Moisture Diffusion Characteristics Research Articles

Evolution and Modelling of the Moisture Diffusion in Walnuts during the Combination of Hot Air and Microwave–Vacuum Drying

To understand the moisture transfer mechanism of walnuts during the combination of hot air (HA) and microwave–vacuum (MV) drying (HA-MVD) process, the drying characteristics and moisture diffusion characteristics of walnut during HA-MVD were investigated. The results indicated that the HA-MVD of walnuts occurred mainly in the falling-rate stage. The value of effective moisture diffusivity (Deff) dropped continuously with the decrease in moisture content (MC) during the HA drying, while switching to MV drying could truncate the decrease in Deff and still maintain a high value until the end of drying. The HA temperature, MC of the transition point, microwave power, and MV thermostatic temperature have significant effects on the moisture diffusion characteristics of walnuts. The values of Deff for walnuts ranged from 2.33 × 10−9 m2/s to 6.89 × 10−8 m2/s. The third-order polynomial prediction model of Deff related to the sample MC and drying conditions was established to describe the dynamic change in the Deff of walnuts during the HA-MVD process. The application of MVD in the final stage of drying could rapidly increase the internal vapor pressure of the walnuts, accelerate the diffusion speed of the internal moisture, and re-enhance the drying rate. The findings have practical value for the development of efficient and energy-saving drying methods in the walnut industry.

Reliability criterion of film capacitor based on moisture diffusion in encapsulation

AbstractThe current reliability criterion (RC) of the film capacitor (FC) is usually a fixed value, which does not consider the staged capacitance loss caused by moisture diffusion under high temperature and/or high humidity. The expected lifetime of capacitors would deviate far from the actual one. The moisture diffusion process of the encapsulated FC is analysed based on Fick's second law. In order to investigate the capacitance loss and moisture diffusion characteristics, 4 types of film capacitors are stressed with AC voltage under different temperatures and humidities. Based on the staged analysis of the capacitance and weight gain curves, the effect of moisture diffusion on capacitance loss is revealed. Results indicate that the linear stage of capacitance loss corresponds to the Fickian moisture diffusion stage of weight gain. The effect of voltage on FC aging is the most significant. The capacitance loss rate increases by 301% when the voltage rises by 30%. An RC calculation method of the FC is further proposed, which could also screen the encapsulation quality and shorten the aging test duration by at least 80%.

Evaluation of moisture diffusion characteristics and the effect of moisture treatment on flexural properties of expanded perlite-based building material

The expanded perlite-based building material for drywall application consisting of sodium silicate solution as a binder was manufactured by varying the degree of compaction and sodium silicate content to investigate moisture diffusion behavior and the effect of moisture treatment on flexural properties of the composites. Moisture treatment was conducted on specimens in a climatic chamber at a temperature of 37°C and a relative humidity of 90% until saturation. Results show that moisture absorption decreased with increasing compaction ratio for a constant sodium silicate content in binder and increased with increasing sodium silicate content in binder for a constant compaction ratio. A range of volume fractions of solid sodium silicate in the foam is identified, in which the fully Fickian diffusion gradually transformed to non-Fickian diffusion as sodium silicate content in foam increased. The concentration-dependent diffusion method was found to be suitable to explain this behavior. The moisture diffusion below this transition range showed an entirely Fickian diffusion and changed to concentration-dependent diffusion above the range. As a result of moisture treatment, the flexural strength of medium density foams was decreased but the lowest- and highest-density foams were not affected while the flexural modulus was increased only for the highest density foam and no significant effects were seen in other cases. The bending failure mechanism of the composite was not affected by the moisture treatment.

