Drying of Pepper Seed Particles in a Superheated Steam Fluidized Bed Operating at Reduced Pressure

A series of drying experiments was performed in a reduced-pressure superheated steam fluidized bed, employing pepper seed particles and some novel data were obtained. Experiments were carried out using different chamber pressures (40–67 kPa), temperatures (90–122°C), steam velocities (2.35–4.10 m/s), and mass flow rates (0.0049–0.0134 kg/s). In the majority of the experiments, the moisture gain observed in some other studies in the warm-up period of the process was prevented through some supplementary heat provided to the column. The drying rate was found to be increasing by operating temperature; however, it was not affected much by the superficial gas velocity and the operating pressure. Nevertheless, the reduced pressure operation increases the degree of superheating that appears as the most important parameter of the process. The experimental results showed that the equilibrium moisture content decreases by the increasing degree of superheating. On the other hand, the critical moisture content assumes higher values for the greater degrees of superheating. It was concluded that a relatively lower temperature process can be achieved through a reduced-pressure superheated steam fluidized bed.     

Drying Characteristics of Carrots Immersed in a Fluidized Bed of Fluidizing Particles Under Reduced Pressure

The drying characteristics and properties (color and shrinkage) of carrots (as a representative agricultural product) were experimentally examined in a fluidized bed under reduced pressure. Dry hot air and superheated steam were used as the drying gases. Rice and carrot powders (0.125–0.355 mm in diameter) were used as the fluidizing particles, in addition to glass beads (0.12 mm in diameter).

It was confirmed that the drying rate using a fluidized bed was much higher than without a fluidized bed (hot-air drying), regardless of the type of fluidizing particles used. Under reduced pressure, both with and without a fluidized bed, the drying rate was higher than that at atmospheric pressure using hot air. The drying rate was sufficiently high for fluidized-bed drying with superheated steam, though the drying rate was higher with hot air than with superheated steam. As the drying temperature increased, the volume ratio (befor/after drying) of the sample increased. At high drying temperatures (373 and 423 K in the present study), the color of the sample changed; in other words, a heat-induced change in the properties of the carrot was observed. At a low drying temperature (333 K in the present study), the drying method did not affect the color of the carrot; i.e., the color of the dried material was maintained even in a fluidized bed under reduced pressure when the drying rate was higher.     

An optimization model of the operating costs of a fluidized bed steam‐drying plant

A model is developed to determine an optimum apparatus geometry and, for given apparatus dimensions, a financially optimal fluidized bed height. The parameters that effect the operating costs are the bed mass, the apparatus diameter and the gas mass flow rate. To implement such cost optimization, a physics‐based mathematical model for describing the thermodynamic processes in fluidized bed steam‐drying is briefly explained and presented. The most important conclusion is not to operate the fluidized bed for a drying process below a certain minimum cost, calculated with the help of the modelling. The problem, when describing the drying process and consequently the mass transfer, is that in the superheated steam drying case studied here, water is evaporated as moisture and withdrawn into an atmosphere of vapor water.     

Drying of soy sauce residue in superheated steam at atmospheric pressure

Soy sauce residue needs drying to avoid fermentation and oxidation during storage and transportation, and its reutilization as a useful resource is expected. Superheated steam drying was applied to investigate the effects of drying conditions on the drying characteristics and the content changes of salt and protein. The results showed that the inversion temperature was about 130°C, beyond which superheated steam drying was faster than hot air-drying. The drying time approaching equilibrium moisture content was reduced with elevated drying temperature as well as higher steam mass flow rate in the present experimental conditions. The effect of bed thickness on drying time was not obvious when drying temperature increased. Interestingly, the salt content of soy sauce residue could be decreased by 34.8% due to condensate water in the initial drying period (wetting), while protein content had no significant loss (p?相似文献   

