Two-phase flows in gas-evolving electrochemical applications

by Wetind, Ruben

This thesis deals with two-phase ows in gas-evolving electrochemical applications. Many important electrochemical processes involves evolution of gas. Some examples are the chlorate process, the chlor-alkali process, aluminium electrolysis in chryolite baths, zinc electrowinning as well as the pickling of steel. The presence of gas can a ect the processes in di erent ways. In the idealised case the rising gas bubbles induce an internal circulation in the reactor which continuously carries fresh electrolyte to the electrodes. On contrary, the worst case involves a dramatic conductivity deterioration in the gas/electrolyte as well as a reduction of active electrode area due to partwise coverage of gas. As a matter of fact, it appears that a small modi cation might be enough to turn a well working process into a hardly working, or the opposite way. The aim of this thesis is to investigate some of the factors which maya ect the e ciency of a gas-evolving electrochemical system. The major part of this thesis involves numerical simulations using two di erent hydrodynamical two-phase models. The rst, a two- uid model with inclusion of turbulence for the liquid phase, is used for a full scale study of the buoyancy induced circulation in a conceptual gas-evolving chlorate reactor. Design matters are discussed and some explicit recommendations, based on results from this study, are presented. An analytical relation for prediction of the electrolyte circulation rate is further suggested. Secondly, a mixture model is used for studies of both forced and free convection of the bubbly ow between two electrodes. Necessary closure laws for the relative velocity between the phases are based on empirical relations for particle suspensions. The mixture model is further extended to include mass-transport of ionic species with the aim to predict non-uniform current densityalong the electrodes. Parameter studies show that the bubble size has a major in uence on both distribution and magnitude of the buoyancy induced velocity. Complementary experimental data from bubbly ow inavertical pipe is presented together with tests of new experimental equipment and measuring techniques. The micro-bubble suspension is investigated by mean of image analysis and the use of a new bre optics velocimeter. Typical results include the distributions of void fraction and velocity. A special device for electrochemical production of hydrogen bubbles is constructed and tested. The measured size distribution of the bubbles shows that the appliance can be used to generate bubbles of size 100 40 m Descriptors: two-phase ow, electrochemical, reactor, chlorate, micro-bubbles, twouid model, numerical modelling, mixture model, drift- ux model, gas-evolving, electrolysis, current distribution, dispersion, image analysis, laser-sheet, bre optics. List of papers Paper 1. Wedin R., Dahlkild A. A numerical and analytical hydrodynamic twophase study of an industrial gas-lift chlorate reactor. Computational technologies for uid/thermal/structural/chemical systems with industrial applications 1, ASME (1999), PVP-Vol. 397-1, 125-136. Paper 2. Wetind R., Dahlkild A. A hydrodynamical model for the dispersal of small bubbles under developing channel ow. Submitted to J. Appl. Electrochem., (2001) Paper 3. Wedin R., Davoust L., Cartellier A., Dahlkild A. A mono-modal beroptics velocimeter for electrochemically generated bubbles. Proceedings of Tenth International Symposium on Applications of Laser Techniques to Fluid Dynamics, Lisbon, (2000) Paper 4. Wedin R., Davoust L., Cartellier A., Byrne P. Experiments and Modeling on Electrochemically generated bubbly ows. Presented at 4th ICMF New Orleans 2001. To appear in special issue of Elsevier journal- ETFS. Paper 5. Wedin R., DahlkildA.Onthe transport of small bubbles under developing channel ow inabuoyant gas-evolving electrochemical cell. Presented at CRE VIII 24-29 June (2001), Barga, Italy To appear in special issue of Ind. Eng. Chem. Res. 2001 Paper 6. Wetind R. Modelling the mass transport and two-phase ow in a gasevolving electrochemical cell. Submitted toJ.Electrochem. Soc. (2001) NOTE: The author has during the course of work altered the family name from Wedin to Wetind.