EFFECT OF DRYING METHOD ON SHRINKAGE AND POROSITY

ABSTRACT

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density p_w, the dry solid density p_s, the dry solid bulk density p_bo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.     

EFFECT OF DRYING METHOD ON SHRINKAGE AND POROSITY

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density p_w, the dry solid density p_s, the dry solid bulk density p_bo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.     

HIGH HUMIDITY DRYING OF CORN: EFFECT ON DRYING RATE AND PRODUCT QUALITY

Drying of corn was studied at high and low humidity condiitions. Drying rate was measured, and test weight (bulk density), solid density, stress cracks, and breakage susceptibility of the dried grain at 15% (w.b.) were measured. Twenty four 350-g samples were dried under eight drying conditions in the 71° -104°C temperature range and 0.8%-80% RH relative humidity range. Each sample was exposed to the drying wnditions by using a semi-closed air flow cycle through a thin-layer of grain. High humidity drying reduced the drying rate by up to 44% and did not increase bulk density or solid density significantly as wmpared with low humidity drying. However, high humidity drying did reduce the multiple stress cracks in the corn kernels by up to 33.0 percentage points, increased the number of kernels with no stress cracks by up to 28.7 percentage points, and reduced Wisconsin and Stein breakage susceptibilities by up to 8.9 and 13.5 percentage poinu, respectively.     

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.     

EFFECT OF OSMOTIC PRE-TREATMENT AND INFRARED RADIATION ON DRYING RATE AND COLOR CHANGES DURING DRYING OF POTATO AND PINEAPPLE

A combination of intermittent infrared and continuous convection heating was used to dry various osmotically pretreated sample of potato (in solutions of 10%, 20% and 30% NaCl) and pineapple (in solutions of 30%, 50%, 70% Brix). The effect of drying conditions on color changes of potato and pineapple was investigated. The Hunter color scale parameters (redness, yellowness and lightness) were measured to quantify the color changes. With appropriate choice of infrared intermittency as well as osmotic pretreatment, it is possible to reduce the overall color change while maintaining high drying rates. As expected, osmotic pretreatment resulted in a shift in the sorption isotherms for both products.     

EFFECT OF OSMOTIC PRE-TREATMENT AND INFRARED RADIATION ON DRYING RATE AND COLOR CHANGES DURING DRYING OF POTATO AND PINEAPPLE

A combination of intermittent infrared and continuous convection heating was used to dry various osmotically pretreated sample of potato (in solutions of 10%, 20% and 30% NaCl) and pineapple (in solutions of 30%, 50%, 70% Brix). The effect of drying conditions on color changes of potato and pineapple was investigated. The Hunter color scale parameters (redness, yellowness and lightness) were measured to quantify the color changes. With appropriate choice of infrared intermittency as well as osmotic pretreatment, it is possible to reduce the overall color change while maintaining high drying rates. As expected, osmotic pretreatment resulted in a shift in the sorption isotherms for both products.     

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.     

EFFECT OF DRYING METHOD ON MESOPOROSITY OF RESORCINOL-FORMALDEHYDE DRYGEL AND CARBON GEL

Resorcinol-formaldehyde hydrogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution. RF cryogels, RF xerogels, and RF xerogels (MW gels) were respectively prepared from RF hydrogels by freeze drying, hot air drying, and microwave drying. Carbon cryogels, carbon xerogels and carbon MW gels were subsequently obtained by pyrolyzing RF drygels in an inert atmosphere. Freeze drying and microwave drying were effective to prepare mesoporous RF drygels and carbon gels. RF cryogels and carbon cryogels showed high mesoporosity over wide ranges of the molar ratio of resorcinol to catalyst (R/C) and the ratio of resorcinol to water (R/W) used in sol-gel polycondensation. Although RF xerogels had a few mesopores, carbon xerogels had no mesopores. RF MW gels and carbon MW gels showed mesoporosity if appropriate values of R/C and R/W were selected.     

EFFECT OF VIBRATION ON THE DRYING RATE DURING THE FALLING RATE PERIOD

The results of an experimental study on the drying characteristics in vibrated fluidized beds of corn plumule, silica gel and citric acid are presented. Application of vibration was found to enhance the drying rate during the falling rate period. In the optimum range of vibration parameters, the critical moisture content decreased significantly as compared with that in conventional fluidized beds of the same material.     

EFFECT OF VIBRATION ON THE DRYING RATE DURING THE FALLING RATE PERIOD

The results of an experimental study on the drying characteristics in vibrated fluidized beds of corn plumule, silica gel and citric acid are presented. Application of vibration was found to enhance the drying rate during the falling rate period. In the optimum range of vibration parameters, the critical moisture content decreased significantly as compared with that in conventional fluidized beds of the same material.     

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Abstract

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air.

The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.     

EFFECT OF STEPWISE CHANGE OF DRYING RATE ON MICROSTRUCTURE EVOLUTION IN POLYMER FILMS

The effect of the drying rate of the solvent on microstructure evolution in thin films of immiscible polystyrene/polycarbonate blends was investigated by applying a stepwise change in air velocity. The polymer blend was dissolved in tetrahydrofuran and cast on a glass substrate to form a thin liquid film. The drying rate of the solvent was measured by a gravimetric technique, and the phase-separated structure in the film was observed by optical microscopy. The results indicated that cellular structures of droplet arrays were formed in the phases evolved primarily during the early stage of drying, and the droplets then coalesced in the intermediate stage to form bigger droplets. The size of the first evolved phase decreased with increasing time at a high drying rate, whereas larger droplets were obtained with longer high drying rate regimes. These results suggested that the droplets were induced by secondary phase separation which was strongly affected by the drying rate of the solvent.     