Analysis of the diffusion characteristics and activation energy of rice drying using low‐field nuclear magnetic resonance

AbstractThis research was conducted to determine the effective moisture diffusion coefficient and activation energy of rice using low‐field nuclear magnetic resonance (LF‐NMR). The results showed that the evaporable water was rapidly removed under high‐temperature drying in the early stages of drying. The diffusion rate of evaporable water at different temperatures in the later stages of drying was not significant. Besides, the activation energy in rice was divided into the activation energy of the conversion of the chemically bound water (15.04–25.69 kJ/mol) to evaporable water and the activation energy of the diffusion of evaporable water from rice (6.42–16.42 kJ/mol), indicating that more energy was needed to dry the chemically bound water from rice. A new method for obtaining the effective moisture diffusion coefficient and the activation energy was proposed based on the relaxation time distribution profiles, revealing the internal relationship between various phase moisture and activation energy. These results will provide a theoretical guidance for the drying process.Practical applicationsThe moisture diffusion characteristics and activation energy of rice were determined using low‐field NMR signal quantities, which were important parameters for optimizing the drying process. In addition, the variable temperature drying method was conducive to the rapid diffusion of moisture in the rice. The chemically bound water within rice required more energy to diffuse than other forms of water components. An internal relationship between moisture content and the activation energy was established to provide a theoretical reference for high‐quality drying techniques.

Study on the performance of a unit dew-point evaporative cooler with fibrous membrane and its application in typical regions

In this paper, a new type of unit dew-point evaporative cooler with fibrous membrane was designed and constructed. The effects of three air state parameters (temperature, humidity, air velocity) on the outlet air temperature, wet-bulb efficiency and dew-point efficiency were studied, as well as the regional adaptability in four typical cities in China. The porous fibrous membrane can be used to improve the moisture permeability and diffusion characteristics of the wet channel. It was found that the wicking height rate of the porous fibrous membrane was 0.203 cm/min, the diffusivity rate was 0.118 cm2/s, which was better than the performance of Kraft Paper. The usage of this novel device in Urumqi and Lanzhou, as regions with lower relative humidity, showed better regional adaptability than in Beijing and Shanghai with higher relative humidity. When the inlet air velocity was 1 m/s, the temperature decreased from 33.4 °C to 24.2 °C and 31.3 °C–24.5 °C respectively in the former area, while, decreased from 33.6 °C to 28.1 °C and 34.6 °C–29.5 °C respectively in the latter area. With the inlet air velocity increasing from 1 m/s to 3 m/s, the outlet air temperature would rise, and the wet-bulb efficiency and dew-point efficiency would decrease, but the cooling capacity would increase.

Application of the Langmuir-type diffusion model to study moisture diffusion into asphalt films

The objective of this study was to investigate the behavioral characteristics of moisture diffusion into asphalt films and to analyze the correlations between diffusion parameters and asphalt components. A Langmuir-type diffusion model was successfully established and applied.Experimental moisture uptake data of four asphalt films immersed in distilled water for 258 h were measured at intervals by the gravimetric method, and the diffusion parameters involved in the Langmuir-type model were exactly fitted by the least square method. Then, a component analysis of the asphalt was conducted by following the solvent precipitation method. Thus, correlations between diffusion parameters and asphalt components were further analyzed.The results show that the behavioral characteristics of moisture diffusion into asphalt films can be well explained by the theory of the Langmuir-type model and that the water molecules absorbed in asphalt films are in mobile and bound phases. The process of moisture diffusion in asphalt films can be divided into two stages: in the first stage, both adsorption of water molecules on the surface of the asphalt (transformation from the mobile phase to the bound phase) and transport of water molecules in the mobile phase through diffusion channels within the asphalt occur simultaneously; in the second stage, adsorption proceeds on the surface of the asphalt, and moisture uptake in the mobile phase becomes constant. The ratio of the adsorption coefficient to the desorption coefficient exhibited an excellent positive correlation with the moisture uptake in the bound phase, as well as with the total equilibrium moisture uptake (R2＞0.99). Among the four components in asphalt, the content of asphaltene was significantly and positively correlated with the adsorption coefficient, while polar aromatics were inversely correlated.

