Adsoiption capacities

The adsoiptive capacity of carbons is obviously finite. Consequently, it is gradually decreased and finally exhausted after the adsorption of the maximum possible amount of a substance. The exhausted carbon is then characterized as spent and has to be regenerated, reactivated, or properly disposed off. The regeneration of spent adsorbents is the most difficult and expensive part of adsorption technology. It accounts for about 75% of total operating and maintenance cost for a fixed-bed GAC operation. 

The search for a suitable adsorbent is generally the first step in the development of an adsorption process. A practical adsorbent has four primary requirements selectivity, capacity, mass transfer rate, and longterm stability. The lequiiement foi adequate adsoiptive capacity icstiicts the choice of adsorbents to microporous solids with pore diameters ranging from a few tenths to a few tens of nanometers. 

Die most important property of adsorbent materials, the property tliat is decisive for the adsorbent s usage, is the pore stincture. The total number of pores, their shape, and size deteiniine tlie adsoiption capacity and even the dynamic adsoiption rate of the material. Generally, pores are divided into macro-, iiieso- and micropores. According to lUPAC, pores are classified as shown in Table 2.2. 

Adsoiptive molecules transport through macropores to the mesopores and finally enter the micropores. The micropores usually constitute tlie largest portion of the internal surface and contribute the most to the total pore volume. The attractive forces are stronger and the pores are filled at low relative pressures in the microporosity, and therefore, most of the adsoiption of gaseous adsoiptives occurs within that region. Tlius, the total pore volume and the pore size distribution determine the adsoiption capacity. 

In conclusion, tlie maximum adsoiption capacity should be measmed in fixed-bed experiments under dynamic conditions, and if models aie applicable, diffusion coefficients should be also deteniiined in fixed-bed appaiams. Due to the fact tliat the equilibrium isotheniis requue extended data series and thus are time-consuming experiments, the latter are quite difficult to be conducted in fixed-bed reactors and from this point of view, it is more practical to evaluated equilibrium isothams in batch reactor systems. Then, it is known that when applying fixed-bed models using an equilibrium isothenn obtained in batch-type experiments, the equilibrium discrepancy (if it exists) can be compensated by a different estimate for the solid diffusion coefficient (Inglezakis and Grigoropoulu, 2003 Weber and Wang, 1987). 

In this woili flic effect of anodic oxidation treatments on activated caibon fibeis (ACFs) was studied in the context of CtfVIX Cu( II), and NKII) ion adsoiption behaviors. Ten wt% phosphoric acid and sodium hydroxide were used fiir acidic and basic electrolytes, respectively. Surfiice properties of the ACFs were determined by XPS. The specific surface area and the pore stnicture were evaluated fiom nitrogen adsoiption data at 77 K. The heavy metal adsoiption rales of ACFs were measured by usii% a UV spectrometer and KT. As a result, the anodic treatments led to an increase in the amount of total acidity by an increase of acidic functional groups sudi as carboxyl, lactone, and phenol, in spite of a decrease in specific surfiare area, due ID the pore bloddug by increased acidic functional groups. The adsoiption amount of anodic oxidation treated ACFs was increased and the adsoiption capacity improved in order of CifVI)>Cu(II)>NKn). It was probably accounted that the surface functional properties on ACFs had a main effect as compared to the structure properties. 

To date, activated carbon is die most universal adsorbent ftn- VOCs control. However, some disadvantages for the plication of activated carbon include its flammability, difficulty in regenerating high-boiling point solvents and required humidity crmtrol. On the positive side, activated carbon fiber has uniform size and dimoision, higher adsoiption capacity, ter adsorption and destuption rates than activated carbon, and ease of handling [1,2]. These features obtain adsorptive system size reduction and added adsorbed vapor selectivity. In diese respects, activated carbon fiber, as alternative to activated carbon inefficiencies, is an excellent micro-pore adsorbent. 

Figure 12-19. Adsoiption capacity of various desiccants versus years of service dehydrating high-pressure natural gas. Curve 1, data of Getty et id. (1963) curve 2, data of Herrmann (1955) curves 3,4,6, and 8, data ofSwerdloff(1967) curve 5, data of Hammerschmidt (19 and curve 7, data of Capped et at. (1944)...

NH2, C2H5) into the nitro compound as a rule leads to a decrease in the potential shift of the catalyst, i. e., a weakening of the adsoiption capacity, whereas electron-acceptor substituents (COOH, CHO) increase the potential shift of the catalyst, i.e., intensify the interaction between the adsorbed molecules of the nitro compound and the catalyst surface. The way in which such a charge in adsorption capacity is reflected in the overall rate of the catalytic process depends on the nature of the catalyst and the conditions xmder which the process operates. 

Finally, we note that the maximum p/p° attained on the adsoiption branch in Figure 12.16 is c. 0.9S. As indicated earlier, exposure of the adsorbent to water vapour at pjp° 1 caused an irreversible partial conversion of the VPI-5 structure to AlP04-8. Furthermore, a significant decrease in the water adsorption capacity was found when the outgassing temperature was taken to 673 K in a stepwise manner (Kenny ef a/., 1992). We conclude that CRTA would be admirably suitable for a more rigorous study of the properties and thermal stability of VPI-5. 

The adsoiption of ions on an ionic solid originates from the normal bonding forces that are responsible for crystal growth. For example, a silver ion at the surface of a silver chloride particle has a partially unsatisfied bonding capacity for anions because of its surface location. Negative ions are attracted to this site by the same forces that hold chloride ions in the silver chloride lattice. Chloride ions at the surface of the solid exert an analogous attraction for cations dissolved in the solvent. 

Tlie maximum adsoiption (or ion-exchange) and breakthrough capacity can be measured using the experimental breakthrough curve (C versus by integration (Peiiy and Gi-een, 1999 Helfferich, 1962) ... 

The diagrams in Figs. 9.4-1 and 9.4-2 are based on the assumption of isothermal adsorption or desorption however, in industrial adsorbers the heat of adsoiption leads to an increase of the temperature which is more pronounced at high loadings. In the case of desorption the temperature is reduced. These heat effects cause a reduction of capacity. Furthermore equihbrium would be reached after an infinite time because the driving force approaches zero. These problems will be discussed later. 

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