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.     

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.     

Comparison of Two Drying Schedules for European Hophornbeam (Ostrya carpinifolia Scop.) Lumber

Two types of conventional kiln-drying schedules (mild and harsh) based on moisture content (MC) were compared with regard to time, drying quality, and energy cost. The results were evaluated according to the classification of the European Drying Group. Proper drying periods of mild and harsh schedules were found to be 550 and 514 h, respectively. Evaluations in terms of drying quality indicated that better results were achieved with the mild schedule, especially when comparing drying defects and final MC. From an energy efficiency point of view, the harsh schedule, by saving 36 h of drying time, reduced electricity by 594 KWh and was therefore found to be $65 more profitable in this trial.     

Impact of rewetting and drying of rough rice on predicted moisture content profiles during in-bin drying and storage

Accurate prediction of moisture content (MC) is vital for effective control of on-farm, in-bin drying and storage of rough rice, especially for systems using recently introduced technology to automate fan run time. The study used simulations, laboratory, and field experiments to investigate the extent to which rewetting and drying, during in-bin drying and storage, affect accuracy of predicted MC—a critical parameter for automated fan control. Vapor sorption analysis (VSA) was used to generate MC prediction models for rough rice. Simulations of in-bin drying and storage, using in-field weather data, were performed while segregating effects resulting from rewetting and drying of the rough rice and the type of fan control strategy used. Predicted MC profiles of rough rice and drying durations were compared with those resulting from using standard constants in the literature for modeling. The root mean square error associated with predicting the MC by model constants developed using the VSA was 0.54% MC and 1.32% MC dry basis (d.b.), for desorption and adsorption, respectively. Deviation in MC logged by in-bin built, field sensors and that simulated by taking into account the influence of rewetting and drying were generally within 1.5% point difference. Therefore, rewetting and drying did not affect drying duration. However, drying duration was significantly influenced by fan control strategy (p?相似文献   

Microwave Finish Drying of Osmotically Dehydrated Cranberries

Combined microwave and hot-air drying characteristics was studied for the drying of cranberries that had been previously partially dehydrated by osmosis in a high fructose corn syrup (76°Brix). A 750 W 2450 MHz microwave oven was used to dry cranberry samples from 57% to 15% moisture content using three different power densities (0.75, 1.0, 1.25 W/g of initial cranberries) and two different power cycles (30 s On/30 s Off and 30 s On/60 s Off). All combinations of these variables were tested in triplicate. Quality of the cranberries was measured using a universal testing machine, chromameter, and with the use of a taste test panel. Drying times ranged from 2.2 to 5.0 h. Power times and power cycles affected the drying time and the quality of the dried cranberries. Lower power densities resulted in cranberries with higher quality. High power densities (125 W/g) resulted in the burning of some cranberries.     

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).     

EVALUATION AND MODELING OF TWO-STAGE OSMO-CONVECTIVE DRYING OF APPLE SLICES

Two-stage drying kinetics of cylindrical pieces of apples were evaluated by subjecting test samples first to various osmotic treatments and then to convective air drying to complete the drying process. Osmotic drying was carried out with cut apple cylinders of three different sizes (12, 17 and 20 mm diameter), all with a length to diameter ratio of 1 : 1, in a well agitated large tank containing the osmotic solution at the desired temperature. Solution to fruit volume ratio was kept greater than 60. After the osmotic treatment, apple slices were further dried in a cabinet drier at an average temperature 58°C. A central composite rotatable design (CCRD) with five levels of sucrose concentrations (34-63°Brix) and five temperatures (34-66°C) was used for osmotic treatment. Half-drying time and solids gain time were used as measures of rate of drying and associated diffusion coefficients for moisture loss and solids gain were evaluated. Half-drying time decreased with an increase in temperature or concentration, or a decrease in sample size. Diffusion coefficients were lower for smaller samples, and were higher for migration of moisture as compared to solids. For a given level of moisture removal, air drying times were shorter than osmotic drying times. Composite models were developed to describe the effect of process variables and particle size on the drying behavior of apple slices.     

