Diameter adsorber

Equipment cost for industrial activated carbon systems were provided in undated vendor material. The Model 4 system contains two 4-ft-diameter adsorbers and contains 2000 lb of granular activated carbon. The Model 8 system has two 8-ft-diameter adsorbers and contains 6000 to 10,000 lb of granular activated carbon. The Model 10 system has two 10-ft-diameter adsorbers, and the Model 12 system contains two 12-ft-diameter adsorbers. Both units contain 20,000 lb of granular activated carbon (D15749X, pp. 19-22). Estimated capital costs for the systems are given in Table 1. 

Removal of SO2 from actual boiler fiue gas was first tested in a 1500 ft /hr, 6-inch diameter unit operating on a slipstream from a 50 MW oil-fired boiler. This equipment was operated satisfactorily around-the-clock for periods as long as one week. Scaled up 18-inch diameter adsorbers, capable of handling gas rates of 20,000 ft /hr, also have been operated satisfactorily on fiue gas from an oil-fired boiler and on simulated Claus tail gas. Data from these continuous units showed SO2-removal capabilities to as low as 50 ppm. 

The hyperbolic C term, Q/uint, expresses the influence of axial diffusion of the solute molecules in the fluid phase. It can only be observed in preparative systems with large diameter adsorbents operated at very low flow rates. In most cases of preparative chromatography it can be neglected, as the velocity of the mobile phase is rather high. As the longitudinal diffusion depends on the diffusion coefficient of the solute, it can be influenced by changing the mobile phase composition. To achieve high diffusion coefficients, low viscosity solvents should be preferred, which will, in addition, result in lower column pressure drops and thus will be favorable. 

Adsorbent particle radius and diameter Adsorbent packed bed void fraction Effective pore diffusion coefficient Superficial gas velocity... 

Solvent / Nonsolvent Particle diameter Adsorb / Desorli Pore size... 

Molar masses Critical range Used samples Solvent / Nonsolvent Particle diameter Adsorb / Desorli Pore size Column dimension Temperature (LC, TLC or SFC separation Pressure and characterization) How rate and/or notices Inj. volume Detector Reference... 

Silica particles, 73 nm in diameter, adsorb onto a PEI-coated silica surface in a controllable way, as shown in Figure 1. Depending on the concentration of llie colloidal particles in the suspension, a monolayer of particles adsorbs onto the PEI-coated silica surface very rapidly, over a time span of several minutes. With a particle concentration of more than 1 wt %, adsorption of a monolayer takes place in less than a minute. For dilute suspensions, say 0.01 wt %, adsorption requires in excess of 1 h to produce a complete monolayer. Tlie extent of this adsorption process is proportion to as scussed later in this section. 

At the Ford Motor Company s spark plug plant in Fostoria, Ohio, trichloroethylene vapor in the air from degreaser units is recovered by adsorption on activated carbon. Two 72-in.-diameter adsorbers are used, each containing 1,300 lb of carbon pellets. The system is reported to be capable of recovering 400 to 450 gal. of liquid trichloroethylene per day with a collection efficiency of over 90%. Operating costs were only about 3% of the value of the recovered solvent (Anon.. 1969). 

The adsorbent, the stationary phase, fills a column of a few decimeters in length and 5 to 10 mm in diameter. The column is swept continually by a solvent or mixture of solvents (the liquid phase). 

A special case of adsorption in cavities is that of clatherate compounds. Here, cages are present, but without access windows, so for adsorption to occur the solid usually must be crystallized in the presence of the adsorbate. Thus quinol crystallizes in such a manner that holes several angstroms in diameter occur and, if crystallization takes place in the presence of solvent or gas... 

Fig. XVII-27. Nitrogen adsorption at 77 K for a series of M41S materials. Average pore diameters squares, 25 A triangles, 40 A circles, 45 A. Adsorption solid symbols desorption open symbols. The isotherms are normalized to the volume adsorbed at Pj = 0.9. (From Ref. 187. Reprinted with kind permission from Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

Islands occur particularly with adsorbates that aggregate into two-dimensional assemblies on a substrate, leaving bare substrate patches exposed between these islands. Diffraction spots, especially fractional-order spots if the adsorbate fonns a superlattice within these islands, acquire a width that depends inversely on tire average island diameter. If the islands are systematically anisotropic in size, with a long dimension primarily in one surface direction, the diffraction spots are also anisotropic, with a small width in that direction. Knowing the island size and shape gives valuable infonnation regarding the mechanisms of phase transitions, which in turn pemiit one to leam about the adsorbate-adsorbate interactions. 

