Effective Mass Diffusivity Research Articles

Comparison of Drying Systems in Terms of Energy Consumption, Effective Mass Diffusion, Exergy Efficiency and Improvement-Sustainability Index in the Valorizations of Waste Tomatoes by Thermal Processes

Comparison of Drying Systems in Terms of Energy Consumption, Effective Mass Diffusion, Exergy Efficiency and Improvement-Sustainability Index in the Valorizations of Waste Tomatoes by Thermal Processes

Impact of ultrasound pretreatment on banana chip production by convective drying using hot air: Consideration of shrinkage and variable effective mass diffusivity in the process description

AbstractIn this study, we used experimental analysis and numerical simulation to describe and compare the convective drying (CD) of banana slices, without and with pretreatment with ultrasound (US) at 25 kHz. Banana slices, with an average initial thickness of about 6.60 × 10−3 m and an average diameter of about 3.01 × 10−2 m, were subjected to CD at temperatures of 60, 70, and 80°C, with air speed of 1.5 m s−1. Considering such dimensions, the one‐dimensional diffusion equation in Cartesian coordinates was numerically solved by the finite volume method, using a fully implicit formulation (direct problem). A solver with a numerical solution was developed for the direct problem. This solver was coupled to an optimizer to determine the parameters a and b of functions that relate the effective moisture diffusivity (D) with the local moisture ratio, and also to determine the convective mass transfer coefficient (h) (inverse problem), considering the shrinkage. The proposed model was used to describe the data obtained. The results revealed that the drying rate increased in samples that were subjected to pretreatment with US, for each drying temperature. The best‐simulated results were those that related D to the local moisture ratio by means of a bcosh (aMR2) function. It could be observed that pretreatment with US allowed saving of time (and consequently energy) of 28.3%, 16.0%, and 12.4% at drying temperatures of 60, 70, and 80°C, respectively.Practical applicationsThe main contribution of this paper is to present a solver and an optimizer to determine the mass transfer properties considering the shrinkage during convective drying of banana pieces pretreated with ultrasound by the inverse problem. With the information obtained for the mass transfer properties and the proposed numerical model with a hyperbolic cosine type function for the effective moisture diffusivity, it is possible to more accurately determine the time and, therefore, the energy savings of processing banana chips pre‐treated with ultrasound.

Pore-scale prediction of the effective mass diffusivity and apparent permeability of carbon gels with adjustable nanoporous characteristics

To explore the structural effects on transport properties in carbon gels, an improved method has been introduced to regenerate their nanostructure and numerically illustrate the adjustability of their porous characteristics with the variation of synthesis parameters. Two lattice Boltzmann equations are applied to investigate the permeation and diffusion in the gel structures at the pore scale, and the apparent permeability is formulated to describe the total mass flux using the dusty gas model. The structural properties of the reconstructed models and calculated apparent permeabilities have been fully validated by various experiments. A decoupled analysis of the impact of structural parameters on transport properties demonstrates that increasing porosity and pore size, while decreasing geometric tortuosity, leads to more pronounced changes in intrinsic permeability compared to gas diffusivity. By utilizing a database that encompasses 240 reconstructed gels, a structural–functional relationship for transport properties in carbon gels could be proposed. Concerning the intrinsic permeability, a near quadratic relationship with the porosity and mean pore size, independent of particle size, could be concluded. For the nondimensional effective diffusivity, a power exponent of 1.85 associated with porosity is proposed, and its independence of pore size could be revealed. In addition, for gels with porosities under 0.65 and mean pore sizes less than 133 nm, diffusion supersedes permeation as the dominant term in total mass transfer, indicating that particle sizes have a more pronounced influence on the apparent permeability. The predictive model offers guidance for tailoring the transfer properties of carbon gels at the stage of preparation.

Recent advances in photoelectrochemical platforms based on porous materials for environmental pollutant detection.

