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Drying kinetics; a comparison of diffusion coefficients from moisture concentration profiles and drying curves

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  • Mathematics


PII: 0009-2509(94)00494-C Pergamon Chemical Enaineerir,a Science, Vol. 50, No. 7, pp. 1187-1191, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009-2509/95 $9.50 + 0.00 0009-2509(94)00494-3 DRYING KINETICS: A COMPARISON OF DIFFUSION COEFFICIENTS FROM MOISTURE CONCENTRATION PROFILES AND DRYING CURVES A. A. J. KETELAARS,* U PEL, *~ W. J. COUMANS* and P. J. A. M. KERKHOF* *Department of Chemical Engineering (TCP), and tDepartment of Architecture and Building Technology (FAGO), Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands (Received 1 August 1993; accepted in revised form for publication 14 October 1994) Abstract--The mathematical formulation of mass transfer in drying processes is often based on the diffusion equation. In principle the diffusion coefficient as a function of moisture content has to be determined experimentally. The most direct approach is to derive the diffusion coefficient from experi- mental moisture concentration profiles in the material during drying. In this work, the diffusion coefficient determined in this way is called the actual diffusion coefficient. Very often, however, an indirect method is used based on drying curves (average moisture content of a sample vs time). Since the diffusion coefficient is determined indirectly from the macroscopic behaviour of the sample, this coefficient will be called the apparent diffusion coefficient. A comparison of diffusion coefficients as a function of moisture content using both methods shows that for porous materials such as clays, the apparent diffusion coefficient depends on experimental conditions. This can be explained by the fact that the apparent diffusion coefficient is in agreement with the actual diffusion coefficient over a limited range of moisture content only. For materials such as clays, drying curves are not suited to derive diffusion coefficients as a function of moisture content in a satisfactory a

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