Fluidized Bed Drying of Aonla (Emblica officinalis)

In the present work an attempt has been made to study the dehydration of aonla (Indian gooseberry) fruits. Aonla fruits, being highly perishable, cannot be kept for long periods. Aonla contains a very high amount of vitamin C, which is highly volatile and susceptible to heat. Sun drying required the longest period of drying (660 min), while the shortest time of drying is with fluidized bed drying at 80°C with 115 m/min air velocity (120 min). The results indicate that there is great loss of most of the ascorbic acid in the aonla slices. This suggests that the drying exposure caused the loss of volatile biochemical compounds. The retention of ascorbic acid in the samples dried in fluidized bed drying is greater compared to those dried under sun and hot air tray.     

Drying Suspensions in a Pulsed Fluidized Bed of Inert Particles

The drying of carbohydrate suspensions on polypropylene particles in a pulsed fluidized bed was studied by means of a 2⁵ experimental design, to determine the effect of the air flow and temperature, suspension flow rate, and free section and rotating speed of the rotary plate on the Nusselt number, the moisture content of the product, and the percentage of solids retained inside the bed (which were minimized to 4.9 and 14.4%, respectively) with an air flow of 600 m³/h at 90°C and 720 mmHg, a suspension flow rate of 6 L/h, and a plate with 6% free section, rotating at 50 rpm.

Additionally, the effects of temperature, air flow, and suspension flow rate on the residence time distribution (RTD) were determined, using the stimulus-response methodology. The RTD was represented by 1.1 to 2 tanks in series, according to this model. The mean residence time of the dried carbohydrate particles was between 5.4 and 8.2 min.

Finally, an egg suspension could be dried at 4 L/h, with air at 90°C, with a mean residence time about 50% longer that that found for drying carbohydrate suspensions.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D_e) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X_o). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

Effects of High-Temperature Drying on the Flavor Components in Thai Fragrant Rice

The effects of high-temperature drying on Thai Jasmine rice were investigated. Paddy was first rewetted and then dried using a two-stage drying process, involving a fluidized bed at high temperature, followed by shade drying (ambient air drying). Three high drying temperatures: 100, 125, and 150°C were studied. Volatile compounds were extracted by distillation and extraction in a Likens and Nickerson apparatus, and analyzed by gas chromatography-mass spectrometry. A total of 94 volatile compounds were identified. These included 21 alcohols, 19 aldehydes, 14 ketones, 12 acids, 9 heterocyclic compounds, 8 hydrocarbons, 5 esters, and 6 miscellaneous compounds. 2-Acetyl-1-pyrroline, a major constituent of volatiles in fragrant rice was identified and tended to increase in concentration with increasing drying temperature. The three different drying temperatures used in the fluidized bed dryer were found to have an effect on the volatile components of Thai Jasmine rice. The increase in drying temperature contributed to the formation of new compounds, as well as the loss of other desirable volatiles.     

Kinetics and Mass Transfer during Atmospheric Freeze Drying of Red Pepper

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

Kinetics of ultrasound pretreated apple cubes dried in fluidized bed dryer

Abstract

This work evaluated the effect of ultrasonic pretreatment on the production of dehydrated apples (Malus domestica L. var Granny Smith) in a fluidized bed dryer. Cube-shaped apple samples were subjected to ultrasound in an ultrasonic bath and dried in a fluidized bed drier. The experimental design evaluated the effect of ultrasound pretreatment time (0 to 30?min) on the soluble solids loss during pretreatment and on the drying time. The ultrasonic pretreatment was carried out in a bath ultrasound operating at 25?kHz and outputting 55?W/m³ of power density. Distilled water was applied in the pretreatment to produce low-calorie apple cubes. Fluidized bed drying was carried out at 30, 40, and 50?°C. Fick’s law was used to model the drying process and to determine the apparent water diffusivity. The soluble solid loss ranged between 8.7 and 21.2% during the pretreatment, and the apparent water diffusivity during air drying ranged from 1.09?×?10^?6 to 2.81?×?10^?6 m²/min. Ultrasound pretreatment increased the apparent water diffusivity up to 58%. Apple cubes subjected to 20?min of ultrasound pretreatment and dried at 50?°C presented the highest apparent water diffusivity and dried to achieve a water activity of 0.4 in 100?min.     