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

In general there are two factors capable of bringing about the reduction in chemical potential of the adsorbate, which is responsible for capillary condensation the proximity of the solid surface on the one hand (adsorption effect) and the curvature of the liquid meniscus on the other (Kelvin effect). From considerations advanced in Chapter 1 the adsorption effect should be limited to a distance of a few molecular diameters from the surface of the solid. Only at distances in excess of this would the film acquire the completely liquid-like properties which would enable its angle of contact with the bulk liquid to become zero thinner films would differ in structure from the bulk liquid and should therefore display a finite angle of contact with it. 

These calculations lend theoretical support to the view arrived at earlier on phenomenological grounds, that adsorption in pores of molecular dimensions is sufficiently different from that in coarser pores to justify their assignment to a separate category as micropores. The calculations further indicate that the upper limit of size at which a pore begins to function as a micropore depends on the diameter a of the adsorbate molecule for slit-like pores this limit will lie at a width around I-So, but for pores which approximate to the cylindrical model it lies at a pore diameter around 2 5(t. The exact value of the limit will of course depend on the actual shape of the pore, and may well be raised by cooperative effects. 

The limits of pore size corresponding to each process will, of course, depend both on the pore geometry and the size of the adsorbate molecule. For slit-shaped pores the primary process will be expected to be limited to widths below la, and the secondary to widths between 2a and 5ff. For more complicated shapes such as interstices between small spheres, the equivalent diameter will be somewhat higher, because of the more effective overlap of adsorption fields from neighbouring parts of the pore walls. The tertiary process—the reversible capillary condensation—will not be able to occur at all in slits if the walls are exactly parallel in other pores, this condensation will take place in the region between 5hysteresis loop and in a pore system containing a variety of pore shapes, reversible capillary condensation occurs in such pores as have a suitable shape alongside the irreversible condensation in the main body of pores. 

The enhancement of interaction energy in micropores was discussed in some detail in Chapter 4. It was emphasized that the critical pore width d at which the enhancement first appears increases with increasing diameter a of the adsorbate molecule, since the relevant parameter is the ratio d/a rather than d itself. The quantity a is involved because the magnitude of the dispersion interaction increases as the polarizability, and therefore the size, of the molecule increases (cf. p. 5). 

Fig. 4. Atom manipulation by the scanning tunneling microscope (STM). Once the STM tip has located the adsorbate atom, the tip is lowered such that the attractive interaction between the tip and the adsorbate is sufficient to keep the adsorbate "tethered" to the tip. The tip is then moved to the desired location on the surface and withdrawn, leaving the adsorbate atom bound to the surface at a new location. The figure schematically depicts the use of this process in the formation of a "quantum corral" of 48 Fe atoms arranged in a circle of about 14.3 nm diameter on a Cu(lll) surface at 4 K.

Adsorbent Pore diameter, nm Particle density, g/cm Specific area, mVg Apphcations... 

Most commercial adsorbents for gas-phase appHcations are employed in the form of pellets, beads, or other granular shapes, typically about 1.5 to 3.2 mm in diameter. Most commonly, these adsorbents are packed into fixed beds through which the gaseous feed mixtures are passed. Normally, the process is conducted in a cycHc manner. When the capacity of the bed is exhausted, the feed flow is stopped to terminate the loading step of the process, the bed is treated to remove the adsorbed molecules in a separate regeneration step, and the cycle is then repeated. 

However, the size of the pores is the most important initial consideration because, if a molecule is to be adsorbed, it must not be larger than the pores of the adsorbent. Conversely, by selecting an adsorbent with a particular pore diameter, molecules larger than the pores may be selectively excluded, and smaller molecules can be allowed to adsorb. 

Pore size is also related to surface area and thus to adsorbent capacity, particularly for gas-phase adsorption. Because the total surface area of a given mass of adsorbent increases with decreasing pore size, only materials containing micropores and small mesopores (nanometer diameters) have sufficient capacity to be usehil as practical adsorbents for gas-phase appHcations. Micropore diameters are less than 2 nm mesopore diameters are between 2 and 50 nm and macropores diameters are greater than 50 nm, by lUPAC classification (40). 

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