DRYING KINETICS AND CHARACTERISTIC DRYING CURVE OF LIGHTLY SALTED SARDINE (SARDINELLA AURITA)

Drying of lightly salted sardine (Sardinella aurita) was accomplished using three air temperatures (35°C, 40°C, 50°C) and three air velocities (0.5 m/s, 1.5 m/s, 2 m/s); the effects of drying conditions on drying kinetics were studied. As for all biological products, air temperature is the main factor influencing the drying kinetics. However, over a given temperature which seems to correspond to protein modification (50°C), and at a high air flow rate (2 m/s and 2.5 m/s) a crust formation on the surface of the fish, due to the combined effect of heat and salt was observed. This phenomenon inhibited the drying rate. From the drying curves, two falling rate periods were observed. The dimensionless drying rate versus a dimensionless moisture content data were regressed by the Marquardt Levenberg non-linear optimization method to obtain an empirical equation describing the salted sardine characteristic drying curve.     

Analysis of the Vacuum Drying Rate for Red Oak in a Hot Water Vacuum Drying System

Red oak boards of 76.2 cm (long) × 7.62 cm (wide) × 2.54 cm (thick) were dried from green moisture content (MC) to 7% MC in the hot water vacuum-drying system. These boards were dried at the pressure of 12 mm Hg and the temperatures ranging from 30 to 50°C within 25 to 70 h. Drying rates were measured and drying curves were calculated. The results showed that the drying rate was higher at higher temperatures. The vacuum drying was faster when wood MC was above 30% than when it was less than 30%. The individual samples did not dry at the same drying rates even at the same drying conditions because of anatomical variations between boards.     

A Vacuum Drying System for Green Hardwood Parts

A vacuum drying system was designed and fabricated and that system was used to dry green rough hardwood dimension. The red oak samples, 76.2 (long) × 7.62 (wide) × 2.54 (thick) cm, were dried from green moisture content (MC) to 7% MC in this system. They were dried at a pressure of 12 mm Hg and temperatures ranging from 30 to 50°C within 25 to 70 h. Drying quality tested included warp, internal checking, and surface checking. Moisture gradients along the length and thickness were measured. The standard prong test was used to assess the drying stresses. Vacuum drying was fast and the drying rate increased as the temperature increased. It was found that the general drying quality was good with no color change. Drying stresses including longitudinal and transverse stresses were small. There were no internal checks.     

A Novel Design of Crop Dryer for Use in Developing Countries

The dryer is required for drying of grain as well as drying of the processed products in small catchment agro processing centers in the developing world. However, due to varied material characteristics of grain and secondary processed product, two entirely different types of dryers are required. The grain is dried in a recirculatory dryer, whereas processed product is dried in a tray dryer, where it is frequently mixed and trays are also intermittently changed. To avoid the need for two dryers, a novel design of a low-cost hot air dryer was developed where just by changing the trays the dryer can be converted from an LSU grain dryer to a tray-type product dryer. The dryer was tested for drying soybean grain as well as processed soy products like blanched soybean dal and soyflakes. The capacity of the dryer was 100 kg/batch in a tray dryer with each tray accommodating 10 kg of wet material. In case of LSU mode, the capacity of the dryer was 250 kg of grain per batch. The drying time required was 5 h for 250 kg of wet soybean from 24 to 10% moisture content, whereas in a tray dryer 100 kg blanched soybean dal was dried from 60 to 10% in 5 h and 100 kg of soyflakes from 25% moisture content to 10% moisture in 1.75 h. The cost of the dryer is estimated at US$580.00 and it can be fabricated in a moderately equipped workshop in developing countries.     

Microwave Heat Treatment of Spinach: Drying Kinetics and Effective Moisture Diffusivity

The effect of microwave drying technique on moisture content, moisture ratio, drying rate, drying time, effective moisture diffusivity, and porosity of spinach (Spinacia oleracea L.) were investigated. By increasing the microwave output powers (180-900 W) and the sample amounts (25-100 g), the drying time decreased from 18 to 3.5 min and increased from 7.7 to 25 min, respectively. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among of the models proposed, Page's model gave a better fit for all drying conditions applied. The activation energy was calculated using an exponential expression based on Arrhenius equation. The relationship between the drying rate constant and effective moisture diffusivity was also estimated and gave a linear relationship.     

