Adsorption ADS

Adsorption is a mass transfer process in which gas molecules are removed from an airstream because they adhere to the surface of a solid. In an adsorption system, the contaminated airstream is passed through a layer of solid particles referred to as the adsorbent bed. As the contaminated airstream passes through the adsorbent bed, the pollutant molecules adsorb or stick to the surface of the solid adsorbent particles. Eventually the adsorbent bed becomes filled or saturated with the pollutant. The adsorbent bed must then be disposed of and replaced, or the pollutant gases/vapors must be desorbed before the adsorbent bed can be reused. 

The relation between the amount of substance adsorbed by an adsorbent and the equilibrium gas (or vapor) partial pressure or concentration at constant temperature is called the adsorption isotherm. The adsorption isotherm is the most important and by far the most often used of the various equilibria data that are available. 

Briefly describe the four major adsorbents employed in air pollution control. 

Four important adsorbents widely used industrially will be considered briefly, namely, activated carbon, activated alumina, silica gel, and molecular sieves. The first three of these are amorphous adsorbents with a nonuniform internal structure. 

Molecular sieves, however, are crystalline and have, therefore, an internal structure of regularly spaced cavities with interconnecting pores of definite size. Details of the properties peculiar to the various materials are best obtained directly from the manufacturer. The following is a brief description of these principal adsorbents. 

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Assume that the total drug concentration [AT] is the sum of the free concentration [Afree] and the concentration bound to a site of adsorption [AD] (therefore, [Afiee] = [Ax] — [AD]). The mass action equation for adsorption is... 

Each process was consisted of conventional PSA steps such as pressurization (PR), adsorption (AD), blowdown (BD), purge (PU), and pressure equalization (PE) steps. However, the flow direction of all the steps except blowdown step was cocurrent. Furthermore, the idle time was applied to keep the cyclic symmehy. 

Figure 7. Initial desorption (IN DES), adsorption (ADS), and seconj ry desorption (SEC DES) isotherms for Douglas-fir. (Adapted from Ref 18.)...

Tab. 22.2 Proton transfer energy, A pj, deprotonation energy, E yp, hypothetical binding energy of NH4 on the deprotonated zeolite surfaces, E p(SH ), and energy of ammonia adsorption, ad( 3) ( J for Bronsted sites in different zeolite frameworks [13].

Table 10.1 Adsorption (AD)-desorption (DS) cycles of the adsorbent. Adsorption condition Pb(ll) 100 ppm, pH 2.0, total volume 20 mL, adsorption dose 100 mg/20 mL, desorption condition stripping solution 2 N HCl, total volume 20 mL, equilibration time 15 h.

Fig. 4-13. Differential molar heat of adsorption Ad.d< adsorption system hydrocarbons/acti-vated carbon) [4.40].

Figure 5.40. Elucidation of the biosorption phenomenon in a reaction scheme including substrate elimination (elim) quantified by COD removal, and degradation (degrad) accompanied by O2 utilization (BOD) and adsorption (ads) following this pseudohomogeneous approach to the L S process.

Adsorption ad- s6rp shsn, - z6rp [ad + absorption] (1882) n. (1) The concentration of molecules of a particular kind at the interface between two phases such as the pigment and vehicle in printing inks. 

Fig. 4.10 Catalyst NH3 storage capacity as a function of adsorption temperature as measured by uptake during adsorption (ads.), release during isothermal desorption (des.) and temperature programmed desorption (TPD), and NO converted by stored NH3 after removing NH3 from the feed gas (NO red.)...

A well-established stepwise procedure was followed [16, 23, 25, 56]. Small successive doses of the adsorptive were admitted and left in contact with the adsorbent until the thermal equilibrium was attained. The 1st run of adsorption performed on the activated sample (pretreated in high vacuum conditions and/or in controlled atmosphere) will be hereafter referred to as ads. I. At any individual dose of gas introduced in the system, the evolved heat AQ " was measured within the calorimetric cells, while the adsorbed amount Anads was measured by volumetry. Ads. I was followed by a desorption run (des. I), performed by simple evacuation of the cell. In such a way the reversibly adsorbed phase was desorbed and either the pristine surface was restored, in case of an entirely reversible adsorption, or the pristine surface was not recovered, in case of a (partially) irreversible adsorption. Ads. II was subsequently performed in order to assess which fraction (if any) of the pristine surface sites was irreversibly occupied by the adsorbed phase (in the adopted conditions). By subtracting the ads. II curve from the ads. I one, the adsorbed fraction not removed by evacuation is evaluated. The ads. n component will be hereafter referred to as the reversible adsorbed phase, whereas the (ads. I - ads. n) component will be referred to as the irreversible phase (in the adopted conditions). Subsequent runs of adsorption (ads. IE, IV etc.) are performed in some cases, if the irreversible modification of the surface is expected/suspected not to be extinguished during the ads. I [21, 23, 26]. Adsorption measurements are usually performed at least twice on a virgin portion of the same batch of the material, activated in the same conditions, to check the experiments reproducibility. The routinely run protocol of adsorption-desorption-adsorption cycles is schematically illustrated in Fig. 1.8. 

Fig. 1.15 Reversible adsorption (ads. II) of NH3 at T = 303 K on defective MFI—SUicalite SH—A(diamond, 1), Sil-B(t triangle, 2), Sil—C(s re, 3) and on perfect (i.e. defect-fiee) MFI-Silicalite Sil—D(citcfe, 4). a Integral heats of adsorption versus NH3 uptake. Curves interpolating the experimental points (1 ) tue polynomials of order 3. b Differential heats of adsorption versus NH3 uptake. Differential heats were obtained by differentiating the section (a) polynomial fimc-tions. All samples were outgassed at r = 673 K. Adapted from Ref. [24] Fig. 4. Note that Sil—A and Sil—D were the same specimens as the ones named in Fig. 1.12 MFI-def and MFI-perf, respectively...

Fig. 6.27 Adsorption of Cd + cations from Cd(N03>2 aqueous solutions of varying concentration at pH 7 onto Spherosil (Sbet = 25 m g ) at 298K [94] a record of successive saturation and desorption runs showing heat absorption and evolution cahbration (cal), adsorption (ads), and desorption (des). For each concentration of the stock solution m , the areas under the adsorption and desorption peaks ate equal...

For low-symmetry adsorption, ad hoc descriptions are given in each individual case, and in these situations the adsorbate-induced relaxations often become quite complicated (asymmetrical) and are not listed in detail it is suggested to consult the Surface Structure Database [99W] for further information. 

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