Water Vapor Adsorption Kinetics Research Articles

Gravimetric analysis of water vapor adsorption kinetics on hydrophilic and hydrophobic Si surfaces using microcantilevers

Understanding the nature of the adsorption of water molecules on a Si surface is of great interest in basic sciences as well as in broad range of industrial processes. Traditional techniques such as Fourier transform infrared spectroscopy cannot capture the adsorption phenomena on a larger surface area of Si. Herein, we report the adsorption kinetics of water molecules on hydrophilic and hydrophobic Si surfaces by means of a precision gravimetric method using microcantilevers (MC). Si MCs with hydrophilic and hydrophobic surfaces were synthesized by chemical route, and their resonance frequency with increasing Relative Humidity (RH) was measured. Shift in resonance frequency of MCs due to adsorption of water molecules was used to estimate the added mass on their surface. In hydrophilic MCs, added mass Vs RH data revealed three distinctive slopes, indicating variation in the molecular configuration of water molecules with increasing RH as predicted by spectroscopic techniques. These results are independently validated by measuring the adhesion force on the MC surface using force-distance curves. In contrast to this, in hydrophobic MCs, added mass due to adsorption was very low and found to increase marginally at higher RH values. An increase in MC surface roughness leading to the clustering of water molecules was shown to be the reason for these observations.

Experimental investigation of innovative composite Maxsorb III adsorbent for cooling and water desalination

This study presents an innovative composite Maxsorb III/CaCl2 (70 wt % of Maxsorb III to 30 wt % of CaCl2) for a relatively high adsorption capacity, cooling capacity, and freshwater production. The raw Maxsorb III (Max) has undergone acid treatment (2 mol HCl) as a pretreatment then it is impregnated with salt hydrates (Max/CaCl2). Raw and composite Max is characterized using XRD, adsorption isotherm utilizing nitrogen, and water vapor adsorption kinetics and isotherm. The isotherm outcomes showed that Max/CaCl2 had a water uptake of 1.35 kgH2O/kgMax/CaCl2. A simulation model of a desalination system employing the adsorption technique, either with or without heat recovery, has been conducted using MATLAB software to show the performance of an adsorption system utilizing the innovative composite adsorbent material. The results showed that Max/CaCl2 attained SDWP of 25.4 m3/tonMax/CaCl2 per day with SCP 717.4 W/kgMax/CaCl2 and COP 0.704 at 85 °C. Additionally, water productivity utilizing condenser-evaporator heat recovery can reach 36.7 m3/tonMax/CaCl2 at 85 °C and 43 m3/tonMax/CaCl2 at 95 °C. The results indicated that the innovative composite material has a promising performance within the AD system and overcomes traditional adsorbents such as activated carbon, silica gel, and zeolite.

Silica Synthesis from Oil Palm Mill Boiler Ash Under Different Concentration of NaOH and Extraction Time

Boiler ash from palm oil mill contains high SiO2compound. The boiler ash used in this study was from PKS Cikasungka PTPN VIII, Bogor. The research objectives were to (i) optimize the conditions of silica synthesis process from boiler ash, (ii) analyze the silica content of the synthesis, (iii) and analyze the performance of silica as a moisture absorber. This research was designed using a central composite design (CCD) and the variables were optimized using the response surface method (RSM), silica was synthesis using sol-gel method and silica content was measured using XRF. The performance of silica as a moisture absorber was also studied to determine the appropriate adsorption kinetics at 32%, 64%, 75%, 85% and 97% RH conditions. The results showed that the optimum process conditions that produced the highest silica yield (41.17%) were solvent concentration of 7.39 M and extraction time 78.13 minutes. The silica gel produced is a white powder with SiO2content of 79.20%. The results showed that the water vapor adsorption kinetics at 64% RH was simulated with a first order model, while other treatments with 32%, 75%, 85% and 97% RH were simulated with a second order pseudo model. At 32% RH had a higher k value of 0.086 g/g/day than the 97% RH of 0.011 g/g/day.

