110th Anniversary: New Volumetric Frequency Response System for Determining Mass Transfer Mechanisms in Microporous Adsorbents

Abstract

A new volumetric frequency response (VFR) system was developed for studying the mass transfer characteristics of gases in microporous adsorbents. For this VFR system, the differential pressure response from a small perturbation in volume was measured in a closed system after equilibrium was established for a gas adsorbate–adsorbent pair at constant temperature and pressure. It operates over a wide range of frequencies from 10–5 to 10 Hz, from atmospheric pressure down to vacuum pressures of 100 Torr, and at temperatures from 5 to 80 °C. The sample chamber holds up to 100 g of adsorbent. These operating ranges make this new VFR system capable of measuring mass transfer characteristics of adsorbate–adsorbent systems at conditions relevant to many commercial separation processes using a relatively large volume of adsorbent in a unique packed bed arrangement. The apparatus and procedure were described in detail, including the use of runs with different gases and different porous and nonporous solid beads and pellets to fully characterize the system in terms of its dynamic behavior especially at high frequencies and in terms of various volumes required in the analyses. It was shown how to analyze these baseline runs to correct for gas compression heating and pressure drop effects in the high frequency region of the pressure change amplitude response curves and to determine intensity (or amplitude ratio) and phase shift (or lag) response curves from which fundamental thermodynamic and kinetic information for an adsorbate–adsorbent pair could be extracted. To demonstrate the utility of this new VFR system, experiments were carried out with CO2 in 13X zeolite beads at 25 °C and 100, 200, and 760 Torr using 32 frequencies at each pressure. Slopes of this isotherm estimated from the intensity response curves at low frequency showed very good agreement with those measured independently. The mass transfer time constant estimated from the maximum in the phase lag response curve also agreed well with that reported in the literature. Unique features of the intensity and phase lag response curves were revealed.

Identification of mass transfer resistances in microporous materials using frequency response methods

“…It also shows that gas compression effect is more severe for monatomic gases with a higher specific heat ratio k (k = 1.67) than that for linear paraffin (k = 1.04 ~ 1.32) under similar experimental conditions [54]. Hossain et al [55] confirmed this unexpected feature at higher frequencies…”

“…The heat released from compression could cause substantial spurious effects for response curves at high frequencies, resulting in interference with mass transfer dynamics. The relative importance of gas compression is found to depend mainly on the gas type, pressure, and heatexchange rate at the adsorber wall from experimental and simulation studies [53][54][55]. Figure 2a shows the simulation results of pressure in a control experiment as a sequence for gas compression-the pressure amplitude exceeds the magnitude vP 0 at higher frequencies for a control experiment, leading to negative values of the in-phase component rather than to zero when the effect of gas compression heating is neglected.…”

“…It is critical to use mathematical treatment in Eqs. 6, 7 to have a relative pressure response ratio because the effect of gas compression on pressure response can be considerably reduced by correcting FR data relative to the blank experiment [33,54,55]. The effect of gas compression was further investigated experimentally by a thermal frequency response (TFR) developed by Bourdin et al [41,53,54].…”

“…Bourdin et al [41,53,54] introduced a thermal FR method to better distinguish thermal transfer from mass transfer limitation through the measurement of both the temperature and pressure of an adsorbent sample subjected to volume modulation. While most investigations were limited to low pressures, Hossain et al [4,55] recently reported a new volumetric system to expand pressures up to 1 bar.…”

Evidence of Combined Heat-Transfer and Surface Barrier Resistances for Propane in Commercial ZIF-8 Crystals Probed by Pressure-Swing Frequency Response

“…Previous studies in volume perturbation FR found that the heat released from compression could cause substantial spurious effects for response curves at high frequencies, interfering with the evaluation of mass-transfer dynamics. − In order to eliminate any effects unrelated to the adsorption kinetics, a blank experiment with quartz particles was performed first at the same operating conditions. A mathematical treatment was then employed to replace the response curves (amplitude ratio and phase lag) of volume and pressure to relative pressure responses in the absence and presence of adsorbent with the same volume perturbation.…”