Dynamics Of Water Adsorption Research Articles

Dynamics of water vapour adsorption by a monolayer of loose AQSOA™-FAM-Z02 grains: Indication of inseparably coupled heat and mass transfer

Dynamics of water adsorption in an adsorption chiller (AC) determines the specific cooling power and the AC size. The dynamics, in its turn, strongly depends on the configuration of an “adsorbent – heat exchanger” (Ad-HEx). Here we considered the simplest, monolayer, configuration of the Ad-HEx and studied the water ad-/desorption dynamics as a function of the grain size (0.2–2.1 mm), temperature drop (16–36 °C), and pressure (7–47 mbar) for a commercial adsorbent AQSOA™-FAM-Z02. The dynamics was measured by a Volumetric Large Temperature Jump method under typical conditions of isobaric stages of real AC cycle.It is found that the initial part of all kinetic curves is exponential and can be described by a single characteristic time τ. This time depends on the grain size R as τ ∼ Rα, where the index α is constant over the whole range of the grain sizes, and its value is 1.33 and 1.20 for adsorption and desorption runs, respectively. The α-constancy may indicate an inseparable coupling of heat and mass transfer in the monolayer configuration of AQSOA™-FAM-Z02 adsorbent bed. For grains of about 2 mm size, quite acceptable initial power (>2 kW/kg) is obtained. Thus, the monolayer configurations may result in fast adsorption dynamics, reasonable specific power and compact AC units.

The bubble effect in bushings – investigations on models

This paper refers to the bubble effect in transformer bushings. The bubble effect consists in releasing water from moistened cellulose insulation after exceeding a critical temperature. The most dangerous result is a pressure increase that leads to an explosion and fire. Research was conducted using laboratory models of OIP (Oil- Impregnated Paper) and RBP (Resin-Bonded Paper) types of bushings immersed in mineral oil and synthetic ester. For all of the investigated systems we determined the dynamics of water adsorption upon contact of the insulating material with air and the temperature characteristics of bubble effect initiation depending on water content in cellulose. In the experiment we investigated the dynamics of changes of water content in dielectric liquid, water content in air over the surface of the dielectric liquid, and air pressure over the surface of the dielectric liquid. In the future, the technical possibilities will be analyzed of using the dynamic changes of the three quantities investigated here to detect the bubble effect in bushings.

Water adsorption dynamics in active carbon probed by terahertz spectroscopy

It is vital to characterize the adsorption dynamics in oil–gas reservoirs and pollution control industry.

Water adsorption dynamics on representative pieces of real adsorbers for adsorptive chillers

Dynamic optimization of adsorbent-heat exchangers (Ad-HExs) represents a key issue for the broader diffusion of adsorption cooling and heating (ACH) technologies. This paper is a sequel to an earlier one (Sapienza et al., 2014) that described the study of an ideal Ad-HEx configuration (loose adsorbent grains placed on a flat metal plate) by a new gravimetric version of the large temperature jump method. In the present paper, the study is extended to analyse the dynamic behaviour of HExs with much more complex geometry, namely, small but representative pieces of a finned flat-tube HEx. The Ad-HEx configuration tested is obtained by filling these small HExs with loose grains of AQSOA FAM Z02. The aim of the study is to evaluate the effect of the HEx geometry, grain size and flow rate of heat transfer liquid. The results obtained are compared with a reference flat Ad-HEx configuration.The ad-/desorption dynamics is found to be nearly exponential that proves the applicability of a modified Linear Driving Force model to characterize the water ad-/desorption rate. The majority of the tests have revealed the existence of the “grain size insensitive” regime for grains of 0.3–0.7mm size. Under this mode, the dynamic behaviour is only function of the ratio (S/m)=〈heat transfer surface〉/〈absorbent mass〉. This leads to a practically important conclusion that it is not necessary to precisely select the adsorbent grain size. When the grain size becomes too small or too large the rate reduction was found that is due to inter- or intra-grain mass transfer resistances.The tested Ad-HEx configuration is proved to be quite efficient: the specific cooling power amounts to 50–66% of that obtained with the reference (ideal) one, and the average specific cooling power W80% can reach 2.3kWkg−1. This power is 6–8 times higher than those reported in the literature for full scale Ad-HExs with similar cell geometry. Thus, the study showed that there is still a big room for significant dynamical improvement of real ACH units.

