Vacuum membrane distillation of liquid desiccants utilizing hollow fiber membranes

Abstract

This paper documents the testing of a vacuum membrane distillation system intended for use with liquid desiccants. Liquid desiccants offer the possibility for low-energy, ambient temperature dehumidification. Effective desalination and purification of diluted desiccants outputs two important products: a concentrated desiccant for reuse in dehumidification and fresh water. In this study, vacuum membrane distillation was used in the laboratory to purify diluted liquid desiccants. Calcium chloride and magnesium chloride were the desiccants selected for testing. Desiccant solutions were pumped through the lumens of poly(vinylidene fluoride) (PVDF) hollow fiber membranes at varying feed inlet temperatures, solution velocity rates and vacuum set points during membrane distillation. An average flux of 8 kg m-2 h-1 was obtained using 30 wt% magnesium chloride solution at a temperature of 50 °C while applying vacuum to achieve 25 mbar absolute pressure on the air side of the membrane. The results are promising for the development of a full-scale vacuum membrane distillation process for desiccant solution regeneration and fresh water recovery. In addition, the recovered condensate was of sufficient quality for use in agricultural irrigation or drinking water.

Gas-Liquid Hollow Fiber Membrane Contactors for Different Applications

“…Desalinated water collected during liquid desiccant regeneration has very low salt concentration and is suitable for use in irrigation. [133] Fibers 2018, 6, 76 The fabricated membrane showed higher water flux than any of in-house made and commercial humidifers, e.g. at T = 30 • C, P=1 bar and gas flow rate 5 L/min the water flux of PES membrane is 2700% higher than a commercial humidifier, Perma Pure ® 1 model PH-60T-24SS.…”

“…Liquid desiccants for air dehumidification can be divided into two broad ranges: (1) aqueous solution of salt desiccants-for example, lithium chloride (LiCl) [42,130,131] or calcium or magnesium chloride (CaCl 2 or MgCl 2 ) [132,133] that usually used for residential and commercial building applications, (2) glycols, like tri-ethylene glycol (TEG), which are frequently used for industrial applications [134]. The work [135] is worth of being noted individually; it employs chilled water as a desiccant for air control in spacecraft applications.…”

Hollow fibre membrane-based liquid desiccant humidity control for controlled environment agriculture

“…Figure 2 SEM images of triple-bore PVDF hollow fibres manufactured in the lab. (Bettahalli et al, 2016) Liquid desiccant humidity control system For each experimental trial, the same bulk desiccant solution was used, however, the solution was regenerated (water removed) between trials via vacuum membrane distillation using the methods described in Lefers et. al.…”

“…al., 2018;Fortunato et al, 2018). In the present work, the liquid desiccant was heated to a temperature of 30° C and vacuum was applied to achieve an absolute pressure of 3 mbar on the air side of the membrane using the setup described in Lefers et. al.…”

“…The VMD flux ranged from ~2.8 kg m -2 h -1 at ~43% ERH to ~7 kg m -2 h -1 at ~76% ERH at a desiccant temperature of 30 °C while applying vacuum to achieve 3 mbar absolute pressure on the air side of the membrane (Figure 6). For a complete discussion of the VMD process, see Lefers et. al.…”