Low-Molecular-Weight Lignin Recovery with Nanofiltration in the Kraft Pulping Process

Kraft lignin is an underutilized resource from the pulp and paper industry with the potential of being a key raw material for renewable fuels and chemicals. The separation of high-molecular-weight lignin from black liquor by ultrafiltration has been widely investigated, while the permeate containing low-molecular-weight lignin has received little attention. Nanofiltration can concentrate the low-molecular-weight lignin. This work, therefore, evaluates nanofiltration for the separation and concentration of low-molecular-weight lignin from the ultrafiltration permeate. For this study, eight flat polymeric sheet membranes and one polymeric hollow fiber membrane, with molecular weight cut-offs ranging from 100 to 2000 Da, were tested. A parametric study was conducted at 50 degrees C, 2.5-35 bar, and crossflow velocity of 0.3-0.5 m/s. At a transmembrane pressure of 35 bar, the best performing membranes were NF090801, with 90% lignin retention and 37 L/m(2)center dot h, and SelRO MPF-36, with 84% lignin retention and 72 L/m(2)center dot h. The other membranes showed either very high lignin retention with a very low flux or a high flux with retention lower than 80%. Concentration studies were performed with the two selected membranes at conditions (A) 50 degrees C and 35 bar and (B) 70 degrees C and 15 bar. The NF090801 membrane had the highest flux and lignin retention during the concentration studies. Overall, it was shown that the nanofiltration process is able to produce a concentrated lignin fraction, which can be either used to produce valuable chemicals or used to make lignin oil.

Automated summary

This study evaluates the use of nanofiltration for the separation and concentration of low-molecular-weight lignin from ultrafiltration permeate. The results show that nanofiltration process can produce a concentrated lignin fraction.