Falling Rate Period Research Articles

Internal and surface temperature profiles of spherical biosolids particles during convective drying

The convective drying of spherical biosolids particles from two wastewater treatment plants (Type 1: anaerobic/anoxic treatment with belt filter press dewatering; Type 2: aerobic/anoxic treatment with centrifuge dewatering) was studied under 10 conditions across 44 trials. A wide spectrum of conditions including three drying temperatures (88, 109, and 138 ° C ), two gas velocities (1.6 and 2 m s − 1 ), and particle sizes of 2 cm and 4 cm were investigated. Measured variables included particle mass, center/internal temperature, surface temperature, shrinkage, and qualitative observations of morphological changes. Most samples showed two drying periods, a constant rate period followed by a falling rate period. Additionally, it was found that drying rates and morphological changes depended significantly on particle size and drying intensity (i.e., drying gas temperature and gas velocity) with higher intensities resulting in larger, more porous particles. Internal temperature profiles displayed a consistent pattern, an initial heating phase where internal temperatures quickly rose to approximately the wet bulb temperature of the drying gas, a quasi-constant temperature phase at that temperature, and a final sharp increase/spike to reach equilibrium with the drying gas temperature. Moreover, IR images showed that sub-surface temperatures (between the core and surface) were significantly lower than surface temperatures. Surface temperatures showed a more linear increase throughout drying until equilibrium with the drying gas temperature. The consistent temperature difference between the center and surface, as well as the internal temperature profile, suggests the existence of a drying front, moving radially inward as the particle dries. This research contributes to a better understanding of biosolids drying dynamics, offering insights to improve thermal treatment strategies thereby contributing to the broader field of waste management.

Study on volumetric heat and mass transfer coefficient in a spray drying process utilizing pressure swirl nozzle: A numerical and experimental approach

This paper applies a sectional and cumulative approach to estimate the volumetric heat and mass transfer coefficients (hV and kV) to characterize the behaviour of the spray drying process. The investigation utilizes a tailor-made spray dryer equipped with a pressure swirl nozzle with a 0.2 mm orifice diameter, utilizing Mannitol solution as the precursor and air as the carrier gas. A comprehensive numerical analysis is conducted using the Adaptive Mesh Refinement (AMR) technique alongside selective experiments. The study considers precursor mass flow rates of 1 g/s, 1.2 g/s, and 1.5 g/s, along with carrier gas velocities of 0.88 m/s and 1.19 m/s at a temperature of 250 °C. Comparisons between predicted and estimated hV and kV are performed, leading to the proposal of a method for identifying constant rate and falling rate periods. The analyses contribute to delineating the solvent evaporation zone, solid particles heating zone, spray penetration length, and drying length. Additionally, experimental temperature measurements in both axial and radial directions at 12 grid points in the chamber are conducted to validate the numerical results, showing acceptable deviation.

Chitosan-Based Oleogels: Emulsion Drying Kinetics and Physical, Rheological, and Textural Characteristics of Olive Oil Oleogels.

Oleogels are of high interest as promising substitutes for trans fats in foods. An emulsion-templated method was used to trap olive oil in the chitosan crosslinked with vanillin matrix. Oil in water emulsions (50:50 w/w) with different chitosan content (0.7 and 0.8% w/w) with a constant vanillin/chitosan ratio (1.3) were air-dried at different temperatures (50, 60, 70, and 80 °C) and freeze-dried (-26 °C and 0.1 mbar) to produce oleogels. Only falling rate periods were determined during air-drying kinetics and were successfully modeled with empirical and diffusional models. At a drying temperature of 70 °C, the drying kinetics were the fastest. The viscoelasticity of oleogels showed that the elastic modulus significantly increased after drying at 60 and 70 °C, and those dried at 50 °C and freeze-dried were weaker. All oleogels showed high oil binding capacity (>91%), but the highest values (>97%) were obtained in oleogels with a threshold elastic modulus (50,000 Pa). The oleogels' color depended on the drying temperature and chitosan content (independent of the drying method). Significant differences were observed between air-dried and freeze-dried oleogels with respect to oxidative stability. Oxidation increased with the air-drying time regardless of chitosan content. The found results indicated that drying conditions must be carefully selected to produce oleogels with specific features.

