Analysis of Control Configuration Changes with Relative Gain Array and Relative Exergy Array for Units in Plants with Recycle

The Relative Gain Array (RGA) provides a quantitative comparison of interactions between control loops. It shows steady state interactions between control loops. The thermodynamic concept of exergy can be used to analyse a process in terms of its efficiency. The Relative Exergy Array (REA) that measures both the relative exergetic efficiency and controllability of a process has been proposed (2010). RGA and REA have been used for a quantitative comparison of interactions and relative exergetic efficiency between control loops of isolated process unit operations e.g. CSTR, Distillation columns, etc. But it is very common that the industrial unit/process includes some recycle loops. The presence of a recycle loop and changes in control configurations can have a significant impact on relative gain arrays and relative exergy gain-arrays and thus effecting the unit operation considered. It is found that the results of RGA and REA can change significantly with the addition of recycle loop and change in the control configuration. This is demonstrated in this paper by two case studies.