Analysis of the Shrinkage Effect on Mass Transfer During Convective Drying of Sawdust/Sludge Mixtures

Convective drying of wastewater sludges and sawdust/sludge mixtures was studied. The first part of this work was an experimental study performed in a cross-flow convective dryer using 500 g of wet material extruded through a disk with circular dies of 12 mm. The results showed that the sawdust addition has a positive impact on the drying process from a mass ratio of 2/8, on a dry basis, with observed drying rates higher than the original sludge. The second part of this work consisted of developing a drying model in order to identify the internal diffusion coefficient and convective mass transfer coefficient from the experimental data. A comparison was made between fitted drying curves, well represented by the Newton's model, and the analytical solutions of the diffusion equation applied to a finite cylinder. Variations of dimensional characteristics, such as the volume and exchange surface of the sample bed, were obtained by X-ray tomography. This technique allowed us to confirm that shrinkage, which is an important phenomenon occurring during sludge and sawdust/sludge mixture drying, must be taken into account. The results showed that both the internal diffusion coefficient and convective mass transfer coefficient were affected by mixing and sawdust addition. The internal diffusion coefficient changed from 7.77 × 10^?9 m²/s for the original sludge to 7.01 × 10^?9 m²/s for the mixed sludge and then increased to 8.35 × 10^?9 m²/s for the mixture of a mass ratio of 4/6. The convective mass transfer coefficient changed from 9.70 × 10^?8 m/s for the original sludge to 8.67 × 10^?8 m/s for the mixed sludge and then increased to 12.09 × 10^?8 m/s for the mixture of a mass ratio of 4/6. These results confirmed that sawdust addition was beneficial to the sludge drying process as the mass transfer efficiency between the air and material increased. Reinforcing the texture of sludge by adding sawdust can increase the drying rate and decrease the drying time, and then the heat energy supply will be reduced significantly. The study also showed that neglecting shrinkage phenomenon resulted in an overestimation for the internal diffusion coefficient for the convective drying of sludges and sawdust/sludge mixtures.     

Microwave drying of wastewater sludge: Experimental and modeling study

This study investigates experimentally and using mathematical modeling the microwave drying of wastewater sludge with determination of moisture diffusivity at different drying conditions. The drying behavior was observed at different power levels (480, 840, and 1,080 W) and different initial masses (90, 120, and 150 g). The observed drying kinetics were divided into three parts: a short adaptation period, a long constant drying rate period, and a falling drying rate period. The maximum drying rate was observed during the constant rate period. Mainly, the results show that the drying rate decreases with the initial mass increase (from 0.45 kg·kg^?1·min^?1 for 90 g to 0.25 kg·kg^?1·min^?1 for 150 g) and increases with an increase in power level (from 0.15 kg·kg^?1·min^?1 at 480 W to 0.45 kg·kg^?1·min^?1 at 1,080 W). The measurement of the sample dimensions shows that shrinkage can occur and, depending on the drying conditions, it ranged between 0.42 and 0.37 of the sample initial volume. Presenting a more accurate solution of the diffusion model by incorporating shrinkage and finite dimensions of the sample is the novelty of this study. The drying conditions influenced the diffusion coefficient, which ranged from 1.53 × 10^?7 to 7.67 × 10^?7 m²s^?1. Similar to the drying rate, the diffusion coefficient was directly proportional to the power level and inversely proportional to the initial mass. Activation energy was determined using an Arrhenius relationship of the diffusion coefficient as a function of the ratio initial mass to the power level.     

