A highly effective multi-string humidifier with a low gas stream pressure drop for desalination

Abstract

The humidification-dehumidification (HDH) process is appealing for small-scale distributed desalination of highsalinity water. We report a new design of compact lightweight humidifiers with high effectiveness and low electric energy consumption for HDH systems. The new design consists of a dense array of vertically aligned strings, along which thin films of a heated liquid feed are allowed to flow under gravity. A counterflowing gas stream makes direct contact with liquid films carrying the generated water vapor to a dehumidifier. The unique geometric configuration of our design affords high interface-to-volume ratios necessary for high heat/mass exchange effectiveness and straight contiguous gas flow paths for reduced gas-phase pressure drops. We constructed a 0.4 m-tall prototype and examined the effects of the liquid flow rate, air velocity and feed liquid salinity on the heat/mass transfer performance and the gas stream axial pressure drop. Compared with previously reported pad humidifiers and spray columns, the present multi-string humidifier achieves almost 5 times the evaporation rate per humidifier volume at comparable gas stream pressure drops or an order of magnitude reduction in the gas stream pressure drop at comparable total evaporate rates. Our work demonstrates a highlyeffective but compact and lightweight multi-string humidifier with a small gas stream axial pressure drop for HDH and related applications.

Water vapor capturing using an array of traveling liquid beads for desalination and water treatment

“…In our recent paper ( 19 ), we reported an early demonstration of thin film flows of water along cotton threads and their potential application in evaporation/humidification. This early demonstration, however, was rather ad hoc and incomplete, as we merely relied on trial and error to select thread diameters and liquid flow rates without proper theoretical understanding of fluid mechanics or heat and mass transfer processes involved.…”

“…Despite offering high interface-to-volume ratios, the multistring array introduces small pressure drops for gas flows in part because it provides straight, open and contiguous gas flow paths and in part because low-profile traveling liquid beads present relatively small form drag (15)(16)(17)(18). In our recent paper (19), we reported an early demonstration of thin film flows of water along cotton threads and their potential application in evaporation/humidification. This early demonstration, however, was rather ad hoc and incomplete, as we merely relied on trial and error to select thread diameters and liquid flow rates without proper theoretical understanding of fluid mechanics or heat and mass transfer processes involved.…”

On travelling waves of a control-volume model for liquid films flowing down a fibre

“…Thin liquid films flowing down a vertical fibre have attracted many interests in the past decades due to their importance in a variety of engineering applications, including heat and mass exchangers, thermal desalination, and vapor and CO 2 capturing [27,28,[35][36][37]. These liquid films are fundamentally driven by Rayleigh-Plateau instability and gravity modulation, spontaneously exhibiting complex interfacial instability and pattern formation [20,21,24,29].…”

Thermally-driven coalescence in thin liquid film flowing down a fibre

“…2017; Zeng, Sadeghpour & Ju 2018), and heat and mass exchangers for vapour, CO and particle capture (Sadeghpour et al. 2019, 2021; Zeng, Sadeghpour & Ju 2019). In addition to gravity and bulk surface tension, changes in liquid properties like surface tension and viscosity, which arise from the presence of a temperature gradient, influence the characteristics of these thin film flows.…”