Effect of non-ideal mixing on control of cooled batch reactors

In this paper, the effect on a temperature control law of unmodelled non-perfect mixing in a cooled batch reactor, in which a strongly exothermic reaction takes place, has been tested. Reactor non-ideality has been described by means of a compartment model, obtained by subdividing the reactor into perfectly mixed interacting zones, while the temperature control scheme has been based on the assumption of ideal reactor behaviour. The proposed model allows to quantify the quality of mixing in terms of a dimensional internal circulation flow rate Xc. Detailed analysis has been carried out by comparing temperature tracking error in different reactor compartments and for different mixing degrees. In addition, the effect of temperature sensor location has been assessed. Main results highlight how the decrease of Xc is responsible for an increase of temperature gradients inside the reactor volume, with consequent worsening of temperature controllability; in correspondence of very poor mixing, run-away conditions can be reached. It can be also observed that, when temperature measures are carried out in peripheral reactor regions, temperature errors are generally larger and the values of Xc at which run-away onsets increase, i.e. a better mixing is required to avoid safety problems. Noteworthy, the effect of Xc on reactant conversion depends on the sensor location as well. In fact, average conversion degree is higher when temperature is measured in peripheral regions, since the temperature controller yields higher average temperatures in the reactor.