Moisture equilibrium of wood and bark chips in superheated steam

This study relating to the steam drying of biofuels suggests that the activities of water in air (defined as relative humidity) and of water in superheated steam (defined as the ratio of the saturated pressure and the saturated pressure at the superheated temperature) are identical. The dependence of the activity in superheated steam on the equilibrium moisture content was studied in various experiments for different wood materials at constant temperatures of 140 and 160°C. The equilibrium moisture content was found to depend on the following factors: the activity of the superheated steam, the temperature and the materials used, the first of these being the most important. The experimentally determined sorption isotherms were compared with different sorption theories. The Dent model gave a good correlation with the experimental data. The sorbed water can be divided into primary water with high binding energy and secondary water with low binding energy. If biofuels are to be dried with superheated steam, an equilibrium moisture fraction of 0.05 seems to be relevant. It will probably be possible to reach this with activities in the range 0.2–0.4 for all real materials.     

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.     

Effect of Fluidizing Particle on Drying Characteristics of Porous Materials in Superheated Steam Fluidized Bed under Reduced Pressure

The hygroscopic porous particle was used as the fluidizing particle for the superheated steam fluidized bed drying under reduced pressure. A relatively large material was immersed in the fluidized bed as the drying sample. The drying characteristics of the sample were examined experimentally and the results were compared with those in the case of inert particle fluidized bed.

The water transfer from the sample to the fluidizing particle bed in the case of hygroscopic porous particle facilitated the drying regardless of pressure and temperature in the drying chamber. The increment degree of the sample temperature at the earlier period of drying was smaller in the case of hygroscopic porous particle than in the case of inert particle, and the phenomenon was more remarkable in the case of superheated steam than in the case of hot air.     

Effect of Fluidizing Particle on Drying Characteristics of Porous Materials in Superheated Steam Fluidized Bed under Reduced Pressure

The hygroscopic porous particle was used as the fluidizing particle for the superheated steam fluidized bed drying under reduced pressure. A relatively large material was immersed in the fluidized bed as the drying sample. The drying characteristics of the sample were examined experimentally and the results were compared with those in the case of inert particle fluidized bed.

The water transfer from the sample to the fluidizing particle bed in the case of hygroscopic porous particle facilitated the drying regardless of pressure and temperature in the drying chamber. The increment degree of the sample temperature at the earlier period of drying was smaller in the case of hygroscopic porous particle than in the case of inert particle, and the phenomenon was more remarkable in the case of superheated steam than in the case of hot air.     

Drying Kinetics of Instant Asian Noodles Processed in Superheated Steam

The objective for this work was to develop a novel technique for creating instant noodles by determining the drying kinetics of noodles undergoing simultaneous drying and processing using superheated steam. The mathematical model of moisture ratio was differentiated to determine the drying rates of noodles during processing. There was a constant rate drying period for all temperatures at a steam velocity of 1.5 m/s but there was no constant rate drying period at a steam velocity of 0.5 m/s. The constant rate drying period suggested by measurement of internal noodle temperature is much longer and well defined for all processing conditions than from the drying curves. The constant drying rate period, was nearly 200 s at 110°C but decreased to 50 s at 150°C. Equilibrium moisture content isobars were determined from mass changes during superheated steam processing. It was determined that isotherm equations for equilibrium moisture content in hot air systems may be utilized to model isobars in superheated steam systems.     

Effect of Fluidized Bed Drying Temperature on Various Quality Attributes of Paddy

