Equilibrium-selective adsorption

The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent While all the adsorbates have access to the adsorbent sites, the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction. This in turn results in higher adsorbent selectivity for one component than the others. One important parameter that affects the equilibrium-selective adsorption mechanism is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate. Specific physical properties of zeolites, such as framework structure, choice of exchanged metal cations, Si02/Al203 ratio and water content can be 

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When developing a liquid phase adsorptive separation process, a laboratory pulse test is typically used as a tool to search for a suitable adsorbent and desorbent combination for a particular separation. The properties of the suitable adsorbent, such as type of zeolite, exchange cation and adsorbent water content, are a critical part of the study. The desorbent, temperature and liquid flow circulation are also critical parameters that can be obtained from the pulse test. The pulse test is not only a critical tool for developing the equilibrium-selective adsorption process it is also an essential tool for other separation process developments such as rate-selective adsorption, shape-selective adsorption, ion exchange and reactive adsorption. 

One of the parameters in the broad class of equilibrium-selective adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. ZeoUtes can be ion-exchanged with a variety of metal cations... 

In a simulation system, we investigate the equilibrium selective adsorption and nonequilibrium transport and separation of gas mixture in the nanoporous carbon membrane are modeled as slits from the layer structure of graphite. A schematic representation of the system used in our simulations is shown in Fig. 11.21(a) and (b), in which the origin of the coordinates is at the center of simulation box and transport takes place along the x-direction in the nonequilibrium simulations. In the equilibrium simulations, the box as shown in Fig. 11.21(a) is employed, whose size is set as 85.20 nm x 4.92 nm x (1.675 + JV) nm in x-, y-, and z-directions, respectively, where JV is the pore width, i.e. the separation distance between the centers of carbon atoms on the two layers forming a slit pore (Fig. 11.21). is the separation distance between two centers of adjacent carbon atom L is the pore length JV is the pore width, A... 

An enlarged view of a crystal is shown in Fig. VII-11 assume for simplicity that the crystal is two-dimensional. Assuming equilibrium shape, calculate 711 if 710 is 275 dyn/cm. Crystal habit may be changed by selective adsorption. What percentage of reduction in the value of 710 must be effected (by, say, dye adsorption selective to the face) in order that the equilibrium crystal exhibit only (10) faces Show your calculation. 

Adsorption The separation achieved depends in part on the selectivity of adsorption at the bubble surface. At equilibrium, the adsorption of dissolved material follows the Gibbs equation (Gibbs, Collected Works, Longmans Green, New York, 1928). 

Although most of the commercial adsorptive separation processes are operated under the selective-equilibrium adsorption mechanism, adsorptive separation may also be based on diffusion rates through a permeable barrier which are designated as rate-selective adsorption processes. In some instances there may be a combination of equilibriums as well as rate selective adsorption. A rate-selective adsorption process yields good separation when the diffusion rates of the feed components through the permeable barrier differ by a wide margin. 

Shape-selective adsorption, also known as molecular sieving, is a process that separates molecules based on inclusion or exclusion from specific zeolite pores. In contrast, the equilibrium- and rate-selective mechanisms are based on adsorb-... 

Na -loess clay, where batch experiments were analyzed by X-ray diffraction and infrared and far-infrared measurements. The adsorption isotherm (Fig. 8.36) shows that loess clay is selective for cesium cations. The raw material contained a large amount of quartz, and the clay material was a mixture of kaolinite and an interstrati-fied iUite-smectite mineral as a result, equilibrium Cs" adsorption data are not consistent with a single site Langmuir model. Cesium adsorption on this particular soil clay occurs by cation exchange on sites with various cesium affinities. At low concentration, far-infrared spechoscopy shows the presence of very selective adsorption sites that correspond to internal collapsed layers. At high concentration, Cs MAS-NMR shows that cesium essentially is adsorbed to external sites that are not very selective. 

The selective adsorption of the hydrogen ions cannot proceed to tr ue equilibrium owing to the electrostatic attraction between the dissimilar ions, consequently chlorine ions are adsorbed in excess of their equilibrium concentration and since the hydrogen ions on adsorption have to do work in increasing the surface concentration of chlorine ions above their proper value, the true adsorption value of the hydrogen ions is not attained. 

Since benzene is not likely to undergo hydrogen bonding, it was expected that Z200H would be suitable. There was, however, some uncertainty about the selective adsorption of the water over the acid, so some very preliminary static equilibrium capacity data were generated. 

