Effective Diffusion Coefficient Of Water Research Articles

Pore‐Scale Modeling of Water and Ion Diffusion in Partially Saturated Clays

AbstractAn accurate mechanistic understanding of solute diffusion in partially saturated clays is critical for assessing the safety of deep geological repositories for radioactive waste. In this study, a pore‐scale numerical framework is developed to simulate water and ion diffusion in partially saturated clays. First, the two‐phase Shan‐Chen Lattice Boltzmann method is employed to establish the liquid‐gas distribution in a reconstructed three‐dimensional pore geometry of a clay. An equivalent solute method is also developed and validated to improve the numerical stability of the solution at the liquid/gas interface corresponding to steep variations of the concentration and diffusion coefficient of the water tracer. By using a mobility‐distance relationship from molecular simulations, Fick's law is numerically solved to simulate water diffusion in nanopores, while the coupled Poisson‐Boltzmann‐Nernst‐Planck equations are solved to simulate ion diffusion under the influence of the electrical double layer (EDL). Our model reveals that the decrease of relative effective diffusion coefficients during the desaturation is more pronounced for ions than for water, due to the additional transport pathway of water tracers in the gas phase. The obtained effective diffusion coefficients of tritiated water and ions agree well with reported data from compacted sedimentary rocks. By comparing the local electric potential and the distribution of ion concentrations in single pores, the simulation results suggest that the EDL in unsaturated clays has a more complex influence on ion distribution than under fully water‐saturated conditions. This study provides critical insights into the coupled transport processes of solutes in partially saturated clays.

Assessment of calcium alginate gels as wall materials for encapsulation systems.

Calcium alginate gels are widely used to encapsulate active compounds. Some characteristic parameters of these gels are necessary to describe the release of active compounds through mechanistic mathematical models. In this work, transport and kinetics properties of calcium alginate gels were determined through simple experimental techniques. The weight-average molecular weight ( = 192 × 103 Da) and the fraction of residues of α-l-guluronic acid ( = 0.356) of sodium alginate were determined by capillary viscometry and 1 H-nuclear magnetic resonance at 25 °C, respectively. Considering the half egg-box model, both values were used to estimate the molecular weight of calcium alginate as = 2.02 × 105 Da. An effective diffusion coefficient of water ( = 2.256 × 10-9 m2 s-1 ) in calcium alginate was determined using a diffusion cell at 37 °C. Finally, a kinetics constant of depolymerization ( = 9.72 × 10-9 m3 mol-1 s-1 ) of calcium alginate was obtained considering dissolution of calcium to a medium under intestinal conditions. The experimental techniques used are simple and easily reproducible. The obtained values may be useful in the design, production, and optimization of the alginate-based delivery systems that require specific release kinetics of the encapsulated active compounds. © 2023 Society of Chemical Industry.

Water Effective Diffusion Coefficient in Dairy Powder Calculated by Digital Image Processing and through Machine Learning Algorithms of CLSM Micrographs.

Rehydration of dairy powders is a complex and essential process. A relatively new quantitative mechanism for monitoring powders' rehydration process uses the effective diffusion coefficient. This research focused on modifying a previously used labor-intensive method that will be able to automatically measure the real-time water diffusion coefficient in dairy powders based on confocal microscopy techniques. Furthermore, morphological characteristics and local hydration of individual particles were identified using an imaging analysis procedure written in Matlab©-R2023b and image analysis through machine learning algorithms written in Python™-3.11. The first model includes segmentation into binary images and labeling particles during water diffusion. The second model includes the expansion of data set selection, neural network training and particle markup. For both models, the effective diffusion follows Fick's second law for spherical geometry. The effective diffusion coefficient on each particle was computed from the dye intensity during the rehydration process. The results showed that effective diffusion coefficients for water increased linearly with increasing powder particle size and are in agreement with previously used methods. In summary, the models provide two independent machine measurements of effective diffusion coefficient based on the same set of micrographs and may be useful in a wide variety of high-protein powders.

