Capillary Condensation Of Water Vapor Research Articles

Laser-ablated synthesis of Ag@TiO2 NPs on Si nanostructure as humidity sensor

ObjectiveThis work aimed to produce Ag@TiO2 nanoparticles/PS ultra-high sensitivity for humidity by laser ablation. Methodswe combine Ag/TiO2 NPs of 59.93 nm with porous silicon (PS) to create a nanocomposite system. On an n-type silicon wafer, the structural, morphological, and optical properties of a silicon bulk and PS layer are detailed in depth. ResultsThe PL regarding the sample PS and Ag/TiO2 NPs is measured in air at various relative humidity (RH) levels. PS’s PL band is associated with quantum confinement; PL for the Ag/TiO2 system has a broad peak. Even though the decline in intensity could be attributed to Ag/TiO2 absorption regarding the excitation and emission radiations, this shows the peak from PS. In the case when RH levels rise, the PL of the PS and Ag/TiO2/PS humidity sensors alter. The intensity of the PL lessens. This effect may be described as capillary condensation of water vapor within PS pores.

Ultrasensitive and reversible room-temperature resistive humidity sensor based on layered two-dimensional titanium carbide

The nanostructured two-dimensional (2D) materials for humidity sensing applications have become increasingly attractive. However, 2D materials have negative aspects of easily stacking and agglomerating multilayers. Here, a novel resistive-type humidity sensor utilizing multi-layer titanium carbide (Ti3C2Tx) films as sensitive material is demonstrated. The humidity sensor exhibits an ultrasensitive and reversible sensing performance in a wide relative humidity range. Furthermore, the ultrafast response and recovery properties are achieved at room temperature. The Fourier transform infrared spectroscopy is used to investigate the adsorption of water vapor on Ti3C2Tx surfaces. A rapid capillary condensation of water vapor on the unique accordion-like microstructures and the hierarchical nanostructures of hydroxyl-riched Ti3C2Tx surface are supposed to be responsible for the super adsorption capability for water vapor. The electrostatic field induced by adsorbed water molecules is proposed to explain the resistance change of the titanium carbide humidity sensor. This work highlights the unique advantages of humidity sensor with layered 2D materials of Ti3C2Tx MXenes, including ultrasensitive, good reversibility and fast recovery at room temperature.

Entropy generation analysis of heat and water recovery from flue gas by transport membrane condenser

Abstract The transport membrane condenser (TMC) has been used for heat and water recovery from coal-fired power plant flue gas. The capillary condensation of water vapor in membrane pore structure is the main gas separation mode. A lumped parameter model was established to study the heat and mass transfer in TMC. Recovered water and heat flow rates, water recovery ratio, heat recovery efficiency, and pressure drops were calculated. The temperature and humidity ratio distributions were displayed. The influences of structural parameters and operating conditions on the water and heat recovery performances were analyzed. In addition, the entropy generation model was proposed to calculate entropy variations and entropy generation components. The aim is to provide insights into TMC parameter selection and operation optimization. Moreover, the relationship between entropy generation components and TMC performances were confirmed. The results show that increasing packing fraction, or decreasing the membrane inner diameter or membrane pore size can improve the heat and water recovery performances. Besides, high water flow rates and low water temperatures have advantages in the operation. Increasing the mass/heat transfer driving force can enhance heat transfer performance, but the heat transfer entropy generation rate also increases. The maximum mass transfer entropy generation rate often corresponds to the best water recovery performance.

Kinetics of Capillary Condensation of Water in Mesoporous Carbon: Nucleation and Meniscus Growth

To examine the mechanisms of nucleation and subsequent meniscus growth in capillary condensation of water vapor in graphitic mesopores that occurs very slowly close to the saturation, we measured the kinetic curves of water uptake in four kinds of mesoporous carbons as a function of temperature. Plotting the amount of uptake against the square root of time gave typical Fickian curves. The plots produced a nonzero intercept with time axis, and for the mesoporous carbons without micropores, the effective value of the intercept increased with an increase in pore size. This suggests that in the initial stage of capillary condensation the number of mesopores with liquid plugs of water at the pore entrances increases with time. In addition, the dependence of the rate constant on pore size strongly suggests that the meniscus growth of water in the graphitic mesopores is controlled by the gradient of chemical potential between the saturated water vapor surrounding the mesoporous carbons and the vapor–liquid equilibrium for the confined water. The rate of water uptake tended to decline rapidly as the temperature approached 0 °C. For the mesoporous carbon with micropores the dependence of the rate constant on pore size and on temperature was different from that without micropores, indicating that the mechanisms of nucleation and meniscus growth are different.

