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

A frequency response device has been designed and built based on previous e fforts by Y. Yasuda, L. C. V. Rees, F. Meunier, and Ph. Grenier. This desig n 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 capac itance manometer. Second, the device uses a servomotor to push and pull a m etal bellows pump, which drives the sinusoidal input function, removing the chance of "floating" the bellows. Third, both sinusoidal and step-change v olume 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 similar to 30 cm, thus removing any averagin g of the pressure signal at higher frequencies. The device is currently cap able 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 t emperature and 473 K and pressures between 0.3 and 300 Torr. Two systems we re used to test the device, n-hexane/silicalite and methanol/silicalite. Th e model developed by Yasuda is used to analyze the frequency response of th ese systems. This model can describe a system as having more than one diffu sivity 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 associat ed with a surface resistance to diffusion were also estimated. (C) 2001 Ame rican Institute of Physics.