Evaluation of water diffusion, water vapor permeability coefficients, physicochemical and antimicrobial properties of thin films of nopal mucilage, orange essential oil, and orange pectin

There is a growing interest in the development of biodegradable, safe materials to extend the shelf life of food and replace non-biodegradable plastics. Several materials have been investigated that can be used as edible coatings to extend the shelf life of fruit and vegetable products. In the present work, the design and characterization of different films of nopal mucilage, orange pectin, and orange essential oil were investigated for their potential use as edible coatings. It was found that the pectin concentration had a significant effect (p<0.05) on the thickness, color, and transparency of the edible films. According to the results, the formulation 0.5% mucilage-0.25 % pectin-0.25 % essential orange oil was chosen. Infrared spectroscopy showed no changes in the chemical compounds of the film formulation. The XRD pattern showed that this thin film is amorphous and contains calcium oxalate of nopal mucilage and oleic acid. In the water vapor permeability tests, the films containing orange essential oil in their formulation showed a higher effective water diffusion and water permeability coefficients as compared to the other formulations. In addition, the concentration of orange essential oil has an antimicrobial effect (p<0.05) on some bacteria of interest for food safety. This work demonstrates a new methodology and measurement technique to determine the effective water diffusion and water permeability coefficients for edible thin films based on the photoacoustic effect.

Effect of ultrasonic power on moisture migration and microstructure of contact ultrasound enhanced far-infrared radiation drying on taro slices

To clarify the moisture migration and microstructure of taro in contact ultrasound enhanced far-infrared radiation drying (CUFRD), low-field nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), paraffin sectioning and microscopic observation techniques were applied to explore the changes of the drying curves, water status, microstructure, porosity, microscopic images and distribution curves of cell microstructure parameters of taro under different ultrasound powers of 0, 40, and 80 W. The results showed that applying contact ultrasound (CU) during drying was beneficial to accelerating the dehydration process and improving the porosity of taro slice. With the augment of CU power, the drying times of taro reduced by 16.67% to 25.00%, and the effective water diffusion coefficient was improved by 14.72% to 31.38%. In addition, the application of CU resulted in an increase firstly and then decrease in the cross-sectional area and perimeter distribution curves of taro cells and a widening of the peak shape of the distribution curve of taro cell roundness. In conclusion, CU application could cause faster internal moisture migration, and higher CU power had more obvious effect on microstructure and cell morphological parameters of taro during CUFRD.

Development of a differential photoacoustic system for the determination of the effective water diffusion and water vapor permeability coefficients in thin films

This paper focused on developing a methodology and metrology using a differential photoacoustic (PA) system to determine the effective water vapor diffusion coefficient (Deff) and the effective permeability coefficient (Π) in thin films as a piece of paper and standard polystyrene for a controlled relative humidity. The methodology proposes a new differential photoacoustic system, including the water reservoir, relative humidity, and temperature detectors. Two cells, reference/sample, were used to obtain the instrumental function to reduce the electronic and environmental noises. A method based on the study of ln[1−(S−S0)/ΔS]=t/τD and the behaviors of R2 as a function of the number of data was proposed to assess the region in which the photoacoustic signal should be processed to determine each effective coefficient. S is the amplitude of the PA signal, S0 is the initial amplitude value, ΔS is the change, t (time), and τD is the water vapor diffusion time. The effective water diffusion coefficient (Deff) for water and polystyrene was 1.90 × 10−11 m2/s and 3.09 × 10−11 m2/s, respectively. The permeability coefficient value for the piece of paper was 4.18 × 10−9 mol kg−1 cm−2 s−1 Pa−1, while for polystyrene, it was 6.80 × 10−9 mol kg−1 cm−2 s−1 Pa−1 for 70% of relative humidity. This methodology can be extended by changing the moisture content on the chamber to obtain the dependence of Deff as a function of relative humidity.

