Characteristic Drying Curve Research Articles

Characteristic drying curve behavior of whey and casein micelle proteins via thin-film drying

Drying characteristics of milk whey protein concentrate (WPC) and micellar caseins (MC) were investigated through a custom-designed mobile thin-film dryer. Several mixtures containing different WPC to MC ratios were tested in drying experiments. Both the constant drying period and falling rate period were identified. It was noticed the initial drying rates at constant drying period were slightly hindered by the presence of protein even at low concentrations. The drying kinetics was investigated by the characteristic drying curve (CDC) method and the drying behavior at the falling rate period was evaluated by a specific parameter n. As the result, while WPC exhibited a uniform master characteristic drying curve through different drying conditions, samples with certain amount of MC showed the more “scattered” curves, which lead to a wide distribution of n. Besides, it was observed that a thin layer of crust formed during the drying process. The analysis of the drying characteristics of MC led to a meaningful correlation with the morphology and structural properties. As the morphology of the crust was determined by the velocity and temperature of the impinging air, it became an inherent limitation for this thin film drying method to investigate materials with such crust formation phenomena.

Economic Evaluation of Drying of Soot Sludge and Sawdust Mixture at Low Temperatures Using the Characteristic Drying Curve Method

Soot sludge is a waste stream formed in the fuel oil gasification of formic acid and hydrogen peroxide production. The soot sludge has a high moisture content (95%) and is presently combusted with heavy fuel oil in order to dispose of the sludge. Experimental tests earlier conducted by the authors have shown that the sludge can be convectively dried with sawdust in a fixed bed. By upgrading the sludge from waste to fuel, the utilization of oil can be decreased. In this study, characteristic drying curves (CDC) are determined for the sludge and sawdust mixture. The CDCs are further used to evaluate the economy of the mixture drying in a belt dryer by using the payback period method. Results show that the linear CDCs of the mixture can be used to extrapolate drying data from specific drying conditions to another when the bed height is 200 or 300 mm, and the inlet air temperature 40–100 °C. The economic analysis shows that drying is economical for all inlet air temperatures if the oil price is ≥350 €/t-oil. Sensitivity analyses reveal that the heat, sawdust and emission prices have no remarkable influence on the economy of drying if the oil price does not fall below c. 300 €/t-oil.

Design and construction of a thin-film drying channel equipment - Modeling the drying kinetics of nanofiltered whey

Drying kinetics of nanofiltered whey was obtained by using thin-film drying experiments in a dryer device made in the laboratory. Four drying temperatures (40, 55, 70, and 85 °C) and two air velocities (0.3 and 0.7 m/s) were used in drying experiments. Drying kinetics were fitted using the characteristic drying curve (CDC) model. Two distinctive periods were observed in the drying curves. The temperature exhibited a significant (P < 0.05) effect on the drying kinetics. An increase in temperature accelerated the drying rate. Air velocity showed a non-significant (P > 0.05) effect on the drying kinetics, possibly due to the air flow was in excess regarding the amount of water vapor removed from the liquid-gas interface. The CDC model described well drying kinetics. The dryer device made in the laboratory was shown as a reliable tool for determining drying kinetics.

Valorization of solar drying process in the production of dried Moroccan sweet cherries

This study presents the convectional drying process of two types of Moroccan sweet cherry, namely, Burlat and Van. 40 g of cherries were taken, pitted, cut in two halves, and were dried using a partially indirect solar convective dryer, which has an area of 2.5 m2. The thin-layer drying of sweet cherry were carried out in July 2017 at three air temperatures of 60, 70 and 80 °C and for two drying air flow rate 150 and 300 m3/h. Moreover, the cherry fruits are sufficiently dried in the ranges between 34 and 40 °C of ambient air temperature, 17–27% of relative humidity, and 200–900 W/m2 of solar radiation. The evolution of the moisture dry content and the drying rate as a function of drying time for different temperatures are shown graphically. Experience shows that as the air drying temperature increases the drying time decreases. During the experiments, pitted cherries were dried and the moisture content was decreased from 82.12 for Burlat and 85.45 for Van to 23 ± 3% (w.b). The drying temperature of 80 °C and air velocity of 300 m3/h were the optimum values for cherry fruit drying in a minimum time of 240 min. The characteristic drying curve (CDC) applicable to both types of cherry has been established as a polynomial of order 3 in reduced moisture content.