Corking and Uncorking a Catalytic Yolk-Shell Nanoreactor: Stable Gold Catalyst in Hollow Silica Nanosphere

Silica templating of a water-in-oil (w/o) microemulsion system containing HAuCl4 results in gold nanoparticles of sizes around 3 nm confined in hollow silica nanospheres. The size of the gold nanocatalyst is stable to high temperature calcination. Its catalytic activity toward CO oxidation is very stable after repeated use and long-time storage. The reaction can be controlled by a corking design based on capillary condensation of water vapor for corking the mesopores on the shell. At higher temperature or low water vapor pressure, pores open after vapor evaporation and water promotes the CO oxidation. The corking and uncorking mechanism of the yolk-shell type nanoreactor is reversible.

Synthesis and water vapor separation properties of pure silica and aluminosilicate MCM-48 membranes

This work reports on the hydrothermal synthesis and gas–vapor separation properties of ordered mesoporous MCM-48 and Al-MCM-48 membranes. Gas permeation data for these membranes is characterized by the Knudsen type permeation mechanism, with a helium permeance of 7×10−8mol/m2Pas and 3×10−8mol/m2Pas, respectively, confirming the mesoporous nature of the membranes synthesized. Separation of a water vapor/oxygen mixture for these membranes was studied to explore potential use of these membranes in air dehumidification and air conditioning applications. These membranes offer high water vapor/oxygen selectivity as a result of capillary condensation of water vapor in the pores of the membranes. In the case of MCM-48, an oxygen permeance of 5×10−9mol/m2Pas is obtained, with a water permeance of around 1×10−7mol/m2Pas. The water vapor/oxygen separation factors as high as 17 are achieved. Al-MCM-48 membranes are seen to provide much higher separation factors reaching as high as 142, with water vapor permeance of 6×10−8mol/m2Pas.

Characteristics of the membrane utilized in a compact absorber for lithium bromide–water absorption chillers

This study aims at investigating experimentally and analytically the characteristics and properties of a membrane utilized to design compact absorbers for lithium bromide–water absorption chillers. The main focus of this study are the factors that influence the water vapor transfer flux into a lithium bromide–water solution in confined narrow channels under vacuum conditions, as well as the properties limits for utilization in compact absorber design. The results indicate that the desired membrane characteristics for this application are as follows: high permeability to water vapor, hydrophobic to the aqueous solution with high liquid entry pressure (LEP) to avoid wettability of the membrane pores and no capillary condensation of water vapor to avoid blocking of the pores. For practical use, this membrane should have a thin hydrophobic microporous active layer with a thickness up to 60 μm, mean pore sizes around 0.45 μm and a porosity of up to 80%. The active layer should be attached to a porous support layer to meet the mechanical strength requirements needed for practical use in the absorber of lithium bromide water absorption chillers application.

Effect of membrane structure on mass-transfer in the membrane gas–liquid contacting process using microporous PVDF hollow fibers

In order to gain a better understanding of the membrane's role in the whole process of mass-transfer in membrane gas–liquid contacting systems, PVDF microporous hollow fiber membranes have been fabricated using three different dope solutions containing N-methyl-2-pyrrolidone (NMP) and different additives. The resultant hollow fibers with different structures were used to make membrane modules, which were then applied as gas–liquid membrane contactors for CO 2 absorption in water. The membranes were characterized and the effect of membrane structure on the mass-transfer was analysed accordingly. It was found that the additives used imposed a significant effect on the final membrane structure under the same spinning conditions. Compared with distilled water, phosphorous acid and glycerol showed a stronger pore-inducing ability. As a result, the membrane #1 or #2 made by the dope with phosphorous acid or glycerol as an additive had a larger pore size and a higher value of MWCO. The finger-like pores also occupied a larger portion on the membranes #1 and #2 than on the membrane #3, which used distilled water as an additive. Moreover, the membranes #1 and #2 presented a much wider pore size distribution than the membrane #3. The different membrane structures turned out to affect CO 2 absorption performance when used as contactors. Since the ratio of the membrane resistance over the overall mass-transfer resistance was increased in the order of #1 < #2 < #3, with respect to the reductions of MWCO and the finger-like pore portion of these three membranes in the same sequence, the CO 2 absorption performance was in the sequence of #1 > #2 > #3. In addition, it was noticed that all the membranes exhibited considerable membrane resistances from 22% to 36% though the model system used was pure CO 2 absorption in distilled water. The partial wetting was probably caused by the capillary condensation of water vapour in the membrane pores instead of water penetration, as the experiment shows that the CO 2 flux of three PVDF membranes was kept almost unchanged over 15 days of operation. In comparison with a commercial double skin layer PVDF membrane, it was found that the membrane with an inner skin-free structure plus a porous substrate is favourable to be used in the membrane gas–liquid contacting process.

