Liquid phase diffusion of branched alkanes in silicalite



This 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+1D23DMB,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. © 2010 American Institute of Chemical Engineers AIChE J, 2011