Modeling ultrafiltration and filtration phenomena applied in chemical pulping processes

Abstract

This thesis is based on seven publications dealing with separation processes applied in the manufacturing of chemical pulp. The first part discusses the correlation of mass transfer phenomena in the ultrafiltration of spent cooking liquor. The second part focuses on the fiber mat formation that takes place in pulp washers and thickeners.

The main interest throughout the thesis is to develop useful and scientifically justified methods for correlating mass transfer and filtration mechanisms in order to interpret the phenomena taking place in the studied unit operations. The methods presented can be used in the designing and dimensioning of industrial-scale equipment and systems.

Two types of basic mass transfer models, the film model and a more complete approach utilizing an integral solution for two-dimensional diffusion, have been introduced for the calculation of the macrosolute concentration at the surface of the ultrafiltration membrane. A pilot-scale test rig was used to measure ultrafiltration of spent cooking liquor. Mass transfer correlations to estimate the permeate flux have been developed based on both models to be used for plant design calculations. The modeling studies indicated that the integral solution method gave a physically more realistic behavior for the macrosolute concentration at the membrane wall than the widely used film model.

It was further shown that dynamic programming may be effectively used to optimize a multistage industrial-scale ultrafiltration-diafiltration process. This method required a shorter calculation time and resulted in slightly better optimal conditions than the direct search algorithm.

The studies on mat formation during pulp washing showed that the washing tester is a useful tool to obtain the required information for the washer. A calculation procedure has been developed to obtain design parameters for the dimensioning of industrial pulp washers. The suggested theory was based on Darcy's law under the conditions of constant pressure filtration of incompressible beds and reasonable accuracy was obtained for purposes of washer design. A more accurate approach for the modeling of the mat formation stage was formulated on the fundamental filtration theory of compressible fiber beds. Two such methods were discussed. In the more general model of mat formation, the equation of continuity during filtration was also taken into account. This model was then used for evaluating the effect of various operational parameters on drum washer performance. This model was also able to predict earlier generated experimental test data rather well. A new method for considering the additional filtration resistance due to suspended gases (e.g. air) was also suggested.