Absorption–evaporation kinetics of water vapour on highly hygroscopic powder: Case of ammonium nitrate

The conditioning of industrial powders or granulates is strongly dependent on their behaviour with respect to the atmospheric humidity. Adsorption and capillary condensation of water vapour at the points of contact between particles change the rheology of the powder and may result in the formation of aggregates with high mechanical resistance. In some cases water vapour causes partial dissolution of powder, as in the case of the hygroscopic fertilizer Ammonium Nitrate. Additives used as caking inhibitors are expected to act in one or both of following directions: – slackening the adsorption of water vapour and/or the dissolution of the particles in conditions of high humidity; – inhibition of the nucleation and growth during the recrystallisation of the powder in a dry atmosphere. An experimental method was developed which allowed following of the kinetics of the cycle, absorption of water vapour and formation of solution – evaporation of water – nucleation and growth of the recrystallized solid. The method is based on the variation of mass of powder sample placed in a flow of water vapour with known partial pressure at constant temperature. A model is proposed, based on the kinetic theory of gases, able to assess quantitatively the sorption/dissolution kinetics of powders exposed to high atmospheric humidity, just as the evaporation of the absorbed water and, eventually, the recrystallisation of the dissolved solid when the humidity is low. The effect of addition of Fuchsine acid (known anti-caking agent for Ammonium Nitrate) on that kinetics is found to be nil as regards the dissolution and the nucleation but slakes the subsequent recrystallisation during powder drying.

Adsorption Equilibrium of Water Vapor on Mesoporous Materials

Adsorption of water vapor on mesoporous materials such as MCM-41, MCM-48, KIT-1, SBA-1, and SBA-15, which have different pore sizes and shapes, was investigated by using a gravimetric system. In addition, adsorption of water vapor on KIT-1 mesoporous material was examined at 293.15 to 353.15 K. A hybrid isotherm model composed of a Henry isotherm and a Sips isotherm was applied to the analysis of water adsorption on mesopores including its surface adsorption and capillary condensation. The proposed isotherm nicely simulated measured adsorption data over the whole range at these experimental conditions. The capillary condensation of water vapor on mesoporous materials was highly dependent on temperature and pore sizes rather than on their pore shapes.

Effect of capillary-condensed water on the dynamic friction force at nanoasperity contacts

A single nanoasperity contact in ambient air is usually wetted by capillary condensation of water vapor and is surrounded by a water meniscus. This phenomenon strongly affects the contact friction, not only by the effect of meniscus loading force (superficial tension and capillary forces), but also by a friction force that accounts for the energy loss in the meniscus movement along with the sliding contact. Occurrence of the water-meniscus-generated friction is experimentally proved by atomic force microscopy measurements of the tip–sample friction force at minimum possible external load (before pull-off). A qualitative explanation for the observed dependence of the friction force on air humidity and solid surface wettability is proposed.

Effect of Humidity and Temperature on Molecular Mobility in Surfactant Monolayers Confined between Two Solid Surfaces

The lateral diffusion coefficient (Ds) of fluorescent-labeled probe molecules in surfactant monolayers of fatty acids confined between two silica surfaces was directly measured as a function of relative humidity (RH) and temperature by the fluorescence recovery after photobleaching (FRAP) technique. The RH dependence of Ds in myristic acid monolayers showed a general trend that the molecules become more mobile with increasing RH. Two distinct RH regimes of different mobility behavior were observed. The molecular mobility characteristics in each regime are explained in terms of the formation of a hydration layer and the capillary condensation of water vapor from the ambient atmosphere. The RH dependences of Ds in stearic acid monolayers and oleic acid monolayers showed similar trends to those in myristic acid monolayers, except that stearic acid monolayers yielded a much smaller Ds and no immobile fraction, whereas oleic acid monolayers gave a somewhat larger Ds and immobile fraction than those in myristic acid monolayers. As the temperature is raised, Ds increases in an exponential manner under dry (0% RH) conditions. At 45 and 55% RH, Ds initially decreases upon heating, and then increases and appears to remain constant above 65 °C. This trend was proposed to be because of the combined effect of RH reduction and thermal energy increase upon heating. Finally, upon repeated heating and cooling, Ds becomes lower, which implies that the monolayers undergo some irreversible structural change. These findings provide new information on the general mobility behavior of surfactant molecules in confined geometry relevant to boundary lubrication.

