Water Adsorption Equilibrium Research Articles

Ising model analysis of water adsorption equilibrium in microporous carbon

Water is ubiquitous and the nature of its behavior within microporous solids impacts the efficacy of important technologies. Rudimentary understanding of this behavior is challenged by disparate observed mechanisms ranging from molecular clustering to condensation. To promote fundamental understanding, this investigation proposes methodology to elucidate this behavior. Using an Ising model, a novel structure–property relation that can predict adsorption equilibrium is proposed for analysis of water in micropores. Additionally, hydrophobicity is partitioned into surface and structural hydrophobicity which are quantified via adsorption equilibrium analysis. Furthermore, the model is employed to discriminate Kelvin-like condensation from pore filling behavior in microporous carbon.

Accelerated Water Desorption of Oligomeric Poly(ethylene glycol) by Addition of Poly(propylene glycol) for Energy-Efficient Water Recovery Systems.

Adsorbents are used to recover water vapor from the atmosphere in desiccant air conditioning (DAC) and atmospheric water harvesting (AWH) systems. Solid adsorbents have been conventionally used in these systems, though liquid adsorbents are considered to be more effective for energy-efficient fluidic thermosystems because of their low regeneration temperatures (45-70 °C). While most previous studies have focused on improving the adsorption performance, the desorption performance of adsorbents can also be a critical factor in improving the energy efficiency of these systems. Thus, this study aimed to improve the water desorption efficiency, focusing on the liquid adsorbents. We found that mixing hydrophobic molecules into a liquid adsorbent decreases the desorption temperature and increases the water-desorption efficiency. Oligomeric poly(ethylene glycol) (PEG), a common moisture-adsorbing liquid oligomer used in detergents and cosmetics, was selected as the liquid adsorbent. Oligomeric poly(propylene glycol) (PPG), which has a structure analogous to PEG and lower hygroscopicity, was selected as the hydrophobic molecule. Water adsorption and desorption experiments showed that the mixture of PPG with PEG promoted the desorption of water molecules beyond that of PEG, while thermogravimetric differential thermal analysis revealed a decrease in the water desorption temperature with increasing PPG content. The improved desorption efficiency was ascribed to the likely water adsorption equilibrium between PEG and PPG in the blend; water molecules are preferentially desorbed from PPG, which has weaker water-adsorbate interactions. The proposed concept is expected to be incorporated into various hygroscopic liquids to develop energy-efficient liquid adsorbents for DAC and AWH.

Solar-driven atmospheric water harvesting with a super-hygroscopic composite modified activated carbon fiber for tropical island ecological farm

Atmospheric water harvesting is a promising strategy to address the water scarcity in islands. In this work, an activated carbon fiber (ACF) templated hybrid water adsorbent ACF-cobalt(II)-ethanolamine (ACF-Co-EA) was fabricated and used to build an ecological farm (Eco-farm) for potential application on tropical coral islands. ACF-Co-EA took the advantage of both the pore structure of ACF and the superabsorbent property of the Co-EA complex, thus, exhibiting superior water harvest capacity over ACF or Co-EA. The equilibrium water adsorption of ACF-Co-EA was 763 ​mg ​g−1 ​at 25 ​°C and 70% RH (typical environment of tropical coral islands). Under 1 ​kW ​m−2 simulated solar irradiation, the surface temperature of ACF-Co-EA increased rapidly to 50 ​°C within 12 ​min and stabilized at 54 ​°C in 30 ​min because of the superior light absorbance of ACF, which made more than 90% of captured water released. ACF-Co-EA-based Eco-farm could harvest 6.9 ​g ​g−1·day−1 of water to ensure plant growth in the tropical coral islands' environment without any additional energy or water supply. The study provided novel ideas to alleviate the problems of freshwater scarcity and food shortage in the tropical coral islands.

