Water Vapor Adsorption Kinetics Research Articles

Mechanical and water barrier properties of agar/κ-carrageenan/konjac glucomannan ternary blend biohydrogel films

Multicomponent hydrogel films composed of agar, κ-carrageenan, konjac glucomannan powder, and nanoclay (Cloisite® 30B) were prepared and their mechanical and water barrier properties such as water vapor permeability (WVP), water contact angle (CA), water solubility (WS), water uptake ratio (WUR), water vapor uptake ratio (WVUR) were determined. Mechanical, water vapor barrier, and water resistance properties of the ternary blend film exhibited middle range of individual component films, however, they increased significantly after formation of nanocomposite with the clay. Especially, the water holding capacity of the ternary blend biopolymer films increased tremendously, from 800% to 1681% of WUR for agar and κ-carrageenan films up to 5118% and 5488% of WUR for the ternary blend and ternary blend nanocomposite films, respectively. Water vapor adsorption behavior of films was also tested by water vapor adsorption kinetics and water vapor adsorption isotherms test. Preliminary test result for fresh spinach packaging revealed that the ternary blend biohydrogel films had a high potential for the use as an antifogging film for packaging highly respiring agricultural produce. In addition, the ternary blend nanocomposite film showed an antimicrobial activity against Gram-positive bacteria, Listeria monocytogenes.

Water vapor adsorption kinetics on small and full scale zeolite coated adsorbers; A comparison

A possibility to enhance both heat and mass transfer characteristics of an adsorber heat exchanger is to apply the adsorbent directly to its surface in form of a consolidated layer. As the majority of the available publications on the effect of both coating technology and adsorbent layer thickness on the adsorption kinetics deals with small scale adsorbent samples, the results obtained can only represent the best case design of an adsorber heat exchanger. This article presents, therefore, a comparison between the adsorption kinetics of water vapour on small as well as two different full scale coated adsorber heat exchanger types with AQSOA-Z02 layers of Mitsubishi Plastics Incorporation under quasi isobaric conditions of adsorption heat pumps. The small scale coated samples have a zeolite dry mass of 200 mg and layer thicknesses of 200, 300 and 500 μm while the full scale adsorbers have a coated zeolite mass between 1.5 and 2.5 kg and layer thicknesses of 150, 200, 300, 400 and 500 μm. In the investigated adsorption heat pump module, up to 52.7 and 57.3% of the equilibrium differential water loading measured with the small scale coated substrates have been obtained after an adsorption-evaporation times of 300 and 600 s, respectively.

Effect of freezing temperature on rehydration and water vapor adsorption characteristics of freeze-dried rice porridge

Effect of freezing temperature of rice porridge on quality properties of freeze-dried rice porridge was investigated at various temperatures. Slow freezing produced porous freeze-dried rice porridges with large pores and more brittle structure than rapid freezing. All the quality properties tested were greatly influenced by both freezing temperature and temperature for the property evaluation. Both the initial rehydration ratio and the dissolution time of the freeze-dried rice porridge decreased with increase in water temperature. Results on the effect of freezing temperature on rehydration ratio and dissolution time indicated slow freezing is more preferable for the freeze-dried rice porridge. The Peleg model was fitted better than the Singh and Kulshrestha model to explain the water vapor adsorption kinetics. The activation energy for the water vapor adsorption of the freeze-dried rice porridge determined by using the Peleg model ranged 16.2–23.1 kJ/mol depending on the freezing temperature.

Preparation of Hemp-Derived Activated Carbon Monoliths. Adsorption of Water Vapor

Activated carbon monoliths have been obtained by the chemical activation of hemp canes with phosphoric acid. The effects of the carbonization temperature and the impregnation ratio on the porous structure and surface chemistry were studied. The porous structure of the activated carbon monoliths was characterized by N2 adsorption−desorption and CO2 adsorption isotherms. The surface chemistry of the carbons was analyzed by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS), and the surface morphology was analyzed by scanning electron microscopy (SEM). Water vapor adsorption kinetics and capacities of the activated carbon monoliths were studied. The surface properties of the activated carbons were found to be strongly dependent on the carbonization temperature and the impregnation ratio. An activated carbon monolith with an apparent surface area of about 1500 m2/g and a significant contribution of mesopores (mesopore volume of 0.6 cm3/g) was obtained at 400 °C with an impregna...

Kinetics of Initial Water Vapor Adsorption by Inonotus obliquus Mushroom Powders

Water vapor adsorption kinetics of Inonotus mushroom powders were investigated in temperature and water activity ranges of 20 to 40℃ and 0.30 to 0.81, respectively. Initial water vapor adsorption rate of mushroom powders increased with increases in temperature and water activity. The temperature dependency of water activity followed the Clausius-Clapeyron equation. The net isosteric heat of sorption increased with an increase in water activity. Water vapor adsorption kinetics of the mushroom powders can be well described by a simple empirical model. Temperature dependency of the reaction rate constant followed the Arrhenius relationship. The activation energy ranged from 56.86 to 91.35 kJ/㏖ depending on water activity. Kinetic compensation relationship was observed between k? and E<SUB>a</SUB> with the isokinetic temperature of 790.27 K.