Adsorption of Carbon Dioxide, Water Vapor, Nitrogen, and Sulfur Dioxide on Activated Carbon for Capture from Flue Gases: Competitive Adsorption and Selectivity Aspects

There is extensive interest in postcombustion flue gas treatment for mitigating CO2 emissions and removal of acid gases. In this study we investigate the adsorption of the main flue gas components (CO2, N2, SO2, and water vapor) on Filtrasorb 400 activated carbon to understand adsorption characteristics of the main components and competitive adsorption effects. The adsorption isotherms of the pure components of flue gas, CO2 (273.15–318.15 K and 0–50 bar), N2 (298.15–313.15 K and 0–150 bar), SO2 (273.15–303.15 K and 0–3.6 bar), and water vapor (293.15–303.15 K and 0–41 mbar), were investigated. The isosteric enthalpies of adsorption were determined to be a function of surface excess. The enthalpies at zero surface coverage have the order SO2 > H2O > CO2 > N2. However, the SO2 isosteric enthalpy decreases with increasing surface excess and is lower than that of water vapor at high surface excess uptake values. The temperature range for CO2 adsorption covers the subcritical to supercritical gas transition. There was no evidence for isosteric enthalpy differences over this temperature range. The adsorption kinetics for SO2 (290.65–303.15 K) and H2O (293.15–303.15 K) adsorption were measured for each isotherm pressure increment. In both cases the adsorption kinetics followed the linear driving force model. The adsorption mechanisms for both SO2 and H2O kinetic trends are discussed in terms of the adsorption mechanisms. The water vapor adsorption kinetics showed a minimum in the region where water molecules form clusters around functional groups, which merge in the pores. The SO2 adsorption kinetics also show a minimum with increasing surface coverage, and this is attributed to dipole–dipole interactions. The activation energies for diffusion of both SO2 and H2O into F400 were very low. Both the N2 and CO2 adsorption kinetics were too fast to be measured accurately by the gravimetric method used in this study. Ideal adsorbed solution theory (IAST) was used to calculate competitive adsorption of SO2/CO2 and CO2/N2 from the isotherms of the pure components. The competitive adsorption of CO2/N2 was investigated by using the integral mass balance (IMB) experimental method, and this was used for validation of the IAST. The results provide an insight into the role of competitive adsorption in the capture of CO2 and SO2 from flue gases by adsorption from both thermodynamic and kinetic perspectives.

Emulsified blend film based on konjac glucomannan/carrageenan/ camellia oil: Physical, structural, and water barrier properties

The objective of this study was to develop a new hydrophobic film based on konjac glucomannan and kappa-carrageenan (KGM-KC) incorporating camellia oil (CO) (2, 4, and 6 %). CO was directly emulsified as a dispersed phase into KGM-KC matrix. The physical, structural, and water barrier properties of the film were studied. The results of Fourier transform infrared and scanning electron microscopy suggested that CO was successfully distributed in KGM-KC matrix by emulsification. Contact angle of the film indicated that addition of CO increased the hydrophobicity and water-resistance properties of film, which corresponding to the moisture content, total soluble mass, water vapor permeability, water vapor adsorption kinetics and water vapor adsorption isotherms. Addition of CO by emulsification improved thermal stability of film, optical properties, and mechanical properties. In conclusion, the incorporation of CO by emulsification is an effective and promising pathway to improve the properties of polysaccharide-based film.

Enhancement mechanisms of water vapor adsorption rate onto silica-gel in acoustic field

This study was focused on the evaluation of the adsorption kinetics of water vapor onto silica-gel with and without an acoustic field, in an objective to reveal the enhancement mechanisms of water vapor adsorption by an acoustic wave. Adsorption uptake curves of water vapor from humidified air onto the silica-gel with and without acoustic field were measured at room temperature under various conditions, after which the adsorption kinetic parameters (fluid film mass transfer coefficient and intraparticle diffusivity) were determined from the uptake curves with the aid of numerical calculations. The results demonstrated that the acoustic field exhibited no effect on the intraparticle diffusion, because the fluid film mass transfer was the rate-limiting step under the condition of this study. In contrast, the fluid film mass transfer was enhanced by the velocity amplitude of the acoustic field. However, the diminished mass transfer was also observed in some cases. For the enhancement and diminution of adsorption rate by the acoustic wave, it was suggested that the ratio of the oscillation flow (U) to the steady flow (u), U’ = U/u, was an important factor. Moreover, the enhanced adsorption kinetics was observed under the conditions of U’ > 2, whereas the adsorption rate was diminished or invariant under the condition of U’ < 2. This would be understood from the viewpoint of the thinning and thickening of fluid film by the oscillation flow caused by the acoustic wave. Even when the direction of the oscillation flow is countercurrent to the steady flow, the thinning of fluid film by the oscillation flow is still possible under U’ = U/u > 2.