Abstract

As reported by many researchers, it was found that fluidized bed paddy drying using high drying air temperatures of over 100°C affected the head rice yield and whiteness of dried rice. However, only a few studies on fluidized bed paddy drying with drying air temperatures below 100°C were so far reported. The main objective of this work was therefore to study the effect of fluidized bed drying air temperature on various quality parameters of Suphanburi 1 and Pathumthani 1 Indica rice. Paddy was dried from the initial moisture contents of 25.0, 28.8, and 32.5% dry basis to 22.5 ± 1.2% dry basis using inlet drying air temperatures between 40 and 150°C at 10°C/step. After fluidized bed drying, paddy was tempered and followed by ambient air aeration until its final moisture content was reduced to 16.3 ± 0.5% dry basis. The results showed that the head rice yield of Suphanburi 1 was significantly related to the inlet drying temperature and initial moisture content whilst there was no significant relationship between the head rice yield, drying temperature and initial moisture content for Pathumthani 1. The whiteness of the two rice varieties was slightly decreased with increase in drying air temperature and initial moisture content. It was also found that the hardness of both cooked rice varieties exhibited insignificant difference (p < 0.05) comparing to rewetted rice, which was gently dried by ambient air aeration in thin layer. The thermal analysis by DSC also showed that partial gelatinization occurred during drying at higher temperatures. Using inlet drying air temperatures in the range of 40–150°C therefore did not affected the quality of cooked rice and paddy. The milling quality of paddy was also well maintained.     

过热蒸汽间歇干燥酒精糟研究

以高湿物料酒精糟为研究对象,进行了过热蒸汽间歇干燥实验研究。利用自行设计的间歇干燥实验台,酒精糟的初始含水质量分数为66.7%,研究了不同实验条件对干燥速率的影响。结果表明:随着干燥介质温度的升高,干燥速率明显加快;被烘物料质量越少,烘干时间越短;随着物料颗粒直径的减小,恒速段的干燥速率基本不变,而降速段的干燥速率明显增加;过热蒸汽质量流量越大,干燥速率越高。因此随着蒸汽过热度的升高、质量流量的增大,过热蒸汽干燥速率加快,干燥效率越高;随着进料质量的增加、颗粒直径的增大,过热蒸汽干燥速率却有所降低。     

Comparison of Drying Rate Curves of Porous Solids in Superheated Steam to Those in Air

ABSTRACT

Drying of porous solids was experimentally investigated in superheated steam as well as in air. Drying rate curves, including critical moisture contents, in steam at subatmospheric pressure, were compared to those for air at atmospheric pressure; moreover, they were compared to those for steam at atmospheric pressure as well. The former comparison was carried out under conditions of sample temperatures of 41.8–42.5°C (which were nearly equal to saturation temperatures of 42.1–42.2°C at pressures of 8.23–8.30 kPa) for the constant rate period in steam and the corresponding sample temperatures of 42.0–45.0°C (which were close to the wet-bulb temperatures) for the constant rate period in air. There were distinct differences between normalized drying rate curves, including critical moisture contents in steam and in air at the above similar sample temperatures for materials of baked clay, firebrick, and cemented glass balloons over the minimum value of 8.3 × 10^?3 µm and up to the maximum value of 1.2 × 10² µm in cumulative pore-size distributions: longer constant rate periods and lower critical moisture contents in steam than in air, and higher drying rates in steam than in air for the falling rate period. Moreover, the latter comparison of the drying rates in steam at subatmospheric pressure to those in steam at atmospheric pressure revealed that the differences in normalized drying rates between subatmospheric pressure and atmospheric pressure were small for both materials under mild external conditions. These findings were common to the baked clay, firebrick, and cemented glass balloons over a wide range of pore-size distributions studied in the present work, as well as sintered coarse glass beads as previously reported.     

Combined Microwave/Fluidized Bed Drying of Fresh Peppercorns

Abstract

A fluidized bed dryer (FBD) and a combined microwave/fluidized bed dryer (CMFD) are used to dry the fresh ripe peppercorns. The average moisture content vs. elapsed drying time, and drying rate vs. average moisture content are experimentally investigated. It is found that the microwave field from the CMFD can increase the potential of the conventional fluidized bed drying. The drying rates of both dryers are dependent on the inlet air temperature and velocity. For the CMFD, the effects of the air velocity on the drying rate are found to be opposite to our previous results tested with white pepper seeds i.e., the drying rates of the fresh ripe peppercorns decreased with increasing air velocity. By using a CMFD, the drying time required to reach the desired moisture content can be reduced to 80–90% of the drying time required for a FBD at the same drying air temperature and velocity. The color of the product dried by a CMFD is also attractive: it becomes flaming yellow, instead of black as obtained from a FBD. The physical structure of the peppercorn, before and after the drying process is also investigated by a metallurgical macroscope and an image analyzer. Different from drying by a FBD, the external form and matter of the white pepper seed are still maintained, even after passing through the drying process.     