Experimental investigation on effect of water film thickness in unsaturated sandstone cores on CO2 transport during geologic storage

Characteristics of the water distribution and film thickness within pores control CO2 transport and determine the effectiveness of geologic storage. Although studies of adsorbed water films on solid surfaces are abundant, reports about the dependence of CO2 transport behavior on film thickness under geologic CO2 storage conditions are still lacking. In this study, the water film thickness was estimated through water adsorption equilibrium experiments on four tight sandstone samples at varying relative humidities. The calculated results reveal that the adsorbed water exerts a greater influence on smaller pores, as micro- and mesopores are first filled with condensed water, while macropores only contain absorbed water films, and that the measured critical brine film thicknesses are strongly controlled by the clay mineral content. The influence of water film thickness on CO2 transport was evaluated based on the DLVO theory for cylindrical pores. Considering the mobility difference of water films in different pores, the effective disjoining pressure measured by the most probable pore radius can be regarded as the preliminary basis for the selection of reservoir or caprock and evaluation of the sealing performance of rock. Increasing the water film thickness results in a smaller effective pore radius and flowable pore space, and the movement of water in pore channel causes an electroviscous effect. Both conditions have a negative effect on CO2 movement and eventually decrease the CO2 effective permeability.

MIL-160 as an Adsorbent for Atmospheric Water Harvesting

Nowadays, the rapidly growing population, climate change, and environment pollution put heavy pressure on fresh water resources. The atmosphere is the immense worldwide and available water source. The Adsorptive Water Harvesting from the Atmosphere (AWHA) method is considered a promising alternative to desalination technologies for remote arid regions. The development of novel adsorbents with advanced water-adsorption properties is a prerequisite for practical realization of this method. Metal–organic frameworks (MOFs) are a novel class of porous crystalline solids that bring a great potential for AWHA due to their extremely high specific surface area, porosity, and tailored adsorption properties. This work addresses MIL-160 as a water adsorbent for AWHA. The water-adsorption equilibrium of MIL-160 was studied by volumetric method, the isosteric heat of adsorption was calculated, and finally, the potential of MIL-160 for AWHA was evaluated for climatic conditions of the deserts of Saudi Arabia, Mongolia, the Sahara, Atacama, and Mojave as reference arid regions. MIL-160 was shown to ensure a maximum specific water productivity of 0.31–0.33 gH2O/gads per cycle. High fractions of water extracted (0.90–0.98) and collected (0.48–0.97) could be achieved at a regeneration temperature of 80 °C with natural cooling of the condenser by ambient air. The specific energy consumption for water production varied from 3.5 to 6.8 kJ/g, which is acceptable if solar heat is used to drive the desorption. The AWHA method employing MIL-160 is a promising way to achieve a fresh water supply in remote arid areas.

Degradation behavior of mixed and isolated aromatic ring containing VOCs: Langmuir-Hinshelwood kinetics, photodegradation, in-situ FTIR and DFT studies

The photodegradation tendencies of mixed and isolated VOCs, e.g., benzene, toluene, and p-xylene were studied using TiO2 P25 as a model photocatalyst. The degradation of VOCs in the mixture is significantly affected by the existence of different organic pollutants. For example, benzene only showed 10% degradation efficiency in the mixture whilst 67% in the isolated mode. The conversion efficiency of benzene was 20% and 27% when mixed with p-xylene and toluene, respectively. It shows that the degradability of benzene is influenced more by the presence of p-xylene than toluene. The dynamic adsorption-desorption experiments and DFT calculations on stoichiometric and defective TiO2 surface revealed that benzene only weakly interacts with the stoichiometric TiO2 surface than toluene and p-xylene. This behavior could be the fundamental factor for the lower degradation efficiency of benzene. Furthermore, the presence of oxygen vacancy (Ov) in TiO2 surface tremendously improved the overall adsorption of VOCs. Several Langmuir-Hinshelwood kinetic models, which are based on different reaction dynamic assumptions, were used to determine rates of reactions, water adsorption equilibrium constant, and VOCs adsorption equilibrium constant. The results indicated that the oxidation of VOCs occurred on the catalyst surface, and the adsorption equilibrium constant of VOCs was higher than water adsorption equilibrium constant. The intermediate formation and hydroxyl groups consumption were further rationalized via in-situ FTIR study. This work provides a comprehensive analysis of VOCs degradation in the mixed and isolated mode, which will increase the possibility of implementing the photocatalytic oxidation technology for the VOCs abatement.

