Mass transfer and thermodynamic aspects of sodium desorption from eucalyptus kraft pulp by acidification using carbon dioxide

Abstract

The thermodynamics and mass transfer aspects associated with the use of CO 2 for the acidification, particularly for the removal of Na + from pulp fibres, were studied. The acidifications with CO 2 and with H 2SO 4 were carried out in a stirred tank reactor under different temperatures, agitation speeds, concentrations of CO 2 and H 2SO 4 and treatment times. The CO 2 acidification of pulp was found to be very fast at Re > 10 4 and N p of 4, and reached equilibrium within 5 min. The volumetric mass transfer coefficient, k L a, of the CO 2 acidification was found to be in the range of 0.0005–0.0083 s −1. Thermodynamically, the CO 2 acidification was achievable only at pH > 4. The extent of the resultant Na + exchange increased with the acidification time, agitator speed and CO 2 flow rate. Under similar experimental conditions, the extent of Na + desorption by CO 2 acidification was found to be slightly lower than that achieved by sulfuric acidification. The equilibrium distribution of Na + in the solution and the fibre phase can be estimated using a model based on the Donnan theory.