DRYING CHARACTERISTICS OF RED CHILLI

Red chilli is widely consumed as a food additive throughout the world. It is blanched/treated to minimize quality loss during processing. This paper reports on various pre-treatments applied before drying and their influence on drying kinetics as well as product quality. Inactivation of peroxidase enzyme was achieved by blanching chillies at 90°C for 3 min in hot water. The physical appearance of the dried product was found to be the best when the blanched samples were soaked in gum acacia solution (0.2% m/v) for 15 min at room temperature. The pre-treated chillies were dried in a tray dryer at selected temperatures (55, 60, 65 and 70°C). Results indicated that drying took place in the falling rate period; the drying kinetics were adequately described by the Page's model. The activation energy for drying was determined to be 41.95 and 41.06 kJ/mol respectively, for blanched and gum-treated chillies. Total pigment content decreased while non-enzymatic browning increased with increase in drying air temperature.     

MOBILE FLUIDIZED BED PADDY DRYER

The objectives of this research are to design, construct and test a mobile fluidized bed paddy dryer with a drying capacity of 2.5-4.0 t/h. Suitable drying conditions are recommended as follows : drying capacity 3.8 t/h, bed velocity 2.8 m/s, average drying air temperature 144 °C, bed height 13.5 cm, fraction of air recycled 0.8. Residence time of paddy was approximately 1.3 minutes. Test results showed that moisture content of paddy was reduced from 32.6 % dry-basis to 25.8 % dry-basis. Consumption of electrical power and diesel fuel was 12.9 kW and 21.71 1/h respectively. Primary energy consumption was 910.9 MJ/h. The dryer could evaporate water 218.8 kg/h. Specific primary energy consumption was 4.2 MJ/kg-water evaporated. Cost of paddy drying was 1.48 baht/kg-water evaporated of which 0.53 was fixed cost and 0.95 was energy cost (US$1 =34baht).     

Attrition of Victorian brown coal during drying in a fluidized bed

Analyzing the attrition of Victorian brown coal during air and steam fluidized bed drying, the change in particle size distribution over a range of initial moisture contents (60% to 0%) and residence times (0 to 60 minutes) was determined. Dried at a temperature of 130°C with a fluidization velocity 0.55 m/s and an initial particle size of 0.5–1.2 mm, both fluidization mediums show a shift in the particle size distribution between three and four minutes of fluidization, with a decrease in mean particle size from 665 µm to around 560 µm. Using differential scanning calorimetry (DSC), the change in particle size has been attributed to the transition between bulk and non-freezable water (approximately 55% moisture loss) and can be linked to the removal of adhesion water, but not to fluidization effects. This is proved through the comparison of air fluidized bed drying, steam fluidized bed drying, and fixed bed drying—the fixed bed drying is being used to determine the particle size distribution as a function of drying. The results show the three drying methods produce similar particle size distributions, indicating that both fluidization and fluidization medium have no impact upon the particle size distribution at short residence times around ten minutes. The cumulative particle size distribution for air and steam fluidized bed dried coal has been modeled using the equation P_d = A₂ + (A₁ ? A₂)/(1 + (d/x₀)^p), with the resultant equations predicting the effects of moisture content on the particle size distribution. Analyzing the effect of longer residence times of 30 and 60 minutes, the particle size distribution for steam fluidized bed dried coal remains the same, while air fluidized bed dried coal has a greater proportion of smaller particles.     

Drying Kinetics and Color Retention of Dehydrated Rosehips

Freshly harvested rosehips (Rosa canina L.) were dehydrated in a parallel flow type air dryer at six air temperatures (30, 40, 50, 60, and 70°C) at air velocities of 0.5, 1.0, and 1.5 m/s. Drying air temperature and velocity significantly influenced drying time and energy requirement. Minimum and maximum energy requirement for drying of rosehips were determined as 6.69 kWh/kg for 70°C at 0.5 m/s, and 42.46 kWh/kg for 50°C, 1.5 m/s. In order to reduce drying energy consumption, it is recommended that the drying air velocity must not be more than 0.5 m/s and drying air temperature should be 70°C. In addition, the influence of drying air temperature and air velocity on the color of dried rosehip has been studied. Hunter L, a, b values were used to evaluate changes in the total color difference (ΔE) on dried rosehips. 70°C drying air temperature and 1 m/s air velocity were found to yield better quality product.     

