Decreasing Drying Rate Period Research Articles

Dehydration of tomato peels and seeds as affected by the temperature

SummaryAn empirical model was developed for dehydrating tomato's peels and seeds (TPS) at three different temperatures (i.e. 50 °C, 60 °C and 70 °C). The model assumes that there is an initial constant drying rate period (first stage) followed by a decreasing drying rate period (second stage). To assess the goodness of fitting procedure, the mean relative deviation modulus was calculated. Results indicate that the model can adequately describe the dehydration data. The robustness of the model was further tested by evaluating the dependence of model parameters on the temperature, using an Arrhenius‐type equation. Recorded findings show that model parameters changed with the temperature, thus confirming that the proposed model can be advantageously used to describe the dehydration kinetic of TPS. The influence of TPS dehydration on temperature was also assessed by calculating the dehydration rate as function of TPS water content. Results demonstrate that the dehydration rate increases as the testing temperature increases too. In both drying stages, the difference between the dehydration rate at 50 °C and the one at the other two temperatures is more marked.

A cluster-based pore network model of drying with corner liquid films, with application to a macroporous material

A pore network model (PNM) of drying in a gravity-dominated macroporous material has been developed. The pore network geometry used for the simulations is extracted from microcomputed tomography scans of porous asphalt (PA), a macroporous, hydrophobic material. The drying of liquid water in PA is modeled using a cluster-based approach with a two-step drying process i.e. elements go first from being fully saturated to having liquid only in pore and throat corners, and then to becoming completely dry. The PNM simulations are validated with gravimetric experiments performed under controlled conditions and the simulations show good agreement with experiments for most of the drying period. From experiments, it is seen that drying in PA completely skips the constant drying rate period (CDRP) and instead begins with the decreasing drying rate period (DDRP) due to the poor hydraulic connectivity in PA as a result of its large and hydrophobic pore space. The PNM simulations exhibit CDRP initially and then transitions to DDRP after a third of the total drying time. The CDRP at the beginning of the PNM simulations is due to the simplified liquid configuration assumed in the network, and its duration can be minimized to an extent by increasing the number of hydrophobic pores in the network that does not retain any liquid after drainage. Although promising, the first results call for a more accurate representation of both the complex pore space of PA and the physics involved in drying of a macroporous material.

STUDYING THE PROPERTIES OF GRAPE POMACE AS OF AN OBJECT OF DRYING

Grape pomace contains a complex of valuable and biologically active compounds. Drying is one of the main ways of microbiological stabilisation and preservation of the nutritional value of this secondary raw material. Kinetic parameters of dehydration of grape pomace from different industrially cultivated and processed varieties have been studied, namely, of the red grape varieties Cabernet and Shiraz, and of the white varieties Chardonnay and Riesling. Dependences of the moisture content in the process of convective drying at different drying agent rates have been obtained at a regulated temperature of 80 °C. The components of such an important technological parameter as the drying time have been determined. These components include the duration of the constant drying rate period Φ2 and the time of the decreasing drying rate period Φ1 of the two zones of the second drying period. The coefficients of the dehydration process have been calculated depending on the type of grape pomace processing. It has been shown that the discrepancy between the calculated and experimental results does not exceed ±3.9%. The specific features of the moisture yield of the pomace have been revealed, the pomace being viewed as a complex heterogeneous system with colloidal and capillary-porous properties. There are different types of its technological preparation: it can be fresh, frozen, fermented, and this makes for the fact that the drying time and drying rate may differ by 1.32–1.46 times. High preservation of valuable properties of grape pomace has been shown. Thus, the concentration of biologically active substances (BAS) in the total polyphenolic compounds is up to 69% of their initial concentration in the grape pomace samples, and the microbial contamination of the samples after drying is reduced by 51–82% of their initial contamination.