Enhancing sludge hot-pressing drying by sawdust: Sensitivity analysis of heat and mass transfer physical properties of sludge, process performance, and kinetics

Hot-pressing drying shows promise in sludge treatment. Optimization often focuses on temperature and external load. However, the impact is restricted by sludge inherent properties, which can be changed by sludge conditioning. Due to coupling effects, altering individual sludge properties is difficult. Key sludge properties that significantly affect heat and mass transfer remain unclear, hampering identification of specific conditioning techniques. To address this gap, the effects of sludge specific heat, thermal conductivity, water activity, moisture diffusivity, and moisture permeability on drying performance were explored using sensitivity analysis. Sawdust conditioning and kinetics were employed to improve drying performance, validate sensitivity analysis findings, and elucidate drying mechanisms. Sensitivity analysis revealed that liquid water permeability significantly positively influenced drying performance by enhancing liquid water convection. Sawdust conditioning and kinetics demonstrated that adding 20 % sawdust increased drying flux from 31 to 62 kgwater·m⁻²·h⁻¹. The drying process included warm-up and constant rate, the first falling rate, and the second falling rate periods. Enhanced moisture transfer was dominated by breakthrough of interfacial gas film, transformation from liquid water diffusion to convection, and facilitated water vapor diffusion, respectively. Hence, this study provided novel insights, strategies, and direction of guidance for revealing sludge drying mechanisms and improving drying performance.

Evaporation Mechanisms and Heat Transfer in Porous Media of Mixed Wettabilities with a Simulated Solar Flux and Forced Convection Through the Media

Abstract An experimental apparatus was designed to study the impacts of wettability on evaporation of water for: 1) a 5.7-cm-thick layer of hydrophilic Ottawa sand; 2) a 5.7-cm-thick layer with 12% hydrophobic content, consisting of a 0.7-cm-layer of n-Octyltriethoxysilane-coated hydrophobic sand buried 1.8 cm below the surface of hydrophilic sand; and 3) a 5.7-cm-thick layer with mixed wettabilities, consisting of 12% n-Octyltriethoxysilane-coated hydrophobic sand mixed into hydrophilic sand. The sand-water mixtures experienced forced convection above and through the sand layer, while a simulated solar flux (i.e., 112 ± 20 W/m2) was applied. Evaporation from homogeneous porous media is classified into the constant-rate, falling-rate, and slow-rate periods. Wettability affected the observed evaporation mechanisms, including the transition from constant-rate to falling-rate periods. Evaporation entered the falling-rate period at 12%, 20%, and 24% saturations for the all hydrophilic sand, hydrophobic layer, and hydrophobic mixture, respectively. Wettability affected the duration of the experiments, as the all hydrophilic sand, hydrophobic layer, and hydrophobic mixture lasted 17, 20, and 26 trials, respectively. Both experiments with hydrophobic particles lasted longer than the all hydrophilic experiment and had shorter constant-rate evaporation periods, suggesting hydrophobic material interrupts capillary action of water to the soil surface and reduces evaporation. Evaporation fluxes were up to 12× higher than the vapor diffusion flux due to enhanced vapor diffusion and forced convection.

Green tea drying in a closed cycle continuous vibro fluidized bed dryer with dehumidified air

There is potential for the development of zeolite as an adsorbent material for drying high-value items. This study aims to compare the green tea drying characteristic utilizing vibro-fluidized bed dryers that operate without dehumidification and with dehumidification. The research was carried out experimentally. The drying chamber was set at temperatures of 50oC, 60oC, and 70oC. In the drying with dehumidification setup, zeolite was placed in a dehumidifier with a weight-to-tea leaf ratio of 1:2.86, 1:3.33, 1:4, and 1:5. The results showed that zeolite's impact on decreasing drying time is effective at a specific temperature, in this case is 60oC. The effect is negligible at low temperatures (50oC), while at high temperatures (70oC) it is minor and drying time is primarily influenced by temperature. Utilizing zeolite for dehumidification can enhance the rate of drying during the falling rate period.