Stationary versus Fluidized-Bed Drying of High-Moisture Paddy with Rest Period

Drying of high-moisture paddy was examined experimentally under stationary and fluidized bed with and without intervening rest periods. Introduction of a rest period between first and second stage of drying improved drying rate and lowered the energy requirement and increased head rice yield. Fluidization further improved the overall drying process. A single-term drying diffusion equation was suitably fitted to first, rest, and second stage drying data of fluidized and stationary bed by applying nonlinear regression method and effective diffusion coefficients were evaluated. During the period of rest stage, paddy grain released a considerable amount of moisture as an effect of residual grain temperature. An appropriate moisture ratio at which resting should start and the length of resting were evaluated by measuring changes in relative humidity in the headspace of mass of paddy and also from the diffusion coefficient values obtained from the experimental drying data. Resting duration between 75 and 90 min at moisture ratio around 0.715 was found suitable for overall good performance in both fluidized and stationary bed drying. A considerable amount of energy (21-44%) can be saved by providing a rest period from 30 to 120 min between the two stages of drying. Fluidization further reduces (≈ 50% against continuous drying under stationary bed) the energy requirement. No significant difference was found in head rice yield obtained from fluidized and stationary bed experiments, though discontinuing drying by providing intervening rest periods considerably improved the percentage head rice when compared with the results from continuous drying.     

OSMO-CONVECTIVE DRYING OF GRAPES

In order to reduce browning of grapes during drying, a special drying method was developed and evaluated using a laboratory scale fluidized bed dryer. Fresh Thompson seedless grapes were initially dried by immersion in a fluidized bed of sugar. The mass ratio of grapes to sugar was 1:1. The flow rate of hot air (at 45 and 60^°C) was sufficient to fluidize the sugar bed, while grapes placed on the screen, 3 cm above the drying air distributor, remained generally stationary.

Due to the simultaneous osmotic and convection drying effects, the drying time was reduced by factor ∼1.5 as compared to drying under the similar conditions without added sugar. A special pre-treatment of dipping of grapes in ethyl oleate (2% solution in 0.5% sodium hydroxide) at 80†deg; C for 30 s further reduced the drying time by factor 2 in both cases. The color of osmo-convective dried grapes were comparable to that of sulfur dioxide treated grapes. The texture of osmo-convective dried grapes was more pliable (softer) than convective dried samples. The major problem associated with die osmo-convective drying of grapes on a sugar bed was the stickiness, caused by sugar, on the fruit surface. This was reduced by partially substituting sugar with semolina (maintaining a 1:1 ratio) to create fluidized bed.     

Drying of Sodium Acetate in a Pulsed Fluid Bed Dryer

Sodium acetate crystals obtained from the reaction of acetic acid with sodium hydroxide are usually dried in rotary or fluidized beds. In this study, a batch pulsed fluid bed dryer with a 0.18 m² cross‐sectional area was used in an attempt to reduce energy consumption and increase productivity. Drying curves of sodium acetate were determined for different conditions: inlet air temperature of 65 and 80 °C and pulsation frequency of 0 rpm (conventional fluidized bed), 500 and 900 rpm (pulsed fluid bed). A 2² factorial design was used to analyze the results. The intermittent flow helped to break agglomerates and provided better contact between particles and the gas. Drying rates were higher under pulsed fluidization when compared to conventional fluidization. Conventional fluidized bed drying consumed 2.5 times more energy at 80 °C. The influence of temperature on the drying rate was also evident.     

