ANALYSIS OF TRANSPORT PHENOMENA DURING THE CONVECTIVE DRYING IN SUPERHEATED STEAM

Abstract

This work focused on high-temperature convective drying (superheated steam drying). The process has been investigated both experimentally and numerically. The experimental analysis was carried out in an aerodynamic return-flow wind-tunnel, with very small cylinders of cellular concrete. For the local analysis, the samples were fitted with thermocouples and pressure sensors. The mean moisture content of the cylinders was measured by simple weighing while the temperature and pressure readings were being taken. Global and. local analysis of heat and mass transfer in small cylinders in superheated steam were carried out. The systematical study for several sizes and aerothermal conditions show a similar behavior for moisture content, pressure and temperature values. A numerical model for high temperature drying, using the finite elements method, in a 2-D configuration, was implemented and validated.     

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.     

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 Coriander Seed Particles in a Reduced Pressure Superheated Steam Fluidized Bed

Drying characteristics of coriander seed particles were experimentally analyzed in a reduced pressure superheated steam fluidized bed. The typical moisture gain, reported in some other studies during the warm-up period of the process, was reduced in most of the cases by supplying additional heat into the column. The experimental results demonstrated that the drying rate increases and the equilibrium moisture content decreases by increasing the operating temperature. However, variation of the operating pressure (40–67 kPa) and the superficial steam velocity (2.3–4.0 m/s) did not present significant effects on the moisture contents. The degree of superheating was found to be the most important parameter for the process. The experiments also showed that the equilibrium moisture content decreases upon increasing the degree of superheating. Finally, employing a reduced pressure superheated steam fluidized bed appears as an option to carry out drying processes at relatively lower temperatures.     

Controllability of Product Moisture Content When Nonscreened Sawdust Is Dried in a Spouted Bed

The control of the dried product's moisture content is of importance in the production of fuel pellets. Tests have been made in air and superheated steam in a spouted bed using the outlet temperature of the drying medium as a control parameter of the dried material's moisture content. For superheated steam different settings for the inlet medium temperature are also used. In the moisture content interval of 8-17% wet base there is a linear correlation between the dried material moisture content and the outlet temperature of the drying medium. When drying nonscreened sawdust in a spouted bed it is recommendable to use the outlet temperature of the drying medium as a control parameter of the dried material moisture content. This is independent of the size of the sawdust, whether superheated steam or air is used as a drying medium.     

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.     

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.     

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.     

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.     

SUPERHEATED STEAM IMPINGEMENT DRYING OF TORTILLA CHIPS

ABSTRACT

Low-fat snack products are the driving forces for the drying of tortilla chips before frying. Super-heated steam impingement drying of foods has the advantage of improved energy efficiency and product quality. The temperature profile, drying curves, and the physical properties (shrinkage, crispiness, starch gelatinization and microstructure) of tortilla chips dried at different superheated steam temperatures and heat transfer coefficients were measured. Results indicated that the steam temperature had a greater effect on the drying curve than the heat transfer coefficient within the range of study. The microstructure of the samples after steam drying showed that higher steam temperature resulted in more pores and coarser appearance. The modulus of deformation and the shrinkage of tortilla chips correlated with moisture content. A higher steam temperature caused less shrinkage and a higher modulus of deformation. The pasting properties showed that samples dried under a higher steam temperature and a higher heat transfer coefficient gelatinized less during drying and had a higher ability to absorb water. Comparison of the superheated steam drying and air drying revealed that at elevated temperatures the superheated steam provided higher drying rates. Furthermore, there was a less starch gelatinization associated with air drying compared to superheated steam drying.     

SUPERHEATED STEAM IMPINGEMENT DRYING OF TORTILLA CHIPS

Low-fat snack products are the driving forces for the drying of tortilla chips before frying. Super-heated steam impingement drying of foods has the advantage of improved energy efficiency and product quality. The temperature profile, drying curves, and the physical properties (shrinkage, crispiness, starch gelatinization and microstructure) of tortilla chips dried at different superheated steam temperatures and heat transfer coefficients were measured. Results indicated that the steam temperature had a greater effect on the drying curve than the heat transfer coefficient within the range of study. The microstructure of the samples after steam drying showed that higher steam temperature resulted in more pores and coarser appearance. The modulus of deformation and the shrinkage of tortilla chips correlated with moisture content. A higher steam temperature caused less shrinkage and a higher modulus of deformation. The pasting properties showed that samples dried under a higher steam temperature and a higher heat transfer coefficient gelatinized less during drying and had a higher ability to absorb water. Comparison of the superheated steam drying and air drying revealed that at elevated temperatures the superheated steam provided higher drying rates. Furthermore, there was a less starch gelatinization associated with air drying compared to superheated steam drying.     

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.     

