ANALYSIS OF OPEN SUN DRYING EXPERIMENTS

ABSTRACT

Open sun drying has lost its previous importance due to the fact that different factors affect its reliability and the quality of the products obtained.

One of the set–backs for the analysis of solar drying exoeriments is their deoendence on a non–controlled source of energy, i.e. solar radiation depends on climatic conditions and experiments are difficult to compare. It is thus necessary to investigate the advantages of a particular set up as well as the climatic influences. Open sun drying could constitute the natural reference, allowing the comparison of different drying strategies.

A new way of standardizing drying times, based on solar radiation inout, is proposed, to allow better evaluation of the experiments. An equivalent time is defined, allowing comparison of experiments carried out under different circumstances. Carrots and potatoes were used in these experiments.

The use of the average daily solar radiation 15.28 MJ m^-2.d^-1 in Palma de Mallorca (39.33°N, 2.37°E), is proposed for comparison purposes. An improvement of more than 12 % in the explained variance was observed, the unexplained variance being lower than 1 %.     

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.     

THIN LAYER SOLAR DRYING OF SOME VEGETABLES

In this study, a solar cabinet dryer consisting of a solar air heater and a drying cabinet, was used in drying experiments. Pumpkin, green pepper, stuffed pepper, green bean, and onion were dried in thin layers. Three different drying air velocities were applied to the process of drying to determine their effects on drying time. Fresh materials were dried by a natural sun drying method. In order to explain drying curves of these products different moisture ratio models were performed and evaluated based on their determination coefficients (R²). Our results revealed that drying air temperature could increase up to about 46°C. Drying air velocity had an important effect on drying process. Drying time changed between 30.29 and 90.43 hours for different vegetables by the solar drying. This drying time was between 48.59 and 121.81 hours for the natural sun drying. Drying curves could be explained by determined thin layer drying models satisfactorily with very high determination coeffcients.     

MASS TRANSFER AND DIFFUSION COEFFICIENT DETERMINATION IN THE WET AND DRY SALTING OF MEAT

Dehydrated salted meat is widely used in Brazil as a very important source of animal protein. The main objective of this kind of processing is water removal. initially by osmotic pressure changes and then by drying, resulting in a product with intermediate moisture levels.

In this work, mass transfer and salt diffusion in pieces of meat submitted to wet and dry salting were studied. Slabs of beef m. trapezius with an infinite plate geometry were salted in a NaCl saturated solution or in a dry salt bed, at two temperatures (10 and 20°C) and different time exposures (120 min and 96 hours). Equilibration studies were extended up to six days.

It was observed that water loss increased with salt uptake, for increasing periods of times. At 20°C the moisture loss was higher than it was at 10°C in both salting processes. On the other hand, the kinetics of salt uptake and moisture loss were of greater importance in the process of dry salting than in that of wet salting.

The salt diffusion coefficient for wet salting was 0.26 × 10^-10m²/s at20°C and 0.25 × 10^-10 m²/s at 10°C and for the dry salting the values were 19.37 × 10^-10 m²/s at 20°C and 17.21 × 10^-10 m²/s at 10°C.     

DETERMINATION OF CROSS-GRAIN PROPERTIES OF CLEARWOOD SAMPLES UNDER KILN-DRYING CONDITIONS AT TEMPERATURE UP TO 140° C

Small specimens of Pinus radiata have been tested to determine the creep strain that occurs during the kiln drying of boards. The samples have been tested over a range of temperatures from 20°C to 140°C. The samples, measuring 150 × 50 × 5 mm, were conditioned at various relative humidities in a pilot-plant kiln, in which the experiments at constant moisture content (MC) in the range of 5-20% MC were undertaken to eliminate mechano-sorptive strains. To determine the creep strain, the samples were brought to their equilibrium moisture content (EMC), then mechanically loaded under tension in the direction perpendicular to the grain. The strain was measured using small linear position sensors (LPS) which detect any elongation or shrinkage in the sample. The instantaneous compliance was measured within 60 sec of the application of the load (stress). The subsequent creep was monitored by the continued logging of strain data from the LPS units.