Shrinkage and deformation during convective drying of calcium alginate

The effect of air temperature (40–70 °C) and gelation time (10–60 min) on drying kinetics and structure of calcium alginate (3 g/100 mL) particles dried in a convective packed bed drier were investigated. All drying conditions showed constant (initially) and decreasing drying rate periods during most of drying time. The drying time was reduced by a factor of 1.8 when increasing air temperature from 40 to 70 °C. Particle diameter decreased by 54–60% and was not affected by air temperature. However, the increase of drying rate due to a temperature beyond 40 °C resulted in formation of internal voids and higher shape deformation, i. e, it increased elongation and decreased roundness of the alginate particles. The formation of internal voids started in the transition between the constant and decreasing drying rate period. Besides, packed bed drying resulted in heterogeneous particles size distribution during intermediate drying times and more homogeneous size distribution in the end of the drying, when particles shrinkage is reduced. Finally, particles crosslinked during shorter gelation times presented less homogeneous structure, with irregular surface and collapsed center after drying, in comparison to longer gelation times.

Paddy Drying in Batch Fluidized Bed and Scale-up Simulation in Continuous Operation Mode

The objective of this research is to develop the industrial-scale fluid bed dryer for paddy by scale-up of lab-scale experimental data. The developed dryer was conducted by simulation using a two phase model. Firstly, the experimental works by using lab-scale batch fluid bed dryer, was conducted to determine the drying curve of paddy (Xin 0.32 kg/kg dry base). In the experimental works,the inlet air temperature was varied (°C): 40; 50; 60. The drying rate curves as a function of moisture content showed only decreasing drying rate period. Then, a very good agreement between the measured and simualtion results of the profile of moisture content in solids was produced by simulator. Finally, asimulated continuous fluidized bed dryer for paddy with dimension 5 m of length and 1.5 of width was succesfully performed, in which the influence of mass solid flow rate 0.1; 0.2; 0.4 tons/h, height of bed 0.25; 0.50; 0.75 m, and air temperature 50; 70; 100 °C on drying process were studied. Keywords: Paddy; fluid bed dryer; batch, contonious; modelling; simulation

Threats to Food Safety and Sustainability of Nike (Awaous melanocephalus) in Gorontalo Province

The objective of this research is to develop the industrial-scale fluid bed dryer for paddy by scale-up of lab-scale experimental data. The developed dryer was conducted by simulation using a two phase model. Firstly, the experimental works by using lab-scale batch fluid bed dryer, was conducted to determine the drying curve of paddy (Xin 0.32 kg/kg dry base). In the experimental works, the inlet air temperature was varied (°C): 40; 50; 60. The drying rate curves as a function of moisture content showed only decreasing drying rate period. Then, a very good agreement between the measured and simualtion results of the profile of moisture content in solids was produced by simulator. Finally, a simulated continuous fluidized bed dryer for paddy with dimension 5 m of length and 1.5 of width was succesfully performed, in which the influence of mass solid flow rate 0.1; 0.2; 0.4 tons/h, height of bed 0.25; 0.50; 0.75 m, and air temperature 50; 70; 100 °C on drying process were studied. Keywords: Paddy, fluid bed dryer, batch, contonious, modelling, simulation

Size Dependence of the Drying Characteristics of Single Lignite Particles in Superheated Steam

Loy Yang lignite particles 10, 5, and 2.5 mm in diameter were dried with superheated steam at temperatures ranging from 383 K to 443 K (110 °C to 170 °C) under atmospheric pressure. The drying rates were obtained by measuring their weights with an electronic balance, while temperatures of the particles were monitored. The following typical drying characteristics were observed: (i) Temperature of lignite rose to 373 K (100 °C) accompanied by condensation of steam on the surface. (ii) A constant drying rate period was then observed, while temperature of the overall particle was maintained at 373 K (100 °C). (iii) A decreasing drying rate period began accompanied by a rise in surface temperature, and eventually the overall particle reached the temperature of the superheated steam. Based on these results, the evaporation rate of water was expressed as a function of particle diameter. A numerical model for simulating the drying process of lignite, which was developed for a large particle in a previous study, was modified to make it applicable to small particles. The model is applicable for simulations of drying behaviors of lignite with size distribution in various dryers when an appropriate heat transfer coefficient is given.

