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

ABSTRACT

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.     

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.     

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.     

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 CHARACTERISTICS OF THE HAZELNUT

Equilibrium moisture content isotherms for Spanish hazelnut (Corylus avellana L.) at different temperatures (30°C-80°C) were determined using static gravimetric method. Thin layer drying experiments were done with forced air circulation and were conducted with different operating conditions to determine the drying characteristics of hazelnuts. The effect of air temperature (30°C-70°C), air velocity (0.5 m/s - 2 m/s) and drying bed loading density (50 kg/m² - 150 kg/m²) on drying of unshelled and shelled hazelnuts was studied. Six mathematical models were used to fit the experimental equilibrium moisture content data, from which the G.A.B. model was found to give the best fit. Diffusion coefficients were determined by fitting experimental thin-layer drying curves to the Fick's diffusion model. Variation of the effective diffusion coefficient with temperature was of the Arrhenius type. The Page equation was found to describe adequately the thin layer drying of hazelnut. Page equation drying parameters k and n were correlated with air temperature and relative humidity.     

DEVELOPMENT OF THE CHARACTERISTIC DRYING CURVE FOR CASSAVA CHIPS IN MONOLAYER

Drying kinetic curves and modelling for cassava chips were determined using a pilot-size air dryer. Operating conditions involved temperatures ranging from 35 to 90 ° C, air flow velocities from 0.5 to 2.0 m/ s, and air moisture content from 0.005 to 0.060 kg water/ kg dry air. Sorption isotherms at temperatures of 23, 45 and 60 ° C were obtained. Results for the drying experiments can be used to calculate the optimal drying conditions for dehydration of cassava roots in multilayers     

Surface Stickiness of Drops of Carbohydrate and Organic Acid Solutions During Convective Drying: Experiments and Modeling

Drying kinetics of low molecular weight sugars such as fructose, glucose, sucrose and organic acid such as citric acid and high molecular weight carbohydrate such as maltodextrin (DE 6) were determined experimentally using single drop drying experiments as well as predicted numerically by solving the mass and heat transfer equations. The predicted moisture and temperature histories agreed with the experimental ones within 6% average relative (absolute) error and average difference of ± 1°C, respectively. The stickiness histories of these drops were determined experimentally and predicted numerically based on the glass transition temperature (T_g) of surface layer. The model predicted the experimental observations with good accuracy. A nonsticky regime for these materials during spray drying is proposed by simulating a drop, initially 120 µm in diameter, in a spray drying environment.     

SIMULATION OF FLUIDIZED-BED DRYING OF CARROT WITH MICROWAVE HEATING

A mathematical model of coupled heat and mass transfer was applied to batch fluidized-bed drying with microwave heating of a heat sensitive material—carrot. Four kinds of microwave heating with intermittent variation were examined. The numerical results show that different microwave heating patterns can affect the fluidized bed drying significantly. Changing the microwave input pattern from uniform to intermittent mode can prevent material from overheating under the same power density. Supplying more microwave energy at the beginning of drying can increase the utilization of microwave energy while keeping temperature low within the particle. For a particle diameter of 4 mm, fluidization velocity of 2 m/s, inlet airflow temperature of 70°C and the bed area factor of 80, the drying time are 750 and 1000 s, respectively, for the two good operating conditions with on/off periods of 125/375 s and 375/375 s. The cumulative microwave energy absorbed by particles at the end of drying is 1415 and 2300 kJ/kg (dry basis), respectively.     

THERMAL DEWATERING OF DILUTED ORGANIC SUSPENSIONS: PROCESS MECHANISM AND DRYING KINETICS

Drying of raw hog manure, a highly diluted suspension of organic and inorganic matter was studied experimentally in the multistage screw-in-trough dryer. Laboratory tests included material characteristics (rheological properties, thermodynamic equilibrium, critical settling point), boiling pattern, and kinetics of convective drying at temperatures from 90 to 120°C and air velocity from 0.5 to 1.5 m/s. Three phases of manure drying were identified, namely boiling and release of volatile compounds, evaporation from viscous liquid, followed by drying of a soft paste, and finish drying of semi-dry granular product. Drying kinetics were quantified in terms of reduced moisture content and a generalized drying curve using the characteristic drying time concept. Field tests were carried out in the pilot multistage screw-in-trough dryer to validate design calculations.     

A Study of the Limitations to Spray Dryer Outlet Performance

The concept of the product moisture locus was tested in this work using a pilot-scale modified Niro spray dryer (diameter 0.8 m, height 2 m), where the residence time of the particles inside this spray dryer is lower compared with larger industrial spray dryers. The moisture contents of skim milk powder produced from spray drying skim milk (solids content 8.8% w/v) at different operating conditions, namely different swirl vane angles (0°, 25°, 30°), inlet air temperatures (170°C, 200°C, 230°C) and process fluid flowrates (1.4 kg h^-1, 1.6 kg h^-1, 1.8 kg h^-1), were compared with the predicted equilibrium moisture contents. In addition, the residence time of the particles was also increased in the spray dryer by decreasing the inlet air mass flowrate from 0.016 to 0.013 kg s^-1. The outlet moisture contents of the skim milk powder for all the 23 runs carried out in this work were within 0.4% of the equilibrium values. Thus, the skim milk powder particles were in close equilibrium with the gas inside the drying chamber. These equilibrium limitations are confirmed by other literature data (Boonyai, P. Comparative Evaluation of Soymilk Drying in a Spray Dryer and Spouted Bed of Inert Particles. M.Sc. Thesis. Asian Institute of Technology: Bangkok, Thailand, 2000; 90 pp; Harvie, D.J.E.; Langrish, T.A.G.; Fletcher, D.F. A computational fluid dynamics study of a tall-form spray dryer. Trans IChemE 2002, in press). The use of this finding to predict spray dryer performance is demonstrated by mass and energy balance calculations.     

