Drying kinetics and evolution of the sample's core temperature and moisture distribution of yam slices (Dioscorea alata L.) during convective hot-air drying

In the present work, the drying kinetics and evolution of sample's core temperature and moisture distribution of yam slices during convective hot-air-drying were investigated. In terms of drying kinetics, the effect of drying temperature (50, 55, 60, 65, 70°C), relative humidity (20, 30, 40, 50%), and sample thickness (5, 7, 9 mm) on drying characteristics of yam slices were studied. Results indicated that all the three factors had significant influence on the drying kinetics, whereas drying temperature gave the most significant effect, followed by relative humidity and sample thickness. Moisture-effective diffusivity and activation energy were calculated, and it was found that the diffusivity was in the range of 5.5454 × 10^?10–1.0804 × 10^?9 m²/s and the activation energy was 29.528 kJ/mol. Heat and mass transfer models were developed based on the finite element method to calculate the core temperature and moisture distribution of yam slices during drying. Model validation exhibited good agreement between predicted and experimental data, which illustrated that the developed models could precisely predict the core temperature profile and moisture distribution of the sample. The current work provides further insights to understand the characteristics and mechanism of drying process of yam slices.     

过热蒸汽携湿在褐煤干燥中的应用

通过调整过热蒸汽温度与流量,得到了理想的褐煤产品,并确定了最佳的携湿参数,为褐煤干燥携湿技术提供了理论依据。     

Assessment of the initial moisture content on soybean drying kinetics and transport properties

Several researchers have developed studies to obtain a mathematical model able to describe grain drying kinetics. However, most of these studies neglect the effect of grain initial moisture content on drying curves. In this study, we assessed the dependence of drying curves and mass transfer coefficients on this initial moisture, air temperature, and its velocity by measuring grain mass losses within time on a tray dryer. Mathematical models were adjusted and results indicated that initial grain moisture content has significant influence on drying curves and mass transfer coefficients.     

The “constant rate period” during the drying of shrinking spheres

Mass transport is described in spheres in which transfer of a component causes a change in volume equal to the (partial) volume of component being removed. Generalized numerical solutions are given for the situation of a constant outside Sherwood-number, a constant driving force and a constant diffusion coefficient in the sphere. A simple graphical method is given predicting the loss of component being removed, until a given concentration at the surface is reached.     

Drying Kinetics and Color Retention of Dehydrated Rosehips

Abstract

Freshly harvested rosehips (Rosa canina L.) were dehydrated in a parallel flow type air dryer at six air temperatures (30, 40, 50, 60, and 70°C) at air velocities of 0.5, 1.0, and 1.5 m/s. Drying air temperature and velocity significantly influenced drying time and energy requirement. Minimum and maximum energy requirement for drying of rosehips were determined as 6.69 kWh/kg for 70°C at 0.5 m/s, and 42.46 kWh/kg for 50°C, 1.5 m/s. In order to reduce drying energy consumption, it is recommended that the drying air velocity must not be more than 0.5 m/s and drying air temperature should be 70°C. In addition, the influence of drying air temperature and air velocity on the color of dried rosehip has been studied. Hunter L, a, b values were used to evaluate changes in the total color difference (ΔE) on dried rosehips. 70°C drying air temperature and 1 m/s air velocity were found to yield better quality product.     

Drying Kinetics and Color Retention of Dehydrated Rosehips

Freshly harvested rosehips (Rosa canina L.) were dehydrated in a parallel flow type air dryer at six air temperatures (30, 40, 50, 60, and 70°C) at air velocities of 0.5, 1.0, and 1.5 m/s. Drying air temperature and velocity significantly influenced drying time and energy requirement. Minimum and maximum energy requirement for drying of rosehips were determined as 6.69 kWh/kg for 70°C at 0.5 m/s, and 42.46 kWh/kg for 50°C, 1.5 m/s. In order to reduce drying energy consumption, it is recommended that the drying air velocity must not be more than 0.5 m/s and drying air temperature should be 70°C. In addition, the influence of drying air temperature and air velocity on the color of dried rosehip has been studied. Hunter L, a, b values were used to evaluate changes in the total color difference (ΔE) on dried rosehips. 70°C drying air temperature and 1 m/s air velocity were found to yield better quality product.     

The Drying Kinetics and Equilibrium Moisture Content of MDF Fibers

An experimental device was constructed to study the drying kinetics of wood fibers under controlled conditions. The device consisted of a drying chamber in which a net basket filled with the fiber material was connected to a load cell. The drying medium was then forced through the basket at controlled levels of humidity and temperature.

