COMBINED DRYING OF CRYSTALLOCOLLOIDAL MATERIALS

ABSTRACT

Principles of a new high-effective technique of granulating and drying some materials are discussed. The material properties are shown to affect the granulation and drying processes. Methods to describe falling and fluidized bed drying are suggested.     

DRYING OF OPTICALLY SEMITRANSPARENT MATERIALS BY COMBINED RADIATIVE-CONVECTIVE HEATING

The objective of this work has been to basically elucidate the drying characteristics of an optically semitransparent material by combined radiative and convective heating. The experiments were conducted for a graphite suspension, a slurry of surplus activated sludge and a wet silica sand. The time-change of the drying rate as well as of the surface temperature of the brimfully wet material layer were measured under the step heating conditions using an infrared lamp bundle and a blast of hot air heated by an electric heater.

The experimental data obtained show satisfactory agreement with the calculated results from unsteady heat and mass transfer equations derived on the basis of a semitransparent drying model during the preheating and the constant drying rate periods.     

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.     

ENHANCEMENT OF SPRAY DRYING OF THERMOSENSITIVE MATERIALS

The ways to enhance spray drying of thermo-sensitive materials such as whole egg (yolk and white) are considered. New approaches to drying agent distribution and liquid spraying processes are suggested. Some experimental results are presented. A spray dryer of capacity 300 kg of water evaporated per hour has been proposed. The dryer operating parameters are compared with those of similar equipment for whole egg drying.     

HEAT PUMP DRYING OF AGRICULTURAL MATERIALS

Heat pump-assisted drying is an energy-efficient process because the heat is recoverable. However, the economic feasibility of the heat pump dryer (HPD) is debatable because high grade energy (electricity) is used. In this study, sawn rubber wood and bananas were dried in an experimental HPD. The moisture extraction rates (MER) and the specific moisture extraction rates (SMER) of wood drying and banana drying decreased rapidly with the drying time but the compressor power was relatively constant. If the final moisture content of wood is less than 10%, the maximum average MER and SMER were 2.854 kg/h and 0.572 kg/kWh, respectively. Banana drying yielded the highest average MER of 2.710 kg/h when the drying load was highest. The corresponding SMER was 0.540 kg/kWh. Economic analyses among the HPD, an electrically-heated hot air dryer and a direct-fired dryer revealed that the HPD had the lowest operating cost.     

PULSED MICROWAVE-VACUUM DRYING OF FOOD MATERIALS

Microwave drying of food materials has been investigated over several years as a potential means for reducing the total drying time. However, some quality loss almost always accompanied when foods were dried completely using microwaves due to nonuniform temperature and moisture distribution. Some strategies used to improve dried product quality include combination of microwave and conventional hot air drying. pulsed or intermittent drying, and microwave-vacuum drying. Combination of pulsing and vacuum drying is a useful technique to maximize energy use efficiency and product quality especially for temperature sensitive products such as fruits. Some results of pulsed, microwave-vacuum drying of cranberries are presented. Pulsed drying is more energy efficient than continuous drying. In pulsed drying, the longer the pulsing ratio (i.e. longer power-off time in relation to power-on time) was more energy efficient. The quality of pulse-dried product was also generally better than that of continuous-dried product. The cycle power-on time and pulsing ratio should be carefully selected to obtain maximize the benefits of pulsed, microwave vacuum drying     

DRYING OF GRANULAR MATERIALS IN AN INCLINED VIBRATED FLUIDIZED BED BY COMBINED RADIATIVE AND ONVECTIVE HEATING

In an inclined-vibrated fluidized bed (IVF-bed), solid particles are fluidized easily at a relatively low air velocity. Good mixing of the fluidized particles in the direction of the air flow and the renewal of the bed surface take place as a result of the vibration effect. Accordingly when the bed surface is heated by thermal radiation from the top surface, radiant heat is absorbed more effectively on the surface than on a stagnant bed surface. This characteristic of an IVF-bed indicates the applicability to a solar dryer and other similar dryers.

