Nanofiltration of kraft black liquor : Process development and techno-economic evaluation

Kraft black liquor (KBL) is a side-stream that is produced during the kraft pulping process, and mainly contains lignin, hemicelluloses, and cooking chemicals, namely sodium hydroxide and sodium sulphide. KBL is usually evaporated and burned in a recovery boiler in order to recover the cooking chemicals and generate energy. However, pulp mills are sometimes limited by the capacity of their recovery boilers. Removing some of the lignin from KBL can decrease the load on the recovery boiler, facilitating an increase in pulp production. Membrane filtration can be used to concentrate lignin from KBL, as it is an energy-efficient separation process that can separate valuable resources from pulping streams based on size or molecular weight.In this study, an ultrafiltration (UF) permeate obtained by filtering KBL in a membrane filtration pilot plant in a Swedish pulp and paper mill was filtered using nanofiltration (NF). The aim was to investigate how commercial NF membranes perform in concentrating lignin from KBL UF permeate, and at the same time to produce a purified liquor stream that could be reused within the kraft process. This study also focused on optimising the cleaning method for membranes used in the NF of KBL UF permeate, as well as examining the interactions between foulants and the membrane surface. Finally, the techno-economic aspects of implementing such a step in a pulp and paper mill and the costs of cleaning were explored.Various commercially available NF membranes were used to filter KBL UF permeate. Their performance was assessed in terms of their flux and lignin retention. The membrane selected after this assessment, NF090801, performed well regarding flux and lignin retention, with an average flux of 25 L/(m2h) and a lignin retention of 94% when operated at 50ºC and 35 bar. At pilot scale, the average flux was 35 L/(m2h) and lignin retention was 82% when operated at 50ºC and 25 bar. A response surface methodology was then used to optimise the cleaning procedure in order to maximise flux recovery following NF of KBL UF permeate using the NF090801 membrane. The factors studied were temperature, time and cleaning-agent concentration. The investigation found that a flux recovery of 80% can be achieved and the main foulants removed by cleaning for 60 minutes at 40°C with a cleaning-agent concentration of 0.8 wt%. The fouling caused by the filtration of KBL UF permeate using the NF090801 membrane was further studied using in-situ monitoring techniques and surface-characterisation methods. Quartz crystal microbalance with dissipation monitoring indicated that a non-uniform layer of lignin, hemicelluloses, and salts became attached to the membrane surface, and was not fully detached by rinsing with sodium hydroxide. Scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis found that the optimised cleaning method successfully removed the foulants from the membrane surface. Finally, two techno-economic evaluations were performed; these investigated the economic feasibility of implementing a membrane filtration plant within a pulp and paper mill, and whether there are any economic benefits to optimising the cleaning procedure. The techno-economic evaluation showed that implementing a membrane filtration plant with an NF step after a UF step in a pulp and paper mill was economically feasible if the NF permeate was reused during the pulping process, but the lignin was not recovered; and that 16% of current cleaning costs can be saved by optimising the cleaning procedure.Overall, the findings of this study demonstrate the viability of NF of KBL UF permeate, and the state of readiness of the process to be applied in the pulping industry.