Human health and ecology are seriously threatened by harmful environmental contaminants. It is essential to develop efficient and simple methods for their detection. Environmental pollutants can be detected using photoelectrochemical (PEC) detection technologies. The key ingredient in the PEC sensing system is the photoactive material. Due to the unique characteristics, such as a large surface area, enhanced exposure of active sites, and effective mass capture and diffusion, porous materials have been regarded as ideal sensing materials for the construction of PEC sensors. Extensive efforts have been devoted to the development and modification of PEC sensors based on porous materials. However, a review of the relationship between detection performance and the structure of porous materials is still lacking. In this work, we present an overview of PEC sensors based on porous materials. A number of typical porous materials are introduced separately, and their applications in PEC detection of different types of environmental pollutants are also discussed. More importantly, special attention has been paid to how the porous material's structure affects aspects like sensitivity, selectivity, and detection limits of the associated PEC sensor. In addition, future research perspectives in the area of PEC sensors based on porous materials are presented.

Probe into the adaptability of local equilibrium/non-equilibrium assumptions in simulating freeze-drying of porous media

A multiphase transport model was formulated to explore the adaptability of local thermal and mass equilibrium/non-equilibrium (LTE/LTNE and LME/LMNE) assumptions in simulating freeze-drying of porous media. Saturated and unsaturated samples with the same initial mass and moisture content but different initial saturations were used in freeze-drying experiments and numerical simulations. Four pairwise combinations between LTE/LTNE and LME/LMNE were tested using the commercial software of COMSOL Multiphysics based on the finite element method. Results showed that the LTE-LMNE assumption achieved excellent agreements between numerical and experimental drying curves with RMSEs being 1.81% for the saturated sample and all less than 1.00% for the unsaturated samples. Based on distributions of saturation and temperature, and variations of effective mass diffusivity and thermal conductivity, the rate-controlling factor was determined to be mass transfer for the saturated material and heat transfer for the unsaturated one. Convective heat transfer played an unimportant role in freeze-drying. The LTE-LMNE based model provided satisfactory predictive capabilities under different conditions. Appropriately increasing the operating temperature could boost the drying rate. Changing the chamber pressure had insignificant effects with drying time differences only below 3%. The smaller the initial saturation, the shorter the drying time. The initially unsaturated frozen material with preformed pores significantly enhanced the freeze-drying process and improved the process economy.

Self-assembled MOF microspheres with hierarchical porous structure for efficient uranium adsorption

Herein, the self-assembled zirconium (Zr)-MOF microspheres formed by the octahedral nano-building blocks (UiO-66 and its derivatives) to adsorb uranium (U) was presented for the first time. The self-assembled MOF microspheres achieved the growth across orders of magnitude from nanometers (∼50 nm) to micrometers (∼2.3 μm), and exhibited ordered hierarchical porous structure (from ∼0.6 nm to ∼340 nm). The self-assembled MOF microspheres combined the advantages of nanostructures with stability and microstructures with hierarchical porous structure for effective mass diffusion. The improved mass diffusion properties of amidoxime (AO)-appended MOF microspheres (P-UiO-66-AO) endowed them with superior adsorption performance and excellent selective adsorption of U compared with AO-appended MOF (UiO-66-AO) octahedron. The adsorption equilibrium was obtained within 1400 min, and the maximum adsorption capacity of U was 283.2 mg/g. The adsorption capacity of U in simulated seawater was determined to be 11.4 mg/g and the ratio of distribution coefficients of U and vanadium (V) (Kd/U/Kd/V) increased 1.75 times than that of UiO-66-AO octahedron, overcoming the challenge of competitive adsorption of U and V with MOF. Overall, the strategy to fabricate self-assembled MOF microspheres with nano-building blocks is expected to expand the diversity and range of potential applications of MOFs.

Coupled surface-volume pyrolysis effects of carbon-phenolic resin composites under hyperthermal non-equilibrium flows

The thermal protection system is essential for the safe and reliable flying of any high-speed aircraft. A carbon fiber-reinforced phenolic resin (PR) composite is one of the most important matrix candidates for ablative materials, and the microscopic understanding of the pyrolysis process of PR, however, still remains poor. The usual numerical approach is based on volumetric pyrolysis while neglecting the influence of external hyperthermal surroundings. A surface-volume coupled reactive molecular dynamics model is established in this work to simulate complicated heat/mechanics/chemistry multi-physical field coupled pyrolysis problems. The pyrolysis process of the carbon fiber-PR composite is investigated in the presence of vacuum and hyperthermal gas impacts and compared with the volumetric pyrolysis simulation only. The consideration of the surface–volume coupling reveals many unique features beyond the volumetric pyrolysis, which include the generation of different pyrolysis products and the dependence of the effective mass diffusivity of pyrolysis gases on impinging gases in the presence of the hyperthermal non-equilibrium environment. With the detailed revelation of the evolutions of PR solid phase and pyrolysis gaseous products, the work is of great help in improving the microscopic pyrolysis mechanisms, especially the “blowing gas effect,” a key phenomenon for improved understanding of the complicated hypersonic boundary layer flow.