Improvement of Convective Drying of Carrot by Applying Power Ultrasound—Influence of Mass Load Density

Power ultrasound is considered to be a novel and promising technology with which to improve heat and mass transfer phenomena in drying processes. The aim of this work was to contribute to the knowledge of ultrasound application to air drying by addressing the influence of mass load density on the ultrasonically assisted air drying of carrot. Drying kinetics of carrot cubes were carried out (in triplicate) with or without power ultrasound application (75 W, 21.7 kHz) at 40°C, 1 m/s, and several mass load densities: 12, 24, 36, 42, 48, 60, 72, 84, 96, 108, and 120 kg/m³. The experimental results showed a significant (p < 0.05) influence of both factors, mass load density and power ultrasound application, on drying kinetics. As expected, the increase of mass load density did not affect the effective moisture diffusivity (D_e, m²/s) but produced a reduction of the mass transfer coefficient (k, kg water/m²/s). This was explained by considering perturbations in the air flow through the drying chamber thus creating preferential pathways and, as a consequence, increasing external mass transfer resistance. On the other hand, it was found that the power ultrasound application increased the mass transfer coefficient and the effective moisture diffusivity regardless of the mass load density used. However, the influence of power ultrasound was not significant at the highest mass load densities tested (108 and 120 kg/m³), which may be explained from the high ratio (acoustic energy/sample mass) found under those experimental conditions. Therefore, the application of ultrasound was considered as a useful technology with which to improve the convective drying, although its effects may be reduced at high mass load densities.     

Kinetics and Mass Transfer during Atmospheric Freeze Drying of Red Pepper

Drying is applied for moisture removal to allow safe and extended storage. Red pepper (Capsicum annum) samples were heat pump dried in fluidized bed at different air temperatures. A slightly modified solution of the diffusion equation was used to describe the kinetics and drying rates of red pepper. The model well described the low- and medium-temperature drying processes. The determined effective mass diffusivities varied from 0.7831 to 4.0201 × 10^?9 m²/s and increased consistently with drying air temperature. The mass diffusivity was correlated to temperature by linear regression with coefficient of determination equal to 0.999 and negligible standard error.     

Coupling CFD and Diffusion Models for Analyzing the Convective Drying Behavior of a Single Rice Kernel

The drying behavior of a single rice kernel subjected to convective drying was analyzed numerically by solving heat and moisture transfer equations using a coupled computational fluid dynamics (CFD) and diffusion model. The transfer coefficients were computed simultaneously with the external flow field and the internal diffusive field of the grain. The model was validated using results of a thin-layer drying experiments from the literature. The effects of velocity and temperature of the drying air on the rice kernel were analyzed. It was found that the air temperature was the major variable that affected the drying rate of the rice kernel. The initial drying rates (in first 20 min) were 7, 12, and 19% per hour at inlet air temperatures of 30, 45, and 60 ^° C, respectively. Important temperature gradients within the grain existed only in the first few minutes of the drying process. The moisture content gradients reached a maximum value of 11.7% (db) mm ^?1 at approximately 45 min along the short axis in the thickness direction. The variation in the inlet air velocity showed a minor effect on the drying rate of the rice kernel. The heat and mass transfer coefficients varied from 16.57 to 203.46 W·m ^?2·K ^?1 and from 0.0160 to 0.1959 m·s ^?1, respectively. The importance of the computation of the transfer coefficients with the heat and mass transfer model is demonstrated.     

The Effect of External Mass Transfer Resistance during Drying of Fermented Sausage

The drying mechanism of fermented sausages (sucuks) that were cylindrical rod shaped, 40 cm long and 4 cm diameter, during ripening under natural convection conditions at different temperatures (15 to 30°C) was examined. To simulate the experimental drying curves, three empirical models and a diffusional model assuming negligible external mass transfer resistance were evaluated. The drying rate curves of sucuk samples were also simulated taking into account the influence of the external mass transfer resistance. The equation was solved using the trial-and-error solution algorithm developed in this study and the mass transfer coefficient, k _c , and effective moisture diffusivity, D _eff , were simultaneously determined (1.44 × 10^?8 to 1.93 × 10^?8 m/s and 4.30 × 10^?10 to 6.85 × 10^?10 m²/s, respectively). The proposed model considering the effect of external resistance allowed the accurate simulation of the experimental drying data of sucuks at different temperatures.     

