Date of Award

8-2004

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

Joseph M. Genco

Second Committee Member

Adriaan Van Heinigen

Third Committee Member

Barbara J.W. Cole

Abstract

Oxygen delignification involves a three phase reaction system; pulp fibers, and aqueous phase containing caustic and an oxygen gas phase. In this system, oxygen dissolves into the aqueous phase and is then transported through the liquid to the pulp fibers.Subsequently the dissolved oxygen diffuses into the fiber wall where it reacts with residual lignin in the presence of caustic. Although the transport of oxygen is an essential component in the overall process, few mass transfer studies have been conducted. The objective of the present study was to characterize the mass transport process in oxygen delignification systems. The present study was conducted in two phases. The focus of the initial work was to develop a mathematical model for the industrial oxygen delignification process. The second phase of the study involved the measurement of the liquid phase mass transfer coefficients in two laboratory reactors that have been widely used to estimate kinetic data. The simulation model was developed by using a liquid phase oxygen mass balance. The model considers mass transfer and chemical delignification reactions taking place in a high shear mixer followed by a medium consistency tower. The model assumes plug flow in the retention tower and utilizes the available literature data for mass transfer coefficients and chemical reactions rates. Typical oxygen delignification conditions were chosen as a base case and the applicability of the model was tested by comparing the predictions to actual oxygen softwood delignification data taken at a Kraft pulp mill. The results of the simulations suggest that very low mass transfer rates exist in the tower and this limits the degree of industrial oxygen delignification. It was concluded that virtually all delignification occurring in industrial systems takes place in the retention tower and very little in the high shear mixer. The liquid phase mass transfer coefficients (k1a) applicable to two laboratory reactors, designated the PAPRICAN and Parr reactors, were determined by using a variant of the sulfite oxidation method. Oxygen gas is allowed to react with sulfite liquor in what is essentially a liquid phase mass transfer controlled process. The total reactor pressure was monitored as a function of time and used to estimate the liquid phase mass transfer coefficients in the reactor. The mass transfer coefficients in the laboratory reactors were determined as a function of consistency, stirring speed and pulp suspension mass. Values for the mass transfer coefficients were determined to vary between 0.003 and 0.01 sec-1 depending upon the reactor geometry and process conditions. The mass transfer rates of the two laboratory reactors are compared using a new term called "specific mass transfer coefficient". A pseudo steady state analysis is also performed to quantify the effect of mass transfer limitations in the laboratory reactors and industrial towers. It was concluded that the oxygen delignification kinetic studies previously ocnducted using these laboratory reactors are not mass transfer limited. It was also concluded that the mass transfer in industrial oxygen delignification systems needs to b e greatly improved in order to reach maximum delignification.

Download

Share