Study on the hygrothermal aging behavior and diffusion mechanism of Ti/CF/PMR polyimide composite laminates

A series of hygrothermal aging experiments were performed on Ti/CF/PMR polyimide (TCPP) composite laminates in this work in order to verify the moisture diffusion characteristics and diffusion mechanism of the laminate. The specimens were subjected to various hygrothermal aging parameters, specifically 70 °C/95RH%, 85 °C/75RH%, 85 °C/85RH% and 90 °C/95RH%. The results illustrate that the temperature affected the diffusion rate of water molecules in the composite laminates, and the relative humidity determined the saturated moisture absorption rate. For the orthogonally laminated TCPP composite laminates, two equations have been obtained to predict the saturated moisture absorption and diffusion rate of water molecules of the prepreg layer. Additionally, the moisture absorption as a function of time has been expressed for the prepreg layer.

Moisture diffusion properties of graded hierarchical structure of bamboo: Longitudinal and radial variations

Determination of moisture diffusion properties of bamboo is necessary to evaluate its growth and development and optimize the manufacturing process of associated products as well. The moisture sorption and diffusion characteristics of bamboo in different locations were tested. Graded hierarchical structure in both longitudinal and radial directions influenced the moisture transportation efficiency of bamboo significantly. Quicker moisture diffusion was found at higher heights or outer radial positions. The high moisture diffusion coefficient (D) could be explained by the increment of the vessel fraction, or the large size of vessel lumen. D logarithmically increased with the increment of moisture content (MC) irrespective of height or radial position. By simplifying the hierarchical structured system of bamboo, moisture-activated diffusion processes in cell wall (DBW) and lumen (DV) were simulated. Calculated values of D, DBW and DV could be used to predict the changes of moisture transport in bamboo, regardless of its graded structure (i.e., location) or MC. Overall, the results of this study elucidated the longitudinal and radial variations of moisture diffusion, exploring possible adaptation strategies during the growth process of bamboo. These findings may also be relevant for the optimization of the manufacturing technology of bamboo-based products.

Moisture diffusion and shrinkage characteristics of broad bean during low-temperature vacuum drying

ABSTRACT Low-temperature vacuum dehydration could avoid heat injury because of high drying temperature, as well as prevent mechanical damage due to ice crystal during freezing. However, it is not clear for the effect of different temperatures and vacuum degrees on the dehydration process. The object of this work was to investigate the drying behavior of broad bean seed and to analyze the effect of moisture diffusion on the structure during low-temperature vacuum drying process. For this purpose, a test bench for low-temperature vacuum dehydration was designed and established. The nine dehydration operating conditions at temperatures of 0°C, 4°C, and 8°C and vacuum degrees of 95, 97, and 99 kPa were conducted. The key parameters including dehydration time, dehydration rate, shrinkage, effective moisture diffusivity were analyzed. Results showed that by increasing temperature and vacuum degree, the dehydration time obviously shortened, and the dehydration rate was significantly increased. The vacuum degree has an obvious influence on product shrinkage, especially in the thickness of broad bean seed. The average effective moisture diffusivity varied from 0.65 to 2.97 × 10−10 m2 s−1 under the tested drying conditions. And the high temperature and low vacuum degree were a better way for reducing the shrinkage of broad bean seed. To avoid structure collapse and cell rupture, the end moisture content of broad bean seed should be higher to 0.05 ~ 0.11.

Mutual transformation of the water binding state and moisture diffusion characteristics of Chinese yams during microwave freeze drying

The moisture distribution changes in Chinese yam slices subjected to microwave freeze drying (MFD) were analyzed using low-field nuclear magnetic resonance (LF-NMR). During this process, the mutual transformation of the water binding state and moisture diffusion characteristics of Chinese yam were analyzed together with changes in the effective water diffusion coefficient, water content, drying rate and pore network structure. Notably, moisture migrated from a high to a low degree of freedom during the MFD process. The effective water diffusion coefficient and water migration velocity increased with increasing microwave power density, and the extent of the conversion of unbound to bound water also gradually increased. During the early stage of drying, moisture diffusion was mainly dominated by liquid phase diffusion. At later stages of drying, moisture diffusion shifted gradually from liquid phase diffusion to gas phase diffusion and was eventually dominated by the latter.

