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control of molecular weight in a batch polymerization reactor using ...

control of molecular weight in a batch polymerization reactor using ...

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Downloaded By: [HEAL-Link Consortium] At: 12:27 29 July 2008 C. KIPARISSIDES et al, TABLE 11 Numerical Values of the Kinetic Rate constants Gel-Effect Parameters introduced to the system. Furthermore, the recovery and recycling of unreacted monomers and solvents significantly contribute to the gradual buildup of impurities in the system. The presence of variable levels of impurities can markedly affect the rate of polymerization and polymer quality in the absence of a corrective feedback control action. It is, therefore, the objective of the present investigation to show that the effects of impurities on the polymerization kinetics can be considerably reduced by operating the reactor under closed-loop control. Several simulation runs of the bulk MMA batch reactor were obtained under LQC, DMC and ESTR control. Kinetic constants used in the simulation studies are reported in Table 11. In all cases, the control objective was to maintain the monomer conversion and number-average molecular weight along some desired trajectories despite the presence of process disturbances in the total initiator concentration. The desired conversion and number-average molecular Time, min FIGURE 1 Desired, uncontrolled and controlled monomer conversion responses for a positive disturbance A/ = 0.025 mol/L in the initial initiator concentration.

Downloaded By: [HEAL-Link Consortium] At: 12:27 29 July 2008 POLYMERIZATION REACTOR CONTROL 1 ESTR 1 Time, min FIGURE 2 Desired, uncontrolled and controlled number-average molecular weight responses for A1 = 0.025 mol/L. weight trajectories were those obtained under isothermal reactor operation (T = 65°C) with M, = 4.75 mol/L and 1, = 0.05 mol/L. In Figure 1 the desired, uncontrolled and controlled monomer conversion responses are plotted against the reaction time for the case of a positive step increase (A1 =0.025 mollL) in the initial initiator concentration. The corresponding number-average molecular weight responses are plotted in Figure 2. The desired responses correspond to isothermal reactor operation (T = 65°C) in the absence of any kind of process disturbances. The uncontrolled responses correspond to isothermal reactor operation (T =65"C) in the presence of a positive disturbance in the total initiator concentration. Finally, the controlled responses illustrate the reactor performance under feedback control (DMC, ESTR and LQC) for the case of a positive step change in the initial initiator concentration. It is evident that all controllers result in an actual reduction of the undesired effects of the initiator disturbance on the output variables (monomer conversion and number-average molecular weight). In Figure 3 the deviation of the manipulated variable (reaction temperature) from its nominal value (T = 65°C) is plotted against the reaction time. The three curves represent the control policies as calculated by LQC, DMC and ESTR algorithms. In all closed-loop studies, the control variable decreases from its isothermal value to compensate for the effect of a positive increase in the initiator concentration. It can be seen that under feedback control both monomer conversion and number-average molecular weight move closer to their corresponding desired values. ~oiever, none of the three controllers can completely eliminate the observed offsets between the desired and actual controlled responses. This is due to the fact that one single control variable is used to regulate two output variables reacting in opposite directions to manipulations of the polymerization temperature. It is interesting to note that the performance of

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