Role of Surface Functional Groups in the Adsorption Kinetics of Water Vapor on Microporous Activated Carbons

Activated carbons have both hydrophilic surface oxygen functional groups, which act as primary adsorption centers for water vapor, and hydrophobic graphene layers on which nonpolar species are primarily adsorbed. The aim of this research was to investigate the effects of oxygen surface functional groups, in activated carbons, on the adsorption characteristics of water vapor. Activated carbon G was oxidized using nitric acid and then heat treated in the range 387-894 K to produce a suite of adsorbents with varying oxygen contents in the range 0.4-21.5 wt % daf, but very similar porous structure characteristics, thereby minimizing effects due to changes in porous structure. The type and concentration of surface oxygen groups present in each sample were assessed using TPD, FTIR, Boehm titration, and analytical methods. Water vapor adsorption at low relative pressure was dramatically enhanced by the presence of functional groups, in particular, carboxylic groups. Kinetic profiles for each pressure increment were modeled using a set of nested kinetic models, which allow the adsorption kinetics to be analyzed in relation to the adsorption mechanism. Relationships between water adsorption kinetics at low surface coverage and the type and concentration of oxygen surface functional groups were observed. A two component double exponential kinetic model was used when carboxylic groups were present in significant amounts with a slow kinetic component associated with adsorption on these groups. In the case of carbons where carboxylic groups were only present in, at most, relatively small amounts, a stretched exponential kinetic model was used and the rate constants in the low-pressure region decreased linearly with increasing Henry's Law constant and oxygen content. The results indicate the importance of adsorbate-adsorbent interactions in water adsorption kinetics and are consistent with a site-to-site hopping mechanism between functional groups.

On the Effect of Grain Size on the Kinetics of Water Vapor Adsorption and Desorption into/from Loose Pellets of FAM-Z02 under a Typical Operating Condition of Adsorption Heat Pumps

The kinetics of water vapor adsorption/desorption under quasi isobaric conditions have been studied on one layer of loose pellets of FAM-Z02 placed on a metal plate. Temperature of the sample holding plate has been changed as it takes place in real adsorption heat pumps (AHPs), while the vapor pressure over the adsorbent was maintained almost constant (saturation pressures corresponding to an evaporator temperature of 5°C and a condenser temperature of 35°C). Adsorption- and desorption-end temperatures have been adjusted to 35 and 90°C, respectively. Measurements have been carried out on grains having the following size distributions: 0.7 to 1.0 mm, 1.4 to 1.6 mm and 2.0 to 2.6 mm. An average equilibrium differential water loading of 19.2 g/100 g has been measured during desorption compared to 16.4 g/100 g during adsorption. The adsorption/desorption rates have been characterized by the times required to reach 50, 80 and 90% of the equilibrium loading. It has been found that the adsorption kinetics are mainly dominated by inter-crystalline diffusion, while the desorption kinetics are mostly influenced by surface resistance to the heat and mass transfer. Average specific evaporator cooling/condenser heating capacities in the order of 1 kW/kg have been estimated for the grain size 0.7 to 1.0 mm, putting in evidence the possibility of developing highly compacted AHPs with FAM-Z02 for heating and cooling applications. The influencing parameters on the combined heat and mass transfer process have been discussed.

Peculiarities in the Water Adsorption and Morphology of Carbon Chars Assessed by NMR

Proton spin–lattice relaxation and 1H NMR spectroscopic data are used for studies of the water adsorption and morphology of several newly synthesized carbon-based chars. Two NMR lines have been observed in accordance with a difference between macropore and micropore water in a system with heterogeneous pore structure. The macropores occupy about the half of the char particle volume. In aqueous suspension fast molecular exchange between water in macropores and bulk water leads to an intense NMR line with a single spin–lattice relaxation time T1, which exhibits a strong particle size effect. This effect has been used for the estimation of T1 of water molecules in the vicinity of paramagnetic centers at the surface of chars, as well as for macropore size estimation. The kinetics of water vapor adsorption in chars as seen by 1H NMR indicates that this process begins in micropores, which serve as primary adsorption centers. Water molecules in micropores exhibit specific properties which are characteristic of liquids in a small, restricted space, and they give rise to the observed accumulation as well as saturation effects.

