The controllability investigation of any kind of chemical process is the interactive and challenging part of process design or development. The investigation works on several levels and uses different methods. First, the control targets are defined, the set of controlled variables are determined and the set of possible manipulated variables are selected. The proper pairing of the controlled and manipulated variables, that is the design of the control structure, consists of first, the study of steady state control indices and then dynamic behaviours in the cases of open and closed control loops of the promising control structures. This investigation is presented on the design of energy integrated separation schemes.

A new challenging method is offered by the Rough Set Theory (RS), which has been already successfully used in several areas of artificial intelligence and data analysis e.g. for the discovery of patterns and dependencies in data. Its use for controllability investigations is a new area at the revision of control structures of existing plants, retrofit modifications, because it can detect the dependencies among the possible controlled and manipulated variables with the analysis of the measured data and helps to fulfil the requirements of the control target. RS theory indicates also the measure of the dependencies among the variables.

The RS theory is used for the improvement of the control structure of a complex chemical process, which is the fluid catalytic cracking unit (FCC). The analysis of the data of existing FCC unit shows that the product quality significantly depends on the temperature in the regenerator unit and its control is necessary. After considering the degrees of freedom, a new control loop is designed for the proper control of the temperature in the regenerator unit, which is accepted by the industry and it will be included in the control structure of the FCC plant.