Drying Rate Period Research Articles

Effect of Temperature and Microwave Power Levels on Microwave Drying Kinetics of Zhaotong Lignite

Microwave drying is a promising and effective way to drying and upgrading lignite. The influence of temperature (100–140 °C) and microwave power levels (500–800 W) on thin-layer drying characteristics of Zhaotong lignite under microwave irradiation were investigated. Fourteen thin-layer drying models were used to analyze the microwave drying process while six thin-layer drying models were used to analyze the hot-air drying process. The microwave drying processes at all temperature (100–140 °C) or low microwave power levels (500–700 W) exhibited four periods: a warm-up period, a short constant period, the first and second falling rate period, while one falling rate period was found during hot-air drying. The effective diffusion coefficient of lignite were calculated and it increases with increasing temperature and microwave power levels. During microwave drying, the two-term exponential model is the most suitable model for all applied conditions, while the Modified Page model is the most suitable model to describe the hot-air drying experiments. The apparent activation energy were determined from Arrhenius equation and the values for the first and second falling rate period are 3.349 and 20.808 kJ·mol−1 at different temperatures, while they are 13.455 and 19.580 W·g−1 at different microwave power levels. This implies the apparent activation energy is higher during the second falling rate period, which suggest that the dewatering of absorbed water is more difficult than capillary water. The value of apparent activation energy in hot-air drying is between the first and second falling rate period of microwave drying. Results indicate that microwave drying is more suitable to dewatering free water and capillary water of lignite.

Banana (Musa acuminata) peel drying and powder characteristics obtained through shade and microwave drying processes

ABSTRACTThe banana peel is a common waste generated when the banana fruit is consumed. Drying is a common method that can preserve the banana peel so that it can be further processed to generate wealth from waste. In this work, the banana peel was dried under shade drying and microwave drying at microwave power settings of 100W, 440W and 1000W. Falling drying rate periods were observed at the end of the drying process for all drying conditions. The shortest drying time and lowest moisture content were obtained when using microwaving drying at 1000W. The banana peel dried under shade has the highest moisture content although having the longest drying period. The flowability of the dried banana peel powders ranges from fair to very, very poor flowability characteristics. The banana peel powder produced under microwave drying at 1000W can aid the preservation by having the lowest final moisture content as well as reducing problems associated with its handling during processing by displaying the best flowability compared to other drying processes studied.

A rationally based model applicable for heat pump tumble dryer

The present study proposes a rationally based heat pump clothes dryer model which is capable of handling transient behaviors of both air-side and refrigerant side. The model can take into account the geometrical variation of the heat exchangers. Yet the proposed model is free from extra constraints that were normally imposed by previous studies. The simulation shows that there exist three stages in a typical drying process, including a preheat stage, followed by a constant evaporation stage, and finally a falling drying rate period. The calculation indicates that the variation of the clothes temperature and the system suction/discharge pressure show an appreciable rise in the first stage, remain nearly unchanged at the second stage, and increase again at the final stage of drying process. Also, the corresponding maximum specific moisture extraction rate (SMER) peaks at the end of the second stage while the corresponding coefficient of performance (COP) drops continuously. The simulation also indicates that increasing the air volumetric flowrate yields a lower discharge/suction pressure, a higher COP value, and a shorter drying time. The results also suggested that a good strategy to maximize COP is initiated by a low volumetric rate during the preheat stage, increasing it during the second stage and lowering it to an intermediate flowrate at the end of the drying period. Increasing heat exchanger size, either by increasing heat exchanger width or the number of tube row, results in a lower suction/discharge pressure and a shorter drying time. Although both approaches adopt larger surface area to promote overall performance, it is found that increasing the width of heat exchanger yields more effective results.

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.

