A non-equilibrium thermodynamics approach to model mass and heat transport for water pervaporation through a zeolite membrane |
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Authors: | Jelan Kuhn Robert Stemmer Freek Kapteijn Signe Kjelstrup Joachim Gross |
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Institution: | aProcess & Energy Department - Engineering Thermodynamics, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands;bCatalysis Engineering - The Pore, DelftChemTech, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands;cDepartment of Chemistry, Norwegian University of Science and Technology, Realfagbygget, D3, NO-7491 Trondheim, Norway |
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Abstract: | Pervaporation through zeolite membranes involves local heat effects and combined heat and mass transport. The current state-of-the-art Maxwell–Stefan (M–S) models do not take these effects into account. In this study, transport equations for the coupled heat and mass transport through a zeolite membrane are derived from the framework of non-equilibrium thermodynamics (NET). Moreover, the assumption of equilibrium between the adjacent bulk phases at the feed and permeate sides of the zeolite layer is abandoned in favor of local equilibrium. The equations have been used to model pervaporation of water through a 2 m thick NaA type zeolite membrane, deposited on an asymmetric -alumina support, at a feed temperature of 348 K. Assuming a flux of 10 kg m−2 h−1(0.15 mol m−2 s−1), the transport through the zeolite layer, as well as the liquid feed side boundary layer and the support layers is modeled. The activity, fugacity, and temperature profiles are calculated with and without taking coupling effects and surfaces into account. The profiles show distinct differences between the two cases. Including the surface effects leads to discontinuities in the activity and temperature at the membrane interfaces. A significantly higher temperature drop of 1.3 K is calculated across the zeolite, compared to 0.4 K when surface and coupling effects are not accounted for. The calculated decrease in temperature over the zeolite layer is dominated by the surfaces. This could indicate that temperature polarization is, to a large extent, a surface effect. The heat flux induces an extra driving force for mass transport, reducing the activity difference over the membrane. A positive jump in activity is observed at the interfaces, revealing the mass transport across the interfaces is governed by the coupling with the heat flux. The support layers contribute significantly to the total mass transport resistance. |
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Keywords: | Zeolite Non-equilibrium thermodynamics Surface Membrane Pervaporation |
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