Structured Adsorbents

The adsorbent particles are normally used as beads, extrudates, or granules (-0.1 -0.3 cm equivalent diameters) in conventional H2 PSA processes. The particle diameters can be further reduced to increase the feed gas impurity mass transfer rates into the adsorbent at the cost of increased column pressure drop, which adversely affects the separation performance. The particle diameters, however, cannot be reduced indefinitely and adsorption kinetics can become limiting for very fast cycles48 New adsorbent configurations that offer (i) substantially less resistance to gas flow inside an adsorber and, thus, less pressure drop (ii) exhibit very fast impurity mass transfer coefficients and (iii) minimize channeling are the preferred materials for RPSA systems. At the same time, the working capacity of the material must be high and the void volume must be small in order to minimize the adsorber size and maximize the product recovery. Various materials satisfy many of the requirements fisted above, but not all of them simultaneously. 

The two promising candidates are adsorbent monoliths and adsorbent sheets. The fabrication of activated carbon and zeolite monoliths are reported in the literature. Zeolite monoliths have also been tested for air separation application by PSA.50 51 However, the use of monoliths for use in H2 PSA is not known to the authors. Monoliths having very high cell density (several hundred to thousand cells per square inch) will be necessary in order to have fast adsorption kinetics as well as reasonable bulk density for a PSA application. Manufacture of such monoliths is complex, and they are not yet commercially available. Gas channeling through the monoliths can also be a problem.52 Adsorbent sheets have been used for air separation by RPSA.53 54 The thickness of the adsorbent sheets and the space between the 

PSA units using adsorbent sheets have been operated by Questair Industries, Inc.46,47,53-55 The sheets were coated with a fine powder of zeolite particles (1-10/rm) and there were spacers between the sheets to establish flow channels in a flow direction tangential to the sheets and between adjacent pairs of sheets. The sheets could be made in various configurations (rectangular, annular stack, spiral-wound, etc.) and included a support for the adsorbent in the form of an aluminum foil, a metallic mesh, or a matrix that could be woven, nonwoven, ceramic, or wool. These structured materials did not fluidize at high gas velocities and exhibited equilibrium and mass transfer properties of the powdered adsorbent. 

The use of activated carbon fabric as the adsorbent in an H2 PSA process has also been reported.56 The advantage of the fabric is that it does not require additional supports like metal mesh, paper, and aluminum foil, and the bulk density of the adsorbent material in the PSA adsorber is increased. 

3 Sorption-Enhanced Reaction Process (SERP) for H2 Production 

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Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

Adsorbents, and activated carbon in particular, are typically characterized by a highly porous structure. Adsorbents with the highest adsorption capacity for gasoline or fuel vapors have a large pore volume associated with pore diameters on the order of 50 Angstroms or less. When adsorption occurs in these pores, the process is comparable to condensation in which the pores become filled with hquid adsorbate. Fig. 5 depicts the adsorption process, including transfer of adsorbate molecules through the bulk gas phase to the surface of the solid, and diffusion onto internal surfaces of the adsorbent and into the pores. 

Using three-dimensionally structured adsorbent with a large number of uniform gas channels, the limitations of mass transfer kinetics and fluidization of the conventional beaded sorbents may be removed. Such structured adsorbents have been described in... 

The question now arises of what simplification is possible in the treatment of orientationally structured adsorbates and what general model can be involved to rationalize, within a single framework, a diversity of their properties. Intermolecular interactions should include Coulomb, dispersion, and repulsive contributions, and the adsorption potential should depend on the substrate constitution and the nature of adsorbed molecules. However difficult it may seem, all these factors can be taken into account if we follow the description pattern put forward in this book. Its fundamentals are briefly sketched below. 

In the present book, we aim at the unified description of ground states and collective excitations in orientationally structured adsorbates based on the theory of two-dimensional dipole systems. Chapter 2 is concerned with the discussion of orientation ordering in the systems of adsorbed molecules. In Section 2.1, we present a concise review on basic experimental evidence to date which demonstrate a variety of structures occurring in two-dimensional molecular lattices on crystalline dielectric substrates and interactions governing this occurrence. 

Overall reaction, 32 281, 285-287 see also Simple overall reactions Overlayer structure, adsorbates on metal surfaces, 29 7-8... 

However, upon heating to 450 K the monolayer orders to form a (6 X 6) surface structure. Adsorbed naphthalene forms a disordered layer also on the Ag(l 11) crystal face at 300 K. However, below 200 K an ordered structure appears with a unit cell (-2 0 a s) sometimes another, less stable monolayer structure is also detectable. 