Microwave vacuum drying and multi-flash drying of pumpkin slices

In this study, the effect of different drying methods on the drying kinetics, product structures, rehydration kinetics and rehydration indices of dehydrated pumpkin slices was evaluated. For that, pumpkin slices were dehydrated by five different methods: i) Microwave multi-flash drying (MWMFD), ii) Microwave vacuum drying (MWVD), iii) Conductive multi-flash drying (KMFD), iv) Freeze-drying (FD), and v) Air-drying (AD). For reaching a moisture content of 0.022 g g−1 (dry basis, db) and water activity of 0.438, the drying times were 1.28 h for MWVD, 1.32 h for MWMFD, 3.50 h for KMFD, 16.33 h for AD and 16.67 h for FD. All the drying methods presented a constant drying rate period and a falling rate period. The highest constant drying rate was observed for MWVD, which was 1.3, 8, 22 and 53 times higher than the observed for MWMFD, KMFD, AD, and FD, respectively. Stereo micrographs of samples from multi-flash drying processes (KMFD and MWMFD) and from MWVD showed large pores in a highly porous structure. Rehydration of dried pumpkin slices was studied at 25 °C and 80 °C. Water temperature influenced samples rehydration ratio (RR = mass of rehydrated sample/mass of dried sample). RR up to 15 was observed at 25 °C, while RR up to 12 was observed at 80 °C. Samples from MWMFD and MWVD, rehydrated at 25 °C presented higher moisture after rehydration than those observed from AD, FD and KMFD samples. At 80 °C, FD samples showed the highest RR. From the whole results, one can conclude that MWVD and MWMFD are the suitable processes for producing dehydrated pumpkin slices in very short drying times.

Kinetics and Mathematical Modeling of Microwave Drying of Sri Lankan Black Pepper (Piper nigrum)

Drying characteristics of black pepper (Piper nigrum) was investigated in a microwave drying system. Drying experiments were carried out at three different microwave power levels, 180, 360 and 540 W and the moisture content was measured at different time intervals. Experimental results were fitted to seven thin layer drying models; Newton, Page, Henderson and Pabis, Logarithmic, Midilli et. al., Weibull and Kaleta et. al.. Statistical indicators; Coefficient of determination (R2), Root mean square error (RMSE) and reduced chi square values (c2) showed Midilli et. al., Weibull and Kaleta et. al. models give better fit to the experimental values. Drying rate constants and the equation constants were compared and analyzed. Similarities of the drying models were observed and discussed with respect to the equation parameters. The absence of the constant drying rate period in drying curves show the drying of black pepper lies totally in the falling rate period where the drying rate is controlled by the moisture diffusion. Maximum drying rates observed were 0.02, 0.05 and 0.08 kg moisture/kg of dry material/ min at 180, 360 and 540 W power levels. Results revealed drying rate constant and the effective moisture diffusivity values increases with the microwave power level. Drying rate constants were 0.03, 0.09 and 0.16 min-1 and the effective moisture diffusivity values were 2.43 x 10-10, 4.87 x 10-10, 1.42 x 10-9 m2/s for power levels of 180, 360 and 540 W respectively. The Activation energy of black pepper calculated based on the Arrhenius equation is 86.7 W/g.

Measurement of constant drying rate of wet material placed in a fluidized bed of inert particles under reduced pressure

ABSTRACTThe objective of this study is to estimate the drying characteristics of a relatively large material immersed in a fluidized bed under reduced pressure by measuring the constant drying rate. The constant drying-rate period in a fluidized bed under reduced pressure is difficult to measure because it is extremely short. To maintain the constant drying-rate period, distilled water is directly supplied to the drying material. Through our experiment, the heat transfer coefficient of the material surface was also determined. The results were compared with data on hot air drying. The constant drying rate is higher for fluidized bed drying than for hot air drying. It suggests that the heat transfer coefficient on the surface of the drying material is much larger for fluidized bed drying than for hot air drying. For fluidized bed drying, the effect of pressure in the drying chamber on the heat transfer coefficient is slight at the same normalized mass velocity of dry air (G/Gmf). The temperature difference between the inside of the drying chamber and the drying material is much smaller for fluidized bed drying than for hot air drying. The constant drying rate increases as the pressure in the drying chamber decreases.

