THIN LAYER METHOD ANALYSIS OF SPOUTED BED DRIED MALAYSIAN PADDY – CHARACTERISTIC DRYING CURVES

A lab‐scale spouted bed dryer was used for the drying of paddy. The Malaysian grown paddy is a sample of Geldart's Group D particle. Drying was carried out in the dryer at two different internal configurations, namely with or without the installation of a draft tube, and at several drying temperatures (60–90C), air flow rates (1.14–1.45 m/s) and bed heights (30–45 cm). Drying kinetics of paddy in spouted bed showed only induction and falling rate periods, without constant drying rate period. Thin layer method analysis shows characteristic drying rate curves of linear and exponential functions for the first and second falling rate period, respectively, in conventional spouted bed drying. A single power function was observed in spouted bed drying with a draft tube, mainly as a result of a better tempering effect that gave a lower drying rate gradient.     

Experimental and parametric study on drying of green peas in spouted bed

Drying of Green Peas (Pisum sativum) was studied experimentally in a lab‐scale spouted bed. Experiments were carried out at different operating conditions, and the effect of inlet air temperature and its flow rate, bed height (initial mass of wet particles) and average diameter of particles were examined on performance of dryer and rate of drying. The obtained experimental results were fitted by an exponential form equation to model the drying kinetic behaviour of green peas in a spouted bed. The fitting parameters are presented in the form of mathematical correlations as a function of operating parameters. As expected, it was found that by increasing inlet air temperature and air‐flow rate, the rate of drying increases, but by increasing the bed height and diameter of particles, the rate of drying decreases. The results can help the design engineers to choose the optimum drying conditions for industrial applications.     

Simulation of drying behaviour of a small spherical foodstuff in a microwave assisted fluidized bed of inert particles

Drying behaviour of a small spherical porous particle was investigated in a microwave assisted fluidized bed of inert energy carriers. A pilot-scale microwave assisted fluidized bed dryer with inert particles was set up for this purpose. Glass beads particles were used as inert materials. Based on the simultaneous heat and mass transfer, a mathematical model was proposed for predicting the temperature variation and moisture distribution in the drying sample during drying. A numerical solution was developed for the proposed model using an implicit finite difference method in one dimensional system. The model was validated with experimental average moisture contents of green peas during drying and the effects of microwave power density, drying air velocity and also drying air temperature were discussed.     

Analysis of giant pumpkin (Cucurbita maxima) drying kinetics in various technologies of convective drying

The paper analyses the results of the research on drying a variety of giant pumpkin (Justynka – 957). The research involved drying kinetics of pumpkin as well as changes in the volume of 10 mm pumpkin cubes, dried at the temperature of 80 °C, using free (natural) convection and in the forced convection in the tunnel dryer, with the drying factor speed of 1.2 m s⁻¹ and in the fluid bed dryer. Drying kinetics of pumpkin pieces dried in the tunnel dryer, fluid bed dryer and during two-stage initial drying in the tunnel, and in the fluid bed dryer were examined. Measurements results were used to verify the theoretical models of drying kinetics of the first period taking volume shrinkage into consideration, and the second period of the examined processes.     

Experimental investigation of drying behaviour and conditions of pumpkin slices via a cyclone‐type dryer

This paper describes an experimental investigation of the drying behaviour and conditions of pumpkin slices via a convective cyclone‐type dryer. Drying experiments were conducted at air inlet temperatures of 60, 70 and 80 °C and air velocities of 1 and 1.5 m s^?1. Samples of 200 g of cylindrical pumpkin slices (thickness 5 mm × diameter 35 mm) were arranged in a single layer on each of two trays. The least squares method was applied to derive the drying curve equation of samples. During the experiments the following parameters were measured: weight loss, temperature, relative moisture and velocity. From these parameters, dimensionless mass loss, moisture content, mass shrinkage and drying rate of samples were calculated and are discussed in detail. Additionally, the transport of water during dehydration was described by the diffusion mechanism, and Fick's equation was used for evaluation of the experimental data. The linear sections of drying curves were analysed by linear regression to obtain the moisture diffusivities. It was found that pumpkin slices would dry perfectly within 340–720 min under these drying conditions. The convective cyclone dryer could also be used to dry other products. Thus it is expected that this new dryer will help farmers and producers to reduce the cost of drying. Copyright © 2003 Society of Chemical Industry     

Construction of a solar drying unit suitable for conservation of food and enhancement of food security in West Africa

Drying is a widely used technique in sub-Saharan Africa which allows food conservation, weight reduction and added value for some products. However, the technique is quite complex, making the rational concept of drying devices challenging. A rational method for the design and the construction of a fruit and vegetable solar dryer is presented. A prototype was designed and constructed at the Université Libre de Bruxelles (Belgium) to test the concept and was found to be capable of drying more than 40 kg of fresh tomatoes in 10 h. A second prototype was constructed in Bandiagara (Mali) by both local craftsmen and students from the Université Libre de Bruxelles, using local materials. Speed of drying and prevention of food spoilage was far superior to that of traditional solar dryers. Based on these prototypes, a dryer industry is being constructed in Bandiagara in order to increase food self-sufficiency.     