Analysis of convective heat and mass transfer coefficients for convective drying of a porous flat plate by conjugate modelling

Convective drying of an unsaturated porous flat plate at low Reynolds numbers (10 3) is analysed by means of conjugate modelling of heat and mass transport in the air flow and the porous material. Conjugate modelling does not require knowledge of convective transfer coefficients (CTCs) but allows determining the CTCs a posteriori, hence identifying their spatial and temporal variability, which is the main focus of this study. Comparison is made with porous-material modelling using spatially and/or temporally constant CTCs. Both spatial and temporal variations of the convective boundary conditions are found to have a distinct impact on the drying behaviour: the spatial CTC variation results in two-dimensional drying of the porous material due to leading-edge effects; the temporal CTC variation identifies distinct maxima in the drying rates at the surface right before the surface dries out locally. The CTCs obtained with conjugate modelling remain approximately constant during the constant and decreasing drying rate period (CDRP and DDRP, respectively), but a distinct variation is noticed at the transition of CDRP to DDRP. The heat and mass transfer analogy is found to be valid during the CDRP and to a lesser extent during the DDRP but large discrepancies are found during the transition of CDRP to DDRP. The advantages of conjugate modelling are indicated in this study but the need for detailed modelling of convective boundary conditions is however strongly dependent on the drying behaviour of the porous material and is not always required.

Analysis of the drying process of a biopolymer (poly-hydroxybutyrate) in rotating-pulsed fluidized bed

A rotating-pulsed fluidized bed (RPFB) dryer was employed to conduct the drying of poly-hydroxybutyrate (PHB) cohesive granules. Along the experiments, it was possible to identify, visually, 3 different dynamic regimes that were related with the temperature profile, the drying kinetics and the fluid dynamic behavior. The drying kinetics of PHB showed a short constant drying rate period followed by a decreasing drying rate period. The constant drying rate ( N c ) and final moisture content (dry basis) were related to the rotation frequency (responsible for the pulsation effect), temperature and velocity of the inlet air. Furthermore, measurements of molecular mass (gel permeation chromatography analysis) and Carr Index (flowability test) on PHB samples were done before and after the drying. The RPFB dryer showed to be appropriate to dry the PHB granules, resulting in an excellent fluid dynamic behavior that provided uniform drying of the solid. The best conditions of drying were identified at 7 Hz of rotation frequency, 90 °C and 0.55 m/s of inlet air temperature and velocity. At these conditions the dried PHB reached final moisture content of 0.56% (wet basis) after 2 h of drying.

Fluid Dynamics and Drying of Cohesive Particles of a Biodegradable Polymer (Poly-Hydroxybutyrate) in a Rotating Pulsed Fluidized Bed

This article presents the results of fluid dynamics and drying of poly-hydroxybutyrate (PHB) granules in a rotating pulsed fluidized bed (RPFB) dryer. Cohesiveness of PHB results in problems when using conventional drying techniques, such as a fluidized bed dryer. The excess surface water on the particles may form agglomerates that damage the flowability of the material inside the dryer. The drying study in RPFB consisted of adjusting mathematical models for predicting the moisture evaporation during the constant drying rate period (linear fit) and the decreasing drying rate period (analytical solution of Fick's second law). The influence of temperature, rotation frequency, and velocity of the drying air on the constant drying rate (Nc), effective diffusivity (Deff), and mass of particles elutriated (me) during drying was evaluated. Particle porosity was analyzed before and after drying and scanning electronic microscopy (SEM) was employed to verify structural changes on the PHB granules after drying. The RPFB dryer proved to be appropriate to dry the PHB granules, resulting in an excellent fluid dynamics behavior of the particles inside the bed and providing uniform drying of the solids. Temperature and velocity of the air had a significant influence on the drying process, but rotation frequency did not affect the process in the ranges analyzed. Fine particle elutriation occurred during the first 10 min of drying. After drying, the porosity changed and breakage of the particles was observed due to attrition inside the bed.