Quality Changes of Superheated Steam–Dried Fibers from Oil Palm Empty Fruit Bunches

Conventional drying of the fibers from oil palm empty fruit bunches (EFB) using flue gas from diesel burners frequently causes browning and dust explosion. Replacing the drying medium with superheated steam is expected to improve the quality of EFB fibers as well as improve the safety of the dryer operation. In this study, the effects of steam temperature and steam velocity on the quality of steam–dried EFB fibers was investigated. The drying experiment was carried out at atmospheric pressure with steam superficial velocity in the range of 0.3 to 0.49 m s^?1 and temperature in the range of 135 to 200°C. Three quality parameters of the EFB fibers, the color, strength, and microstructure, were used to assess the changes in EFB fiber quality as a result of superheated steam drying. The color of the EFB fiber was either improved or not significantly degraded. The strength of the superheated steam–dried EFB fibers was higher than that of undried and hot air–dried EFB fibers. The microstructure of fresh undried EFB fibers as seen by scanning electron microscopy (SEM) showed the presence of round silica particles of 10–20 µm in diameter all over the EFB fiber strand, which complicates pulping and bleaching. Superheated steam drying successfully removed the silica particles from the EFB fibers at temperatures of at least 200°C and a velocity of steam of at most 0.49 m s^?1, which is better than hammering, which can only remove 88% of the silica particles. The high temperature of the superheated steam loosened the silica particles from their craters. The EFB fibers cracked and split at steam velocities at or above 0.49 m s^?1 and high superheated steam temperatures at or above 200°C and as a consequence became weaker at these conditions. The removal of silica particles by superheated steam drying makes the EFB fiber amenable to pulping and bleaching. Superheated steam drying is therefore found to improve the overall quality of EFB fibers compared to hot air drying.     

Effects of High-Temperature Fluidized Bed Drying and Tempering on Kernel Cracking and Milling Quality of Vietnamese Rice Varieties

Studies on the effects of high-temperature fluidized bed drying and tempering on physical properties and milling quality of two long-grain freshly harvested Vietnamese rice varieties, A10 (32±1% wet basis moisture) and OM2717 (24.5±0.5% wet basis moisture), were undertaken. Rice samples were fluidized bed dried at 80 and 90°C for 2.5 and 3.0 min, then tempered at 75 and 86°C for up to 1 h, followed by final drying to below 14% moisture (wet basis) at 35°C by thin-layer drying method. Head rice yield significantly improved with extended tempering time to 40 min. Head rice yield tended to increase with decreasing cracked (fissured) kernels. The hardness and stiffness of sound fluidized bed dried rice kernels (in the range of 30–55 N and 162–168 N/mm, respectively) were higher than that of conventionally dried ones (thin layer dried at 35°C). The color of milled rice was significantly (P < 0.05) affected by high-temperature fluidized bed drying, but the absolute change in the value was very small.     

Combined Microwave/Fluidized Bed Drying of Fresh Peppercorns

A fluidized bed dryer (FBD) and a combined microwave/fluidized bed dryer (CMFD) are used to dry the fresh ripe peppercorns. The average moisture content vs. elapsed drying time, and drying rate vs. average moisture content are experimentally investigated. It is found that the microwave field from the CMFD can increase the potential of the conventional fluidized bed drying. The drying rates of both dryers are dependent on the inlet air temperature and velocity. For the CMFD, the effects of the air velocity on the drying rate are found to be opposite to our previous results tested with white pepper seeds i.e., the drying rates of the fresh ripe peppercorns decreased with increasing air velocity. By using a CMFD, the drying time required to reach the desired moisture content can be reduced to 80-90% of the drying time required for a FBD at the same drying air temperature and velocity. The color of the product dried by a CMFD is also attractive: it becomes flaming yellow, instead of black as obtained from a FBD. The physical structure of the peppercorn, before and after the drying process is also investigated by a metallurgical macroscope and an image analyzer. Different from drying by a FBD, the external form and matter of the white pepper seed are still maintained, even after passing through the drying process.     

