The design and construction of a frequency response apparatus to investigate diffusion in zeolites

Abstract

A frequency response device has been designed and built based on previous efforts by Y. Yasuda, L. C. V. Rees, F. Meunier, and Ph. Grenier. This design uses the best attributes of the three previous devices and is improved in five specific ways. First, the device makes use of the fast response capacitance manometer. Second, the device uses a servomotor to push and pull a metal bellows pump, which drives the sinusoidal input function, removing the chance of “floating” the bellows. Third, both sinusoidal and step-change volume perturbations can be studied. Fourth, the overall cost of the device was kept below $30 000. Finally, the pressure transducer is mounted only 8 cm from the sample, instead of ∼30 cm, thus removing any averaging of the pressure signal at higher frequencies. The device is currently capable of measuring frequencies between 0.005 and 5 Hz, a range of three full orders of magnitude. The system can operate at temperatures between room temperature and 473 K and pressures between 0.3 and 300 Torr. Two systems were used to test the device, n-hexane/silicalite and methanol/silicalite. The model developed by Yasuda is used to analyze the frequency response of these systems. This model can describe a system as having more than one diffusivity and accounts for the presence of a surface resistance to diffusion. The results for n-hexane/silicalite and methanol/silicalite are compared to those of van den Begin and Nayak, respectively, and are in good agreement. Also, the diffusivities measured using this device are comparable to those estimated in 1997 by Rees using the frequency response method. In addition to measuring the diffusion rate processes, the kinetic parameters associated with a surface resistance to diffusion were also estimated.

Measurement of Mass Transfer Rates in Adsorbents: New Combined-Technique Frequency Response Apparatus and Application to CO2 in 13X Zeolite

“…Such a pump has been used previously in a VSFR apparatus. 34 Figure 1 shows a schematic representation of the new apparatus. Flow of gas into the system is controlled by two mass flow controllers (MKS type 1479A).…”

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

“…In general, the kinetics or mass transfer rate of adsorption and desorption of a pure gas in a microporous adsorbent can be controlled by a single mechanism, like micropore diffusion, macropore diffusion, or macropore advection, or any combination of these mechanisms . In recent decades, frequency response (FR) methods − have proven to be one of the best macroscopic techniques for investigating the kinetic behavior of gas adsorbate–adsorbent systems due to their ability to discriminate among different rate limiting mechanisms.…”

“…The objective of this work is to introduce a unique VFR system based largely on the simple schematic in the work of Reyes et al 35 with some ideas taken from Tuner et al 36 This is a closed, batch system wherein the differential pressure response from a small perturbation in volume is measured after equilibrium is established for a gas adsorbate−adsorbent pair at constant temperature and pressure using a relatively large volume of adsorbent. A detailed description of the apparatus, its operating procedure, including the use of baseline runs with different gases and different porous and nonporous solid beads and pellets that fully characterize the system in terms of its dynamic behavior especially at high frequencies and in terms of various volumes required in analyses, is provided.…”

Nonlinear Frequency Response measurements of gas adsorption equilibrium and kinetics: New apparatus and experimental verification

“…The pressure response is measured as a differential pressure between an adsorption and a reference cell (a cell without adsorbent at the same steady-state temperature and pressure). This concept, introduced by Turner et al (2001), enables large range of operating pressures to be covered with only one (costly) differential pressure transducer. The infra red (IR) measurements of the particle temperature, already used by Bourdin et al (1998), are simplified by using an IR detector of the newest generation which has small size and enables easy mounting.…”

“…Certain modifications of this basic concept have also been reported. Turner et al (2001) improved the accuracy of the input sine wave approximation by using servomotor with feedback control to drive the metal bellows. Bourdin et al (1998) have used an infra red (IR) detector to measure the adsorbent temperature during the volume variation and included a temperature response function in the analysis.…”