Assessment of Different Model Approaches

The ideal model provides good accuracy only if the column efficiency is very high (N 1000). If band broadening is significant, the ideal model as well as the stage models, which can only reproduce the number of stages for a single component, are not suited to describe multicomponent separations accurately. 

The equilibrium-dispersive model offers an acceptable accuracy only for N3 100, which may be sufficient for many practical cases. However, sometimes the number of stages is lower and more sophisticated models have to be applied. 

Whitley, and Wang (1996) and Whitley, Van Cott, and Wang (1993) have provided methods to decide if effects such as pore and surface diffusion or adsorption kinetics have to be considered in a model. Their approach is based on the qualitative assessment of measured breakthrough curves, which result from a step input for different feed concentrations and flow rates. When the physical parameters are known or can be estimated, the value of dimensionless parameters defined in the mentioned publications may be used to select a model. 

Since the main focus of this book is on adsorption chromatography (low-affinity adsorbents), influences of finite adsorption kinetics as well as of surface diffusion can be neglected. Thus, the reaction models and models including surface diffusion are no more considered in the follovdng. 

Gonsequently, the two remaining groups considering columns with a low number of stages are the lumped and the general rate models. Under the simplification mentioned above, general rate models take into account the individual mass transfer resistance in the liquid film/boundary layer (Equation 6.80) and inside the pore 

---