Experiments were performed with spruce fibers and the drying medium at varying temperature (50–170°C) and relative humidity (1–86%). In general, the drying rate increased with increasing temperature and decreasing relative humidity. A constant drying rate period was observed in all cases. The critical moisture content was approximately 1.25. The characteristic drying curve has a slight downward concave shape. The equilibrium moisture contents obtained at ambient temperature agree well with data in the literature.     

Frozen mirabelle plum drying: Kinetics, modelling and impact on biochemical properties

The aim of this research was to study and model the kinetics of the hot air drying of frozen mirabelle plums. Effects of temperature (50-85 °C), air velocity (0.6-1.2 m/s) and a pre-treatment (ascorbic acid + sucrose-saturated solution) were investigated. The sorption isotherm and heat of sorption of mirabelle plum were mathematically described respectively by the Guggenheim, Anderson and de Boer (GAB) model and a new one. Depending on temperature and air velocities, the effective diffusivity and the energy of activation were respectively in the range of 8-16.8 × 10⁻¹⁰ m² s⁻¹ and 43.7-55.14 kJ mol⁻¹. To establish the model best adapted to describe this process the fit quality of five models (Newton, page, modified page, logarithmic and diffusion) were evaluated and compared. The diffusion model led to low values of reduced χ-square, mean square root and mean relative deviation modulus with the highest linear regression coefficient. This model was validated in a large range of operating conditions and represents an excellent tool to predict the duration of this process. The pre-treatment preserves biochemical properties of the dried mirabelle plums (reducing sugar content, total phenolic compounds and anti-oxidant activity).     

Influence of sawdust addition on drying of wastewater sludges: Comparison of structural characteristics

ABSTRACT

This work aims to investigate the structural characteristics of wastewater sludges and sludge–wood mixtures, and how the addition of sawdust affects them in a positive way for the drying process. The study was performed for a fixed bed at the pilot scale, with sludges from three different wastewater treatment plants from the Liège province in Belgium, namely, Oupeye, Grosses Battes, and Embourg. X-ray tomography was chosen to evaluate variations in the structural characteristics before and after drying, which included volume, void fraction, and exchange surface of the bed. Results first confirm that the drying rate is raised after the sawdust addition operation for all three sludges, but the increase amount is smaller while the moisture of the sludge is high. With sawdust, the average drying rates increase by factors of 1.475 (Oupeye), 1.342 (Grosses Battes), and 1.162 (Embourg), respectively. It also increases initial volume as well as void fraction and surface exchange, however after drying this addition leads to less shrinkage and void fraction, and higher exchange surface. The initial void fraction increases by 7.25 (Oupeye), 8.30 (Grosses Battes), and 14.01% (Embourg), but the final void fraction decreases 1.56 (Oupeye), 10.83 (Grosses Battes), and 25.61% (Embourg). With high initial moisture content, we observed lower porosity and higher shrinkage.     

Drying Kinetics of Tomato by Using Electric Resistance and Heat Pump Dryers

Drying kinetics of tomato was studied by using heat pump dryer (HPD) and electric resistance dryers with parallel and crossed airflow. The performance of both systems was evaluated and compared and the influence of temperature, air velocity, and tomato type on the drying kinetics was analyzed. The use of HPD showed to be adequate in the drying process of tomatoes, mainly in relation to the conversion rate of electric energy into thermal energy. The heat pump effective coefficient of performance (COP_HT,EF) was between 2.56 and 2.68, with an energy economy of about 40% when compared to the drying system with electric resistance. The Page model could be used to predict drying time of tomato and statistical analysis showed that the model parameters were mainly affected by drying temperature.     

Drying Kinetics of Tomato by Using Electric Resistance and Heat Pump Dryers

Abstract

Drying kinetics of tomato was studied by using heat pump dryer (HPD) and electric resistance dryers with parallel and crossed airflow. The performance of both systems was evaluated and compared and the influence of temperature, air velocity, and tomato type on the drying kinetics was analyzed. The use of HPD showed to be adequate in the drying process of tomatoes, mainly in relation to the conversion rate of electric energy into thermal energy. The heat pump effective coefficient of performance (COP_HT,EF) was between 2.56 and 2.68, with an energy economy of about 40% when compared to the drying system with electric resistance. The Page model could be used to predict drying time of tomato and statistical analysis showed that the model parameters were mainly affected by drying temperature.     

