The Zeolite ZSM-5 membrane study; synthesis, permeation and modeling.
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The Zeolite ZSM-5 membrane study; synthesis, permeation and modeling. Bovornlak Oonkhanond, Michael E. Mullins Chemical Engineering Department Michigan Technological University 1400 Townsend Drive Houghton, MI 49931 ABSTRACT Zeolite particles formed in the synthesis solution assume a negative charge due to electrical double layer effects. The membranes synthesized via electrophoretic deposition produce a thin continuous zeolite ZSM-5 layer on the porous alumina substrates. The thickness of membranes can be controlled by varying the precursor concentration, applied potential, and synthesis time. The permeation for a variety of gases through the membranes is examined. The diffusion properties are measured at near ambient temperature and pressure. Selectivity of nbutane over iso-butane is observed on every membrane. The diffusion coefficient of gases on ZSM-5 is also evaluated by gas adsorption on zeolite powders. The results of these studies show an ability to predict the effective diffusion coefficients. The estimation of molar fluxes across the membranes using the Maxwell-Stefan approach predicts a higher flux for thinner membranes. However, the calculations also show that even minor defects in the membrane have a great effect on the permeation rates. INTRODUCTION Zeolite membranes offer the possibility of true molecular “sieving” during separation of large and small molecules. However, traditional hydrothermal zeolite membrane fabrication techniques produce a membrane layer on the order of 10 to 100 microns, yielding low fluxes for gas separation processes. Thinner, continuous membranes might provide higher, more practical molecular fluxes. To accomplish this goal, improved membrane growth processes are required. Since the zeta potentials of both zeolite ZSM-5 and the α-alumina substrate in the synthesis solution are negative, -38 and –62 mV respectively, the deposition of ZSM-5 crystals onto the surface can be inhibited [1]. By incorporating electrophoretic deposition during the regular hydrothermal synthesis process, the zeolite crystals can be induced to the substrate surface. The authors have previously shown that thinner, high quality ZSM-5 membranes may be produced in this fashion. This paper is concerned with measuring and modeling the diffusive flux (Ns) of molecules across the membranes prepared. The permeation of various gases across the ZSM-5 membranes has been compared to the model based on the Maxwell-Stefan approach suggested by Krishna [2,3]. (1) N s = − ρ p εq sat D1s ∇θ 1 D1s =
D1sv 1 − θ1
(2)
Diffusion coefficients (D) are estimated based on either bulk diffusion or surface diffusion depending on the size of the gas molecule. Argon which has a relatively small kinetic diameter (~3.4 Å) compared to the pore opening of the zeolite may be described by a bulk
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diffusion mechanism. By considering the gas phase concentration change in the system, the partial differential equation for mass transfer in a sphere based on Fick's law is used. P=0 ∂P ∂ P =D 2 ∂t ∂r 2
0≤r
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