Experimental studies on 3A and 4A zeolite molecular sieves regeneration in TSA process: Aliphatic alcohols dewatering–water desorption

Abstract The objective of this work are in-depth experimental studies of the thermal regeneration of the zeolite molecular sieves 3A and 4A used to dewatering of aliphatic alcohols: ethanol, n-propanol and n-butanol. The experimental results of water desorption during regeneration stage of temperature swing adsorption (TSA) process, following the dewatering stage of alcohols on adsorbent fixed bed contained in column are presented. Multitemperature Langmuir-Freundlich and dual-site Langmuir-Langmuir equilibrium models were used to correlate experimental equilibrium data of water adsorption from gas phase on 3A zeolite at temperatures 100–250 °C. The thermal stability, water adsorption capacity, adsorption selectivity and thermal desorption efficiency of zeolites were studied using DTA-TGA, XRD, IR and UV–Vis-NIR methods. DTA-TGA investigations, carried out in the range of temperature 20–1000 °C revealed that water removing from 3A zeolite molecular sieve takes place up to 400 °C and from 4A up to 500 °C. However, in temperatures higher than 240 °C (3A zeolite) and 230 °C (4A zeolite) decrease of the water desorption rate was observed. Investigated zeolites reveal structural stability up to at least 500 °C, and heating above this temperature lead to an irreversible collapse of structure and a loss of adsorptive capacity. The influence of the inlet purge air temperature (200–250 °C), and air mass flux density (0.170–0.306 kg/m2s) on the zeolite dynamic water adsorption capacity, desorption and cooling steps duration, were studied. The lowest purge air and energy consumption were measured for regeneration temperature of 250 °C. Dynamic adsorption equals to 65% of the equilibrium adsorption capacity.

Adsorption Dynamics in Adsorptive Heat Transformers: Review of New Trends

This review addresses recent advances in understanding and trends in studying dynamics in adsorptive heat transformers (AHTs), which are considered as an alternative to common compression and absorption machines. Experimental data have been obtained mainly by a new large temperature jump method (LTJM) specifically developed to imitate the conditions of isobaric stages of AHTs. Here we consider merely a simple but quite realistic configuration of an “adsorbent–heat exchanger” (Ad-HEx) with n-layers of loose adsorbent grains (n = 1–8). Water, methanol, and ammonia are used as adsorbates. Several recommendations following from this experimental study have been checked with prototypes of adsorptive chiller in ITAE-CNR (Italy) and the University of Warwick (UK). These tests clearly demonstrated that the dynamic performance of AHT can be significantly improved by a proper management of AHT cycle and Ad-HEx geometry. The most important findings and general regularities that have been revealed by systematic studying of the adsorption dynamics of water, methanol, and ammonia in AHT systems are summarized, illustrated, and discussed in this review.

Modulation of electron carrier density at the n-type LaAlO3/SrTiO3 interface by water adsorption

We investigate the energetic stability and dissociation dynamics of water adsorption at the LaAlO3 surface of the n-type LaAlO3/SrTiO3 (LAO/STO) interface and its effect on the electronic properties of the interface by carrying out first-principles electronic structure calculations. In an ambient atmosphere at room temperature the configuration of 1 monolayer (ML) of water molecules including 3/4 ML of dissociated water molecules adsorbed at the surface is found to be most stable, whereas the configuration of 1 ML of dissociated water molecules is metastable. Water molecule dissociation induces an up-shift of the valence band maximum (VBM) of the LAO surface, reducing the gap between the VBM of the LAO surface and the conduction band minimum of the STO. For the LAO/STO interface with three LAO unit-cell layers, once the coverage of dissociated water molecules reaches 1/2 ML the gap is closed, the interface becomes metallic and the carrier density at the LAO/STO interface increases with increasing coverage of dissociated water molecules. Our findings suggest two ways to control the conductivity at the LAO/STO interface: (I) an insulator–metal transition by adsorbing an amount of water at the bare surface; (II) a carrier density change by the transition between the most stable and the metastable adsorption configurations for 1 ML coverage in an ambient atmosphere at room temperature.

Humidity Effects on the Wetting Characteristics of Poly(N-isopropylacrylamide) during a Lower Critical Solution Transition

Poly(N-isopropylacrylamide) (PNIPAM) is expected to find utility in tissue engineering and drug delivery, among other biomedical applications. These applications capitalize on the intrinsic lower critical solution temperature (LCST) of the polymer: below the LCST, enthalpic gain from intermolecular hydrogen bonding between PNIPAM and water molecules dominates the solvation; above the LCST, entropic effects resulting from the intramolecular hydrogen bonding between the carboxyl and amide groups of PNIPAM lead to water expulsion. The dependence of the LCST upon the molecular weight, solvent, and solution activity (i.e., solute concentration) has been studied extensively. However, what has not been previously explored is the effect of humidity on the characteristic properties of the polymer. Herein, we show that the relative humidity affects the water adsorption dynamics of PNIPAM as well as the magnitude of the transition that occurs at the LCST of the polymer. In short, the magnitude of the LCST transition decreases with an increasing relative humidity, and the time period over which adsorption occurs decreases with the temperature.

