Date of Award

Spring 5-6-2022

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor

Adriaan van Heiningen

Second Committee Member

Douglas W. Bousfield

Third Committee Member

M. Clayton Wheeler

Additional Committee Members

Thomas J. Schwartz

Mehdi Tajvidi

Abstract

Oxygen delignification is a pre-bleaching stage that provides environmental benefits for bleached pulp production by replacing about 50% of the chlorine-based bleaching chemicals with oxygen. The resulting reduction in chlorine-based chemicals leads to a significantly lower release of chlorinated organics in the pulp bleaching effluent. However, the main disadvantage of applying oxygen delignification is the low delignification/cellulose degradation selectivity compared to chlorine dioxide bleaching. Modern conventional oxygen delignification is limited at level 60% delignification to maintain an acceptable cellulose Degree of Polymerization (DP) and thus fiber strength as indicated by the pulp intrinsic viscosity. The increase in the degree of delignification from 60 to 80% leads to a reduction by a factor 2 of the required bleaching chemicals and also reduces the environmental impact of the bleach plant effluent by at least a factor of two. However, to achieve this increase in the degree of delignification the delignification/cellulose degradation selectivity must be improved, and therefore the objective of the present research is to understand and improve the selectivity especially at high degrees of delignification.

The technical objective of the present PhD project was to perform extended oxygen delignification up to 80% of conventional Southern Pine kraft pulp (about 24 kappa) while maintaining acceptable cellulose DP as calculated from the measured pulp intrinsic viscosity. More importantly, the fundamental objective of the research is to understand how to increase the delignification/cellulose degradation selectivity during oxygen delignification of conventional softwood kraft pulp (about 24 kappa) in particular at very high degrees of delignification.

In the present study, we found that the oxygen delignification of industrial softwood pulp consists of two contributions; phenolic delignification and alkaline leaching delignification.

In addition, we could extend the oxygen delignification up to 80% of conventional Southern Pine kraft pulp (about 24.4 kappa number) while maintaining acceptable intrinsic viscosity (>700 mL/g) in the presence of 2.186 g/L Na2CO3 at 100 oC for 90 minutes of oxygen delignification. But, for the conventional kraft pulp, the selectivity is not affected by the addition of 100 ppm NaBH4 because of the absence of carbonyl groups on cellulose in the kraft pulp, and no improvement in pulp viscosity and selectivity is seen.

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