Water Vapor Diffusion Coefficient Research Articles

Water Vapor Permeability through Porous Polymeric Membranes with Various Hydrophilicity as Synthetic and Natural Barriers.

The article is devoted to the analysis of sorption kinetics, permeability, and diffusion of water vapor in porous polymeric membranes of different hydrophilicities and through-porosities. The water transport measurement with a constant gradient of partial pressure allows the authors to obtain reliable characteristics for porous membranes, films, artificial leathers, and fabrics of various chemical natures (synthetic and bio-based) and phase structures. All the kinetic permeability curves were determined and effective diffusion coefficients, as well as their apparent activation energies, were calculated at the stationary and non-stationary stages of the mass transfer. The relationship between the sorption–diffusion characteristics of the polymer barriers and their vapor permeability is traced. Within the framework of a Zolotarev–Dubinin dual dispersive model, an analytical equation is obtained that relates permeability to diffusion coefficients of water vapor in the pore volume, polymer skeleton material using such characteristics as porosity and the solubility coefficient. It is proposed to use this equation to predict the sorption properties for barrier and porous materials of complex architecture specifically in food packaging.

Hygric properties of Cement-Lime Plasters with Incorporated Lightweight Mineral Admixture

Cement-lime plasters are among the most often applied materials for surface finishing of interior and exterior surfaces of buildings. In order to improve their properties, especially the thermal insulation performance, common silica filler is often replaced with lightweight filler. The main positive effect of the thermal insulating plaster is reflected primarily in winter, preventing freezing of water in the masonry porous structure. In this paper, novel cement-lime plasters with lightweight aggregate are studied. In plasters composition, silica sand was replaced with perlite. The replacement ratio was 25 %, 50 %, 75 %, and 100 % by volume. The effect of the sand substitution on plasters structural characteristics, such as bulk density, matrix density, and total porosity, was tested. As one of the plasters functional properties is to protect the masonry from the water penetration, i.e., from the moisture related damage, and at the same time the moisture from the structure must be efficiently drained, determination of their moisture transport parameters is of the particular importance. In this respect, the water vapour transmission rate in the developed materials was characterized by water vapour permeability, water vapour diffusion coefficient, and water vapour resistance factor. For the description of the liquid moisture transport in the studied plasters, a 1-D vertical water uptake experiment was performed. Based on this experiment, moisture profiles that characterize moisture distribution along the sample length at the specific time of water suction were determined. Using inverse analysis of moisture profiles, the moisture dependent moisture diffusivity was calculated. Since the admixed lightweight filler increased the moisture diffusivity of the developed plasters, their samples were in the second set of tests provided with a hydrophobic coating based on silicone oil. This improvement helped to reduce the moisture diffusivity of plasters to the value of the reference mixture. In summary, presented experimental analysis of hydrophobised lightweight plasters proved their increased porosity and reduced moisture diffusivity, which can improve plasters thermal performance and prolong the durability of both the plasters themselves and the masonry they cover.

Direct numerical simulation of sublimating ice particles

A thermodynamically consistent numerical framework for the simulation of three-dimensional sublimation processes of ice at temperatures below the triple point and low pressure is presented. To this end, a novel description of the local thermodynamic equilibrium of frozen water (ice) and a mixture of water vapour and an inert gas is derived, where anisotropic surface energy densities can be considered. A new correlation for the diffusion coefficient of water vapour in nitrogen is proposed based on equilibrium molecular dynamics simulations. The simulation results are in excellent agreement with experimentally obtained sublimation rates.