Mathematical modelling of infrared-dried kiwifruit slices under natural and forced convection.

In this work, the effect of the radiation intensity, slice thickness, and the distance between slices and infrared lamps under natural drying air and the effect of slice thickness and air velocity under forced drying air on the moisture diffusion characteristics and the drying rate of kiwifruit slices during infrared drying were investigated. The drying of kiwifruit happened in the falling rate period, and no constant‐rate period was observed in the drying curves. One hundred models were fitted to the drying data. Among the models, the exponential dsecay function model and modified two‐term exponential‐V model and the artificial neural networks with 4‐5‐7‐1 and 3‐5‐5‐1 topologies, hyperbolic tangent sigmoid transfer function, and Levenberg‐Marquardt training algorithm presented the best results and showed the goodness of fit with the experimental data for the former and latter systems, respectively. The diffusivities varied between 1.216 × 10−10–8.997 × 10−10 m2⁄s and 2.567 × 10−10–10.335 × 10−10 m2⁄s for natural and forced drying air systems, respectively.

Moisture diffusion and damage characteristics of carbon fabric reinforced polyamide 6 laminates under hydrothermal aging

This paper presented an investigation into the hydrothermal aging behavior and mechanical durability of carbon fabric reinforced polyamide 6 (CF/PA6) laminates. Moisture absorption, tensile and flexural tests of CF/PA6 laminates were conducted by immersing in distilled water under different temperatures. Based on our experimental observation, a novel extended Fickian model was proposed to capture the post-saturation diffusion behavior of CF/PA6. Diffusion coefficients of CF/PA6 composites were obtained for 1D and 3D Fickian model and the extended one. Furthermore, dynamic mechanical analysis demonstrated activated chain motion induced by absorbed water at higher aging temperatures. Micro-cavities on the surface and micro-cracks in the cross-section of moisture uptake samples aged at 60 °C-water revealed that hot/wet environment accelerated the damages of CF/PA6 laminates. Mechanical results showed that tensile properties were well-maintained, however, flexural properties were attenuated under hydrothermal aging, and the failure mode of unaged and aged flexural specimens were discussed.

Moisture Diffusion in Silica/Clay/Natural Rubber Hybrid Composites

This study investigated moisture diffusion in natural rubber (NR) hybrid composites filled with silica and bentonite clay. Natural bentonite (BNT) was treated with tetradecyldimethylammonium chloride and coco diethanolamide to produce modified bentonite (M-BNT). Varied proportions of silica, M-BNT, and BNT fillers were added to raw NR according to a third-degree simplex lattice mixture design of experiment. The addition of fillers affects the vulcanization characteristics, mechanical properties, and hardness of NR hybrid composites. Moisture diffusion behavior was studied by monitoring the water uptake of NR composites during immersion in deionized water at 80°C. Data from sorption experiments were fitted on the classical Fickian and Langmuir-type diffusion models. The Fickian model overestimates and underestimates the water uptake of NR composites in the early and later stages of moisture diffusion, respectively. On the other hand, the Langmuir-type model adequately captures the anomalous diffusion behavior of moisture in NR composites. Parameters of the Langmuir model (equilibrium water uptake and diffusion coefficient) vary with the composition of hybrid fillers. Optimum proportions of silica, M-BNT, and BNT in rubber composites were obtained by considering the effect of fillers on mechanical properties and moisture diffusion characteristics of NR.

