A critical overview of adsorption kinetics for cooling and refrigeration systems

Tailored SAPO-34/Graphite Adsorbent Composite Coatings on Aluminum Substrate for Energy Sustainable Sorption Technologies

This paper explores a novel composite adsorbent coating applied to an aluminum support. This coating incorporates SAPO-34 and exfoliated graphite fillers within a sulfonate polyether ether ketone (S-PEEK) matrix, offering a promising avenue for energy-efficient adsorption technologies. Composite coatings, containing SAPO-34 zeolite as the primary adsorbent (80-95 wt.%) and exfoliated graphite as a conductive additive (5 wt.%) were produced. A drop-casting technique was employed to deposit the composite mixtures onto aluminum substrates. The coatings exhibited excellent adhesion to the metal substrate, as proved by a pull-off strength higher than 1.0 MPa. The morphological characterization revealed a uniform dispersion of both additives within the host material. To evaluate their adsorption/desorption behavior, equilibrium water vapor adsorption isobars were determined at a constant pressure of 11 mbar across a 30-120 °C temperature range. The adsorption/desorption tests showed the composite coatings reached 26-30% water uptake, indicating that the matrix did not obstruct the water vapor mass transfer, and the zeolite exhibited active participation in the adsorption/desorption process. These results suggest that this material may be a promising candidate for energy saving systems.

Precisely controlled electrostatically sprayed sodium alginate/carboxymethyl chitosan hydrogel microbeads as super-adsorbent for adsorption of cationic dye

In this pioneering study, electrostatic spraying (ES) technology with high voltages is proposed to reduce the size of hydrogel microbeads further, aiming to enhance the adsorption rate of cationic methylene blue (MB) dye. The increased voltages, ranging from 0.0 to 13.0 kV, further decreased the size of electrostatically sprayed hydrogel microbeads crosslinked by hydrogen bonds between sodium alginate (SA) and carboxymethyl chitosan (CMCS) in hydrochloric acid. The size of SA/CMCS hydrogel microbeads was successfully reduced from 2000 ± 121 μm (SC-2000) to 400 ± 15 μm (SC-400). Notably, SC-400 exhibits the highest maximum adsorption capacity (qm) and rate constant (k2) at 840.3 mg/g and 0.0598 g/mg/min, respectively, at pH 9.0 and a temperature of 25 °C in the absence of ionic compounds, which is three times higher than that of SC-2000, due to their high specific surface area and pore volume. Through a series of adsorption studies and characterization analyses, SA/CMCS hydrogel microbeads displayed heterogeneous adsorption behaviors towards MB dye through electrostatic interactions between the deprotonated carboxylic groups and cationic MB molecules, where MB adsorption efficiency could be significantly influenced by pH and ionic strength. These findings suggest that ES technology is effective in synthesizing smaller SA/CMCS hydrogel microbeads with enhanced MB removal rates and stable adsorption capacities and their applications could be further explored for removing other organic dyes and toxic metals in subsequent research studies.

Development of Shaped MIL-100(Fe) Granules for High-Performing Adsorption Desalination: From Formulation Optimization to System Test

Adsorption desalination (AD) driven by low-grade renewable energy or waste heat is a sustainable solution to the water crisis. Recently, metal-organic frameworks (MOFs) with excellent water adsorption performances have been recognized as some of the most promising candidates for AD. However, previous studies mainly focused on MOFs in powder form, causing pipe clogging and a drastic pressure drop, which inspire the development of shaped MOFs for industrial use. In this work, MIL-100(Fe) with high water stability, high adsorption capacity, and mild synthesis conditions was chosen, and the optimal formulation of the shaped MIL-100(Fe) granules using different binders was explored. The high-performing MIL-100(Fe)@5PVB granule containing 5% polyvinyl butyral (PVB) with outstanding adsorption performance and mechanical strength was selected and massively prepared for AD system testing. It is found that, although binder content decreased the surface area, pore volume, and water uptake of MIL-100(Fe), the mechanical strength and adsorption kinetics of shaped MIL-100(Fe)@5PVB were enhanced, which favor its performance in an AD system. Moreover, system testing demonstrated that the desalination performance of the AD system based on the adsorption beds of MIL-100(Fe)@5PVB outperformed both silica gel and MIL-100(Fe) powder. The specific daily water production (SDWP) of the AD system based on MIL-100(Fe)@5PVB (28.74 m3/ton/day) is 30% higher than that based on MIL-100(Fe) powder (19 m3/ton/day). Such a phenomenon is mainly contributed by the improved water adsorption dynamics of MIL-100(Fe)@5PVB granules that favors the mass transfer efficiency in the adsorption bed. This work opens up the possibility for the development of high-performing shaped MOFs for adsorption desalination from the perspectives of formulation optimization and system testing.

Innovative solid desiccant dehumidification using distributed microwaves

Dehumidification is one of the key challenges facing the air conditioning (AC) industry in the treatment of moist air. Over many decades, the dual role of heat exchangers of AC chillers for the sensible and latent cooling of space has hindered the thermal-lift reduction in the refrigeration cycle due to the requirements of water vapor removal at dew-point and heat rejection to the ambient air. These practical constraints of AC chillers have resulted in the leveling of energy efficiency of mechanical vapor compressors (MVC) for many decades. One promising approach to energy efficiency improvement is the decoupling of dehumidification from sensible processes so that innovative but separate processes can be applied. In this paper, an advanced microwave dehumidification method is investigated in the laboratory, where the microwave (2.45 GHz) energy can be irradiated onto the dipole structure of water vapor molecules, desorbing rapidly from the pores of adsorbent. Results show a significant improvement in performance for microwave dehumidification, up to fourfold, as compared to data available in the literature.