Drying characteristics and modeling of yam slices under different relative humidity conditions

The drying characteristics of yam slices under different constant relative humidity (RH) and step-down RH levels were studied. A mass transfer model was developed based on Bi-Di correlations containing a drying coefficient and a lag factor to describe the drying process. It was validated using experimental data. Results showed that the drying air with constant RH levels of 20, 30, and 40%, temperature of 60°C, and air velocity of 1.5 m/s had an insignificant effect on drying time. This phenomenon was likely attributed to the fact that higher RH led to a rapid increase in sample’s temperature. The higher sample temperature could provide an additional driving force to water diffusion and thereby promote the moisture movement, which could minimize the negative effect of lower the drying rate in the initial drying stage. Applying air with 40% RH for 15 min in the initial stage achieved the desired color and reduced the drying time by 25% compared to the drying time under continuous dehumidification from an initial RH of 40%. Using the developed Bi-Di correlation, the estimated Biot number, effective moisture diffusivity, and mass transfer coefficient ranged from 0.1024 to 0.1182, 1.1133 × 10^?10 to 8.8144 × 10^?9 m²/s, and 1.8992 × 10^?9 to 1.7364 × 10^?7 m/s, respectively. A rather high correlation coefficient of determination (R² between 0.9871 and 0.9971) was determined between the experimental and predicted moisture contents. The present findings contribute to a better understanding of the effect of relative humidity on drying characteristics. The developed Bi-Di correlation provided a new method to determine the effective diffusivity of moisture in drying.     

AN APPLICATION OF SIMULTANEOUS HEAT AND MASS TRANSFER IN A CYLINDER USING THE FINITE-DIFFERENCE METHOD

ABSTRACT

A study of simultaneous heat and mass transfer during drying an infinite cylinder shape material (twigs of ilex paraguayenais saint hilaire) was carried out. The finite-difference method was used to solve the drying model and a simultaneous heat and mass balance in each node was made. Models with different assumptions were tested and the external mass transfer coefficient was used as a parameter to fit the model to experimental data. The thickness of the node and the time step were selected considering the system stability.

Drying temperature, twig diameter and air velocity were selected as study variables. The models results were in good agreement with experimental measurements giving mass coefficient values between 1.97 10^?4and 9.55 10^?4 Kg/m² s.     

Dielectric Properties and Optimization of Parameters for Microwave Drying of Petroleum Coke Using Response Surface Methodology

The dielectric properties of petroleum coke at five temperatures between 20 to 100 ^° C, covering different moisture content levels at 2.45 GHz, were measured using an open-ended coaxial probe dielectric measurement system. The effects of drying temperature, duration of drying, and sample mass on the moisture content and dehydration rate of petroleum coke was assessed utilizing the response surface methodology. The dielectric constant, loss factor, and loss tangent were all found to increase nearly linearly with increase in moisture content. Three predictive empirical models were developed to relate the dielectric constant, loss factor, and loss tangent of petroleum coke as a linear function of moisture content from 3–10%. An increase in temperature between 20 to 100 ^° C was found to increase the dielectric properties. The penetration depth was observed to increase linearly with decrease in moisture content in the range of 3 to 10%. A predictive empirical model was developed to calculate penetration depth for petroleum coke. Two mathematical models were established and analyzed using RSM to describe the relationship between the microwave drying conditions and the responses, moisture content, and dehydration rate. Statistical analysis with response surface regression showed that microwave drying temperature, duration of drying, and sample mass were significantly related to moisture content and dehydration rate. Based on the RSM analysis, the optimum process conditions were estimated to be a microwave drying temperature of 75 ^° C, drying duration of 10 sec, and sample mass of 60 g, with the resultant moisture content being 0.34 at a dehydration rate of 2.94 g/min.     

Analysis of Heat and Mass Transfer during High-Temperature Drying of Pinus radiata

This article describes the analysis of heat and mass transfer coefficients for a single board of Pinus radiata (D. Don) timber over a range of high temperature and superheated steam drying conditions. The calculated heat transfer coefficients were in the range 20 to 60 W m^?2 K^?1. The mass transfer coefficients were of the order of 2 × 10^?8 to 3 × 10^?7 kg m^?2 s^?1, based on the vapor pressure difference, and of the order of 0.002 to 0.04 m s^?1 (expressed in terms of mass transfer velocity) based on vapor concentration difference between the surface of the board and the bulk drying medium.     