Effects of sample length on the transient measurement results of water vapor diffusion coefficient of porous building materials: A case study of autoclave aerated concrete (AAC) with various porosities

The effects of sample length on the transient measurement results of the water vapor diffusion coefficient of autoclave aerated concrete (AAC) were studied based on the transient technique that solves an inverse problem of one-dimensional mass transfer in a semi-infinite domain. Tests were performed on square-rod-shaped AAC samples having various porosities. The axial sample length was varied from 120 mm to 250 mm while maintaining a constant cross-sectional area of 50 mm × 50 mm. Humidity sensors having two sizes of 6 mm and 12 mm were also adopted, and the transient measurement results were compared with reference values obtained using standardized steady-state method. Results showed that smaller humidity sensors are always preferred and that the appropriate sample length varies with the type of materials. For the specific types of AAC samples studied, the appropriate sample length was found to lie between 150 mm and 200 mm. The use of a shorter (or longer) sample was shown to cause overestimation (or underestimation) of the transient results. It was suggested that the appropriate sample length should be kept short as long as the semi-infinite domain assumption is valid, which could guide the sample length selection for other porous building materials.

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

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

A New Approach to Determine Gas Diffusion Coefficients in Porous Solids by EIS: Application for NH3 and CO2 Adsorption on Zirconia and Zeolite Type 5A

A new theoretical approach has been established to define transport coefficients of charge and mass transport in porous materials directly from impedance data; thus four transport coefficients could be determined. In case of ammonia adsorption on sulfated zirconia, the diffusion coefficient was figured out to be approximately the mobility diffusion coefficient of ammonium ions: 1.2 x 10-7 cm2/s. The transport of carbon dioxide was examined for samples of zeolite type 5A in different hydration states. By impedance spectroscopy measurements, the diffusion coefficient of water vapor at 373 K is estimated to be about 7 x 10-6 cm2/s. The influence of carbon dioxide adsorption on diffusion coefficients is studied based on two pellet types of zeolite 5A. The difference between polar and non-polar gas adsorption in porous solids is considered as changed characteristic of impedance.

Long-term weathering rate of stained-glass windows using H and O isotopes

The understanding of glass alteration is a biogeochemical, industrial, societal (radioactive waste confinement), and cultural heritage issue. Studies have been mainly performed in aqueous conditions. However, glass reactivity under hydraulically unsaturated conditions may be more important than previously recognized. In this context, we evaluate here the role of the alteration layer formed on medieval stained-glass windows on the ongoing alteration in unsaturated conditions. H2O adsorption isotherms were measured to study the relation between the vapor sorption and the relative humidity inside the alteration layer. From it, the average pore radius was calculated, yielding a water vapor diffusion coefficient of 7.8 × 10-7 m² s-1 inside the pore network. Experiments using doped water vapor (D218O) confirm the vapor transport up to the alteration front via fractures and pore network. They also demonstrate that the alteration mainly progresses via an interdiffusion mechanism. The calculated interdiffusion coefficients at 20 °C are 3.6 × 10-20 m2 s-1 at 70% RH and 4.9 × 10-20 m2 s-1 at 90% RH, which is similar to the values measured on model stained-glass samples altered in short durations (1–4 years). Therefore, this study highlights that, given its morphology, the alteration layer is not protective against vapor transport and interdiffusion.

Effect of immobilization methods and the pore structure on CO2 separation performance in silica-supported ionic liquids

The CO2 separation performance in ionic liquids modified porous silica gels prepared by the impregnation and grafting methods were investigated. The effects of impregnation amount and pore size distribution of adsorbents with different types of ionic liquids on the CO2/N2 selectivity and water vapour adsorption and diffusion coefficient were analyzed in detail. The porous structures were characterized by CO2 adsorption isotherms at 0 °C. Adsorption behavior of CO2 (at 25, 40 and 60 °C) and N2 (at 40 °C) was measured using a gravimetric method. Compared with the impregnated samples, the grafting of ILs on the support surface only causes a weak loss of microporosity, leading to a slight decrease in CO2 adsorption capacity, and has much better recycling. CO2/N2 selectivity and water resistance are however enhanced, especially samples with the longer chain on imidazolium cation. A conventional homogeneous surface diffusion model for ILs grafted silica was used to estimate the diffusivities and good agreement between experimental values and fitting curves were obtained. For ILs modified silica prepared with impregnation method, the CO2 diffusivity constants include diffusion coefficients in ionic liquid phase and silica phase simultaneously. The diffusion coefficients are at a level of 10−7-10−9 m2 s−1 and about two or three orders of magnitude higher than that of neat ILs. The grafted silica exhibits diffusivities faster than those of the impregnated silica, indicating that more ionic liquids accessed into inner pore is unfavorable for CO2 diffusion.

