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.     

Numerical Investigation on Dielectric Material Assisted Microwave Freeze-Drying of Aqueous Mannitol Solution

Abstract

The dielectric material assisted microwave freeze-drying was investigated theoretically in this study. A coupled heat and mass transfer model was developed considering distributions of the temperature, ice saturation and vapor mass concentration inside the material being dried, as well as the vapor sublimation-desublimation in the frozen region. The effects of temperature and saturation on the effective conductivities were analyzed based on heat and mass flux equations. The model was solved numerically by the variable time-step finite-deference technique with two movable boundaries in an initially unsaturated porous sphere frozen from an aqueous solution of mannitol. The sintered silicon carbide (SiC) was selected as the dielectric material. The results show that dielectric material can significantly enhance microwave freeze-drying process. For case of the dielectric field strength, E = 4000 V/m under typical operating conditions, the drying time is 2081 s, 30.1% shorter and 47.2% longer, respectively, than those for E = 2000V/m and E = 6000 V/m. The heat and mass transfer mechanisms during the drying process were discussed.     

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

ABSTRACT

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.     

Effect of Dielectric Material on Microwave Freeze Drying of Skim Milk

Abstract:

A simultaneous heat and mass transfer model of the dielectric material–assisted microwave freeze drying was derived in this study considering the vapor sublimation-desublimation in the frozen region. The mathematical model was solved numerically by using the finite-difference technique with two moving boundaries. Silicon carbide (SiC) was selected as the dielectric material, and the skim milk was used as the representative solid material in the aqueous solution to be freeze-dried. The results show that the dielectric material can significantly enhance the microwave freeze drying process. The drying time is greatly reduced compared to cases without the aid of the dielectric material. Profiles of the temperature, ice saturation, vapor concentration, and pressure during freeze drying were presented. Mechanisms of the heat and mass transfer inside the material sphere were analyzed. For an initially unsaturated frozen sample of 16 mm in diameter with a 4-mm-diameter dielectric material core, the drying time is 288.2 min, much shorter than 380.1 min of ordinary microwave freeze drying and 455.0 min of conventional vacuum freeze drying, respectively, under typical operating conditions.     

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.     

Batch drying kinetics of corn in a novel rotating jet spouted bed

The batch drying kinetics of corn as a test material were investigated experimentally in a novel rotating jet spouted bed (RJSB) using both continuous and intermittent (on/off) spouting and heating schemes. The parameters investigated include inlet air temperature, bed height, superficial air velocity, nozzle diameter, distributor rotational speed and intermittency of spouting and heat input. The results indicate that the drying kinetics are comparable with conventional spouted and fluidized beds for slow drying materials and that intermittent drying can save up to 40% of the thermal energy as well as air consumption with better quality product.     

MOBILE FLUIDIZED BED PADDY DRYER

ABSTRACT

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).     

Drying Efficiency of Indonesian Lignite in a Batch-Circulating Fluidized Bed Dryer

This research explores the production of low-moisture, high-rank coal using a batch-type, laboratory-scale, circulating fluidized bed to dry low-rank Indonesian coal with a high moisture content of 35 wt%. The operation was performed using air as a fluidization gas in a riser (a 4-m-tall pipe with an inner diameter of 0.04 m) at a gas velocity ranging from 2.0 to 2.7 m/s and a riser temperature of 80 to 150°C. The electric heaters were installed in the upper part of a downcomer to prevent the condensation of the evaporated moist- ure. The drying rate of the coal was investigated in terms of the inlet gas temperature, the gas velocity, and the drying time in order to determine the optimum operating conditions. Changes in the moist- ure content of the coal, before and after the experiments, were char- acterized by a proximate analysis, an ultimate analysis, the higher heating value (HHV), the lower heating value (LHV), a particle size analysis, and by the equilibrium moisture content. The results show that 70 to 80 wt% (wet basis, wb) of the total moisture can be reduced when the gas velocity of the riser is 2.0 m/s and the gas temperature is 150°C. In experiments, a simple mathematical model based on the heat and mass balances and a thin-layer drying model were simul- taneously used to predict the drying behavior of coal under the given operating conditions. The results of the model are similar to those of the experiment.     