Thin-film friction and adhesion studies using atomic force microscopy

An atomic force microscope is used to study the effect of tip radius and humidity on the adhesive force and coefficient of friction. Samples studied are Si (100), 2 nm thick bonded perfluoropolyether lubricant (Z-DOL) coated Si (100), 20 nm thick diamond-like carbon (DLC) coated Si (100) and plasma etched DLC coated Si (100). Tip diameters ranged from 100 nm to 14.5 μm. It is observed that both adhesive force and coefficient of friction are strongly dependent on the tip radius and humidity. The change is caused by the formation of meniscus bridges from capillary condensation of water vapor. DLC coated surface is rather insensitive to the changes in the radius and humidity because of its passivated surface. Etching of DLC activates the surface and makes it sensitive to the environment. In addition, a technique is developed to get adhesive force maps of surfaces, which can be used to obtain an estimate of film thickness only a couple of monolayers thick. Examples and applications of this technique are presented.

Moisture-induced ageing in granular media and the kinetics of capillary condensation

In 1773 Coulomb1 recognized that the static properties of granular systems can be discussed in terms of the frictional properties between different layers2, leading to his relationship between the angle of repose of a granular pile (θ0) and the coefficient of static friction µ s: tan θ0 =µs. Two centuries later, solid friction and granular media still present many puzzles. One such is that the coefficient of static friction depends on the time during which the solids remain in contact before the measurement. Here we show that this ageing effect is manifested too in the angle of repose of granular media and originates from capillary condensation of water vapour between the packed particles, leading to the formation of water bridges. By assuming that the kinetics of this process are governed by the thermally activated nucleation of bridges, we can reproduce both the time- and humidity-dependence of the ageing behaviour. Our results also clarify the kinetics of adsorption in porous media more generally.

Environmental effects on the streaming mode performance of metal evaporated and metal particle tapes

A commercial Hi-8 video cassette recorder was instrumented to measure friction force between the rotary heads and tape, rms head output, and signal dropouts to sub /spl mu/s duration. Streaming (play) mode experiments using metal evaporated (ME) and metal particle (MP) tapes were performed at design tension under equilibrium and nonequilibrium conditions inside of an environmental chamber at various temperature and specific humidity (SH=ratio of the weights of water vapor to dry air in the mixture). Interface stability and recording performance at a 0.6 /spl mu/m recording wavelength were measured to bit level resolution using a dropout counter and changes in rms head output were correlated to changes in head-to-tape spacing using the Wallace equation. ME tape performed best at moderate SH (from 0.009 to 0.013) in the operating temperature range of 15.6 to 32.2/spl deg/C, whereas MP tape performed best at low temperature. At a given temperature, higher SH increased normal and friction forces and decreased head-to-tape spacing due to spontaneous water meniscus formation between contacting and noncontacting asperities on the head, tape, and debris particle surfaces. Dropout frequency and interface stability were sensitive to both SH and temperature. Humidity dependence was governed by the relative size difference between wear debris particles and spontaneously formed water menisci. Competing mechanisms of increased lubricant mobility and spontaneously formed water menisci of smaller radii of curvature at higher temperature governed temperature dependence. A model based on capillary condensation of water vapor onto surfaces in a sliding contact is developed to explain experimental data. An expression for meniscus force consisting of both Laplace and surface tension contributions is developed for nanometer size contact spots and wear debris particles. The model predicts that meniscus force will increase and head-to-tape spacing will decrease with increasing SH which was observed experimentally. Four lubrication regimes are defined for wear debris particles passing through the contact interface. Both SH and extent of deformation of a particle determine whether menisci will form around the particle, and presence or absence of associated meniscus forces explains trends in dropout frequency data. The model allows determination of meniscus height from which minimum values of Kelvin radius and relative humidity (RH) in the contact interface can be calculated. Proximity and intimate contact of the surfaces increases RH and SH in the contact interface over that maintained in the environmental chamber to near their saturation values.

Water Adsorption Properties on Porous Silica Glass Surface Modified by Trimethylsilyl Groups

Nitrogen and water adsorption properties on the porous silica glass surfaces modified by different concentration of trimethylsilyl (TMS) groups were studied. The specific surface areas and parameterCvalues in the BET equation estimated from nitrogen adsorption decreased with increase in the surface concentration of the TMS groups and became constant values above 1.0 TMS group per nm2. The water adsorption property suggested that a highly silylated sample still represented hydrophilic character in initial adsorption process. Water molecules interact with the residual hydroxyl groups existing under the umbrellas of the TMS groups and form clusters. Above the concentration of 1.0 TMS group per nm2the capillary condensation of water vapor did not occur. The TMS groups interrupt to build a two-dimensional network of each cluster. The formation of multilayer adsorption of water molecules depends on the magnitude of hydrophilic patch areas which are surrounded by TMS groups.