Far‐infrared drying influence on machine learning algorithms in improving corn drying process with graphene irradiation heating plates

AbstractDrying methods often suffer from appropriate drying process modeling, low heating efficiency, and high‐energy consumption. However, graphene may provide significant improvements during the drying process due to its ability to save energy and convert electro‐thermal energy effectively. In this study, random forest (RF), surface vector machine (SVM), artificial neural network (ANN), and k‐nearest neighbor (kNN) were the four machine‐learning algorithms employed to analyze the drying process of two types of corn (ZD88 and RP909) at varying temperatures of 35, 45, and 55°C in a graphene far‐infrared drying. The study found that the drying process of corn showed a decrease in moisture ratio during the falling rate period from 35 to 55°C. The moisture diffusivity increased from 35 to 55°C with the rise in the temperature, ranging from 2.74 to 4.36 × 10−8 m2/s for ZD88 and 2.05 to 3.07 × 10−8 m2/s for RP909. The ZD88 showed a heat efficiency of 81.62% at 55°C. The results of the machine learning algorithms revealed that 1–6 ANN was the best architecture. Normalized PUK was the best for SVM filters. A k = 3 and s‐fold = 5 were the best values for k‐NN and RF, respectively. Among the computed algorithms, the tested data for normalized Pearson universal kernel SVM filters proved to be the most effective in computing the process of drying corn. Overall, a graphene far‐infrared dryer showed improved heating efficiency and less energy consumption, providing a new concept for developing a drying process through machine learning algorithms strategies.Practical applicationsDrying methods often suffer from appropriate drying process modeling, low heating efficiency, and high‐energy consumption. This study provides significant improvements due to the ability of graphene irradiation heating plates to save energy and convert electro‐thermal energy effectively. The study achieved a heating efficiency of 81.62% at an infrared temperature of 55°C. Machine learning algorithms are intelligent models that minimize the limitations in describing the drying process of agricultural materials. The study found the normalized Pearson universal kernel was the most effective computation SVM filter. Therefore, using a graphene far‐infrared dryer showed a better drying alternative for industrial applications, as it has the potential to improve heating efficiency and consume less energy, providing a new concept for developing drying equipment through machine learning algorithms modeling strategies.

Tray Dryer's Performance in the Drying of Banana Slices Using LPG and Wood Stove

An evaluation was carried out on a simple tray dryer of banana slices in the "sale pisang" production. The dryer is designed to use gas fuel and a wood-burning stove as a source of energy. The dryer's performance is analyzed to evaluate its ability to dry the product efficiently and effectively to prevent excessive heat use. The research aims to analyze the temperature distribution in the drying chamber, the profile of the product's moisture content, the drying rate on each shelf, as well as the efficiency of dryers using gas fuel and wood-burning stoves. At an air speed of 0.0176 m3/s, the drying chamber reaches a maximum temperature of 76.2 ℃ using gas and 66.23 ℃ using a wood stove. The use of Liquid Petroleum Gas (LPG) gives an average room temperature of 73.46 ℃, while the furnace gives a less stable pattern with an average temperature of 63.02 ℃. Drying using LPG occurs more quickly with a constant drying rate and a falling rate period of 0.8262%/hour and 0.01504%/hour, higher than using wood fuel which is 0.5482%/hour and 0.0098 %/hour. Within 400 minutes, the product’s moisture content reached 24.64% using LPG and 36.762 using the furnace. The efficiency for heating the drying air is 28.51% and the drying process efficiency is 27% using 8.6 kilograms of LPG as fuel. Meanwhile, using a furnace energy source, the dryer provides a heating efficiency of 15.9% and a drying efficiency of 7.37% with a fuel consumption of 34.12 kg of wood.

