The energy optimisation of a highly integrated chemical process is a complex subject. The effect of the numerous interactions is not easily understood. It is shown that close co-operation of an experienced process engineer and an expert in the field of computer aided process engineering with a sound technological background forms the basis to a successful energy study. The use of CAPE-tools is essential, but the process engineer keeps a central position throughout the study. This approach results in a significant reduction in the consumption of energy from 26 MW to 6 MW in the example project.