FLASH DRYING OF FISH MEALS WITH SUPERHEATED STEAM: ISOTHERMAL PROCESS

The work considers the application of the flash drying to the moisture removal of fishmeal using superheated steam as transport medium. Heat, momentum and mass transfer equations were applied and an algorithm based on these equations was developed and solved. The model was validated using experimental data obtained in a pilot pneumatic dryer ( total length 60 m) provided with a steam jacket to maintain the superheated steam at a constant temperature. The drying time was less than 10 s to decrease the fish meals moisture content from 53.5% to 28% d.b. and in a second pass by the dryer the moisture down to 16.9% using superheated steam at It 1 ° C and 130 ° C in the jacket. The computational results are in agreement with the experimental data.     

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.     

FLASH DRYING OF FISH MEALS WITH SUPERHEATED STEAM: ISOTHERMAL PROCESS

ABSTRACT

The work considers the application of the flash drying to the moisture removal of fishmeal using superheated steam as transport medium. Heat, momentum and mass transfer equations were applied and an algorithm based on these equations was developed and solved. The model was validated using experimental data obtained in a pilot pneumatic dryer ( total length 60 m) provided with a steam jacket to maintain the superheated steam at a constant temperature. The drying time was less than 10 s to decrease the fish meals moisture content from 53.5% to 28% d.b. and in a second pass by the dryer the moisture down to 16.9% using superheated steam at It 1 ° C and 130 ° C in the jacket. The computational results are in agreement with the experimental data.     

Drying with Internal Vaporisation : Introducing the Concept of Identity Drying Card (IDC)

ABSTRACT

Different drying configurations (convective drying with moist air and superheated steam, microwave drying and vacuum drying) on different materials (isotropic and anisotropic) were experimentally studied in order to model and visualise the evolution of internal pressure and temperature. To be able to do so, in addition to measuring the average moisture, a method which can determine internal-local pressure and temperature simultanously by using specially designed sensors was developed. In combination with the experiments, the numerical code TRANSPORE has been used to simulate drying processes. A less comprehensive but more comprehensible analytical model was also provided to facilitate the better understanding of internal phenomena. Based on the results gained from measurement and numerical analysis, the dynamic distribution and development of local temperature and pressure inside seasoned medium are coupled together by a temperature-pressure graph, which is herewith called “Identity Drying Card” (IDC), a new concept initialid in the paper. By using IDC, the internal profile of temperature and pressure, the dominant transport properties (penncability and difisivity), the mechanism of transport (diffusion, convection or both) and the phase transitions during drying can be visualised. More specifically, the amount of dry air, the moisture content in the hygroscopic rcgion or the danger due to internal mechanical loads of handled materials can be figured out with the aid of IDC.     

Drying with Internal Vaporisation : Introducing the Concept of Identity Drying Card (IDC)

Different drying configurations (convective drying with moist air and superheated steam, microwave drying and vacuum drying) on different materials (isotropic and anisotropic) were experimentally studied in order to model and visualise the evolution of internal pressure and temperature. To be able to do so, in addition to measuring the average moisture, a method which can determine internal-local pressure and temperature simultanously by using specially designed sensors was developed. In combination with the experiments, the numerical code TRANSPORE has been used to simulate drying processes. A less comprehensive but more comprehensible analytical model was also provided to facilitate the better understanding of internal phenomena. Based on the results gained from measurement and numerical analysis, the dynamic distribution and development of local temperature and pressure inside seasoned medium are coupled together by a temperature-pressure graph, which is herewith called “Identity Drying Card” (IDC), a new concept initialid in the paper. By using IDC, the internal profile of temperature and pressure, the dominant transport properties (penncability and difisivity), the mechanism of transport (diffusion, convection or both) and the phase transitions during drying can be visualised. More specifically, the amount of dry air, the moisture content in the hygroscopic rcgion or the danger due to internal mechanical loads of handled materials can be figured out with the aid of IDC.     

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   

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。     

MEASURED GAS VELOCITY AND MOISTURE CONTENT DISTRIBUTION IN A CONVECTIVE VACUUM KILN

Transferring the necessary heat of evaporation to the stack is the bottleneck in convective vacuum drying of wood. Higher gas velocities are applied to compensate for the lower gas density and to obtain similar heat and mass transfer characteristics as under normal pressure. Like in conventional kiln drying the region with the most unfavorable drying conditions determines drying time and product quality. To use the full potential of the meanwhile established superheated steam vacuum drying technology, it is therefore necessary to work on an improved uniformity of process conditions in the kiln.

To evaluate the fluid dynamics and its influence on the final moisture content, experimenls in a laboratory convective vacuum kiln were carried out. For different total pressures the profiles of dynamic pressure in the stack entry section were measured in a dry atmosphere. At normal pressure the profiles were determined between the board layers throughout the whole stack. For the same slack configuration vacuum drying tests were used to assess the impact of the velocity distribution in the slack on the final moisture content distribution-Regions of low gas velocities coincided well with regions of high final moisture content.