Predicting water removal during vacuum dewatering from fundamental fibre property data

Control of the solids content after vacuum dewatering is vital for efficient control and energy use of the paper machine. Dewatering is a complex function of many parameters such as fibre and pulp properties as well as settings on the paper machine and a predictive model for dewatering will require extensive and detailed studies of the occurring physical phenomena. The hypothesis investigated in the present study is that fibre width and fibre length density measured for fibres suspended in water can be used for estimating the achievable dryness in vacuum dewatering. The hypothesis was tested by comparing model predictions to experimental data for 11 different pulps. The results show that the dryness after vacuum dewatering can be estimated from the fibre length density and the fibre width for the fresh pulps using an elliptical fibre cross-section model with a ratio of 0.48 between the short and long axes. For the dried pulps a lower value was needed to reach a good agreement.