A study on drying models and internal stresses of the rice kernel during infrared drying

Due to the limited penetration of infrared, it is very difficult to develop an infrared drying model of rice kernels. In this study, two kinds of simplified drying models, which assumed the penetration depth is infinity and zero, were developed to investigate the effects of penetration on drying characteristics of thin layer infrared drying. The results show each model can predict temperature and moisture contents (MC) accurately. The maximum temperature difference of rice kernels in both models was always less than 1.5°C, so it is reasonable to exclude the influence of thermal stresses due to nonuniform temperature. This study also developed the internal stresses model with the mechanical properties from literatures. Mechanical properties were with the changes of temperature and MC. These models were solved with COMSOL Multiphysics and there are two stress concentration areas. One is near the surface of the endosperm, another close to the center. Comparison between the von Mises stress distribution and the moisture gradient was made after the simulation. The maximum MC in the endosperm appeared at its surface, which reached 190 1/m at 110?s, and the maximum stress appeared at the same place, which, a little later, reached 7?MPa at 160?s. Moisture gradient at the center was zero due to the existence of symmetry, while there was a significant stress, which reached 3.2?MPa during drying.     

Inhibition of hydrogasification of brown coals by moisture: Influence of heating rate, temperature and pressure

Two types of Rheinische Braunkohle with different mineral matter contents, each with two different moisture contents plus a coke produced from the coal with the lower ash content, were gasified at total pressures between 0.2 and 5 MPa with pure or dry hydrogen, hydrogen/water vapour and argon/water vapour mixtures. In studies with controlled heating (4 K min⁻¹ up to 850 °C) it was found that: 1. methane formation rates and methane yields during gasification in dry hydrogen are drastically lowered with increased moisture of the coals but only at high pressures which reduce evaporation of water; 2. methane formation rates and methane yields during gasification with wet hydrogen (x_H2o = 0.02) are generally lowered with all materials; 3. increasing the water content does not further lower the yields or lead to water vapour gasification. Studies at constant temperature (after rapid heating, 100 K s⁻¹) confirmed these results. It was found that increasing the temperature to 950 °C does not eliminate the inhibiting effect of moisture (in hydrogen) if hydrogen pressure is low ≈ ≤ 1 MPa. It was also determined that raising the temperature above 850 °C with a simultaneous increase in pressure up to 5 MPa hydrogen effectively prevented the inhibition by moisture. It was concluded that extremely stable ether bridges are blocking the active sites at the carbon suface and are therefore responsible for the inhibitory effect of moisture in hydrogasification.     

Correlation between dewatering and hydro-textural characteristics of sewage sludge during drying

The aim of this work consists in analysing the dewatering and the shrinkage of a residual urban sludge during convective drying with the objective to find correlations between dewatering and hydro-textural characteristics of the sludge. Laboratory drying experiments are performed in immobile atmosphere, at a temperature of 30 °C, with various relative humidities. Kinetics curves of dewatering and deformation are obtained. A coupled analysis of dewatering and induced volume shrinkage is proposed. This analysis allows: (i) to distinguish the respective parts played by the intrinsic characteristics of the sludge and the process parameter and (ii) to find correlations between the hydro-textural characteristics of the sludge, and its drying and shrinkage aptitudes. Attention is given to the fact that the process parameter, i.e. relative humidity, controls the first constant rate period of the dewatering and that the hydro-textural state of the sludge determine the transitions between the different dewatering periods. The relationships between the hydro-textural characteristics and the dewatering and shrinkage aptitude allow predicting the sludge behaviour during drying according to its characteristics, which is essential information in the choice of the drying strategy.     

Application of the reaction engineering approach (REA) for modeling intermittent drying under time-varying humidity and temperature

A ‘good’ drying model is important for the design of dryer, evaluation of dryer performance and prediction of product quality. Among the available models, the reaction engineering approach (REA) is a lumped model, proven to be simple, robust and accurate to model drying of several materials. In this paper, the REA is implemented to model intermittent drying, which is usually practiced for saving energy consumption and maintaining product quality during drying, under time-varying drying air temperature and humidity, which is a challenging drying case to model. For this purpose, the equilibrium activation energy (ΔE_v,b) is defined according to the drying settings in each time period and combined with the relative activation energy (ΔE_v/ΔE_v,b) generated from the convective drying experimental data obtained under constant drying conditions. The mass and heat balances also implement the corresponding drying settings in each time period during the intermittent drying. The results indicate that the REA can describe both the moisture content and temperature profiles of the intermittent drying under time-varying drying air temperature and humidity well. The accuracy, simplicity and robustness of the REA for the intermittent drying under time-varying drying air temperature and humidity are proven here. This has provided a major and significant extension of the REA on modeling challenging drying cases.     