Drying kinetics modeling of hot air drying of emulsion templated oleogels employing hydroxypropyl methylcellulose as structuring agent

This work focused on the drying of O/W emulsions employed to obtain templated structured oils with hydroxypropyl methylcellulose (HPMC) as oleogelator. Sunflower oil/water emulsions with different HPMC content (1%–3% w/w) were dried at different air temperatures (from 70 to 100 °C) employing two different initial thicknesses (0.25 and 0.50 cm). Drying kinetics showed the existence of an initial constant drying rate period followed by a falling drying rate period below critical moisture content. This critical content was constant (0.38 kg water/kg dry solid) independently of drying conditions and HPMC content. Drying rate decreased linearly with moisture content during the falling rate periods. A 3-parameter model was proposed to simulate drying kinetics in the range of drying temperature and HPMC content of tested oleogels. Physical properties of fresh and stored (20 days) oleogels such as oil binding capacity, color, and hardness were measured to evaluate drying conditions effect on product quality. Temperatures above 70 °C were necessary to promote the HPMC gelation. Best characteristics of oleogels in terms of texture, color and oil binding capacity were obtained with HPMC content of 2% w/w, 0.25 cm of emulsion thickness and 80 °C of drying temperature. These results are useful for the design of large-scale dryers for this new kind of products.

An energy-efficient solution to sludge drying and combustion process through Camellia oleifera shells amended foaming

Foaming pretreatment has been proven effective in promoting sludge drying, however, the variation in sludge properties significantly influences the foaming efficiency. Inspired by foam stabilizer of solid particles, Camellia oleifera shells (COS) was screened out from various biomasses as an additive incorporated with the CaO for promoting the sludge foaming. For the introduction of COS, this study analyzed the drying behaviors of foamed sludge, quantified the surface cracks information, characterized the combustion performance, and evaluated the energy consumption. The results indicated that 46.72–50.10% of time could be saved in foaming the sludge to 0.70 g/mL by addition of 3.0 wt% COS. Compared with the original sludge (OS), the 0.70 g/mL foamed sludge saved 47.43% of time for sludge drying at 80 °C, and this value further increased to 53.14% with 3.0 wt% COS addition. Combining the multifractal spectra and drying kinetics analysis, the foaming promoted the formation of complex surface cracks in the warm-up period, while COS further improved the complexity of cracks in the constant rate period, and the shrinkage of isolated sludge blocks in the falling rate period, thus enhanced the moisture diffusion and heat transfer. Furthermore, the appropriate porous structure and additional volatile matters promoted the combustion performance. The 0.90 g/mL foamed sludge with COS presented the lowest activation energy of 180.362 kJ/mol in combustion. Overall, compared with OS, the 0.70 g/mL foamed sludge with COS saved 40.65% energy consumption during the foaming, drying and combustion processes, providing an energy-efficient solution for the sludge treatment and disposal.

Enhancing mass transfer and preserving heat-sensitive quality of mango through foam mat drying

Mango puree was foamed using hydroxypropyl methylcellulose to achieve foam densities ranging from 0.3 to 0.8 g/cm3. Subsequently, these mango foams were dried at temperatures ranging from 60 to 80 °C. The experimental results showed that mango foam drying was in the falling rate period. The effective moisture diffusivity (D eff ) was well described by the Arrhenius equation, with the pre-exponential factor (D 0) and activation energy (Ea ) closely related to the void area fraction in mango foam. This relationship led to higher D eff and shorter drying times at higher void area fractions. The minimum loss of total phenolics content (TPC) was observed in mango foams with an initial foam density of 0.3 g/cm3 dried at 60ºC, with only a 7% loss. Increasing drying temperatures to reduce drying time did not mitigate TPC loss. Similarly, the minimum reductions in antioxidant activities, as measured by both the 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid and ferric reducing antioxidant power assays, were observed in mango foams with an initial foam density of 0.3 g/cm3 dried at 60 °C, with values of 14% and 18%, respectively. Dried mango foams with lower densities exhibited more pronounced color changes due to the presence of more sponge-like voids. However, the drying temperature did not significantly affect the product color. In addition, dried mango foams with lower initial foam densities displayed a more brittle and spongy texture.

