Hiromichi Shibata

STEAM DRYING TECHNOLOGIES: JAPANESE R&D

ABSTRACT This report reviews some fundamental and practical aspects of steam drying technologies based mainly on studies published in Japan. Steam drying kinetics, and some industrial technologies particularly for drying of foods, textiles and sludges are reviewed with focus on quality of dried material, drying time, dryer selection and energy recovery from steam dryer exhaust. For energy recovery, heat pump technologies are outlined along with characteristics of different types of steam compressors. A new process is proposed for steam drying; it combines a direct-indirect dryer followed by a direct steam dryer for internal moisture removal.

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.

Drying Rate Curves of Porous Solids in Steam and in Air under Low-Pressure Conditions

Drying of porous solids such as sintered glass beads, baked clay, and cemented glass balloons in both steam and air streams was investigated under low-pressure conditions. There was no significant difference between the normalized observed drying rate curves in air at low pressures of 0.71–1.19 kPa and those in steam at low pressures of 0.97–0.99 kPa. However, lower critical moisture contents and higher drying rates in superheated steam at subatmospheric pressures of 8.27–8.33 kPa were observed compared to those in steam at pressures in the range of 0.97–0.99 kPa. Moreover, two models were validated for the prediction of drying rate curves of sintered glass beads at subatmospheric and low pressures. The patterns of the drying rate curves, which depend on the drying medium and its pressure, were common to these materials.

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.

Effect of Segment Scale in a Pore Network of Porous Materials on Drying Periods

Drying periods of porous glass disks of nonhygroscopic MPG glass and hygroscopic Vycor glass were investigated. A simple model of dimensionless segment sizes is proposed where a dimensionless segment size is equal to the value of the tortuosity factor of gas diffusion. This model was applied to the estimation of the segment size and the drying periods of a porous Vycor glass disk. The drying rate curve of the Vycor glass disk showed that the first falling rate period appeared to be included in the constant rate period and that the second falling rate period started right after the constant rate period ended, whereas the drying rate curve of the MPG glass disk estimated from previous models showed that the first falling rate period and the second falling rate period were clearly distinguished. The difference in drying periods between two porous glass disks was caused by a large difference in the respective segment sizes.

Vacuum Drying of Porous Solids under Supercooling

Vacuum drying of sintered glass beads, cemented glass balloons, and baked clay was investigated under supercooling. Glass beads and glass balloons, which were made of a common glass material, were used for comparison. Concerning sintered glass beads, there was no effect of supercooling on drying rates at a pressure of 0.93 kPa. In cemented glass balloons as well as in baked clay, however, a prominent peak of drying rates emerged in drying rate curves and at the same time, a stepwise increase in the surface temperature appeared under supercooling at a pressure of 1.19 kPa; no peak of drying rates emerged in drying rate curves of sintered glass beads. The total amount of latent heat of vaporization and sensible heat generated at the peak of drying rates was 39–59% of the latent heat of solidification. A part of the latent heat of solidification evaporated the water in the sample, producing a peak of drying rates as well as a stepwise increase in the surface temperature of the sample.

SUPERHEATED STEAM DRYING OF SINTERED SPHERES OF GLASS BEADS UNDER VACUUM

ABSTRACT Drying of sintered spheres of coarse glass beads with a wide sintering range in superheated steam under vacuum was studied. In samples with sintered angles of 7.5° –27°, the experimental normalized drying rates in superheated steam at pressures of 7.3-7.9mmHg were smaller than those for 56.0-767.6 mmHg in the vicinity of the critical moisture cotents for 56.0-767.6GmmHg. As reported in an earlier paper, there were

Mechanism of the First Falling Rate Period: An Alternative Approach by Image Analysis

An alternative experimental approach using the image luminosity of a drying surface photographed by a bore scope was conducted in order to elucidate the drying mechanism for the first falling rate period including the critical moisture content. The observed luminosity was in fair agreement with the luminosity predicted from the model of the configuration of water, indicating that the configuration of water plays a critical role for the drying mechanism, especially for the first falling rate period and the critical moisture content. The parameters of the configuration of water estimated in this work could be effective for prediction of drying rates along with other transport properties.

Measurement of Physical Characteristics of Particle Beds for Drying

Among physical characteristics of porous materials, properties related to liquid flow in porous materials play a critical role in the prediction of drying curves. In the present work, a simple experimental method was developed to simultaneously measure the capillary pressure and the relative permeability of particle beds by a simple drainage method. The observed data of permeability at low saturation of less than 70% was in fair agreement with a range of two lines that were predicted from the Campbell model. The purpose of the development of the present drainage method along with the drying method described in a previous paper is to provide a group of physical properties which are needed for prediction of drying curves by use of minimal measurement devices.