Measurement and prediction of dewatering characteristics for mechanical pulpsusing optical fibre analysis

Abstract: On?line measurement of relevant fibre and pulp characteristics is necessary in order to increaseproductivity and to maintain uniform quality. The drainage properties within the wire and the presssection are important factors since they affect the dry content after the press section. The higher thedry content, the less the steam consumption and thus less energy is consumed. In some cases thedrier section has a limiting capacity and thus decreased web dryness will reduce the production. Therunnability in the paper machine is also affected by the dry content after the press section, becauseweb breaks might occur in the drier section or in the calendar.The long term aim of this work was to obtain an on?line measurement of dewatering behaviour inpaper machines based on optically measured fibre and fines characteristics. However, due to thedifficulty in obtaining pulps with sufficient distribution in dewatering properties and the difficulty invarying the pulps characteristics on one single paper machine, a comparative study between fourdifferent laboratory dewatering methods were conducted as a first step. Optical measured fibrecharacteristics were used to attempt to predict the dewatering behaviour of the different laboratoryequipments for different mechanical pulps. In addition, a designed experiment was conducted inorder to further evaluate the quality of the optical fibre and fines measurement.The results showed that there are rough correlations between the dewatering equipments; howeverthey rank the pulps differently depending on the wood raw material used and whether the refiningconditions are gentle or harsh. The prediction models formulated for the dewatering equipmentsbased on optically measured fibre characteristics showed rather good correlation between themeasured versus the calculated values; however, not sufficiently good for use in on?line applications.It was also found that the same measured fines amounts show different dewatering behaviour,depending on the quality of the fines used. The difference in fines quality was, however, notreflected in the optical measurement and it was thus concluded that there is a need for higherresolution of the measurement equipments in order to make it possible to measure the shape andthe exact amount of the fines.The results obtained from this work have provided an increase in both knowledge and understandingand can hopefully be utilized in characterizing paper machine dewatering with on?line measurementsof fibre properties in the future.