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.     

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.     

STUDIES ON STIFLING AND DRYING OF COCOONS FOR LONGER STORAGE

ABSTRACT

Long term storage of stifled cocoons is an important problem during unfavourable seasonal conditions. Cocoons could be stifled within 20 minutes by using steam al 90°C. The shade drying of steam stifled cocoons in thin layers reduced the moisture content from 228 per cent (db) to 3 1.7 per cent (db) within 10 days. By hot air conditioning method the cocoons could be stifled and dried simultaneously. Using hot air at 90°C the cocoons could be stifled within 10 minutes, and dried to the equilibrium moisture content within 2 hours. The cocoons stifled by the hot air conditioning method could be reeled even just after stifling.     

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.     

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 Regeneration of a Desiccant Wheel—Experimental Results and Comparison with Air Regeneration

A commercial zeolite desiccant wheel is tested with atmospheric pressure superheated steam regeneration over a range of air inlet conditions, steam inlet temperatures, and wheel rotation speeds. Results are compared with those from high-temperature air regeneration experiments on the same wheel obtained from the literature. For both cases the air stream to be dried was relatively hot and moist with inlet temperature and absolute humidity values of 50°C and 25 g · kg^?1 chosen to reduce heat carryover. Using steam at 160°C to regenerate the wheel leads to the same dehumidification as using hot air at approximately 90°C. The benefit of superheated steam drying is that a nearly closed-loop regeneration process can be used with potential energy savings on the order of 30%.     

Drying Kinetics and Quality of Shrimp Undergoing Different Two-Stage Drying Processes

Abstract

An innovative two-stage drying concept is presented in this article. The work considered drying of shrimp using a superheated steam dryer followed by a heat pump (SSD/HPD) or a hot air dryer (SSD/AD) both from drying kinetics and dried product quality points of view. The experiments were performed using the first-stage superheated steam drying temperature of 140°C while the second-stage heat pump drying (or hot air drying) was performed at 50°C. The moisture content of shrimp at the end of the superheated steam drying stage was varied between 30 and 40% (w.b.). The effect of tempering between SSD/HPD was also investigated. Shrinkage, color, rehydration behavior, texture (toughness and hardness), and microstructure of dried shrimp were measured. The results showed that SSD/HPD dried shrimp had much lower degree of shrinkage, higher degree of rehydration, better color, less tough and softer, and more porous than single-stage SSD dried shrimp. It was also found that SSD/AD gave redder shrimp compared to shrimp dried in a single-stage superheated steam dryer. No improvement in terms of shrinkage and rehydration behavior was observed, however.     

Experimental Investigation and Practical Application of Superheated Steam Drying Technology for Softwood Timber

Abstract

Due to several advantages, superheated steam drying of timber has attracted great attention. However, the technology is still restricted to some special cases, partly due to the lack of fully understanding of the drying process. In this work, experiments were conducted to dry radiata pine timber using superheated steam under vacuum and at pressure. In the first part of the experiments, softwood timber was dried in a superheated steam kiln with drying rates, steam temperature across the stack and wood temperature being measured during drying. In the second part of the work, experimental studies were performed to investigate potential applications of the superheated steam drying at ultra-high temperatures (UHT) and pressurized steam conditioning of kiln dried timber. Compared to normal drying temperatures, the UHT drying can reduce the drying time by a factor of 5 to 10 and it is also more energy efficient. The pressurized steam conditioning has been proven to be a promising technology to relieve drying stresses and to reduce twist of the dried timer.

     

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.     

Some Considerations In Simulation Of Superheated Steam Drying Of Softwood Lumber

ABSTRACT

A mathematical model for high-temperature drying of softwood lumber with moist air has been modified and extended to simulate wood drying with superheated steam. In the simulation, differences between the two types of drying are considered, these include: external heat and mass transfer processes and calculation of equilibrium moisture content. The external mass transfer coefficient in the perheated steam drying was found to be much higher than that in the moist air drying, however, the heat ransfer coefficients for these two cases were of the same order. The predicted drying curves and wood temperatures from the superheated steam drying model were compared with experimental data and there was close agreement. Further studies will apply the model to development of commercial drying schedules for wood drying with superheated steam.     

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.     

