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10 Introduction are employed, mass transfer effects become important since here the cycle times are similar to the time scale for adsorption [14]. In order to have a high internal mass transfer in pellets, it would thus be preferable to have small pellets/beads comprised of small crystals. On the other hand, the pressure drop in a packed bed consisting of small pellets would become unmanageable, and, with increasing velocity, bed fluidization may also occur [33]. Hence, it is evident that in PSA processes employing the conventional pellets there is a trade-off between mass transfer kinetics and pressure drop, whereas larger adsorbent particles result in lower pressure drop, but increased mass- and heat- transfer limitations, and vice-versa, as discussed above. Structured adsorbents in the form of monolith adsorbents or coated monoliths described below may offer a solution to the mass and heat transfer problem, as they are characterized by a low pressure drop and lower mass and heat transfer resistances than the traditionally used beads or pellets [34]. 1.4.1 Monolith adsorbents Recently, the preparation and evaluation of monolith adsorbents in adsorption systems have been reported [35, 36, 37, 38]. Monolith adsorbents combine low resistance to mass transfer (short diffusion path in the adsorbent wall) with low pressure drop. However, in order for monolith adsorbents to be competitive with packed beds, they must have sufficient adsorption capacity. The CO2 adsorption on carbon monolith adsorbents with wall thicknesses of 1.6 or 2.8 mm was studied using the Zero Length Column (ZLC) method [38]. The ZLC data showed that the dispersion in the monolith was controlled by mass transfer resistance rather than axial mixing. Zeolite monoliths consisting of 5A zeolite and Na-bentonite with square lattice channels and a wall thickness of 0.98 mm were prepared by Li et al. [36, 39]. The O2 adsorption separation performance of the zeolite monolith was compared with that of 5A zeolite pellets with a diameter of 1.5 mm and legnth of 3.6 mm. The main outcomes of the work were that the adsorption capacity per column volume was of the same magnitude as for pellets whilst the pressure drop over the zeolite monolith was 3-5 times lower than that for the packed bed. The lower pressure drop resulted in a 3-5 times faster pressurization time when the PSA unit was loaded with the zeolite monolith as compared to when it was loaded with pellets. Shorter pressurization time is an advantage [39, 36, 14] and allows faster PSA cycles which re- sults in a higher productivity and should compensate for the lower adsorbent loading when using monoliths instead of traditional adsorbents. However, due to the large wall thicknesses of these monoliths, the mass transfer resistance is still significant and compa- rable to conventional beads. Possible ways of improving the separation performance of the zeolite monoliths would thus be to reduce the wall thickness and increasing the cell density of the zeolite monolith [6, 14]. Alternatively, thin zeolite films may be grown on the walls of ceramic cordierite monoliths, which was the approach used in this work.PDF Image | Structured Zeolite Adsorbents for PSA Applications
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