Use of empirical and diffusion models in the description of the process of water absorption by rice

PurposeTo describe water absorption by the rice grains over time, diffusion and empirical models were used. Also, an optimization software was developed in this study to determine parameters and their uncertainties for the diffusion models (LS Optimizer, for partial differential equations). Parameters (and their uncertainties) for empirical models were determined by LAB Fit Curve Fitting Software.Design/methodology/approachHeat and mass diffusion phenomena are found in various processes of technological interest, including pasteurization, drying and water immersion of agricultural products, among others. The objective of this work was to study the process of water absorption by rice grains with and without husk, using diffusion and empirical models to describe the absorption kinetics. Rice grains were immersed (approximately 10 g for each experiment) in drinking water maintained at constant temperatures of 28, 40 and 50 C. In the experiments, the water contents absorbed by rice grains over time were obtained by the gravimetric method.FindingsAmong empirical models, Peleg was the most satisfactory to describe the kinetics of water absorption by rice without husk, while the Silva et alii model had the best statistical indicators for rice with husk. It was also verified that a diffusion model with boundary condition of the first kind showed the best (or equivalent) results in the description of all processes of kinetics of water absorption by rice grains, with and without husk. For grains without husk, the effective mass diffusivities were (1.186 ± 0.045) × 10−9, (1.312 ± 0.024) × 10−9 and (2.133 ± 0.028) × 10−9 m2 min−1, for the immersion temperatures of 28, 40 and 50C, respectively. For grains with husk, diffusivities were (0.675 ± 0.011) × 10−9 and (1.269 ± 0.017) × 10−9 m2 min−1, for temperatures of 28 and 50 C, respectively.Originality/valueThis work developed a solver for the diffusion equation in cylindrical geometry and presented the LS Optimizer software developed to determine differential equation parameters through experimental data sets.

Multi-scale modelling on PEM-based electrolyte dehumidifier: Transient heat and mass transfer in anode catalyst layer with microstructures

As an independent and ultra-compact method, PEM-based dehumidifiers are promising, while accurate modelling of anode-side transport is still challenging. The theoretical model developed in this study is transient, two-dimensional and non-isothermal based on the lattice Boltzmann method, emphasizing the anode-side catalyst layer (CL) with reconstructed microstructures. Then the pore-scale CL modelling was coupled with the macro-scale simulation of air channel and diffusion layer (DL), and experimentally validated by a 3.5 cm2 dehumidifier. Results showed that the CL microstructure significantly affects the air outlet parameters of PEM dehumidifiers, especially in the beginning state. Although the uniform distribution of vapor concentration can be achieved immediately in CL, the time required for the air outlet to reach transient equilibrium is much longer, which is around 1000s for the moisture content and 7000s for the temperature. The moisture transfer resistance at the beginning state mainly exists in CL, then it moves to both CL and DL (close to the interface) when it reaches equilibrium. Due to the continuous heat generation, the temperature flux in CL dominates in the anode side, and increases by 5-6 times during dehumidification. The calculated values of effective mass diffusivity and thermal conductivity of CL are 7.71×10−6 m2/s and 0.063 W/m•K. The randomly arranged catalyst particles makes CL the hottest (2~3°C higher) and driest (half the DL concentration) part of PEM element, which severely limits the whole performance. This modelling reveals the interaction between anode-side CL with microstructures and the performance, considering the transient heat and mass transfer.

Effective thermal conductivity of composite materials made of a randomly packed densified spherical phase

The prediction of the effective thermal conductivity of composite materials is of paramount importance for many applications. While there exist correlations for bi-phase materials made of rigid spherical inclusions in a continuum matrix, there is lack of correlations for composites whose spherical phase is densely packed, as in densified granular materials or in foams with overlapping bubbles. In this study we combine a packing algorithm with a finite difference solver to determine the effective thermal conductivity when the spherical phase is highly overlapping, with volume fraction larger than 55%, for all the conductivity ratios of the two phases. Three fit relations are proposed to encompass the whole range of conductivities, showing good accuracy for both simulated and experimental data and outperforming existing relations commonly used for the estimation of the effective thermal conductivity. Our relations can also be used for estimating the effective electric conductivity, magnetic permeability and mass diffusivity in continuum regime.

