Forward models disadvantages

The feedforward control strategy (Fig. lb) addresses the disadvantages of the feedback control strategy. The feedforward control strategy measures the disturbance before it affects the output of the process. A model of the process determines the adjustment ia the manipulated variables(s) to compensate for the disturbance. The information flow is therefore forward from the disturbances, before the process is affected, to the manipulated variable iaputs. 

A Feed-forward extension. One possible extension could be to create a flow ratio controller FrC between Fa and Fa this is a form of feed-forward control, the aniline outlet concentration could in that case reset the ratio. A general disadvantage of feed-forward control is that it is model-based (prone to modeling errors), in this case of ratio control the two models from Fa and Fp to the aniline outlet concentration are similar, which is an advantage. Another disadvantage is that variations in aniline concentration XA,m me not taken care of by the flow-ratio feed-forward controller. 

A step forward in modelling is provided by the use of activity coefficient models and group contribution methods. One of the most valuable features of these methods is their applicabihty to multi-component systems imder the assumption that local compositions can be described in this case by a relationship similar to that obtained for binary systems. However, one of the main disadvantages of these methods is that they depend on an extremely large amount of experimental data. Furthermore, the absence of the volume and surface p>arameters p>oses a hindrance in the calculation of the binary interaction parameters for UNIQUAC and UNIFAC models. These limitations can be overcome by the use of quantum-based models, such as COSMO-RS (see, for instead, the works of Shah et al., (2002) and of Guo et al. (2007)). In this method no experimental data is needed as an input to model the ionic hquids, being the main constraint the extensive computational time and also that, in some cases, the comparison with experimental data is only qualitative. 

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