Fiber energy efficiency Part IV: Multi-scale modeling of refining processes

Models of thermomechanical pulp processes must handle a large number of physical variables and machine specific parameters at different scales. Some variables, measured in the refining zone, will be invaluable when spanning the material and energy balances and thereby they provide essential information to the models. In this paper, measurements of temperature profile and plate gaps from a full-scale CD-refiner are used as model inputs together with process variables such as motor load, production, dilution water, inlet pressure and casing pressure. Two independent models, a micro-model describing fluid mechanics between the refining bars and an extended entropy model (macro-model) describing the material and energy balances for a complete refiner are used to get a realistic estimate of the pulp dynamic viscosity inside the refining zone. The models comprise a multi-scale concept and it is shown that the variability in the estimated pulp dynamic viscosity is directly related to the fiber energy efficiency in a micro-as well as in a macro perspective. To produce homogeneous pulp quality at energy efficient process conditions, it is concluded that large variations in the pressure pulses must be reduced. This can be performed by optimizing refining segments as well as stabilizing conditions by improved control strategies