Chemical Processing - June 2008 - (Page 35)

Jacket inlet temperature Rea TT 30 25 0 SP is the controller set point PV is the measurement process variable Reactor temperature, °C 60 55 70 Use steam METHODS FOR SUCCESS 60 50 40 30 20 10 0 20,000 In the typical chemical plant, there’re several obstacles 50 to achieving optimal control. Plant design and construc45 tion often emphasize chemistry, cost and safety instead of 40 control. Academic control courses typically leave the plant 35 engineer ill-prepared due to their emphasis on complex 30 mathematics or sole focus on continuous processes. Early tuning methods, still taught in the industry, were designed 25 to deliberately make the loop oscillate. e jacket and0reactor temperature relationship includes integrating dynamics, making controller tuning less intuitive than for self-regulating (e.g., ﬂow) loops. Complex control systems have been developed to handle various reactor hardware, speciﬁc types of chemical reactions and production constraints [1]. For the fastest possible set-point response, you may want to consider a nonlinear control strategy as described in Ref. 2. However, for reactors controlled by simple cascade strategies (Figure 1), many problems can be prevented by applying the following ﬁve steps to each loop: 1. Make the process dynamics as linear as possible. 2. Minimize dead time. 3. Measure the process dynamics. 4. Choose the right controller algorithm to compensate for the process dynamics. 5. Tune for the speed required, without oscillation. Use chilled water Jacket controller output Add ﬁnal ingredient 5,000 10,000 Time, sec 15,000 Jacket controller output, % wasn’t possible to ﬁ nd the best controlCase 1 ler tuning parameters by hand calculation. Instead the consultant built a 50 computer simulation using test data Set point acquired from manual step responses. 40 is led to much better tuning of the controller and allowed the operators to keep the reactor loop in auto mode — 30 as designed. e tests also identiﬁed several limitations in the jacket response (Fig20 ure 5). e response was much faster to cooling than to heating. e cooling response showed initial oscillation fol10 lowed by a very slow attempt to recover 0 600 1,200 to 40° C. e asymmetry in heating versus cooling indicates the need for a gain scheduling controller, which applies one set of tuning parameters for cooling and another for heating. Manual step testing of the jacket also showed that inappropriate derivative and ﬁ ltering values had been installed for the jacket controller. Finally, ideal cooling re75 sponse would require a diﬀerent inherent ﬂow characteristic Reactor set point 70 in the cooling valve. Fixing all these problems in the jacket 65 loop would further improve the reactor response. Reactor temperature, °C OUT is the controller output PV with default PID algorithm PVs with alternative PID algorithms 1,800 Time, sec 2,400 3,000 3,600 Figure 2. Overshoot afﬂicted setpoint steps on an 800-L reactor with the reactor loop in auto and the jacket loop in cascade. 100 Temperature PV 90 80 http://www.intersystems.net

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Chemical Processing - June 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Achieve Model Operations Bolster Your Condition Monitoring Toolbox Particle Analysis Makes Solid Progress Improve Batch Reactor Temperature Control Improve Your Job Security Plant InSites Process Puzzler Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - June 2008

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