Water Vapor Sorption Kinetics Research Articles

Comprehensive analysis of water vapor sorption kinetics and mechanisms using biosorbent pellets from canola meal and oat hull

Agricultural residues, specifically canola meal (CM) and oat hull (OH), have been innovatively utilized to develop biosorbents for sorbing water vapor from the air. These biomaterials are comprised of hydrophilic functional groups that effectively perform air dehydration. Air, being non-polar, was used as a model gas in this study to simulate gas dehydration. In the current research, these materials were formed into pellets to produce high-quality biosorbents with controlled size, shape, and enhanced moisture uptake capacity. CM (309.48 mg/g) and OH (233.07 mg/g) pellets had higher or comparable water sorption capacities than commercialized adsorbents used for drying gases. The mixed-order kinetic model described the sorption process well and identified both mass transfer and sorption on active site steps (R 2 > 0.991 and χ 2 < 16.0). Regarding the OH pellet, sorption on active sites was the predominant kinetic mechanism at the beginning, followed by intraparticle diffusion until equilibrium. However, sorption on CM pellets was delayed at 4.2 min at the initial stage, owing to the external mass transfer resistance; then, intraparticle diffusion controlled the process until equilibrium. Adding sodium lignosulfonate (LSNa) lowered the initial sorption rate but enhanced the water uptake and strength of pellets. The addition of LSNa resulted in a 25% and 14% increase in the water uptake capacity of oat hull and canola meal pellets, respectively; conversely, it also caused an increase in the delay time for sorption on CM pellets at the beginning of the process, extending it to 9.50 min.

Water Vapor Sorption Kinetics of Beech Wood Modified with Phenol Formaldehyde Resin Oligomers

Beech is an important tree species in Europe. This study aimed to elucidate the influence of four molecular weights of phenol formaldehyde (PF) resin (266, 286, 387, and 410 g/mol) on the sorption behavior of unmodified and modified beech wood samples using a dynamic vapor sorption (DVS) apparatus. The variations in the environmental relative humidity and moisture content (MC) of the samples were recorded, and the DVS isotherms were plotted from the equipment. During the sorption process, the MC of the modified samples decreased in comparison to that of the unmodified samples, and both apparently decreased with the increasing molecular weight of the PF resin. The DVS isotherm hysteresis plot illustrated a reduction in sorption hysteresis for the modified wood with varying PF resins compared to the unmodified samples. Based on the DVS isotherm adsorption and desorption plots, the decrease in the equilibrium of the MC can be attributed to there being fewer sorption sites in the modified samples, which exhibited the lowest hygroscopicity. Overall, the moisture sorption mechanism for both types of samples was clarified, highlighting a clear correlation between the molecular weight of the applied PF resin and its influence on moisture sorption behavior.

Discrepancy of Effective Water Diffusivities Determined from Dynamic Vapor Sorption Measurements with Different Relative Humidity Step Sizes: Observations from Cereal Materials

Water diffusivity, a critical parameter for cereal processing design and quality optimization, is usually concentration-dependent. dynamic vapor sorption (DVS) system provides an approach to establishing the relationship between water concentration and diffusivity. However, the usual relative humidity (RH) jump during practical sorption processes is usually greater than that adopted in DVS measurements. Water vapor sorption kinetics of glutinous rice grains, glutinous rice flour and wheat flour dough films were measured using the DVS system to verify if varying RH step sizes can obtain identical diffusivities within the same range. The effective diffusivities were determined according to Fick's second law. The results revealed that increasing RH step size led to a higher estimated diffusivity, regardless of whether the water concentration gradient or potential chemical gradient was considered a driving force for water diffusion. This finding was further confirmed by a linear RH scanning DVS measurement. The water concentration-dependent diffusivity obtained from a multi-step DVS measurement, according to Fick's second law, will overestimate the required time for practical cereal drying or adsorption. Thus, this paradoxical discrepancy needs a new mass transfer mechanism to be explained.

