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Adsorption (2008) 14: 687–693 Fig. 1 Low magnification SEM images of (a) of the 13X beads and (b) of the cross-section of a 400 cpsi cordierite monolith images were recorded on samples covered with a thin layer of gold (few nm) previously deposited by sputtering. X-ray diffraction data (XRD) were recorded with a Siemens D 5000 powder diffractometer equipped with a Cu-target in the Bragg–Brentano geometry. The specific carbon diox- ide adsorption capacity (mmol/gsample) of the structured ad- sorbents and NaX beads was measured at 0 °C by using a Micromeritrics ASAP 2010 instrument and was used to de- termine the adsorption capacity of a PSA column (2 cm in diameter and 30 cm long) loaded with NaX beads or three coated cordierite monoliths (2 cm in diameter and 10 cm long) connected in series. Liquid nitrogen sorption at liquid N2 temperature was performed on the structured and tradi- tional adsorbents by using a Micromeritrics ASAP 2010 in- strument. The samples were outgassed at 300 °C under vac- uum prior to analysis. The pore size distribution was deter- mined by the Barret-Joyner-Halenda (BJH) method applied to the desorption branch. CO2 breakthrough curves were de- termined by performing step experiments on a single PSA column loaded with three structured samples (0.99 gzeolite in total) or NaX beads (33.7 gzeolite). The samples were ac- tivated in-situ and heated at a heating rate of 2 °C/min to 200 °C for 2 hours under a N2 flow and cooled thereafter to room temperature at a cooling rate of 2 °C/min. The feed consisted of 10% CO2 in N2 and was injected at a volumet- ric flow-rate of 0.2 L/min. The data were collected by a CO2 analyzer (Briage analyzers INC) using Advantech GeniDaq data acquisition software. The pressure drop was measured as a function of air flow rates for the zeolite beads and the structured adsorbents in a single PSA column (each 2 cm diameter and 30 cm long). 3 Results and discussion 3.1 Characterization by SEM, weight gain and XRD Figure 1 shows low magnification SEM images (a) of the NaX beads and (b) of the cross-section of a 400 cpsi porous cordierite monolith. The beads are quasi-spherical with a diameter of about 700 μm (Fig. 1a). The thickness of the cordierite monolith wall is about 100 μm and the channel 689 Fig. 2 SEM top-view image of the NaX molecular sieve zeolite beads width is about 1.1 mm (Fig. 1b). Macropores with a size up to several μm are observed inside the walls of the ceramic cordierite support. Figure 2 shows the cross-sectional image of the NaX beads. It is possible to identify the typical octahedral shape of the zeolite X crystals. A zeolite film was grown in a longer hydrothermal treat- ment of 6 h and 40 min (not shown). The film was not continuous, and the presence of pin-holes and etched crys- tals may be due to the decreased hydrothermal stability of the zeolite crystals. Large sediments on top of the film and hydroxysodalite (HS) crystals were also detected by SEM. Hydroxysodalite probably formed because of the combined effect of the Al-leaching from the support and the thermo- dynamics of the system (HS is a more stable zeolite than NaX). As observed in a previous work by Lassinantti et al. (2000) on the growth of zeolite Y films on α-Al2O3 wafers, the film thickness as a function of time had a maximum and decreased upon long hydrothermal treatments, due to disso- lution of the crystals in the film and formation of zeolite P. Sediments and zeolite crystals other than NaX may be a dis- advantage in PSA applications in terms of reduced separa- tion performance. In order to minimize the detrimental effects of Al- leaching and sedimentation, a multiple step synthesis pro- cedure was performed. The synthesis duration of 6 h and 40 min was divided in 5 steps of 1 h and 20 min. Top-view and cross-section images of the NaX film grown in the clear solution on the 400 cpsi cordierite supports in 5 steps of 1 hPDF Image | Structured Zeolite Adsorbents for PSA Applications
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