Measuring of isothermal water vapor adsorption/desorption rate using QCM method and its mass transfer resistance of a layer coated with silica-gel micro particles in a moist air

The increase in the future use of adsorption chillers and dehumidification systems will majorly depend on their energy consumption, compaction and general efficiency. Adsorbents are generally coated on heat exchangers to increase their adsorption/desorption rate with enhanced heat transfer. Consequently, researchers have noted the importance of decreasing the vapor transfer resistance in the concentration boundary layer taking into consideration the influence of flow rate on the resistance. This necessitates the need to measure the mass of the adsorbed water vapor which is currently a challenge. To date, several techniques have been devised to measure the amount of the adsorbed vapor and the adsorption rate. However, these methods are limited due to several challenges. As such, researchers have been looking for alternative methods and have identified the quartz crystal microbalance as a promising one.

Dr. Yoshinori Hamamoto, Mr. Takehiro Nakamori, and Professor Hideo Mori from Kyushu University recently used a highly sensitive quartz crystal microbalance to measure the amounts of adsorbed and desorbed water vapor in moist air to a thin layer of the adsorbent. The adsorbent was consolidated with silica-gel microparticles on the quartz crystal coated with thin gold electrodes. The work is published in the International Journal of Refrigeration

Briefly, the authors started by cross-examining the possibility of achieving stable measurements by comparing the data with the catalog date of the desorption equilibrium of silica-gel particles. Additionally, the adsorption/desorption rates were measured taking into account the changes in the surrounding relative humidity conditions. Finally, the influence of the airflow velocity on the mass transfer resistance relative to mass diffusion in the particles and mass transfer in the boundary layer was investigated.

At constant temperatures, the measured relative equilibrium adsorption reproduced the catalog data of the silica gel. The time constants of the reactions reflected the characteristics of the mass transfer resistance of the reactions and the resistance was observed to decrease with the increase in the velocity. Numerical simulations of the temperature change of the layer were conducted during adsorption and desorption reactions. Based on the results, the adsorbents were regarded as isothermal. On the other hand, it was necessary to separate the resistance into diffusion resistance inside the adsorbent and mass transfer resistance in the concentration boundary. Specifically, the resistance through the boundary layer was reported to be negligible at airflow velocity above 0.1 ms^-1. Furthermore, these resistances could be predicted by the proposed correlations and fittings parameters.

In summary, the Kyushu University scientists successfully applied the quartz crystal microbalance method to measure the isothermal water vapor adsorption and desorption rate and its mass transfer resistance of a layer in the moist air. In a statement to Advances in Engineering community, Dr. Yoshinori Hamamoto highlighted the importance of the study especially in revolutionizing the future heating and cooling systems.