ISA 2012 Automation Week Track: Control Performance Wednesday, 26 September, 3:15-4:45 pm, Room 204B Flávio P. Briguente (Monsanto) and Greg McMillan (CDI Process & Industrial) Temperature control is one of the four most common types of loop. In some cases, temperature is the critical condition for chemical reactions to take place. In batch processes, control becomes more complicated because of the variability of level, pressure, and concentrations. For this particular case, the process consists of a batch reactor, an external shell-and-tube heat exchanger with cooling tower water flow on the shell side.
The main raw materials are charged in the reactor, and the contents are continuously circulated through the heat exchanger tubes. Then a flow of gas is initiated, pressuring the vessel to 90 psig, starting the exothermic reaction. Pressure is maintained constant during the batch. As raw materials are converted, the product concentration increases. The product has very low solubility in water.
The temperature in the batch must be ramped up high enough to avoid precipitating the product and plugging the heat exchanger tubes. On the other hand, high temperature directly impacts side-reaction kinetics and consequently production yield. Since the plant start-up in 1998, this process consisted of a manual reactor temperature control by manipulation of a butterfly control valve installed in the cooling tower water inlet pipeline. This work aims to implement an automatic temperature control strategy for a batch reactor. After many previews trials, the strategy proposed is a cascade loop with a primary reactor temperature controller and a secondary heat exchanger temperature controller.
The primary controller set point is calculated using the product solubility curve. The solubility curve is a relationship between product concentration and temperature. The product concentration is calculated as a function of totalized amount of gas added to the batch. The primary controller output is the set point for the secondary controller. The primary controller output considers a limitation of maximum delta between the heat exchanger inlet-outlet temperature and the temperature of solubility curve itself.
This strategy allows the control valve to be almost totally open at the beginning of the batch, when high amount of heat is generated due to high raw material concentration, and almost closed at the end of batch when the main reaction slows down. As a result, the batch average temperature was reduced by 2.5ºC, improving significantly the production yield of the process. This project achieved significant financial savings and improved the product quality by reducing impurities formation.
