Investigating pulp quality improvements in the NSSC cooking process

Heinola Fluting Mill has four continuous digester lines that utilize the NSSC cooking process. It has been noticed that yield variation between the digesters is high even though the aim is to produce similar pulp. The objective of this thesis was to improve pulp quality at the mill with a priority to identify the cause for high variation in quality. Through data analysis it could be verified that the variation in pulp quality was clearly higher on two of the four digester lines. By conducting experiments, it was concluded that pulp quality was dependent on production rate on these two deviant digester lines. An inaccurate production calculation was identified to be a one reason for this, as it causes variation in quality because cooking liquor is dosed according to production rate. Additionally, it was concluded that better pre-steaming improves pulp quality while too short or too long impregnation time causes decline in pulp quality. Because of this, pre-steaming should be kept on a high level and impregnation time should be optimized to maximize pulp quality.

Quality improvements were also investigated by focusing on new measurement techniques. Continuous cooking result measurements are not generally available for NSSC cooking process which means that efficient process control is not feasible. Therefore, alternative cooking result measurements were searched using literature, and the most potential finding was the degree of sulphonation. It correlates well with pulp strength properties which could make it one of the most important quality indicators during process optimization. However, it cannot be measured continuously. As a result, a combination of a refractometer and a conductivity measurement was suggested as a possible indicator for cooking result. In addition, one objective was the correction of the H-factor for the NSSC cooking process which was done successfully using literature.