Chemical Processing - June 2008 - (Page 34)

Common temperature control strategy Jacket (Slave) TIC PV Cooling Heating TT SP SP limits Reactor (Master) TIC PV SP From operator or batch recipe OUT OUT temperature. The root causes are nonlinearity in the jacket loop from selecting inappropriate control valves and excessive dead zones in the split range strategy. No tuning of the feedback controller will 60 eliminate limit cycles. Reactor temperature and output, ˚C Case 2 55 Temperature set point Controller output, °C TY Reactor temperature Reactor temperature, °C 40 35 30 30 20 10 Jacket controller output 34 JUNE 2008 chEmicalprocEssiNg.com Add ﬁnal ingredient Jacket controller output, % At another plant, temperatures of eight reactors 50 were oscillating. Figure 3 shows a set-point response and a load response for one reactor temperature 45 Heating/Cooling loop. With the reactor set point initially at 30°C, Temperature PV ﬂuid supply 40 the slow oscillation caused the jacket to continuously and alternately consume significant quantities 35 of steam and chilled water. Later, after the exothermic reaction, the jacket controller output began 30 swinging almost full scale up and down. Average Jacket inlet temperature energy consumption was much greater than that 25 TT theoretically required1,000maintain the reactor temto 0 2,000 3,000 4,000 5,000 perature. There also was a smaller faster oscillation Time, sec of the jacket loop. SP is the controller set point The main problems identified by the consultant PV is the measurement process variable and corrected were: OUT is the controller output • oscillatory tuning of the reactor temperature controller; • oscillatory tuning of the jacket temperature Figure 1. 50 PV trend charts. We’ll Various split-range controller; seen from the operators’ with default PID algorithm show (TY) configurations • excessive dead zone in the jacket split range symptoms of common problems and examples of Set point can be used to logic; and benefits achieved. regulate jacketed 40 glass-lined batch • control valve setup problems. reactors. The plant personnel hadn’t been trained in Case 1 30 modern loop-tuning methods such as Lambda A plant was starting up a new building with all new reactors, instruments and PID algorithms Control tuning, which gives nonoscillatory response at the Distributed PVs with alternative speed required by the production objectives. The System (DCS). A consultant applied Lambda tun20 tests required for systematic tuning also revealed ing (which we’ll discuss later) to give smooth fast the nonlinearities in the split range logic and set-point and load responses without oscillation. control valves. After applying corrections to three However, as shown in Figure 2, the default Pro10 reactors, energy savings on steam alone paid for the portional + Integral + Derivative (PID) algorithm 0 600 1,200 1,800 2,400 3,000 3,600 consulting project in less than three months. produced temperature overshoot that exceeded the recipe specifications on sec set-point steps. The overTime, shoot is due to the presence of integral action in Case 3 both the controller and the process. Dominance of The as-found auto response was too slow, taking integration (slow ramping) in the reactor temperamore than two hours to reach the new set point (Figture process may confuse the engineer, technician or ure 4). Note for the reactor (master) loop the units auto-tuner responsible for finding the best control- 100 the SP, PV and output all are °C. For integrating of 75 ler set point parameters. Integral action is needed in tuning processes, fast closed-loop response requires driving Temperature PV Reactor 70 90 the controller to correct for load disturbances. In a the output beyond the PV for some period of time. 65 modern control system it’s easy to choose alterna- 80 Due to the slow tuning, the operators preferred to 60 tive algorithms (Figure 2) to prevent or reduce this 70 make frequent manual adjustments to the jacket Use steam overshoot. set 55 60 point until the correct reactor temperature was If we waited longer for the set-point responses 50 achieved. This interfered with the operators’ primary 50 to settle, we’d see a slow limit cycle of ±0.5°C on duties such as sampling for quality control. Use chilled water 45 40 the reactor temperature and ±5°C on the jacket Due to nonlinearities in the control logic, it Reactor temperature, °C 6,000 7,00 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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