Effect of particle size on adsorption kinetics of water vapor on porous aluminium oxide material

Influence of the grain size of aluminium oxide material, being a product of centrifugal thermal activation of hydrargillite, on adsorption kinetics of water vapors was studied. The material was characterized by the BET method and X-ray phase analysis (XRD). Influence of gas flow rate on adsorption dynamics was studied on a laboratory installation using McBain-Bakr quartz balance. It was shown that with the fraction size greater than 0.5-1.0 mm, the rate of water vapor adsorption on this adsorbent decreased, which was connected with the influence of internal diffusion resistance. On the base of the first-order kinetic equation for the water adsorption mathematical modeling was carried out. The kinetic parameters of the equation for the various grain size samples (0.25-0.5 mm and 0.5-1.0 mm and 3.7 x 6 mm granule) were determined.

Influence of alkaline modification on adsorption properties of alumina

The kinetics of water vapor adsorption on alumina adsorbents at a temperature of 25 °C was studied. It was shown that modification by cations of alkali metals (K, Na) of the adsorbent based on alumina allows increasing its efficiency in the process of water vapor adsorption. The mathematical treatment of the experimental data by the dependence of water vapor adsorption on the time with the help of the Linear Driving Force (LDF) model and the dependence of adsorbent’s adsorption capacity on the water vapor concentration (adsorption isotherm) by Dubinin-Astakhov equation was carried out. Constants included in the equations were defined.

Kinetics of Water Vapor Adsorption and Desorption in MIL-101 Metal–Organic Frameworks

The adsorption–desorption isotherms and relaxation curves of water in chromium terephthalate metal–organic frameworks (MOFs), MIL-101, were measured by the gravimetric method at 298 K and 1 atm. Th...

Equilibrium and Kinetics of Water Vapor Adsorption on Shale

Water vapor adsorption and desorption isotherms and kinetics studies on three Sichuan Basin shale samples were performed at 298 K by an accurate gravimetric method. The adsorption equilibrium data were fitted using both Dent model and Modified Dent model to estimate the adsorption characteristic of water on the primary and secondary sites. The primary site adsorption is restricted to a monolayer while the secondary site adsorption is associated with multilayer sorption. A positive correlation was found between clay mineral content and monolayer sorption content. The isosteric heats of sorption of water were determined from the equilibrium data and they decreased with the increase of adsorption amount. The adsorption/desorption hysteresis were studied with the pore structure. The kinetics of water vapor adsorption was studied with the unipore model and linear driving force mass transfer (LDF) model. The effective diffusivity and kinetic rate constant varied with the increase of relative pressure, which suggested diffusion of water vapor on shale corresponding to a combination of adsorption on primary sites, adsorption on secondary sites, formation of water clusters, and capillary condensation.

Pore-size resolved water vapor adsorption kinetics of white cement mortars as viewed from proton NMR relaxation

The dynamic pore structure of partially saturated cement-based material is rather essential to quantify its mass transport and many other properties. To clarify the influence of water removal and re-entry, the low-field nuclear magnetic resonance technique is employed to characterize the pore structure of two white cement mortars during isopropanol exchange and water vapor adsorption kinetics. It is found that, after isopropanol replacement the pore structures of mortars become remarkably coarsen due to the significant collapse of C-S-H interlayer pores, which are only partly reversible at water re-saturation. Consistently, water vapor adsorption is not a simple progressively filling process from finer to coarser pores, but accompanied by continuous expansion of C-S-H gel adsorbing water in priority. Furthermore, the irrecoverable contraction of C-S-H gel is the physical root of irreversible shrinkage. The smaller BET surface area using nitrogen than water is also attributed to the collapse of interlayer pores after drying.

Silver-Exchanged Mordenite for Capture of Water Vapor in Off-Gas Streams: A Study of Adsorption Kinetics

Reduced silver-exchanged mordenite (Ag0Z) has been recognized as the benchmark adsorbent for radioactive iodine removal from the nuclear fuel reprocessing off-gases. It has also been shown to have a considerable adsorption capacity for water vapor, which is a major component (tritiated water) in the off-gases of spent nuclear fuel reprocessing facilities. Therefore, understanding the kinetics of water vapor adsorption on Ag0Z is necessary for a better design of off-gas treatment systems. The adsorption kinetics were studied through adsorption experiments of water vapor on Ag0Z pellets and analyses of the kinetic data with adsorption models that describe processes of mass transfer and inter/intracrystalline diffusion. Uptake curves of water vapor on Ag0Z pellets were obtained with a continuous-flow adsorption system at temperatures of 25, 40, 60, 100, 150, and 200 °C and dew points from −53.6 °C to 12.1 °C. The diffusion-controlling factors were determined experimentally and analytically. It was found that the diffusion process of water vapor in Ag0Z pellets was controlled by macropore diffusion. Gas film mass-transfer resistance also contributed to the adsorption process of the 0.9 mm Ag0Z pellets, but it could be minimized with a high gas velocity and small pellet radius. Kinetic models including macropore diffusion (MD), linear driving force (LDF) and shrinking core (SC) were used to fit the uptake curves. The macropore diffusivity for water vapor adsorption on Ag0Z pellets was determined using the three models. It was found that the LDF and SC models could well describe the kinetic process, while the fitting with the MD model was not quite as good due to the existence of external mass-transfer resistance for the 0.9 mm Ag0Z pellets.