The results of these experiments are consistent with previous studies of Wu and Milota (1995) on Douglas-fir (Pseudotsuga menziesii). An increase in temperature or moisture content causes a rise in the creep straw while the sample is under tension. Values for the instantaneous compliance range from 1.7 × 10^-3 to 1.28 × 10^-7 MPa^-1 at temperatures between 20°C and 140°C and moisture content in the range of 5-20%. The rates of change of the creep strains are in the Order of magnitude 10^-7to10^-8s^-1 for these temperatures and moisture contents. The experimental data have been fitted to the constitutive equations of Wu and Miloia (1996) for Douglas-fir to give material parameters for the instantaneous and Creep strain components for Pinus radiata.     

RELATIONSHIP BETWEEN A SOLAR DRYING MODEL OF RED PEPPER AND THE KINETICS OF PURE WATER EVAPORATION (I)

Drying of red pepper under solar radiation was investigated, and a simple model related to water evaporation was developed. Drying experiments at constant laboratory conditions were undertaken where solar radiation was simulated by a 1000 W lamp.

In this first part of the work, water evaporation under radiation is studied and laboratory experiments are presented with two objectives: to verify Penman's model of evaporation under radiation, and to validate the laboratory experiments. Modifying Penman's model of evaporation by introducing two drying conductances as a function of water content, allows the development of a drying model under Eolar radiation.

In the second part of this paper, the model is validated by applying it to red pepper open air solar drying experiments.     

A GRAPHICAL DESIGN METHOD FOR A PARTIAL SOLAR DRYER FOR CORN

This paper presents the application of a design method for a partial solar heating system of polyvalent modular dryers called “GJ-ABAQUE” to the drying of thick layers of grains.

This method is based on the use of charts or polynomial correlations. In the actual case where the drying air is not recycled, we only need one chart which allows one to determine the fraction of the monthly heating load supply by solar energy as a function of two dimensionless parameters. The latter implies the use of monthly average radiation data, the collector surface and estimates of drying loads.

The “GJ-ABAQUE” method was applied for drying 777 kg of corn, corresponding to 1 m³ of fresh product, in a thick layer in each modular dryer.     

PROTOTYPE HIGH TEMPERATURE/HIGH PRESSURE KILN FOR THE EVALUATION OF WOOD DRYING SCHEDULES

A new laboratory kiln was developed and built to perform over a very wide range of drying conditions. For example, the dry bulb temperature can vary from 30°C to 150°C and the dew point can be adjusted between 20°C and 130°C. Obviously, with such a high level of dew point, pressures over atmospheric pressure may be induced inside the chamber. For this reason, the kiln has been designed to withstand pressure of up to 3 bars. This kiln can also perform vacuum drying.

A programmable controller allows the temperature levels to be maintained within ± 0.2°C. Because the whole kiln can be heated only through the agitated water present at the bottom of the kiln, the load temperature can be increased up to 130°C in saturated conditions, without any change of moisture content.

The kiln has various sensors attached and is capable of withstanding severe conditions (high temperature, saturated vapour and elevated pressures). At present, air and water temperatures as well as temperature at different locations within the board can be collected during the drying process. A load cell and pressure gauges are also available. The first tests performed using this equipment are presented at the end of the paper.     

Thin Layer Drying Models for Osmotically Pre-dried Young Coconut

Thin layer convection drying was performed on osmotically pre-dried young coconut, strips, both thin and thick. A drying air temperature range of 50-70°C and an airflow of 0.25 m s^-1 was used to dry samples soaked in three sugar solution concentrations (40, 50, and 60°B) during the osmotic drying phase, with the convection drying alone serving as control. An analysis of variance (ANOVA) revealed that sugar concentration and thickness significantly affected osmotic drying rates as shown by their final moisture contents. While the drying air temperature and slab thickness significantly affected the average drying rate and the sugar concentration was an insignificant factor during convective drying phase. Effective diffusivity of water during hot air drying varied from 1.71 to 5.51 × 10^-10 m²s^-1 over the temperature range investigated, with energy of activation equal to 1173.0 kJ/kg. Three mathematical models available in the literature were fitted to the experimental data, with the Page model giving better predictions than the single or double term exponential model. The temperature dependence of the diffusivity coefficients was satisfactorily described by a simple Arrhenius type relationship.     

