A non-contact microwave sensor for monitoring the interaction of zeolite 13X with CO2 and CH4 in gaseous streams

Abstract

Gas sensing and analysis through the monitoring of dielectric properties of adsorbent materials is a robust, sensitive, inexpensive emerging technique with contactless operation capability. Planar resonator sensors are particularly favored for this purpose due to their simple structure and operational principles. In this work, an application of zeolite 13x combined with a planar microwave microstrip resonator sensor, reinforced by regenerative feedback loop, was investigated for real-time monitoring of the bed and CO2 and CH4 gas detection for concentrations between 1% to 50% (balance N2). The sensing was completed by detecting the adsorbent’s permittivity change during adsorption of the target gases on zeolite 13x. The results obtained indicate that the change in the permittivity was due to the cumulative effect of increased temperature (heat of adsorption) and occupied pores of the adsorbents. Real-time monitoring of the resonant frequency shift of the microwave sensor also illustrates the higher adsorption capacity of zeolite 13x when exposed to CO2 as compared to similar concentrations of CH4, which confirmed higher selectivity of the sensor towards CO2. Additionally, a two-step modeling analysis based on Maxwell Garnett equation was performed in MATLAB and FEM analyzer, to demonstrate the necessity of the high resolution sensor principle for very small permittivity-change detection. Finally, by correlating the resonant frequency shift with the concentration of the target gas, the sensor was able to characterize sample concentrations of CO2 and CH4.