The Adsorption Wave

Adsorption Chromatography. The principle of gas-sohd or Hquid-sohd chromatography may be easily understood from equation 35. In a linear multicomponent system (several sorbates at low concentration in an inert carrier) the wave velocity for each component depends on its adsorption equihbrium constant. Thus, if a pulse of the mixed sorbate is injected at the column inlet, the different species separate into bands which travel through the column at their characteristic velocities, and at the oudet of the column a sequence of peaks corresponding to the different species is detected. 

Fig. 3. Passage of the adsorption wave through a stationary bed during the course of an adsorption cycle. The progressing S-shaped curves indicate the nonadsorbed vapor concentration by position in the bed at different time periods. represents the maximum permissible oudet concentration for release...

Electrochemical reduction of iridium solutions in the presence azodye (acid chrome dark blue [ACDB]) on slowly dropping mercury electrode is accompanied by occurrence of additional peaks on background acetic-ammonium buffer solutions except for waves of reduction azodye. Potentials of these peaks are displaced to cathode region of the potential compared to the respective peaks of reduction of the azodye. The nature of reduction current in iridium solutions in the presence ACDB is diffusive with considerable adsorptive limitations. The method of voltamiuetric determination of iridium with ACDB has been developed (C 1-2 x 10 mol/L). 

In a GAC column, dynamic adsorption occurs along an adsorption wave front where the impurity concentration changes. 

Copper(II) ions in the presence of chloride ions are reduced at the dropping mercury electrode (dme) in two steps, Cu(II) -> Cu(I) and Cu(I) -> Cu(0) producing a double wave at -1-0.04 and 0.22 V versus sce half-wave potentials. In the presence of peroxydisulphate , when the chloride concentration is large enough, two waves are also observed the first limiting current corresponds to the reduction of the Cu(II) to Cu(I) plus reduction of a fraction of peroxydisulphate and the total diffusion current at a more negative potential is equal to the sum of the diffusion currents of reduction of Cu(II) to Cu(0) and of the peroxydisulphate. There is evidence that peroxydisulphate is not reduced at the potential of the first wave because of the adsorption of the copper(I) chloride complex at... 

Based upon the concepts of the adsorption of the anode reaction product, the share of the anodic curve, on which the carbamide oxidation processes is reflected as a wave, can be explained. It may be assumed that the adsorption of the reaction product inhibits the direct oxidation of carbamide. To verify this conclusion, the anode was polarized to the electrolysis product formation potential, and the reverse sweep was stopped before the electrolysis product was reduced at the electrode. Then the carbamide oxidation process was completely inhibited on the subsequent forward sweep, and the curve exhibited only a current increase at the chlorine ion oxidation potential. 

Similar mechanisms appear to apply for the reduction in methanol of R3SnX and Ar3SnX. Polarography over Hg characteristically involves three reduction waves the first, for adsorption, is followed by two reduction stages89-91. The first two waves comprise a first step of the reaction in which bis(trialkylstannyl)mercury or bis(triarylstannyl)mercury is produced. For example ... 

Reduction to the neutral radical appears as an irreversible wave at -0.9 V. Neither anodic peak exhibits the shape characteristic of stripping a solid coating from the electrode hence precipitation of the radical cation or neutral radical on the electrode is not evident (11-13). The sharp peaks at +0.46 V are tentatively assigned to desorption and adsorption of the CiebpyMe2 there are no anticipated redox reactions at that potential. 

Figure 17.16. The distribution of adsorbate concentration in the fluid phase through a bed (a) Development and progression of an adsorption wave along a bed. (b) Breakthrough curve...

Equation 17.75 is important as it illustrates, for the equilibrium case, a principle that applies also to the non-equilibrium cases more commonly encountered. The principle concerns the way in which the shape of the adsorption wave changes as it moves along the bed. If an isotherm is concave to the fluid concentration axis it is termed favourable, and points of high concentration in the adsorption wave move more rapidly than points of low concentration. Since it is physically impossible for points of high concentration to overtake points of low concentration, the effect is for the adsorption zone to become narrower as it moves along the bed. It is, therefore, termed self-sharpening. 

Effect of the shape of the isotherm on the development of an adsorption wave through a bed with the initial distribution of adsorbate shown at t = 0... 

All concentrations move at the same velocity. If z0 = 0 at t = 0 for all concentrations, the adsorption wave propagates as a step change from the inlet to the outlet concentration,... 

It may be noted that, when the adsorption wave begins to emerge from the bed, the bed is saturated in equilibrium with the inlet concentration. 

As C increases, f(C) decreases and points of higher concentrations are predicted to move a greater distance in a given time than lower concentrations. It is not possible for points of higher concentrations to overtake lower concentrations, and if zo = 0 for all concentrations, the adsorption wave will propagate as a step change similar to case a. 

The relatively poor conductivity of a packed bed makes it difficult to get the heat of regeneration into the bed, either from a jacket or from coils embedded in the packing. This is more easily achieved by preheating the purge stream. Even in the best conditions, it takes time for the temperature of the bed to rise to the required level. Thermal regeneration is normally associated with long cycle times, measured in hours. Such cycles require large beds and, since the adsorption wave occupies only a small part of the bed on-line, the utilisation of the total adsorbent in the unit is low. 

The inter-pellet air velocity = 0.233 m/s. The velocity uc with which the adsorption wave moves through the column may be obtained from equation 17.79. 

The left-hand side of equation 17.123 is the velocity of a point of fixed concentration on the adsorption wave. For a linear isotherm and if longitudinal diffusion is neglected, all points of concentration will move at the same velocity. Changing the pressure will affect u and, to a lesser extent, Ka. 

The existence of multiple peaks for molecular desorption has been attributed to lateral interactions among adsorbed species 62-64). As discussed previously, adsorption onto the surface lattice may occur preferentially in next nearest neighbor sites to form p(2 x 2) structures. Even at low coverages, attractive forces may cause adatoms to occupy next nearest neighbor positions, so that clusters of adsorbate form which have local twofold periodicity 65) with respect to the surface. Such effects are entirely consistent with the perturbations of the surface electronic wave functions due to adsorption 66-68) which show that these binding sites represent the... 

Note that the product /A yields the scan rate of the square-wave potential modulation. If the delay period is sufficiently long, the additional adsorption during the potential scan is negligible. Otherwise, the additional adsorption complicates the theoretically expected dependencies, in particular the relationships between the net peak currents and potentials on the frequency [114]. 

The speed of the adsorption wave can be readily derived by introducing the linear isotherm assumption and the chain mle derivative of q with respect to t. The wave speed results because the assumptions turn Eq. (9.10) into a kinematic wave equation and the wave speed W is instantly recognized as ... 

When no analyhcal soluhon can describe the process satisfactorily it may be possible, working from Eq. (9.18) (which describes the length of the wave) and either Eq. (9.11) or (9.13) (the expression for the velocity of the adsorption wave), to assemble a simple wave mechanics solution that approximates the length and movement of the mass transfer front in the bed. As with analytical solutions this method can deliver useful results that may approximate the wave shape inside the bed and thus can be used to describe the shape and duration of the breakthrough curve that occurs as the wave intercepts and crosses the end of the bed. Such methods are generally only applicable for one or at most two adsorbable components. 

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