Stepwise Change of Operating Pressure in Vacuum Fluidized Bed Drying: An Experimental Study

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An analytical model for internal moisture content during the decreasing drying rate period

AbstractA system of analytical expressions for internal moisture distribution during the falling drying rate period for porous nonhygroscopic slabs is presented. This model is derived by resolution of thermal balance and mass flux continuity equations and on the assumption of a parabolic internal moisture profile in the liquid transport zone. This study is based on the assumption of the existence of a receding evaporative front within the product during the decreasing drying rate period and on heat transfer being the limiting mechanism. The model addresses the cases where heat transfer is the limiting mechanism during drying and applies to flat slabs which are classified into two groups, namely, thick products and thin products, depending on their thicknesses and drying conditions. Overall, the model predictions were in a slight deviation from experimental data obtained on plaster. © 2007 American Institute of Chemical Engineers AIChE J, 2008

Numerical and experimental investigation of convective drying in unsaturated porous media with bound water

The heat and mass transfer in an unsaturated wet cylindrical porous bed packed with quartz particles was investigated theoretically for relatively low convective drying rates. Local thermodynamic equilibrium was assumed in the mathematical model describing the multi-phase flow in the unsaturated porous media using the energy and mass conservation equations to describe the heat and mass transfer during the drying. The drying model included convection and capillary transport of the free water, diffusion of bound water, and convection and diffusion of the gas. The numerical results indicated that the drying process could be divided into three periods, the temperature rise period, the constant drying rate period and the decreasing drying rate period. The numerical results agreed well with the experimental data verifying that the mathematical model can evaluate the drying performance of porous media for low drying rates. The effects of drying conditions such as the ambient temperature, the relative humidity, and the velocity of the drying air, on the drying process were evaluated by numerical solution.

Drying of Dielectric Resin Coatings in the Presence of Polycondensation

ABSTRACT Drying of dielectric resin coatings is accompanied simultaneously by evaporation of multicomponent solvents and polycondensation from monomers. The characteristic of the drying is studied experimentally. As a test sample, a vanish consisting of trimellitic acid anhydride and 4.41-diphenylmethane diisocyanate dissolved in the mixture of N-methylpyrrolidone and xylene is coated on an aluminum pan. The sample is subjected in drying in two types of dryers: hot air heating and radiation heating. The constant drying rate period is not observed in any run. The maximum drying rate of the sample is lower than the evaporation rate from the solvent layer with no resin. There are remarkable fluctuations in the drying rate in the decreasing drying rate period. The fluctuations are caused by bubble formation. The progress of the reaction can be followed by IR spectroscopic analysis. From these results it is suggested that removal of the solvent and the product is inhibited by the formation of a polymer skin on the surface and it makes control of drying difficult.

Modelling and Simulation of a Direct Contact Rotary Dryer

ABSTRACT A mathematical model able to predict solid and drying gas temperature and moisture content axial profiles along a direct contact rotary dryer was developed. The study was focused on the drying kinetics based on phenomenological models. Two different drying mechanisms in the decreasing drying rate period were tested: proponional to the unbound moisture content and moisture diffusion inside the particle. Experimental data collected in a pilot-scale direct contact rotary dryer was used to validate the model. Soya and fish meals were used as drying material.

Mutual diffusion coefficient of the polycarbonate-methylene chloride system at 140.DEG.C.

The drying rate of methylene chloride from glassy polycarbonate (Tg=155°C) was measured by the thermobalance method at 140°C. It was found that the stage of drying was the decreasing drying-rate period. Analyzing the data with Fick's model, it was confirmed that the mutual diffusion coefficient of the polycarbonate-methylene chloride system is proportional to the second power of concentration. Further, it was found that the present data could be fitted using the correlation equation derived by Duda et al., based on the free volume theory.

