Modelling Fundamentals

Models are an integral part of any kind of human activity. However, we are mostly unaware of this. Most models are qualitative in nature and are not formulated explicitly. Such models are not reproducible and cannot easily be verified or proven to be false. Models guide our activities, and throughout our entire life we are constantly modifying those models that affect our everyday behavior. The most scientific and technically useful types of models are expressed in mathematical terms. The use of models in chemical engineering is well established, but the use of dynamic models, as opposed to the more traditional use of steady-state models for chemical plant analysis, is much more recent. This is reflected in the development of new powerful commercial software packages for dynamic simulation, which has arisen owing to the increasing pressure for design validation, process integrity and operation studies for which a dynamic simulator is an essential tool.

Indeed it is possible to envisage dynamic simulation becoming a mandatory condition in the safety assessment of plant, with consideration of such factors as start up, shutdown, abnormal operation, and relief situations assuming an increasing importance. Dynamic simulation can thus be seen to be an essential part of any hazard or operability study, both in assessing the consequences of plant failure and in the mitigation of possible effects. Dynamic simulation is thus of equal importance in large scale continuous process operations, as in other inherently dynamic operations such as batch, semi-batch and cyclic manufacturing processes. Dynamic simulation also aids in a very positive sense in enabling a better understanding of process performance and is a powerful tool for plant optimization, both at the operational and at the design stage. Furthermore steady-state operation is then seen in its rightful place as the end result of a dynamic process for which rates of change ha ve become eventually zero.

Large scale commercial software packages for chemical engineering dynamic simulation are now very powerful and contain highly sophisticated mathematical procedures, which can solve both for the initial steady-state condition as well as for the following dynamic changes. They also contain extensive standard model librates and the means of synthesizing a complete process model by combining standard library models. Other important aspects are the provision for external data interfaces and built-in model identification and optimization routines, together with access to a physical property data package. An essential stage in the development of any model is the formulation of the appropriate mass and energy balance equations. One of the most important features of modeling is the frequent need to reassess both the basic theory, and the mathematical equations, representing the physical model, in order to achieve agreement, between the model prediction and actual p rocess behavior.