Equilibria of Binary Gas Mixture Adsorption by Active Carbon

Abstract

Separation of gas mixtures by adsorption has become an important unit operation in the petrochemical industry. To design an apparatus for separating gas mixtures by adsorption, it is necessary to know the adsorption equilibria of gas mixtures.Equilibria of adsorption, by active carbon, of various gas mixtures such as CO2-C2H4, CO-CO2, CO-C2H4, H2-CO2 and H2-C2H4 were measured at various temperatures and total pressures. And equations expressing the isothermal adsorption of each component gas from these mixtures were examined.ExperimentalThe binary adsorption equilibria were measured by using the apparatus shown in Fig. 1 (a). 1.2 or 4.0gr of active carbon was packed in a vessel, A, and the gas mixture was circulated through the bed of the adsorbent by means of a circulating pump, D. The circulating pump was equipped with a glass valve, E, having a piece of iron in it and with an electromagnet, F, through which intermittent direct current was sent. This intermittent direct current was obtained from the ordinary 100 volt alternative current by the use of a relay shown in Fig. 1 (b). After the adsorption reached the state of equilibrium, the gas composition in the adsorption system was measured with the interferometer, I, which was modified specially for use in vacuum system. The chamber volume of the interferometer was about 7.5cm3. The accuracy of the analysis varied with pressure, temperature and difference in refractivity of the two pure gases, and the errors at 0°C, calculated in terms of pressure, were as follows: 0.84mm Hg for CO2-C2H4, 2.0mm Hg for CO-CO2, 0.59mm Hg for CO-C2H4, 0.39mm Hg for H2-C2H4, and 0.73mm Hg for H2-CO2. Material balance revealed the composition of the adsorbed phase.The adsorption equilibria were measured at 0, 25, 30 and 50°C, and 200, 300, 450, 600 and 760mm Hg for CO2-C2H4, CO-CO2, CO-C2H4, H2-C2H4 and H2-CO2 gas mixtures.Results and DiscussionThe relations among x-y equilibria for various gas mixtures are shown in Fig. 3 and in Table 1. From these experimental results was calculated the relative adsorptivity, α, in Eq. (7), which is employed as a measure of separability of these gasses as shown in Table 2.In order to find out the equation of isothermal adsorption for the gas mixtures, it was examined, in the first place, whether the Markham-Benton Equation (Eqs. (1) and (2)) of the ideally mixed adsorption was applicable to the adsorption of these gases.The adsorptions of CO2 and C2H4 from CO2-C2H4 gas mixture were expressed by the Markham-Benton Equation (Figs. 7 and 8). In the case of CO-CO2 and CO-C2H4 gas mixtures, the adsorptions of CO2 and C2H4 were expressed by Eq. (1), While the adsorption of CO could not be expressed by Eq. (1).Consequently it was supposed that the adsorption of CO was affected by the presence of adsorbed molecules of CO2 or C2H4. Therefore, Eq. (1) was empirically modified as follows:(1) On the assumption that the adsorbed molecules of component B were already uniformly distributed on the surface, and that the extent of the adsorption of the component A were varied within a certain distance from the adsorbed molecules B by the repulsion or attraction forces of the adsorbed molecules B, Eq. (1) would be re-written as Eq. (8), (2) On the assumption that the adsorbed molecules of B