DESORPTION ISOTHERMS AND DRYING CHARACTERISTICS OF SHRIMP IN SUPERHEATED STEAM AND HOT AIR

Desorption isotherms for shrimp were determined at the temperatures of 50, 60, 70 and 80°C. Amongst the moisture equilibrium predictions between the BET and GAB models, the latter has a better predictable capability. The GAB parameters are correlated with the temperatures by the Arrhenius expression. Drying characteristics of shrimp in drying media at the temperature range of 120-180°C for superheated steam and of 70-140°C for hot air have been examined. Drying rate and effective diffusion coefficient are used to quantify quantitatively the difference between the superheated steam and the hot air dryings. The temperature is more important effect on drying rate and effective diffusion coefficient in the superheated steam than in the hot air. Inversion temperature exists between 140 and 150°C. Comparing to the hot air, the shrimp dried by the superheated steam shows a lower degree of shrimp shrinkage. In addition, product colours are slightly different to those from the commercial sources.     

DRYING OF SOY PULP (OKARA) IN A BED OF INERT PARTICLES

Drying of okara, an insoluble pulp residue waste byproduct of tofu production, was investigated in a continuously moving bed of inert particles subjected to vortex-like motion. The experimental variables in their respective ranges included the mass of Teflon pellets used as inert particles (0.4-1.2 kg), feed rate (0.5-1.4 kg/h), inlet air temperature (100-145°C) and airflow rate (195-271 m³/h). The dryer showed good performance in general and produced dry okara with moisture content ranging from 5 to 33% wb depending upon the operating conditions. The product recovery ranged from 80 to 90% on dry basis in most experiments. The specific water evaporation rate in okara drying increased with increasing of the feed rate and mass of Teflon pellets. However, the specific heat consumption decreased with an increase in the okara feed rate. Results showed that specific heat consumption for okara drying in a bed of inert particles was about 3 to 4 times higher in comparison with that of free water.     

EFFECT OF TEMPERATURE AND RELATIVE HUMIDITY ON THE DRYING RATES AND DRYING TIMES OF GREEN BELL PEPPERS (CAPSICUM ANNUUM L)

Green bell pepper dices were dehydrated at different dry bulb air temperatures (55°, 60°, 65°, 70° and 75°C) and relative humidities (15, 20, 25, 30, 35 and 40%). The effects of temperature and relative humidity (RH) on the drying rates and drying period of diced green bell peppers were determined. Drying rate curves were characterized by a short induction (heating) phase followed by a falling rate period. Near constant rate drying was observed only at 55°C at 15% RH and at 65°C (15% RH). Drying rates generally increased with increasing temperatures and decreasing RH. The effect of temperature on the drying rates became less pronounced with increasing RH Drying rate maxima at 70°C and RH of 15, 20 25 and 40% exceeded those at 75°C, possibly due to case-hardening.     

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.     

WATER TRANSFER IN POTATO DURING AIR DRYING

Drying data (moisture vs time) were obtained on slabs of potato in an air dryer operated at 70°C and an air velocity of 1.6 m/s. The considerable differences between the experimental data and the receeding front: and Fickian diffusion models for moisture profiles were found. The temperature in the potato sample increased with increasing drying time. This increase which results in starch gelation might be the main reason in the formation of moisture profile peaks.     

From Laboratory Experiments to Design of a Conveyor-Belt Dryer via Mathematical Modeling

A conveyor-belt dryer for picrite has been modeled mathematically in this work. The necessary parameters for the system of equations were obtained from regression analysis of thin-layer drying data. The convective drying experiments were carried out at temperatures of 40, 60, 80, and 100°C and air velocities of 0.5 and 1.5 m/sec. To analyze the drying behavior, the drying curves were fitted to different semi-theoretical drying kinetics models such as those of Lewis, Page, Henderson and Pabis, Wang and Singh, and the decay models. The decay function (for second order reactions) gives better results and describes the thin layer drying curves quite well. The effective diffusivity was also determined from the integrated Fick's second law equation and correlated with temperature using an Arrhenius-type model. External heat and mass transfer coefficients were refitted to the empirical correlation using dimensionless numbers (J_h, J_D = m · Reⁿ) and their new coefficients were optimized as a function of temperature. The internal mass transfer coefficient was also correlated as a function of moisture content, air temperature, and velocity.     

EFFECT OF TEMPERING ON SUBSEQUENT DRYING OF PADDY USING FLUIDISATION TECHNIQUE

Drying and tempering models for paddy drying by a fluidised bed technique have been developed to describe the moisture movement inside a single paddy kernel. The grain shape was considered as a finite cylinder. The internal diffusion is an important contribution to control the drying rate of paddy. The dependence of effective diffusion coefficient on drying temperature can be adequately explained based on Arrhenius form. The parameters of this equation were evaluated in the range of temperatures between 110°C and 170°C by using the regression analysis with 189 experimental drying data. As compared with no tempering, the faster drying rate can be obtained by tempering treatment between drying stages. The effect of degrees of tempering on determining the moisture reduction in the second stage has also been explored. According to the simulation results, a prediction equation of the required tempering time for the tempering index of 0.95 has been established in which the drying air temperature, initial moisture content and drying time are taken into account. The tempering time for 35 min is recommended for the continuous fluidised bed dryers being operated in rice mills.     

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.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.