The present study reports the result of model experiments carried out to confirm the effectiveness of radiative heating on drying of wet granular materials. Further, for quantitative evaluation of the radiation effect, a theoretical model is offered and its validity is examined.     

ENHANCEMENT OF SPRAY DRYING OF THERMOSENSITIVE MATERIALS

The ways to enhance spray drying of thermo-sensitive materials such as whole egg (yolk and white) are considered. New approaches to drying agent distribution and liquid spraying processes are suggested. Some experimental results are presented. A spray dryer of capacity 300 kg of water evaporated per hour has been proposed. The dryer operating parameters are compared with those of similar equipment for whole egg drying.     

PULSED MICROWAVE-VACUUM DRYING OF FOOD MATERIALS

ABSTRACT

Microwave drying of food materials has been investigated over several years as a potential means for reducing the total drying time. However, some quality loss almost always accompanied when foods were dried completely using microwaves due to nonuniform temperature and moisture distribution. Some strategies used to improve dried product quality include combination of microwave and conventional hot air drying. pulsed or intermittent drying, and microwave-vacuum drying. Combination of pulsing and vacuum drying is a useful technique to maximize energy use efficiency and product quality especially for temperature sensitive products such as fruits. Some results of pulsed, microwave-vacuum drying of cranberries are presented. Pulsed drying is more energy efficient than continuous drying. In pulsed drying, the longer the pulsing ratio (i.e. longer power-off time in relation to power-on time) was more energy efficient. The quality of pulse-dried product was also generally better than that of continuous-dried product. The cycle power-on time and pulsing ratio should be carefully selected to obtain maximize the benefits of pulsed, microwave vacuum drying     

DRYING OF GRANULAR MATERIALS IN AN INCLINED VIBRATED FLUIDIZED BED BY COMBINED RADIATIVE AND ONVECTIVE HEATING

In an inclined-vibrated fluidized bed (IVF-bed), solid particles are fluidized easily at a relatively low air velocity. Good mixing of the fluidized particles in the direction of the air flow and the renewal of the bed surface take place as a result of the vibration effect. Accordingly when the bed surface is heated by thermal radiation from the top surface, radiant heat is absorbed more effectively on the surface than on a stagnant bed surface. This characteristic of an IVF-bed indicates the applicability to a solar dryer and other similar dryers.

The present study reports the result of model experiments carried out to confirm the effectiveness of radiative heating on drying of wet granular materials. Further, for quantitative evaluation of the radiation effect, a theoretical model is offered and its validity is examined.     

RADIATIVE DRYING MODEL OF POROUS MATERIALS

ABSTRACT

A transient one dimensional first principles model is developed for the drying of a porous material (paper) that includes both heat and mass transfer. All three modes of heat transfer are considered; conduction, convection and radiation. The conduction is assumed to be in one dimension, through the porous material. The convection is assumed to exist only at the surface as a boundary condition. The radiation is assumed to be a volumetric phenomenon, so that the material internally absorbs, emits, and scatters energy. The absorption and scattering coefficients are spectrally dependent. Furthermore, the material is considered to have a non-unity refractive index with diffuse surfaces. In the mass transfer it is assumed that water exists in three phases: bound, free and vapor. The results provide profiles within the material for each moisture phase, temperature, and pressure and the effect of radiation on these distributions.     

RADIATIVE DRYING MODEL OF POROUS MATERIALS

A transient one dimensional first principles model is developed for the drying of a porous material (paper) that includes both heat and mass transfer. All three modes of heat transfer are considered; conduction, convection and radiation. The conduction is assumed to be in one dimension, through the porous material. The convection is assumed to exist only at the surface as a boundary condition. The radiation is assumed to be a volumetric phenomenon, so that the material internally absorbs, emits, and scatters energy. The absorption and scattering coefficients are spectrally dependent. Furthermore, the material is considered to have a non-unity refractive index with diffuse surfaces. In the mass transfer it is assumed that water exists in three phases: bound, free and vapor. The results provide profiles within the material for each moisture phase, temperature, and pressure and the effect of radiation on these distributions.     

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.