Cellulose nanocrystal/plant oil polymer composites with hydrophobicity, humidity-sensitivity, and high wet strength

The preparation of high-performance cellulose nanocrystals (CNCs)/plant oil-derived polymer composites is still a challenge, due to their poor compatibility. Here, by designing amide groups and epoxy groups on sunflower oil derived polymers, appropriate interfacial hydrogen bond interactions between the polymers and CNCs were constructed, where CNCs were homogenously dispersed in polymer matrix. Tensile tests and DMA results revealed that the incorporation of CNCs into sunflower oil derived epoxy polymers significantly enhanced the tensile strength and storage modulus. More importantly, nanocomposites with 50 wt% CNCs are still hydrophobic, which not only show a fast and reversible humidity induced modulus switch, but also exhibit high wet strength (19.9 MPa) after equilibrium water adsorption. The present work revealed that proper designed CNCs/plant oil polymer nanocomposites are good candidates for high performance and functional materials, which are able to replace petroleum-based materials in various fields.

Tuning of adsorbent properties – oxidative hydrophilization of activated carbon monoliths for heat storage applications

In this work, methods and processes for the production of hydrophilized and hydrothermally stable activated carbon honeycomb structures based on cost-efficient raw materials for the use in heat storage applications are presented. As a reference, activated carbon (AC) was hydrophilized using nitric acid. In a second step, AC monoliths with a channel substructure were prepared by an extrusion process and consequently oxidized using either nitric acid or ozone gas in order to restore microporosity and create hydrophilic groups on the inner surface of the material.The influence of the shaping and hydrophilization procedures towards substrate chemistry, pore structures, equilibrium water adsorption capacity, thermophysical properties such as heat capacity and conductivity as well as the multi-cycle ad-/desorption stability are comprehensively discussed.The successful hydrophilization by the nitric acid treatment was demonstrated for a powder AC sample. After 4 h of treatment a water adsorption capacity of 0.28 gg−1 at relative pressures p/p0 = 0.4 was observed, which represents a multiplication of capacity by 50 or 70 times compared to the raw material. In case of the AC monoliths, a capacity of 0.12 gg−1 at p/p0 = 0.4 is achieved by nitric acid treatment compared to 0.07 gg−1 for the ozone treatment.

Water adsorption equilibrium and dynamics of LICL/MWCNT/PVA composite for adsorptive heat storage

Recently, a new composite “LiCl inside Multi-Wall Carbon NanoTubes” (LiCl/MWCNT) has been suggested as water sorbent for Adsorption Thermal Energy Storage (ATES), because it has a large thermal storage capacity of 1.7 kJ/g for a daily heat storage cycle. This work addresses the results of the study of water sorption dynamics on the novel composite loaded into representative small scale fragments of a common finned flat-tube HEx. The study consists of four parts: (1) shaping the LiCl/MWCNT composite as grains using polyvinyl alcohol (PVA) as a binder; (2) analysis of sorption equilibrium for the pair “LiCl/ MWCNT/PVA – water”; (3) measuring the thermal storage capacity of the granulated LiCl/ MWCNT/PVA composite; (4) investigation of water sorption dynamics on the LiCl/MWCNT/PVA under typical conditions of the daily storage cycle. It is shown that the thermal storage capacity of the LiCl/MWCNT/PVA composite equals 1.5–1.6 kJ/g. The specific power reaches 4.2 and 9.8 kW/kg of the heat release and thermal storage stages, respectively. The results obtained show that the working pair “LiCl/MWCNT/PVA – water” appears to be advantageous for ATES.

