Ultrafiltration of Bleach Plant Efflunet from a Sulphite Pulp Mill-Influence of operating parameters

Sammanfattning: Stora Enso Nymölla Mill is an integrated pulp and paper mill located in south Sweden near the Baltic Sea. The mill uses ultrafiltration (UF) to remove compounds with a high chemical oxygen demand (COD) from the bleach plant effluent. UF is a separation technology using membranes. There is no phase change during the separation, which makes UF less energy demanding than other separation processes. UF can be used for many purposes in the pulp and paper industry; treatment of bleach plant effluent, white water, deinking effluent and separation of compounds in the spent pulping liquor. The UF plant at Nymölla was built in 1995 and due to changes in the production, the feed and concentration to the UF is higher today than originally. Studies have shown that the effluent has a COD above 10 g/l. Flux, transmembrane pressure (TMP), crossflow velocity (CFV) and concentration in the different stages of the UF plant at the hardwood line were measured. A simulation tool in MATLAB, and experimental data from laboratory experiments with liquid withdrawn at different positions of the UF plant, was used to study the influence of pressure, CFV and module configuration on membrane performance. The simulation showed that the UF plant is not running optimally today: the entire membrane length was not used in the last two stages. Three options to increase the flux was obtained from this study: • Increase the driving force, which means increasing the transmembrane pressure. • Decrease the frictional pressure drop by decreasing the crossflow velocity • Decrease the frictional pressure drop by changing the module configuration to 18 membranes in parallel instead of two parallel flows with nine membranes in series. Increasing the pressure and changing the module configuration means an investment cost, but these two options will increase the performance of the last stages the most.