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 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.     

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.     

EFFECT OF MATURITY AND HARVEST METHOD ON DRYING RATE OF WHEAT

Drying rates of cereals have a significant influence on the drying capacity of dryers. Drying rates are influenced by the variety and type of cereal, past history of the grain, maturity at harvest and the harvest method. The effect of the last two parameters on the drying rate of wheat was investigated by drying a single-kernel-thick layer of wheat in an experimental dryer. The laboratory results indicated that grain in the swath had a higher drying rate than grain standing in the field on a particular day of harvest. The effect of maturity or harvest date on the drying rate did not show any particular trend. In the field, the grain in the swath dried faster than grain standing in the field.     

INTERMITTENT DRYING OF CARROT IN A VIBRATED FLUID BED: EFFECT ON PRODUCT QUALITY

ABSTRACT

Drying of diced carrot was studied experimentally in a laboratory batch-type vibrofluidized bed dryer (VFB) The kinetics of the degradation reaction of P-carotene chosen as the dominant quality index were determined for various time-temperature conditions. Relationships were obtained for instantaneous concentration of β-carotene, and for the degradation rate constant as a function of moisture content and material temperature. It was shown that the best quality product is obtained when carrot is dried in a VFB dryer down to the critical moisture content, than conditioned (tempered) in a single layer exposed to ambient air, and then finish dried to the final moisture content in a VFB dryer.     

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.     

抽风率对旋风分离器主要性能参数的影响

根据冷模试验结果 ,探讨了抽风率对压力损失、分离效率与切割粒径等旋风分离器的主要性能参数的影响 ,分析了造成这种影响的原因 ,从而可为开发新型的旋风分离器提供参考     

EFFECT OF DRYING MODE ON DRYING KINETICS OF ONION

Onion slices 3 mm thick were dried in different modes. Drying at 60°C by convection was taken as a reference process. reviseddate=It has been shown that increase in air temperature increases the rate of drying. Effective diffusivity increases with increasing temperature, but it is strongly dependent on water content. Energy of activation for diffusion is also strongly affected by water content. acceptdate=Stepwise changes in temperature do not result in beneficial effects which could be expected. Extension of drying time was even observed in respect to that obtained at constant 60°C. printpubdate=Drying with infrared energy or assisting convective drying with microwave energy results in increased drying rates and substantial shortening of the drying time.     

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.     

氧气流速对水煤浆气化烧嘴性能的影响规律研究

针对水煤浆烧嘴设计过程中氧气流速的取值问题,采用数值模拟的方法研究了氧气流速对水煤浆烧嘴工作性能的影响规律.结果表明,提高氧气流速有利于强化水煤浆的雾化效果,从而提高气化反应的效率,但过高的氧气流速会使回流区上移,造成气化炉拱顶超温,不利于气化炉的安全运行.     

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.     

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

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 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.     

CONVECTIVE DRYING OF AGRICULTURAL PRODUCTS. EFFECT OF CONTINUOUS AND STEPWISE CHANGE IN DRYING AIR TEMPERATURE

Samples of banana were dried in a two-stage heat pump dryer capable of producing stepwise control of the inlet drying air temperature while keeping absolute humidity constant. Two stepwise air temperature profiles were tested. The incremental temperature step change in temperature of the drying air about the mean air temperature of 30 °C was 5 °C. The total drying time for each temperature-time profile was about 300 minutes. The drying kinetics and color change of the products dried under these stepwise variation of the inlet air temperature were measured and compared with constant air temperature drying. The stepwise air temperature variation was found to yield better quality product in terms of color of the dried product. Further, it was found that by employing a step-down temperature profile, it was possible to reduce the drying time to reach the desired moisture content.     

CONVECTIVE DRYING OF BANANA, GUAVA AND POTATO PIECES : EFFECT OF CYCLICAL VARIATIONS OF AIR TEMPERATURE ON DRYING KINETICS AND COLOR CHANGE

Pieces of banana, guava and potato were dried in a two-stage heat pump dryer capable of precise control of air humidity with predetermined cyclic variations of air temperature entering the drying chamber. The air temperature variations tested were : a cosine, a reversed cosine and three different square wave profiles with peak-to-valley variations from 20^°C to 40^°C. The cycle time was about 60 minutes with drying time of approximately 300 minutes. The drying samples were placed on trays in a thin layer. With appropriate choice of temperature-time variation, it is possible to reduce the overall color change while maintaining high drying rates.     

SIMULATION OF THE EFFECT OF AIR VELOCITY IN THE DRYING OF HARDWOOD TIMBER

The effects of different air velocities on the drying of Australian hardwood timber have been investigated using a drying model based on Fickian diffusion. The air velocities studied were 0.05, 0.5, and 2 m.s^-1, corresponding to typical velocities covering the range used from pre-drying to normal kiln conditions. Decreasing tbe air velocity from 2 m.s^-1 to 0.05 m.s^-1 reduces the maximum strain experienced with an optimised drying schedule by 34%, although if the lower velocity is used throughout the drying period, the drying time is predicted to be 40% longer. Explorations with a program to optimise drying schedules suggest that there may Dot be any significant advantage in moving from a low air velocity of 0.05 m.s^-1 to a higher one (2 m.s^-1) in terms of reducing drying time, for the same maximum strain during drying, compared with using a constant air velocity of 0.5 m.s^-1.     

SIMULATION OF FLUIDIZED-BED DRYING OF CARROT WITH MICROWAVE HEATING

ABSTRACT

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.     

CONVECTIVE DRYING OF BANANA,GUAVA AND POTATO PIECES : EFFECT OF CYCLICAL VARIATIONS OF AIR TEMPERATURE ON DRYING KINETICS AND COLOR CHANGE

ABSTRACT

Pieces of banana, guava and potato were dried in a two-stage heat pump dryer capable of precise control of air humidity with predetermined cyclic variations of air temperature entering the drying chamber. The air temperature variations tested were : a cosine, a reversed cosine and three different square wave profiles with peak-to-valley variations from 20^°C to 40^°C. The cycle time was about 60 minutes with drying time of approximately 300 minutes. The drying samples were placed on trays in a thin layer. With appropriate choice of temperature-time variation, it is possible to reduce the overall color change while maintaining high drying rates.