Kinetics of water vapor adsorption by vacuum-dried jujube powder

Water vapor adsorption kinetics of vacuum-dried jujube powder were investigated in temperature and relative humidity ranges of 10 to 40℃ and 32 to 75%, respectively. Water vapor was initially adsorbed rapidly and then reached equilibrium condition slowly. Reaction rate constant for water vapor adsorption of vacuum-dried jujube powder increased with an increase in temperature. The temperature dependency of water activity followed the Clausius-Clapeyron equation. The net isosteric heat of sorption increased with an increase in water activity. Good straight lines were obtained with plotting of 1/(m-m0) vs. 1/t. It was found that water vapor adsorption kinetics of vacuum-dried jujube powder was accurately described by a simple empirical model, and temperature dependency of the reaction rate constant followed the Arrhenius-type equation. The activation energy ranged from 50.90 to 56.00 kJ/mol depending on relative humidity. Arrhenius kinetic parameters (Ea and k0) for water vapor adsorption by vacuum-dried jujube powder showed an effect between the parameters with the isokinetic temperature of 302.51 K. The information on water vapor adsorption kinetics of vacuum-dried jujube powder can be used to establish the optimum condition for storage and processing of jujube.

CaCl2-containing composites as thermochemical heat storage materials

The purpose of this paper is to enhance the heat storage and mass transfer performances of mesoporous materials by adding a hygroscopic salt. Silica-gel, alumina, and bentonite were chosen as supports of 15wt% of calcium chloride. Comparison between the three series of composites was done by analyzing the impact of the salt addition on the physico-chemical and sorption properties of the three host matrixes. Moreover, the water sorption/desorption kinetics on the different composites under operating conditions, close to those of heat pump application, has been studied. The thermal behavior of the composite materials was examined by TG/DSC analysis.The addition of calcium chloride increased the quantity of sorbed water and released heat. The composite silica gel/CaCl2 showed the highest sorption capacity (0.27gH2Ogsample−1) accompanied with the highest heat storage (746Jgsample−1). The activation energy determined for the desorption process was following the silica-gel/CaCl2 < bentonite/CaCl2 < alumina/CaCl2 order, opposite to the pore size of the various composites, that was the larger for silica-gel based materials. Finally, the experimental results of rehydration were fitted with kinetics model in order to describe the kinetics of water vapor adsorption of the three mesoporous composites. Moreover, using the experimental data of rehydration a kinetic model was applied to describe water vapor diffusion in the three mesoporous composites.

Adsorption kinetics of water vapor on hypercrosslinked polymeric adsorbent and its comparison with carbonaceous adsorbents

In the removal of volatile organic compounds (VOCs) from gas, the unavoidable presence of water can degrade the performance of adsorbent because water competes well with VOCs. In this study, the adsorption kinetic behavior of water vapor on hypercrosslinked polymeric adsorbent (HPA) at three temperatures (308 K, 318 K and 328 K) was studied through thermogravimetric analysis (TGA), and compared with granular activated carbon (GAC) and activated carbon fiber (ACF). The results showed that the kinetic rate constants decreased with the increase of relative pressure, but increased with the increase of temperature for three adsorbents. Under the same condition, the kinetic rate constants were as following: HPA > ACF > GAC, which was just opposite with the concentration of surface functional groups. Activation energies were found to increase with the increase of relative pressure, indicating the development of barriers to diffusion associated with the formation of water clusters. A compensation effect was observed, where activation energy and ln(preexponential factor) parameter exhibited a linear correlation.

