Qualitative Reasoning Methodology for the Generation of Process Plant Operating Procedures

Abstract

The analysis of operating procedures in the early stages of design can lead to safer and higher performance plants. Qualitative reasoning techniques hold considerable promise in supporting generations of operating procedures, since they are able to describe possible trajectories of a system based on non-quantitative information and provide explanation about process behaviour in a way which gives insight into the underlying physical processes. Despite this potential, existing techniques still present limitations related to the tendency for generating non-real behaviour patterns and the inability to describe distributed parameter systems. This study presents a qualitative reasoning methodology, weighted digraph (WDG) approach, for describing the dynamics of complex chemical processes, and in particular of distributed parameter systems, with a considerable reduction in the generation of spurious solutions. It is based on a generalisation of the signed digraph approach and retains its main advantages, such as the ability to easily represent intuitive and causal knowledge and a graph structure which makes apparent the flow of information between variables. In addition, it incorporates several new features, making use of functional weighting, differential nodes and temporal edges, which enable the procedure to qualitatively describe complex patterns of behaviour. The effectiveness of the approach is demonstrated by considering the qualitative modelling and simulation of the dynamic behaviour of several chemical processes: heat-exchanger, CSTR with and without temperature control and distillation column. The proposed weighted digraph approach is used to support generation of start-up procedures with reference to two case studies: a network of heat-exchangers and an integrated system composed of a CSTR and a feed/effluent heat-exchanger. It is shown that the digraph based strategy has the ability to generate feasible operating procedures in the presence of operational constraints and identify the need for modifications of the process topology in order to allow the start-up of the system. Results also indicate that work is still needed in order to further improve the methodology and create an interactive computer based interface to help with reasoning about complex patterns of behaviour