Foam-mat convective drying of kiwiberry (Actinidia arguta) pulp

The purpose of this work has been to determine the optimal conditions for the processing of kiwiberry fruit pulp based on the relevant thickness of the layer and air temperature, that will target the maximal reduction of drying time and minimal energy consumption. The research was designed to use the response surface methodology (RSM). The effects of independent variables (factors) such as the thickness of the layer (4, 8, and 12 mm) and the temperature of the drying air (50, 60, and 70°C) were studied by the means of the response variables. In this study, the drying kinetics was evaluated based on the drying time, drying rate, and effective water diffusion coefficient (D&lt;sub&gt;eff&lt;/sub&gt;). Based on the simplified Fick's second law of diffusion, D&lt;sub&gt;eff&lt;/sub&gt; was determined, while the relative water content [moisture ratio (MR)] was predicted using empirical models. It was found that as the temperature of the drying process increased, the process time was reduced, and the drying process intensified, which was manifested by the increment of the drying rate as well as the effective moisture diffusivity (varied from 5.58 × 10&lt;sup&gt;–10&lt;/sup&gt; m&lt;sup&gt;2&lt;/sup&gt; s&lt;sup&gt;–1&lt;/sup&gt; to 27.2 × 10&lt;sup&gt;–10&lt;/sup&gt; m&lt;sup&gt;2&lt;/sup&gt; s&lt;sup&gt;–1&lt;/sup&gt;). The mathematical model, developed using the RSM, showed a good fit for the data obtained in the study, which allowed to predict the response of the effective water D&lt;sub&gt;eff&lt;/sub&gt; of the foamed kiwiberry fruit pulp with R&lt;sup&gt;2&lt;/sup&gt; &amp;gt; 0.97.

Characteristics and mathematical modeling of apple slice drying in an electrohydrodynamic system with a needle‐plate electrode

AbstractThe drying of apple slices in an electrohydrodynamic (EHD) system with a needle‐plate electrode was characterized and mathematically modeled at voltages of 30, 40, and 50 kV and with distances between needles of 5, 6, and 7 cm. The results showed that the drying rate, effective water diffusion coefficient, and rehydration ratio of the apple slices, and specific energy consumption of drying, all increased with increasing voltage. Initially, there was a period when the drying rate was approximately constant in the low‐voltage EHD, but not in the high voltage conditions. At the same voltage, the drying rate was highest when the distance between needles was 6 cm. The voltage and distance between needles had a significant (p &lt; .05) interactive effect on the effective water diffusion coefficient and specific energy consumption, but not on the rehydration ratio. General mathematical models for drying can be used to represent the drying behavior of apple slices. The Midilli and Kucuk model and the Logarithmic model had the best fits, with correlation coefficients of 0.998803 and 0.998267, respectively. Predictive functions for the moisture ratio of apple slices based on voltage and distance between needles were established and verified. The results indicated that the Logarithmic model could more accurately predict the moisture ratio of apple slices dried using the EHD method.Practical ApplicationsAs a new drying technology, electrohydrodynamic (EHD) drying has unequaled advantages. According to the results, using the appropriate voltage and distance between needles improves not only drying efficiency, but also the rehydration ratio of apple slices, which reflects the quality of dried products. The Logarithmic model can accurately predict the EHD drying behavior of apple slices, which is of great significance in optimizing actual production.

Blanching effects on convective air-drying and sorption properties of papaya

This study investigated the influence of blanching on the drying kinetics of papaya from the perspective of the microstructure of the tissue and the water sorption behavior. Drying kinetics and water effective diffusion coefficients were determined based on Fick’s law considering shrinkage. Intense cellular changes were observed in blanched tissues images obtained by light microscopy and transmission electron microscopy. Heat pretreatment increased the water diffusion coefficients and the drying rates in comparison with fresh fruit. Severe color changes were induced by blanching, resulting in color loss during drying. At intermediate moisture ranges, sorption isotherms of blanched papayas showed lower water sorption capacity than those of fresh fruit, therefore the critical moisture for microbial growth in fresh papayas is higher than in blanched ones. The compensation theory was satisfied for fresh fruit but not for blanched fruit, which presented distinctive sorption behavior attributed to cellular damage caused by heat pretreatment.