Water in Wood: A Review of Current Understanding and Knowledge Gaps

Wood-water interactions are central to the utilization of wood in our society since water affects many important characteristics of wood. This topic has been investigated for more than a century, but new knowledge continues to be generated as a result of improved experimental and computational methods. This review summarizes our current understanding of the fundamentals of water in wood and highlights significant knowledge gaps. Thus, the focus is not only on what is currently known but equally important, what is yet unknown. The review covers locations of water in wood; phase changes and equilibrium states of water in wood; thermodynamics of sorption; terminology including cell wall water (bound water), capillary water (free water), fiber saturation point, and maximum cell wall moisture content; shrinkage and swelling; sorption hysteresis; transport of water in wood; and kinetics of water vapor sorption in the cell wall.

Physical stability of co-freeze-dried powders made from NaCl and maltodextrins – Impact of NaCl on glass transition temperature, water vapour sorption isotherm and water vapour sorption kinetics

NaCl is commonly added into food powders as a humectant or taste enhancer with little consideration of its impact on the physical properties and stability of powders. In this study, maltodextrins of dextrose equivalence (DE) 5, 20 and 45 and 10%w/w NaCl were first dissolved in water and then freeze-dried to produce powder mixtures. After freeze drying, maltodextrin DE 20-NaCl and maltodextrin DE 45-NaCl mixtures were amorphous based on powder X-ray diffraction and scanning electron microscopy-energy dispersive X-ray spectroscopy analyses while NaCl partially recrystallized in the maltodextrin DE 5 mixture. Compared to the corresponding pure maltodextrin, freeze-dried maltodextrin DE 5-NaCl blend had a lower glass transition temperature (Tg). Furthermore, the Tg depression effect of NaCl increased with maltodextrin molecular weight. Also, freeze-dried maltodextrin 20/45-NaCl mixtures had greater hygrocapacities than their respective maltodextrins throughout the aw range investigated (0.05–0.50). The impact of molecular mixing via freeze drying versus dry mixing on the sorption behaviour of maltodextrin-NaCl blends was also investigated. Freeze-dried maltodextrin-NaCl absorbed the most moisture (from 20% to 70%RH), followed by dry-mixed blends, which absorbed more moisture than predicted after 60, 50 and 40%RH for maltodextrin DE 5, 20 and 45-NaCl mixtures, respectively. In all, formulating NaCl with maltodextrin caused freeze-dried maltodextrin-NaCl blends to be more temperature- and moisture-sensitive.

Effect of interactions of locust bean gum and rosehip juice on the physical properties of gum tragacanth composite films

The aim of the study was to demonstrate the synergistic effect of natural polysaccharide hydrocolloids of locust bean gum and tragacanth gum combined with rosehip juice. It was shown that tragacantha gum has an excellent film-forming properties at low concentration and forms water-soluble, thin and transparent edible films with high application potential, which was demonstrated by selected physical properties of composite films obtained from a mixture of gum tragacanth and locust bean gum. Optical (color and opacity), thermal (thermogravimetric analysis), water vapor permeability and sorption kinetics were determined. The thickness of the films depends on the composition of the films, the process conditions, as well as the application techniques. The addition of locust bean gum resulted in an increase in the water content and consequently more elastic and less brittle gum tragacanth films. Films made only of gum tragacanth were characterized by greater hygroscopicity, as opposed to composite films. As the concentration of gum tragacanth increases, the value of water vapor permeability also increases, and the share of locust bean gum in the films caused the permeability to decrease. Composite films had a lighter color compared to those of pure gum tragacanth, and the carob contributed to a more transparent film and slower thermal degradation. In the films obtained, a synergistic effect was observed between the biopolymers in all the ratios used, and the addition of locust bean gum and rosehip juice had a strong influence on the properties of the obtained edible films. Practical applications The potential of using gum tragacanth to create edible films and coatings through their appropriate modification and optimization of the composition ratio of the created composites can become a challenge for their use in the food packaging industry. This study will be constructive for further research work focused on films and coatings and their application as protective coatings or bio-based packaging films.