A Vacuum Drying Model for Mango Pulp

Vacuum drying of mango pulp at varying conditions of pulp thickness (2, 3, and 4 mm) and vacuum chamber plate temperature (65, 70, and 75°C) was carried out under 30-50 mm of mercury absolute pressure. A model based on moisture diffusivity was found to give close prediction to moisture content of the pulp at different times of drying with correlation coefficient varying between 0.98-0.99 for pure mango pulp and pulp with ingredients. Color change of reconstituted pulp made from mango powder was found to depend more on pulp thickness than plate temperature. For getting low color change vacuum drying should be carried at maximum pulp thickness of 2.6 mm and vacuum chamber plate temperature of 72.3°C.     

Effect of Variety on Drying Characteristics and Selected Quality Attributes of Dried Carrots

The effects of variety on drying characteristics, color, and water absorption of carrots were investigated. Six different varieties of carrots, viz. Kazan, Maxima, Nandor, Nektarina, Simba, and Tito were evaluated. The hot air drying characteristics of carrot cubes dried under forced convection conditions were determined and drying data were analyzed to obtain parameters of Page and first-order kinetic models as well as moisture diffusivity. Color characteristics were determined for fresh, dried, and dehydrated samples by measuring lightness (L*), redness (a*), and yellowness (b*). Water absorption data were analyzed for ground samples. It was observed that drying characteristics, such as color and water absorption were significantly influenced by variety. The varieties of Kazan and Nektarina were found to be characterized by highest and lowest moisture diffusivity of 7.52 × 10^-9 and 3.31 × 10^-9m²/s respectively. Kazan variety was also characterized by shortest drying time. The lowest changes in color caused by drying were observed for Tito variety. The variety of Kazan was characterized by the highest resistances to color changes affected by drying followed by rehydration. Nandor and Tito varieties displayed the highest water absorption near to 560 g/100 g. The best drying characteristics and good water absorption accompanied by the high color attributes of dried and rehydrated samples implies that Kazan variety is expected to be the most useful to drying industry.     

Moisture Nonuniformity in Drying Paper: Measurement and Relation to Process Parameters

Local nonuniformity of moisture content during drying is doubly disadvantageous. Moisture nonuniformity reduces drying effectiveness, increasing drying cost or reducing drying capacity, and may decrease product quality for printing through effects such as cockling and curl. These problems can occur with the now dominant process of cylinder drying, but become much more acute with the various higher intensity drying techniques which are competing to become elements of the higher speed hybrid dryer sections of the future. For quantitative characterization of moisture nonuniformity a novel method applicable to paper being dried by various processes was developed, then demonstrated for one high intensity air convection process, through air drying. During drying, the local moisture content was determined for many 2.5 mm diameter sensing areas by very rapid scanning with a custom design 3-wavelength infrared paper moisture instrument. For local moisture nonuniformity thus determined, the coefficient of variation provides a standard basis for comparison between paper types and drying conditions. For kraft paper from 20 to 120 g/m², the effects on local moisture nonuniformity were evaluated for drying intensity and for several paper parameters—formation, grammage, and initial moisture content. The drying nonuniformity index was found to parallel both formation quality and drying intensity. An unexpected finding was that over the 20-120 g/m² range, local moisture nonuniformity passes through a maximum at 30 g/m². Sheet initial moisture content produces the largest effect, with moisture nonuniformity index being both high and very sensitive to this parameter for initial moisture content above the fibre saturation point. Another finding from these unique measurements is that, for the same 2.5 mm diameter sensing areas, there is a clear correlation between moisture nonuniformity index and local grammage.     

SIMULATION OF THE EFFECT OF AIR VELOCITY IN THE DRYING OF HARDWOOD TIMBER

The effects of different air velocities on the drying of Australian hardwood timber have been investigated using a drying model based on Fickian diffusion. The air velocities studied were 0.05, 0.5, and 2 m.s^-1, corresponding to typical velocities covering the range used from pre-drying to normal kiln conditions. Decreasing tbe air velocity from 2 m.s^-1 to 0.05 m.s^-1 reduces the maximum strain experienced with an optimised drying schedule by 34%, although if the lower velocity is used throughout the drying period, the drying time is predicted to be 40% longer. Explorations with a program to optimise drying schedules suggest that there may Dot be any significant advantage in moving from a low air velocity of 0.05 m.s^-1 to a higher one (2 m.s^-1) in terms of reducing drying time, for the same maximum strain during drying, compared with using a constant air velocity of 0.5 m.s^-1.     