DRYTNG OF PEPPER SEEDS USING A COMBINED MICROWAVE/FLUIDIZED BED DRYER

Abstract

The drying rates curves in terms of moisture content versus elapsed drying time for white pepper seeds were obtained experimentally using a fluidized bed and a combined microwave/fluidized bed. The combined microwave/fluidized bed employs a microwave field to assist convective drying. For both procedures, the drying rates were found to be dependent on the inlet air temperature and velocity. Significantly improved drying rates were achieved utilized a combined microwave/fluidized bed drying compared with a conventional fluidized bed.     

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

ABSTRACT

Microwave drying characteristics of sliced foods were investigated using potatoes (Solarium tuberosum) as a test model. Sliced samples were dried to 7-10% moisture content at microwave power levels between 2.2 W/g and 3.6 W/g (raw material). Moisture and temperature changes during drying were monitored. Semi-empirical models were developed that followed temperature and moisture changes during microwave drying. Sliced potatoes experienced three distinct periods: a warming-up period with little removal of moisture; a constant temperature period in which most of the drying took place; and a heating up period in which the drying rate decreased and sample temperature increased rapidly, often causing partial charring. Product temperature during the second period of microwave drying increased with sample thickness and microwave power. Drying rates were not affected by slice thickness, but increased with the microwave power/mass ratio. Product charring towards the end of drying may be avoided by reducing microwave power and increasing ambient air velocity.     

内热式惰性粒子流化床中膏状物料干燥模型

通过对惰性粒子流化床中膏状物料干燥机理的分析,得到了干燥时间及单位面积床层水分汽化量的数学计算式,可对干燥器的性能进行预测。采用带浸没加热管的惰性粒子流化床对膏状钛白物料进行干燥中试研究,采用气流式喷嘴将膏状物料分散成200～400μm的小液滴喷洒在惰性粒子表面进行干燥,探索了适宜的干燥条件,测定和确定了最佳的干燥工艺参数、操作参数和设备参数。结果表明:该干燥工艺能强化床内传热传质,促进高黏性膏糊状物料很好地分散,床层温度分布均匀,干燥器的操作弹性大,热量消耗低,干燥强度高,传热系数可达300W·m-2·K-1以上。     

A Study of the Power Density Distribution generated during the Combined Microwave and Convective Drying of Softwood

ABSTRACT

Investigations into new and innovative drying strategies can lead to the development of more efficient and effective drying processes. The commercialisation of these processes would prove invaluable to the drying industry as a whole and the associated technology would generate worldwide interest. Combined microwave and convective drying is one such process which offers great potential, with benefits that include : reduced drying times and increased drying rates; volumetric heating; higher fluxes of liquid to the drying surface; high temperature and internal pressure buildup within the material which enhances the overall moisture migration rate; and preferential heating of wetter areas. Numerical simulation can elucidate on the intricate details of the heat and mass transfer henomena that occur during the drying process, thus eliminating the need for performing numerous time consuming and expensive experiments. The simulations can predict the evolutionary behaviour of the moisture, temperature and pressure distributions, and can provide a detailed analysis of how microwaves interact with materials during drying and heating operations at a fundamental level. The research presented in this paper uses a comprehensive mathematical model to study the behaviour of the iniernal microwave power density distribution that is generated during the microwave enhanced convective drying of softwood. The configuration understudy concerns a plane wave microwave source irradiating the wood in the transverse direction.     

SOLAR DRYING OF RAISING

The drying of Sultana seedless grapes was investigated under intermittent and continuous operating conditions in a laboratory solar installation involving a thermal storage bed and an auxiliary heater. The effect of pretreatment, and of the a i r velocity on the drying rate of the grapes at constant temperature was also studied in relation t o the quality of the dried product.

Solar drying of the grapes was accomplished in 30.5 to 60.5 h of intermittent operation, or 19 to 60 h of continuous drying, involving the thermal storage bed and the auxiliary heater. The shortest drying time (19 h ) and the highest quality dried product were obtained with grapes dipped in a hot (80 C ) solution of sodium hydroxide and ethyl oleate, which were dried continuously at 42°C and 2m/s, air temperature and velocity respectively.

The mean apparent diffusivity of moisture in raisins at 6o°C and air velocity 2m/s was estimated as 1.0.10-10 m2/s.     