A DIFFUSION MODEL WITH A MOISTURE DEPENDENT DIFFUSION COEFFICIENT FOR PARBOILED RICE

A mathematical model describing moisture migration by diffusion in a solid sphere with variable diffusion coefficient is proposed. An analytical expression for dependence of the diffusion coefficient with moisture content was derived based on the assumption that the activation energy for diffusion varies linearly with the desorption energy.

The expression for moisture dependence of diffusion coefficient was used to simulate drying of parboiled rice in the temperature range 50-90°C. The mathematical model shows good agreement between observed and predicted drying rate curves.     

STEAM CONDENSATION IN A VERTICAL CORRUGATED DUCT

An experimental investigation was made on the condensation of water steam in a vertical corrugated duct. The data have been correlated as follows

Co=5.11Re^-0.431 150 ≤ Re ≤ 350 Co = 0.034311Re^-0.425350 ≤Re ≤l000

The vertical corrugated duct is constructed of two corrugated plates with corrugation inclination angles of β = 0 and β = 45° respectively (relative to the overall flow direction).

The condensation heat transfer coefficient in the corrugated duct is more than two times higher than that of bulk condensation on a vertical plate. A physical model was proposed to explain the heat transfer enhancement. Attention was also paid to the effect of exit steam velocity on the heat transfer during partial condensation. It was demonstrated that the heat transfer in the corrugated duct was strongly affected even at a low exit velocity, which was different from the case of bulk condensation on a vertical plate. Experimental apparatuses and the method for examining their reliability are described in detail.     

DRYING OF GRANULAR PARTICLES IN A MULTISTAGE INCLINED FLUIDIZED BED WITH MECHANICAL VIBRATION

A mathematical model for the drying rate of granular particles in a multistage inclined fluidized bed(IFB) is presented from the standpoint of simultaneous heat and mass transfer, with taking the effect of mechanical vibration added vertically into consideration.

Steady-state distributions for the temperatures and concentrations of the particles and the heating gas, and for the moisture content of the particles are numerically calculated based on the present model. The calculated results show fairly good agreement with the experimental data, which were obtained from the drying experiments of brick particles in a three-stage IFB using comparatively low temperature air(40-60°C) as the heating gas.

It has been found within the range of the experimental conditions employed that, the mechanical vibration added vertically enhances the over-all drying rate of the particles and its effect can be considered equivalent to an increase in the air velocity.     

SOLAR DRYING OF FRUITS IN A MEDITERRANEAN CLIMATE

Two twin forced convection dryers of 1.5 m³ were built in Majorca (Spain). They are of a mixed kind, with solar air collectors and a green house type chamber. A wooden frame supports polycarbonate walls. After two years operation they have been proven weather resistant. Six solar air collectors 2.12 × 1.05 m were used in each dryer.

Apricots were processed in both dryers and at open sun. Three different tray heights were tested 5, 9 and 12 cm. The best results were obtained with 12 cm trays. Recycling part of the exhaust air improves the efficiency of the dryer. Blanching the fruits makes no difference to the dehydration rate. The rate of SO₂ loss during the drying process is higher within the chamber.     

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 Characteristics of Barley Grain Dried in a Spouted-Bed and Combined IR-Convection Dryers

A study was performed to determine the drying characteristics and quality of barley grain dried in a laboratory scale spouted-bed dryer at 30, 35, 40, and 45°C and an inlet air velocity of 23 m/s^-1, and in an IR-convection dryer under an infrared radiation intensity of 0.048, 0.061, 0.073, and 0.107 W cm^-2 at an air velocity of 0.5 m/s^-1. The results show that the first, relatively short, phase of a sharp decrease in the drying rate was followed by the phase of a slow decrease. The time of barley drying depended on temperature of inlet air in a spouted-bed dryer and on radiation intensities in an IR-convection dryer. Barley drying at 45°C in a spouted-bed dryer was accompanied by the lowest total energy consumption. The average specific energy consumption was lower and the average efficiency of drying was higher for drying in a spouted-bed dryer. The effective diffusivities were in the range 2.20-4.52 × 10^-11 m² s^-1 and 3.04-4.79 × 10^-11 m²/s^-1 for barley dried in a spouted-bed and in an IR-convection dryer, respectively. There were no significant differences in kernel germination energy and capacity between the two drying methods tested.     

