EFFECTS OF OPERATIONAL CONDITIONS ON DRYING CHARACTERISTICS IN CLOSED SUPERHEATED STEAM DRYING

The effects of operational conditions on the drying performance in closed superheated steam drying were examined theoretically and experimentally. The vapor generated from the sample was circulated in the drying chamber. In the theoretical analysis, the replacement of air with vapor in drying chamber and the convective vapor transfer in sample were considered. At the start of drying, the drying chamber was filled with air. As the drying proceeded, the air was replaced with the vapor generated from sample. The calculated results explained the characteristics of experimental data. The pore diameter of sample had little effect on the drying characteristics. During the internal evaporation period, the evaporation occurred in the narrow zone, which moved from the surface to the bottom of sample. The convective vapor transfer in sample had a significant influence on the drying performance. The excess increments in temperature and velocity of drying gas hardly contributed to shortening the drying time.     

SUPERHEATED STEAM DRYING: A BIBLIOGRAPHY

An experimental setup was developed to study the through air–drying characteristics of permeable grades such as tissue and towel under commercially relevant conditions of basis weight, airflow rate, temperature, and humidity conditions. The experimental setup is capable of evaluating the transient fluid flow, heat, and mass transfer characteristics of relatively larger samples (TAPPI standard hand sheets; 0.1524 m) and is capable of studying the effect of local heterogeneity and structure on convective heat and mass transfer. The system is capable of airflow rates of 0.5–10 m/s with corresponding high-speed data collection and acquisition for measuring important variables such as exhaust air humidity. To study the effect of nonuniformity, local temperature and velocity profiles can also be measured using grid of thermocouples and hot wire anemometers. The instantaneous drying rate and airflow characteristics during through air drying was measured and dry permeability, wet permeability, and convective heat and mass transfer characteristics were then calculated. The experimental results were verified by comparing with the results from literature. Typical experimental results were presented to show the effect of sheet basis weight, initial moisture content, and airflow rates on the drying characteristics for two different types of paper samples.     

EFFECT OF FLUIDIZING PARTICLE ON DRYING CHARACTERISTICS OF POROUS MATERIAL IN FLUIDIZED BED

The drying characteristics of porous material in fluidized bed were examined theoretically and experimentally. The brick ball was used as the sample and immersed in the fluidized bed. The glass beads were used as the fluidizing particles and the particle diameters were changed. When the pore diameter of sample was relatively large, the fluidizing particles were adhered on the sample surface. In the theoretical analysis, the heat and mass transfers in adhered particle layer were considered. The fluidizing particles were adhered on the sample surface during the earlier period of drying. The sample temperature largely decreased when the mass of adhered particle decreased. The calculated results are in good agreement with experimental data. The diameter of fluidizing particle had a small effect on the drying time. The excess increments in drying gas temperature hardly contributed to shortening the drying time.     

OPTIMUM OPERATING CONDITIONS IN DRYING FOODSTUFFS WITH SUPERHEATED STEAM

It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pressure at which the initial drying rate is a maximum. Finally, the initial drying rate and optimum condition equations are applied to three model dryers, a dryer for a flat sheet, a fixed bed dryer and a rotary dryer. In each case, numeric values are computed and plotted as drying rate versus pressure curves, in which the optimum drying rate is also included. Also presented is a chart to compare the optimum pressures as functions of temperature and steam velocity for the three dryers.     

EXPERIMENTAL INVESTIGATIONS OF BEET PULP DRYING IN SUPERHEATED STEAM UNDER PRESSURE

ABSTRACT

Beet pulp drying in superheated steam under pressure makes it possible to save energy in sugar factories. A new concept of a two-stage convective steam drier is presented. To obtain kinetic data on beet pulp drying, an experimental setup was built. Beet pulp samples were dried at steam pressure up to 4 bar and temperature up to 220° C.     

DRYING OF SAWDUST IN AN ATMOSPHERIC PRESSURE SPOUTED BED STEAM DRYER

The results presented in this paper show the possibility to control the outgoing moisture content using the exhaust temperature as a control parameter. A model for heat exchange is used to support visual observations of the spouting quality in the bed. The model is also used in a discussion about the heat and mass transfer coefficients. Two efficiency concepts are defined and experimental results presented. The temperature efficiency is used to present the possibility to integrate a steam dryer into energy systems such as boilers and heat sinks. Energy efficiency is used to show the variation of the recoverable heat and to point out suitable outgoing moisture contents for steam dryers. It is important to keep the temperature after the dryer as low as possible in order to achieve a high temperature efficiency. It is also shown that energy efficiency improves with decreasing outgoing moisture content and increasing inlet steam temperature.     

裂解炉过热蒸汽温度控制的影响因素和控制算法的探讨

主要阐述了裂解炉过热蒸汽温度控制的影响因素和控制方案，并对蒸汽温度约束条件的非线性化问题解决方案做了初步探讨。     

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.     

DESORPTION ISOTHERMS AND DRYING CHARACTERISTICS OF SHRIMP IN SUPERHEATED STEAM AND HOT AIR

Desorption isotherms for shrimp were determined at the temperatures of 50, 60, 70 and 80°C. Amongst the moisture equilibrium predictions between the BET and GAB models, the latter has a better predictable capability. The GAB parameters are correlated with the temperatures by the Arrhenius expression. Drying characteristics of shrimp in drying media at the temperature range of 120-180°C for superheated steam and of 70-140°C for hot air have been examined. Drying rate and effective diffusion coefficient are used to quantify quantitatively the difference between the superheated steam and the hot air dryings. The temperature is more important effect on drying rate and effective diffusion coefficient in the superheated steam than in the hot air. Inversion temperature exists between 140 and 150°C. Comparing to the hot air, the shrimp dried by the superheated steam shows a lower degree of shrimp shrinkage. In addition, product colours are slightly different to those from the commercial sources.     

THE INFLUENCE OF CONSTANT AND VARIABLE CONDITIONS ON THE DRYING KINETICS OF APPLES

Abstract

The purpose of this work was to examine the influence of constant and variable temperatures or velocities on apples' drying kinetics. The step changes of temperature influenced forcefully on apples1 temperature in all situations. The drying curves -6f apples showed the influence of air's temperature as well velocity on the water content. The largest difference between the apple drying curves and absolute value of drying rate are obtained after introducing step changes or rectangle impulses of air's temperature or velocity at the beginning of the process.     

INFLUENCE OF PROCESS CONDITIONS ON THE DRYING RATE OF CASSAVA CHIPS IN A SOLAR SIMULATOR

ABSTRACT

Experiments were carried out in a solar simulator to study the influence of air temperature (25-40°C), air relative humidity (40-80%), air velocity (0.95-2.2 m/s), radiation intensity (0-916 W/m²), and loading density (10-30 kg/m²) on the drying rate of a bed of cassava chips (2×2×2 cm). Well-known thin-layer drying equations were fitted to the experimental data, and the empirical constants were used in a statistical analysis of the influence of process conditions on the drying rate. The air temperature, air velocity, radiation intensity, and loading density influenced the drying rate significantly (p=0.05). The effects of the air temperature and the radiation intensity were attributed to the temperature-dependent diffusion of moisture within the chips, while the effect of the air velocity was ascribed to the resistance to mass transfer at the air-chip interface. Equations were presented to express the empirical constants as functions of the process variables.