In each case, as the temperature was raised to 200 K, a markedly changed VEEL spectrum was observed, which was attributed to the formation of cyclopentene by dehydrogenation (see Section VI.B). Avery s study of the adsorption of cyclopentane was continued to 260 K, whereby a much simpler spectrum was obtained, convincingly attributed to the formation of the 175-C5H5 (i75-cyclopentadienyl) structure adsorbed flat on the surface. The 200 K spectrum of the species on Ru(0001) may even contain some features characteristic of this species (strong bands at 758 and 3057 cm 1). 

McGillivray et al. [71] have also observed stable layered structures adsorbed at the silicon-solution (and air-solution) interface for didodecyl dimethylammo-nium bromide (DDAB) and the corresponding diundecyl (DUDAB) cationic surfactants, in the concentration range 0.2-2 wt.%. Similar to AOT, the surface structures that are found are highly sensitive to temperature, with the repeat distance decreasing with increasing temperature. A notable difference between these systems and AOT [69] is that for the DDAB and DUDAB, the repeat distances are much larger, 600-1500 A. Furthermore, the observed structures... 

Abstract Electron transmission through chiral molecules induced by circularly polarized light can be very different for mirror image structures. This behavior is described in terms of current transfer, the transfer of both charge and momentum. We review recent theoretical developments on the theory of current transfer and discuss related experimental studies of electron transmission through chiral molecular structures adsorbed on surfaces. 

Shablakh et al. (1984) investigated the dielectric properties of bovine serum albumin and lysozyme at different hydration levels, at low frequency. Besides a relaxation attributed to the electrode—sample interface, they detected a further bulk relaxation that can be confused with a d.c. conduction effect. The latter relaxation was explained by a model of nonconductive long-range charge displacement within a partially connected water structure adsorbed on the protein surface. This model has nonconventional features that differ from the assumptions of other more widely accepted models based on Debye relaxations. 

We divide this section into three main themes structure, adsorbed amount, and polydispersity. In sec. 5.7a we discuss the structure of the adsorbed layer, paying attention to the volume fraction profile, the composition in terms of loops and tails, the bound fraction, and the layer thickness. Section 5.7b deals with the adsorbed amount as a function of the polymer concentration, the chain length, the solvency and the surface affinity. These two sections are mainly concerned with monodisperse polymer. Polydispersity, where competition between different chain lengths shows up, is treated In sec. 5.7c. 

Repeatedly from the 1980s onwards, and especially when I was grappling with many of the then unknown aspects of zeolite science, I often consulted Rabo et al. The paper gave authoritative accounts of certain features pertaining to structure, adsorbability and reactivity of various kinds of cation-exchanged zeolites. It also highlighted some of the properties of these solids that were enigmatic or poorly understood. 

Polymeric adsorption resins — A series of polymeric adsorbents has recently been developed that can adsorb hydrocarbons from aqueous systems. They derive their adsorptive properties from a combination of macroreticular porosity, pore size distribution, high surface area, and the aliphatic nature of their structure. Adsorbent resins can produce water containing less than 1 ppm oil. 

The authors have studied the molecular structure adsorbed in micropores about various molecules such as CCl/N,", O,", NO SO, He , and H,0" " using activated carbon liber (ACF) which has considerably uniform slit-shaped hydrophobic microporcs. Polar molecule, for example, H,0 molecule in carbon micropore interacts strongly between H,0 molecules than between H,0 and carbon walls. liyama et al. reported that H,0 molecules confined in... 

Recent developments in rapid pressure swing H2 PSA processes are described. The rapid cycles are used to reduce adsorbent inventories, to reduce vessel size, and to lower the overall cost of the PSA unit. Research in the area of rotary valves and structured adsorbents, which enable the RPSA cycles, is outlined. 

Fig. 16 Power spectral density (PSD) from AFM data (circles) and intensity of out of plane scan from X-ray scattering at grazing incidence (squares) from an adsorbed diblock polyampholyte (Mn 15,000 g mol-1, PMAA/PDMAEMA weight ratio 33/67) of highly regular lateral structure adsorbed at different pH on a plane silicon wafer (a) 6.1, (b) 9.4, and (c) 8.5. The most prominent length scale is marked with an arrow...

Lin, Y.S. Deng, S. Sol-gel preparation of nano-structured adsorbents. In Adsorption and Its Application in Industry and Environmental Protection, Dabrowski, A., Ed. Elsevier Amsterdam, 1998 vol.l20A, 653-686. 

Figure 20.12 Scheme of the 1-dodecanethiol structure adsorbed on h hly oriented pyrolytic graphite (HOPG). The axis of the hydrocarbon chain is oriented parallel to the surface, although partially extended. 

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