Practical Adsorbents

The requirement for adequate adsorptive capacity restricts the choice of adsorbents for practical separation processes to microporous adsorbents with pore diameters ranging from a few Angstroms to a few tens of Aflgstroms. This includes both the traditional microporous adsorbents such as silica gel, activated alumina, and activated carbon as well as the more recently developed crystalline aluminosilicates or zeolites. There is however a fundamental difference between these materials. In the traditional adsorbents there is a distribution of micropore size, and both the mean micropore diameter and the width of the distribution about this mean are controlled by the manufacturing process. By contrast, the micropore size of a zeolitic adsorbent is controlled by the crystal structure and there is virtually no distribution of pore size. This 

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AH practical adsorbents have surfaces that are heterogeneous, both energetically and geometrically (not all pores are of uniform and constant dimensions). The degree of heterogeneity differs substantially from one adsorbent type to another. These heterogeneities are responsible for many nonlinearities, both in single component isotherms and in multicomponent adsorption selectivities. 

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). 

The practical adsorbents used in most gas phase appHcations are limited to the following types, classified by their amorphous or crystalline nature. 

Practical adsorbents may also be classified according to the nature of their surfaces. 

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 long-term stabiUty. The requirement for adequate adsorptive capacity restricts the choice of adsorbents to microporous soUds with pore diameters ranging from a few tenths to a few tens of nanometers. 

The separation of fmctose from glucose illustrates the interaction between the framework stmcture and the cation (Fig. 5) (50). Ca " is known to form complexes with sugar molecules such as fmctose. Thus, Ca—Y shows a high selectivity for fmctose over glucose. However, Ca—X does not exhibit high selectivity. On the other hand, K—X shows selectivity for glucose over fmctose. This polar nature of faujasites and their unique shape-selective properties, more than the molecular-sieving properties, make them most useful as practical adsorbents. 

In addition to the fundamental parameters of selectivity, capacity, and mass-transfer rate, other more practical factors, namely, pressure drop characteristics and adsorbent life, play an important part in the commercial viabiUty of a practical adsorbent. 

Adsorption (qv) is a phenomenon in which molecules in a fluid phase spontaneously concentrate on a sohd surface without any chemical change. The adsorbed molecules are bound to the surface by weak interactions between the sohd and gas, similar to condensation (van der Waals) forces. Because adsorption is a surface phenomenon, ah practical adsorbents possess large surface areas relative to their mass. 

In theoretical derivations of adsorption isotherms, giving the amount of adsorbed molecules as a function of the equilibrium pressure of the coexistent gas or the equilibrium concentration in the coexistent solution, it is mostly assumed that the heat of adsorption is the same over the entire surface of the adsorbent. As we discussed in Sec. V,12 such a conception can hardly be maintained for practical adsorbents. 

Since adsorption is essentially a surface phenomenon, a practical adsorbent must have a high specific surface area, which means small diameter pores. Conventional adsorbents such as porous alumina, silica gel, and activated carbon have relatively wide pore size distributions, spanning the entire range from a few angstroms to perhaps 1 /xm. For convenience the pores are sometimes divided into three classes ... 

R. B. Anderson (McMaster University, Hamilton, Ontario, Canada) Could the higher diffusivity in practical adsorber be attributed to the temperature of the bed being higher than that of the gas ... 

Films deposited from nonpolar solvents are relatively thick (>1000 A) and resistant to desorption films from polar solvents are generally thinner (<100 A) and easily disrupted by polar solvents. An adsorbed silane film can consist of different strata a silane interface with covalent bonding [10], a relatively cross-linked intermediate layer, and a superimposed layer of relatively un-cross-linked material. In practice, adsorbed films on both glass and metals are discontinuous and consist of discrete islands or agglomerates, called the button-down theory [37]. 

Practical adsorbents are inherently heterogeneous, and therefore to properly account for kinetics in such adsorbents, we need to develop a mathematical model to allow for the energetic heterogeneity. First we will address single component systems to uncover the various features of the heterogeneous model. 

The diffusion time constants (D/R ) are shown in Table 10.19. These values would not cause diffusion limitation for practical adsorber operations. 

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