Chemical Engineering Journal 151 (2009) 302–307 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Mass transfer and thermodynamic aspects of sodium desorption from eucalyptus kraft pulp by acidification using carbon dioxide Denny Vitasari ∗ , Kien L. Nguyen, Vinh Dang Australian Pulp and Paper Institute, Department of Chemical Engineering, Monash University, Clayton VIC 3800, Australia article info Article history: Received 10 June 2008 Received in revised form 28 February 2009 Accepted 5 March 2009 Keywords: Pulp acidification Desorption Carbon dioxide Eucalyptus Kraft pulp abstract The thermodynamics and mass transfer aspects associated with the use of CO 2 for the acidification, par- ticularly for the removal of Na + from pulp fibres, were studied. The acidifications with CO 2 and with H 2 SO 4 were carried out in a stirred tank reactor under different temperatures, agitation speeds, concen- trations of CO 2 and H 2 SO 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 acidifi- cation. The equilibrium distribution of Na + in the solution and the fibre phase can be estimated using a model based on the Donnan theory. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The greater demand of environmentally friendly processes in the pulp and paper industry generates a need to replace existing chlorine-based process with oxygen-based processes for bleach- ing chemical pulps. Unfortunately the selectivity of oxygen-based bleaching processes depends on the deactivation or removal of metal ions in the pulp fibres. Acidification, which provides the source of proton for the replacement of metal ions sorbed on fibres, is one of the most effective methods to remove metal ions from the fibres. The use of mineral acids has disadvantages due to corrosion problem and the increasingly stringent environmental regulations on mill effluents and discharges [1]. Dissolved CO 2 in water forms a weak acid named carbonic acid. The carbonic acid provides an alternative proton source for acidification with less corrosion risk. CO 2 acidification method has been used in some commercial appli- cations [2] and the addition of CO 2 in pulp washing to lower the pH become a common practice in North European and North American Kraft mill [3]. However, details of the kinetics and thermodynamics of the process, particularly the application in the pulp and paper industry, have not been reported. The CO 2 to be dissolved can be obtained from the mill effluent itself. Therefore, using CO 2 for pulp ∗ Corresponding author. Tel.: +62 271 3060648. E-mail addresses: denny vitasari@ums.ac.id, denny.vitasari@gmail.com (D. Vitasari). acidification is not only eliminate the cost of chemical, but also solve the problems regarding CO 2 disposal. Wood fibres carry a negative charge when suspended in water due to the presence of ionisable acidic groups in the hemicelluloses, lignin, and cellulose [4,5]. The main ionisable groups of fibres are carboxyl groups, sulphonic acid groups, catecholic groups, phenolic groups, and hydroxylic groups [6]. Typically, the negative charge sites attract cations and ion exchange could happen [7]. Fibre - X + + Y + ↔ Fibre - Y + + X + (1) Under an acidic condition proton plays an important role in the displacement of cations from the fibres. Among the ionisable groups the carboxylic group ionises in neutral or weakly acidic conditions, while the phenolic group ionises in alkaline condition. The alcoholic hydroxyl groups are weak acids; therefore they are only ionised in a presence of a strong alkali [7]. The aim of this paper is to develop a method based on thermo- dynamic and mass transfer principles for studying the kinetics and equilibrium of Na + exchange in pulp fibres using CO 2 acidification. 2. Model development for CO 2 acidification There are three mechanistic steps involved in CO 2 acidification. The first step is the transfer of CO 2 from the gas phase into the liquid phase. The second step is the formation of carbonic acid from the hydrolysis of the CO 2 in the liquid phase. The last step is the exchange of proton with the metal ions sorbed in the fibres. 1385-8947/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.03.008