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688 Adsorption (2008) 14: 687–693 2001; Li et al. 1998b). The long diffusion paths through meso- and macropores in the zeolite beads may induce heat—and mass—transfer limitations due to temperature and concentration gradients, thus decreasing the overall per- formance of the PSA process (Li et al. 1998b). Recently, parallel flow monolith structures have received considerable attention for their use in adsorption processes. These materials have parallel channels with controllable shape and wall thickness (Cybulski and Moulijn 1998; Williams 2001). In addition, zeolite monoliths, consist- ing solely of zeolite and binder material (Lee et al. 2000; Li et al. 2001), have been prepared and tested in an air sep- aration application. Monoliths have been widely used in catalytic convert- ers since the 1960s (Cybulski and Moulijn 1998; Williams 2001) rather than in PSA processes. Ceramic cordierite monoliths are mainly used as catalysts substrates in the au- tomotive industry for oxidation of CO and CHx, reduction of NOx, and as diesel particulate filters (DPF). The main advantage offered by monoliths compared to packed beds is the low pressure drop. Lower pressure drop is very im- portant in PSA processes (Todd 2003) in terms of reduced power consumption, high product recovery, purity and pro- ductivity. In another study, the CO2 adsorption and diffu- sion on a carbon monolith adsorbent were studied using the Zero Length Column (ZLC) method (Brandani et al. 2004). The ZLC curves show that at low purge rates the adsorp- tion mechanism is equilibrium controlled, and at high flow rate kinetic-controlled. Further, the combined effect of axial mixing and resistance to mass transfer in the carbon mono- lith walls result in spreading of the CO2/N2 breakthrough curve. The adsorption performance of 5A zeolite monoliths (made from zeolite with a binder rather than by coating a substrate) with square lattice channels and a wall thickness of 0.98 mm was compared to that of 5A zeolite pellets in the separation of oxygen from air (Li et al. 1998a, 1998b, 2001). The adsorption capacity and pressure drop in the ze- olite monolith were slightly lower than in the pellets. The authors assign the poorer oxygen separation performance of the 5A monolith to the reduced ability to transfer the mole- cules from the gas phase to the adsorbent surface, as a result of the increased diffusion path in the thick walls of the ze- olite monolith. However, the use of monoliths in adsorption processes may represent a competitive alternative to packed beds, leading to a more effective and lower cost processes by reducing the power demand and cycle time. Possible ways of improving the separation performance of the zeolite mono- liths might be to reduce wall thickness, to increase cell den- sity, or to use hexagonal rather than square cell shape (Patton et al. 2004). Due to the high mass transport resistance in traditional zeolite adsorbents (beads, powder and extrudates) or in thick walls of whole zeolite monoliths, structured adsorbents in the form of monoliths coated with NaX films having a con- trollable thickness were developed in this work. CO2 ad- sorption capacity and pressure drop were evaluated for the structured adsorbent and compared with NaX zeolite beads. 2 Experimental section Porous cordierite monoliths (400 cpsi, Corning) were used as structured substrates for the zeolite film growth. The sub- strates were first rinsed in toluene, acetone and several times in a 0.1 M NH3 solution. Prior to hydrothermal treatment, the monoliths were dried in a ventilated furnace at 110 °C, cooled thereafter in a desiccator and then the weight was recorded. Growth of adsorbed 80 nm faujasite (FAU) seeds on the walls of the substrates was performed in a clear solution with molar composition of 80Na2 O:1Al2 O3 :9SiO2 :500H2 O, as described by Öhrman et al. (2004a) about the growth of thin ZSM-5 film on cordierite monoliths. To prepare the clear solution, sodium metasilicate (Na2 SiO3 × 9H2 O, Sigma-Aldrich) was mixed with pelletized sodium hydrox- ide (99.9% NaOH, Merck) and distilled water. An aque- ous solution of aluminium sulphate (Al2(SO4)3 × 18H2O, Riedel-de-Häen) was added under vigorous stirring. The synthesis of the NaX film was performed in 1 step of 6 h and40minorin5stepsof1hand20minintheclear solution, which was heated in an oil bath at 100 °C and at- mospheric pressure under reflux. NaX crystals were also prepared in the clear synthesis solution for 1 h and 20 min, and used as a reference for the determination of the zeo- lite loading. Prior to characterization the samples were dried at room temperature and in a ventilated furnace at 100 °C over night, and cooled thereafter in a desiccator. The zeolite loading (gzeolite /gsample ) of the structured adsorbents was de- termined by the weight gain, as the weight increase of the cordierite supports after hydrothermal treatment. NaX (13X) molecular sieve zeolite beads (Qingdao JIT Corporation, Qingdao, China) with a diameter of 0.7 mm were used for comparison with the NaX film coated cordierite monoliths. The zeolite loading of the beads was determined by liquid N2 adsorption on a Micromeritrics ASAP 2010 instrument, as the ratio between the BET surface area of the beads and the reference NaX crystals, as described by Öhrman et al. (2004b) for thin ZSM-5 film coated alu- mina beads. A Philips XL 30 scanning electron microscope (SEM) equipped with a LaB6 emission source was used for studies of surface morphology and to measure film thick- ness. The monoliths were cut in the channel direction in or- der to characterize the films. The NaX beads were mounted in a phenolic resin (Phenolic Resin Black, Buehler LTD) and polished in order to obtain cross-sectional images. SEMPDF Image | Structured Zeolite Adsorbents for PSA Applications
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