Combined macroscopic and pore scale modeling of direct contact membrane distillation with micro-porous hydrophobic membranes

The transport processes of a direct contact membrane distillation (DCMD) module are investigated with a multiscale approach in this study. A stochastic, numerical reconstruction algorithm is first developed to generate 3D virtual membranes based on the membrane's pore size and fiber orientation distributions. Pore-scale simulations are then performed on the reconstructed membrane model. The membrane's effective thermal conductivity and mass diffusivity are computed using AVIZO software and OpenFOAM solver, respectively. A 2D macroscopic, multiphysics model considering the conservation of mass, species, momentum, and energy is developed, where the effective transport properties obtained from pore-scale simulations are utilized, to study the heat and mass transfer through the membrane. Two commercially available polytetrafluoroethylene (PTFE) membranes (GE-Osmonics/0.22 μm and MS-3010/0.45 μm) are chosen as examples for the present multiscale approach. The computed transport properties of the membranes are found to yield good distillate flux predictions with data reported in the literature. The MS-3010/0.45 μm membrane is found to produce 24% higher freshwater on average than GE-Osmonics/0.22 μm membrane. Finally, the performance of DCMD of three hypothetical membranes with different specifications is compared to the reconstructed MS-3010/0.45 μm membrane to study the factors that need to be taken into account for optimal design of the membrane for DCMD applications.

Molecular Dynamics Approach to Calculate the Thermodiffusion (Soret and Seebeck) Coefficients of Salts in Aqueous Solutions.

An approach to investigate the physical parameters related to ion thermodiffusion in aqueous solutions is proposed herein by calculating the equilibrium hydration free energy and the self-diffusion coefficient as a function of temperature, ranging from 293 to 353 K, using molecular dynamics simulations of infinitely diluted ions in aqueous solutions. Several ion force field parameters are used in the simulations, and new parameters are proposed for some ions to better describe their hydration free energy. Such a theoretical framework enables the calculation of some single-ion properties, such as heat of transport, Soret coefficient, and mass current density, as well as properties of salts, such as effective mass and thermal diffusion, Soret and Seebeck, coefficients. These calculated properties are compared with experimental data available from optical measurements and showed good agreement revealing an excellent theoretical predictability of salt thermodiffusion properties. Differences in single-ion Soret and self-diffusion coefficients of anions and cations give rise to a thermoelectric field, which affects the system response that is quantified by the Seebeck coefficient. The fast and slow Seebeck coefficients are calculated and discussed, resulting in values with mV/K order of magnitude, as observed in experiments involving several salts, such as K+Cl-, Na+Cl-, H+Cl-, Na+OH-, TMA+OH-, and TBA+OH-. The present approach can be adopted for any ion or charged particle dispersed in water with the aim of predicting the thermoelectric field induced through the fluid. It has potential applications in designing electrolytes for ionic thermoelectric devices in order to harvest energy and thermoelectricity in biological nanofluids.

On the Study of Osmotic Dehydration and Convective Drying of Cassava Cubes

This paper aims to study the hybrid process of osmotic dehydration and convective air drying of foods. Emphasis has been put on cassava cubes (Manihot esculenta Crantz.). Convective drying kinetics of fresh and osmotically dehydrated cassava cubes was evaluated at the following hot air-drying conditions: temperature 50°C, velocity 1.35 m/s, and absolute humidity 0.060 dry water/g. Experimental results of the moisture loss, solids gain, and incorporation of sodium chloride are shown and analyzed. For estimation of the effective mass diffusion coefficient, experiment data of average moisture content of cassava cubes (fresh and osmotically dehydrated) was fitted to the simplified Fick model and a good agreement was obtained. The effective mass diffusivity of the osmotically dehydrated cassava cube was 2.75 x10-10 m2/s and to fresh cassava cubes 5.45x10-10 m2/s.

