spent catalyst.

This paper investigates the application of two techniques to improve the proportional-integral (PI) control performance when implemented to stabilize the operation of gas-phase polyethylene reactors around an optimal operating point which is open-loop unstable. The first technique deals with improving the performance of a single input-single output control loop through employing an on-line adaptive tuning strategy. The purpose of the tuning strategy is to force the closed-loop response to fit inside a desired time-domain specification envelope by automatically adapting the PI setting values. The second technique deals with improving the multiple input-multiple output control performance by simple selection of the proper control structure design. The objective of the proposed control design is to avoid the procedure of screening various control structure candidates. Moreover, because there are more inputs than outputs, a splitrange configuration is used to utilize all available manipulated variables. This configuration allows for tight control. Simulation results demonstrated the success of the proposed methods to provide a better regulatory control performance when compared to those that have been reported in earlier work.