Displacement washing of softwood pulp cooked to various levels of residual lignin content

This study investigates the influence of the degree of delignification of kraft spruce pulp cooked at seven different kappa numbers, ranging from 18.1 to 50.1, on the efficiency of displacement washing under laboratory conditions. Although the pulp bed is a polydispersive and heterogeneous system, the correlation dependence of the wash yield and bed efficiency on the Péclet number and the kappa number of the pulp showed that washing efficiency increased not only with an increasing Péclet number, but also with an increasing kappa number. The linear dependence between the mean residence time of the solute lignin in the bed and the space time, which reflects the residence time of the wash liquid in the pulp bed, was found for all levels of the kappa number. Washing also reduced the kappa number and the residual lignin content in the pulp fibers.

DISPLACEMENT WASHING OF KRAFT PULP WITH VARIOUS CONSISTENCY

The paper deals with the displacement washing of kraft softwood and hardwood pulps with various consistency of the pulp fibre bed. The results obtained revealed differences between the two types of kraft pulp with various morphological properties of fibres. The washing efficiency decreased with decreasing Péclet number for both kraft pulps, but, due to lower longitudinal dispersion of the wash liquid, greater washing efficiency was achieved for the short-fibred hardwood pulp. On the other hand, the long-fibred softwood pulp exhibited lower hydraulic resistance of the pulp fibre bed. The change in pulp consistency and, similarly, the volumetric mass transfer coefficient, characterising the rate of leaching of alkali lignin from pulp fibres, did not have a significant effect on the washing efficiency. However, the mass transfer coefficient decreased with increasing specific resistance of the pulp bed. With increasing bed consistency of both softwood and hardwood pulps, as the amount of black liquor in the inter-fibre pores decreased, the difference between the space time, characterising the holding time of wash liquid, and the mean residence time of alkali lignin increased.

How to use total dissolved solids measurements to evaluate the performance of diffuser washers—A mill study

Various types of pulp washing equipment are available. Each washing device has a unique mechani-cal construction, and the washing principle is often a combination of dilution, thickening, and displacement washing. In this work, the performance of the pressure diffuser washer is studied. In stepwise trials, the effect of the feed and discharge consistencies on the performance of the diffuser was stud-ied. The effect of the downward velocity of the screen on the pressure diffuser’s washing efficiency was also stud-ied. The measurement of total dissolved solids (TDS) by a process refractometer was used as a wash loss measure-ment unit and the refractometer’s results were used in the calculations of standardized Nordén efficiency (E10) values. The chemical oxygen demand (COD) and conductivity values were also measured and their results com-pared to the TDS results. The results indicated that feed consistency has a significant effect on the performance and effectiveness of the diffuser washers in the mill. It can also be stated that when the downward velocity of the screen is adjusted to too high a level, the washing efficiency of the pressure diffuser decreases. As a conclusion from the mill tests, it can be stated that even small process parameter changes can provide enhanced diffuser washing at the beginning of the washing line, which has a direct effect on the performance of post-oxygen washing.

Displacement washing of soda rapeseed pulp

The paper deals with the displacement washing of unbleached pulp cooked from rapeseed straw by soda pulping under laboratory conditions. Pulp fibres were characterised by their average length, as well as by effective specific volume and surface. Using the step function input change method, the washing breakthrough curves measured for alkali lignin as a tracer were described by the dispersed plug flow model containing a dimensionless criterion, the Péclet number. Besides the wash yield, the dispersion coefficient as well as the mean residence time and space time were evaluated. Preliminary results obtained for soda rapeseed pulp were compared with those for kraft hardwood (beech) and softwood (spruce, pine) pulps published earlier. The wash yield measured for soda pulp was found to be lower than that for hardwood and softwood pulps which manifested lower hydraulic resistance. The presence of silique valves in rapeseed straw resulted in lower mean residence time of lignin removed from the pulp bed in comparison with pulp manufactured from stalks only.