Improving extended delignification technology for kraft pulping, Part II: Co-current two-stage continuous cooking

Some older Kamyr(R) continuous digesters cannot be converted to EMCC(R), Lo -Solids(R) or ITC(R) cooking procedures that use most of the wash zone as a cooking zone. Instead, we propose co-current, two-stage isothermal continu ous cooking in which the two-stage concept we developed for batch cooking i s applied to continuous cooking. Pilot-plant simulation of this two-stage c ooking was performed in a batch digester at 160 degrees C. Delignification of black spruce could be extended to kappa numbers in the low 20s with the same cooking time (or H-factor) and chemical charge as those used in conven tional batch cooking to kappa number 30. The pulp viscosity improvement was about 10%. The yield of pulp from two-stage cooking was 0.3 to 0.4% lower than that from conventional cooking to low 20s kappa numbers. The two-stage pulp was slightly better in tear-tensile strength. Comparing co-current tw o-stage isothermal cooking at 160 degrees C and conventional cooking at 170 degrees C, two-stage cooking provides a lower kappa number without decreas ing the production rate. Other advantages include improved pulp yield and v iscosity at a given kappa number. Modifications required to convert a conve ntional Kamyr digester to co-current, two-stage isothermal continuous cooki ng might be similar to those required for Lo-Solids(R), pulping.