CFD simulations of droplet and particle agglomeration in an industrial counter-current spray dryer

In this study, CFD simulations of particle and droplet agglomeration in an industrial counter-current spray dryer are presented. For this purpose, a modified form of the stochastic collision agglomeration model is proposed. This model takes into account droplet–droplet collision as well as wet and dry particle interaction. These events are coupled with heat, mass and momentum transfer. A comprehensive moisture evaporation model based on the concept of characteristic drying curve (CDC) was applied to predict the drying kinetics of the detergent slurry. Due to high instability in air flow inside the drying chamber, simulations were carried out under transient condition. A comparison between time-averaged simulation results and measurements, which were performed on an industrial spray drying installation, shows a good agreement. This finding proves the correctness of the developed agglomeration and drying models. The presented methodology of CFD simulations of agglomeration can be used to design or optimise spray-drying installations and to predict the final particle size distribution of the product.

Superheated steam drying of single wood particles: A characteristic drying curve model deduced from continuum model simulations and assessed by experiments

ABSTRACTComputational fluid dynamics (CFD) and CFD coupled with discrete element method (DEM) are powerful approaches to describe the drying behavior of real drying towers. In these approaches, the heat and mass interaction between fluid phase and the wet solid is required as an essential input. In this work, a new methodology for establishing simple drying model of single wood particles is presented. First, a spatially resolved continuum-scale model that describes the coupled heat and mass transfer within a single porous wood particle during superheated steam drying under atmospheric pressure is developed. The thermophysical properties of wood particle required as input data for the continuum model are determined experimentally. Then sophisticated continuum model is reduced to a simpler lumped model, which is referred to as the characteristic drying curve (CDC) model. The continuum model simulations are performed in a board range of operating drying conditions and the associated results are used to establish correlations for the CDC model parameters. The results of both sophisticated and reduced models are validated against the experimental observations made using a magnetic suspension balance. The sensitivity analysis performed with the continuum model indicates a strong dependency of the critical moisture content on particle size and drying condition. The established CDC model can be implemented in the CFD or CFD-DEM model of superheated steam dryers.

Experimental determination of the effective moisture diffusivity and activation energy during convective solar drying of olive pomace waste

The drying of olive pomace waste was performed using a partially indirect solar convective dryer operating in forced convection. This comparative study is focused on the drying kinetics of two types of residue, namely, raw olive pomace and deoiled olive pomace. These products have been spread out in thin layers on perforated circular racks before they be placed in the dryer. The sample thicknesses considered are 0.5, 1.0 and 1.5 cm. Kinetic measurements are carried out for three temperatures (40, 60 and 80 °C) and two drying air flow rates (0.042 and 0.083 m3 s−1). The relative humidity varies between 28% and 65% throughout the drying period. The increase of temperature in the environment reduces significantly the drying time. The characteristic drying curve (CDC) applicable to both types of olive pomace has been established as a polynomial of order 4 in reduced moisture content. Data obtained from dried products were used to determine the effective diffusivity values during the drying period with decreasing curve. In this period, the moisture transfer from the pomace was described by applying the Fick diffusion model. Effective diffusivity varies between 1.6 × 10−8 and 34.7 × 10−8 m2 s−1 with the increase of the medium air drying temperature but also with the sample thickness. The activation energy value is estimated at 29.06 kJ mol−1.