Diffusion-Controlled Batch Drying of Particles in a Novel Rotating Jet Annular Spouted Bed

Abstract

Results are presented for batch drying of rewetted wheat (used as model particles) in a newly developed annular spouted bed in which the drying air jet rotates slowly thus spouting bed intermittently. For particles drying in the falling rate period intermittent spouting is shown to result in significant energy savings with small increase in the drying time. A finite element simulation is employed to evaluate the effective moisture diffusivity of the test material which is nonspherical in geometry. Further, advantages of variable spouting and heating of the spouting air are demonstrated by a parametric study of the simulation model.     

DEFORMATION AND SlRESS ANALYSIS OF POROUS CAPILLARY BODIES DURING INTERMITTENT VOLUMERIC THERMAL DRYING

ABSTRACT

lntermittent heating in a thermal drying is one way to improve energy utilization and to enhance the quality for some products. Using the finite element method and Luikov's theory. the temperature, moisture and stress distributions at different instances of intermittent drying were obtained. Several examples were analyzed using this method. The numerical results are compared with prior experimental and theoretical results. This numerical simulation method could be used to devise an intermittent drying process schedule optimal operation.     

Drying and Quality Characteristics of Shredded Squid in an Infrared-Assisted Convective Dryer

Drying is one of the most common methods for processing and preserving squids. A novel forced convective dryer based on infrared heating was developed with an online temperature control. By setting the drying medium temperature of 50°C, we studied the effects of infrared wavelength and air velocity on drying characteristics of the shredded squid and qualities of dried squid products. We also compared it with the conventional hot-air drying (HAD) and advanced microwave vacuum drying (MVD). The infrared heating rate increase was faster than that of HAD. The heating and drying at the wavelength of 2.5–3.0 µm were more effective than those at the infrared wavelength of 5.0–6.0 µm. Specific energy consumption linearly increased with the air velocity. Microstructure observation showed that the infrared-dried rehydrated sample displayed a muscle fiber structure similar to the fresh sample. The infrared-dried squids had less drying shrinkage, brighter color, and better rehydration capacity than HAD products. Their sensory qualities were better than HAD and MVD products. Above all, infrared drying with wavelength of 2.5–3.0 µm and air velocity of 0.5 m/s was suggested as the best drying condition for squids in this study.     

EXPERIMENTAL INVESTIGATION AND MODELING OF THE INFLUENCE OF INDIRECT HEATING ON FLUIDIZED BED DRYING

Experimental results of fluidized bed drying with additional energy input by indirect heating are presented. A clear influence of moisture on wall-to-bed heat transfer coefficients is observed for particle diameters of 250 μm and 50 μm. Two different reasons for this influence may be distinguished in both, the measurements and theoretical considerations: 1 The change of bed and particle properties and 2 the increase of the local wall-to-particle heat transfer.     

Optimization of Heat Pump–Assisted Intermittent Drying

In the present study, heat pump–assisted drying of salak fruit was optimized by dividing the dehydration process into three distinct phases, namely, the initial, intermittent, and final stages. Drying variables considered for the optimization were the intermittent duration (X ₁), intermittent ratio (X ₂), and intermittent cycle (X ₃); the response variables studied were the total drying time (Y ₁), total heating time during intermittent drying (Y ₂), total heating time after intermittent drying (Y ₃), total color change (Y ₄), ascorbic acid content (Y ₅), and total phenolic content (Y ₆). Response surface methodology was used to determine the best combination of the drying variables that could provide the shortest drying period and premium product quality. Experimental results showed that all of the response variables were improved under the optimized intermittent drying conditions compared to the conventional method using constant drying conditions. The optimized heat pump–assisted intermittent drying reduced the drying time by 36% and improved phytochemicals retention with ascorbic acid and total phenolic content recorded at 18.4 ± 1.8 mg ascorbic acid/100 g dw and 43.3 ± 2.2 mg gallic acid equivalent (GAE)/g dw, respectively. The color change of the final product was minimum with a ΔE* value of 7.26 ± 2.03.     

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.     

OPTIMIZATION OF CONVECTIVE DRYING PROCESS BASED ON ECONOMIC INDICATORS

ABSTRACT

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.