Ph.D. Proposal Presentation by Muhammad K. Akbar
Monday, November 11, 2002
(Drs. S. M. Ghiaasiaan and Seppo Karrila, co-advisors)
"Mass Transfer and Oxidation Kinetics in Low and Medium Consistency Fibrous Suspensions"
Oxygen and ozone-treatment of slurries is relatively common in environmental and waste management applications, and is gaining widespread use in paper bleaching. Bleaching is a process in paper-making in which residual lignin is oxidized and removed from cellulose fibers. Bleaching has been performed in the past using chlorine as the oxidizing agent. Oxygen and ozone-based bleaching are replacing chlorine-based methods due to environmental considerations, however. The low solubility of oxygen and ozone in water, the tendency of fibers to form three-dimensional networks and flocks, and the tendency of ozone to cause carbohydrate degradation in addition to delignification, render the bleaching process extremely complicated. The objective of this study is to investigate the basic mass transfer and chemical kinetics associated with low (LC) and medium consistency (MC) aqueous pulp suspensions.
The hydrodynamics and mass transfer in a low consistency suspension will be studied in a test facility that includes an instrumented, vertical test section 5 cm in diameter. In one group of tests, a mixture of N2 (the inert component) and CO2 (transferred species) will be used as the gas phase, and alkalized fiber suspensions of various consistencies, in the range of LC, as the liquid phase. By measuring the absorption rate of CO2, and using the Danckwerts’ surface renewal model, the interfacial area concentrations will be measured and modeled and/or correlated. The interfacial mass transfer resistance associated with the transport of oxygen will be measured and modeled and/or correlated in another test group, using a mixture of N2 and O2 as the gas phase, and using the cobalt-catalyzed sulfite oxidation technique. In both of test groups, the flow patterns will be visualized, and the void fraction will be measured by Gamma-ray densitometry.
MC pulp suspensions act as solid-like wet fiber networks, and need high shear
to flow and mix. Oxidation kinetics of a MC system will be studied using a scaled
high-shear mixer that has a mixer dissipative power density similar to prototypical
systems. The relevant characteristics of pulp fibers will be microscopically
measured before and after oxygen and ozone treatment, and the extent of lignin
oxidation and fiber degradation will be quantified as functions of various system
parameters. Using flash X-ray photography, the occurrence and characteristics
of macroscopic three-dimensional structures in the slurry produced by the mixer
will be examined. The relative significance of mass transfer resistances associated
with diffusion in water, diffusion in the oxidized layer of material covering
the intact fibers, and the kinetics of the oxidation process, will be assessed.
The applicability of a recent theory (the shrinking core model) to the data
will be examined.