Investigating the influence of bed structure on pulp washing using a novel measurement technique
Advancing the chemical engineering fundamentals
Filtration - I (T2-11a)
Keywords: washing, dispersion, experimental, paper pulp, bed structure
In the chemical paper pulping process, the pulp is washed after the delignification stage in order to recover spent cooking chemicals and remove dissolved organic compounds. There are two ways to wash pulp, either by dilution/separation or by displacement. In an ideal displacement washer all of the chemicals and dissolved organic compounds are recovered by one void volume of wash water. This is, however, not the case in a real washer. There are several different phenomena responsible for this deviation from the ideal case, e.g. adsorption, as well as various phenomena related to the structure of the cake and the flow conditions; these, together with diffusion, are collectively called dispersion. Much work has been performed on determining what factors influence the washing efficiency. The results have however been somewhat contradictory and a full explanation on how factors such as different pulp types, temperature and pulp consistency influence pulp washing is yet to be presented. One explanation for this might be the experimental technique used in practically all of the earlier experiments. This technique involves measuring the concentration of some interesting specie, usually Na+ or lignin in the collected filtrate. The drawback of this experimental method is that it gives very little information about what happens inside the cake itself. A new method of measuring concentrations within the pulp bed during the washing process was therefore developed and presented in (Lindau et al., 2006a). In (Lindau et al., 2006b) the new measurement technique was used to study the influence of pulp types on displacement washing.
In this paper the novel experimental technique has been used in order to investigate the influence of bed structure on pulp washing. Even though it well-known that the structure of the pulp cake influence the washing it has not been studied extensively since it has been difficult to determine the structure of the pulp cake. However, this problem has been eliminated with the new measurement technique since also the porosity can be measured locally. Highly flocculated pulp beds have been investigated as well as pulp beds with a porosity gradient. Both mechanical and chemical pulps have been used in the washing experiments. Preliminary results show that a porosity gradient does not decrease washing efficiency as much as flocculation.
Lindau, J., Theliander, H. and Sedin, P. (2006a) A new experimental technique for determining local dispersion coefficients in displacement washing, Chemical Engineering Research and Design (accepted for publication)
Lindau, J., Theliander, H. and Sedin, P. (2006b) Investigating the influence of pulp types on washing using a new measurement technique, Nordic Pulp and Paper Research Journal (accepted for publication)
Presented Monday 17, 12:11 to 12:30, in session Filtration - I (T2-11a).
