Mass transfer with physical adsorption

A fascinating application of enhancing mass transfer by physical adsorption on activated carbon recently has been published by Jana-kiraman and Sharma [2l]. These authors studied the role of addition of activated carbon particles with d in alkaline hydro-... 

The intrinsic rate of physical adsorption is very rapid so the overall sorption rate is generally controlled by the diffusional resistances associated with mass transfer to the adsorption site. Commercial zeoUte-based catalysts and adsorbents consist of small (micron-sized) zeolite crystals formed into macro-porous (milUmeter-sized) particles, generally with the aid of a clay binder. Such materials offer at least three and in some cases four distinct mass transfer resistances (see Fig. 10) [35] ... 

In contrast, physical adsorption is a very rapid process, so the rate is always controlled by mass transfer resistance rather than by the intrinsic adsorption kinetics. However, under certain conditions the combination of a diffiision-controUed process with an adsorption equiUbrium constant that varies according to equation 1 can give the appearance of activated adsorption. 

External mass transfer is the only process of the three involved in adsorption that can be predicted with reasonable accuracy from physical data. Mass transfer from the bulk gas to the particle surface can be considered by the film resistance approach. The rate of mass transfer is proportional to the external surface area of the adsorbent particles and the adsorbate concentration difference between the bulk gas and the particle external surface. The proportionality constant is the mass transfer coefficient, the reciprocal of the resistance to... 

To represent the adsorption dynamics in column, the linear driving force (LDF) approximation model for overall mass transfer coefficient was applied. The LDF model for gas adsorption dynamics is frequently and successfully used for analysis of column dynamics because it is simple, analytic, and physically consistent [7J. We assumed that the velocity of the gas in column is constant, and radial temperature, concentration and velocity gradients within the bed are negligible in this model. With the ideal gas-law assumption, the set of equation for this work is as follow ... 

Drinkenburg and Rietema (D17) have presented a numerical computation of /cob based on the stream functions given by Davidson and Harrison (D3) and by Murray (M47). The bubble-void resistance to mass transfer has been neglected. Enhancement of gas transfer rate by diffusion with simultaneous chemical reaction (Fig. 5 of D17) is reasonably well expressed by Eq. (6-11), the enhancement being expressed as the Hatta number. Enhancement by physical adsorption (Fig. 2 of D17) is also approximated by Eqs. (6-22) or (6-23) for smaller particles. 

We are considering a situation in which the several processes have come to a steady relationship. Thus the concentration may differ from Cj because the molecules of A, are being adsorbed and desorbed at the surface. In fact, the rate of these processes will depend on the and the concentration of immediately above the surface. Let this be denoted by Cj, then Cja may not be the same as Cj because there is a resistance to mass transfer between the bulk phase and the surface. If Aj is a reactant we would expect Cj to be greater than to provide the potential for driving it to the surface where it is adsorbed and it reacts if Aj is a product, it is formed at the surface and so Cj, would tend to be greater than Cj, However, the only place that there is any change from one species to another is by reaction at the surface and here the rate of production of Aj is proportional to its stoichiometric coefficient Now when the different physical processes have reached a steady relationship with one another, the rate of transfer of a reactant species from the bulk phase to the reaction surface and the rate of adsorption must equal the rate at which it reacts, otherwise there would be a continual accumulation or depletion at the surface. Similarly, for a product species the rate of transfer or desorption must be the rate of formation. Thus in the bulk phase, in the volume element as a whole, the rate of production of Aj must be proportional to its stoichiometric coefficient a/, we can write Cj — Cjq... 

Although many physical processes of interest to chemical reaction engineers involve absorption, heterogeneous reaction, surface mass transport, and interfacial mass transfer at moving and deforming interfaces, their main focus is concerned with the phenomena occurring at two particular types of interface systems. These are (1) the adsorption and reaction processes taking... 

Such characteristic horizontal bands, as shown in Figure 5.43, might be termed adsorption multiplicity since they arise primarily from the interactions of external film heat and mass transfer resistances with the adsorption resistance. For smaller adsorption resistance, the horizontal bands disappear giving way to the more familiar multiplicity regions, arising from the interactions of physical transport and surface reaction resistances. 

Rate processes, on the other hand, are limited by the rate of mass transfer of individual components from one phase into another under the influence of physical shmuli. Concentrahon gradients are the most common stimuli, but temperature, pressure, or external force fields can also cause mass transfer. One mass-transfer-based process is gas absorption, a process by which a vapor is removed from its mixture with an inert gas by means of a liquid in which it is soluble. Desorption, or stripping, on the other hand, is the removal of a volatile gas from a Hquid by means of a gas in which it is soluble. Adsorption consists of the removal of a species from a fluid stream by means of a solid adsorbent with which it has a higher affinity. Ion exchange is similar to adsorption, except that the species removed from solution is replaced with a species from the solid resin matrix so that electroneutrality is maintained. Lastly, membrane separations are based upon differences in permeability (transport through the membrane) due to size and chemical selectivity for the membrane material between components of a feed stream. 

In chapter 4 we discussed the physical properties of chromatographic adsorbents. In this chapter we will discuss their chemical properties. The most important aspect of the chemistry of a packing is the character of the adsorbing surface. But the chemistry of the packing also plays a role with respect to its hydrolytic stability or whether and to what degree it shrinks and swells in various solvents. In addition, the chemistry of the packing influences its physical strength. The chemistry of the surface influences adsorption kinetics and mass transfer. 

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