New 100 Hz Volumetric Frequency Response System: Design, Operation and Analysis

Frequency response methods have been developed as an alternative technique for the determination of mass transfer mechanisms controlling the transport of gases into and out of microporous adsorbent materials. Recent results in the literature are showing that this technique seems to be the best for discriminating the controlling mechanism. It is also possible to reveal two or more different controlling mechanisms that may dominate in different frequency ranges. This presentation will focus on the design and operation of a new 100 Hz volumetric frequency response (VFR) system, and the analysis of the resulting amplitude and phase lag data.

Like the previous 10 Hz system constructed at USC, the apparatus consists of two sections, the gas handling/adsorption section and the driving section. The first section is comprised of the tubing for gas dosing and vacuuming into the adsorption chamber, differential and absolute pressure sensors, and all the necessary isolation valves. The second section includes the metal bellows for volume perturbation, motor, eccentric cam, position indicator, and electrical and data acquisition systems. It operates like the previous one using LabView hardware and software, but up to much higher frequencies.

With respect to analysis, the effects of and corrections for three undesirable physical phenomena observed at intermediate (0.001-10 Hz), high (> 1 Hz) and very high (> 20 Hz) frequencies will be shown and discussed. The first of these three phenomena is well described in the literature and is associated with heat that is generated due to gas compression in sections of the working volume that do not have the ability to remove heat. The other two phenomena are probably due to local pressure drop and mechanical vibration. The ability of the instrument to measure amplitude and phase lag data up to about 90 Hz will be demonstrated for BPL activated carbon in both pelleted and structured forms.