KINETICS AND MECHANISM OF THE REACTION BETWEEN OZONE AND O-CRESOL IN AQUEOUS SOLUTIONS

The stopped-flow spectrophotomctric method was applied to study the kinetics of the ozonation of o-cresol in aqueous solutions of pH values varying from 2 to 9 at 10 to 40°C. The fast reaction between o-cresol and ozone is second order overall with first order in each reactant. The ozonation rale increases with the temperature and pH value of the solution. In neutral solutions, the overall rate constant increases from 422,000 M^-1S^-1 at 10°C to 1,549,000 M^-1 S^-1 at 40°C. The activation energy is about 30 KJ/mol in the neutral solutions; it increases slightly with the acidity of the solution.

A mechanism based on the initial attack of ozone moleculesat the ortho and para positions of o-cresol is proposed for the ozonation reaction. According to this mechanism, three moles of ozone are required to react with each mole of o-cresol resulting in the rupture of the aromatic ring and production of various acids. The oxidation products were identified by these and other investigators. The proposed mechanism yields a second order kinetics for the overall reaction, as confirmed by the kinetic experiments.     

Microwave Pretreatment of Backsawn Messmate Boards to Improve Drying Quality: Part 1

The findings of an investigation into the microwave (MW) modification and conventional kiln drying of backsawn/flatsawn messmate stringybark (Eucalyptus obliqua L'Herit) are summarized. The project was conducted in two parts. Part 1, the subject of this article, investigates whether low-intensity microwave modification results in more or less check formation than occurs during conventional drying of eucalypts. Part 2 of the investigation will determine the acceleration in kiln drying that can be achieved through microwave modification of the wood structure and in turn improving permeability.

Significant improvement in microwave modified samples compared to controls in relation to check (internal/honeycombing and surface) formation during drying was observed, with Schedule 1 (70 kWh/m³) having 55% of samples without checks compared to the control samples at 31%.

The study also found that there is a significant reduction in surface checking (number and depth) in MW modified boards compared to the controls. The control samples exhibited the highest proportion of surface checking, with 65% possessing one or more surface checks, of which 82% extended more than one quarter the width of the sample. This was in marked contrast to Schedule 2 (75 kWh/m3), which resulted in 43% having one or more surface checks, of which 25% extended more than one quarter the width of the sample.

The control samples also returned the highest rate of internal/honeycombing checking with 28% of samples affected, while samples pretreated according to MW Schedule 2 yielded the lowest rate, with 11% affected.     

SOLAR DRYING OF GRAPES

Mass losses and low quality are the most serious disadvantages of traditional grape drying methods. For the production of high quality raisins an increase in the drying rate is required and the grapes should be protected from rain, dust and insects during drying.

Under the terms of a joint German-Greek research program low-cost solar grape dryers were developed in the Institute of Agricultural Engineering of Hohenheim University and were tested in Greece in cooperation with the Crete Agricultural Research Center.

The required data basis for optimizing solar grape dryers was obtained by additional laboratory tests measuring the influence of various drying parameters on drying rate and quality.

Tests with the solar dryers have shown that it is possible to reduce the drying time and improve the quality significantly compared to the traditional drying methods. Also mass losses due to rain can be prevented.     

AMARANTH GRAIN DRYING IN A FLUIDIZED BED

An empirical equation as a function of drying time and temperature was developed to calculate the moisture ratio required Tor balch fluidized bed dryers design for amaranth grain drying

The relative deviations of the moisture ratio values calculated with the proposed equation with regard to the experimental ones ranged between 0.014 and 0.095 for a drying air temperature between 60 and 100 ° C, and a grain initial moisture content between 23.5 and 16.6 % wb.     

EFFECT OF AIR FLOW RATE ON CARROT DRYING

The influence of air flow rate on the kinetics of drying 10x10x10 mm carrot cubes is presented. For this geometry kinetic equations are available, for the first falling rate drying period.

Drying air flows of 1000, 2000, 2500, 3000, 4000, 5000, 6000, 8000 and 9000 kg/m ² h were employed. It was found that for flow rates above 6000 kg/m² h the value of D/r² remains almost constant, thus indicating that when the air flow rate is higher it has no influence on the drying rate. The influence of air flow rate on carrot drying has been determined, hence allowing optimal flow rate calculation under economic constrictions.