On the surface diffusion of adsorbable gases through porous media

Ponzi, Marta I.; Papa, José; Rivarola, J. B.; Zgrablich, G.

doi:10.1002/aic.690230319

Cited by 22 publications

(10 citation statements)

References 12 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…1, the slope of the characteristic kinetic curve of the adsorbent at each point in Figure 8 coefficient with progressive surface coverage in the range studied is probably due to the advantageous effect of the increasing repulsion of adsorbed molecules, which is in agreement with the conclusions of other authors (Gilliland et al, 1958Ponzi et al, 1977;Sudo et a]., 1978). On the other hand, the sharp increase of the surface migration coefficient for values of the slope K* lower than 200 can be explained by taking into account that this range of slopes corresponds only to adsorbates of low adsorption capacity, such as methane in our case, which has a much higher surface diffusion coefficient than ethane and ethylene ( Table 4).…”

Section: Characteristic Kinetic Curvesupporting

confidence: 91%

Adsorption of gaseous hydrocarbons on activated carbon: Characteristic kinetic curve

1985

View full text Add to dashboard Cite

The internal diffusion coefficients, Di, of pure methane, ethane and ethylene as well as some of their binary and ternary mixtures, have been calculated at 20°C for solid particles of a commercial activated carbon. It has been observed that the contribution of the surface migration mechanism to the global mass transfer process inside the adsorbent particles can be as much as 7040%. Values for the surface migration coefficient Ds have also been calculated from the relation Di = Dg + KD,, where K is a dimensionless mean slope factor. Values found for both coefficients are of the same order of magnitude as those reported in the literature for similar systems.All the values for the internal diffusion coefficients of these pure components and their mixtures fit into a single correlation curve, the characteristic kinetic curve of the adsorbent. SCOPEThe increasing importance of adsorption processes for the separation of gaseous mixtures is due to the high selectivity and adsorption capacity of solid adsorbents, the less extreme operation conditions needed, and the small energy consumption required. Adsorption is becoming a competitive operation that can advantageously substitute for other separation operations such as distillation or liquid-liquid extraction. The advantages are especially attractive when the problem is the separation of light gases such as methane, ethane, ethylene and other typical hydrocarbons of gaseous refinery streams, since their separation by distillation requires expensive high pressure units.Although there are some commercial processes for the separation of hydrocarbons using gas adsorption (Milton, 1963; Priegnitz, 1973; Yatsurugi, 1974;Broughton, 1977), the extension of this method to other cases of practical interest is generally limited by the lack of reliable fundamental data for estimating the feasibility of a given separation and for reducing the empiricism of the present design of adsorbers. Among these fundamental data, the equilibrium adsorption isotherms and the internal diffusion coefficients of individual species of the mixture should be emphasized. Although substantial attention has been paid in the literature to the adsorp tion isotherms of pure compounds and their mixtures, even to the point of predicting multicomponent adsorption equilibria (Lewis et al., 1950 shown that the adsorption rates of pure propane, n-butane, ibutane, l-butene and other light gases into molecular sieves were lower when these compounds were present in binary mixtures. Feng et al. (1973Feng et al. ( ,1974 have extensively studied the diffusion of gaseous mixtures inside the pores of solid adsorbents, using several diffusion models. Their results, which were achieved by assuming a single pore size for the adsorbent and a reasonable tortuosity factor, were erroneous for not considering surface migration. We present here new values for the internal diffusion coefficients of methane, ethane, and ethylene, both as pure components and as part of the mixtures of methane-ethylene, ethaneethylene, and methane...

show abstract

Section: Characteristic Kinetic Curvesupporting

confidence: 91%

Adsorption of gaseous hydrocarbons on activated carbon: Characteristic kinetic curve

1985

View full text Add to dashboard Cite

show abstract

“…However, a molecule that jumps to the activated state can instead jump to the gas phase, thereby desorbing completely. We shall discuss briefly the way we can get the coverage dependence for a generalized concept of A. On the basis of a random walk with no return to the origin, X is found to be (Ponzi et al, 1977). (37) where Pef is the effective probability for a molecule to be captured when passing over an unoccupied site.…”

Section: Mean Free Path For Surface Diffusionmentioning

confidence: 99%

“…Several models to describe the surface flux of adsorbed gases on solids have been developed and tested during the past three decades (Babbit, 1950;Gilliland et al, 1958Gilliland et al, , 1974Higashi et al, 1963;Horiguchi et al, 1971;Weaver et al, 1966;Smith et al, 1964;Roybal et al, 1972;Sladeck et al, 1974;Ponzi et al, 1977;Horas et al, 1980).…”

Section: Introductionmentioning

confidence: 98%

Connectivity effects for surface diffusion of adsorbed gases

et al. 1986

View full text Add to dashboard Cite

Nonlocal models for the surface transport of adsorbed gases on solids are proposed and compared to experimental data. These models take into account connectivity effects due to heterogeneous adsorption potential surfaces and introduce a more realistic calculation of the surface mean free path. SCOPESeveral models exist that describe the surface flux of adsorbed gases in solids to various degrees of accuracy. However, there are two aspects that have not been taken into account in the literature:1. The cooperative nature of the effects of the surface adsorptive potential's heterogeneity, which means that the activation energy needed for a molecule to migrate through a distance R is determined not only by the adsorptive potential at a point but also by what we call the energetic topography of the surface within a distance R.2. The fact that a migrating molecule stops contributing to the surface flux not only when it is readsorbed on a site, but also when it evaporates from the surface into the gas phase. This particularly affects the calculation of the mean free path.The objective of this study is to develop a general surface diffusion model that is based on the connectivity concept of percolation theory and that takes these two aspects into account. CONCLUSIONS AND SIGNIFICANCEBy developing a close analogy of the surface diffusion of adsorbed gases on a heterogeneous surface with the conduction of electrons in an amorphous semiconductor, we were able to take into account the cooperative nature of the phenomenon and arrive at a remarkably simple equation for the surface permeability, Eq. 35: lS = I, exp {-(TJ T ) "~ r r ( e )~-~/~/ .We concluded our work by improving the calculation of the mean free path X(0) by including the evaporation contribution to it. We show that the model fits the experimental data for several gas-solid systems very well and that the parameters obtained are fully significant.In short, we have developed a model that leads to a very simple equation for the surface permeability, one that takes into account more physical facts than previous models and that fits experimental data at least as well as other models.

show abstract

“…The models most commonly used to describe the surface flow mechanism are the hydrodynamic model (Babbit, 1950Gilliland et al, 1958) and the hopping model (Hill, 1956Higashi et al, 1963Smith and Metzner, 1964;Weaver and Metzner, 1966;Ponzi et al, 1977;Okazaki et al, 1981). The dependence of the surface flow coefficient on the amount adsorbed and the temperature for many experimental results can be correlated by these models.…”

Section: Introductionmentioning

confidence: 98%