The subject of this Ph.D. thesis is to investigate and develop methods for modelling, simulation and control applicable in large scale termal industrial plants. An ambition has been to evaluate the results in a physical process. Sugar production is well suited for the purpose. In collaboration with The Danish Sugar Corporation two subsystems in the production have been chosen for application - the evaporation process and the crystallization process. In order to obtain information about the static and dynamic behaviour of the subsystems, field measurements have been performed. A realtime evaporator simulator has been developed. The simulator handles the normal working conditions relevant to control engineers. A non-linear dynamic model based on mass and energy balances has been developed. The model parameters have been adjusted to data measured on a Danish sugar plant. The simulator consists of a computer, a data terminal and an electric interface corresponding to the interface at the sugar plant. The simulator is operating in realtime and thus a realistic test of controllers is possible. The idiomatic control methodology has been investigated developing a control concept for the evaporation process. A theoretical efficiently working concept has been developed taking inspiration from the simulation results. While this concept requires investments in the steam supply system, a slightly simplified control concept has been created and implemented in a Danish sugar plant, causing large energy retrenchment. A realtime simulator for a crystallization process in a sugar plant has been developed. The sections of the actual processes are internally modelled as separate modules according to the system apparatus. All components are modelled by ordinary differential equations and algebraic equations based on mass and energy balances. A special hierachic numerical integration method has been used to solve the model equations. The concept has been transferred to a simulation tool DISTSIM, which has been used in the simulator construction. DISTSIM is a tool developed for realtime simulator construction. The simulation concept submits the demand of a modular and hierachic structure. Separate programming of algorithms and configuration is described. Distribution of the simulation code to distinct computers is possible and the simulation system operates under several operating systems.