Mass transfer in adsorption

Diffusion in porous solids is usually the most important factor con-troUing mass transfer in adsorption, ion exchange, drying, heterogeneous catalysis, leaching, and many other applications. Some of the... 

The phenomenological aspects of diffusional mass transfer in adsorption systems can be described in terms of Fick s law ... 

Although adsorptive processes have been extensively studied for gas separation, catalysis, it is only recently that they have been proposed for heat management. M. Tather et al. [51] developed a mathematical model for a novel arrangement proposed in order to cope with the drawbacks originating from the inefficient heat and mass transfer in adsorption heating pumps with Zeolite 4A used as the adsorbent. L. Bonaccorsi et al. [52] have successfully synthesized zeolite coatings on metal supports with thickness ranging from few to several tens of microns, which had important technical applications in adsorption heat pumps. 

Yao, C, and Tien, C, Approximation of intraparticle mass transfer in adsorption processes Linear and nonlinear systems, Chem. Eng. Sci., 47(2), 457-474 (1992). 

Georgiou, A., and Kupiec, K., Nonlinear driving force approximations of intraparticle mass transfer in adsorption processes The effect of pellet shape, Dev. Chem. Eng. Mineral Process., 4(3), 235-244 (1996). 

FIGURE 7.24 Mass transfer in adsorption processes (a) fixed beds and (b) intrapellet mass transfer. (Reproduced from C. Tien. Butterworth-Heinemann Series in Chemical Engineering. Butterworth-Heinemann, Boston, 1994. With permission.)... 

Adsorption. In the design of the adsorption step of gas-phase processes, two phenomena must be considered, equiUbrium and mass transfer. Sometimes adsorption equiUbrium can be regarded as that of a single component, but mote often several components and their interactions must be accounted for. Design techniques for each phenomenon exist as well as some combined models for dynamic performance. 

Fernandez, M. A., Laughinghouse, W. S., and Carta, G. J. Chromatogr. A, 746 (1996) 184-198. Characterisation of protein adsorption by composite silicapolyacrylamide gel anion exchanges II mass transfer in packed columns and predictability of breakthrough behaviour. 

A1 Duri, B., 1995, Adsorption Modeling and Mass Transfer. In Use of Adsorbents for the Removal of Pollutants from Wastewaters, G. McKay (ed.), CRC, Boca Raton, Elorida, pp 133-173. 

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

Adsorption According to Fernandez and Carta (1996), who studied mass transfer in agitated reactors, the relative importance of external and intraparticle mass transfer resistances is strongly dependent on the solution composition. They used the following dimensionless number ... 

The third term of Equation 2.80 accounts for resistance to mass transfer in liquid phase. An obvious way of reducing this term is to reduce the liquid film thickness d. This causes a reduction in k and an increase in the term k1/(1 + k )2- However, using thinly coated column packings increases the probability of adsorption of solute molecules on the surface of support material, which might result in peaks tailing. 

The effects of physical transport processes on the overall adsorption on porous solids are discussed. Quantitative models are presented by which these effects can be taken into account in designing adsorption equipment or in interpreting observed data. Intraparticle processes are often of major importance in adsorption kinetics, particularly for liquid systems. The diffusivities which describe intraparticle transfer are complex, even for gaseous adsorbates. More than a single rate coefficient is commonly necessary to represent correctly the mass transfer in the interior of the adsorbent. 

Finally, intraparticle diffusion appears to be an important factor in adsorption kinetics for many types of systems. In the past it has been customary to define such mass transfer quantitatively in terms of an effective diffusivity. However, even in gas-solid systems more than one process can be involved for porous particles. Thus, two-dimensional migration on the pore surface, surface diffusion, is a potential contribution. Liquid systems appear to be more complex, and, with electrolytes, it has been shown that the electric potential induced by counter-diffusing ions should be taken into account. A realistic description of intraparticle mass transfer in such cases requires more than a single rate coefficient for a binary system. 

Solute-solute Interactions may affect the diffusion rates In the fluid phase, the solid phase, or both. Toor (26) has used the Stefan-Maxwell equations for steady state mass transfer In multicomponent systems to show that, in the extreme, four different types of diffusion may occur (1) diffusion barrier, where the rate of diffusion of a component Is zero even though Its gradient Is not zero (2) osmotic diffusion, where the diffusion rate of a component Is not zero even though the gradient Is zero (3) reverse diffusion, where diffusion occurs against the concentration gradient and, (4) normal diffusion, where diffusion occurs In the direction of the gradient. While such extreme effects are not apparent in this system, it is evident that the adsorption rate of phenol is decreased by dodecyl benzene sulfonate, and that of dodecyl benzene sulfonate increased by phenol. 

When we started selling molecular sieves, little was known about heat and mass transfer in fixed bed adsorption/desorption. Design was an art. G.J. Griesmer, J.J. Collins, F.W.Leavitt, W.F. Avery, and K. Kiyonaga made it a science, a unit process we understand and can optimize. Griesmer led much of this activity and was primarily responsible for Union Carbide s Iso Sieve process for separating normal from iso paraffins [37]. 

Note Since the model is linear for the special case considered, the same equation is also satisfied by the other three variables.) The following observations may be made from Eq. (98) that expresses the dimensionless dispersion coefficient A (i) The first term describes dispersion effects due to velocity gradients when adsorption equilibrium exists at the interface. We note that this expression was first derived by Golay (1958) for capillary chromatography with a retentive layer, (ii) The second term corresponds to dispersion effects due to finite rate of adsorption (since this term vanishes if we assume that adsorption and desorption are very fast so that equilibrium exists at the interface), (iii) The effective dispersion coefficient reduces to the Taylor limit when the adsorption rate constant or the adsorption capacity is zero, (iv) As is well known (Rhee et al., 1986), the effective solute velocity is reduced by a factor (1 + y). (v) For the case of irreversible adsorption (y — oo and Da —> oo), the dispersion coefficient is equal to 11 times the Taylor value. It is also equal to the reciprocal of the asymptotic Sherwood number for mass transfer in a circular... 

---