Effects of High-Temperature Fluidized Bed Drying and Tempering on Kernel Cracking and Milling Quality of Vietnamese Rice Varieties

Studies on the effects of high-temperature fluidized bed drying and tempering on physical properties and milling quality of two long-grain freshly harvested Vietnamese rice varieties, A10 (32±1% wet basis moisture) and OM2717 (24.5±0.5% wet basis moisture), were undertaken. Rice samples were fluidized bed dried at 80 and 90°C for 2.5 and 3.0 min, then tempered at 75 and 86°C for up to 1 h, followed by final drying to below 14% moisture (wet basis) at 35°C by thin-layer drying method. Head rice yield significantly improved with extended tempering time to 40 min. Head rice yield tended to increase with decreasing cracked (fissured) kernels. The hardness and stiffness of sound fluidized bed dried rice kernels (in the range of 30–55 N and 162–168 N/mm, respectively) were higher than that of conventionally dried ones (thin layer dried at 35°C). The color of milled rice was significantly (P < 0.05) affected by high-temperature fluidized bed drying, but the absolute change in the value was very small.     

Drying Behavior of Carrots Dried in a Spout–Fluidized Bed Dryer

The effect of water blanching treatment and the inlet air temperature on drying kinetics as well as the quality attributes of carrot cubes dried in a spout–fluidized bed dryer at 60, 70, 80, and 90°C were analyzed. The material shrinkage and the rehydration potential were calculated to assess the changes in quality of dried carrots. It was found that the value of the air velocity during the drying of carrot cubes in a spout–fluidized bed dryer should be related to the moisture content of the carrot particles. A high value of air velocity at the beginning of the drying cycle and a lower value for the later stages were also required. The linear equation was correlated to the data of shrinkage of raw and blanched carrots. Blanching significantly influenced the coefficients in the shrinkage model derived for drying of carrot cubes in a spout–fluidized bed dryer, while drying temperature did not influence the shrinkage of carrot particles. The intensity of heat and mass transfer during spout–fluidized drying of carrot cubes was dependent on the drying temperature. A correlation was developed to calculate the values of effective moisture diffusivity of dried carrot cubes as a function of the moisture content and temperature of the material. It was observed that for any given time of rehydration, both the moisture content and the rehydration ratio calculated for samples dried at 60°C were higher than for samples dried at temperatures of 60, 70, 80, and 90°C.     

OSMO-CONVECTIVE DRYING OF GRAPES

ABSTRACT

In order to reduce browning of grapes during drying, a special drying method was developed and evaluated using a laboratory scale fluidized bed dryer. Fresh Thompson seedless grapes were initially dried by immersion in a fluidized bed of sugar. The mass ratio of grapes to sugar was 1:1. The flow rate of hot air (at 45 and 60^°C) was sufficient to fluidize the sugar bed, while grapes placed on the screen, 3 cm above the drying air distributor, remained generally stationary.

Due to the simultaneous osmotic and convection drying effects, the drying time was reduced by factor ～1.5 as compared to drying under the similar conditions without added sugar. A special pre-treatment of dipping of grapes in ethyl oleate (2% solution in 0.5% sodium hydroxide) at 80?deg; C for 30 s further reduced the drying time by factor 2 in both cases. The color of osmo-convective dried grapes were comparable to that of sulfur dioxide treated grapes. The texture of osmo-convective dried grapes was more pliable (softer) than convective dried samples. The major problem associated with die osmo-convective drying of grapes on a sugar bed was the stickiness, caused by sugar, on the fruit surface. This was reduced by partially substituting sugar with semolina (maintaining a 1:1 ratio) to create fluidized bed.     

Effect of Pulsed Electric Field and Osmotic Dehydration Pretreatment on the Convective Drying of Carrot Tissue

The influence of pulsed electric field (PEF) and subsequent centrifugal osmotic dehydration (OD) on the convective drying behavior of carrot is investigated. The PEF was carried out at an intensity of E = 0.60 kV/cm and a treatment duration of t_PEF = 50 ms. The following centrifugal OD was performed in a sucrose solution of 65% (w/w) at 40°C for 0, 1, 2, or 4 h under 2400 × g. The drying was performed after the centrifugal OD for temperatures 40-60°C and at constant air rate (6 m³/h).

With the increase of OD duration the air drying time is reduced spectacularly. The dimensionless moisture ratio Xr = 0.1 is reached for PEF-untreated carrots after 370 min of air drying at 60°C in absence of centrifugal OD against 90 min of air drying after the 240 min of centrifugal OD. The PEF treatment reduces additionally the air drying time. The total time of dehydration operations can be shortened when OD time is optimized. For instance, the minimal time required to dehydrate untreated carrots until Xr = 0.1 is 260 min (120 min of OD at 40°C and 140 min of drying at 60°C). It is reduced to 230 min with PEF-treated carrots.

