A new combined-technique frequency response apparatus has been developed for studying mass transfer in porous adsorbents. This new apparatus can perform pressure-swing, volume-swing, and concentration-swing frequency response experiments, enabling it to investigate mass transfer of pure components and binary gas mixtures in adsorbents over a wide frequency range. To demonstrate the capabilities of the apparatus, pressure-swing and volume-swing frequency response experiments were performed in tandem to study transport of pure CO 2 in 13X zeolite beads over the frequency range from 10 −4 Hz to 10 Hz at pressures from 0.125 to 1 bar. Frequency response spectra show that transport in this system is governed by a nonisothermal macropore diffusion resistance with diffusion occurring by a Knudsen-type mechanism. Macropore diffusion control is confirmed by the existence of particle size dependence of the system's dynamic response. In smaller zeolite particles, the macropore diffusion resistance is lower, and macropore diffusion becomes less important to the system response.

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Transport of binary O 2 /Ar gas mixtures in a carbon molecular sieve (CMS) has been studied using concentrationswing frequency response. The rate data for this binary gas system are described well by the transport models (surface barrier for Ar and surface barrier plus micropore diffusion for O 2 ) and parameters that were found to govern transport of the respective pure gases on the same CMS. These results suggest that transport of O 2 is not impeded by the presence of Ar. However, the experimental data do not rule out the possibility of some enhancement of the O 2 transport rate. Furthermore, O 2 and Ar isotherms on this CMS were observed to be linear in the region over which frequency response experiments were performed, suggesting that the micropore diffusion component of O 2 transport behavior is unaffected by the presence of Ar. Because transport of Ar in this CMS is much slower than O 2 , the dynamic response of this system owes the majority of its character to O 2 adsorption, and the effect of O 2 on Ar transport remains hidden. INTRODUCTIONCarbon molecular sieve (CMS) materials have received attention as potential adsorbents for the production of high-purity (>99%) O 2 from air by pressure swing adsorption (PSA). 1−14 In traditional zeolite-based PSA processes, the O 2 product is often limited to 94−95% purity, as the similar adsorption behavior of O 2 and Ar on many zeolites results in a substantial Ar impurity in the product. 1,2 In CMS, however, O 2 adsorbs much more rapidly than Ar, allowing separation of these two gases to be effected by the difference in their adsorption rates.To rigorously design a PSA process utilizing CMS or any other adsorbent, accurate knowledge of the adsorption rate behavior of the relevant adsorbent/gas system is desirable. Adsorption rates are generally limited by mass transfer, which can occur by one or more of a variety of possible mechanisms including micropore diffusion, macropore diffusion (ordinary diffusion, Knudsen diffusion, or Poiseuille flow), transport across a surface barrier, and external mass transfer. The existence of cross-coefficients introduced by multicomponent transport models can further complicate adsorption rate behavior. As a result, adsorption rates are an important and nontrivial subject worthy of thorough investigation.As CMS has long been recognized for its ability to produce purified N 2 from air, many of the studies of adsorption rates of atmospheric gases on CMS have been focused on O 2 and N 2 . 15−28 However, with the exception of a few studies, 24,25,29 adsorption rates of Ar have been largely ignored. Moreover, though adsorption rates of pure gases are generally presumed to differ from those of gases in multicomponent mixtures, studies of adsorption rates in such mixtures have been infrequent. 26 We thus conclude that there remains a need for further study, especially regarding adsorption rates of mixtures of O 2 and Ar.In a recent work, 30 we studied pure component adsorption rates of O 2 , N 2 , and Ar on two varieties of CMS using pr...

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The mixture of isobutyric acid and water has a consolute point at a temperature of 25.75 °C and mole fraction 0.1148 isobutyric acid. When charcoal is added to this mixture, the concentration of isobutyric acid is reduced by adsorption. We have measured the action of charcoal on solutions of isobutyric acid and water as a function of isobutyric acid mole fraction at temperatures of 25.85 and 32.50 °C. At the higher temperature, the specific adsorption density (y(2)(α)/m) satisfies the Freundlich equation (y(2)(α)/m)=KX(2)(1/n), where y(2)(α) is the mass of isobutyric acid adsorbed, m is the mass of charcoal, X(2) is the equilibrium mole fraction of isobutyric acid, n is the Freundlich index, and K=K(T) is an amplitude that depends upon the temperature T. At 25.85 °C, a critical endpoint is located at an isobutyric acid mole fraction X(2)(ce)=0.09. When compared with the Freundlich equation at this temperature, a plot of the specific adsorption density as a function of X(2) in the vicinity of the critical-endpoint composition assumes a shape which is reminiscent of the derivative of a Dirac delta function. Using critical-point scaling theory, we show that this divergent pattern is consistent with the principle of critical point universality.