Adsorption front

In the irreversible limit R < 0.1), the adsorption front within the particle approaches a shock transition separating an inner core into which the adsorbate has not yet penetrated from an outer layer in which the adsorbed phase concentration is uniform at the saturation value. The dynamics of this process is described approximately by the shrinldng-core model [Yagi and Kunii, Chem. Eng. (Japan), 19, 500 (1955)]. For an infinite fluid volume, the solution is ... 

Abstract To design an adsorption cartridge, it is necessary to be able to predict the service life as a function of several parameters. This prediction needs a model of the breakthrough curve of the toxic from the activated carbon bed. The most popular equation is the Wheeler-Jonas equation. We study the properties of this equation and show that it satisfies the constant pattern behaviour of travelling adsorption fronts. We compare this equation with other models of chemical engineering, mainly the linear driving force (LDF) approximation. It is shown that the different models lead to a different service life. And thus it is very important to choose the proper model. The LDF model has more physical significance and is recommended in combination with Dubinin-Radushkevitch (DR) isotherm even if no analytical solution exists. A numerical solution of the system equation must be used. 

Fig. 17.1 Adsorption front (left) and breakthrough curve right)...

The shape of the adsorption front, the width of the MTZ, and the profile of the effluent concentration depend on the nature of the adsorption isotherm and the rate of mass transfer. Practical bed depths may be expressed as multiples of MTZ, values of 5-10 multiples being economically feasible. Systems that have linear adsorption isotherms develop constant MTZs whereas MTZs of convex ones (such as Type I of Figure 15.1) become narrower, and those of concave systems become wider as they progress through... 

Since taking samples of adsorbent from various positions in the bed for analysis is difficult, it is usual to deduce the shape of the adsorption front and the width of the MTZ from the effluent concentration profile which may be monitored with a continuous analyzer-recorder or by sampling. The overall width of the MTZ, for instance, is given in terms of the exhaustion and breakthrough times and the superficial velocity as... 

The combination supercritical fluid volatility amplification-molecular sieve process has the potential for producing two valuable products simultaneously from gas oil-lube oil petroleum stocks. In a flow-through molecular sieve bed, the leading adsorption front operates in the lean molecular sieve loading range, providing the conditions needed for the n-paraffin removal needed to produce low pour lube oil, while the trailing adsorption... 

One simple way to analyze the performance of a fixed-bed adsorber is to prepare a breakthrough curve (Figure 10.12) by measuring the solute concentration of the effluent as a function of time. As the solution enters the column, most of the solute will be adsorbed in the uppermost layer of solid. The adsorption front will move downward as the adsorption progresses. The solute concentration of the effluent will be virtually free of solute until the adsorption front reaches the bottom of the bed, and then the concentration will start to rise sharply. At this point (tb in Figure 10.12), known as the break point, the whole adsorbent is saturated... 

Since the movement of the adsorption front in a granule is relatively small, the adsorbate diffusion into the granule may be regarded as a pseudosteady process. 

The adsorption front having progressed for distance x, the steady diffusion rate will be... 

The adsorption front will travel a distance dx toward the center of the granule in a time dt at this particular distance from the center, the additionally filled volume being 47t (R — x)Mx. This volume is proportional to the adsorption increase dm, i.e.. 

This behavior is caused by the fact that the retention of the chemical decreases with increasing concentration in solution. In the case of the adsorption front, the retention decreases with increasing concentrations, and this effect leads to an instability and to the development of a narrow, self-sharpening front. In the case of the desorption front, the retention increases in time and leads to a broad, diffuse front (Biirgisser et al., 1993). 

From Fig. 5, it is apparent that the adsorption fronts are considerably less steep than the desorption fronts, and that the adsorption fronts simulated for different initial concentrations of the spots overlap. Similar behavior is apparent in the typical experimental densitograms, given in Figs. 3 and 4. In all these densitograms, the adsorption fronts for the different concentrations of acid also overlap. 

Satisfactory qualitative agreement between experimental and theoretical concentration profiles for polar analytes suggests their retention is substantially affected by lateral interactions, which are probably even more complex than is assumed in this isotherm model. Overlapping of the adsorption fronts can be explained solely on the basis of the lateral interactions among the adsorbed molecules. 

Stability Catalyst replacement between batches to overcome rapid poisoning Possibility of continuous renewal. Must have good attrition resistance Essential for fixed bed operation. Plug flow may establish a poison adsorption front... 

For complete separation the desorption fronts of the two components must not exceed points 1 and 2 respectively, which are located one column downstream the desorbent and extract port. Since the concentration profile displayed in Fig. 7.16 demonstrates the situation at the end of a switching interval, all ports will move one column downstream in the very next moment. In the case were the desorption front of component B does exceed point 2, the extract stream, meant to withdraw the more retained component A only, will be polluted with B after the ports have been switched. The same applies to point 1. If component A is shifted into section IV the adsorbent will transfer it to the raffinate port and the raffinate will be polluted. For the adsorption fronts, components A and B must not violate points 3 and 4, respec-... 

A higher feed rate at constant eluent consumption can be realized when the adsorption front of component A exceeds point 3 and/or the desorption front of component B exceeds point 2. 

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