COMBINED MICROWAVE AND CONVECTIVE DRYING OF A POROUS MATERIAL

A model is formulated to describe the drying of a slab of porous material in a combined microwave and convective environment. The model describes the evolution of temperature, pressure, moisture and power distributions that occur during the drying process. The microwave internal heat source is calculated from electromagnetic theory with varying dielectric properties. The inclusion of pressure in the model allows the physical phenomena of “water pumping”, often observed in microwave drying systems, to be accounted for. The influence of sample size; on the drying kinetics 1s examined and found to be an important parameter during the drying process. In particular the effect of resonance on the moisture and temperature profiles and the need for careful consideration of surface mass transfer coefficients are investigated. Simulation results are presented for the combined microwave and convective drying of a homogeneous, isotropic porous material.     

COMBINED CONVECTIVE AND INFRARED DRYING OF A CAPILLARP POROUS BODY

Abstract

This work investigates the effect of spray drying conditions on some properties of tomato powder prepared by spray drying of tomato pulp. A pilot scale spray dryer (Buchi, B-191) with cocurrent regime and a two-fluid nozzle atomizer was employed. Sixty-four different experiments were conducted keeping constant the feed rate, the feed temperature, and the atomizer pressure, and varying the compressed air flow rate, the flow rate of drying air, and the air inlet temperature. Tomato powders were analyzed for moisture, solubility, density (bulk and packed), and hygroscopicity. Analysis of experimental data yielded correlations between powder properties and the above-mentioned variable operating conditions. Regression analysis was used to fit a full second order polynomial, reduced second order polynomials and linear models to the data of each of the properties evaluated. F values for all reduced and linear models with an R ² ≥ 0.70 were calculated to determine if the models could be used in place of full second order polynomials.     

ANALYSIS OF HEAT AND MASS TRANSFER DURING COMBINED MICROWAVE CONVECTIVE SPOUTED-BED DRYING

Heat and mass transfer phenomena during the combined microwave-convective batch spouted bed drying are analyzed. Wheat was chosen as a test material. The governing equations, including consideration of internal heat generation and thermodiffusion are formulated and solved using the numerical method of lines. The model allowed variable material transport and dielectric properties. The parameters investigated include electric field strength, electromagnetic field frequency, inlet air temperature, and superficial air velocity. Representative drying and temperature curves as well as moisture and temperature profiles are presented and discussed.     

ANALYSIS OF HEAT AND MASS TRANSFER DURING COMBINED MICROWAVE CONVECTIVE SPOUTED-BED DRYING

Heat and mass transfer phenomena during the combined microwave-convective batch spouted bed drying are analyzed. Wheat was chosen as a test material. The governing equations, including consideration of internal heat generation and thermodiffusion are formulated and solved using the numerical method of lines. The model allowed variable material transport and dielectric properties. The parameters investigated include electric field strength, electromagnetic field frequency, inlet air temperature, and superficial air velocity. Representative drying and temperature curves as well as moisture and temperature profiles are presented and discussed.     

COMBINED OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES

Strawberries pretreated with 2% ethyl oleate and 0.5% NaOH were osmotically dehydrated and their osmotic dehydration rate is compared with untreated berries. It was found that treated berries dehydrated belter compared to untreated berries. Osmotically dehydrated berries were convective and microwave dried at different power levels and results were compared with respect to drying time and rate. The rehydration ratio, texture, color and sensory values are compared with freeze dried strawberries with the same pretreatment. It was found that microwave drying was short in time and the quality parameters of the microwave dried berries were comparable to those of freeze dried berries.     

COMBINED OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES

ABSTRACT

Strawberries pretreated with 2% ethyl oleate and 0.5% NaOH were osmotically dehydrated and their osmotic dehydration rate is compared with untreated berries. It was found that treated berries dehydrated belter compared to untreated berries. Osmotically dehydrated berries were convective and microwave dried at different power levels and results were compared with respect to drying time and rate. The rehydration ratio, texture, color and sensory values are compared with freeze dried strawberries with the same pretreatment. It was found that microwave drying was short in time and the quality parameters of the microwave dried berries were comparable to those of freeze dried berries.     