Dynamics of water adsorption on TiO2 monitored by work function spectroscopy

Water adsorption dynamics on two TiO2 (110) rutile surfaces at room temperature has been investigated using the work function (WF) change as a function of time. The first surface was prepared in a standard way using sputtering/annealing cycles, whereas the second one was long term annealed at 620K in moderate vacuum conditions (the residual gas pressure of about 1×10−7mbar) and cleaned afterwards. The WF change show striking difference as compared to those obtained for highly reduced TiO2 (110) rutile or the (2×1) reconstructed surfaces. For the first kind of surface we show that the observed adsorption dynamics can be qualitatively explained by the present understanding of the water adsorption on non-reconstructed TiO2 (110) rutile surface according to which the bridging oxygen vacancies and Ti rows are the main adsorption sites. Although generally similar to the former results, water adsorption dynamics on the second kind of the surface has an additional feature that can be only explained by a new adsorption site, which we suggest to be due to (2×1) reconstructed regions coexisting with the non-reconstructed TiO2 (110) surface.

Investigation of hydrogen bonds and temperature effects on the water monolayer adsorption on rutile TiO2 (110) by first-principles molecular dynamics simulations

Density Functional Theory (DFT), based on both static and Born–Oppenheimer Molecular Dynamics approaches, has been used to investigate the effect of hydrogen bonds and temperature on the water monolayer adsorption on the rutile TiO2 (110) face. It was demonstrated that the difference between some previous theoretical results and experimental data is due to too slim slab thickness model and/or too small surface area. According to the present static calculations, water monolayer adsorbs molecularly on the five-fold titanium atoms of an optimised five-layer slab thickness, due to the stabilising lateral hydrogen bonds between molecules. From the molecular dynamics simulations, two adsorption mechanisms were described as a function of temperature. Finally, it was pointed out that the dynamics of water adsorption is strongly influenced by the structural model used. When temperature increases, the monolayer dissociates gradually. However, because of the periodic boundary conditions, the 1×1 surface unit needs to be extended to at least 2×5 to get an accurate representation of the monolayer dissociation ratio. In these conditions, this ratio is around 20%, 25% and 33% at 270, 350 and 425K, respectively.

Adsorption dynamics of water on the surface of TiO2 (110)

Rutile titanium dioxide TiO2 is used in a number of technological areas. Therefore, in surface science, it has become the most studied oxide surface. Water adsorption on rutile TiO2 (110) has been investigated using the X-ray photoelectron spectroscopy (XPS) and the work function study (WF): water adsorption induces formation of a dipole layer, which locally changes the work function. This can be experimentally observed as the onset shift of the secondary electron energy spectrum. While XPS seems to be insufficiently sensitive to monitor water adsorption on TiO2, there is a clear work function change undoubtedly attributed to the water adsorption. The measurements were done for different water vapour pressures, exposure times, sample temperatures and general surface conditions. Time evolutions of the work function change and the H2O partial pressure, enable us to successfully model the adsorption dynamics and help us understand the observed results. The analysis clearly shows existence of at least three different adsorption sites. Their interplay governs the work function time evolution, while the relative contributions depend on the surface temperature and, presumably, its topography. These results will be discussed in the light of several recent experimental and theoretical studies of this system done by other authors.

Effect of Residual Gas on Water Adsorption Dynamics Under Typical Conditions of an Adsorption Chiller

In this article the effect of a non-adsorbable gas (air) on kinetics of water adsorption on loose grains was studied for three adsorbents promising for adsorption chilling: SWS-1L (silica KSK modified by calcium chloride), silica Fuji type RD, and FAM-Z02. The experimental conditions were fixed similar to real operating conditions during an isobaric adsorption stage of the basic cycle of an adsorption chiller (AC). Reduction of the adsorption rate was revealed even at a partial pressure of residual air as low as 0.06 mbar. Dependence of the characteristic adsorption time on air partial pressure was found to be linear for partial pressures greater than 0.4 mbar, with the slope depending on the adsorbent nature. Desorption stage was less affected by the residual air. Specific power released in an AC evaporator during the adsorption process was estimated as a function of the partial pressure of residual air, and recommendations how to improve cooling performance are made.