CLOSE AND OPEN CYCLE ADSORPTION KINETICS: DEVELOPMENT OF CORRELATION FOR DESICCANT AIR-CONDITIONING

Adsorption kinetics by desiccant-water pairs always play curial role in the development of low-cost thermally driven desiccant air-conditioning systems. It also influences various operating parameters of desiccant air-conditioning systems e.g. switching/cycle time, desiccant/system size, and regeneration temperature etc. Usually it is difficult to measure open-cycle adsorption kinetics precisely in adsorption heat pump systems. Therefore, present study provides an experimental comparison of close and open cycle based water vapor adsorption kinetics for the development of desiccant air-conditioning system. In this regard, a polymer based hydrophilic sorbent was used for close and open cycle adsorption measurements. Gravimetric method based magnetic suspension adsorption measurement unit was used for close-cycle adsorption kinetics measurements. However, the open-cycle adsorption kinetics was determined from the instantaneous measurement of air relative humidity and dry-bulb temperature in and out to the desiccant block while assuming the equilibrium between air and desiccant sides. Both experimental results were analyzed using linear driving force (LDF) model which presents the best regression of the experimental data. Furthermore, an empirical correlation of close to open cycle water vapor adsorption kinetics was established. Results revealed that the open-cycle kinetics of water vapor adsorption was 0.65 times slower (on average) than the close-cycle, which was more likely due to the complex mechanism of adsorption kinetics in open system. This study can be useful to determine the open-cycle adsorption kinetics from the close system data for designing the desiccant air-conditioning system.

Kinetics of Cluster-Mediated Filling of Water Molecules into Carbon Micropores

Understanding the kinetics of water adsorption/desorption on activated carbon is significant for chemical applications in which competitive adsorption of water occurs. In this study, we investigated the water adsorption process by determining the adsorption rate constant using the novel pressure feedback method (PFM), which measures the rate of adsorption directly with high precision (∼10 nmol s–1) by controlling the introducing and outgassing flow rates. The PFM was used to investigate water vapor adsorption kinetics on activated carbon fibers (ACFs) of different pore sizes and to obtain a correlation of rate constant with pore size. The systems show good agreement with the stretched exponential model. The adsorption rate constants were found to be lower for ACFs with a larger pore width (average pore width; w = 1.03 nm) as compared with those for smaller pore systems (w = 0.57, 0.72 nm).

Kinetics of Water Vapor Adsorption on Single-Layer Molecular Sieve 3A: Experiments and Modeling

The objective of the current work was to shorten the gap for fundamental adsorption kinetic data required for the development of advanced adsorption unit-operation models to be incorporated into an overall plant-level model for spent nuclear fuel reprocessing. The kinetics of water-vapor adsorption on molecular sieve 3A was investigated at 25–80 °C and water dew points from −69 to 17 °C. Water uptake curves were fitted with three kinetic models including the linear-driving-force model, the shrinking-core model, and the Langmuir kinetic model. The results suggest that the water-vapor adsorption on molecular sieve 3A under the investigated experimental conditions was controlled by both external film resistance and internal macropore resistance. The contribution of the external film resistance varied from 25% to 50% of the total mass-transfer resistance depending on the adsorption temperature. It was also found that the Langmuir kinetic model fitted individual sets of kinetic data very well, but the Langmuir adsorption constant obtained from curve fitting decreased with increasing adsorption temperature and with increasing water vapor pressure. This result indicates a significant surface heterogeneity of molecular sieve 3A and also implicitly verifies that the Langmuir isotherm model is unable to represent isotherms of water adsorption on molecular sieve 3A.

Kinetics of water vapor sorption on porous glass as a method of porometry

Equilibrium low temperature nitrogen adsorption and a new method based on the kinetics of water vapor sorption at room temperature are used for a comparative study of pore morphology of high-silica porous glass. The values of the total pore volume, the specific surfaces, the effective mesopore diameters, and the fraction of each mode in the total mesopore volume are established using both techniques. It is found that the rate of water vapor sorption at constant pressure grows in inverse ratio to the diameter of cylindrical menisci in pores. The time dependence of the volume of adsorbed water is linear, while the characteristic time of filling for each pore mode varies directly with the square of the effective pore mode diameter. The proposed new kinetic porometry technique based on analyzing water vapor adsorption kinetics at room temperature is quite simple as it requires no complicated vacuum equipment and allows the simultaneous examination of large numbers of porous material samples.

Kinetics of water molecule adsorption on the porous silicon surface

The kinetics of water vapor adsorption on the porous silicon surface is studied by the gas relaxometry method. The process multistepping which is reflected in sequential changes in H2O molecule diffusivities into the porous matrix, is found. Phenomenological models describing the experiment are considered.