mixture, whereas the chemical equilibrium constant Ka (Treac ) only depends on temperature but not on composition (Sandler,2006). For homogenously catalysed esterification reactions Riechert et al. (2015) found out that Kx strongly dependents on the amount of alcohol in the reaction mixture, while Ka remains constant. Considering the esterification in a reactive distillation column, where the composition profile changes significantly along the column height, the application of the chemical equilibrium constant Ka is favoured to describe the reaction kinetics. The chemical equilibrium constant Ka can be calculated according to Eq. (5.1) as the ratio of the activities ai eq of the reactants and products in the equilibrium.