Drying kinetics and quality attributes analysis of moringa (Moringa oleifera) leaves under distinct drying methods

AbstractThe present work aimed to analyze the drying kinetics and color degradation of moringa leaves in selected drying methods. Also, quantitatively analyze the retention of phytochemicals and antioxidant capacity. Moringa leaves were dried using three methods, namely, freeze‐drying (−42 ± 2°C, 0.024 ± 0.001 mbar), microwave drying (200, 400, and 600 W), and hot air oven drying (55, 65, and 75°C). Drying kinetics revealed that the drying of moringa leaves occurred in the falling rate period, which means the diffusion process controlled the moisture removal from leaves. The color degradation was most minor in freeze‐dried leaves, followed by microwave‐dried leaves at 200 W. The value of total phenolic content was found maximum (57.42 ± 2.49 mg GAE/g dm) under microwave drying at 200 W, while the same was found least in quantity (24.62 ± 1.10 mg GAE/g dm) for samples dried under hot air oven drying at 65°C. A similar pattern was observed for total flavonoid content. The antioxidant capacity of the samples dried in the freeze dryer was highest at 5.94 ± 0.25 mg GAE/g dm. This research revealed that microwave drying at 200 W is best regarding the overall quality attributes of moringa leaves.Practical ApplicationsDrying is one of the essential processes in the food industry, for example, drying is crucial for the production of herbal medicines, nutritional supplements and extracts, leaf‐based culinary ingredients, aroma and essential oils, natural dyes and pigments, etc. It is used to increase the shelf‐life of agricultural produce; it facilitates the transportation and storage of food products. This paper may help in the selection of the drying method and condition for leaves drying to have desired characteristics in the final product like color, bioactive compounds, or antioxidant properties; May help in the trade‐off among quality, time, and cost of the production. Moreover, it may be helpful in the designing of the multi‐mode dryer which may produce superior quality products with great energy efficiency in less time. Which further facilitates the manufacturing of nutritionally rich food products.

Mentha spicata drying via infrared: Mathematical modeling and evaluation of two‐step temperature potential for improving drying characteristics

AbstractTo assess the effects of a two‐step temperature method (TTM) on dehydration parameters, color deterioration of dried samples, and specific energy consumption (SEC), spearmint leaves were dried using infrared (IR) undergoing the TTM. Moreover, 14 mathematical models were compared based on the adjusted correlation coefficient, reduced chi‐square, and root mean square error to estimate drying curves. Changes in the color of the dried product at two‐step temperatures of 70to60°C, 60to50°C, and 50to40°C led to decreases of 2.69, .81, and 3.34 compared to fixed temperatures of 70, 60, and 50°C, respectively. On the other hand, the specific energy consumption (SEC) increased by 5.43, 10.92, and 22 MJ.g−1 H2O at 70to60°C, 60to50°C, and 50to40°C compared to 70, 60, and 50°C, respectively. To interpret the results from another point of view, three basic objectives of shortening drying time, reducing energy consumption, and raising the quality of the dried product are achieved in the TTM. For instance, the drying time and energy consumption are reduced at a two‐step temperature of 70to60°C compared to the fixed temperature of 60°C, along with a decrease in the color change compared to the fixed temperature of 70°C. The results indicated that Midilli et al. model resulted in the best fitness in explaining the IR drying behavior of spearmint leaves.Practical applicationsDuring the falling rate period in drying process and especially at higher temperatures and powers, there is a potential for a decrease in the quality of the dried product, and even the risk of thermal runaway due to excessive temperature increase. Certainly, at lower powers and temperatures, the quality of the dried product can be enhanced. However, in this situation, the duration of the process and the energy consumed, which are two crucial factors in the drying process, increase, leading to financial losses for the producer. In the two‐step temperature method (TTM), due to the temperature reduction during the critical period of falling rate, the quality of the dried product improves by considering the processing time and energy consumption.

Multi-agent simulation of doughnut deep fat frying considering two domain heating media and sample flipping

Two domain heating media and sample flipping during processing were considered when developing an agent-based model to explain coupled heat and mass transfer phenomena during deep fat frying of doughnuts. The model was validated by comparing the moisture content, oil content and temperature profiles obtained from the experimental results with those obtained from the model. Results of this study showed that the water content of crumb raised to 60% (based on dry weight) whereas, it decreased to less than 10% in the case of doughnut crust during deep fat frying. Simulated profile of oil penetration illustrated that the oil content of different parts of crust were not equal and were affected by frying temperature and crust structure. In general, as the surface of doughnut (a porous material) was heated from the surface, evaporation zones were formed in the thinner parts of the crust and gradually formed oil penetrating areas. Moreover, experimental and simulated data indicated that flipping of samples in the middle of processing time had an important effect on heat and mass transfer during frying. Variation of thermophysical properties in each part of doughnut had a unique behavior. The changes in the density, specific heat capacity and thermal conductivity of crumb followed a sigmoid pattern; whereas, a dominant falling rate period with some variations was observed in crust. Moreover, any changes in moisture content and temperature of crust occurred faster than the crumb. The output of simulation was in a good agreement with the experimental data. With the power of simulation now available for design, the results of this study greatly improve the design of fried foods and frying processes.

