Experimental validation of closed loop identification of a multivariable system

The digital control of concentration in a two-input-two-output system was u sed in order to test experimentally the applicability of a closed loop iden tification method proposed by Melo and Friedly (1992). The problem consiste d of controlling the concentration of product at each exit of two isotherma l continuous stirred tank reactors connected in series. The control variabl es were the flowrates of the limiting reactant fed to each tank. A series o f experiments involving step changes in the controller set point were carri ed out to obtain the concentration-time response. The nonparametric frequen cy response was calculated from the closed loop step response and then it w as fit with a parametric linear first order plus dead time model. The resul ting model compared favorably with models obtained from open loop responses and from first principles.