Modeling of the changes in some physical and chemical quality attributes of potato chips during frying process

In the present study, the heat and mass transfer parameters related to the potato chips and frying medium were determined, and the changes in some physical and chemical quality attributes of the potato chips were investigated during frying at 175 °C. The effective water diffusion coefficient-(Deff) was determined as 2.067×10−8 m2/s for potato chips, while the heat transfer coefficient-(h) was calculated as 251.5 W/m2.°C for the frying medium. The core temperature of the potato chips was equilibrated around 100 °C during frying, and under this constant temperature phase, the water evaporation rate was determined ∼8 g H2O/m2.s. HMF and furfural accumulations in the potato chips fit the best with the first-order mass formation kinetics model, and the formation rates were calculated as 0.1862 and 0.1386 1/min, respectively. High and significant correlations were determined between the color attributes of the potato chips and furfural derivatives. Consequently, the collected results by the suggested mathematical approaches can be used to optimize frying conditions along with obtaining the fried potato snacks compatible with consumer expectations.

Nuclear magnetic resonance footprint of Wharton Jelly mesenchymal stem cells death mechanisms and distinctive in-cell biophysical properties in vitro.

The importance of the biophysical characterization of mesenchymal stem cells (MSCs) was recently pointed out for supporting the development of MSC‐based therapies. Among others, tracking MSCs in vivo and a quantitative characterization of their regenerative impact by nuclear magnetic resonance (NMR) demands a full description of MSCs’ MR properties. In the work, Wharton Jelly MSCs are characterized in a low magnetic field (LF) in vitro by using different approaches. They encompass various settings: MSCs cultured in a Petri dish and cell suspensions; experiments‐ 1D‐T 1, 1D‐T 2, 1D diffusion, 2D T 1‐T 2 and D‐T 2; devices‐ with a bore aperture and single‐sided one. Complex NMR analysis with the aid of random walk simulations allows the determination of MSCs T 1 and T 2 relaxation times, cells and nuclei sizes, self‐diffusion coefficients of the nucleus and cytoplasm. In addition, the influence of a single layer of cells on the effective diffusion coefficient of water is detected with the application of a single‐sided NMR device. It also enables the identification of apoptotic and necrotic cell death and changed diffusional properties of cells suspension caused by compressing forces induced by the subsequent cell layers. The study delivers MSCs‐specific MR parameters that may help tracking MSCs in vivo.

Mathematical modelling of mass transfer phenomena for sucrose and lactitol molecules during osmotic dehydration of cherries

The diffusion phenomena of sucrose and lactitol in cherries using different proportions during osmotic dehydration was quantified by means of a mathematical model based on Fick's second law. The average effective diffusion coefficient for soluble solids in skin and flesh are 5.37 10−11 m2∕s and 1.24 10−10 m2∕s. Whereas, for water in skin and flesh are 9.27 10−09 m2∕s and 5.48 10−08 m2∕s. A significant difference for water diffusion coefficients (p < 0.05) was observed between the treatments. This could indicate that the diffusion between species and treatments is differential. Effective diffusion coefficients for water in skin and flesh are 2 orders of magnitude greater than effective diffusion coefficients for soluble solids. This is probably due to its lesser molecular weight. Furthermore, the effective diffusion coefficients for water and soluble solids in cherry skin are between 1 and 2 orders of magnitude lower than effective diffusion coefficients for both in cherry flesh, possibly due to the barrier effect exerted by the cherry skin.