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

Air-drying and rehydration characteristics of the brown seaweeds, Ascophylum nodosum and Undaria pinnatifida

The water sorption isotherms (from 5 to 65 °C) and air-drying kinetics (from 35 to 75 °C) of the brown seaweeds, Ascophylum nodosum and Undaria pinnatifida (UNP), were determined. Rehydration kinetics of dried seaweeds at different temperatures were obtained at room temperature. The Caurie model was successfully applied to the experimental water adsorption and desorption data. Air-drying kinetics of both seaweeds were satisfactorily modelled using the Page model. The critical moisture contents (dry basis), 1.3 ± 0.2 for A. nodosum and 5.0 ± 0.4 for UNP, were invariant from 35 to 75 °C. Heat and mass transfer coefficients during constant drying rate period and effective coefficients of water diffusion during falling drying rate period were calculated, assuming cylinder geometry for A. nodosum and plane-sheet geometry for UNP, considering volumetric shrinkage in both cases. The Peleg model was employed for rehydration kinetic modelling at 25 °C employing dried seaweeds at 35 and 75 °C. A. nodosum showed adequate rehydration properties recovering its colour and final moisture content after drying at low temperature (35 °C).

Drying kinetics of granulated cork: Effect of air drying stream conditions and granule size

Granulated cork is a by-product of natural cork stoppers manufacturing which is incorporated into highly valued products such as agglomerated cork stoppers and expanded cork insulation. Prior to this, the cork granules must be dried.This work presents an experimental study for the characterization of wet granulated cork drying kinetics with different morphologies: a powder and granules with different sizes (2–2.8, 2.8–4 and 4–5.6 mm). A drying process with hot air stream flowing tangentially to the solid was used for a systematic analysis of the influence of air stream conditions (temperature and flowrate) on drying kinetics. Under these conditions, the influence of the stream temperature was much more relevant than its velocity with the drying time almost halving when the temperature rose from 35.0 °C to 56.5 °C. The drained humid solid with higher granulometry was dried entirely during a falling drying rate period. All others products presented a constant drying rate period before the falling drying rate period.The results revealed that the falling drying rate period is conditioned by water migration through the solid and presents a similar shape for all products. Thus, the behavior can be described by a single characteristic drying curve, allowing predicting drying rate as a function of the solid normalized humidity, despite the cork granules dimensions. This normalized curve is a useful tool for dryer equipment dimensioning in the cork manufacturing industry.To predict the time of drying for defined air drying conditions, a simple procedure was proposed using the characteristic drying rate obtained.

Thin-layer infrared drying characteristics of construction gypsum plaster and selection of a suitable drying model

In this study, infrared drying behaviour of construction gypsum plaster samples was investigated. Experimental studies were carried out by using an infrared dryer at different temperatures (50–85°C) and at atmospheric conditions. As a result, the falling rate period was the drying rate period for the drying of gypsum plaster samples at low temperatures studied (50°C and 65°C). But, three drying rate periods consisting of beginning, constant, and falling periods occurred at the high temperature studied (85°C). FTIR and SEM analyses were performed to determine microstructural and surface properties of raw material and dried gypsum samples. A typical FTIR spectra for construction gypsum plaster samples have been obtained. It has been observed that dried gypsum plasters at the temperatures of 50°C and 65°C have a porous structure. On the contrary, raw material and dried gypsum at temperature of 85°C were no almost porous structure according to the gypsum plasters at other temperatures studied. Thus, it may be said that the drying process for construction gypsum plaster sample should be performed at temperatures below 85°C. In this work, the experimental drying data have also been fitted to various model equations by statistical testing. The most suitable model equations for drying of construction gypsum plasters have been determined by analysing the statistical test parameters such as chi-square, RMSE, and RSS at the temperatures studied. It has been founded that these models are Page and Diffusion equations. Also, derived forms of Page and Diffusion equations have been used to obtain an equation containing both drying time and temperature by multiple nonlinear regression analysis. As a result, it has been determined that a model derived Diffusion equation is the best representing the drying of CGP. Finally, the effective diffusivities and activation energy for the drying process of construction gypsum plaster samples have been calculated. The effective diffusivity values were 2.9920×10−9m2/s, 3.7527×10−9m2/s, and 3.9386×10−9m2/s at temperatures of 50°C, 65°C, and 85°C, respectively. The activation energy value was also 7.3795kJ/mol.