CFD modelling of moisture evaporation in an industrial dispersed system

A methodology is presented here for the mathematical modelling of moisture evaporation in a dispersed system in an industrial tower. An empirical model using characteristic drying curves was applied to calculate moisture evaporation from a droplet and particle. A simple method was developed to calculate the agglomeration process of the dispersed phase in the drying towers, using transient functions between the initial and final particle size distributions, at the nozzle and in the final product. The developed model and simulation results were validated on the basis of industrial spray tower experiments. High instability of the airflow due to the geometry of the dryer and the construction of the air inlets was observed. The general methodology applied within this CFD model is universal, and can be applied to the scaling-up of installations for dewatering in dispersed systems in order to determine configurations of feeding systems and control the product quality and safety of the process.

Thermophysical characterization and mathematical modeling of convective solar drying of raw olive pomace

This experimental study is focused on the convective solar drying of raw olive pomace. In this investigation, the desorption/adsorption equilibrium water content of this product has been determined at the drying temperatures of 30°C, 40°C and 50°C, by using the gravimetric static method. Moreover, four mathematical models have been used to predict the hygroscopic behavior of the product during drying process. From a thermodynamic point of view, more significant changes are felt in the thermodynamic properties of natural raw olive pomace in terms of differential enthalpy and entropy of sorption. The characteristic drying curve (CDC) of raw olive pomace has also been experimentally determined. This kinetic measurement is carried out for three temperature values of drying air (45±0.1°C, 60±0.1°C and 80±0.1°C) and also for two values of drying air flow rate Dv (0.042±0.002m3s−1 and 0.083±0.002m3s−1). The drying rate is empirically determined from the characteristic drying curve, and four mathematical models have been used to describe drying curves. The Midilli–Kucuk model seems to be the appropriate model for drying of the product studied here.

Characteristic Drying Curve of Oil Palm Fibers

The drying of crushed oil palm fronds was studied in a fluidized bed dryer assisted with mechanical agitation at different inlet air temperature, superficial air velocity, bed loading and agitation speed. The drying kinetics of the fibers under various drying conditions could be described by a single characteristic drying curve. A combined exponential and power law model is proposed to represent the characteristic drying curve which described both the increasing rate and the falling rate periods. The proposed model is also tested for drying kinetics of oil palm empty fruit bunch from previous researcher. It was found that the characteristic curve for both oil palm fronds and empty fruit bunch fibers has similar shape and that the proposed model is acceptable for describing the complete drying characteristics of the fibers.

CFD Evaluation of Droplet Drying Models in a Spray Dryer Fitted with a Rotary Atomizer

Computational fluid dynamics (CFD) modeling of spray dryers requires a simple but sufficiently realistic drying model. This work evaluates two such models that are currently in discussion; reaction engineering approach (REA) and characteristic drying curve (CDC). Two versions of the CDC, linear and convex, drop in drying rate were included. Simulation results were compared to the overall outlet conditions obtained from our pilot-scale experiments. The REA and CDC with a linear drop in drying rate predicted the outlet conditions reasonably well. This is contrary to the kinetics determined previously. Analysis shows that the models exhibit different responses to changes in the initial feed moisture content. Utilizing different models did not result in significantly different particle trajectories. This is due to the low relaxation time of the particles. Despite the slight differences in the drying curves, both models predicted similar particle rigidity depositing the wall. For the first time in a CFD simulation, the REA model was extended to calculate the particle surface moisture, which showed promising results for wet particles. Room for improvement was identified when applying this concept for relatively dry particles.

Comparative study of droplet drying models for CFD modelling

CFD simulation is used to study wall deposition and agglomeration phenomena commonly encountered in industrial spray dryers. This paper initially provides a comparison of two drying kinetics models: Characteristic Drying Curve (CDC) and Reaction Engineering Approach (REA). Comparisons are made with experimental data with application to carbohydrate droplet drying obtained from past workers. These models were then extrapolated to actual drying conditions to assess their performance. The REA model predicts the progressive reduction in drying rate better than the CDC model for the carbohydrate droplets. A modified CDC model incorporating a convex falling rate produced better agreement than the conventional linear falling rate model. Further analysis showed that the REA model can be extended to simulate the particle surface moisture which may affect the agglomeration process. The proposed concept was compared with reported simulation results from a diffusion model which showed reasonable fit with data.

