Experimental and Theoretical Assessment of Water Sorbent Kinetics

Abstract

The kinetics of water adsorption in powder sorbent layers are important to design a scaled-up atmospheric water capture device. Herein, the adsorption kinetics of three sorbents, a chromium (Cr)-based metal–organic framework (Cr-MIL-101), a carbon-based material (nanoporous sponges/NPS), and silica gel, have been tested experimentally, using powder layers ranging from ∼0 to 7.5 mm in thickness, in a custom-made calibrated environmental chamber cycling from 5 to 95% RH at 30 °C. A mass and energy transfer model was applied onto the experimental curves to better understand the contribution of key parameters (maximum water uptake, kinetics of single particles, layer open porosity, and particle size distribution). Open porosity (i.e., the void-to-particle ratio in the sorbent layer) shows the highest influence to improve the kinetics. Converting the sorbent kinetics data into a daily yield of captured water demonstrated (i) the existence of an optimal open porosity for each sorbent, (ii) that thinner layers with moderate open porosity performed respectively better than thicker layers with high open porosity, and (iii) that high maximum water uptake and fast single-particle kinetics are not necessarily predictive of high daily water yield.

All‐Day Multicyclic Atmospheric Water Harvesting Enabled by Polyelectrolyte Hydrogel with Hybrid Desorption Mode

“…Specifically, we simulated the dynamic sorption behavior of the sorbent based on the Fickian diffusion (FD) model [ 52 ] (Note S2, Supporting Information). The FD model shows a satisfactory agreement between the theoretical and experimental data derived from both milligram‐scale samples (Figure S30, Supporting Information) and bulk samples with a specific weight of 0.45 g cm −1 (Figure 4c).…”

A universal strategy for efficient synthesis of Zr‐based MOF nanoparticles for enhanced water adsorption

“…Furthermore, Figure 9B showed that the reduced particle size results in faster adsorption rate. On one hand, the reduced particle size can promote the internal diffusion process 69–71 . On the other hand, the smaller particle size enlarges the external surface area (the external surface area of UiO‐66‐70 nm, UiO‐66‐162 nm, and UiO‐66‐2.5 μm are 178, 112, and 60 m 2 g −1 , respectively) and allows more adsorption sites to be exposed on the surface, 40,71,72 which can facilitate the adsorption of water molecules.…”

“…On one hand, the reduced particle size can promote the internal diffusion process. [69][70][71] On the other hand, the smaller particle size enlarges the external surface area (the external surface area of UiO-66-70 nm, UiO-66-162 nm, and UiO-66-2.5 μm are 178, 112, and 60 m 2 g À1 , respectively) and allows more adsorption sites to be exposed on the surface, 40,71,72 which can facilitate the adsorption of water molecules. At the same time, we found that the maximum water adsorption capacity was determined by the total pore volume, and the smaller the particle size, the larger the total pore volume (Figure 9C and Table 1).…”

Hygroscopic Porous Polymer for Sorption‐Based Atmospheric Water Harvesting

“…In this case, the intergranular space is filled by nearly stationary air, which increases the overall thermal resistance of a material. [63,76] During the desorption process, the captured water is desorbed into water vapor and ejected into the ambient. The existence of air thermal resistance degrades sorbent's thermal conductivity, further disturbing uniform desorption temperature.…”