Parametric study of methane catalytic combustion in a micro-channel reactor: Effects of porous washcoat properties

A washcoat resolved numerical model of catalytic micro combustor fueled by methane is developed. Detailed gas phase mechanism and alumina dioxide supported Pt-based surface mechanism are adopted. This model proves to capture flow, temperature and species fields precisely including impermeable flow and products accumulation in porous washcoat layer. The novelty and superiority of this research compared to traditional modeling method in which instantaneous diffusion onto catalytic wall is assumed lies in that: it is possible to investigate effects of porous washcoat properties with this full-scale numerical model. Computational results show that homogeneous reactions (HR) are impaired and heterogeneous reactions (HTR) are strengthened as channel confinement increases. Da number analysis reveals that the micro combustor works under a mixed controlled mode in which catalytic reaction rate and internal mass diffusion rate are comparable. When washcoat layer thickness increases from 15 μm to 75 μm, overall catalyst usage roughly decreases from 0.34 to 0.05. It is concluded washcoat thickness can’t be too thin in order to maintain HTR intensity and too thick since catalyst usage is extremely low. Porous washcoat properties impact combustion characteristics in a similar mechanism: modifying effective mass diffusion coefficient. In general, increasing porosity, average pore diameter and decreasing tortuosity will enhance HTR and impair HR. In real application, these parameters can be utilized to modify combustion temperature, emission performance and stability operation map.

Description of Cumbeba (Tacinga inamoena) Waste Drying at Different Temperatures Using Diffusion Models.

One approach to improve sustainable agro-industrial fruit production is to add value to the waste generated in pulp extraction. The processing of cumbeba (Tacinga inamoena) fruits generates a significant amount of waste, which is discarded without further application but can be a source of bioactive compounds, among other nutrients. Among the simplest and most inexpensive forms of processing, convective drying appears as the first option for the commercial utilization of fruit derivatives, but it is essential to understand the properties of mass transfer for the appropriate choice of drying conditions. In this study, cumbeba waste was dried at four temperatures (50, 60, 70 and 80 °C). Three diffusion models were fitted to the experimental data of the different drying conditions. Two boundary conditions on the sample surface were considered: equilibrium condition and convective condition. The simulations were performed simultaneously with the estimation of effective mass diffusivity coefficients (Def) and convective mass transfer coefficients (h). The validation of the models was verified by the agreement between the theoretical prediction (simulation) and the experimental results. The results showed that, for the best model, the effective mass diffusivities were 2.9285 × 10−9, 4.1695 × 10−9, 8.1395 × 10−9 and 1.2754 × 10−8 m2/s, while the convective mass transfer coefficients were 6.4362 × 10−7, 8.7273 × 10−7, 8.9445 × 10−7 and 1.0912 × 10−6 m/s. The coefficients of determination were greater than 0.995 and the chi-squares were lower than 2.2826 × 10−2 for all simulations of the experiments.

Increase of mass transfer rates during osmotic dehydration of apples by application of moderate electric field

Abstract Osmotic dehydration (OD) is a drying process that consists in placing the food in contact with concentrated solutions of soluble solids to reduce its water activity. The use of moderate electric field (MEF) may promote increase of the mass transfer rates due to the non-thermal effects of electroporation and permeabilization of the cells. In this context, the objective of this study was to investigate the mass transfer process kinetics during apples OD assisted by MEF, evaluating the non-thermal effects of this emerging technology. The experiments were conducted with sucrose solutions (40, 50 and 60%, m/m) at 40 °C. Samples were submitted to electrical field strength (0, 5.5 and 11.0 Vcm−1), according to an experimental design. Results indicated that the application of MEF favoured water loss and solid gain. The effective mass diffusivities of water and solids increased as voltage applied increases. Moreover, MEF negatively influenced color and reducing capacity of the samples.

Dual active sites-dependent syngas proportions from aqueous CO2 electroreduction

Electroreduction of aqueous CO2 is a sustainable approach to produce tunable syngas, which demands the rational design of dual-active-sites catalysts for individual CO2 reduction reaction (CRR) and hydrogen evolution reaction (HER). Here, the self-supporting nickel@nitrogen-doped carbon nanotubes grown on carbon nanofiber (Ni@NCNTs-CNF) is developed with Ni single atoms (SAs) and nanoparticles (NPs) functioning as CRR and HER sites respectively. Benefiting from the hierarchical nanostructures for effective electron transport and mass diffusion, as well as the high accessibility of dual-active sites, Ni@NCNTs-CNF catalysts can mediate aqueous CO2 into CO/H2 (2:1) with a 100 % Faraday efficiency and a high current density (25 mA cm−2) at low working potential (-0.8 V). By manipulating the Ni SAs/Ni NPs ratio, the CO/H2 proportion is easily modulated in a wide range from 1:4 to 5:1, independence of the working potential (-0.6 to -0.8 V). Our findings provide fresh insights into the design of dual-active-sites catalysts.