Predicting equilibrium moisture content of mushrooms by NARX neural network and first order kinetic modelling approaches

Soft-cellular and porous agricultural products continuously exchange water vapor with their surrounding environment. The aim of this work was to model the water vapor sorption kinetics of mushrooms by strictly pragmatic and numerical approaches. A nonlinear autoregressive with exogenous inputs (NARX) neural network was used to simulate water vapor sorption kinetics over a wide range of relative humidity at temperatures of 20-40°C. The predictive power of the neural network was based on physical data, namely time-series of relative humidity at constant temperature. In addition, the time dependence of water vapor sorption was fitted by a first-order kinetic model, which was applicable for all relative humidity steps. Both models were fed with a total sample size of ca. 11000 data points generated by an automated gravimetric dynamic vapor sorption system. A neural network with a seven-node hidden layer was found to have the best performance. The results revealed the potential efficient use of artificial neural networks in simulating water vapor sorption kinetics of mushrooms and consequently predicting equilibrium moisture content with a high accuracy in the entire range of relative humidity and temperature covered by the experiments.

Relationship between Water Vapor Sorption Kinetics and Clay Surface Properties

AbstractA complete understanding of soil–water interactions and the corresponding mechanical behavior of unsaturated soils requires differentiating adsorptive and capillary components of water rete...

Adapting gravimetric sorption analyzer to estimate water vapor diffusivity in micrometric size cellulose particles

This study aims at developing a reliable and simple-to-implement methodology for the estimation of apparent water vapor diffusivity at the scale of micrometric size cellulose particles based on gravimetric measurements. The water vapor apparent diffusivity value was evaluated by using a quartz crystal microbalance (QCM) on a cellulose sample of 1 µg was 4 × 10−12 m2 s−1. Water vapor sorption kinetics at successive relative humidity (RH) steps were measured using a dynamic vapor sorption (DVS) microbalance by testing two types of sample preparation, i.e. either a powder bed or a compressed tablet. Water vapor apparent diffusivity was identified at each RH step by employing an analytic solution corresponding to the sample type: plane sheet for both powder bed and tablet considered as porous media and finite cylinder for a population of particles. The impact of the initial cellulose sample mass was also investigated. Water vapor diffusivity values varied from 6 × 10−15 to 4 × 10−10 m2 s−1 depending on the sample mass, sample preparation mode, analytic solution and/or sample porosity. The DVS-based methodology was compared to the value obtained from QCM measurements.

Kinetics of Water Vapor Sorption in Wood Cell Walls: State of the Art and Research Needs

Water vapor sorption is the most fundamental aspect of wood-moisture relations. It is directly or indirectly related to the physical properties of wood and the onset of wood-damage mechanisms. While sorption properties of cellulosic materials have been utilized since antiquity, the time-dependent transition from one moisture content to another (i.e., sorption kinetics) has received much less attention. In this critical review, we present the state-of-the-art of water vapor sorption kinetics in wood. We first examine different experimental methods that have been used to measure sorption kinetics, from the quartz helix vacuum balance beginning in earnest in the 1930s, to automated sorption balances used recently. We then give an overview of experimental observations and describe the physical phenomena that occur during the sorption process, which potentially govern the following kinetics: boundary layer mass transfer resistance, heat of sorption, cell wall diffusion, swelling, and polymer mobility. Finally, we evaluate theoretical models that have been proposed for describing sorption kinetics, considering both experimental data and the physical processes described in the previous section. It is clear that no previously developed model can phenomenologically describe the sorption process. Instead, new models are needed. We conclude that the development of new models will require more than simple gravimetric measurements. In addition to mass changes, complementary techniques are needed to probe other important physical quantities on multiple length scales.

Investigating water vapour sorption kinetics of aluminium MOFs by powder X-ray diffraction

The water sorption behaviour of aluminium MOF CAU-10 and CAU-15-Cit was followed by in situ powder X-ray diffraction.

Gas barrier and wetting properties of whey protein isolate‐based emulsion films

The aim of this research was to investigate the effect of rapeseed oil concentration (1–3% w/w) on the water vapor, oxygen and carbon dioxide permeability, water vapor sorption and surface properties of whey protein isolate emulsion‐based films. The water contact angle as affected by oil content, film side and time was analyzed. The effect of temperature (5 and 25°C) on the water vapor permeability (WVP), water vapor sorption kinetics and diffusion coefficient was also studied. The results showed that the incorporation of a lipid phase to whey protein film‐forming solutions was able to decrease the WVP, water hydrophilicity (increasing water contact angle) and water transfer of whey protein films. However, the films containing oil were more permeable to oxygen and carbon dioxide. Significantly higher values of WVP and diffusion coefficient were obtained at 5°C than at 25°C, indicating that storage temperature should be taken into account when designing the composition of edible films and coatings for food applications. POLYM. ENG. SCI., 59:E375–E383, 2019. © 2018 Society of Plastics Engineers