Investigation of the moisture classification and moisture removal behavior of diamond wire saw silicon powder waste cake

When processing silicon wafers for photovoltaic power generation, approximately 35 % of high-purity silicon is lost as a diamond wire saw silicon powder (DWSSP) waste cake with a high moisture content. The high moisture content of DWSSP results in the oxidation of high-purity silicon, which hinders the recovery and utilization of silicon resources. Disappointingly, a method for the removal of moisture from the DWSSP waste cake, which is crucial for silicon recovery, has not yet been revealed. In the present study, a novel investigation was conducted to determine the moisture classification of a DWSSP waste cake and to reveal the moisture removal behavior. The results indicate that the moisture content in the DWSSP waste cake can be divided into two distinct categories: surface adsorbed water and capillary water. Moreover, the kinetics analysis and simulation demonstrated that the elimination of moisture predominantly takes place during the constant-rate period and the falling-rate period. Increasing the drying temperature and reducing the equivalent diameter of the DWSSP waste cake will contribute to improving the efficiency of moisture removal. The application of the findings of this study can help to reduce the harmful environmental impacts of DWSSP and achieve the efficient recovery of silicon resources.

Comparative Study On Shade Drying, Sun Drying And Direct Solar Drying Of Mint Leaves

Aromatic herbs are in high demand in food processing industries, Cosmetics, biotechnology and pharmaceuticals. Mint leaves can be used as a flavoring agent in food; they are also valued for their antibacterial and antioxidant properties. Mints (Mentha spp.) are a well-known medicinal and aromatic plant. The mint leaves are used as fresh due to high moisture content. Drying helps to inhibit bacterial growth and decrease water activity. The purpose of this study was to enhance their shelf life and save packaging and shipping costs. A comparative study on the drying process of mint leaves was conducted under shade drying, sun drying and direct solar dryer. Fresh mint leaves were collected from the medicinal block of Krishi Vigyan Kendra Vaishali, Bihar, India. They were washed and the excess amount of water was removed from the surface of the leaves. The various parameters recorded hourly in the experiments were temperature and relative humidity inside and outside the direct solar dryer; the weight of the mint leaves was recorded hourly. Temperature plays a great impact in the drying process. The maximum temperature reached in solar drying up to 560 C as compared to 420 C in sun drying. The minimum relative humidity reached in the direct solar dryer was 17 percent as compared to 28 percent in sun drying.The drying curve data suggest that the drying displays a falling rate period. Henderson pabis model was fitted for drying processes in shade drying, sun drying and direct solar drying. The highest sensory score is reached with color (8.7), flavor (8.5), appearance (9) and overall acceptability (8.7) when dried in a Solar dryer. To fabricate a low -cost environment friendly solar dryer is quite a challenging task. The solar dryer was built by locally available ply-wood and low- cost materials. The dryer consists of transparent glass, exhaust fan, wire mesh screen, solar cell. Another challenge is to use the maximum solar radiation inside the solar dryer. Inclination angle of the Transparent glass was made 15o to get maximum radiation. Future scope: World is suffering from harmful gases originated from burning of fossil fuels. Solar energy is a safer and cleaner energy source. Solar dryer can also reduce up to 34 % CO2 emission. This study aims to maintain best sensory quality of dehydrated herbs and spices.it is also aiming for low -cost clean drying technology and sustainable development.