Single Layer Drying Characteristics of Half Fruit Tomato

Drying kinetics of half fruit tomatoes were investigated in a laboratory dryer at the temperature of 35–65°C and air velocity of 0.13–1.00 m/s. The optimum drying conditions in terms of drying rate, colour and growth of microorganisms were also identified. Drying rate and shrinkage increased significantly with the increase in air temperature. But the red colour (a?) decreased significantly with the increase in air temperature. Drying rate increased with the increase of air velocity up to 0.75 m/s. The optimum drying air conditions was the temperature 55°C and air velocity 0.75 m/s, when drying time, colour, ripeness and growth of microorganisms were considered. Microorganisms (molds and fungus) were grown at or below 40°C temperature and it could be prevented by sodium metabisulphite pretreatment. The experimental data were fitted to six different single layer drying models and Midilli-Kucuk-Yapar model was found the best to predict the moisture content.     

Effects of pretreatments and drying air temperature on drying behaviour of peach slice

In this study, the effect of pretreatments (potassium meta bisulphite and ascorbic acid) and drying air temperature (55 and 65 °C) on drying behaviour of peach slices were investigated. The peach slices were dried in a cross-flow tunnel dryer. Drying of peach occurred in falling rate period. Pretreated samples had higher drying rate. To select a suitable drying curve, six thin-layer drying models were fitted to the experimental data. The logarithmic model best described the drying behaviour of peach slices with high correlation coefficient values. The effective moisture diffusivity of treated samples was higher than the untreated samples.     

Drying and Rehydrating Kinetics of Green and Red Peppers

ABSTRACT: The effects of air temperature, air velocity, and pretreatments (blanching, sulphiting, and sodium chloride dipping) on drying kinetics of green- and red-pepper slices were investigated. Drying experiments were performed in a fluidized bed dryer. In the falling rate period, moisture transfer from peppers was described by applying the unsteady state Fickian diffusion model, and the apparent moisture diffusion coefficients (D_a) were calculated. The effect of temperature on D_a could be interpreted according to Arrhenius law. Drying rate and therefore D_a values were found to be affected by pretreatments. Rehydration rates of dried peppers at 25 and 45 °C were also determined and found to be independent of drying conditions and rehydration temperature. Predrying treatments were found to improve partly the rehydration characteristics of peppers.     

Analysis of dryer performance for the improvement of small‐scale litchi processing

Litchi is a significant cash crop in Thailand, but marketing is constrained by the perishable nature of the fruit. Drying can extend the shelf life of the fruit; however, in Thailand drying technology is still in its early stages of development. With the aim of improving locally available small‐scale drying equipment, the performance of a litchi batch‐dryer by a farmers’ cooperative near Chiang Mai was studied. Drying conditions, energy consumption and product quality were monitored over three trials. Energy performance was analysed using instantaneous indices, leading to suggestions for possible design‐modifications. About 29.8% of the heat input was used for moisture evaporation, 38.4% was lost via unsaturated exhaust air and 17.6% was lost to the ambient environment. An optimum air flow rate, one that minimises exhaust heat losses without increasing the drying time, was calculated. Temperature and air flow within the drying chamber varied, resulting in a non‐uniform batch. To improve performance, a new design for the drying chamber air inlet is recommended.     

SOLAR OSMOVAC-DEHYDRATION of PAPAYA

A solar osmotic dryer was designed and constructed to test the utilization of solar energy in the two-step osmovac-dehydration of papaya. Drying rates measured in solar versus nonsolar osmotic dehydration experiments showed that solar osmotic drying had higher drying rates and sucrose uptake in the papaya samples (0.6 cm thick × 4 cm long × 2 cm wide) than in the nonsolar runs. In separate experiments, drying rates from solar vacuumdrying (as a second step of the osmovac-drying process) were about twice those of nonsolar vacuum-drying. Drying rates of both solar and nonsolar vacuum-drying reached the end of the constant-rate period in about 3 h. Sensory qualities of solar osmovac-dried papaya were comparable to those of vacuum-dried or solar osmotic-nonsolar vacuum-dried. These results suggest the possibility of drying rate increase and quality retention in utilizing solar energy in the osmovac-dehydration process.     