Humidity regulation by stretched PP and PLA films with dispersed CaCl2

ABSTRACTPolymer materials that regulate the relative humidity in their environment are relevant for applications in the packaging and building sectors. By integration of salts in polymer structures, such materials are able to absorb and desorb high amounts of water vapor. In this study, films of polylactic acid and polypropylene with dispersed calcium chloride (2 and 4 wt %) were produced and biaxially stretched to induce the formation of cavities. The resulting cavities in these films account up to 10 vol % and are able to contain emerging calcium chloride solution formed by water vapor absorption. These films absorb reversibly up to 15 wt % water vapor at 75% relative humidity at 23 °C. This absorption behavior is described by effective diffusion and effective sorption coefficients. Using a simple model, the effective water vapor diffusion coefficient of these films can be estimated from the permeation coefficient of the polymer and the sorption coefficient of the absorber. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45713.

Analysis of Energy Saving during Food Freeze Drying

Most of researches focus on freeze drying curves of some certain food, that is, how to control temperature and pressure during freeze drying process. About why to make such freeze drying curves and why to take some kinds of pretreating and pre-freezing measures, there are less researches dealing with. In this paper, based on equations of heat transfer and mass transfer, factors affecting freeze drying rate are analyzed systematically, which provides theoretic basis for how to do pretreating and pre-freezing and how to control temperature and pressure during freeze drying so as to shorten drying time. And the results are verified by two freeze drying figures. The results show (1) in order to shorten drying time and save energy, food should be cut into proper size and can be pretreated with ultrasound or other technologies which make food structure porous. (2) From the point of raising freeze drying rate, slow pre-freeze is better than quick pre-freeze, especially at the late stage of freeze drying which is controlled by mass transfer. (3) at the earlier stage of drying (generally controlled by heat transfer), temperature of food’s sublimation interface should be relatively low and absolute pressure of drying box relatively high so that there is enough heat transfer driving force and heat transfer coefficient. (4) at the late stage (always controlled by mass transfer), temperature of food’s sublimation interface should be relatively high, and absolute pressure of drying box relatively low to ensure there are enough mass transfer driving force and water vapor diffusion coefficient.

Permeation of water vapour through polyethylene terephthalate (PET) films for back-sheets of photovoltaic modules

Different polyethylene terephthalate films were investigated as to the permeation and diffusity for water vapour and as to structure-property correlations. Water vapour transmission rate, permeability and diffusion coefficient were measured at temperatures 23, 38, 60 and 80 °C using a special test device with a mass spectrometer detection unit. While the permeability was significantly dependent on temperature and specimen thickness, the effect of TiO2 white pigmentation was negligible. Based on the empirical data a model for the temperature and film thickness dependency of the water vapour permeability was established and validated.

Experimental determination of the water vapor diffusion coefficient of autoclaved aerated concrete (AAC) via a transient method: Effects of the porosity and temperature

In this Short Communication, the water vapor diffusion coefficient of autoclaved aerated concrete (AAC) samples with various apparent densities was measured using a transient method that is based upon solving an inverse problem of one-dimensional water vapor diffusion. The transient measurements were performed over a relative humidity (RH) range from 1% to 90% at temperatures between 15°C and 45°C. It was revealed that the dependence of water vapor diffusion coefficient on the presence of pores cannot be simply correlated to either apparent porosity or open porosity. The pore structure, as quantified by pore size distribution, was found to be responsible for the relative comparison among the various AAC samples. It was also exhibited that the water vapor diffusion coefficient is enhanced at higher temperatures, whereas its RH-dependence is significantly suppressed at lower temperatures.

Water Diffusion from a Bacterial Cell in Low-Moisture Foods.

