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itanium silicate molecular sieves are a new class of materials, which provide uniform pore size like aluminosilicate zeolites. Over the last ten years, Engelhard Corporation has developed and patented a family of titanium silicate (aNa2O:bTiO2:ySiO2:zH2O) molecular sieves and has named them ETS-4, ETS-10 and ETS-14. |
Small pore ETS-4 is structurally more interesting. It has a distinct octahedral-tetrahedral structure, as shown in Figure 1. The structure has faults in the [100] and [001] directions, that is, along the a and c axes. In other words, the 12-membered ring (12MR) is always blocked and is therefore inaccessible to diffusing molecules. This faulting, however, does not block the 8-ring pores (8MR) in the b direction. Thus, the channels formed by the 8MR along the b direction are the sole channels for transport of diffusing molecules.
| Figure 1: Schematic representation of ETS-4 structure. (a) View in the a-b plane
showing titania chains and 12-membered ring (12MR), (b) view in the a-c plane
showing 8MR along the b direction. |
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The uniqueness of ETS-4 and its ion exchanged variants is the contraction of its 8MR channel at a molecular scale with increasing dehydration temperature. Methane-nitrogen separation for natural gas upgrading is one leading example of industrial importance that can benefit from exploitation of aforementioned transport channel contraction in ETS-4.
The effect of pore shrinkage on strontium and barium exchanged ETS-4 (denoted as Sr-ETS-4 and Ba-ETS-4, respectively) on adsorption and diffusion of methane and nitrogen was studied volumetrically in the linear range of the adsorption isotherm to identify the condition of optimum selectivity. To begin with, the equilibrium selectivity was in favor of methane, while the diffusivity ratio was in favor of nitrogen. With progressive dehydration at increasing temperature, there was a reversal in equilibrium selectivity in favor of nitrogen. While the diffusivity ratio remained practically unchanged in Ba-ETS-4 up to the point of equilibrium reversal, it dropped considerably in case of Sr-ETS-4. In other words, maxima in equilibrium and diffusivity ratios were synchronous in Ba-ETS-4, but were out of phase in Sr-ETS-4. As a result, a very high kinetic selectivity over 200, defined as the product of Henry’s constant ratio and square root of diffusivity ratio, was achieved in Ba-ETS-4 dehydrated at 400°C.
This selectivity is significantly higher than the best value reported so far in the literature. Representative uptake results are shown in Figure 2.
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Olefin-paraffin separation (ethylene from ethane, propylene from propane, etc), which is currently the most energy intensive distillation process in the petrochemical industry requiring low temperature, high pressure and over 100 trays, is another potential area where a custom designed ETS-4 can significantly impact the economics. In fact, the unique pore contraction property of ETS-4 and its ion exchanged variants opens up practically unlimited opportunities in gas separation.
This work was done in collaboration with two graduate students, R P Marathe and B Majumdar.
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Figure 2: Effect of dehydration on uptake of nitrogen and methane in
Ba-ETS-4. |
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Shamsuzzaman Farooq received his BSc and MSc degrees in Chemical Engineering from BUET (Bangladesh) and his PhD from the University of New Brunswick (Canada). A faculty member in the Chemical and Biomolecular Engineering Department at the National University of Singapore since 1991, his primary research interest is in the area of adsorption science and technology related to gas separation. He is a coauthor of the book Pressure Swing Adsorption (VCH, 1994).
Email: chesf@nus.edu.sg |
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