Application of simulation in determining suitable operating parameters for industrial scale fluidized bed dryer during drying of high impurity moist paddy

A systematic approach has been developed for selecting the suitable drying parameters to be used for drying of high moisture and high impurity paddy with an industrial fluidized bed paddy dryer (10–20 t h⁻¹ capacity) based on targeted specific air flow rate and residence time during two typical paddy drying seasons. A mathematical model was developed by modifying an existing model and was simulated and validated with observed industrial drying data as well as data reported in the literature. Comparison between the observed and simulated results showed that the mathematical model is capable of predicting outlet paddy moisture content and air temperature well. Suitable operating parameters were determined for reducing any initial paddy moisture content (mc) down to 24–25% dry basis (db), the safe mc level after fluidized bed drying to maintain rice quality, to achieve maximum possible throughput capacity of the dryer with corresponding energy consumption. Based on these criteria, bed thickness at 10 cm, specific air flow rate of 0.05 kg kg⁻¹ s⁻¹ (for corresponding bed air velocity of 2.3 m s⁻¹), air temperature of 150 °C and residence time of 1.0 min were found to be suitable drying conditions for reducing paddy mc from 30 to 24.30% (db) in one season while the maximum throughput capacity of 15.7 tonne per hour (t h⁻¹) might be achieved. The specific electrical and thermal energy were 0.48 and 6.15 MJ kg⁻¹ water evaporated, respectively. On the other hand, the dryer capacity was found to be limited to 7.4 t h⁻¹ during drying paddy of higher initial mc (35% db). This approach might provide easy and comprehensive guidelines for selecting suitable sets of operating parameters for any industrial fluidized bed dryer at its possible maximum throughput capacity for drying of freshly harvested high moist paddy with a high level of impurities.     

Fluidised bed drying of soybeans

The fluidised bed drying characteristics of soybeans at high temperatures (110-140 degrees C) and moisture contents, 31-49% dry basis, were modelled using drying equations from the literature. Air speeds of 2.4-4.1 m/s and bed depths from 10 to 15 cm were used. The minimum fluidised bed velocity was 1.9 m/s. From a quality point of view, fluidised bed drying was found to reduce the level of urease activity which is an indirect measure of trypsin inhibitor, with 120 degrees C being the minimum required to reduce the urease activity to an acceptable level. Increased air temperatures caused increased cracking and breakage, with temperatures below 140 degrees C giving an acceptable level for the animal feed industry in Thailand. The protein level was not significantly reduced in this temperature range. The drying rate equations and quality models were then combined to develop optimum strategies for fluidised bed drying, based on quality criteria, drying capacity, energy consumption and drying cost. The results showed that from 33.3% dry basis, soybean should not be dried below 23.5% dry basis in the fluidised bed dryer, to avoid excessive grain cracking. The optimum conditions for minimum cost, minimum energy and maximum capacity coincided at a drying temperature of 140 degrees C, bed depth of 18 cm, air velocity of 2.9 m/s and fraction of air recirculated of 0.9. These conditions resulted in 27% cracking, 1.7% breakage and an energy consumption of 6.8 MJ/kg water evaporated.     

Experimental study on drying characteristics of pomegranate peels

Drying kinetics of pomegranate peels has been experimentally investigated in a cabinet dryer. Drying experiments were performed at constant air velocity of 2.0 m/s and initial thickness of 2.8 cm for pomegranate peels, and 3 drying air temperatures of 50, 60, and 70°C. The drying time decreased with increase in drying air temperature. Experimental data were fitted to 10 mathematical models. The fit quality of models on experimental data were evaluated using 3 statistical tests, coefficient of determination, reduced chi-square, and root mean square error. The statistical analysis concluded that the best model in terms of fitting performance was the Midilli et al. model. The effective diffusivity varied from 4.02 to 5.31×10⁻⁹m²/s over temperature range. Temperature dependence of the diffusivity was well documented by Arrhenius-type relationship. The activation energy was found to be 12.72 kJ/mol for pomegranate peels.     