Removal of VOCs from humidified gas streams using activated carbon cloth

This research investigates the effects of relative humidity (RH) on the adsorption of soluble (acetone) and insoluble (benzene) volatile organic compounds (VOCs) with activated carbon cloths (ACC). A gravimetric balance was used in conjunction with a gas chromatograph/mass spectrophotometer to determine the individual amounts of water and VOC adsorbed on an ACC sample. RH values from 0 to 90% and organic concentrations from 350 to 1000 ppmv were examined. The presence of water vapor in the gas-stream along with acetone (350 and 500 ppmv) had little effect on the adsorption capacity of acetone even at 90% RH. Water vapor in the gas stream had little effect on the adsorption capacity of benzene (500 ppmv) until about 65% RH, when a rapid decrease resulted in the adsorption capacity of benzene with increasing RH. This RH was also about where capillary condensation of water vapor occurs within ACC pores. Water vapor condenses within the ACC pores, making them unavailable for benzene adsorption. Increasing benzene concentration can have a significant effect on the amount of water vapor adsorbed. At 86% RH and 500 ppmv, 284 mg/g water was adsorbed, while at 86% RH and 1000 ppmv, only 165 mg/g water was adsorbed. Water vapor was more inhibitory for benzene adsorption as benzene concentration in the gas stream decreased.

Adhesive forces between fibers due to capillary condensation of water vapor

A theoretical analysis is presented of the capillary condensation of water which takes place around the contact between two cylindrically shaped fibers of radii R 0 that cross at an arbitrary angle, α. We verify that within the vapor pressure range 0.78 ⩽ p p 0 ⩽ 0.99 , the predominant adhesion force contribution is given to a satifactory approximation by the Derjaguin expression F 1 = 4 πR 0 γ cos θ/sin α, where γ is the surface tension of water and θ the contact angle. This adhesion force is due to the negative Laplace pressure in the condensate and it strengthens a network of hydrophilic cellulose fibers in a sheet of paper during its formation. The corresponding contribution to the water adsorption isotherm from capillary condensation at fiber contacts is shown to be negligible.

Sulphur dioxide control by reactive photografted membranes immobilizing high surface area calcium oxide

The effects of temperature (313–423 K), of feed SO 2 concentration (985–2200 ppm (v/v)), of pore size (0.04–1.9 μm) and of surface area (11–39 m 2 g −1) of CaO sorbent on the reactivity of composite membranes, immobilizing 30 wt.% of sorbent in an epoxy-diacrylate resin matrix photografted onto cellulose, towards SO 2, have been studied at constant flow rate (1.85 cm min −1) and pressure (1 bar gauge). Moisture in the test gas was kept constant, by saturation with water at 343 K. With high surface area CaO and at the dew point temperature the % of SO 2 captured was always greater than 50%, approaching 95–98 % at 313 K where capillary condensation of water vapour occurred. In the lower temperature range, the presence of liquid water in the membrane pores slackens gas diffusion, enhances SO 2 solubility and consequently favours reactivity. The dependency of sorption rate on temperature and on pore size allows us to find experimental conditions, in which to operate with a sorption efficiency significantly lower than unity, but with an equally satisfactory overall yield, in a multistage membrane process, provided high surface area sorbents are used, and membranes immobilizing these sorbents are manufactured by a technique, such as photografting, able to maintain this surface area almost entirely active.

Capillary condensation of water vapor within a particulate bed

AbstractFor situations in which capillary condensation of water occurs (e.g., within porous media), the thermodynamics can usually be studied by means of the Laplace and Young equations and a compressibility equation. We show here that the compressibility equation for the liquid does not reflect the criteria of equal gas and liquid phase potential changes at equilibrium, for departures from a specified reference state.Carman (1953) concluded that the capillary condensate can exist in a state of tension for large vapor‐liquid pressure differentials and may exhibit physical properties that differ substantially from normal values. The familiar Kelvin equation is used to calculate the capillary curvature and is derived from the general Laplace and Young relationship. Melrose (1966) has obtained this relationship by matching the coefficients of the internal free energy and hydrostatic balances of the system shown in Figure 1.