Modeling Superheated Steam Vacuum Drying of Wood

Abstract

A two-dimensional mathematical model developed for vacuum-contact drying of wood was adapted to simulate superheated steam vacuum drying. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady-state mass conservation of dry air. A drying test conducted on sugar maple sapwood in a laboratory vacuum kiln was used to infer the convective mass and heat transfer coefficients through a curve fitting technique. The average air velocity was 2.5 m s^?1 and the dry-bulb temperature varied between 60 and 66°C. The ambient pressure varied from 15 to 11 kPa. Simulation results indicate that heat and mass transfer coefficients are moisture content dependent. The simulated drying curve based on transfer coefficients calculated from boundary layer theory poorly fits experimental results. The functional relation for the relative permeability of wood to air is a key parameter in predicting the pressure evolution in wood in the course of drying. In the case of small vacuum kilns, radiant heat can contribute substantially to the total heat transfer to the evaporative surface at the early stages of drying. As for conventional drying, the air velocity could be reduced at the latter stage of drying with little or no change to the drying rate.     

Non-Isothermal Drying of Medicinal Plants

An intelligent system for non-isothermal drying of medicinal plants, based on machine vision, sensor fusion, and neural network, was developed. Air temperature, velocity, and humidity, along with material size and moisture content were inputs to the neural model for diffusivity. Temperature, time, mass, volume, and color were inputs to the neural model for quality. Isothermal low-temperature drying of ginseng root and blueberry showed extremely low effective diffusivity (0.2–0.75)*10^?10 m²/s. In contrast, non-isothermal drying demonstrated a potential to increase diffusivity and prevent quality losses. Testing of the intelligent drying system showed reduced drying time from 240 to 60 hours for ginseng, and from 110 to 30 hours for blueberry with desired product quality.     

Hot air drying of purple-fleshed sweet potato with contact ultrasound assistance

A drying technique using a combination of a contact ultrasound apparatus and a hot air dryer is developed to investigate the strengthening effect of contact ultrasound on hot air drying. The effects of drying parameters such as ultrasound power and drying temperature on drying characteristics, effective moisture diffusivity (D_eff), microstructure, glass transition temperature (T_g), rehydration ratio, and color difference are discussed. The results show that the application of contact ultrasound causes a significant acceleration of internal mass transfer, and higher ultrasound power applied leads to faster drying rate. The effect of ultrasound power on drying rate decreases along with the reduction of moisture content during drying process. The increase in drying temperature significantly reduces drying time but has a little negative influence on the strengthening effect of ultrasound. D_eff values range from 1.0578?×?10^?10 to 5.4713?×?10^?10?m²/s in contact ultrasound-assisted hot air drying of purple-fleshed sweet potato and increase significantly with an increase in drying temperature as well as ultrasound power. The microstructure of purple-fleshed sweet potato is greatly different at different ultrasound powers during contact ultrasound-assisted hot air drying and shows more microchannels and dilated intercellular spaces in the cross-section of purple-fleshed sweet potato micrographs at higher ultrasound power. Contact ultrasound application during hot air drying could improve the mobility of water and consequently reduce glass transition temperature. Lower color difference and higher rehydration ratio could be achieved as drying temperature decreases and ultrasound power increases. The increase in contact ultrasound power could reduce energy consumption of drying process up to 34.60%. Therefore, contact ultrasound assistance is a promising method to enhance hot air drying process.     

Characterization of isotherms and thin-layer drying of red kidney beans,Part II: Three-dimensional finite element models to estimate transient mass and heat transfer coefficients and water diffusivity

