This project demonstrates the advantages of applying hierarchical multivariate modelling in the petrochemical industry in order to increase knowledge of the total process. The models indicate possible ways to optimise the process regarding the use of energy and raw material, which is directly linked to the environmental impact of the process. The refinery of Nynäs Refining AB (Gothenburg, Sweden) has acted as a demonstration site in this project. The models developed for the demonstration site resulted in: * Detection of an unknown process disturbance and suggestions of possible causes. · Indications on how to increase the yield in combination with energy savings. * The possibility to predict product quality from on-line process measurements, making the results available at a higher frequency than customary laboratory analysis. * Quantification of the gradually lowered efficiency of heat transfer in the furnace and increased fuel consumption as an effect of soot build-up on the furnace coils. * Increased knowledge of the relation between production rate and the efficiency of the heat exchangers. This report is one of two reports from the project. It contains a technical discussion of the result with some degree of detail. A shorter and more easily accessible report is also available, see IVL report B1586-A