Adsorption of gaseous hydrocarbons on activated carbon: Characteristic kinetic curve

Abstract

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...

Study of the adsorption and desorption of multiple adsorbates in a fixed bed

“…1 1 are 7.5 x lo-* and 9.1 x m2/s, respectively. Costa et al (1985) reported the internal diffusion coefficient of ethane on AC-40 activated carbon to be 1.0 x m'/s a t 2OoC, which compares well with the value calculated here.…”

“…where The gas-phase concentration profile within the particle is assumed to be parabolic (Tsai et al, 1983): The surface tortuosity factor T, is often assumed equal to rp in the absence of a better approximation (Satterfield, 1981). and 1.5 x m'/s for the surface diffusivities of ethane and propane, respectively, on silica gel a t 323 K. Costa et al (1985) reported an average value of 3.8 x lo-'' m'/s for ethane on AC- …”

“…This high surface area provides a chance for high surface concentration, and thus the surface diffusion dominates the flux in the pores. Costa et al (1985) observed that the contribution of surface diffusion to the global mass transfer inside the activated carbon could be as much as 70-8096 for methane, ethane, and ethylene at 293 K and pressures below 40 kPa.…”

Kinetic study of time-dependent fixation of UVI on biochar

“…It has been successfully applied to describe time-dependent sorption processes in materials, including soil [19,20], minerals [21] activated carbon [22] and waste materials e.g. 'bottom ash' and de-oiled soya [23].…”

Dynamic Measurement of Copper (II) Ion Adsorption in Activated Carbon Fixed Bed Columns

“…Although in the earlier works [1,4] , it was found that diffusion contributed little to breakthrough in adsorption from the gas phase, this can no longer be assumed for adsorption from liquids. Indeed, a large literature discusses the importance of diffusion in the latter system [5,8] . Diffusion through a boundary layer on the outer surfaces of the activated carbon particles may also contribute to the mass transfer resistance.…”