An experimental study on liquid regeneration process of a liquid desiccant air conditioning system (LDACs) based on vacuum membrane distillation

In this paper, a liquid regeneration method by vacuum membrane distillation (VMD) is proposed for the liquid desiccant air conditioning system (LDACs), and the experimental study on this method is carried out. VMD regeneration experiments were carried out with LiCl solution. The effects of temperature, concentration of feed solution, length, number of fiber membranes and vacuum pressure on the membrane flux, mass transfer coefficient, rejection rate and regeneration ability were studied. The results show that the error between experimental and calculation results is reduced from less than 15% to less than 5% by the optimized calculation model. The temperature of feed solution has a great influence on the regeneration performance of VMD, and the regeneration ability of VMD process increases approximately exponentially from about 0.1% to 0.8–1.2% with the increase of regeneration temperature. The VMD regeneration process of LiCl solution is the result of Poiseuille flow and Knudsen diffusion, and Poiseuille flow dominates. Under the same regeneration capacity, the regeneration temperature of 30 wt% LiCl solution is about 7 °C higher than that of 20 wt% LiCl solution, and this temperature difference also increases as the target regeneration amount increases. In order to improve the regeneration effect of high concentration solution, the regeneration temperature can be increased appropriately. The water flux of the membrane decreases with the increase of the length of the membrane. The membrane length of Type1 is 2.1 times longer than Type 2, but regeneration capacity of Type 1 is only 1–1.7 times higher than Type 2. Further, both the water flux and regeneration ability of the solution decrease first and then increase with the increasing number of membranes. Therefore, the reasonable selection of number of fiber membranes can significantly save materials and also improve the regeneration ability.