Three-dimensional finite element models with consideration of shrinkage and irregular shape were developed to estimate the relationships among the transient heat and mass transfer coefficients, the transient water diffusivity, and the temperature and moisture content of the red kidney beans being dried under different drying conditions. An equation was developed to calculate the transient mass transfer coefficient using the measured time–moisture content data. This calculated transient mass transfer coefficient was further used to calculate the transient heat transfer coefficient. To verify the predicted temperature on the surface of the red kidney beans, surface temperature was measured using a handhold infrared thermometer. These measured temperature and time–moisture content data were used to determine the transient water diffusivity using the least square method when the red kidney bean kernel experienced a shrinkage during drying. Strong relationship among the transient heat and mass transfer coefficients, the water diffusivity, and the ratio of the transient heat and mass transfer coefficients was revealed. This relationship could be used to predict temperature and moisture content of the red kidney beans during the entire drying period. The Lewis number?=?27, and the ratio of the transient heat over mass transfer coefficients was 10765?J?m^?3?k^?1 at 30 and 40°C, and 10729?J?m^?3?k^?1 at 50°C. Shrinkage did not significantly influence the value of the estimated transient water diffusivity.     

QUANTITATIVE ANALYSIS OF DRYING BEHAVIOUR OF FRUITS AND VEGETABLES

ABSTRACT

In this study, the drying characteristics of seven vegetables (garlic, potato, bean. ginger, leek, onion, and carrot) and five fruits (avocado, banana, sultana, kiwi fruit and apple) were studied under idealised ‘constant’ controlled drying conditions using an automatic thermogravimetric analyser. Drying-rate curves were constructed and quantified in a systematic way using the least-squares method. This allowed the drying behaviour of each product to be expressed in terms of three variables; gradient of the “constant rate” stage, gradient of the falling rate stage and critical moisture content (CMC)

The drying curves of fruits and vegetables were found to vary greatly indicating the nature of foodstuffs to have a strong effect on the drying kinetics. The gradient of the constant rate period was not truly constant but had an average gradient of 3.1 × 10^?4 per second with bean having the lowest gradient (1.9 × 10 ^?4/) and garlic having the highest gradient (5.3 × 10^?4/s). This was expected as bean had the highest moisture content (93% wet basis) and was able to feed the surface with sufficient water to maintain a near constant rate of evaporation

The falling-rate period ranged from 10 × 10^?4 per second with avocado and garlic having the lowest gradient while apple had the highest gradient of 30 × 10^?4 per second. The CMC for most of the fruits and vegetables studied was about 1 kg/kg dry mass. This means that, at the CMC, the mass of water was the same as the mass of dry matter but with a spread in values from 0.7 for a fruit like banana to 1.8 for avocado.     

Efficient drying and oxygen-containing functional groups characteristics of lignite during microwave irradiation process

To remove the high moisture of ZhaoTong lignite, the efficient drying characteristics and oxygen-containing functional groups changes in lignite during microwave irradiation process were highlighted in this study. As the microwave absorbers, lignite char and NaNO₃ were added to microwave drying of ZhaoTong lignite. The minimum chemical oxygen demand of waste water generated from microwave drying process of lignite was 99.89?mg?O₂?L^?1. The effects of microwave power, lignite mass, the weight ratio of lignite char to lignite and NaNO₃ content on the drying rate, and moisture diffusion coefficient of lignite were investigated during lignite microwave irradiation process. It was found that the drying rate and moisture diffusion coefficient of lignite increased with increasing microwave power, the weight ratio of lignite char to lignite and NaNO₃ content, but decreased with increasing lignite mass. Lignite char and NaNO₃ were mixed with lignite that can enhance the instantaneous surface temperature of lignite sample under microwave irradiation. Compared with addition of lignite char to lignite, the addition of NaNO₃ to lignite can decrease the unit electric power consumption of moisture evaporating. And the minimum unit electric power consumption of moisture evaporating was 9.44?Wh?g^?1. The FTIR technology was used to investigate the oxygen-containing functional groups changes in lignite during microwave drying process. The oxygen-containing functional groups of lignite were effectively removed with increasing microwave power.     