EFFECT OF STEPWISE CHANGE OF DRYING RATE ON MICROSTRUCTURE EVOLUTION IN POLYMER FILMS

The effect of the drying rate of the solvent on microstructure evolution in thin films of immiscible polystyrene/polycarbonate blends was investigated by applying a stepwise change in air velocity. The polymer blend was dissolved in tetrahydrofuran and cast on a glass substrate to form a thin liquid film. The drying rate of the solvent was measured by a gravimetric technique, and the phase-separated structure in the film was observed by optical microscopy. The results indicated that cellular structures of droplet arrays were formed in the phases evolved primarily during the early stage of drying, and the droplets then coalesced in the intermediate stage to form bigger droplets. The size of the first evolved phase decreased with increasing time at a high drying rate, whereas larger droplets were obtained with longer high drying rate regimes. These results suggested that the droplets were induced by secondary phase separation which was strongly affected by the drying rate of the solvent.     

共沉淀喷雾干燥法制备YSZ粉料及其对烧结密度的影响

用共沉淀结合喷雾干燥法制得了可直接用于煅烧、成型的Y_2O_3稳定的ZrO_2粉料。用扫描电镜观察了喷雾干燥得到的粉料团聚体的尺寸、形貌,以及与喷雾干燥条件的关系。发现了共沉淀悬浮液的球磨对喷雾干燥后粉料的热行为的影响。喷雾干燥制得的粉料在1350、1500℃烧结后,相对密度分别达96.5％、97.8％;而烘箱干燥的粉料在相应温度下烧结,相对密度只达88％、93％。     

EFFECT OF CREEP AND MECHANO-SORPTIVE EFFECT ON STRESS DEVELOPMENT DURING DRYING

Constitutive equations to quantify wood deformation under combined mechanical loading and moisture content change (1] were coupled with the moisture distribution developed during drying to predict stress and strain in 50 by 190-mm Douglas-fir heartwood lumber.

Two combinations of temperature and relative humidity were used to dry the wood. The overall board shrinkage and the immediately released and set strains were measured as a function of time. Those strains were compared with analytic results, which showed good agreement.

The roles that four strain components played in the development of stress-both at board surface and center were compared for different drying conditions. The significance of creep and mechano-sorptive strain in relieving the stress was demonstrated by varying the model parameters.     

EFFECT OF A TEMPERING PERIOD ON DRYING OF CARROT IN A VffiRO-FLUlDIZED BED

ABSTRACT

Drying of diced carrot in a vibrofluidized bed was studied experimentally for various air temperatures, bed heights and size of the cubes. Effect of a tempering period that mav be implemented into ¦ drying cycle was examined with respect to drying kinetics and energy consumption. Although two stage drying with a tempering period increases the overall drying time, it shortens the drying time in a VFB dryer thus reduces energy consumption     

EFFECT OF CREEP AND MECHANO-SORPTIVE EFFECT ON STRESS DEVELOPMENT DURING DRYING

ABSTRACT

Constitutive equations to quantify wood deformation under combined mechanical loading and moisture content change (1] were coupled with the moisture distribution developed during drying to predict stress and strain in 50 by 190-mm Douglas-fir heartwood lumber.

Two combinations of temperature and relative humidity were used to dry the wood. The overall board shrinkage and the immediately released and set strains were measured as a function of time. Those strains were compared with analytic results, which showed good agreement.

The roles that four strain components played in the development of stress-both at board surface and center were compared for different drying conditions. The significance of creep and mechano-sorptive strain in relieving the stress was demonstrated by varying the model parameters.     

EFFECT OF DRYING METHODS ON PHYSICOCHEMICAL AND SENSORY QUALITIES OF FUFU

ABSTRACT

Fufu was dried using rotary drier, cabinet drier and sun drying methods. The effects on the physical, chemical and sensory properties of fufu by the different drying methods were evaluated. The drying methods did not appreciably affect the yield, proximate composition or acidity of fufu, but affected pasting properties. Panellists (P < 0.05) rated the rotary dried fufu best.     

EFFECT OF DRYING ON FLUIDISATION BEHAVIOUR OF PACKED BEDS OF SHREDDED COCONUT

Drying of shredded coconut is usually carried out commercially in order to facilitate storage over reasonable periods of time and to obtain advantages of reduction of weight and volume in transport and packaging. Fluidised bed drying of materials is generally accepted to be an efficient method of drying. Experiments were carried out to investigate the behaviour of fluidisation of shreddet coconut at various moisture contents. The pressure drop was measured across random packings of shredded coconut. It is seen that shredded coconut does not fluidise easily at moisture contents greater than 0.55 (55 weight percent moisture). It is also seen that fluidisation can be easily achieved by drying shredded coconut to moisture contents between approximately 0.25 to 0.55. The pressure drop characteristics within this region is seen to closely resemble the theoretical behaviour of a fluidised bed. It is also seen that particles of shredded coconut in beds of moisture contents less than 0.25 tend to undergo pneumatic transport if efforts are made to fluidise such beds.