Low-field nuclear magnetic resonance for the determination of water diffusion characteristics and activation energy of wheat drying

The characteristics of moisture diffusion during wheat drying were determined through low-field nuclear magnetic resonance. The feasibility of variable temperature drying was validated by identifying the variation in the characteristics of evaporable water (EW) and each water component in wheat. During drying, chemically bound water in wheat was converted into EW, which comprised physically bound water and free water components that can evaporate and gasify directly. EW was removed from wheat through diffusion. The analysis of the average rate of the reduction in EW and the variation in the characteristics of weakly chemically bound water () at different wheat drying stages showed that EW was rapidly removed from wheat at the drying temperature was set at 80 °C during the early stage of drying. The highest average drying rate of was observed during the later stage of drying at the drying temperature of 70 °C. As inferred from the behavior of chemically bound water during conversion and the behavior of EW during diffusion in wheat, the activation energy required for the removal of EW from wheat comprises the activation energy of the diffusion of EW from wheat () and the activation energy of the conversion of chemically bound water to EW (). Calculations showed that and fell within the ranges of 25.72–53.75 and 1.09–24.17 kJ/mol, respectively. The maximum value of was less than the minimum value of This result confirmed that the energy consumption of removal of chemically bound water is bigger than that of EW. A method for calculating the drying activation energy of wheat () was developed in reference to the characteristics of the stable component contents of mature wheat. The relationship between and the moisture content of biomass was revealed.

Characterization of moisture mobility and diffusion in fresh tobacco leaves during drying by the TG\u2013NMR analysis

Curing is an essential procedure in tobacco primary processing. The moisture mobility and diffusion characteristics during drying are the key factors that affect the curing condition optimization. In the present study, the thermogravimetric analysis (TG) was integrated to the online nuclear magnetic resonance analysis (NMR) to investigate the drying characteristics of fresh tobacco leaves (FTL) during hot air drying at 50 and 60 °C. The results of TG were used to analyze the drying kinetics and obtain the effective diffusion coefficient of moisture of FTL during drying. The moisture diffusion coefficient of FTL was 4.1782E−09 m2 s−1 at 50 °C and 5.7777E−09 m2 s−1 at 60 °C, respectively. The moisture mobility of the samples at different drying stages was monitored by the NMR analyzer. T2 spectra showed that the initial ratios of semi-bound water and bound water in FTL were 93.2 and 6.8%, respectively. The drying rate showed a decreasing trend with the reduction in relaxation time of T22 peak from about 80 ms to about 30 ms, especially for the ones dried at 60 °C. The reduced relaxation time of T22 peak indicated that water activity in FTL was weakened during drying. Moisture distribution obtained by magnetic resonance imaging in FTL at different drying stages can reveal the moisture transfer path.

Bio-based coatings for reducing water sorption in natural fibre reinforced composites

In this study, bio-based coatings were used for reducing water sorption of composites containing flame retardant treated natural fibres and phenolic resin. Two types of coatings; polyfurfuryl alcohol resin (PFA) and polyurethane (PU) were used on the composites and compared with a water resistant market product. Uncoated and coated samples were conditioned at 90 °C and relative humidity of 90% for three days and the relative moisture content and mechanical properties after conditioning were analysed. In addition, the changes in the weight loss of the conditioned samples were also investigated by thermogravimetric analysis. The moisture diffusion characteristics of coated laminates were also studied at room temperature under water immersion conditions. PFA coated samples showed better moisture resistance and mechanical performance than other bio-based coatings when subjected to long term environmental aging.