Simulation and Validation of Microwave-Assisted Fluidized Bed Drying of Soybeans

A general mathematical model of heat and mass transfer was developed to simulate the microwave-assisted fluidized bed drying of bulk grain. The model was solved using the well-known Runge-Kutta-Gill method. The model is capable of predicting the moisture content of soybean as well as the drying air parameters (i.e., drying air temperature and moisture content) during drying. The values of mean relative deviation (MRD) were less than 8 and 10% for prediction of grain moisture content and outlet air parameters, respectively, which reflects an acceptable accuracy. In comparison with conventional fluidized bed drying of soybean, microwave-assisted fluidized bed drying led to 83.39–98.07% savings in drying time and 82.07–95.22% savings in specific energy consumption when reducing soybean moisture content from 18.32 to 12% (db).     

过热蒸汽流化床流化特性的实验研究

在底部直径为120 mm的锥型流化床中,以玻璃珠为流化颗粒,过热蒸汽为流化介质,研究了固体颗粒在过热蒸汽流化床中的流化特性,考察了操作温度和压力对临界流化速度(umf)的影响.结果表明,过热蒸汽流化床的流化行为与热空气相似,临界流化速度(umf)随床层温度的升高而减小,随床内压力的增大而减小;在相同温度条件下,过热蒸汽流化床的临界流化速度比热空气大.     

DRYING OF SLICED RAW POTATOES IN SUPERHEATED STEAM AND HOT AIR

Drying experiments were conducted on raw potato slices, using atmospheric pressure superheated steam and hot air as drying media at 170 and 240°C. Mass changes of the material were continuously measured, the conditions of cross section near the surfaces were observed with an electron microscope, also color changes of their surface were measured during drying. The respective drying methods and temperature conditions were compared and it was found that, in the case of superheated steam drying, moisture content temporarily increases due to steam condensation in the initial stage of drying, therewith, as well as starch gelatinization rapidly develops. Meanwhile, in case of hot air drying, starch gelatinization occurs more slowly than with superheated steam drying and that non-gelatinized starch granules remain on the surface when drying was completed. Furthermore, surface color measurements showed that samples dried by superheated steam were more reddish than ones dried by hot air and the surfaces were more glossy, because no starch granules remain on the surface in case of superheated steam drying.     

Newly designed superheated steam dryer bearing heat recovery unit: Analysis of energy efficiency and kinetics of Kelp drying

In this work, a newly designed superheated steam dryer (SSD) bearing exhaust heat recovery unit (RD) was fabricated. The dryer was designed for the superheated steam (SS) to be generated by foodstuff being dried, and the latent heat of the exhaust SS was recovered by microtube radiators and exploited to heat the RD unit. The system was then applied in Kelp drying, in which the drying kinetics was analyzed and fitted with mathematical models. The energy efficiency by the system was further evaluated and compared with hot air (HA)-drying. The results showed that the Kelp drying process in SSD could be described by two stages: heat-upstage (stage I) and superheated steam stage (stage II). While in stage I, different heat-up times of 10, 15, and 20?min were required to generate superheated steam at temperatures of 110–150°C, in stage II, the moisture content in Kelp was decreased to approximately 50?±?5% (wet basis) within 60, 50, and 30?min at 110, 130, and 150°C, respectively. Moreover, the Midilli and Kucuk model best described Kelp drying curve in both stages, whereas the logarithmic model best fitted with that in stage II. Finally, the energy efficiency for SSD-RD was in the range of 1.127–1.425?kWh/kg_water compared with 2.406–2.508?kWh/kg_water for HA operating under the same conditions, demonstrating that the SSD-RD was able to reduce the energy input by at least 46.14%.