Membrane fouling in direct contact membrane distillation for liquid desiccant regeneration: Effects of feed temperature and flow velocity

Membrane fouling is a major challenge of membrane distillation when employed as a regenerator in liquid desiccant air-conditioning systems. The present work investigated the impacts of feed temperature and flow velocity on the occurrence of membrane fouling using a bench-scale direct contact membrane distillation (DCMD) system to regenerate a 27.5 wt% of lithium chloride (LiCl) solution. The membranes used at different operating conditions were characterized and compared by scanning electron microscopy and water contact angle measurement. The results showed that increasing feed inlet temperature significantly improved the DCMD regeneration capacity but reduced the membrane hydrophobicity. The DCMD operated at a higher cross flow velocity exhibited a higher propensity of membrane fouling. Membrane fouling tended to accumulate from loose LiCl crystals to a dense scaling layer with an increase in both feed inlet temperature and cross flow velocity. Membrane cleaning with deionized water was an effective mitigation strategy to remove the LiCl deposition from the membrane surface. It can effectively restore the DCMD regeneration performance and increase the hydrophobicity of the fouled membrane by 10.6%. However, using the post-cleaning membrane increased the risk of membrane wetting in the DCMD regeneration process. [Display omitted] •Two membrane fouling patterns, i.e. crystal cluster and cake layer, were prominent.•Increasing feed temperature and/or flow rate beyond a range exacerbated fouling.•Membrane cleaning restored regeneration performance and membrane hydrophobicity.