Drying regimes in homogeneous porous media from macro- to nanoscale

MRI visualization down to nanometric liquid films in model porous media with pore size from micro-to nano-meter enables to fully characterize the physical mechanisms of drying. In larger pores, we identify an initial constant drying rate period, probing homogeneous desaturation, followed by a falling drying rate period. This second period is associated with the development of a gradient in saturation underneath the sample free surface that initiates the inward recession of the contact line. During this latter stage, the drying rate varies in accordance with vapor diffusion through the dry porous region; possibly affected by Knudsen effect for small pore size. However, we show that for sufficiently small pore size and/or saturation the drying rate is increasingly reduced by the Kelvin effect. Subsequently, we demonstrate that this effect governs the kinetics of evaporation in nanopores as a homogeneous desaturation occurs. Eventually, under our experimental conditions, we show that the saturation unceasingly decreases in a homogeneous manner throughout the wet regions of the medium regardless of pore size or drying regime considered. This finding suggests the existence of continuous liquid flow towards the interface of higher evaporation, down to very low saturation or very small pore size. Paradoxically, even if this net flow is unidirectional and capillary driven, it corresponds to a series of diffused local capillary equilibrations over the full height of the sample, which might explain that a simple Darcy's law model does not predict the scaling of the net flow rate on the pore size observed in our tests.

Influence of stationary and non-stationary conditions on drying time and mechanical properties of a porcelain slab

This work deals with a numerical study of the response of a porcelain slab when subjected to convective drying in stationary and non-stationary conditions. The used model describes heat, mass, and momentum transfers is applied to an unsaturated viscoelastic medium described by a Maxwell model. The numerical code allows us to determine the effect of the surrounding air temperature on drying kinetics and on mechanical stress intensities. Von Mises stresses are analysed in order to foresee an eventual damage that may occur during drying. Simulation results for several temperatures in the range of [30 °C, 90 °C] shows that for the temperature from 30 °C to 60 °C, Von Mises stresses are always lower than the yield strength. But above 70 °C, Von Mises stresses are higher than the ultimate strength, and consequently there is a risk of crack at the end of the constant drying rate period. The idea proposed in this work is to integrate a reducing temperature phase when the predicted Von Mises stress intensity exceeds the admissible stress. Simulation results shows that a non-stationary convective drying (90–60 °C) allows us to optimize costs and quality by reducing the drying time and maintaining Von Mises stress values under the admissible stress.

Experimental study on freeze drying of Loy Yang lignite and inhibiting water re-adsorption of dried lignite

Loy Yang lignite with a high water content was dewatered by freeze drying (FD). The drying kinetics of FD was calculated using thin-layer drying models given in the literature. Residual water content, re-adsorption or desorption behaviors, and pore size distributions of all of the samples were investigated. Furthermore, coating with different amounts of kerosene by direct mixing or adsorption methods to restrain water re-adsorption was also investigated. The results showed that FD dewatering contained three steps: a fast dewatering period (∼2h), followed by a reduced drying-rate period (2–3h) and an apparently falling-rate period (>3h). The Midilli-Kucuk model described the drying process perfectly and could be employed to predict residual water content in the sample at any time during the FD dewatering process. The moisture holding capacity (MHC) of the FD-treated samples was lower than that of raw lignite, which is because of the effect of the water–lignite bond strength and the diffusion adsorption or desorption force. Moreover, adding kerosene by either adsorption or direct-mixing methods can both decrease MHC because kerosene is coated on the surface and in the pores of the lignite. These prevent water re-adsorption. The adsorption method is better than the direct-mixing method because it consumes less kerosene.

Performance Evaluation of a Biomass Fuelled Cabinet Dryer

In an attempt to reduce post-harvest losses of agricultural produce to considerable level in rural area in developing countries, a study was conducted to evaluate the performance of a developed biomass fuelled cabinet dryer. The effect of drying air temperature on unripe plantain sizes at 5, 10 and 15 mm dried in a thin-layer dryer was investigated within a temperature range of 50–70 °Cat regular interval of 30 minutes until equilibrium moisture content were reached. The performance evaluations of the experimental biomass cabinet dryer were conducted for thermal efficiency of charcoal and sawdust fuels respectively. It was found that experimental curves of the plantain drying rate versus time showed that drying rate increased when hot air drying increased with time. Moisture transfer occurred during the falling rate period of drying. Moisture loss of the plantain slices at 5 ,10 and 15 mm reached an equilibrium moisture content at 150, 180 and 240 minutes respectively when charcoal fuel was fired. While moisture loss on the plantain slices at 5, 10 and 15 mm reached an equilibrium moisture content at 120, 240 and 270 minutes respectively when sawdust fuel was fired. The maximum overall drying efficiency of the biomass fuel cabinet dryer when plantain slices were dried was calculated to be 89.67 %.

