Dynamic process models are useful for a number of purposes during
design and operation of chemical processes. In this paper the general
principles for model development are outlined, and these principles
are applied to a simple ash tank (which appears to be a lot more
interesting than one might believe at first sight) and to a distillation
column example. Detailed numerical simulation is discussed only
briefly, and it is shown that one in some cases may get an "index"
problem which may pose problems for standard integration methods.
The emphasis of the paper is to demonstrate how one by linearization and simplifications can obtain simple low-order models. For example, we derive simple analytical expressions for the time constants of the pressure and concentration response for the ash tank and distillation column. Such simple models may yield invaluable process insight. In addition, simple models are often needed for process control or for simplified simulation studies (eg., training simulators).