We used a Fick's unsteady state diffusion equation to estimate the time required for a single spherical shaped bacterium (assuming Enterococcus faecium as the target microorganism) in low-moisture foods to equilibrate with the environment. We generated water sorption isotherms of freeze-dried E. faecium. The water activity of bacterial cells at given water content increased considerably as temperature increased from 20 to 80 °C, as observed in the sorption isotherms of bacterial cells. When the water vapor diffusion coefficient was assumed as between 10(-12) and 10(-10) m(2) /s for bacterial cells, the predicted equilibration times (teq ) ranged from 8.24×10(-4) to 8.24×10(-2) s. Considering a cell membrane barrier with a lower water diffusion coefficient (10(-15) m(2) /s) around the bacterial cell with a water diffusion coefficient of 10(-12) m(2) /s, the teq predicted using COMSOL Multiphysics program was 3.8×10(-1) s. This result suggests that a single bacterium equilibrates rapidly (within seconds) with change in environmental humidity and temperature.

Moisture diffusion coefficients of mortars in absorption and desorption

Water vapor diffusion coefficients have been determined with a new set-up of the cup method for mortars with three binders (OPC, OPC+70% slag, OPC+10% silica fume) and two water/binder-ratios (0.4, 0.5). The measurements were made as a function of relative humidity with samples on both the absorption and desorption limbs of the sorption isotherm. For OPC samples the results show a clear difference between absorption and desorption. For the samples with supplementary cementitious materials the dependence on RH is small.

Mechanical, durability and hygrothermal properties of concrete produced using Portland cement-ceramic powder blends

Blended Portland cement-ceramic powder binder containing up to 60 % fine-ground waste ceramics from a brick factory is used in concrete mix design as an environmentally friendly alternative to the commonly used Portland cement. The experimental analysis of basic physical characteristics, mechanical and fracture-mechanical properties, durability properties and hygrothermal characteristics shows that the optimal amount of ceramic powder in the mix is 20 % of the mass of blended cement. The decisive parameters in that respect are compressive strength, liquid water transport parameters and resistance to de-icing salts, which are not satisfactory for higher ceramics dosage in the blends. In the case of other parameters studied, the limits for the effective use of ceramic powder are higher: 40 % for effective fracture toughness and specific fracture energy, 60 % for frost resistance and chemical resistance to MgCl2, NH4Cl, Na2SO4, HCl and CO2. The water vapour diffusion coefficient is found to increase with increasing ceramics content, which for wet envelopes can be considered as a positive feature, but may have a negative effect for dry envelopes. The thermal conductivity of all mixes increases fast with growing moisture content; differences of up to 50 % between the dry and water-saturated state values are observed. This has to be taken into account in energy-related calculations.

Development and Performance Evaluation of Thermal Insulation Materials Composed of Textile Waste in Terms of Moisture Properties Resulting from Absorption and Diffusion of Humidity

This study focus on the insulating thermal properties of an insulating composed of textile waste in order to exploit the many advantages by the implementation of such material: matter gain, environmental and ecological advantages, creation of job sites and finding a new field of exploitation of recycled textile waste. In this work, we have used two structures of textile waste samples comminuted: Textile linters of waste, “WL”, and a tablecloth of textile waste, “WT” to insulate the building by blowing or by recovery. This paper describes the results of research of hydrous behavior of such support and its impact on thermal properties, in particular, the diffusion coefficient of water vapor, the kinetic absorption, the moisture uptake and its effect on the thermal conductivity. Compared with other insulators, they present competitive thermal properties: Their conductivities varied between 0.033-0.059 w/mK for tablecloth and 0.039-0.063 W/mK for the linters respectively, when the relative humidity varies from 50% to 90%. Spite of their loss of insulation ability with increasing relative humidity, they could be considered, always, as civil construction insulation materials. They react rapidly to absorb a considerable rate of humidity (15.5% for the Tablecloth and 23.6% for linters of their dried weight), so moderate the climate by capturing the excess of moisture. Their diffusion coefficients of water vapor