A PRELIMINARY STUDY OF STEAM DRYING OF SILKWORM COCOONS

ABSTRACT

An experimental study was conducted to explore the feasibility of silkworm cocoon drying with superheated steam. The influence of steam drying on the drying kinetics and the technological characteristics of the dried cocoons is discussed. A promising new technology of cocoon drying is suggested to improve cocoon quality and decrease raw cocoon consumption in silk production.     

THE INFLUENCE OF DRYING MEDIUM,TEMPERATURE, AND TIME ON THE RELEASE OF MONOTERPENES DURING CONVECTIVE DRYING OF WOOD CHIPS

ABSTRACT

The release of volatile organic components (VOC) during the drying of wood chips was studied experimentally. The drying medium was hot dry air with a pressure of 1 bar and velocity of 1 m/s. Four different temperature levels of the drying medium, i.e., 120, 140, 160 and 180°C, and two different materials, Scots Pine and Norway Spruce, were investigated. It was found that the main components released consist of various types of monoterpenes, with α-pinene dominating in each of the two materials. The amount released is time-dependent, showing a maximum at shorter times. The release rate is more intensive at higher drying temperatures. The mass-balance for α-pinene was checked by comparing the amount present in the wood chip before and after drying with the amount obtained by integrating the release curve. A good agreement was obtained. Environmental Scanning Electron Microscope pictures were taken of both the spruce and pine chip in order to investigate differences in the wood structure before and after drying with air at 160°C. No obvious structural change could, however, be observed. The influence of the drying medium on the release rate of monoterpenes (air and superheated steam) was also studied. The results showed that the release rate was faster when drying with superheated steam.     

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.     

SOLAR DRYING OF RAISING

The drying of Sultana seedless grapes was investigated under intermittent and continuous operating conditions in a laboratory solar installation involving a thermal storage bed and an auxiliary heater. The effect of pretreatment, and of the a i r velocity on the drying rate of the grapes at constant temperature was also studied in relation t o the quality of the dried product.

Solar drying of the grapes was accomplished in 30.5 to 60.5 h of intermittent operation, or 19 to 60 h of continuous drying, involving the thermal storage bed and the auxiliary heater. The shortest drying time (19 h ) and the highest quality dried product were obtained with grapes dipped in a hot (80 C ) solution of sodium hydroxide and ethyl oleate, which were dried continuously at 42°C and 2m/s, air temperature and velocity respectively.

The mean apparent diffusivity of moisture in raisins at 6o°C and air velocity 2m/s was estimated as 1.0.10-10 m2/s.     

Energy Analysis on Use of Air and Superheated Steam as Drying Media

ABSTRACT

The physical properties of air and superheated steam were analysed in a range of temperature applied in paper and paperboard drying processes. On the basis of tests carried out on a pilot stand the values of energy indices for air and steam drying processes are compared. With the drying media temperature as T_m = 300°C, nozzle velocity v= 60m/s and using the Huang and Mujumdar model as well as relationships given by Chance a compartive analysis of the results has been carried out Variation of several indices in the range of temperatures 100-600°C and various nozzle velocities was studied.     

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.     

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.     

HIGH TEMPERATURE DRYING OF WOOD SEMI-INDUSTRIAL KILN EXPERIMENTS

We briefly introduce the main results obtained in our laboratory about high temperature drying kinetic of softwood and hardwood. We point out during drying the acceleration effect of pressure gradient in vapour phase which develops in the wood.

Then we give the main experimental results carried out on a small industrial kiln which can dry wood boards with moist air and superheated steam at high temperature till 180°C and velocity above to 6 m/s. The drying process is completely automated. Tested species are : beech, poplar, fir and maritime pine.

We show that the drying kinetics are almost the same using the industrial kiln or the wind tunnel and we define for different board thicknesses, temperatures and velocities the optimal sequences of the process. The influence on the quality of the pretreatment, the various drying periods and the post-steaming is of importance.     

RELATIONSHIP OF PROCESS PARAMETERS AND STACK WIDTH IN TIMBER DRYING

ABSTRACT

Drying schedules applied in kiln drying, especially for hardwood species should be supplemented with values of air velocity. When dryers have no air velocity control their drying schedule can be suitably corrected. The correction should take into account factors related to dried material, i.e. wood moisture content, timber thickness, dried timber volume and primary width of a stack. Drying efficiency may be the criterion of modifications. The applied procedure of drying efficiency calculations lets to determine quantitative relationships between process and material factors.