Characterization of a High-Performance Biosorbent for Natural Gas Dehydration

Dehydration of gases is crucial in industry. Current dehydration methods have concerns about high energy consumption and environmental pollution. In this work, natural gas, an important energy source, was selected as a model gas to investigate dehydration using a cost-effective biosorbent in a pressure swing adsorption process. The biosorbent was developed from flax shives, a byproduct of the flax industry, and are representative of renewable cellulose materials. The morphology, surface functional groups, and thermal stability of the biosorbent were investigated by FE-SEM, XPS, and TGA. The biosorbent has higher water adsorption capacity (up to 0.9 g/g) and higher water selectivity compared to those of conventional adsorbents. Adsorption of the main component of natural gas, i.e., nonpolar methane, was negligible. In addition, the most significant operation factors and interaction among them were determined with regards to their effects on water adsorption capacity. The water adsorption equilibrium data w...

Production of anhydrous biobutanol using a biosorbent developed from oat hulls

Bio-butanol is an important source of renewable biofuels. It contains significant amount of water in the production process through fermentation. The current technologies to remove water from butanol are energy intensive and costly. In this work, oat hulls, representative of cellulosic material abundantly generated as agricultural by-product, was used to dehydrate butanol in a packed column in order to produce biofuel product. The results demonstrate that the oat hull based biosorbent is able to effectively remove water from butanol. The biosorbent was characterized by Brunauer, Emmett and Teller (BET) surface area, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscope (SEM). The adsorption performance and breakthrough curve of the single component (butanol or water) adsorption on the biosorbent were investigated separately. Besides, the separation factors of water over butanol on the biosorbent were determined according to the equilibrium data in the butanol-water binary system. The effects of feed concentration and temperature on adsorption capacity were investigated. The water adsorption equilibrium data in the single water system and the butanol-water binary system were fitted to the Dubinin-Polanyi model. The approximate sorption site energy distribution was calculated to analyze the adsorption equilibrium data on the biosorbent. The results indicate the oat hulls based biosorbent has the capability for butanol dehydration.

Thermodynamic Characterization of the Hydroxyl Group on the γ-Alumina Surface by the Energy Distribution Function

Controlling water adsorption on γ-alumina, i.e., the quantity and nature of the surface hydroxyl groups, is essential for adjusting the acidic/basic properties of the surface. IR and DFT studies have shown that different OH groups, each of them characterized by its own chemical environment and adsorption properties, can be present on the surface of γ-alumina. However, quantifying this surface heterogeneity and predicting the influence of the synthesis and activation conditions is still a challenging problem. In this paper, a detailed experimental study is conducted on a γ-alumina sample obtained by thermal decomposition of a commercial boehmite. Using a thermogravimetric setup, both water-adsorption equilibrium and desorption kinetics were acquired in a large range of controlled experimental conditions (1 Pa < partial pressure of water <1400 Pa ; 100 °C < temperatures <600 °C). The energy distribution function (EDF) of water-adsorption enthalpy is evaluated on the basis of the theories developed for stron...

Adsorption Equilibrium and Modeling of Water Vapor on Reduced and Unreduced Silver-Exchanged Mordenite

This work is related to the removal of tritiated water and radioactive iodine from off-gases released during spent nuclear fuel reprocessing. Specifically, it is focused on the adsorption equilibrium of water on reduced silver mordenite (Ag0Z), which is the state-of-art solid adsorbent for iodine retention in the off-gas treatment. As the off-gases contain different gas species, including iodine and water, Ag0Z would take up iodine and water simultaneously during the adsorption process. Therefore, understanding the adsorption of water on Ag0Z is important and necessary for studying the performance of Ag0Z in off-gas treatment processes. The isotherms of water (nonradioactive water) on Ag0Z were obtained at temperatures of 25, 40, 60, 100, 150, and 200 °C with a continuous-flow adsorption system. The data were analyzed using the Heterogeneous Langmuir and generalized statistical thermodynamic adsorption (GSTA) models, and thermodynamic parameters of the isotherms were obtained from both models. Both models...