Spouted bed drying of cork stoppers

The spouted bed is commonly used to dry granular particles as it displays some special characteristics which render it capable of performing cyclic operations, with particles that are very difficult to fluidize under a different type of bed. Information on the characteristic drying rates of cork stoppers is still very limited in the technical and scientific literature, and until now no published data was found on the spouted bed drying of cork stoppers. The characterization of the drying performance of Port wine cork stoppers was carried out in a laboratory scale spouted bed. Characteristic drying curves were determined with 29 mm × 21 mm cork stoppers under several operating conditions. Values for the effective diffusivity were obtained in the range of 1.36 × 10 −9 to 4.44 × 10 −9 m 2/s, with the temperature dependence represented by an Arrhenius-type relationship. The activation energy obtained was 28.72 kJ/mol.

Characteristic drying curves for cellulosic fibres

The tray drying of cellulosic fibres from citrus fruits has been studied here, for hot air drying at dry-bulb temperatures from 40–80 °C, a wet-bulb temperature of 40 °C, and a cross-stream air velocity of 1.3 m s −1 . It appears that a single characteristic drying curve may be adequate to describe the drying behaviour of the fibres, whether the fibres are mixed with water or with hibiscus extract. The amount of hindrance to moisture movement seems to be small relative to many other materials, and the characteristic curve has the approximate form f = Φ 0.5 . Comparison of the characteristic curve with the normalized drying rates measured by previous workers suggests that the characteristic curve remains applicable for sugar beet fibres dried in hot air over a wider range of temperatures, from 130 to 183 °C. The importance of the basic cellulosic component in the fibres in controlling the drying behaviour may explain the absence of any significant difference between the normalised drying rates for sugar beet fibres and those for citrus fibres. In the context of drying the fibres by other methods, including spray drying, compared with the tray drying used here, the absence of substantial additional hindrance to the movement of moisture caused by the extract (compared with water) may be useful.

Microwave Assisted Vacuum Drying and Endpoint Determination Using Mass Spectrometry, Part II, On-Line Moisture Content Profiling

Quantitative methods were developed to compute on-line conventional loss-on-drying (LOD) profiles from mass spectrometry (MS) measurements. Good agreement between calculated LOD values and actual LOD measurements were obtained for single and multiple solvent wet cakes. Characteristic drying curve profiles were determined for calcium carbonate, sand, cellulose, starch, and silica gel solids, and the corresponding critical moisture contents were estimated. Although this work was developed for microwave drying, the methods apply to conventional dryer monitoring as well.

COMPARISON OF DIFFERENT DRYING KINETICS MODELS FOR SINGLE PARTICLES

ABSTRACT Different kinetic drying models applied to non-shrinking materials are scrutinised and compared. Rigorous mechanistic models are difficult to apply because of the large number of unknown parameters; hence simpler alternatives have been applied instead. This paper focuses on the two most commonly used kinetic models, namely; the diffusion model and the characteristic drying curves. These were compared by plotting the predicted drying curves (moisture content versus time) in the same diagram as a reference curve. The response to different changes in drying conditions and sensibility to extrapolated conditions were tested. The parameters studied were the temperature, velocity and humidity of the gas and the size and initial moisture content of the panicles. The comparison was based on two approaches; one theoretical where the reference drying curves were generated by an rigorous drying model; and one experimental where the reference drying curves were measured using a thin-layer kinetics rig. The materials under study were softwood, ceramic clay, silica gel and purolit. The results were promising in the sense that the characteristic drying curves (CDCs) and diffusion model could both predict the response to most changes in external conditions. The predictions for drying above the boiling point were however less accurate than the ones below it, probably due to internal overpressure effects.

