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

The thermodynamics and mass transfer aspects associated with the use of CO2 for the acidification, particularly for the removal of Na+ from pulp fibres, were studied. The acidifications with CO2 and with H2SO4 were carried out in a stirred tank reactor under different temperatures, agitation speeds, concentrations of CO2 and H2SO4 and treatment times. The CO2 acidification of pulp was found to be very fast at Re > 104 and Np of 4, and reached equilibrium within 5 min. The volumetric mass transfer coefficient, kLa, of the CO2 acidification was found to be in the range of 0.0005–0.0083 s−1. Thermodynamically, the CO2 acidification was achievable only at pH > 4. The extent of the resultant Na+ exchange increased with the acidification time, agitator speed and CO2 flow rate. Under similar experimental conditions, the extent of Na+ desorption by CO2 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.