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The use of drying experiments in the study of the effective thermal conductivity in a solid containing a multicomponent liquid mixture
Faculty of Chemical Engineering, National University of Engineering (UNI), Managua, Nicaragua.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Transport Phenomena. (Drying & Process Technology)
2012 (English)In: Chemical engineering research & design, ISSN 0263-8762, E-ISSN 1744-3563, Vol. 90, no 11, 1765-1778 p.Article in journal (Refereed) Published
Abstract [en]

The effective thermal conductivity of a porous solid containing multicomponent liquid mixtures has been studied. To achieve this, the liquid composition, liquid content and temperature distributions have been measured in a cylindrical sample dried by convection from the open upper side and heated by contact with a hot source at the bottom side. A quasi-steady state reached at high source temperatures permits to calculate the total heat flux from temperatures measured on the surface and the gas stream. The simulations performed and compared with experimental data made it possible to estimate the adjusting geometric parameter of Krischer's model for the effective thermal conductivity. The effective thermal conductivity has been widely studied for two-phase systems, mostly with regard to thermal insulation elements. The calculation of this transport parameter includes the contribution to heat transfer of the evaporation–diffusion–condensation mechanism undergone by the multicomponent mixture. The influence of liquid composition and temperature on the thermal conductivity due to the evaporation–diffusion–condensation mechanism and the effective thermal conductivity is described. The results reveal that in this case the resistance to heat transfer seems to correspond to a parallel arrangement between the phases.

Place, publisher, year, edition, pages
2012. Vol. 90, no 11, 1765-1778 p.
Keyword [en]
Diffusion, Evaporation, Heat transfer, Phase equilibrium, Temperature gradient
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-28894DOI: 10.1016/j.cherd.2012.04.003ISI: 000312431400008Scopus ID: 2-s2.0-84868354876OAI: oai:DiVA.org:kth-28894DiVA: diva2:390483
Note

QC 20121214

Available from: 2011-01-21 Created: 2011-01-21 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Transport Coefficients during Drying of Solids containing Multicomponent Mixtures
Open this publication in new window or tab >>Transport Coefficients during Drying of Solids containing Multicomponent Mixtures
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This study investigated the transport coefficients involved in mass and heat transfer during the drying of a porous solid partially saturated with multicomponent mixtures.  It included the coefficients governing liquid transport through the solid, the matrix of multicomponent diffusion coefficients in the liquid phase, and the effective thermal conductivity.  As it is not possible to determine these coefficients by theoretical considerations alone and considerable experimental work is required to determine them in a broad range of process conditions, the principle of this study has been the use of mathematical models complemented with some empirical parameters.  These empirical parameters were determined by comparison between measurements in specially designed experiments and the results of mathematical models that describe the process.  In addition, the application of the multicomponent diffusion coefficients is described in two cases where liquid diffusion is important: convective evaporation of a multicomponent stationary liquid film and a falling film.

To study liquid transport through the solid, isothermal drying experiments were performed to determine the transient composition profiles and total liquid content of sand samples wetted with ternary liquid mixtures with different initial compositions and temperatures.  A mathematical model including mass transfer by capillary movement of the liquid and interactive diffusion in both the gas and liquid phases was developed.  To simulate the capillary movement of liquid mixtures, parameters experimentally determined for single liquids were weighed according to liquid composition. A fairly good agreement between theoretical and experimental liquid composition profiles was obtained considering that axial dispersion was included in the model.

To study the matrix of multicomponent diffusion coefficients in the liquid phase, the redistribution of liquid composition in a partially filled tube exposed to a longitudinal temperature gradient was analysed.  Experimental work was carried out using two main ternary mixtures with different initial compositions and temperature gradients.  Experimental data were compared with the results of a theoretical model that describes the steady-state liquid composition distribution in a partially filled non-isothermal tube to find the empirical exponent that modifies the matrix of thermodynamic factors.  Correlations for the exponents as a function of temperature were determined for each particular multicomponent mixture.

The effective thermal conductivity of a porous solid containing multicomponent liquid mixtures was studied by measuring the liquid composition, liquid content and temperature distributions in a cylindrical sample dried by convection from the open upper side and heated by contact with a hot source at the bottom side.  Simulations performed at a quasi steady state were compared with experiments to estimate the adjusting geometric parameter of Krischer’s model for effective thermal conductivity, which includes the contribution of the evaporation-diffusion-condensation mechanism. The results revealed that a resistance corresponding to a parallel arrangement between the phases seems to dominate in this case.

In the study of the convective drying of a multicomponent stationary liquid film, the equations describing interactive mass transfer were decoupled by a similarity transformation and solved simultaneously with a conduction equation by the method of variable separation.  Variations of physical properties along the process trajectory were taken into account by a stepwise application of the solution in time intervals with averaged coefficients from previous time steps.  Despite simplifications, the analytical solution gives a good insight into the selectivity of the drying process and is computationally fast.  On the other hand, numerical simulations of the convective evaporation of the multicomponent falling liquid film into an inert gas with a co-current flow arrangement of the phases almost always revealed a transition from liquid-phase-controlled conditions to a process in which neither the gas nor the liquid completely controls the evaporation.

The results obtained in this work would be useful in implementing models to improve the design, process exploration and optimisation of dryers by incorporating the solid-side effects to describe the drying of liquid mixtures along the whole process.

Place, publisher, year, edition, pages
Stockholm: KTH, 2011. xii, 85 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 5
Keyword
capillary, conduction, convection, diffusion, evaporation, heat transfer, hydraulic conductivity, liquid film, liquid transport, mass transfer, Maxwell-Stefan diffusion coefficients, molar fluxes, phase equilibrium, temperature gradient, ternary mixture, thermodynamic factors
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-28897 (URN)978-91-7415-850-2 (ISBN)
Public defence
2011-02-11, E3, entréplan, Lindstedtsvägen 3, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110124Available from: 2011-01-24 Created: 2011-01-21 Last updated: 2012-03-23Bibliographically approved

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