The use of mixing-sensitive chemical reactions for the study of pulp fibre suspension mixing

• Main Author: Thangavel, V. K.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/5445

The competitive, consecutive azo coupling between 1-naphthol and diazotized sulphanilic acid was used to investigate the mixing of a pulp fibre suspension in a stirred tank reactor operated in a semi-batch mode. The distribution between the mono and bis substituted product dyes depended upon the turbulent intensity in the reaction zone and was used to indicate the degree of mixing in the vessel. Coupling reactions were carried out under conditions where macromixing did not influence the product distribution and were used to indicate the turbulent dissipation and microscale mixing in the reaction zone. When fibres were present in the reactor some of the product dyes were adsorbed onto the fibres. Adsorption rate was not proportional to the equilibrium concentration of the dyes and it was necessary to correct the measured product distribution to account for the differential adsorption rates. Once this correction had been made the product distribution correctly indicated the degree of mixing. As the dyes were found to have different affinities for different pulps, separate correlations would have to be developed for each pulp tested. All results reported here were obtained with a commercially produced fully bleached kraft pulp. Tests were conducted in the stirred vessel using aqueous media and at fibre suspension concentrations up to 2.5% by mass. Test results in aqueous media compared favourably with past work and a predictive model developed by Baldyga and Bourne. In the presence of fibres a reduction in turbulent energy dissipation was detected under most operating conditions when compared with the aqueous tests. When the suspension mass concentration was increased to the point where the vessel contents ceased to be in complete motion the product distribution began to increase due to macroscale mixing effects. The technique could not be applied successfully to a high-shear mixer due to the high energy dissipation rates and dye adsorption, both of which reduced the product distribution below the analytical detection limit. While other mixing sensitive chemical reactions are available to permit mixing determination at these high dissipation rates, the extension of the reaction system to more than 3 adsorbable components would make the measurements difficult to conduct and interpret.