Effective free water diffusion coefficient of cement paste internally cured with superabsorbent polymers

A hybrid model, comprised of analytical and phenomenological models, was developed to quantify the effective free water diffusion coefficient of cement hydration with superabsorbent polymers (SAP) as internal curing. NMR/MRI measurements were used to estimate the water types and distribution, and the cement degree of hydration. Employing reverse engineering, the estimated temporal free water distribution in cement hydration with SAP was reproduced using 1-D finite element transient flow model and a tortuosity model. The computed free water diffusion coefficient and effective diffusion length of cement paste with 0.2% and 0.3% SAP is 5.48 × 10−12 m2/s and 4.46 × 10−12 m2/s, and 0.25 mm and 0.19 mm, respectively. The computed efficacy of SAP dropped by 8% and 27%, for 0.2% and 0.3% SAP content respectively, when accounting for particles agglomeration.

Modeling the Tablet Disintegration Process Using the Finite Difference Method.

The purpose of the study is to present the finite difference method (FDM) and demonstrate its utility in modeling mass transport processes that are pharmaceutically relevant. In particular, diffusion processes are ideally suited for FDM because the governing equation, Fick's second law of diffusion, can be readily solved using FDM over a finite space and time. The method entails the mesh creation, space and time discretization, and solving Fick's second law at each node using finite difference-based numerical schemes. We applied FDM to study tablet disintegration, in which the tablet water uptake was simulated with an effective water diffusion coefficient; the tablet disintegration was controlled by a designated critical water content parameter, beyond which the node is treated as being disintegrated from the tablet. The resulting simulation agreed with the experimental tablet disintegration behaviors, under both disintegration-controlled and water uptake-controlled conditions. This study highlighted the unique advantage of FDM, capable of providing spatial-temporal information on water uptake and evolution of tablet size and shape during tablet disintegration, which was otherwise not available using other methods. The FDM method enabled more in-depth tablet disintegration studies. The model also has the potential to be calibrated and incorporated in tablet formulation DoE studies.

Multi-scale study water and ions transport in the cement-based materials:from molecular dynamics to random walk

The transport properties of water and ions play a key role in the durability of cement-based materials. However, little work has been done to study the diffusion behaviors of water and ions in a multi-scale view. Therefore, the current paper presented a multi-scale molecular dynamics fed random walk (MD-RW) simulation scheme for the purpose of obtaining the diffusion coefficients of water and ions in the calcium-silicate-hydrate (C–S–H) gel. At nanoscale, MD simulations were used to obtain the diffusion coefficients of water and ions in the C–S–H with nanopore size ranging from 0.5 nm to 5 nm since pore size has a crucial effect on the diffusion behavior of water and ions. Then these results were utilized as the input parameters for random walk simulation to scale the confining effect up and derive the effective diffusion coefficients of water and ions in the C–S–H gel at mesoscale. The results acquired from mean square displacement, atomic intensity distribution function and radial distribution function show that water molecules and Cl ions would form Ca–Cl, Ca–O ion-clustering and hydrogen bonds with C–S–H substrate which will restrict the transport of water molecules and Cl ions. In addition, with the size of gel pore increases from 0.5 nm to 5 nm, the diffusion coefficient of water increases from 0.016 to 1.85 × 10−9 m2 s−1 and the diffusion coefficient of Cl ions increases from 0.002 to 0.75 × 10−9 m2 s−1. In the random walk simulation, the diffusion coefficients of water and Cl ion were derived as 1.01 × 10−12 m2 s−1 and 1.2 × 10−13 m2 s−1 in the FCC packing model and 1.55 × 10−11 m2 s−1 and 1.77 × 10−12 m2 s−1 in the BCC packing model respectively, which reaches reasonable agreement with the previous studies results. Unlike traditional reverse methods which back-calculate these coefficients from macroscopic experimental results and hypothesized microstructure, this study presents the first bottom-up method deriving the transport properties of C–S–H gel directly from the physicochemical characteristics and microstructure, which has relevant significance for the study of concrete resistance to ion erosion and provides a basis for designing durable concrete.