Modeling for Drying of Thin Layer of Native Cassava Starch in Tray Dryer

Purpose: The drying of a thin layer of native cassava starch in a tray dryer was modeled to establish an equation for predicting the drying behavior under given conditions. Methods: Drying tests were performed using samples of native cassava starch over a temperature range of 40-60°C. We investigated the variation in the drying time, dynamic equilibrium moisture content, drying rate period, critical moisture content, and effective diffusivity of the starch with temperature. The starch diffusion coefficient and drying activation energy were determined. A modification of the model developed by Hii et al. was devised and tested alongside fourteen other models. Results: For starch with an initial moisture content of 82% (db), the drying time and dynamic equilibrium moisture content decreased as the temperature increased. The constant drying rate phase preceded the falling rate phase between 40-55°C. Drying at 60°C occurred only in the falling rate phase. The critical moisture content was observed in the 40-55°C range and increased with the temperature. The effective diffusivity of the starch increased as the drying temperature increased from 40 to 60°C. The modified Hii et al. model produced randomized residual plots, the highest R2, and the lowest standard error of estimates. Conclusions: Drying time decreased linearly with an increase in the temperature, while the decrease in the moisture content was linear between 40-55°C. The constant drying rate phase occurred without any period of induction over a temperature range of 40-55°C prior to the falling rate period, while drying at 60°C took place only in the falling rate phase. The effective diffusivity had an Arrhenius relationship with the temperature. The modified Hii et al. model proved to be optimum for predicting the drying behavior of the starch in the tray dryer.

Microwave-hot air drying applied to selected cassava cultivars: drying kinetics and sensory acceptance

Summary The effects of three cassava cultivars produced in Brazil were assessed (IAC Espeto, IAC 14 and IAC Caapora) in order to obtain chips by precooking and hot air–microwave drying (70 °C, 95 W). The difference in the composition of cultivars has not influenced the drying of cassava slices. The drying kinetics presented a constant drying rate period followed by a decreasing drying rate with effective diffusion coefficient of 2.8–3.0 × 10−10 m2s−1. The hedonic means of all attributes ranged from ‘neither like nor dislike’ to ‘like moderately’. Therefore, the choice of the more suitable cultivar can be based on plant productivity and product yield. The IAC 14 cultivar resulted in the highest root production per plant, product mass per roots and chips yield.

Evaluation of thin-layer models for describing drying kinetics of poplar wood particles in a fluidized bed dryer

ABSTRACTIn this study, fluidized bed drying experiments were conducted for poplar wood particles (Populus deltoides) at temperatures ranging from 90°C to 120°C and air velocities ranging from 2.8 m s−1 to 3.3 m s−1. The initial moisture content (MC) and the bed height of the poplar wood particles were 150% (on an oven-dry basis) and 2 cm, respectively. The results showed that the drying rate increased by increasing the drying temperature and air velocity. The constant drying rate period was only observed at the early stages of the drying process and most of the drying processes were found in the falling rate period. The experimental data of the drying process were put into e11 models. Among these models, Midilli, Kucuk, and Yapar (2002) and Henderson and Pabis (1961) were found to satisfactorily describe the drying characteristics of poplar wood particles. The effective moisture diffusivity of wood particles increased from 7E-6 to 8.46E-6 and 7.65 E-6 to 1.44E-5 m2 s−1 as the drying air temperature increased from 90°C to 120°C for 2.8 m s−1 and 3.3 m s−1 of velocities, respectively. Also, the activation energies of diffusion were 34.08 kJ mol−1 and 64.70 kJ mol−1 for the air velocities of 2.8 m s−1 and 3.3 m s−1, respectively.