Chemical Processing - June 2008 - (Page 36)

10 0 600 1,200 1,800 Time, sec 2,400 3,000 3,600 Case 2 Reactor temperature and output, ˚C or the time required for a new f luid mixture to pass from the 75 100 Temperature PV control valve to the measuring Reactor set point 70 90 element. Minimize this dead 65 80 time by appropriate sensor loca60 70 tion and by installing a circulatUse steam 55 60 ing pump as shown in Figure 1. Also, the effect of filters added 50 50 in the transmitter or the conUse chilled water 45 40 troller may look like dead time 40 30 to the PID algorithm. In the Jacket controller output 35 20 reactor loop, we minimize dead 30 10 time by getting the fastest linear Add ﬁnal ingredient response of the jacket loop, in25 0 cluding allowing one overshoot 0 5,000 10,000 15,000 20,000 on the jacket set-point response. Time, sec Process dynamics is a model of the shape and size of the PV Figure 3. Oscillation response to output changes, which we need to Linear means the process temperature PV reoccurred during set-point step on a sponds consistently regardless of the size, direction optimally tune the controller. For most loops this 40,000-L reactor with can’t be calculated before construction and is most or history of the controller output changes. In the the reactor loop in conveniently determined from step tests with the jacket loop, achieving linearity requires selecting auto and the jacket loop in cascade; a controller in manual. The process dynamics can appropriate control valves and minimizing nonload disturbance be manually calculated from trend charts or time linearities in the control strategy, e.g., dead zones (exothermic reaction) also took place. series data acquired from an OPC server or, in in the split range logic. Figure 5 shows another example of nonlinearity: on the heating step, after the some cases, automatically by software running in the DCS. The two simplest types of process initial overshoot, recovery to 50° C was extremely dynamics are: slow due to the control-valve flow characteristic. • self-regulating processes (those that eventually Sometimes the limiting nonlinearity is in the settle at a new value during manual step tests); and utilities supplying the jacket, e.g., a steam-header • integrating processes (those that ramp at varipressure control loop. In the reactor loop, linearity ous slopes during manual step tests). means getting symmetrical set-point responses from The tests to measure these process dynamics the jacket. If the cooling and heating responses of Jacket (Slave) Reactor (Master) the jacket are asymmetrical (as in FigCase 3 SP ure 5 or for steam versus cold water), SP From operator SP TIC TIC limits or batch recipe OUT OUT consider a gain scheduling controller PV 60 PV to compensate. This doesn’t require any special coding because it can be Cooling TT Reactor temperature 55 Temperature set point Controller output, °C TY easily configured by drag-drop-andHeating tune in modern control systems. 50 Dead time is the time measured 45 from an output change before Heating/Cooling Temperature PV anything happens on the PV. It’s ﬂuid supply 40 inherently destabilizing in a feedback control loop. In the jacket, one 35 cause of dead time is transport delay Reactor temperature, °C 30 25 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 Jacket inlet temperature Figure 4. Response was far too slow for a setTT point step on a 3,600-L reactor with the reactor loop in auto and the jacket loop in cascade. Jacket controller output, % Time, sec SP is the controller set point PV is the measurement process variable JUNE 2008 OUT is the controller output chEmicalprocEssiNg.com 36 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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