Assessing the potential of quartz crystal microbalance to estimate water vapor transfer in micrometric size cellulose particles

This study aims at assessing the use of a quartz crystal microbalance (QCM) coupled with an adsorption system to measure water vapor transfer properties in micrometric size cellulose particles. This apparatus allows measuring successfully water vapor sorption kinetics at successive relative humidity (RH) steps on a dispersion of individual micrometric size cellulose particles (1 μg) with a total acquisition duration of the order of one hour. Apparent diffusivity and water uptake at equilibrium were estimated at each step of RH by considering two different particle geometries in mass transfer modeling, i.e. sphere or finite cylinder, based on the results obtained from image analysis. Water vapor diffusivity values varied from 2.4 × 10−14 m2 s−1 to 4.2 × 10−12 m2 s−1 over the tested RH range (0–80%) whatever the model used. A finite cylinder or spherical geometry could be used equally for diffusivity identification for a particle size aspect ratio lower than 2.

Performance evaluation of hydrophilic organic polymer sorbents for desiccant air-conditioning applications

The present study provides performance evaluation of two kinds of crosslinked hydrophilic organic polymer sorbents (PS-I and PS-II) for desiccant air-conditioning applications. In this regard, optimum temperature and humidity zones are established for various air-conditioning applications which include (i) humans’ thermal comfort, (ii) animals’ thermal comfort, and (iii) postharvest storage of fruits/vegetables. Honeycomb-like desiccant blocks composed of PS-I/PS-II are assumed for numerical simulation analysis. The numerical simulation model is programmed into MATLAB which utilizes the scientific relationships of adsorption isotherms, adsorption kinetics, isosteric heat of adsorption, and thermophysical properties for each sorbent. A particular desiccant air-conditioning system design is proposed, and numerical simulation has been conducted for the performance evaluation of PS-I and PS-II. According to the results, PS-I enables higher dehumidification than PS-II at low regeneration temperature (50℃) and cycle time of 60:90 min. It is because the PS-I possesses better water vapor sorption kinetics as compared to PS-II. Although the PS-II enabled higher steady-state adsorption amount but it could not influence the overall system performance. On the other hand, the optimum performance by the PS-II is limited to relatively long cycle time and higher regeneration temperature (≥80℃). It has been concluded that the PS-I is relatively better choice for desiccant air-conditioning, and consequently can be considered for various air-conditioning applications. Furthermore, effects of mass flow rate, isosteric heat of adsorption, regeneration temperature, and cycle time on air humidity ratio and air temperature profiles have been discussed in order to highlight the performance variability of desiccant air-conditioning system.

Influence of temperature on water vapour sorption isotherms and kinetics of hardened cement paste and concrete

Water vapour sorption isotherms (WVSIs) have contributed significantly towards current knowledge of the porous nanostructure of cementitious materials and are an important component in modern durability calculations. In this paper, two newly developed multi-chamber testing apparatus for determining WVSIs of hardened cementitious materials are described. The adsorption, desorption and scanning behaviour of hardened cement paste and concrete are studied in detail over temperatures of 23°C to 80°C. Experiments provide confirmation of a monotonic reduction in sorption hysteresis with increasing temperature and show a disappearance of low humidity hysteresis by 60°C. Time and history dependence of apparent isosteric enthalpy of adsorption is demonstrated. Typically overlooked anomalous (non-Fickian) sorption kinetics are observed to significantly influence the equilibrium process. The magnitude and effect of temperature and humidity on kinetics is examined. Evidence supporting the role of microstructural change in observed size-independent kinetics and WVSI changes with temperature are discussed.

The water vapour sorption characteristics and kinetics of different wool types

The water vapour sorption behaviour of a range of sheep wool types and alpaca was studied using dynamic vapour sorption. Sorption isotherms were interpreted using the polymer sorption model developed by Vrentas and Vrentas. Satisfactory fits were obtained for absorption and desorption isotherms with the adjustment of parameters outside the scope of what is allowed. This is possibly because the underlying Flory–Huggins approach does not take into account any clustering of sorbate within the polymer. Water clustering in the wool fibre, determined using the Zimm–Lundberg clustering function, starts above a fibre moisture content of approximately 20%. Sorption kinetics were analysed using the parallel exponential kinetics model, providing excellent fits and allowed for calculation of a fibre modulus at different relative humidities; the values were reasonable at the upper end of the hygroscopic range, but were overestimated at the lower end of the range.