Analysis of the Decreasing Drying Rate Period of Granular and Powdered Materials (II)

In our former report we discussed the relation between water content and material temperature at the decreasing drying rate period under the constant drying condition. The relations among the gas temperature, t, material temperature, tm, and water content, F, under the unsteady drying condition-in a dryer of continuous counter current or parallel current-were studied, too.For drying granular and powdered materials, the high temperature gas (400700°C) is frequently used. In such a case, as in the case of drying a material containing organic solvents, the sensible heat needed for heating the evaporated vapor from the evaporating temperature to the temperature of the main gas stream (the so-called Ackermann effect), cannot be overlooked.In this paper, the relations among t, tm and F shall be discussed, taking into consideration the presence of this effect in the decreasing drying rate period.I. Under the steady drying condition:The heat at the surface of the material, qnd, (Fig. 1) may be calculated by solving the differential equation, Eq. (11).qnd=h(t-tm)b/(eb-1) (15)b=RdCp/hThe decreasing drying rate, Rd, is shown by Eq. (20).Rd=W0(-dw)/A0dθ=Rc(F/Fc)=(h/Cp)(lnD)(F/Fc) (20)The heat transferred from the main gas stream to the boundary film, qtd, (Fig. 1) is represented by:qtd=h(t-tm)beb/(eb-1) (17)The heat, qnd, causes the drying and the rise of the temperature of the material.qnd=h(t-tm)b/(eb-1)=(W0/A0)(-dw/dθ)rm+(W0/A0)(c+cww)(dtm/dθ) (19)From Eqs. (19) and (20), (dtm/dw)={rm-Cp(t-tm)/(DF/Fc-1)}/(c+cww) (21)Eq. (21) shows the relation between tm and w in the decreasing drying rate period. This equation can be solved by the numerical method. When the sensible heat of water at critical water content (cwwc) is small as compared with the specific heat of the dried material (c>cwwc), and rm_??_rw, Eq. (21) can be solved analytically:(t-tm)=rwFc/cln, D∞Σk=11/n-k(DF/Fc-1/DF/Fc)k+(D/D-1)n(DF/Fc-1/DF/Fc)n{(t-tw)-rwFc/clnD)∞Σk=1(D/D-1)k} (23)n=(CpFc)/(clnD), D=Cp(t-tw)+rw}/rwUnder the ordinary drying condition, the terms below the third one may be discarded.II. Under the unsteady drying condition:The heat balance in the differential small area of a continuous counter or parallel current dryer is given by:±GCHdt=qtdAdθ (25)+ count. cur., - para. cur.From Eqs. (20)', (17) and (25)

Drying Process Analysis of the Decreasing Drying Rate Period of Granular and Powder Material

I. On the stational drying conditions:The drying rate curves for various materials and methods from which Eq. 1 is derived are shown in Figs. 1-4. It is clear from these that the decreasing drying rate of granular material is proportional to the water content of the material. The heat transfer between air and material is shown by Eq. (2). When the temp. gradient of the material is negligibly small, Eq. (3) is obtained. As shown in Figs. 5 and 6, the temp. gradient can be neglected for the granular material whose diameter is below 2-3mm. Eq. (4) derived from Eqs. (1) and (3) may be solved numerially, e.g., by Runge-Kutta's method. When the sensible heat of water (wc·cw) contained in the material is small as compared with the specific heat of the dried material and rm≅rw, Eq. (5) can be solved analytically:(6)In case (wc·cw) has a value comparable to the specific heat of the material, Fcrw/(c+cww )(t-tw)>>1 and rm≅rw, the following approximate equation, Eq. (8), can be obtained.(8)Eqs. (6) and (8) give the relation between tm and w.II. On the unsteady drying conditions:The unsteady drying which takes place in a continuous (parallel or counter current) dryer, such as a rotary, pneumatic conveying, spary or fluidized-bed dryer, can be presented by Eqs. (9)-(12). From these are derived Eqs. (13) and (14), whose solutions show the relations among t, tm and w in the dryer.These equations could be solved numerially. The calculated examples are shown in Table 1 and Figs. 9 and 10.The drying rate and the drying time in the continuous adiabatic dryer which can be easily calculated by using these relations may help to decide the dryer volume.The application of this calculation method to the dryer design would be expatiated upon in our report.