Hardwood alkaline pulping processes and systems

Abstract

Hardwood pulp rejects at a given Kappa number can be reduced by subjecting the hardwood pulp to specified cooking conditions at the end of cooking. In this regard, it has been discovered that s-lignin reacts faster than g-lignin and that there exists a higher proportion of g-lignin at the end of a hardwood cook as compared to the g-lignin present at the beginning of the cook. Thus, when cooking hardwoods, relatively harsher cooking conditions (e.g., higher cooking temperatures) at the end of the cooking have been discovered to decrease the g-lignin content which in turn decreases the percentage of pulp rejects at a given Kappa number. In preferred embodiments, methods and systems are provided for continuously producing chemical cellulose pulp from a slurry of comminuted hardwood material by subjecting the slurry of comminuted hardwood material to a first cooking stage under conditions sufficient to reduce syringyl lignin (s-lignin) content in the hardwood material as compared to guaiacyl lignin (g-lignin) content therein, and thereafter subjecting the slurry of comminuted hardwood material to a second cooking stage under conditions sufficient to reduce the g-lignin content remaining therein after the first cooking stage.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application is based on and claims domestic priority benefits under 35 USC §119(e) from U.S. Provisional Application Ser. No. 60 / 791,431 filed on Apr. 13, 2006, the entire content of which is expressly incorporated hereinto by reference.FIELD OF THE INVENTION[0002]The present invention relates generally to the field of chemical treatments of comminuted cellulosic fibrous material. In especially preferred embodiments, the present invention relates to processes and apparatus for the alkaline pulping of comminuted hardwoods.BACKGROUND AND SUMMARY OF THE INVENTION[0003]Chemical cooking of hardwoods can typically result in greater pulp rejects as compared to pulp obtained from the chemical cooking of softwoods at Kappa numbers of 17 and greater. In this regard, the present applicants have discovered that the middle lamella in hardwoods have substantial amounts of guaiacyl lignin (g-lignin) on a cellular level in the middle lamella as compa...

AI Technical Summary

Benefits of technology

[0005]It has now been discovered that rejects from cooking hardwoods at a given Kappa number can be reduced by subjecting the hardwoods to specific cooking conditions at the end of cooking. According to the present invention, therefore, alkaline cooking processes and apparatus, especially those useful for hardwoods, are provided which aim to increase the Kappa number of the hardwood pulp at the end of cook to levels above 17, but with much lower rejects levels. The present invention may be combined with an effective oxygen delignification stage to reduce the kappa number below 17. The present invention therefore contributes to benefits in respect to overall pulping yield and pulp strength.

Problems solved by technology

In order to increase pulp yield it would be desirable to cook hardwoods to a higher Kappa number but such a practice is conventionally not possible since the amount of rejects would increase to a point that would not be commercially feasible.

Thus, when cooking hardwoods, relatively harsher cooking conditions (e.g., higher cooking temperatures) at the end of the cooking have been discovered to decrease the percentage of pulp rejects at a given Kappa number as a result of g-lignin breakdown.

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