Continuous and Intermittent Drying of Rough Rice: Effects on Process Effective Time and Effective Mass Diffusivity

The choice of the drying process plays a key role in reducing the costs of electricity consumption in the food industry. Thus, this study aimed to evaluate continuous and intermittent drying of rough rice, using empirical and diffusion models to describe the drying kinetics, considering only effective time of operation to compare and evaluate these processes. Experiments were carried out during the month of April 2018, in Campina Grande, Paraiba Brazil, and were conducted with continuous and intermittent drying of rough rice grains (about 20 g, each experiment) using a fixed-bed dryer with constant power, at temperatures of 50 and 70 °C. For the intermittent experiments, the intermittency ratio was α = 2/3 and the drying periods were 10 and 20 min, with intermittency periods of 20 and 40 min, respectively. Comparison between continuous and intermittent drying kinetics indicated reduction in the effective time of all intermittent drying experiments, reaching up to 32.2%, hence promoting energy saving. It was also found that a one-dimensional diffusion model with boundary condition of the first kind properly described all rough rice drying processes and that the effective mass diffusivity is higher in intermittent drying, compared to continuous drying at the same temperature.

Structures of laminar lifted flames in a non-premixed jet and their relationship with similarity solutions

The concept of an edge flame structure has been adopted to explain the laminar lifted flames in a non-premixed jet. Two similarity solutions of velocity and fuel concentration have been used to explain flame stability. Recently, it was shown that the experimental results from studies of jet velocities and lift-off heights could be converted to a relationship between the edge flame speed and the fuel concentration gradient. However, this was not sufficient to explain actual structures and behaviors of lifted flames. In this study, related theories and their similarity solutions were improved to get more realistic results through an efficient process. Experiments were conducted with various parameters such as jet velocity, fuel type, tube diameter, air-premixing ratio, and fuel dilution. Effective mass diffusivities and effective Schmidt numbers were estimated based on the experimental results. Using these values, the theoretical basis for a new relationship between the edge flame speed and the fuel concentration gradient was obtained. It was conclusively found that non-monotonic variation in the edge flame speed occurs. From this, flame structures were classified into three regimes based on flame structures and heights: ordinary edge flame in a lower-regime, merged edge flame in a middle-regime, and premixed flame in a higher-regime. Flame speeds of the ordinary edge flames in the lower-regime were selected for comparison with the results of previous studies. From this work, the relationship between the similarity solutions and the experimental results can now be understood in greater detail, and the deviations due to the flame structure can be distinguished.

Self-propelled nanofluids a coolant inspired from nature with enhanced thermal transport properties

Self-propelled nanofluids (SPNFs), are suspensions that contain active particles, that self-propel by converting some form of energy to mechanical work. This theoretical investigation considers the heat transfer mechanisms that may exist in an SPNF. Equations describing the effective mass diffusivity of spherical and rod-shaped particles were taken from the literature and used in analysis of particles of different shapes, aspect ratios, swimming velocities and suspended in water and in ethylene glycol. The analysis showed that the effective mass diffusivity of the particles was up to three orders of magnitude higher than the thermal diffusivity of the solvent. The enhancement of the mass diffusivity leads to a significant enhancement of thermal conductivity of up to an order of magnitude higher compared to that of the pure solvent. It is further discussed that in SPNFs, dispersion and clustering of particles and active turbulence mechanisms may add extra enhancement to the thermal transport. Recent experimental investigations supporting our findings are discussed. Combining this enhancement in thermal transport with the reported reduction of the viscosity observed for an SPNF consisting of Artificial Bacterial Flagella (ABF) particles, will help to create a highly efficient coolant. This can help to reduce energy consumption in a wide variety of the economic sectors, especially in the computing and data storage.