This paper outlines the use of a process model directly in a control algorithm. The process considered, a forced circulation single-stage evaporator, is a nonlinear interacting process. The control strategy employing a process model derived from fundamental mass and energy balances is shown to outperform single loop and predictive control strategies by a significant amount. The control structure is first presented in general form and then specifically applied to this process.Nous dCcrivons ici I'utilisation d'un modBle de procCdt directement dans un algorithme de rCgulation. Le procedi considCrC, un Cvaporateur ti une phase circulation forcCe, est un procCdC d'interaction non IinCaire. Nous montrons que la stratCgie de contr8le employCe qui utilise un modble de procede calcul6 h partir des bilans de masse et d'hergie, permet de surpasser largement la stratCgie de boucle unique et la stratCgie de contr8le prkdictif. La structure de contrBle est d'abord prksentte sous sa forme gCnCrale, puis specifiquement appliquCe au procCdC.

The effect of overload on a two-phase high rate anaerobic wastewater treatment system with recycle was determined experimentally and simulated dynamically using a structured model. The experimental system consisted of a well mixed continuous reactor, controlled at pH 6 by sodium hydroxide addition, as the acidification stage and a fluidized bed reactor for the methanogenic stage, with an additional recycle connection from the second to the first reactor. Step changes in the feed concentration as well as in the feed flow rate were investigated and compared to simulation predictions. Operation without recycle was modelled accurately, with the simulation data of most process variables matching the experimental results quantitatively. The application of the same model to recycle operation showed significant discrepancy between the experimental and simulated data for the sodium hydroxide consumption rate and the gas flow rate in the acidification reactor. Although the actual values were different, the qualitative responses of the experimental and simulated step changes were similar. Other simulated variables showed good agreement with the experimental measurements. The model provides a useful tool for design and control studies in systems without recycle and, with some modification, for operation with recycle.

The effect of lowering the pH of the acidification reactor on the overall performance of a two-stage anaerobic wastewater treatment system was determined both experimentally and through simulation of a dynamic structured model. The system operated at low pH was then subjected to a short-term step increase in feed concentration. The measured and the predicted dynamic responses of reactor variables to the shock load were evaluated. The result indicated that lowering the pH setpoint from 6.0 to 5.3 led to a reduction in the external alkali addition by 30% without any significant deterioration in the final effluent quality. The model prediction showed good agreement with the experimental results for most reactor variables. The dynamic response of the system to a concentration shock load indicated a decrease in effluent quality during the disturbance, but the system did recover quickly as soon as the shock load terminated. The comparison between the experimental and the simulation results demonstrated the feasibility of the model to be applied for reactor design and operational evaluation purposes.

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