Drying of yellow pea starch on inert carriers: Drying kinetics,moisture diffusivity,and product quality

This study aimed at investigating the drying of yellow pea starch dispersions on inert solid carriers and determining the drying kinetics, moisture diffusivity and the product quality, quantified through damage index and final moisture content. Drying kinetics accomplished in a convective drying tunnel show that the overall mass transfer is controlled by internal migration of moisture within the starch particles. For a given inlet air temperature from 100 to 180 °C, the apparent diffusion coefficient derived from the drying curves increases exponentially with the instantaneous moisture content, with values ranging from 4 × 10⁻¹¹ to 3 × 10⁻⁸ m²/s. Due to low diffusivity and the thin coat formed on the surface of solid carriers, the resistance to internal diffusion is negligible as compared to the overall mass transfer resistance when drying of starch dispersions takes place on inert solid carriers. Drying of yellow pea starch dispersion on Teflon particles as inert carriers was studied in laboratory and pilot fast spouted bed dryer for inlet air temperatures from 140 to 240 °C, and initial solid content of 38%, mass (d.b.). The starch damage index for targeted product moisture content was below 2.5% in the inlet air temperature range from 120 to 210 °C, when atomizing from the bottom of dryer.     

DRYING of COFFEE BERRIES IN A VIBRATED TRAY DRYER OPERATED WITH SOLIDS RECYCLE and SINGLE‐STAGE1

A vibrated tray dryer was tested for drying coffee berries. It consists of a vertical chamber (0.33 m × 0.18 m × 1.30 m) provided with four trays. the trays are supported by a vibrated shaft. Experimental tests were carried out at a vibration amplitude of 1 mm and at a variable frequency. the process itself consisted of feeding the vibrated tray dryer with an initial mass of coffee berries: after leaving the dryer through the discharge channel, the berries were fed again to the top of the dryer by a bucket elevator (intermittent operation with recycle of the coffee berries). Drying tests were carried out with drying air velocity of 2.4 m/s. the drying air control temperature was related with the temperature of the coffee berries at the discharge (under 45C). the drying time required for coffee berries to reach 12.6% (wb) moisture was 28 h, initial mass (moisture of 67% wb) was 5.54 kg. In this work the influence of vibration in single‐stage drying is also shown. the single‐stage operation shows that vibration (besides promoting the conveyance of coffee berries) also reduces the time of drying.     

Modeling Effective Moisture Diffusivity of Wheat (Tajan) During Air Drying

Drying conditions can greatly affect the physical and mechanical properties of grains. The drying process must be controlled to reduce or minimize drying damage. In this paper the thin layer drying behavior of wheat (Tajan) in a convective dryer is experimentally investigated. The mathematical modeling by using thin layer kinetics drying models available in literature was performed. Drying experiments were conducted at inlet drying air temperatures of 35, 45, 50, 60 and 70°C and at a fixed drying air velocity of 0.3 m/s and initial moisture content of 0.26–0.27 (d.b.). The effects of drying air temperature on the model's parameters were predicted by a linear regression analysis. The constants and coefficients of this model were be explained in terms of drying air temperature. A correlation coefficient (r) of 0.99 was found for the multiple regression model of moisture content during the drying process using different temperature values. Values of the diffusion coefficients for the whole kernel ranged from 2.28 × 10^?11 to 1.14 × 10^?10 m²/s.     

Industrial paddy drying and energy saving options

Paddy drying is an energy-intensive process and influences rice quality. In this study, the energy consumption of paddy drying in a large-scale milling plant was investigated. Furthermore, some drying experiments were conducted in a laboratory. The aims were to gain practical information and to propose more economical drying options to the industry. The results indicated that the current drying in the plant consumed specific primary energy of between 3.874 and 4.421 MJ/kg of water evaporated. The experimental results showed that two-stage drying with tempering by using a fluidised bed dryer at 100–110 °C in the first stage and drying with ambient air using a solar dryer for the second stage provided rice quality that was comparable to that of the plant. Also, from the calculation, the energy cost of the plant could be reduced if an in-store dryer was used after the first-stage drying by LSU dryer.     

Performance analysis of drying of green olive in a tray dryer

This paper deals with the performance evaluation of a single layer drying process of green olives in a tray dryer using exergy analysis method. Green olive was used as the test material being dried. Drying process was realized at four different drying air temperatures (40, 50, 60 and 70 °C) and a constant relative humidity of 15%. The effects of temperatures and mass flow rates were investigated. Maximum exergy efficiency of the drying chamber was obtained at a temperature of 70 °C and a drying air mass flow rate of 0.015 kg/s with 0.0004 kg/s of olive. The exergy efficiency values were found to be in the range of 68.65%–91.79% from 40 °C to 70 °C with drying air mass flow rates of 0.01 kg/s–0.015 kg/s.