It is recognized that filtration is operational, that colloidal-bound PCB congeners are not retained by the filter, and that operational dissolved measurements may be biased positively by colloidal material. Techniques to measure truly dissolved PCBs include gas sparging, differential diffusion into membrane-bound lipids (e.g., semipermeable membrane devices, [230]), and selective adsorption (e.g. non-equilibrium solid phase microextraction [231, 232]). Unfortunately, none of these techniques has sufficient sensitivity to reliably and unambiguously measure truly dissolved PCB congeners at the levels present in the Great Lakes. 

Assuming non-selective adsorption and observing that the equilibrium catalyst Fraction A exhibits a crystallinity of 28% with a micropore volume of 0.075 cc/g, the expected increase in density due to a given change in micropore volume, Ampore, is ... 

Plots of selectivity factor (calculated using Equation 2 and the data from Table I) for mephenytoin and hexobarbital enantiomers versus CD concentration are shown in Figure 3 a,b (22) The profiles of relation oC vs [(3-CD] for these two compounds are different because two different factors determine resolution of their enantiomers difference in K- values for hexobarbital and difference in kl t ftnn values for mephenytoin. The latter case represents 5nuinteresting example the resolution of its enantiomers arises from the great differentiation in the adsorption of diastereoisomeric (3-CD complexes. The calculated selectivity factor for these complexes is ca 3 (see Table I). In this particular case selectivities of the two processes adsorption and com-plexation in the bulk mobile phase solution are opposite to each other enantioselectivity arising from selective adsorption dominating over differentiation in the solution. Unfortunately the stabilities of diastereoisomeric -CD mephenytoin complexes are relatively small and solubility of -CD in the mobile phase solution is rather limited. Therefore one cannot shift the comple-xation equilibrium... 

The primary requirement for an economic separation process is an adsorbent with high selectivity and capacity. The selectivity may depend upon differences in either kinetics or thermodynamic equilibrium of adsorption. Differences in diffusion rates between molecules, due to steric effects, can be large enough to provide transient selectivity. The separation factor is the ratio between the diffusion coefficients of the molecules. 

Desiccants. A solid desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or liquid) stream. The main requirements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are silica gel, activated alumina, and the aluminum rich zeolites (4A or 13X). The equilibrium adsorption isotliemis for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different applications. 

The method of predicting the mixture adsorption isotherms is to first select the feed mole fractions of interest and to pick an adsorption level within Region II. The pure component standard states are determined from the total equilibrium concentration that occurs at that set level of adsorption for the pure surfactant component adsorption isotherms. The total equilibrium mixture concentration corresponding to the selected adsorption level is then calculated from Equation 8. This procedure is repeated at different levels of adsorption until enough points are collected to completely descibe the mixture adsorption isotherm curve. 

Counterdiffusion of cumene and 1-MN occurred readily in type Y zeolite, as shown by several studies. The 1-MN is selectively adsorbed relative to cumene thus, when the zeolite was initially saturated with cumene and placed in 1-MN, essentially 100% of the cumene diffused out but when the zeolite was saturated with 1-MN and placed in cumene, only about 74% of the 1-MN diffused out. The same end point was also reached when SK-500 saturated with cumene was placed in a mixture of 1-MN and cumene in the proper ratio. This selective adsorption equilibrium value was essentially independent of temperature and, except for the cerium form of type Y, was independent of the nature of the cation... 

The other mechanism of the action of surfaces in altering the position of equilibrium is that removal of one of the products of reaction by selective adsorption may swing the reaction towards producing more of the adsorbable component. In the synthesis of the condensed product of Wasteneys and Borsook (68), and Section VI, for example, the surface probably acts in this way. 

The concept that the removal of an undesired reaction product by selective adsorption from the reaction zone of an equilibrium-controlled reaction increases the conversion and the rate of formation of the desired component (based on Le Chatelier s principle) was used to develop a novel PSA process concept called SERP for direct production of fuel cell-grade hydrogen by steam reforming of methane (CH4 + 2H20 44 C02 + 4H2).57 61 The concept uses a physical admixture of a reforming (noble metal on alumina) catalyst and a chemisorbent (K2C03 promoted hydrotalcite), which selectively and reversibly chemisorbs C02 from a gas at a temperature of -450 °C in the presence of steam. The cyclic SERP steps consisted of the following ... 

Here Xa, Ya are strictly equilibrium mole fractions for component A in the adsorbed phase and adsorbate (fluid) phase, respectively as are Xb, Fb for component B. For equilibrium-based adsorptive separation process, the adsorbent selectivity is the same as the separation factor as defined in Eq. (1). Apparently, this definition is not applicable to other processes based on kinetic and steric effects. In a kinetically controlled adsorption process, the adsorbent selectivity depends on both equilibrium and kinetic effects. A simplified definition for adsorbent separation factor is given by Ruthven et al. ... 

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