Chemical Processing - June 2008 - (Page 37)

Asymmetrical response in the control logic (for example, the SP limiter Controller ouput, % block in Figure 1), a non50 60 Use heater linear computer simulation can accelerate finding the 48 50 best controller parameters Jacket set point Use chilled ﬂuid — today’s control systems 46 40 come with built-in simulation tools. In some cases 44 30 a Fuzzy Logic Controller 42 (FLC) can give control 20 superior to PID. When Jacket temperature PV 40 10 there’s no jacket but only electrical heating of the re38 0 actor, the FLC can provide 1,550 2,050 2,550 3,050 the fastest possible heating Time, sec of the reactor with no overshoot or oscillation. Again, the modern control system Figure 5. Jacket responded faster to cooling than heatmakes it easy to install this controller. also will reveal the nonlining as shown with reactor earities in the process. loop in manual and jacket loop in cascade mode. With a PID controller ACHIEVE BETTER CONTROL the type of process deterSome plants have experienced oscillatory or slugmines how to compensate gish response of batch reactor temperature. is for the process dynamics. For purely self-regulating can be caused by the integrating process response, processes we mainly use integral action in the con- limitations in the control system and lack of traintroller. For purely integrating processes we mainly ing. With a modern control system, a plant can set use proportional action. Derivative action normally up the controllers to give fast set-point response isn’t needed in the jacket controller but may be ap- without overshoot or oscillations. is oﬀ ers propriate in the reactor controller. the opportunity to maximize product quality, e Lambda tuning method is one way of minimize batch cycle time and eliminate utilichoosing the PID parameters to tune for the speed ties waste — all positive impacts on your plant’s required, without oscillation. For process dynamproﬁtability. ics that are purely self-regulating or integrating, simple algebraic tuning rules developed for MARK COUGHRAN is a senior process control consultant continuous processes [3] have proven applicable to for Emerson Process Management, Austin, Texas. E-mail him batch processes. ese rules can be taught to engi- at Mark.Coughran@Emerson.com. neers, technicians and operators as a time domain method — without the need to use Bode plots or transfer functions. We must observe the rule of cascade by tuning REFERENCES the jacket (slave) control loop ﬁ rst and faster than 1. Process Control and Optimization, Volume the reactor (master) control loop. e Lambda VIII in “Instrument Engineers’ Handbook,” p. tuning method provides explicitly for tuning by the 1,664, 4th ed., B.G. Liptak, ed., CRC Press, cascade rule because we can set the response time Boca Raton, Fla. (2007). (λ) of each control loop as: 2. Boudreau, M.A. and G.K. McMillan, “New Diλjacket << λreactor rections in Bioprocess Modeling and Control,” More-advanced tools are helpful for some situp. 91, ISA, Research Triangle Park, N.C. (2007). ations. For instance, if the temperature process 3. Bialkowski, W.L., in “The Control Handbook,” response has elements of both self-regulating and p. 1,234, W. Levine, ed., CRC Press, Boca integrating dynamics, a different tuning rule is Raton, Fla. (1996). needed. When there’re significant nonlinearities 52 70 37 Jacket controller output, % Jacket temperature, °C CHEMICALPROCESSING.COM JUNE 2008 http://CHEMICALPROCESSING.COM

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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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