Effects of temperature and moisture on the anaerobic digestion of refuse

The influence of the first-tier variable temperature on the anaerobic decomposition of refuse was investigated by use of laboratory models. Temperature was found to exert significant effects on the interrelated metabolic processes of acidogenesis, solventogenesis and methanogenesis. A temperature elevation from a mean ambient regime (18.7°C) to 30°C resulted in a 2.6-fold increase in the methanogenic rate while a 7.8-fold increase was observed when the temperature was raised from 18.7 to 40°C. The rate of methanogenesis at 40°C was approximately three-fold higher than at 30°C. Further temperature elevation to 55°C, however, resulted in the cessation of methanogenesis although solventogenesis was promoted. An attempt was made to extrapolate the results obtained to actual landfill, with specific reference to the role of refuse density in thermogenesis.     

Analysis of the Drying Kinetics of Brachiaria brizantha (Hochst. Stapf) Grass Seeds at Different Drying Modes

Forage grass seeds have a high economical importance in the Brazilian bovine cattle breeding and seed drying is a fundamental stage of processing to guarantee their stability and allow their storage for long periods. In this context, the objective of this work was to develop a pilot-scale belt dryer, which operates also as a fixed and fluidized bed. Brachiaria brizantha seeds dehydration was analyzed under different air velocities and temperatures. Experimental data of moisture content variation along the drying time was successfully fit to a one-term exponential model. The experimental drying rate points were calculated by approximating the derivatives to finite differences. Its behavior was accomplished fitting curves of the drying rate versus water content and time. Finally, fissure, germination, and vigor rates were analyzed as a function of the drying conditions so that the experimental conditions combine the best process efficiency with the best physiological quality maintenance.     

Analysis of the Drying Kinetics of Brachiaria brizantha (Hochst. Stapf) Grass Seeds at Different Drying Modes

Forage grass seeds have a high economical importance in the Brazilian bovine cattle breeding and seed drying is a fundamental stage of processing to guarantee their stability and allow their storage for long periods. In this context, the objective of this work was to develop a pilot-scale belt dryer, which operates also as a fixed and fluidized bed. Brachiaria brizantha seeds dehydration was analyzed under different air velocities and temperatures. Experimental data of moisture content variation along the drying time was successfully fit to a one-term exponential model. The experimental drying rate points were calculated by approximating the derivatives to finite differences. Its behavior was accomplished fitting curves of the drying rate versus water content and time. Finally, fissure, germination, and vigor rates were analyzed as a function of the drying conditions so that the experimental conditions combine the best process efficiency with the best physiological quality maintenance.     

Numerical approach to describe continuous and intermittent drying including the tempering period: Kinetics and spatial distribution of moisture

Continuous and intermittent drying experiments were performed with whole bananas, using hot air at 70°C. The intermittent drying experiments were performed with intermittency ratio equal to 1/2 and tempering times of 0.5, 1.0, and 2.0?h. The conditions imposed to the experiments permitted to investigate the influence of these tempering times on the processes. A one-dimensional numerical solution of the diffusion equation coupled with an optimizer was used to determine the process parameters for four experiments. To describe the processes, a model was proposed. Model includes shrinkage, variable effective mass diffusivity, and two values for convective mass transfer coefficient (within and outside the dryer), enabling to consider moisture loss during the tempering period. For all experiments, the simulation of the drying kinetics has resulted in good statistical indicators. Proposed model also made it possible to predict moisture distributions during the entire processes, including the migration of moisture from the central part to the peripheral region of the cross section of the bananas, during the tempering period. The results indicated that, for the same effective operation time and intermittency ratio, increasing the tempering time implied moderate decrease in the final average moisture content.     

Drying of Citrus sinensis Peels in an Inert Fluidized Bed: Kinetics,Microbiological Activity,Vitamin C,and Limonene Determination

Citrus sinensis peel drying kinetics in a fluidized bed with inert material were investigated. Drying impact on microbiological activity as well as limonene and vitamin C content was also studied. Drying parameters studied were as follows: temperatures of 40°C, 50°C, and 60°C; air velocities of 0.73 m/s and 0.85 m/s; and orange peel:sand weight ratios of 1:0, 1:1, and 1:2. High temperatures, high air velocity, and the presence of inert material increased the drying rate. Nine thin-layer drying models were fitted to the experimental data of Citrus sinensis peels. The Midilli et al. model was found to be the most suitable model for describing the drying kinetics of Citrus sinensis peels. Vitamin C and limonene content were higher in the product dried using a fluidized bed than in the sun-dried product. Drying of Citrus sinensis peel in a fluidized bed also may reduce microorganism growth, increasing storage life.