Hygric Properties of HPC with Natural Pozzolana

The water vapour transport and the liquid water transport of high performance concrete (HPC) with the content of natural pozzolana are the described in this paper. Studied properties are presented by means of water vapour diffusion coefficient, water vapour diffusion resistance factor and absorption coefficient respectively. The natural pozzolana (NP), namely natural zeolite, is used as supplementary cementitious material which affects the durability properties of the end product. The obtained results revealed the effectiveness of NP in the terms of both studied properties when used in small amounts, i.e. at about 20% of cement weight. On the other hand substitutions higher than 40% result in worsening of the resistance of the concrete to water and water vapour ingress and its movement trough the material.

Water distribution in high temperature polymer electrolyte fuel cells

In this work a high temperature polymer electrolyte fuel cell based on a phosphoric acid doped polybenzimidazole membrane is operated at 160 °C on dry air and dry hydrogen. The anodic stoichiometry was varied to resemble typical operation with pure hydrogen or reformate gas. At the outlet of the fuel cell liquid water was collected by condensers. The resulting amounts of liquid water from cathode and anode were computationally analyzed. The results yielded an effective diffusion coefficient of water vapor of D≈2.7⋅10−7 m2 s−1, which is the average value for the cell based on the water partial pressures at the outlets. From the calculated water partial pressures at the anode and cathode it can be concluded that the concentration of phosphoric acid differs significantly within catalyst layer of the anode and cathode. Finally, the crossover of hydrogen and oxygen was shown to depend on the swelling of the membrane.

CFD simulation of heat and mass transport for water transfer through hydrophilic membrane in direct-contact membrane distillation process

In this study, direct-contact membrane distillation (DCMD) process is simulated using computational fluid dynamics. The DCMD process is typically used to provide pure water from salt water. The system consists of a flat-sheet membrane, a feed channel with salt water as the food, and a permeate channel. The cross-current flow pattern is used in this process. The governing equations to simulate the process include the heat, mass, and momentum balance equations. The developed equations are solved numerically using the finite element scheme. To improve accuracy of the simulation results and precision of water vapor diffusion coefficient estimation in membrane pores, a composition of Poiseuille and Knudsen diffusion coefficient is applied. The simulation results are compared with experimental data obtained from the literature and a satisfactory agreement is obtained (average error less than 5%).

A thermophysical study on the freeze drying of wooden archaeological artifacts

Abstract The protection of unearthed waterlogged archaeological artifacts is a heavy task when large-scale construction is carried out in today's China. Freeze drying offers an efficient way for preserving wooden artifacts. In order to rationally design the freeze drying process, thermophysical studies were carried out in this paper. The measurements of water vapor diffusion coefficient and thermal conductivity were made. The impacts of cooling velocity and grain orientation on effective diffusion coefficient of water vapor, D, in dried wood were investigated. At low cooling velocity, 0.2 K/min, D was (4.8 ± 0.4) × 10 −4 m 2 /s in parallel grain direction, or (1.9 ± 0.4) × 10 −4 m 2 /s in perpendicular grain direction; at higher cooling velocity, 3.0 K/min, D was (4.3 ± 0.2) × 10 −4 m 2 /s in parallel grain direction, or (1.2 ± 0.1) × 10 −4 m 2 /s in perpendicular grain direction. The thermal conductivity of the dried sample was in the range of 0.78–1.06 W/(m·K). With the values attained, one-dimensional pseudo-steady analysis were made for the primary drying process of a cylinder and a flat slab. It was found that the process was mass-transfer controlled and the difference between surface temperature and sublimation temperature was very limited. Thus only mass transfer equation needed to be solved with assumption of constant sublimation temperature. The primary drying of a two-dimensional object was also simulated to predict the time required. Monitoring the surface temperature of each artifact and handling artifacts with similar drying time in a batch are proposed for the freeze drying of archaeological artifacts. The freeze-dried wooden artifacts by the authors yielded very small deformations.