Analysis of the Drying Processes of Osmotically Dehydrated Apple Using the Characteristic Curve Model

ABSTRACT Round slices (2 mm thick) of fresh apple and of apple subjected to osmo-dehydration processes in sucrose syrups until reaching aw values of 0.979, 0.968. and 0.958. were air-dried at 50. 60. and 70°C. Analysis of the drying curves showed that the products do not present a constant rate period (except for fresh apple) but showed two falling rate periods. Initial aw greatly affects the fraction of water lost during the drying processes. The use of the Fick's Second Law was not adequate to describe the experimental drying curves. However, apple drying processes can be modeled by the Characteristic Drying Curve model, using several equations (polynomial and logarithmic) with a high confidence level. The parameters of each equation showed a great dependence on product's aw These models can be used to predict drying curves within the studied limits in each case, presenting great advantages when compared to classical models based on effective diffusion coefficients.

Characteristic Drying Curves of Cocoa Beans

ABSTRACT The characteristic drying curves of cocoa beans are determined by using a tunnel drier where conditioned air passes a single cocoa berm suspended from an electronic balance in the test section. Weight loss, and temperatures of air, testa and nib of the cocoa bean is monitored on personal computers. The nornmalised drying rate versus the normalised moisture content is regressed by least square method to fit a new polynomial model for the penetration falling rate period and a linear model for the regular regime falling rate period. It can be concluded that there are three drying periods for cocoa beans namely the constant drying rate period, the penetration falling rate period and the regular regime falling rate period. The polynomial model estimates the penetration period quite well whereas the linear model estimates the regular regime quite well as well. There is no observable influence of relative humidity and air temperature on the characteristic drying curve of cocoa beans. However, the air ve...

An Experimental Test of the Concept of the Characteristic Drying Curve Using the Thin-Layer Method

ABSTRACT The drying behaviour of paticles ( purolit and silica gel) was studied using the thin-layer method described by Langrish et al ( 1). The experiments covered inlet air temperatures between 100 and 150°C, inlet air humidities from 0.02 to 0.052 kgkg1 superficial air velocities between 3.8 and 10.8 ms-1, with layer thicknem of 2 – 10mm. No constant mle period war observed. Characteristic drying curves were found to fall within a narrow band fur these ranges of process variables, for material of uniform size and shape and with relative moisture content defined in terms of the end of the induction period. Small changes in panicle shape, particle size distribution and uniformity of particle layers had negligible infuence on the drying kinetics. However, reduction in particle size from 5.2mm diameter to 0.86mm had a marked effect: the normalised drying rate at a given relative moisture content became larger as the particle size became smaller. This phenomenon is attributed to an increase in available contact area per unit volume with diminishing particle size. The thin-layer technique thus appears to be a useful and robust way of obtaining a general characteristic drying curve for a given particulate material. A review of various works ( Keey, 2) has shown that the concept ofa characteristic drying curve may be used to describe the drying kinetics of paniculate materials below 20mm in size for modest changes in process variables ( air temperature, humidity and velocity). This concept has found to be very useful to help model drying processes of a wide variety of particulates, cross-circulated slabs, heaped loaw fabric fibres, hygroscopic ceramic cylinders and discrete vermiculite particles. The drying of a single particle has been related lo the drying kinetics of a fluidized bed by the use of this ida. ( Tsotsas, 7). A grater understanding of the properties of the characteristic drying curve will provide a greater confidence in applying thir concept more generally to process design and the analysis of industrial drying equipment. The goal of this study was to examine further the experimental and theoretical foundations of the characteristic drying curve, using thin-layer methods.

Drying Kinetics of Pinus Rqdiata Boards at Elevated Temperatures

ABSTRACT From the numerical results of a rigorous, mathematical model, the rate–o–drying curves for high–temperature seasoning of Pinus radiata board show that the drying process can be divided into three periods. Based on the mechanistic picture of moisture movement and differences in the movement in each period, this paper simplifies the model by using the concept of the haracteristic drying curve. In heartwood drying, the first period is very short (5–15 minutes), hence it can be neglected, whereas for sapwood, the first period is substantial with the drying rate being taken as constant in the analysis. Since the physical characteristics of the second and third drying periods are different, separate characteristic drying curves have been developed for each period. It has been found that common normalised drying curves are able to describe the results generated from the earlier model for normal range of kiln conditions over the examined ranges in dry–bulb temperatures of 1 10–140°C, wet–bulb temperature...