DIELECTRICALLY-ASSISTED DRYING OF A NONHYGROSCOPIC POROUS MATERIAL

Experiments and simulations for dielectrically-assisted drying of a nonhygroscopic material that exhibits negligible capillary moisture transfer have been conducted in order to ascertain phenomena associated with the transient drying process. Results and predictions indicate that, in general, dielectrically-assisted drying is characterized by three drying regimes. An initial regime occurs in which the material is heated convectively and dielectrically, followed by a transition regime and ultimately, a final regime during which the material is dielectrically heated and convectively cooled. During the final regime, an inverted temperature distribution in the material induces internal condensation which influences transport rates and the thermal response of the system. Measured and predicted drying histories are bracketed by algebraic expressions derived from simple energy balances under a range of conditions where volumetric dielectric heating is strong, relative to surface convective heating or cooling.     

DIELECTRICALLY-ASSISTED DRYING OF A NONHYGROSCOPIC POROUS MATERIAL

Experiments and simulations for dielectrically-assisted drying of a nonhygroscopic material that exhibits negligible capillary moisture transfer have been conducted in order to ascertain phenomena associated with the transient drying process. Results and predictions indicate that, in general, dielectrically-assisted drying is characterized by three drying regimes. An initial regime occurs in which the material is heated convectively and dielectrically, followed by a transition regime and ultimately, a final regime during which the material is dielectrically heated and convectively cooled. During the final regime, an inverted temperature distribution in the material induces internal condensation which influences transport rates and the thermal response of the system. Measured and predicted drying histories are bracketed by algebraic expressions derived from simple energy balances under a range of conditions where volumetric dielectric heating is strong, relative to surface convective heating or cooling.     

COMPARISON OF CONVECTIVE, VACUUM, AND MICROWAVE DRYING CHLORPROPAMIDE

Drying kinetics of convective, vacuum, and microwave drying of a pharmaceutical product, chlorpropamide, has been investigated on a laboratory scale, in the temperature interval from 40°C to 60°C, and the range of microwave heating power from 154 W/kg_dm to 385 W/kg_dm.

The experimental data obtained were approximated with the “thin-layer” equation and a two parameter exponential model. In order to compare convective, vacuum, and microwave drying, effective diffusion coefficients and specific heat consumption were calculated for each drying method.

Higher rates and shorter drying times were achieved at a higher temperature and microwave heating power. The highest drying rates and the lowest specific heat consumption were achieved with microwave drying. This leads to the conclusion that microwave heating is the most appropriate method for drying of chlorpropamide. The quality of product was not changed for all applied methods.     

SHRINKAGE OF POROUS MATERIALS DURING MICROWAVE DRYING

The dissipation of electromagnetic energy inside a material creates a thermal imbalance state producing some reactions different from those observed through slow classic drying processes. Important drying rates obtained by microwave application can be understood by taking into account induced pressure gradients which greatly accelerate the thermomigration mechanism and thereby modify the physical properties of the product. The shrinkage of porous materials during the drying stage is very sensitive to the internal vapour pressure.The quality of such products depends on the shrinkage behaviour, and it is therefore interesting to study this phenomenon in order to control the characteristics of the product. This study is an experimental approach leading to a theoretical model describing the shrinkage mechanism. This model is developed from results obtained by a computer controlled measurement system allowing to regulate the drying kinetics.     

SHRINKAGE OF POROUS MATERIALS DURING MICROWAVE DRYING

The dissipation of electromagnetic energy inside a material creates a thermal imbalance state producing some reactions different from those observed through slow classic drying processes. Important drying rates obtained by microwave application can be understood by taking into account induced pressure gradients which greatly accelerate the thermomigration mechanism and thereby modify the physical properties of the product. The shrinkage of porous materials during the drying stage is very sensitive to the internal vapour pressure.The quality of such products depends on the shrinkage behaviour, and it is therefore interesting to study this phenomenon in order to control the characteristics of the product. This study is an experimental approach leading to a theoretical model describing the shrinkage mechanism. This model is developed from results obtained by a computer controlled measurement system allowing to regulate the drying kinetics.     

HYBRID DRYING OF GRANULAR MATERIALS BY COMBINED RADIATIVE AND CONVECTIVE HEATING

ABSTRACT

As one of the basic steps of the R&D on hybrid drying by combined radiative and convective heating, drying experiments of a stagnant horizontal wet granular layer heated simultaneously by infrared lamps and hot air were conducted throughout a whole period of the drying. An opaque model, in which the incident radiation is assumed to be absorbed only on the surface of the layer, was introduced to explain experimental trends. Then, the effects of combined heating on drying characteristics of three kinds of samples, a silica sand, a brick and a brown coal were discussed by comparing the experimental data with the calculated results.

