Characteristics of heat and mass transfer in moist air-ionic liquid desiccant bubbly systems: Parametric assessment of regeneration performance

Ionic liquids (ILs) have recently become an increasingly popular working medium for liquid-desiccant deep dehumidification as applied to low-humidity industries and, in particular, the use of IL desiccants in bubble columns regeneration has been proven to be an effective way to promote their deep dehumidification potential. The present work considers a moist air-IL desiccant bubbly deep dehumidification/regeneration system, focusing specifically on the heat and mass transfer characteristics of regeneration in bubble column. An experimental apparatus is constructed and used to explore the effects of key operating parameters (i.e., liquid height H, superficial velocity vs, solution temperature T-s) on regeneration performance. Furthermore, a semi-analytical model of bubbly regeneration heat and mass transfer is developed and used to perform parametric assessments. The results indicate that elevating the liquid height, as well as higher superficial velocities and solution temperatures all contribute to improved regeneration rates, each with distinct influencing mechanisms. A higher superficial velocity enhances the volumetric transfer coefficient through a combined effect (i.e., gas-liquid specific interfacial area, heat and mass transfer coefficients). The effect of the solution temperature on the volumetric transfer coefficient is mainly reflected in the heat and mass transfer coefficient as determined by the transfer potential difference, while the hydrodynamics play a relatively minor role in the system of interest. Bubbly regeneration differs from deep dehumidification in terms of the volumetric transfer and heat/mass transfer coefficients. Notably, the Lewis number of bubbly regeneration is generally between 0.6-1. In practical applications, it is necessary to consider the air-side pressure drop, droplet entrainment, and low-grade thermal energy utilization. Under recommended conditions (e.g., H = 0.3 m, vs = 3.0 cm/s, T-s = 60 degrees C), a regeneration rate of 49 g/h and moisture effectiveness of 79 % can be achieved. This study provides guidance for the design of moist air-IL desiccant bubbly deep dehumidification/regeneration systems.