Osmotic dehydration kinetics of fresh and frozen blueberries considering volume shrinkage in a novel ternary solution

Osmotic dehydration is a nonthermal processing technology, that is limited in blueberries because of the osmotic agent, temperature and efficiency. This study was to prepare a novel ternary solution considering low-sugar, low-salt and low-viscosity requirements and to dehydrate blueberries at 20 °C. The mass transfer kinetics of fresh and frozen blueberries were explored in ternary solution considering the volume shrinkage of blueberries. The results showed that the 40%/50% xylitol solution had the lowest viscosity in polyols at 20 °C. The addition of 5%/10% CaCl2 had no significant effect on the viscosity of the ternary solution. Frozen blueberries had higher OD efficiency and less equilibrium time than fresh blueberries at 600 rpm vibration speed and 50%xylitol/10%CaCl2 concentration. There was competition between xylitol and CaCl2 transfer at shorter dehydration times for fresh blueberries, which was not significant in frozen blueberries and with longer times. Considering volume shrinkage of the blueberries, the effective diffusion coefficient of water and solute was calculated. The vibration speed had a significant effect on the effective diffusion coefficient, which governed the mass transfer efficiencies of the blueberries in the ternary solution.

Correlating 3D porous structure in polymer films with mass transport properties using FIB-SEM tomography

Porous polymer coatings are used to control drug release from pharmaceutical products. The coating covers a drug core and depending on the porous structure, different drug release rates are obtained. This work presents mass transport simulations performed on porous ethyl cellulose films with different porosities. The simulations were performed on high spatial resolution 3D data obtained using a focused ion beam scanning electron microscope. The effective diffusion coefficient of water was determined using a diffusion chamber. Lattice Boltzmann simulations were used to simulate water diffusion in the 3D data. The simulated coefficient was in good agreement with the measured coefficient. From the results it was concluded that the tortuosity and constrictivity of the porous network increase with decreasing amount of added hydroxypropyl cellulose, resulting in a sharp decrease in effective diffusion. This work shows that high spatial resolution 3D data is necessary, and that 2D data is insufficient, in order to predict diffusion through the porous structure with high accuracy.

Hot-air drying characteristics of Millettia speciosa Champ. slice and establishment of the kinetic model

[Objective] To explore the influence of different hot air temperatures, slice thicknesses and material loads on Millettia speciosa Champ. slice hot air drying rate, establish model of thermal wind drying kinetic, and provide reference for perfecting M. speciosa Champ. drying processing technology. [Method] Taking hot air temperature (50, 60, 70, 80 °C), slice thickness (2,4,6,8 mm) and material load (100,200,300 g) as investigating factors,real-time measurement of moisture changes during hot air drying of M. speciosa Champ. slices under various conditions. The 5 kinds of drying models were screened and fitted, calculated the effective water diffusion coefficient and theactivation energy in the drying process. [Result] With the increase of the hot air temperature, the slice thickness and material load decreased, the moisture content of dry basis was greatly reduced, and the drying rate was greatly increased. The hot-air drying process of M. speciosa Champ. slices was divided into two stages : acceleration and deceleration, and most of the drying process was the deceleration stage. The kinetic model of hot-air drying of M. speciosa Champ. slices conformed to the Page model, and the predicted value of the Page model had a good fit with the experimental value ( R 2=0.969). the fitting equation ln (-lnMR)= -3.174-0.242 H+0.029 T-0.006 L+(0.721+0.015 H+0.002 T)ln t, k=e -3.174-0.242 H+0.029 T-0.006 L , n=0.721+0.015 H+0.002 T, the effective diffusion coefficient D eff of M. speciosa Champ. under different drying conditions was 1.62114×10 -10-12.96913×10 -10 m 2/s, with the increase of hot air temperature and slice thickness, the material load decreased, and the overall trend was increasing, and the activation energy was 60.7388 kJ/mol. [Conclusion] The Page model can better reflect the moisture change law of hot air drying process of M. speciosa Champ. slice with different slice thicknesses, and by fitting the equation, the content of water ratio in the hot air drying process at a certain time can be accurately predicted. 摘要：【目的】探讨不同热风温度、切片厚度及装载量对牛大力切片热风干燥速率的影响, 并建立牛大力切片热风 干燥动力学模型, 为牛大力干燥工艺探索提供理论依据。 【方法】以热风温度 (50、60、70、80 °C)、切片厚度 (2、4、6、8 mm)和装载量 (100、200、300 g)为考察因素, 实时测定各条件下牛大力切片热风干燥过程中水分变化, 对常见的5种 干燥模型进行筛选, 并计算干燥过程中的有效水分扩散系数和活化能。 【结果】随着热风温度的升高, 切片厚度和装载 量的降低, 牛大力切片的干基含水量明显减少, 干燥速率明显增加。牛大力切片在热风干燥过程分为加速和降速2个 阶段, 其中大部分干燥过程为降速阶段。牛大力切片热风干燥动力学模型符合Page模型, 该模型预测值与试验值拟 合度较高( R 2=0.969), 拟合方程为ln(-lnMR)=-3.174-0.242 H+0.029 T-0.006 L+(0.721+0.015 H+0.002 T)ln t, 可求得K=e -3.174-0.242 H+0.029 T-0.006 L , n=0.721+0.015 H+0.002 T, 不同干燥条件下牛大力切片的有效水分扩散系数在1.62114×10 -10~12.96913×10 -10 m 2/s, 均随着热风温度的升高和切片厚度的增加, 总体呈上升趋势; 活化能为60.7388 kJ/mol。 【结论】Page模型可 较好地描述不同切片厚度的牛大力切片热风干燥过程中水分的变化规律, 且通过拟合方程能较准确预测热风干燥过 程中某时刻牛大力切片的水分比。

