DRYING WITH SUPERHEATED STEAM

The principle of drying with superheated steam is known for a long time already, and different designs have been worked out and partly realised. The principle of the steam drying described in this paper starts with superheated steam that is blown on to the products to be dried. The superheated steam transfers its sensible heat to the product and the water to be evaporated. The superheated steam acts both as heat source and as drying medium to take away the evaporated water. The majority of the superheated steam has to be recirculated and reheated. The excess steam from this process, corresponding to the evaporated water, can be used elsewhere in the process or plant, thus making an efficient energy recovery possible. Possible emissions from the drying process can be effectively removed by condensation. Drying by means of superheated steam impingement is one of the possible designs that are in progress now for the paper industry. Work is going on with respect to this type of drying for other materials, especially foodstuffs. At present experimental research at the laboratory steam dryer of TNO-MEP is carried out for vegetables, fries, herbs, cacao nuts, wheat, flour, etc. Besides the energy savings and environmental benefits, the oxygen free steam atmosphere and the higher product temperatures in the dryer appear to have positive effects on the product qualities. For foodstuffs combinations of steam drying with blanching, pasteurisation, sterilisation, etc. are possible and make the steam drying very attractive for food producers. Modelling of the processes in foodstuffs during steam drying is established, both on macro and micro scale. In this paper examples and results of research on steam drying for specific products will be presented.     

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.     

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.     

AN EXPERIMENTAL STUDY OF THE FALLING RATE PERIOD OF SUPERHEATED STEAM IMPINGEMENT DRYING

ABSTRACT

The drying of paper under impinging jets of superheated steam and air during the falling rate period was investigated in the range of jet temperatures 150 < Tj < 450^oC and basis weights 30 < B < 150 g/m². The equilibrium moisture content of Kraft and TMP paper was measured. The adsorption energy of water on pulp fibers near the boiling point appears lower than the value extrapolated from Prahl s (1968) measurements made in air at lower temperatures. The critical moisture content was determined for superheated steam and air impingement drying. Complete drying rate - moisture content histories are presented for a series of typical conditions.     

DRYING PAPER IN SUPERHEATED STEAM

The industrial potential for drying paper in superheated steam has been investigated in a series of studies designed to treat paper properties - drying process relations as well as process engineering aspects. Properties measurements show that for paper made from mechanical pulps, drying in superheated steam produces a better bonded sheet which is thereby stronger and has a lower scattering coefficient. Surface properties of such steam dried paper are improved, including reduced tinting. Desorption equilibrium shows that completely dry paper can be obtained at very low superheats. Impingement drying rates can be about twice as high for drying in superheated steam as in air. A hybrid cycle was developed in which drying is partially by superheated steam impingement drying and partially by conventional cylinder drying. Preliminary design and techno-economic evaluation indicates that three factors, paper properties, dryer size and energy efficiency, in various combinations specific to each case, may provide industrial potential for this new technology for drying paper.     

DRYING PAPER IN SUPERHEATED STEAM

ABSTRACT

The industrial potential for drying paper in superheated steam has been investigated in a series of studies designed to treat paper properties - drying process relations as well as process engineering aspects. Properties measurements show that for paper made from mechanical pulps, drying in superheated steam produces a better bonded sheet which is thereby stronger and has a lower scattering coefficient. Surface properties of such steam dried paper are improved, including reduced tinting. Desorption equilibrium shows that completely dry paper can be obtained at very low superheats. Impingement drying rates can be about twice as high for drying in superheated steam as in air. A hybrid cycle was developed in which drying is partially by superheated steam impingement drying and partially by conventional cylinder drying. Preliminary design and techno-economic evaluation indicates that three factors, paper properties, dryer size and energy efficiency, in various combinations specific to each case, may provide industrial potential for this new technology for drying paper.     

STUDIES OF STEAM DRYING IN A FLUIDIZED BED

In general, drying processes are described by the quantity of air humidity of the exiting gases. This approach is not possible however by the drying medium of water in steam drying, since the air humidity naturally possesses a constant value of 100%. This paper presents a model which represents the drying processes on the basis of the observation of temperature profiles of the material and energetic balancing of all components involved as well as the wall of the apparatus. The modeling differentiates three intervals: the condensation phase, the 1st drying period and the 2nd drying period. In addition, a validation of the model on the basis of experiments in an experimental plant DN100 belonging to the university is dealt with. The satisfying concurrence of the theoretical and practical results shows that, with the help of the theoretical model, discontinuous steam drying processes can be theoretically described with sufficient accuracy.     

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 OF SUGAR BEET FIBER WITH HOT AIR OR SUPERHEATED STEAM

A comparative study of drying thin layers of sugar beet fiber with hot air or with superheated steam in a specially designed pilot dryer is reported. Our present interests are focused on drying rate and on the quality of the dried product. Steam superheated at 130-150°C yields 90 % of dry matter (DM), however steam condensation on the cold product at the beginning of the process may penalize the drying time. The color of the fiber is not modified until reaching 80 % DM. Drying with air heated at temperatures lying in the 40 to 105°C range does not alter the white color up to 90 % DM. The water retention capacity of the original fiber, 14 g water/g DM remains unchanged whatever drying agent is used. The scaling up of the primary hot air drying set up to a ten times larger experimental dryer introduce no modification of drying time, water retention capacity or fiber color. Thick layer drying experiments permit to define parameters of a belt dryer for an industrial fiber processing plant.     

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.     

SUPERHEATED STEAM DRYING OF SINTERED SPHERES OF GLASS BEADS UNDER VACUUM

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     

DRYING OF GRANULAR PARTICLES IN A MULTISTAGE INCLINED FLUIDIZED BED WITH MECHANICAL VIBRATION

A mathematical model for the drying rate of granular particles in a multistage inclined fluidized bed(IFB) is presented from the standpoint of simultaneous heat and mass transfer, with taking the effect of mechanical vibration added vertically into consideration.

Steady-state distributions for the temperatures and concentrations of the particles and the heating gas, and for the moisture content of the particles are numerically calculated based on the present model. The calculated results show fairly good agreement with the experimental data, which were obtained from the drying experiments of brick particles in a three-stage IFB using comparatively low temperature air(40-60°C) as the heating gas.

It has been found within the range of the experimental conditions employed that, the mechanical vibration added vertically enhances the over-all drying rate of the particles and its effect can be considered equivalent to an increase in the air velocity.     

EQUILIBRIUM BEHAVIOR OF KRAFT BLACK LIQUOR IN SUPERHEATED STEAM

The boiling point rise (BPR) of black liquor with solids content of 85 to 98% was measured in superheated steam. The BPR for 85 and 95% solids content liquor is 28 and 52°C, respectively. A surprisingly large amount of liquor swelling occurs in this environment. A swelling mechanism is proposed.     

EQUILIBRIUM BEHAVIOR OF KRAFT BLACK LIQUOR IN SUPERHEATED STEAM

The boiling point rise (BPR) of black liquor with solids content of 85 to 98% was measured in superheated steam. The BPR for 85 and 95% solids content liquor is 28 and 52°C, respectively. A surprisingly large amount of liquor swelling occurs in this environment. A swelling mechanism is proposed.     

射流循环管流化床的适宜循环条件

研究了射流循环管流化床的几何结构和操作条件对固体循环速率的影响。结果表明，循环管中最小气速与循环管面积无关，约为０．３６－０．５Ｕｔ；给出确定最小射流喷嘴气量的定量方程以及沉降室的充气范围，进一步提出了适用于计算射流循环管流化床中沉降室节通涌时的充气气速修正关联式，为工程设计与操作提供了重要依据。     

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.