Modeling of Flow Dynamics for Control of Distillation Columns

In the modeling of distillation columns for control purposes, the flow dynamics are usually assumed to be independent of and much faster than the concentration dynamics. Nearly always, it is also assumed that the energy effects can be neglected, or at least that it is sufficient to model only the steady-state behavior. These assumptions are related with the popular assumption of "constant flows" in the column sections. Because the heat of vaporisation of the mixture distilled usually is dependent on its composition, especially if mass units are used, the assumption of constant flows is not valid in general. This also means that the total and component material as well as the energy balances are coupled. In the modeling of flow dynamics presented in this paper, the coupling of the balance equations is taken into account. The results clearly show how the flow dynamics, and especially the steady-state gains, are affected when the assumption of constant flows does not apply.