Transient temperature and concentration fields in highly loaded packed bed adsorbers
Final Report Abstract
Adsorption in packed beds has been investigated under severe operating conditions of high load, and high wall and thermal effects, as frequently occurring in applications of the process and environmental industry. To this purpose, relatively large zeolite particles were packed in a relatively narrow, wall-cooled tube, with water as the adsorptive and air as the inert gas. Due to the influence of the wall and to the thermal effect, radial profiles of concentration and temperature exist within as well as at the outlet of the adsorber. Consequently, the breakthrough of the adsorptive must be analysed not only in respect to time, but also in respect to the radial position. To achieve this task, a novel tomographic technique of near infra-red spectroscopy has been developed and applied. This technique is non-invasive, enables higher time and space resolutions than conventional methods, and might be of interest also for other applications, especially in the field of drying technology. On the other hand, transient concentration and temperature fields have been calculated numerically by means of a two-dimensional model for heat and mass transfer in the packed bed. The model considers the increase of porosity and flow velocity near the tube wall, expresses the effective transport coefficients as functions of the radial coordinate, and has been extensively validated for pure transport phenomena or wall-cooled, catalytic packed bed reactors. Necessary data on particle-side adsorption kinetics and equilibrium have been determined separately by means of a magnetic suspension balance. This technique is also interesting for other applications in, e.g., drying or catalysis. Both the measurements and the calculations reveal that the described two-dimensional modelling is mandatory in order to reliably simulate the operation of packed bed adsorbers with tube-to-particle diameter ratios of less than about twenty. Such adsorbers can not be treated with conventional, onedimensional approaches. Wall (channelling) effects, thermal effects and the influence of intraparticle kinetics combine in the investigated region of operating conditions in an intimate and complex way. Specific experiments with dilution and annular geometry have been designed and conducted in order to weaken the influence of heat release or intraparticle diffusion in relation to the wall effect, coming closer to important computational limiting cases and practical applications. The model predicts correctly the experimentally observed trends and performs in the total stisfactory. The conducted research provides a mature state of continuous, quasihomogeneous modelling of packed bed adsorption and progresses in the experimental techniques of near infrared tomography and magnetic suspension gravimetry. Further advancement of modelling is still possible by more detailed description of the stationary phase, more fundamental calculation of topology and flow, better coupling between transport phenomena, and better consideration of the dynamic character of the process. Large potential is seen in the application of the model in order to account for thermal effects in chromatography and electrophoresis. Additional applications of the experimental techniques are seen in drying and catalysis.
Publications
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Kwapinski, W., Tsotsas, E., Determination of kinetics and equilibria for adsorption of water vapour on single zeolite particles by a magnetic suspension balance, Chem. Eng. Technol., 27, 681-686,2004
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Kwapinski, W., Winterberg, M., Tsotsas, E., Mewes, D., Modeling of the wall effect in packed bed adsorption, Chem. Eng. Technol., 27, 1179-1186, 2004
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Salem, K., Kwapinski, W., Tsotsas, E., Mewes, D., Nah-Infrarot Tomographie zum Messen von Konzentrationsfeldern im Festbett, Chem. Ing. Tech., 76, 9, 2004b
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Salem, K., Tsotsas, E., Mewes, D., Tomographie measurement of breakthrough in a packed bed adsorber, Chem. Eng. Sei., 60, 517-522,2005