Effect of residual gas on the dynamics of water adsorption under isobaric stages of adsorption heat pumps: Mathematical modelling

A mathematical model of the coupled heat and mass transfer in an adsorbent layer was developed to study the effect of a non-adsorbable gas (air, hydrogen) on kinetics of water adsorption on loose grains of the composite adsorbent SWS-1L (silica modified by calcium chloride). The adsorbent monolayer was placed on the surface of an isothermal metal plate at T = 60 °C and equilibrated with the mixture of water vapor at constant P = 10.3 mbar in the presence of the non-adsorbable gas at a variable partial pressure P A = 0.06–14.3 mbar. After that the metal plate is subjected to a temperature drop down to 35 °C that initiates water adsorption. It is shown that the adsorption of water causes effective gas sweeping to the surface where it was accumulated as a gas-rich layer. This results in dramatic slowing down of the adsorption and heat transfer processes.

Dynamics of water adsorption on Pt{110}-(1×2): A molecular dynamics study

Recent experimental studies of water adsorption on Pt{110}-(1x2) using supersonic molecular beams [F. R. Laffir et al., J. Chem. Phys. 128, 114717 (2008)] have revealed that the translational energy dependence of the initial sticking probability is a stepwise function with a threshold energy of 5 kJ/mol. The initial sticking probability increases sixfold from approximately 0.1 (at translational energies less than 5 kJ/mol) to approximately 0.64 (at translational energies greater than 10 kJ/mol). The aim of this work is to study the adsorption dynamics of water using classical molecular dynamics simulation in order to assess what physical factors are responsible for the observed behavior of the initial sticking probability. The simulations were performed using a purpose-designed code; water molecules were modeled using the well-known TIP4P water model, whereas the water-platinum potential energy function was determined using the ab initio density functional theory calculations. We conclude that the main factor controlling the initial sticking probability is a relatively weak energy transfer between the water molecule and the surface substrate during collision. This energy transfer is enhanced when the total energy of the water molecule increases. The assumption of an exponential increase of the probability of the energy transfer as a function of total energy of water molecule gives initial sticking probabilities very similar to those experimentally obtained. The same model was applied for the simulation of the coverage dependent sticking probability using a hybrid method comprising molecular dynamics and kinetic Monte Carlo approaches. We found a reasonable agreement between our results and the experimental data. The sticking probability as a function of coverage initially increases due to an increasing amount of the adsorbate island edges; it reaches a maximum and finally decreases as the islands merge together at high coverage. The saturation coverage was determined to be 2.8 ML at surface temperature 165 K, where water forms a puckered almost regular lattice with each water molecule having four nearest neighbors. At the studied temperature we did not observe the existence of stable water multilayers on the surface which is consistent with the experimental findings.

Wetting behavior of electrospun poly(L‐lactic acid)/poly(vinyl alcohol) composite nonwovens

AbstractPoly(L‐lactic acid) (PLLA) fibers have been extensively studied for various applications. In this work, PLLA and poly(vinyl alcohol) (PVA) were prepared by coelectrospinning to form composite nonwoven materials. The structures and diameter distribution of the electrospun PLLA/PVA composite nonwovens were examined by atomic force microscopy (AFM) and scanning electronic microscope (SEM). The wetting behavior of the electrospun PLLA/PVA composite nonwovens was also investigated using static contact angles and dynamic water adsorption measurements. It was observed that the addition of PVA in the electrospun PLLA/PVA composite nonwovens significantly alerted the contact angles and water adsorption of the composite materials. It was also found that the increase in the content of PLLA led to the increase in the surface contact angle and decrease in water adsorption of the electrospun PLLA/PVA nonwoven materials. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Adsorption dynamics of water on Pt{110}−(1×2)

The dynamics of H(2)O adsorption on Pt{110}-(1 x 2) is studied using supersonic molecular beam and temperature programed desorption techniques. The sticking probabilities are measured using the King and Wells method at a surface temperature of 165 K. The absolute initial sticking probability s(0) of H(2)O is 0.54+/-0.03 for an incident kinetic energy of 27 kJmol. However, an unusual molecular beam flux dependence on s(0) is also found. At low water coverage (theta<1), the sticking probability is independent of coverage due either to diffusion in an extrinsic precursor state formed above bilayer islands or to incorporation into the islands. We define theta=1 as the water coverage when the dissociative sticking probability of D(2) on a surface predosed with water has dropped to zero. The slow falling H(2)O sticking probability at theta>1 results from compression of the bilayer and the formation of multilayers. Temperature programed desorption of water shows fractional order kinetics consistent with hydrogen-bonded islands on the surface. A remarkable dependence of the initial sticking probability on the translational (1-27 kJ/mol) and internal energies of water is observed: s(0) is found to be essentially a step function of translational energy, increasing fivefold at a threshold energy of 5 kJ/mol. The threshold migrates to higher energies with increasing nozzle temperature (300-700 K). We conclude that both rotational state and rotational alignment of the water molecules in the seeded supersonic expansion are implicated in dictating the adsorption process.