Isotherms for Water Adsorption on Molecular Sieve 3A: Influence of Cation Composition

This work is part of our continuing efforts to address engineering issues related to the removal of tritiated water from off-gases produced in used nuclear fuel reprocessing facilities. In the current study, adsorption equilibrium of water on molecular sieve 3A beads was investigated. Adsorption isotherms for water on the UOP molecular sieve 3A were measured by a continuous-flow adsorption system at 298, 313, 333, and 353 K. Experimental data collected were analyzed by the Generalized Statistical Thermodynamic Adsorption (GSTA) isotherm model. The K+/Na+ molar ratio of this particular type of molecular sieve 3A was ∼4:6. Our results showed that the GSTA isotherm model worked very well to describe the equilibrium behavior of water adsorption on molecular sieve 3A. The optimum number of parameters for the current experimental data was determined to be a set of four equilibrium parameters. This result suggests that the adsorbent crystals contain four energetically distinct adsorption sites. In addition, it w...

Application of the novel ETS-10/water pair in cyclic adsorption heating processes: Measurement of equilibrium and kinetics properties and simulation studies

The ETS-10/water pair was explored for the first time for cyclic adsorption heating purposes, with modelling and simulation studies. Measurements of water adsorption equilibrium properties were carried out, and, for the first time, the effective thermal conductivity and specific heat capacity of ETS-10 were measured. The experimental results were used for the modelling and simulation of an adsorption heating unit. A model was developed, which contemplates adsorption equilibrium, one-dimensional heat and mass transfer in the bed, heat transfer in the external film, and intraparticle mass transport. From the numerical simulations, the coefficient of performance (COP) and specific heating power (SHP) were calculated, which allowed evaluating the heating performance of the adsorption unit. The bed thickness, adsorbent regeneration temperature, and heating thermal fluid temperature influence considerably the cycle time and cyclic adsorption loading swing, thus impacting on COP and SHP. For three simulated cycles differing in bed thickness, COP values in the range 1.36–1.39 were obtained, which are close to the estimated ideal value of 1.41; the corresponding SHP ranged from 934 to 249 W kgs−1. Based on sensitivity studies, a good compromise is required between the bed thickness, regeneration temperature, and the heating fluid temperature in order to meet superior performances of the system.

Modeling of Microwave-Assisted Regeneration of Selected Adsorbents Loaded with Water or Toluene

In this work, the microwave-assisted desorption of water or toluene from Sorbonorit 4 activated carbon and zeolite HiSiv 3000 has been studied. The nonequilibrium, nonisothermal, and nonadiabatic process described by a mathematical model for the microwave-assisted desorption including the internal heat source capacity is formulated and solved numerically by means of a numerical method of lines. The adsorption equilibrium of water–Sorbonorit 4 is described by a multitemperature Qi-LeVan isotherm, water–HiSiv 3000 by a multitemperature Langmuir-Freundlich isotherm, and toluene–Sorbonorit 4 and toluene–HiSiv 3000 by a multitemperature Toth isotherm. Computed results show that heat and mass transport mechanisms occurring during the microwave-assisted desorption process are properly incorporated into the proposed model. Such a comprehensive mathematical modeling of microwave-assisted desorption has not previously been discussed in the literature.

Heterogeneously Catalyzed Esterification Reaction: Experimental and Modeling Using Langmuir- Hinshelwood Approach

The esterification reaction of ethyl alcohol and acetic acid catalyzed by the ion exchange resin, Amberlyst 15, was investigated. The experimental study was implemented in an isothermal batch reactor. Catalyst loading, initial molar ratio, mixing time and temperature as being the most effective parameters, were extensively studied and discussed. A maximum final conversion of 75% was obtained at 70°C, acid to ethyl alcohol mole ratio of 1/2 and 10 g catalyst loading. Kinetic of the reaction was correlated with Langmuir-Hanshelwood model (LHM). The total rate constant and the adsorption equilibrium of water as a function of the temperature was calculated. The activation energies were found to be as 113876.9 and -49474.95 KJ per Kmol of acetic acid for the esterification reaction and the heat of adsorption of water. These results agreed well with the previous published data.

