Method and a system for production of high molecular weight lignin

A cooking method for production of high molecular weight lignin having a molecular weight of more than 2300 and pulp comprising:

• i. cooking a cellulosic fiber source comprising wood chips in a cooking liquor at a temperature selected from a range between 120° C. and 180° C. to a kappa number selected from a range between 100 and 5 to form a cooked cellulosic fiber source;
• ii. compressing the cooked cellulosic fiber source at the cooking temperature with a pressure selected from a range between 35 kPa and 1000 kPa, to provide a compressed cellulose fraction with a consistency higher than 10% by weight and a density selected from a range between 350 kg/m3 and 2000 kg/m3 in a black liquor comprising high molecular weight lignin comprising a molecular weight of more than 2300;
• iii. displacing the black liquor with a displacing liquor by pressing the compressed cellulose fraction in the black liquor with a pressure selected from a range between 1 kPa and 250 kPa to form a column having a consistency of at least 5% by weight, or a column porosity of at least 0.25, followed by displacing the black liquor with a displacing liquor having a lignin content lower than that in the black liquor, and wherein the pressure is lower than that in step ii;
• iv. recovering a high molecular weight lignin fraction having a molecular weight of more than 2300 from the displacing liquor;
• v. and recovering pulp from the compressed cellulose fraction.