An apparatus for adsorption dynamics studies using infrared measurement of the adsorbent temperature

Abstract

An apparatus has been built for kinetics measurement in adsorption processes. It consists of a variable volume chamber where the adsorbent sample is placed. During the adsorption process the pressure and the sample temperature are measured. The temperature is measured by infrared detection achieving fast, sensitive, and nonintrusive measurements. Two methods are used for adsorption kinetics measurement: the volume step and the thermal frequency response. In the volume step method the volume is suddenly reduced and the adsorption process occurs at constant volume. Due to the sorption heat released during adsorption, the adsorbed mass is closely related to the sample temperature. This method permits one to determine the relevant thermodynamic parameters of the system and the kinetics up to 20 ms. The different mass transfer modes may then be separated. The thermal frequency response (TFR) method consists of achieving a periodic volume of the chamber. The amplitude and phase lag of the temperature, taking the pressure function as a reference, may be used to measure with great accuracy very fast kinetics (up to 1 ms). Moreover, it permits the easy discrimination of surface barrier from Fickian diffusion. Finally it allows one to measure microporous and macroporous mass transfer in the same experiment. It is shown that substantial improvements are obtained with the TFR method compared with the pressure frequency response method which was, until now, the best macroscopic method. Moreover, the TFR method has produced results in good accordance with the microscopic pulsed field gradient-nuclear magnetic resonance method. Volume step and TFR methods are limited to low pressure (less than ∼4 kPa) in consequence of nonisothermal gas compression. Experimental results are given to illustrate the possibilities of both methods.