Influence of Nano‐Calcium Carbonate Particles on the Moisture Absorption and Mechanical Properties of Epoxy Nanocomposite

AbstractThe moisture absorption and diffusion characteristics of epoxy‐based nanocomposites containing nano‐calcium carbonate (nano‐CaCO3) are investigated. Although the moisture absorption decreased with increasing nano‐CaCO3 content except for 8 wt%, the moisture absorption and diffusion behaviors were different from each other. It was found that the incorporation of 2 wt% and 4 wt% nano‐CaCO3 decreased moisture absorption capacity effectively. This is due to the more uniform distribution of nano‐CaCO3 particles, which created a longer diffusion path of moisture molecules in the nanocomposites. For the diffusion rate of moisture, there is no significant change observed on adding nano‐CaCO3. The effect of moisture absorption on the mechanical properties of nano‐CaCO3‐filled epoxy nanocomposites were investigated as a functional property of nano‐CaCO3 content. Due to moisture absorption, the flexural strength of epoxy/nano‐CaCO3 composites significantly impaired when compared to dry nanocomposites. However, the effect of nano‐CaCO3 on these mechanical properties was found to be positive, which allows the application of modified epoxy resin in high‐humidity environments.

Moisture diffusion and permeability characteristics of hydroxypropylmethylcellulose and hard gelatin capsules

The primary objective of this paper is to compare the sorption characteristics of hydroxypropylmethylcellulose (HPMC) and hard gelatin (HG) capsules and their ability to protect capsule contents. Moisture sorption and desorption isotherms for empty HPMC and HG capsules have been investigated using dynamic vapour sorption (DVS) at 25°C. All sorption studies were analysed using the Young–Nelson model equations which distinguishes three moisture sorption types: monolayer adsorption moisture, condensation and absorption. Water vapour diffusion coefficients (D), solubility (S) and permeability (P) parameters of the capsule shells were calculated. ANOVA was performed with the Tukey comparison test to analyse the effect of %RH and capsule type on S, P, and D parameters. The moisture uptake of HG capsules were higher than HPMC capsules at all %RH conditions studied. It was found that values of D and P across HPMC capsules were greater than for HG capsules at 0–40 %RH; whereas over the same %RH range S values were higher for HG than for HPMC capsules. S values decreased gradually as the %RH was increased up to 60% RH. To probe the effect of moisture ingress, spray dried lactose was loaded into capsules. Phase evolution was characterised by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and differential scanning calorimetry (DSC). The capsules under investigation are not capable of protecting spray dried lactose from induced solid state changes as a result of moisture uptake. For somewhat less moisture sensitive formulations, HPMC would appear to be a better choice than HG in terms of protection of moisture induced deterioration.

Topological disorder of microstructure in fiber-reinforced polymer composites: Diffusion response

Influence of microarchitectural fiber matrix arrangement in representative volume element is studied to assess the effect of mass diffusion in the fiber-reinforced polymer composites. A novel approach has been proposed to illustrate the quantitative definition of the representative volume element heterogeneity using topological descriptor of clustering. Average fiber center distance from the fixed representative volume element center is described to compute the clustering in different representative volume elements, followed by finite element method simulation to model and visualize two-dimensional microstructures of fiber-reinforced matrix composites with moisture boundary conditions. The suggested topology descriptor shows good correlation with diffusion response in relation to saturation time, whereas a fair degree of correlation was observed for mass accumulation and mass flux characteristics. The results indicate that microstructural heterogeneity has a strong influence on typical moisture diffusion characteristics and hence, an overall diffusion-induced damage. The observations should serve as a clue to design the fiber-reinforced polymer composites with the moisture barrier properties.

Investigation of hygroscopic and mechanical properties of nanoclay/epoxy system: Molecular dynamics simulations and experiments

In this study, the moisture related hygroscopic characteristics and mechanical properties of polymer–clay nanocomposites (PCNs) were investigated using molecular dynamics (MD) simulations and experiments as a function of the weight fraction of nanoclay. The hygroscopic and mechanical properties including the moisture saturation amount, moisture diffusivity, work of adhesion, and elastic modulus of the PCNs were studied using MD simulations and compared to experimental results as a function of the nanoclay content. It was demonstrated that the proposed MD simulation technique can be successfully used for the prediction of the effects of the nanoclay on the moisture diffusion characteristics as well as mechanical improvements of PCNs. It is expected that the simulation technique applied in this work can be widely used to evaluate the performances of nanocomposite materials.