Ethanol Dehydration in a Fixed Bed Using Canola Meal

Ethanol dehydration was investigated in a fixed-bed packed with canola meal biosorbent. Breakthrough curves were used to investigate the effects of feed flow rate, column temperature, and ethanol feed concentration on water and ethanol adsorption capacity. Experimental results demonstrated that fuel grade ethanol (over 99 wt %) was achieved through water adsorption in the fixed bed with ethanol feed concentrations of 75 wt %, and 92 wt %, respectively, at a feed flow rate of 2.9 mL/min and column temperature of 85 °C. Both water and ethanol were adsorbed on canola meal. Water adsorption isotherm was described as type II isotherm. Peudo-equilibrium ethanol adsorption isotherm was also discussed in this work. Water selectivity over ethanol was higher at the breakthrough point defined at 99 wt % ethanol than that at water adsorption equilibrium. Separation factor can be optimized through controlling feed flow rate, ethanol feed concentration, and column temperature.

Antifungal activity and humidity sensitivity of quaternized cellulose synthesized in NaOH/urea aqueous solution

In this work we report the fabrication of cellulose-based humidity responsive material with antifungal activity. The quaternized cellulose (QC) derivatives with low degree of substitution (DS) values of 0.08–0.37 were synthesized in NaOH/urea aqueous solution. Water insoluble QC membranes (c-QCM) were prepared by casting from QC aqueous solutions, followed by crosslinking with glutaraldehyde. The c-QCMs were disintegrated in acid solutions, but were able to keep membrane shape in neutral and mild basic solutions with pH value of 7.2 and 9.7. The equilibrium water adsorption ratios of c-QCMs were in the range of 66–98%, depending on the DS values of quaternary ammonium groups and the pH value of the aqueous solutions. The antifungal activity of QC was evaluated and found that QC could effectively inhibit the reproduction of Rhizopus stolonifer, Aspergillus flavus and Penicillium digitatum, with minimum inhibitory concentration of 5, 10, and 7.5 mg/mL, respectively. The resistivity of the c-QCM changed for about 65–134 times corresponding to the change of environmental relative humidity from 20 to 99%; and the performance of c-QCM as a resistive-type humidity responsive material was consistent in the cycling of relative humidity.

Biocompatibility of polypropylene non-woven fabric membrane via UV-induced graft polymerization of 2-acrylamido-2-methylpropane sulfonic acid

This work described the graft polymerization of a sulfonic acid terminated monomer, 2-acrylamido-2-methylpropane sulfonic acid (AMPS), onto the surface of polypropylene non-woven (NWF PP) membrane by O 2 plasma pretreatment and UV-induced photografting method. The chemical structure and composition of the modified surfaces were analyzed by FTIR-ATR and XPS, respectively. The wettability was investigated by water contact angle and equilibrium water adsorption. And the biocompatibility of the modified NWF PP membranes was evaluated by protein adsorption and platelet adhesion. It was found that the graft density increased with prolonging UV irradiation time and increasing AMPS concentration; the water contact angles of the membranes decreased from 124° to 26° with the increasing grafting density of poly(AMPS) from 0 to 884.2 μg cm −2, while the equilibrium water adsorption raised from 5 wt% to 75 wt%; the protein absorption was effectively suppressed with the introduction of poly(AMPS) even at the low grafting density (132.4 μg cm −2); the number of platelets adhering to the modified membrane was dramatically reduced when compared with that on its virgin surface. These results indicated that surface modification of NWF PP membrane with AMPS was a facile approach to construct biocompatible surface.