Optimization of Fluidized Bed Drying Process of Green Peas Using Response Surface Methodology

The fluidized bed drying process of green peas was optimized using the response surface methodology for the process variables: drying air temperature (60–100°C), tempering time (0–60 min), pretreatment, and mass per unit area (6.3–9.5 g/cm²). The green peas were pretreated by pricking, hot water blanching, or chemical blanching. Product quality parameters such as rehydration ratio, color, texture, and appearance were determined and analyzed. Second-order polynomial equations, containing all the process variables, were used to model the measured process and product qualities. Rehydration ratio was influenced mostly by pretreatment followed by tempering time, temperature, and mass per unit area. Pretreatment and mass per unit area significantly affected color and texture. Higher levels of temperature and lower levels of tempering time and mass per unit area increased the rehydration ratio. The optimum process conditions were derived by using the contour plots on the rehydration ratio and sensory scores generated by the second-order polynomials. Optimum conditions of 79.4°C drying air temperature, 35.8-min tempering time, pretreatment of the once pricked peas with chemical blanching in a solution of 2.5% NaCl and 0.1% NaHCO₃, and mass per unit area of 6.8 g/cm² were recommended for the fluidized bed drying of green peas. At these conditions the rehydration ratio was 3.49.     

Processes and kinetics of mass gain in archeological brick following drying and reheating

The mass gain behavior of archeological bricks was examined following drying (130°C)/reheating (500°C) and aging at a range of temperatures (25°C, 35°C, 45°C). For drying or reheating, samples exhibit a two‐stage mass gain behavior, the second stage, Stage 2, continuing indefinitely and better described by a t^1/n model (1/n=1/6‐1/2); a correlation between the 1/n value and the specific surface area/pore volume demonstrates diffusion mechanisms with some pore geometry/morphology dependence. Stage 2 is shown to have an Arrhenius temperature dependence with activation energies of similar orders of magnitude following both drying and reheating. Supported by thermogravimetric‐mass spectrometry (TG‐MS), Stage 2 is demonstrated as likely due to the recombination of chemisorbed water, previously removed, whereas following reheating due to two components, a chemisorbed component associated with drying and a component associated with rehydroxyls removed at higher temperatures during reheating. Differences between activation energies of chemisorption and rehydroxylation components support this. Evidence for a fundamental compositional relationship between these processes is presented by a strong linear relationship between the drying and reheating mass gain rates. Stage 1, following drying or reheating, is shown to be likely associated with physisorption processes alone.     

A kinetic study of microwave and fluidized-bed drying of a Chinese lignite

The drying kinetics of Chinese lignite in nitrogen fluidized-bed, superheated steam fluidized-bed and microwave were investigated. The changes in the mass as a function of drying time were measured under various drying conditions. The variations of moisture ratio with time were used to test ten different thin-layer empirical drying models given in the literature. In studying the consistency of all the models, some statistical tests, such as χ², residual sum of squares (RSS) and F-value were also used as well as coefficient of determination R². In nitrogen fluidized-bed and superheated steam fluidized-bed, the Midilli–Kucuk model best described the lignite drying process. Drying data in microwave were best described by the Page model, indicative of a difference in kinetics between the two drying methods. This difference was attributed to different heat transfer mechanisms under conventional and microwave drying conditions. The effects of drying parameters in nitrogen fluidized-bed, superheated steam fluidized-bed and microwave drying on the constants and coefficients of the selected models were studied by multiple regression analysis. The apparent diffusion coefficient of moisture in samples was obtained from the kinetics data and the apparent activation energies under nitrogen fluidized-bed, superheated steam fluidized-bed and microwave drying were found to be rather similar.     

Predicting drying kinetic and undesired agglomeration during drying of granules containing amorphous water-soluble substances in a continuous horizontal fluid bed dryer

Various industries use fluid bed dryers for the drying of larger granules produced by extrusion, pan agglomeration or spray granulation. During such drying processes secondary agglomeration is undesired due to an increasing amount of oversize particles. In the current study the drying of granules containing amorphous substances is modelled. The model is based on the laws of heat and mass transfer as well as heat and mass balances around a differential volume element of the fluid bed. For each location in the bed and for different drying parameters the calculated moisture and temperature values are used to estimate the surface viscosity of the granules. The calculated viscosity allows estimating the risk of secondary agglomeration of granules. Viscosity values which are estimated for different drying conditions are compared with the experimentally determined amount of oversize particles. An increasing amount of oversize particles can be observed if the viscosity of the amorphous substance is below 10⁴ Pa s.