Improve batch reactor temperature control

By Mark Coughran

ChemicalProcessing.com

Understand the likely causes and fixes for common problems in reaching set points

Reactor temperature control typically is very important to product quality, production rate and operating costs. With continuous reactors, the usual objectives are to:

• hold the temperature within a certain band around the set point, preferably without oscillation;
• reduce operator intervention as much as possible; and
• minimize consumption of utilities.

Figure 1. Various split-range (TY) configurations can be used to regulate jacketed glass-lined batch reactors.

Batch reactors generally demand some additional objectives such as:

• fast heatup or cooldown to a new set point without oscillation and with minimal overshoot; and
• stable response to load disturbances, e.g., exothermic reaction.

Achieving these objectives requires paying attention to many details of the equipment and controller logic. Systematic testing and optimization of the feedback control loops also can speed the startup of a new plant.

Figure 1 shows a common control system for glass-lined batch reactors where the slave loop operates on the jacket inlet temperature to protect the lining. The heating/cooling supply can have various split-range (TY) configurations such as control valves to hot/cold headers (which we’ll call Case 1), control valves to steam and chilled-water heat exchangers (Case 2) and control valve on the chilled fluid and variable electrical heating (Case 3). Here, we’ll look at some challenges and opportunities based on real data from three such reactors as seen from the operators’ trend charts. We’ll show symptoms of common problems and examples of benefits achieved.

Case 1

Figure 2. Overshoot afflicted set-point steps on an 800-L reactor with the reactor loop in auto and the jacket loop in cascade.

A plant was starting up a new building with all new reactors, instruments and Distributed Control System (DCS). A consultant applied Lambda tuning (which we’ll discuss later) to give smooth fast set-point and load responses without oscillation. However, as shown in Figure 2, the default Proportional + Integral + Derivative (PID) algorithm produced temperature overshoot that exceeded the recipe specifications on set-point steps. The overshoot is due to the presence of integral action in both the controller and the process. Dominance of integration (slow ramping) in the reactor temperature process may confuse the engineer, technician or auto-tuner responsible for finding the best controller tuning parameters. Integral action is needed in the controller to correct for load disturbances. In a modern control system it’s easy to choose alternative algorithms (Figure 2) to prevent or reduce this overshoot.

If we waited longer for the set-point responses to settle, we’d see a slow limit cycle of ±0.5°C on the reactor temperature and ±5°C on the jacket 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 eliminate limit cycles.

Case 2

Figure 3. Oscillation occurred during set-point step on a 40,000-L reactor with the reactor loop in auto and the jacket loop in cascade; a load disturbance (exothermic reaction) also took place.

At another plant, temperatures of eight reactors were oscillating. Figure 3 shows a set-point response and a load response for one reactor temperature loop. With the reactor set point initially at 30°C, the slow oscillation caused the jacket to continuously and alternately consume significant quantities of steam and chilled water. Later, after the exothermic reaction, the jacket controller output began swinging almost full scale up and down. Average energy consumption was much greater than that theoretically required to maintain the reactor temperature. There also was a smaller faster oscillation of the jacket loop.

The main problems identified by the consultant and corrected were:

• oscillatory tuning of the reactor temperature controller;
• oscillatory tuning of the jacket temperature controller;
• excessive dead zone in the jacket split range logic; and
• control valve setup problems.

The plant personnel hadn’t been trained in modern loop-tuning methods such as Lambda tuning, which gives nonoscillatory response at the speed required by the production objectives. The tests required for systematic tuning also revealed the nonlinearities in the split range logic and control valves. After applying corrections to three reactors, energy savings on steam alone paid for the consulting project in less than three months.

Case 3

Figure 4. Response was far too slow for a set-point step on a 3,600-L reactor with the reactor loop in auto and the jacket loop in cascade.

The as-found auto response was too slow, taking more than two hours to reach the new set point (Figure 4). Note for the reactor (master) loop the units of the SP, PV and output all are °C. For integrating processes, fast closed-loop response requires driving the output beyond the PV for some period of time. Due to the slow tuning, the operators preferred to make frequent manual adjustments to the jacket set point until the correct reactor temperature was achieved. This interfered with the operators’ primary duties such as sampling for quality control.