Optimizing Nanofiltration for Kraft Black Liquor fractionation: Pilot scale results and techno-economical evaluation

Introduction.Kraft black liquor is a side-stream produced during the kraft pulping process containing lignin, hemicelluloses and cooking chemicals. The stream is usually evaporated and burned in the recovery boiler to recover the cooking chemicals and generate energy. The mill is, however, often/sometimes limited by recovery boiler capacity, and removing some of the lignin from kraft black liquor could allow an increase in pulp production. Membrane filtration can be used to separate lignin from kraft black liquor. Membranes are energy-efficient and can separate valuable resources from the pulping streams based on size or molecular weight. The aim of this study is to investigate how commercial nanofiltration membranes would perform in separating lignin from kraft black liquor ultrafiltration permeate and at the same time, produce a purified water stream that could be reused within the kraft process. Moreover, this study has looked into optimizing the cleaning procedure of membranes used for filtration of kraft black liquor. The techno-economic aspects of implementing such a step in a pulp and paper mill have also been explored. Experimental/methodology. First, multiple commercially available nanofiltration membranes were used for filtering kraft black liquor ultrafiltration permeate. Their performance was assessed according to flux and lignin retention. The chosen membrane was tested at pilot-scale experiments in a membrane filtration unit that was installed at a Swedish pulp and paper mill. The lignin, total solids and ash retention were evaluated to assess the performance of the membrane. Next, response surface methodology was used to find a cleaning procedure that would maximize the flux recovery after filtration of black liquor. The factors studied were temperature, time and cleaning agent concentration. Finally, a techno-economic evaluation which contemplated the installation of a membrane filtration plant within the mill, the possibilities of using the nanofiltration retentate and permeate within the pulping process, and if there were any economical benefits to optmizing the cleaning procedure. Results and discussion. The membrane selected after the screening was the NF090801 manufactured by Solsep BV, which showed a good performance regarding flux in combination of lignin retention. At laboratory scale, the membrane had an average flux of 25 L/(m2h) and a lignin retention of 94%, whereas at pilot scale, it had an average flux of 35 L/(m2h) and a lignin retention of 82%. Even though at pilot scale the lignin retention was lower, it was proved that nanofiltration of kraft black liquor is a viable option. Optimizing the cleaning of the process showed that a flux recovery over 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%. Moreover, 16% of the cleaning costs can be saved by optimizing the cleaning procedure. The techno-economic evaluation showed that implementing a nanofiltration step after a ultrafiltration step in a pulp and paper mill was economically feasible if the nanofiltration permeate was reused during the pulping process, but the lignin was not recovered. Overall, the findings of this study demonstrate the viability of nanofiltration of kraft black liquor and the readiness of the process to be applied within the pulping industry.