Zheng Dang

Date of Award

8-2002

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

Joseph M. Genco

Second Committee Member

Adriaan R.P. van Heiningen

Third Committee Member

Barbara J.W. Cole

Abstract

Oxygen delignification has developed steadily since it was introduced commercially in 1970 in South Africa and virtually all new fiber lines incorporate an oxygen delignification system following the digester prior to the bleach plant. Currently, the degradation of cellulose and consequently the loss of pulp strength limit the drop in kappa number that can be obtained using an oxygen process. Pulp selectivity, given by the selectivity coefficient (α), relates the reduction in the molecular weight of cellulose to the reduction in the ligin content of the pulp when measured by the kappa number. An evaluation of selectivity of deligification data published by Agarwal (1998) for mixed southern hardwood treated in a single stage oxygen process showed that temperature has little effect on pulp selectivity within the temperature range of 90 to 110°C. Rather, pulp selectivity is determined primarily by the addition rate of caustic and thus, the caustic concentration present in the reactor during the oxygen process. The objective of this thesis was to investigate three pulp pretreatment techniques known to improve pulp selectivity by removing metal ions that enhance the formation of hydroxyl free radicals that degrade cellulose by random hydrolysis reactions. The techniques investigated were pretreatment of the pulp using DTPA, termed a Q-stage, pretreatment by washing the pulp with acid (A-stage), and acid washing the pulp with a small amount of Cl02 present (AD-stage). In addition to removing metals prior to oxygen delignification, the use of Cl02 in an AD-stage is thought to activate the lignin in the pulp to promote a high level of delignification. The three pretreatment techniques investigated were compared to control experiments that were conducted with no pulp pretreatment. Each of the four delignification conditions was investigated in both a stainless steel reactor and in glass to test the hypothesis that metals are leached from the stainless steel reactor and influence the results. All of the experiments were performed in quadruplicate with and without H2O2 reinforcement in the oxygen stage. Response variables in this investigation were the kappa number, intrinsic viscosity, degree of delignification, and the selectivity coefficient (α). The effectiveness of the various conditions on the response variables was determined using the student t-test. This thesis was limited to the study of northeastern brownstock krafi pulp. The results of the experimental investigation showed that all of the pretreatment techniques improved the selectivity of oxygen delignification. Metals present in the stainless reaction vessel contributed little to the reduction in kappa number except for the case of the AD stage where the stainless steel reactor improved the delignification. Also, use of the stainless steel reactor decreased the selectivity only in the DTPA pretreatment case. Use of the AD pretreatment stage led to additional delignification compared to the no pretreatment case and also when compared to the Q- and A-stage pretreatment techniques. Lastly the use of H2O2 to reinforce an oxygen stage resulted in an improvement in the level of delignification but a loss in selectivity. This was true for all of the cases evaluated except for the oxygen stage with AD pretreatment where no difference was detected in delignification when the reaction was conducted in stainless steel. DTPA was extremely effective in controlling metals and no drop in selectivity was observed even for the case of using H2O2 reinforced oxygen delignification.

Download

Share