Convective drying of papaya (Carica papaya L. 'Red Maradol') and banana (Musa acuminata (AAA Group) 'Gros Michel')

In this study, the drying kinetics of papaya (Carica papaya L. 'Red Maradol') and banana (Musa acuminata (AAA Group) 'Gros Michel') was experimentally investigated in a lab-scale tunnel dryer. The drying experiments were performed at three air temperatures (50, 60, and 70 °C) and three air velocities (1.0, 1.5, and 2.0 m/s). A surface area shrinkage linear model from the literature was used to include the shrinkage effect on the drying process. From the drying curves, no constant rate period was observed and drying occurred in a falling rate period. It was found that the changes in air velocity had a slight effect on the drying process. In addition, a non-linear regression analysis was employed to determine the characteristic drying curve.

Convective drying of sweet pepper (Capsicum annuum L. 'Tres Cantos') and yellow onion (Allium cepa L. 'Yellow Granex F1')

In this study, the drying kinetics of sweet pepper (Capsicum annuum L. 'Tres Cantos') and yellow onion (Allium cepa L. 'Yellow Granex F1') was experimentally investigated in a lab-scale tunnel dryer. The drying experiments were performed at three air temperatures (50, 60, and 70 °C) and three air velocities (1.0, 1.5, and 2.0 m/s). A volumetric shrinkage linear model from the literature was used to include the shrinkage effect on the drying process. From the drying curves, no constant rate period was observed. The drying process took place in a falling rate period. It was found that the changes in air velocity had essentially no effect on the drying process. In addition, a non-linear regression analysis was employed to determine the characteristic drying curve.

Thin-layer mathematical modelling of maize in a biomass-powered inclined bed dryer

In this study, the drying kinetics of maize dried in a biomass-powered inclined bed dryer were studied using ten (10) thin-layer mathematical drying models. The drying system recorded an average plenum temperature of 73.54 oC during the drying experiment compared to an average ambient temperature of 28.41 oC. Maize grains with an initial average moisture content of 23.25 % on a wet basis were reduced to 13.61 % average final moisture content over a drying period of 2 hours 40 minutes. The results revealed a moisture extraction rate, drying rate, and drying efficiency of 6.70 kg/h, 9.50 kg/h, and 71.37 %, respectively. The two-term model best describes the thin-layer drying kinetics of maize in the biomass-powered inclined bed dryer based on the coefficient of determination (R2) and root mean square error (RMSE) values of 0.998 and 0.00738, respectively. The two-term model showed a better fit between the experimental and the predicted moisture ratios. The drying process occurred in the falling rate period with an effective moisture diffusivity of 4.65x10-9 m2/s and activation energy of 21.31 kJ/mol. The two-term model was able to imitate the behaviour of the drying process of maize in the drying system. The model would assist in predicting the drying time for different moisture contents of maize in the scale-up of the drying system and, accordingly, help farmers and agro-processors in planning drying schedules. The drying kinetics of other staples like cassava and rice are recommended for further studies using the biomass-powered inclined bed dryer.

Analysis of Drying Front Propagation and Coupled Heat and Mass Transfer During Evaporation From Additively Manufactured Porous Structures Under a Solar Flux.

Drying front propagation and coupled heat and mass transfer analysis from porous media is critical for soil-water dynamics, electronics cooling, and evaporative drying. In this study, de-ionized water was evaporated from three 3D printed porous structures (with 0.41 mm, 0.41 mm, and 0.16 mm effective radii, respectively) created out of acrylonitrile butadiene styrene (ABS) plastic using stereolithography technology. The structures were immersed in water until all the pores were invaded and then placed on the top of a sensitive scale to record evaporative mass loss. A 1000 W/m2 heat flux was applied with a solar simulator to the top of each structure to accelerate evaporation. The evaporative mass losses were recorded at 15 min time intervals and plotted against time to compare evaporation rates from the three structures. The evaporation phenomena were captured with a high-speed camera from the side of the structures to observe the drying front propagation during evaporation, and a high-resolution thermal camera was used to capture images to visualize the thermal gradients during evaporation. The 3D-structure with the smallest effective pore radius (i.e., 0.16 mm) experienced the sharpest decrease in the mass loss as the water evaporated from 0.8 g to 0.1 g within 180 min. The designed pore structures influenced hydraulic linkages, and therefore, evaporation processes. A coupled heat-and-mass-transfer model modeled constant rate evaporation, and the falling rate period was modeled through the normalized evaporation rate.