Kinetic Study of Ozone Treatment on Chemical Pulp, 1996 Pulping Conference Proceedings

Peter X. Long and Jeffery S. Hsieh

Georgia Institute of Technology

Traditional bleaching technologies, such as chlorine and chlorine dioxide are facing increasingly strong environmental pressure from both government and public. New bleaching technologies have been investigated by numerous researchers for the past decade. Oxygen-based technology, such as oxygen, ozone, and peroxide are attracting lots of attention. This paper focuses mostly on ozonation of chemical pulp. Ozone has been used for chemical pulp bleaching commercially. Using ozone has lots of advantages, i.e., shorting production time, lowering operating cost and gaining technology advantages. However, there are several problems associated with using ozone. Low solubility of ozone in water/pulp slurry increases mass transfer resistance for effectively ozone bleaching. Fast reaction rate of ozone also makes it difficult to control the bleaching process. Moreover, ozone decomposition reaction in pulp slurry also increases mill’s operating cost. Currently, kinetic process of ozone brightening is not clear. The percentage of ozone decomposed in the pulp slurry is not known neither. These processes are very difficult to monitored and some of the radicals generated during brightening and decomposition reactions are impossible to measure. However, these reactions are very important to mill’s operation. Thus an effective model is needed to elucidate the mechanism from a kinetic process point of view. This paper has investigated ozone bleaching kinetics of chemical pulp. Ozone gas is a very strong oxidant. However, due to ozone’s low solubility in water, the reaction between ozone and lignin is mass-transfer limited. An estimated overall mass transfer coefficient was used for the model. Two water layers (two films) were used to simulate the bleaching conditions on the fiber surface. A two-film theory model, incorporated with ozone self decomposition reaction, was used to model the bleaching process. Ozone self decomposition was modeled as a one and half order reaction with respect to ozone concentration in the pulp slurry. The pH condition was modeled using OH• calculated from decomposition mechanism. Comparison between literature experimental and model data was given. An overall rate equation was given with regressed values for rate constants, activation energy and reaction order for medium consistency. This paper will also try to apply the model to the actual mill process. In conclusion, with this model, we are able to predict the actual ozone operating conditions for the mill and develop certain means to control the operating cost. It also provide us with a better understanding of the kinetic process.