Surface Tension and Dynamic Contact Angle of Water in Thin Quartz Capillaries

The surface tension of water has been measured in quartz capillaries with radii from 200 down to 40 nm. It appears that the surface tension does not differ from the known (bulk) values in the temperature range from 8 to 70°C, within 1% experimental error. The dynamic contact angle, θd, vanishes when the capillary surface is covered with a wetting film left behind the receding meniscus. In the case of a dry surface, θd depends on the velocity of the meniscus motion. The results obtained do not agree with presently available theoretical predictions from hydrodynamic theories of dynamic contact angles. Rather the kinetics of water vapor adsorption ahead of the moving meniscus seems to be the major controlling agent of the dynamic contact angle.

Diffusion Barriers in the Kinetics of Water Vapor Adsorption/Desorption on Activated Carbons

The adsorption of water vapor on a highly microporous coconut-shell-derived carbon and a mesoporous wood-derived carbon was studied. These carbons were chosen as they had markedly different porous structures. The adsorption and desorption characteristics of water vapor on the activated carbons were investigated over the relative pressure range p/p° = 0−0.9 for temperatures in the range 285−313 K in a static water vapor system. The adsorption isotherms were analyzed using the Dubinin−Serpinski equation, and this provided an assessment of the polarity of the carbons. The kinetics of water vapor adsorption and desorption were studied with different amounts of preadsorbed water for set changes in pressure relative to the saturated vapor pressure (p/p°). The adsorption kinetics for each relative pressure step were compared and used to calculate the activation energies for the vapor pressure increments. The kinetic results are discussed in relation to their relative position on the equilibrium isotherm and the ...

Kinetics of Water Vapor Adsorption on Activated Carbon

The adsorption of water vapor on a highly microporous carbon derived from the carbonization of coconut shell has been studied. This material was characterized by the adsorption of nitrogen at 77 K and carbon dioxide at 273 K. The micropore size distribution was determined using probe molecule vapors at p/p0 = 0.5 and 301 K. The adsorption and desorption characteristics of water vapor on the activated carbon were investigated over the pressure range 0−2.41 kPa (p/p0 0−0.9) in a static water vapor system. The adsorption and desorption kinetics were studied with different amounts of preadsorbed water for changes in vapor pressure of 0.303 kPa. The adsorption rate constants were also studied for three relative humidities for a dynamic flow system at a constant temperature. In these experiments the changes in vapor pressure were much higher than in the static vapor pressure experiments. The kinetic results for both the static atmosphere and dynamic flow systems are discussed in relation to their relative position on the equilibrium isotherm and the adsorption/desorption mechanism.

Effect of surface treatment on the adsorption kinetics of water vapor in a vacuum chamber

Adsorption kinetics of water vapor in a vacuum chamber can be characterised by the sticking probability, S, the mean sojourn time, τ, and the amount adsorbed, σ, of water molecules on the surface in the chamber. Among them, s can be obtained through the analysis of pressure change in the chamber caused by the pressure pulse with a very short time duration. The pulse was generated by pulsed-laser-beam irradiation of water vapor condensed on the cold surface. On the surface of a stainless steel chamber polished electrolytically, s was about 0.01 during the pumping at room temperature, and increased to about 0.1 after the degassing at 250 °C. When the chamber was coated with TiN s increased by about 0.04 to 0.2 during the pumping at room temperature. Temperature dependence of s was measured on both surfaces degassed thoroughly. Change of s on those surfaces caused by the exposure with humid nitrogen or air was also observed.

Kinetics of water vapor adsorption by ion exchange polyacrylonitrile fibres

Kinetics of water vapor adsorption by ion exchange PAN fibres has been studied. Adsorption can be described by an exponential kinetic equation. Experimental results permit the assumption that the surface of the materials studied behaves as uniformly nonhomogeneous with more strongly expressed entropy nonhomogeneity, connected with cross-linking of the modified fibres. The dependence between the kinetic characteristics and the solubility of the materials in DMF shows that the main factor affecting unfavorably the adsorption kinetics is cross-linking. Increase of N-containing anion exchange groups reduces the effect of adsorption delay due to cross-linking.

Adsorption of water and the micropore structures of carbon adsorbents Communication 6. Effect of the micropore structure parameters of activated carbons on the kinetics of water vapor adsorption

1. A study has been made of the water vapor adsorption by microporous carbon adsorbents, working at constant temperature and rate of air-vapor flow, with 75 and 90% humidity. 2. A method for constructing the kinetic curve for water vapor adsorption from the micropore structure parameters of the activated carbon is proposed.

The determination of the diffusion coefficient of water vapor in granulated zeolites by adsorption from a carrier-gas current

1. We investigated the adsorption kinetics of water vapor on NaA and NaX granulated zeolites at a concentration of 1.75 mg/liter, 20°C and different carrier-gas velocities. 2. At a current velocity of 1.4 m/sec the approximate value of the diffusion coefficient may be calculated from the equation for internal diffusion, i.e., on the assumption that external mass exchange is infinitely rapid. 3. A measurement procedure with more efficient consumption of carrier-gas is recommended.