Numerical study on the moisture transfer characteristics of membrane-based liquid desiccant dehumidifiers: Resistance distribution and concentration polarization

Abstract

Membrane-based liquid desiccant dehumidifiers have received increasing attentions due to their main advantage of preventing liquid carryover in air handling process, but the low dehumidification efficiency limits their further applications. The underlying mechanism of their moisture transfer deterioration is not entirely clear yet, and further efforts are needed to effectively identify and reduce the mass transfer resistances in the dehumidification process. In this work, considering impressionable transmembrane permeation flux, the resistance distribution of moisture transfer in the quasi-counter flow flat-plate membrane-based liquid desiccant dehumidifier is studied by a three-dimensional conjugate heat and mass transfer model. The velocity, temperature and concentration fields in dehumidifying process are analyzed in baseline condition, especially on the membrane surfaces. Analogy to a thermal circuit, the normalized moisture transfer resistances through gaseous, solid and liquid phases are defined to describe their relative importance. Causes of these normalized resistances in variable conditions are analyzed in terms of concentration polarization and boundary layer theory. Furthermore, the resistance variation is compared with the latent effectiveness, to reveal their inherent relation. The results suggest that moisture transfer resistances are mostly originated from the membrane and air side, and thus dehumidifying enhancement is more likely to emerge from the air-side concentration polarization alleviation in synergy with the appropriate improvement of membrane porosity. Among all operating parameters, the air inlet temperature and solution flow rate have relatively limited effect on the total resistance, while increasing solution concentration, followed by decreasing solution temperature, can significantly reduce the moisture transfer resistances at a nondecreasing latent effectiveness. In addition, the latent effectiveness shows a similar variation tendency to that of the air-side moisture transfer resistance, but it cannot reflect the total resistance variation. These results can help researchers and designers make decisions on where and how to improve the performance of membrane-based liquid desiccant dehumidifiers.