The moisture effective diffusivity D_eff is calculated for the convective air drying based on Fick's law. The centrifugal OD pretreatment increases drastically the value of D_eff. For instance, 4 h of centrifugal OD permitted increasing the value of D_eff from 0.93 · 10^-9 to 3.85 · 10^-9 m²/s for untreated carrots and from 1.17 · 10^-9 to 5.10 · 10^-9 m²/s for PEF-treated carrots.     

A Small-Scale Pneumatic Conveying Dryer of Rough Rice

This article studies the possibility of reducing the high initial moisture content of wet rough rice using a small-scale low-cost pneumatic conveying dryer as a first stage dryer. The parameters investigated are final moisture content, surface temperature of rough rice, head rice yield, drying rate, power consumption per unit mass of evaporated water, and physical characteristics of rice. Parametric effects of the following variables are examined: velocity of drying air from 20 to 30 m/s, feed rate of rough rice from 150 to 350 kg/h, initial moisture content from 22 to 26% (wet basis), and drying air temperature from 35 to 70°C. From the experimental results, it is found that this drying method can be used for fresh rough rice with an initial moisture content of over 24% (wet basis). The drying process is able to lead to very rapid drying without any grain quality problems such as cracks in the rice kernel. The moisture content can be reduced to approximately 18% (wet basis) or about 5-6% of the initial moisture content within 3-4 s. The optimal drying air temperature is in the range of 50 to 60°C. A comparison of pneumatic conveying drying data obtained from the present study with fluidized bed drying data reported in the open literature is also discussed.     

Effect of Fluidized Bed Drying Temperature on Various Quality Attributes of Paddy

Abstract

As reported by many researchers, it was found that fluidized bed paddy drying using high drying air temperatures of over 100°C affected the head rice yield and whiteness of dried rice. However, only a few studies on fluidized bed paddy drying with drying air temperatures below 100°C were so far reported. The main objective of this work was therefore to study the effect of fluidized bed drying air temperature on various quality parameters of Suphanburi 1 and Pathumthani 1 Indica rice. Paddy was dried from the initial moisture contents of 25.0, 28.8, and 32.5% dry basis to 22.5 ± 1.2% dry basis using inlet drying air temperatures between 40 and 150°C at 10°C/step. After fluidized bed drying, paddy was tempered and followed by ambient air aeration until its final moisture content was reduced to 16.3 ± 0.5% dry basis. The results showed that the head rice yield of Suphanburi 1 was significantly related to the inlet drying temperature and initial moisture content whilst there was no significant relationship between the head rice yield, drying temperature and initial moisture content for Pathumthani 1. The whiteness of the two rice varieties was slightly decreased with increase in drying air temperature and initial moisture content. It was also found that the hardness of both cooked rice varieties exhibited insignificant difference (p < 0.05) comparing to rewetted rice, which was gently dried by ambient air aeration in thin layer. The thermal analysis by DSC also showed that partial gelatinization occurred during drying at higher temperatures. Using inlet drying air temperatures in the range of 40–150°C therefore did not affected the quality of cooked rice and paddy. The milling quality of paddy was also well maintained.     

Drying of Turnip Seeds with Microwaves in Fixed and Pulsed Fluidized Beds

This article presents experimental and modeled drying kinetics of turnip seeds as a function of the type of air-particle contact (fixed bed, pulsed fluidized bed), humidity of the air, and three levels of microwave irradiation. The effects of those factors on the drying time required to reach a moisture content of 0.1 kg/kg d.b. were determined with an experimental design of 12 experiments by using the software Statgraphics. It was found that in drying, a significant factor was the application of microwaves to a pulsed fluidized bed. The effect of the humidity of the air only became noticed when the moisture contents of the seeds was below 0.2 kg/kg d.b. The simplified variable diffusivity model (SVDM) gave the least deviations for the experimental data. The effective diffusivity values determined in this work are similar to those informed in the literature for experiments without microwave application. The application of microwaves in combination with pulsed fluidization of the turnip seeds resulted in a fourfold increase of the effective diffusivity.