Dynamics of Water Adsorption onto a Calcite Surface as a Function of Relative Humidity

We have developed a molecular dynamics scheme in order to understand the dynamics of water adsorption on calcite surfaces as a function of relative humidity. In contrast to previous studies where either a monolayer or bulk water was assumed to cover the surface, we observe the formation of two to three prominent layers of water depending on the relative humidity. Due to the fact that these simulations are at room temperature, the distribution of water molecules on the surface is inhomogeneous and nonuniform. Our simulation results agree well with recent grazing incidence X-ray diffraction studies. The free energy of adsorption of a single water molecule onto the bare calcite (101̄4) surface is predicted to be −10.6 kcal/mol at room temperature while the enthalpy for the same process is −21.3 kcal/mol. The time scale for the bare calcite surface to become in dynamic equilibrium with water vapor at 100% relative humidity is determined to be close to 6 ns, and the adsorption follows a BET (Brunauer−Emmet−Teller)-like isotherm, in that multilayers form. From our orientational distribution functions, we are able to determine the existence of three binding modes for water. The mobility of water adsorbed on the calcite surface is greater in directions parallel to the surface. Motion perpendicular to the suface is slower. The diffusivity of water significantly increases with increasing relative humidity.

Structural characterization and dynamic water adsorption of electrospun polyamide6/montmorillonite nanofibers

AbstractA facile compounding process, which combined nanocomposite process with electrospinning for preparing novel polyamide6/organic modified montmorillonite (PA6/O‐MMT) composite nanofibers, is reported. In this compounding process, the O‐MMT slurry was blended into the formic acid solution of PA6 at moderate temperatures, where the nanosized O‐MMT particles were first dispersed in N,N‐dimethyl formamide solvent homogeneously via ultrasonic mixing. Subsequently the solution via electrospinning formed nanofibers, which were collected onto aluminum foil. The O‐MMT platelets were detected to be exfoliated at nanosize level and dispersed homogeneously along the axis of the nanofibers using an electron transmission microscope. Scanning electron microscope and atomic force microscope were used to analysis the size and surface morphology of polyamide6/O‐MMT composite nanofibers. The addition of O‐MMT reduced the surface tension and viscosity of the solution, leading to the decrease in the diameter of nanofiber and the formation of rough and ridge‐shape trails on the nanofiber surface. The behavior of the dynamic water adsorption of composite nanofibers was also investigated and discussed in this article. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Dynamic Water Adsorption Characteristics of Distillers Dried Grains with Solubles (DDGS)

ABSTRACTDistillers dried grains with solubles (DDGS) is one of the coproducts obtained from dry‐grind ethanol manufacturing. As the ethanol industry is growing exponentially, the production of DDGS has been significantly increasing as well. To optimize the use of DDGS, it has to be economically transported from one part of the country to other parts, and stored efficiently. But DDGS has some flow issues, which often makes storage and transportation very problematic. So the objective of this study was to investigate the dynamic water adsorption characteristics of DDGS with four soluble levels at four temperatures and four relative humidities. Three mathematical models were then used to fit the adsorption data (Peleg, Pilosof, and Singh‐Kulshrestha). As there was no model available for describing the water adsorption characteristics of DDGS with varying soluble levels at various temperature and relative humidity conditions, a new comprehensive model was developed. The new model, Ganesan‐Rosentrater‐Muthu (GRM) model, encompassed soluble level, temperature, and relative humidity effects, along with time and moisture content. The GRM model (R2 = 0.94; F =16503.90) provided a good description of DDGS water adsorption behavior and can be used to predict the dynamic adsorption of water in DDGS for a broad range of storage conditions.

Dynamic water adsorption behaviour of plasma-treated polypropylene nonwovens

Plasma surface treatment has been extensively applied in the textile industry for the modification of polymer materials. In this study, polypropylene (PP) nonwovens were treated with cold oxygen gas plasma for improving the water adsorption properties of the materials. The effects of plasma treatment on wetting and water adsorption behaviour of the materials were examined using environmental scanning electron microscopy (ESEM) and dynamic adsorption measurements (DAM). ESEM observations revealed the behaviour of water droplets formed on the fibre surfaces. DAM also showed the effect of plasma treatment on the dynamic water adsorption behaviour of the materials. The investigation revealed that plasma treatment could significantly improve the dynamic water adsorption properties of PP nonwovens. It was also found that the fibre fineness influenced the water adsorption behaviour of the plasma-treated materials.