Estimation of the effective water diffusion coefficient in Nafion® membrane by water balance measurements

AbstractParameters estimation is important in fuel cell modelling and it remains a challenging task to predict. One of the most discussed parameters is the water diffusion coefficient in Nafion® membrane, which has a huge variation (10−9 and 10−5 cm2 s−1), based upon the viewpoints of different authors. In this article, the estimation of the effective water diffusion coefficient through water balance method is presented and the effects of relative humidity gradient and cell temperature are explained. The membrane used in this work is Nafion® NRE 211 tested at a temperature of 60°C and 70°C with air flows. The value of water flux passing through the membrane is used to calculate the effective diffusion coefficient. A value in the range of 1 × 10−7 to 4 × 10−7 cm2 s−1 is reported and it shows that the water balance method can be applied easily to measure an effective membrane water diffusion coefficient.

Drying characteristics and quality of Stevia rebaudiana leaves by far-infrared radiation

The far-infrared drying technique was used to dry Stevia rebaudiana leaves, and the drying rate, drying kinetics, the glycoside content of dried Stevia rebaudiana leaves and surface structure of the dried material were explored at different drying temperatures (45 °C、50 °C、55 °C、60 °C、65 °C), irradiation height (60 mm、120 mm、180 mm、240 mm、300 mm). Under different conditions, the moisture rate decreased and the drying rate increased significantly with the increase in drying temperature and the decrease in irradiation height, Weibull model fits the drying curves well (R2 > 0.99). The range of the estimated water diffusion coefficient Dcal was 0.841 8 × 10−4–2.945 3 × 10−4 m2 s−1, and the range of the effective water diffusion coefficient Deff was 0.835 9 × 10−6–3.629 1 × 10−6 m2 s−1. The activation energy was 28.79 kJ mol−1 determined by Arrhenius equation. By comparing the glycoside content of dried products under different drying conditions, it was found that far-infrared drying could better retain the glycoside content. The drying temperature have the batter effect on active ingredients of samples when it increased from 45 °C to 55 °C, the stevioside and rebaudioside A increased from 7.01 to 7.88 mg to 7.44 and 8.58 mg, respectively. Far-infrared drying technology could increase the number of micropores on the surface and inside the materials and improve the quality of dried products. This study explored the law of water transfer during the drying process of Stevia rebaudiana leaves and provided a theoretical reference for the research and application of Stevia rebaudiana leaf drying technology.