Determination of air-to-air energy wheels latent effectiveness using humidity step test data

Desiccant wheels are capable of recovering a considerable amount of moisture difference between two air streams. This study is aimed at determining the latent effectiveness of a rotary desiccant wheel through small-scale, transient testing. A coating of mesoporous silica gel particles (55μm particle size-pore width 77.5Å) was deposited on an aluminum substrate. The physical properties (particle size, pore width, surface area, and surface functional groups) of the silica gel were characterized and its sorption properties were investigated by nitrogen gas adsorption at 77K. In addition, the equilibrium condition and the kinetics of water vapor sorption on the silica gel sample were studied at 23°C. A test facility was developed to obtain the exchanger transient response during the dehumidification process and the data was used to determine the latent effectiveness of a regenerator constructed with the silica gel coated aluminum. Moreover, the latent effectiveness was determined using a cyclic test with both dehumidification and regeneration cycles. Good agreement was observed between the latent effectiveness values obtained through the single dehumidification and cyclic tests. A correlation from the literature was used to calculate the latent effectiveness of the silica gel coated regenerator. Comparison between the latent effectiveness determined by transient test data and the correlation showed good agreement when Ntum<4.3 and ω>4 rpm.

Water vapor sorption kinetics of polymer based sorbents: Theory and experiments

Water vapor sorption kinetics of two polymer based sorbents has been experimentally measured at adsorption temperatures of 20, 30, 50, 70 and 80°C using a magnetic suspension adsorption measurement unit. The experimental data is employed to commonly known adsorption kinetics approximations i.e. Linear driving force (LDF) model, Fickian diffusion (FD) model, and Semi-infinite model. All these models could not approximate the adsorption kinetics of polymer based sorbents, however, the LDF model is modified which successfully predicts the experimental kinetics for short-time and long-time estimation. For both sorbents, the diffusion time constant has been calculated at each adsorption temperature, and consequently, activation energy and pre-exponential constant are found from Arrhenius plot. The variation of diffusion time constant with relative pressure and adsorption temperature is discussed in relation with typical behavior of polymer/water pairs.

Kinetics of water vapor diffusion in resins

The water vapor sorption and desorption kinetics were monitored in methyl methacrylate–acrylonitrile–butadiene–styrene copolymer (MABS), bromobutyl, ethylene–propylene–butadiene monomer (EPDM) rubber, and butyl rubber at various temperatures by gravimetric method. Modified thermogravimetric analysis system was built and used. The geometry of the samples was approximating that of the infinite two-sided plane sheet of certain thickness. The coefficients of water vapor diffusion in polymers were deduced by comparing the experimentally monitored kinetics and kinetics computed using diffusion coefficients as adjustable parameters. The activation energies of water vapor diffusion in these polymers were deduced in the temperature range from 30 to 75 °C. The activation energy of diffusion was 45.9 kJ mol−1 in bromobutyl rubber, 46.4 kJ mol−1 in butyl rubber, 60.3 kJ mol−1 in EPDM rubber, and 43.5 kJ mol−1 in MABS. Compensation effect was observed for water vapor diffusion in this series of polymers. Compensation effect parameters were determined to be, a = −23.9/ln(cm2 s−1) and b = 0.49/(mol kJ−1) × ln(cm2 s−1), in fair agreement with published data for water diffusion in polymers.

Evidence for anomalous water vapor sorption kinetics in cement based materials

We used dynamic sorption balance measurements to evaluate the diffusivity for cement pastes with three different binders (OPC, OPC+70% slag, OPC+10% silica fume). The diffusion of water vapor in cement based materials is normally assumed to follow Fick's law of diffusion, but our results clearly show that Fick's law cannot completely describe the sorption process in our materials. In this paper we report the evidence for this anomalous sorption behavior and discuss a possible method to evaluate diffusivities from such measurements.