Currently, standard feedback controllers $($PI or PID$)$ are employed for controlling the variables, which have been found to be less effective in controlling the variations in some of the control loops. It is suggested that model$-$based control can help to improve the performance of certain critical loops. As an appointed control engineer, you have identified that the available process control loops of the Reppe Process follow Tennessee Eastman Process Flowsheet as in FIGURE Q$2.$

FIGURE Q$2$: Tennessee Eastman Process Flowsheet

a$.$ From the available information, select a minimum of TWO $(2)$ control loops that would benefit the most from the model$-$based control strategies, and design model$-$based controllers of your choice for those loops. With appropriate justifications based on relevant process knowledge and proper references, you may assume the process models for the associated control loops to be firstorder system.

$[15$ marks$]$

b$.$ Operability is important because of process conditions change. For the Reppe process in FIGURE Q$1,$ identify a minimum of THREE $(3)$ possible sources of variability that require operability analysis. Cite all references.