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Chapter 3 Results and discussion 3.1 General properties of structured adsorbents 3.1.1 Selection of synthesis mixture, Papers B-C Figure 3.1 (a) shows a top-view SEM image of a NaX film grown in a gel (Table 3.1) on a 400 cpsi cordierite monolith. The film appears to be well intergrown and typical NaX crystals with octahedral habit and with a size ranging between 600 and 2000 nm are observed. The average thickness of the film is about 1.1 μm, as shown in Figure 3.1 (c) and reported in Table 3.1. Figure 3.1 (e) shows a top-view SEM image at low magnification of the same sample. The arrows indicate aggregates with radii ranging from ca. 0.5 to 10 μm on top of the film. These aggregates were probably deposited by sedimentation from the synthesis gel. Sediments deposited on top of and embedded in the film may be a disadvantage in PSA applications due to the increased diffusion path of the adsorbing species in the relatively large aggregates, as will be discussed later in the thesis. In order to avoid sediment deposition, a clear synthesis solution was used for film growth in the following experiments. Figure 3.1 (b) shows a top-view SEM image of a NaX film grown in a clear synthesis mixture in 5 steps of 1 hour and 20 minutes. The film is well intergrown; however, a few hydroxysodalite crystals with a ball-of-yarn habit are observed, which will be discussed in the next paragraph. The film grown in the clear synthesis mixture has an average thickness of 1.5 μm, as shown in Figure 3.1 (d) and reported in Table 3.1. A much lower amount of sediments was observed for the film grown in the clear synthesis mixture, see Figure 3.1 (f). These results were supported by weight gain measurements; although the NaX film grown in the gel has a lower film thickness than the one grown in the clear synthesis mixture, the weight gain is higher for the former sample, see Table 3.1, which suggests the presence of more sediments on the film grown in the gel, in accordance with SEM observations. Figure 3.2 shows XRD patterns of the NaX samples grown in the gel and in the clear 19PDF Image | Structured Zeolite Adsorbents for PSA Applications
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