Dual-functional marine algal carbon-based materials with highly efficient dye removal and disinfection control

The design and simple, green preparation of dual-functional materials for the decontamination of both hazardous dyes and pathogenic microorganisms from wastewater remain challenging currently. Herein, a promising marine algal carbon-based material (named C-SA/SP) with both highly efficient dye adsorptive and antibacterial properties was fabricated based on the incorporation of sodium alginate and a low dose of silver phosphate via a facile and eco-friendly approach. The structure, removal of malachite green (MG) and congo red (CR), and their antibacterial performance were studied, and the adsorption mechanism was further interpreted by the statistical physics models, besides the classic models. The results show that the maximum simulated adsorption capacity for MG reached 2798.27 mg/g, and its minimal inhibit concentration for Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) was 0.4 mg/mL and 0.2 mg/mL, respectively. The mechanistic study suggests that silver phosphate exerted the effects of catalytic carbon formation and pore formation, while reducing the electronegativity of the material as well, thus improving its dye adsorptive performance. Moreover, the MG adsorption onto C-SA/SP showed vertical orientation and a multi-molecular way, and its adsorption sites were involved in the adsorption process with the increase of temperature. Overall, the study indicates that the as-made dual-functional materials have good applied prospects for water remediation.

Enhanced Water Adsorption of MIL-101(Cr) by Metal-Organic Polyhedral Encapsulation for Adsorption Cooling

Metal-organic frameworks (MOFs) are one of the most promising adsorbents in the adsorption cooling system (ACS) for their outstanding water adsorption performance. Notwithstanding that fact, numerous reports pay more attention to the ACS performance improvement through enhancing equilibrium water uptake of MOFs. However, adsorption cooling performance, including specific cooling power (SCP) and coefficient of performance for cooling (COP_C) of MOF/water working pairs, always depends on the water adsorption kinetics of MOFs in ACS. In this work, to increase the water adsorption rate, the preparation of MOP/MIL-101(Cr) was achieved by encapsulating hydrophilic metal-organic polyhedral (MOP) into MIL-101(Cr). It was found that the hydrophilicity of MOP/MIL-101(Cr) was enhanced upon hydrophilic MOP3 encapsulation, resulting in a remarkable improvement in water adsorption rates. Furthermore, both SCP and COP_C for MOP/MIL-101(Cr)-water working pairs were also improved because of the fast water adsorption of MOP/MIL-101(Cr). In brief, an effective approach to enhance the water adsorption rate and cooling performance of MOF-water working pairs through enhancing the hydrophilicity of MOFs by encapsulating MOP into MOFs was reported in this work, which provides a new strategy for broadening the application of MOF composites in ACS.

New Analysis Method for Adsorption in Gas (H2, CO)-Solid (SnO2) Systems Based on Gas Sensing

The chemisorption phenomenon is widely used in the explanation of catalysis, gas-solid reactions, and gas sensing mechanisms. Generally, some properties of adsorbents, such as adsorption sites and dispersion, can be predicted by traditional methods through the variation of the chemisorption capacity with the temperature, pressure, and gas-solid interaction potential. However, these methods could not capture the information of the interaction between adsorbents, the adsorption rate, and the competitive adsorption relationship between adsorbents. In this paper, metal oxide semiconductors (MOSs) are employed to study the adsorption behavior. The gas sensing responses (GSRs) of MOSs caused by the gas adsorption process are measured as a new method to capture some adsorption behaviors, which are impossible for the traditional methods to obtain. The following adsorption behaviors characterized by this new method are presented for the first time: (1) distinguishing the adsorption type using an example of two reducing gases: the adsorption type of the two gases is single-molecular layer adsorption in this work; (2) detecting the interaction between different gases: this will be a promising method to provide original characterization data in the fields of gas-solid reaction mechanisms and heterogeneous catalysis; and (3) measuring the adsorption rate based on the GSR.

CFD Analysis of Elements of an Adsorption Chiller with Desalination Function

This paper presents the results of numerical tests on the elements of an adsorption chiller that comprises a sorption chamber with a bed, a condenser, and an evaporator. The simulation is based on the data and geometry of a prototype refrigeration appliance. The simulation of this problem is unique and has not yet been performed, and so far, no simulation of the phenomena occurring in the systems on a real scale has been carried out. The presented results are part of the research covering the entire spectrum of designing an adsorption chiller. The full process of numerical modeling of thermal and flow phenomena taking place in the abovementioned components is presented. The computational mesh sensitivity analysis combined in the k-ε turbulence model was performed. To verify and validate the numerical results obtained, they were compared with the results of tests carried out on a laboratory stand at the AGH Center of Energy. The results of numerical calculations are in good agreement with the results of the experimental tests. The maximum deviation between the pressure obtained experimentally and by simulations is 1.8%, while for temperatures this deviation is no more than 0.5%. The results allow the identification of problems and their sources, which allows for future structural modifications to optimize the operation of the device.