The calculated results were found to be in a close agreement with the experimental data obtained on the time behaviors of the drying rate and the temperature distribution in the- layer during not only the preheating and the constant rate periods but also the falling rate period. In the case of the coal layer, however, the falling drying rate different from that in ordinary convective drying had to be considered since it appeared that the radiative heating had an effect to enhance the falling drying rate of the coal owing to some interactions of the coal and water.     

HYBRID DRYING OF GRANULAR MATERIALS BY COMBINED RADIATIVE AND CONVECTIVE HEATING

As one of the basic steps of the R&D on hybrid drying by combined radiative and convective heating, drying experiments of a stagnant horizontal wet granular layer heated simultaneously by infrared lamps and hot air were conducted throughout a whole period of the drying. An opaque model, in which the incident radiation is assumed to be absorbed only on the surface of the layer, was introduced to explain experimental trends. Then, the effects of combined heating on drying characteristics of three kinds of samples, a silica sand, a brick and a brown coal were discussed by comparing the experimental data with the calculated results.

The calculated results were found to be in a close agreement with the experimental data obtained on the time behaviors of the drying rate and the temperature distribution in the- layer during not only the preheating and the constant rate periods but also the falling rate period. In the case of the coal layer, however, the falling drying rate different from that in ordinary convective drying had to be considered since it appeared that the radiative heating had an effect to enhance the falling drying rate of the coal owing to some interactions of the coal and water.     

COMBINED SUPERHEATED STEAM AND MICROWAVE DRYING OF SINTERED GLASS BEADS: DRYING RATE CURVES

A new drying method of combined superheated steam and microwave drying is being proposed. The drying rates of sintered glass beads in combined superheated steam and microwave drying are experimentally and theoretically investigated. Drying experiments have been carried out in a waveguide where a standing wave is formed to uniformly heat a small sample. Concerning drying rate curves in combined superheated steam and microwave drying, a distinct constant rate period has been observed. For the falling rate period, high drying rates have been observed. For both periods, the drying rates in combined superheated steam and microwave drying are higher than those in superheated steam alone. Also, in comparison with the results of combined nitrogen and microwave drying, the normalized drying rates in combined superheated steam and microwave drying are higher than those at less than the critical moisture content in combined nitrogen and microwave drying. Moreover, theoretical drying rates for the falling rate period (predicted by a modified receding evaporation front model) in combined superheated steam and microwave drying, are in good agreement with the observed drying rates. The combined superheated steam and microwave drying method can attain higher drying rates under mild external conditions.     

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.     

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.     

非饱和含湿多孔介质微波冷冻干燥过程传热传质分析

基于升华冷凝模型,对非饱和含湿多孔介质微波冷冻干燥过程作了数值计算．结果表明,干燥过程中不饱和含冰区内的冰饱和度有较大变化．通过与不考虑升华冷凝区相比较,表明升华冷凝区的存在不可忽略．     

EFFECT OF THE VARIABILITY OF HEAT AND MASS TRANSFER COEFFICIENTS ON CONVECTIVE AND CONVECTIVE-RADIATIVE DRYING OF POROUS MEDIA

A numerical investigation was conducted to study two-dimensional heat and mass transfer during convective drying of a clay brick. The established numerical code has allowed us to determine the effect of heat and mass transfer coefficients variability on state variables and on the drying kinetic.     

CONVECTION HEAT TRANSEER COEFFICIENTS AT CONVECTIVE DRYING OF POROUS MATERIALS*

Our measurements proved that the convective heat transfer coefficient (h) has a larger value h_wet at the constant drying rate period and after that it falls down to a minimum one: h_dry in the equilibrium dried state (see Fig.1). Our measurements showed also that the heat of vaporisation in the last phase of the falling drying rate period is far greater than it was in the constant drying rate period (see Fig.10).

Our first measurements were made on a gypsum plate. Afterwards we carried out measurement research with fine glass powder and cement-perlite plate and determined h_wet and h_dry heat transfer coefficients as function of Reynolds number. All of these measurements confirmed our establishment that h_wet is far greater than h_dry.