Enhancing the mass transfer and anti-corrosion performance of falling film dehumidifier/regenerator by plate surface treatment and liquid desiccant modifications

Abstract

The falling film type dehumidifier/regenerator is widely used in the liquid desiccant air-conditioning system (LDAC) and its heat and mass transfer characteristics significantly affects the system mass transfer ability. In practice, falling film shrinks along the flow direction in a plate dehumidifier which greatly reduces the desiccant/air contact area for mass transfer and deteriorates the moisture removal ability. In addition, the current liquid desiccant may corrode the metal-made dehumidifier/regenerator which further reduce the system reliability and system performance. Accordingly, the present study developed a novel theoretical model and a 3D CFD simulation model to accurately describe the simultaneous heat and mass transfer processes in the dehumidifier/regenerator, and proposed novel methods by plate surface treatment and liquid desiccant modification to enhance the mass transfer performance and to alleviate the metal corrosion problem. Firstly, concerning film shrinkage on the falling film dehumidifier, a novel theoretical model was developed with the consideration of both falling film shrinkage shape and vapor condensation, and a novel 3D CFD simulation model was also successfully developed. After experimental validation, the theoretical model was employed to analyze the influence of vapor condensation on the whole dehumidification performance. Compared with the situation without condensation, the relative increment of absolute moisture change ranges from 1.2% to 10.7% when concerning vapor condensation for different operational conditions. For CFD simulation, the newly developed 3D model could predict the absolute moisture removal accurately. Falling film of liquid desiccant shrank gradually along the flow direction, which led to an inhomogeneous water vapor absorption process in the dehumidifier. The intense absorption occurred at the phase interface, resulting in large concentration gradient of solution and moisture content in the zones near the interface. However, mass transfer in the form of diffusion in other zones lowered the whole dehumidification performance. To alleviate the film shrinkage and enlarge the mass transfer area between solution and air, this study successfully introduced a new surfactant PVP and successfully fabricated a novel and stable LiCl/H₂O-MWNTs nanofluid for the falling film solution. The new kind of non-volatile, odorless and nontoxic additive is polyvinyl pyrrolidone (PVP). After the addition of this surfactant into liquid desiccant, the dehumidification rate and regeneration rate had an average relative increment of 22.7% and 26.3% respectively due to the reduction of contact angle and the increase of surface energy. A comparison study was conducted to investigate the performance of LiCl/H₂O solution, LiCl/H₂O-PVP solution and LiCl/H₂O-MWNTs nanofluid. Experimental results showed that LiCl/H2O-PVP solution and nanofluid could enhance the dehumidification rate by up to 26.1% and 25.9% and regeneration rate by up to 24.9% and 24.7% as a result of contact angle reduction. However, the mass transfer enhancement of nanofluid can be only attributed to the adding of surfactant, and the adding of 0.1wt% MWNTs had undetected effect on the mass transfer performance in present study. In the aspect of anti-corrosion, the present study introduced a novel falling film dehumidifier/regenerator by adopting anodized aluminum, and successfully fabricated a new mixed liquid desiccant solution. Electrochemical test results showed that the anodized aluminum could alleviate the erosion significantly. Compared with the normal aluminum dehumidifier/regenerator, the anodized one had an average enhancement of 25.3% for dehumidification and 23.7% for regeneration in terms of effectiveness, respectively due to the increase of surface energy and the decreasing of surface energy. Also, this study first developed a new mixed liquid desiccant solution via the addition of hydroxyethyl urea into lithium chloride solution to reduce its causticity. The formula of the new solution—25% LiCl, 39% hydroxyethyl urea and 36% water—was determined according to the vapor pressure. Its basic thermal properties were measured and fitted as polynomial correlations. Electrochemical test results demonstrated the lower corrodibility of the new mixed solution. For dehumidification/regeneration effectiveness, it also had a significant relative improvement of 12.9% and 14.1% respectively. The mass transfer improvement was attributed to the larger wetting ratio and greater fluctuation of falling film. In addition, correlations were also newly proposed to predict the effectiveness of the mixed solution. The novelties of present study lie in: firstly, this study developed a theoretical model for dehumidification concerning water vapor condensation and a 3D simulation model to accurately examine and understand the actual dehumidification/ regeneration process. Moreover, the newly proposed methods by adding surfactant and nanoparticles were proved to be effective ways for the improvement of wettability and mass transfer performance. Furthermore, the anodizing surface treatment technology and mixed liquid desiccant were successfully introduced to alleviate the metal corrosion and system performance improvement. Overall, the theoretical, numerical and experimental studies conducted in this study give meaningful and feasible guidance for the understanding of falling film dehumidification/regeneration process and other falling film thermal components.