A continuous process consists of two sections, A and B$,$ shown in the figure overleaf. Feed of composition X$1$ enters section A where it is extracted with a solvent $($S$)$ pumped at a rate L$1$ to A$.$ The raffinate $($R$),$ i$.$e$.,$ the $$residue$$ from the extraction process, is removed from A at a rate L$2,$ whilst the extract is pumped to a cracking section B$.$ Hydrogen is added at the cracking stage at a rate L$3$ whilst heat is supplied at a rate Q$.$ Two products are formed, having compositions X$3$ and X$4.$ The feed rate to A$,$ along with L$1$ and L$2,$ can be kept constant easily, but it is known that fluctuations in X$1$ can occur. Consequently, a feed$-$forward control system is proposed to keep X$3$ and X$4$ constant vis$--$vis variations in X$1,$ using L$3$ and Q as controlling variables. Experimental frequency response analysis yielded the following transfer functions: