Liquid phase diffusion of branched alkanes in silicalite

Abstract

International audienceThis work provides a new mass transfer model based on the Maxwell-Stefan theory, especially adapted to represent adsorbed phase multicomponent diffusion at high-adsorbent loading. In our model - contrarily to the well-known model developed by Krishna et al. (Chem Eng Sci. 1990;45:7:1779-1791; Gas Sep Purif. 1993;7:91-104; J Phys Chem B. 2005;109:6386-6396) - the hypothesis that the micropores are saturated does not imply a dependency between the adsorbed phase diffusion coefficients. Experimental liquid phase breakthrough curves of 2-methylpentane (2MP), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB), and 2,2-dimethylbutane (22DMB) were measured at 458 K in silicalite. The self-diffusion coefficients and Langmuir parameters of the different species were determined using binary exchange breakthrough curves. The Maxwell-Stefan diffusion coefficients obtained for the different isomers are in the order D3MP,nc+1 > D2MP,nc+1 D23DMB,nc+1, and vary between 4 × 10-15 m2 s-1 for 3MP to 6 × 10-16 m2 s-1 for 23DMB. The 22DMB diffusion coefficient is so low that it could not be estimated (the quantity of 22DMB entering silicalite during the experiment is not significant). The model was then validated by comparing experimental breakthrough curves at different feed concentrations and simulations using the independently estimated parameters. Even though the diffusion coefficients of the different isomers vary by one order of magnitude, the agreement between simulated and experimental curves is very satisfactory, showing the good predictive power of our model

A transient method for mass‐transfer characterization through supported zeolite membranes: Extension to two components

“…A similar behavior has also been experimentally obtained for diffusion of branched paraffins in silicalite‐1 by using the inverse chromatographic technique in the linear domain . On the one hand, by considering the Langmuir monosite model, on the other hand, by assuming that the MS friction coefficient is proportional to the vacant site loading according to , the Fick diffusion coefficient should be a function of . One can see in Figure that this behavior seems to be experimentally confirmed.…”

“…where is the loading of the j component with respect to a maximum loading that can possibly depend on the adsorbate . In this formulation, the friction force due to the adsorbent is assumed to be proportional to the vacant sites' fraction . These equations can be inverted in order to express the flux as a function of ∇μ z,1 and ∇μ z,2 In order to use the change of state variable that we propose, the chemical potential gradient is simply expressed as as the fictitious gaseous phase at concentrations is assumed to be ideal.…”

Sieving di-branched from mono-branched and linear alkanes using ZIF-8: experimental proof and theoretical explanation

“…[5][6][7] Especially zeolites are extensively studied in this context, due to their well-defined pores, the diameters of which are close to those of alkane molecules. 7 The most studied structures for separating hexane isomers are ZSM-5, [8][9][10][11] silicalite-1, [11][12][13][14][15][16][17][18][19][20][21][22] zeolite beta, [23][24][25][26][27][28][29] and mordenite. [30][31][32] Compared to this, there are relatively few studies on applying MOFs for separating hexane isomers, mainly using UiO-66(zr), 33 Zn(BDC)(DABCO) 0.5 , 34,35 MIL-47, 36 Cu(hfipbb)(H 2 hfipbb) 0.5 , 37 and Zn(BDC)(4,4 0 -Bipy) 0.5 (MOF-508).…”

Integrated Stefan–Maxwell, Mean Field, and Single-Event Microkinetic Methodology for Simultaneous Diffusion and Reaction inside Microporous Materials

“…Herein, K i L constitutes the Langmuir physisorption coefficient of i. Even though equivalent in nature, eqs 10 and 11 are slightly different from those reported by Lettat et al, 42,43 who considered volume fractions instead of site occupations. In cases where physisorption is considered a form of friction against mass transport, the surface Stefan−Maxwell or corrected diffusion coefficient D ̃i in eq 10 is related to the inverse of a drag coefficient quantifying the drag force between sorbate species i and the sorbent only.…”

“…The corrected diffusion coefficient estimates, D ̃i, are situated around the lower limit of the reported range of diffusion coefficients obtained from macroscopic measurements. 42,72,86 The discrepancy between results from macroscopic and microscopic measurement techniques has been the subject of much research 67−69 and is attributed to the experimental technique itself and not to the methodology used to extract the corrected diffusion coefficients from the obtained data. This was demonstrated by Jobic et al 87 and Millot et al, 73 who applied the Stefan−Maxwell formulation to determine the corrected diffusion coefficient of various alkanes from respectively quasi-elastic neutron scattering (QENS) and permeation measurements, the former being a microscopic technique while the latter is classified as a macroscopic technique.…”