ON THE STUDY OF TIME-VARYING TEMPERATURE DRYING—EFFECT ON DRYING KINETICS AND PRODUCT QUALITY

The ability of heat pump dryer to produce controlled transient drying conditions, in terms of temperature, humidity and air velocity, has given it an edge over other drying systems. Exploiting this characteristic, we studied and compared the effect of different temperature-time profiles on the quality of agricultural products in a tunnel heat pump dryer capable of providing up to 14.6 kW of cooling capacity. The product quality refers to the color change of the products. Samples of banana and guava were dried in batches in a two-stage heat pump dryer. The effects of the starting temperature of a selected profile and the cycle time on both drying kinetics and product quality were studied. It was observed that by employing a step change in drying air temperature with the appropriate starting temperature and cycle time, it was possible to reduce significantly the drying time to reach the desired moisture content with improved product color.     

PERFORMANCE STUDY OF A RE-CIRCULATING CABINET DRYER USING A HOUSEHOLD DEHUMIDIFIER

The performance and operating characteristics of a low temperature re-circulating cabinet dryer using a dehumidifier loop were studied using alfalfa. Chopped alfalfa, initially at 70% moisture content, was dried to 10% moisture content in the dryer. Two dryer setups were used. The dryers in each case had a partitioned cabinet with trays of material on one side and a stack of one or two small household dehumidifiers on the other side. Air was re-circulated through the material from bottom to the top and back through the dehumidifiers. Two drying configurations were tested. In one, the material was left on the trays until drying was complete (batch or fixed tray drying). In the other configuration, the trays were moved from top to bottom, introducing a new tray at the top while removing an old tray from bottom. Drying air temperature ranged from 25 to 45°C. The average air velocity through the material was 0.38 m/s. Alfalfa chops dried in 5 h in the fixed tray drying and in 4 h in the moving tray drying. The specific moisture extraction rate ranged from 0.35 to 1.02 kg/kWh for batch drying and stayed at an average value of 0.50 kg/kWh for continuous/moving tray drying.     

Constant and Intermittent Drying Characteristics of Olive Cake

The drying kinetics of olive cake, the solid by-product of the olive oil extraction process, has been experimentally investigated in a small-scale tray dryer using both constant and intermittent (on/off) heating schemes. The parameters investigated include inlet air temperature and intermittency of heat input. The drying kinetics was interpreted through two mathematical models, the Page equation and the Lewis equation. The Page equation was most appropriate in describing the drying behavior of olive cake. A diffusion model was used to describe the moisture transfer and the effective diffusion coefficient at each temperature was determined. The dependence of the effective diffusion coefficient on drying temperature can be adequately explained based on an Arrhenius-type relation. The effective diffusion coefficient varied between 7.6 × 10^-8 and 2.5 × 10^-7 m²/min with an activation energy of 38.55 kJ/mol. Comparison of time evolution of material moisture content due to intermittent and constant drying is also made.     

Oven-Drying Characteristics of Soils of Different Origins

The accurate measurement of the moisture content of a soil is an important step in characterizing its engineering behavior. However, the oven-drying method can cause some chemical reaction (oxidation or loss of water of crystallization) to occur in certain soil types. The level of oxidation of the solid particles was studied over the drying temperature range of 60 to 140°C for different soils. The period of oven drying necessary for the specimen mass to equilibrate was also examined. The standard practice of oven drying the soil specimens at 110 ± 5°C or 105±5°C over a period of 24 h was confirmed as giving accurate moisture content values for inorganic soils. Oven drying of peat and other highly organic soils over a period of 24 h at 80°C produced similar levels of accuracy in the moisture content measurements as that for inorganic soils at the standard oven drying temperatures of 105 or 110°C. Some oxidation of the organic fraction commenced at between 80 and 90°C.