Superheated Steam Vacuum Drying of Rubberwood

The effect of superheated steam vacuum drying (SSVD) on the drying time and mechanical properties of rubberwood was studied. Rubberwood boards with dimensions of 1000 mm × 76.2 mm × 25.4 mm were dried at 86.7–89.3 kPa vacuum pressure (14.6–12.0 kPa absolute) and temperatures of 60, 70, and 80°C. Superheated steam at 110°C was injected intermittently to relieve stress buildup in wood and eliminate cracking. The prong test was used to evaluate the initial acceptability of the dried wood and the mechanical properties of wood were measured. From this study, the total drying time was reduced from 168 h to less than 20 h (MC reduction from 0.80 to 0.06 db). In addition, compared to the reference values shown in the parentheses, the shear parallel-to-grain, the compression parallel-to-grain, the compression perpendicular-to-grain, the modulus of rupture (MOR), the modulus of elasticity (MOE), and the hardness for the optimum drying temperature of 70°C were 28.87 (11.0) MPa, 59.09 (32.0) MPa, 21.09 (5.0) MPa, 101.97 (66.0) MPa, 9838.5 (9240.0) MPa, and 6475 (4350) N, respectively. Thus, the vacuum-dried wood showed a 32% increase in hardness, a 12% increase in compression parallel-to-grain, and an 88% increase in shear parallel-to-grain.     

Evaluating the Use of Humidification Systems During Heat Treatment of MPB Lumber

Lumber produced from lodgepole pine logs attacked by the mountain pine beetle (MPB) infestation in British Columbia, Canada, exhibits very low initial moisture content (MC). Depending on the time elapsed since attack, the initial MC can be significantly lower than the fiber saturation point (FSP ～30%). Lumber exhibiting 19% MC or less is considered ready for the dimension lumber market and does not necessarily need to be kiln dried. However, phytosanitary regulations require that lumber products be heat treated before delivery to customers. For lumber already quite dry, the Canadian Food Inspection Agency (CFIA) kiln drying/heat treatment schedules may be too long and can result in overdrying. For the significant volume of MPB lumber with an initial MC below 19%, the use of low-pressure steam or cold water spray could allow this lumber to be heat treated without causing further drying. This approach should result in improved lumber quality because overdrying would certainly be minimized and probably eliminated.

In this study, two sorts (dry and wet sort) of MPB lumber were dried in a pilot laboratory kiln humidified with low-pressure steam or cold water spray. Twelve drying runs were carried out. The experimental results indicated that the times to reach the temperature of 56°C in the core of the lumber were shortened and warp for the dry-sort lumber group was reduced when the low-pressure steam or cold water spray system was used. Thus, for MPB lumber with a low initial MC, the utilization of lower pressure steam or cold water spray during heat treatment represents an attractive alternative to reduce kiln residence time, minimize or eliminate overdrying, and improve lumber quality.     

Establishing Kiln Drying Schedule for Beech (Fagus orientalis) at 5-cm Thickness from the Neka Region

To establish a kiln drying schedule for beech (Fagus orientalis) lumber, 5-cm-thick boards were kiln dried down to a final moisture content of 8%. Three replications were made utilizing three kiln schedules of T5-C3, T5-C4, and T6-C4. With due attention to the effect of thickness on wood drying intensity, the t-test showed no significant difference between the thicknesses of the three drying schedules at a significance level of 99%. Therefore, the results of this study can be applied for 5-cm-thick boards.

The primary dry bulb temperature in each of the three schedules was adjusted to 41°C and the final dry bulb temperatures were adjusted to 71, 71, and 82°C, respectively. The schedule offering the shortest drying time for the desired quality was chosen. Specific gravity and dry specific gravity were measured as 0.52 and 0.61, respectively. Longitudinal, radial, tangential, and volumetric shrinkage were 0.46, 5.8, 10.2, 16.48%, respectively. The extent of defects including crook, bow, twist, and three longest surface checks of the lumber was determined for each drying schedule. Quality control graphs were used to analyze the lumber defects in order to determine the best drying schedule.

Analysis of the results indicates that with either of three kiln schedules the extent of defects before and after drying was not statistically different. However, the distribution of defects in the third schedule (T6-C4) was more uniform with respect to the average line compared to other two schedules. At the end of this schedule, a 17-h equalization and 24-h conditioning treatment is recommended.     

Hybrid Drying of Rubberwood Using Superheated Steam and Hot Air in a Pilot-Scale

A pilot-scale rubberwood dryer was constructed and injected with superheated steam and hot air to study the effect of the hybrid system on the drying rate and mechanical properties of the wood. A total of 300 pieces of rubberwood boards, each with dimensions of 1000 mm long × 76.2 mm wide × 25.4 mm thick, were stacked in 1.0 m × 1.0 m × 1.7 m (1.7 m³) pallet. The stack was impinged with alternating cycles of superheated steam and hot air. The time required for conventional drying was 168 hours, but the drying time for the hybrid system was only 64 hours, resulting in a 62% reduction.

After being dried, the rubberwood boards were mechanically tested for static bending, compression strength, hardness, and shear strengths. From the mechanical tests, the hybrid drying system using superheated steam and hot air had no significant effect on the mean shear strength parallel-to-grain; however; the mean compression strength parallel-to-grain was reduced by 24.2% and the mean MOR by 21.4%. Nonetheless, the mean MOE was increased by 30.4% and the mean of hardness by 16.4%.     

Impact of conventional drying and thermal post-treatment on the residual stresses and shape deformations of larch lumber

Quality evaluation and grading of thermally treated wood products are of fundamental importance to their commercial utilization. The combined impacts of conventional drying, thermal post-treatment, and transverse dimensions of lumber over the residual stresses and shape deformations of larch (Larix gmelinii) wood were examined. Larch specimens with two different thicknesses (25 and 40?mm) and three different widths (100, 150, and 200?mm) were dried using conventional technology and thereafter thermally treated at 200°C for 1?h at atmospheric superheated steam conditions. Drying and residual stresses and shape deformations of both kiln-dried and thermally post-treated lumbers were measured and statistically analyzed. The influential mechanisms of lumber thickness and width over the drying stresses and shape deformations of thermally post-treated lumbers were revealed. The drying and residual stress measurement based on the prong test was recommended as a potential quality evaluation strategy for the thermally post-treated larch lumbers. As the lumber width increased from 100 to 150?mm, and then to 200?mm, the cupping deformation of thermally post-treated larch lumber increased substantially. These results provided some practical foundations for the quality evaluation of thermally treated wood.     

Dynamic Viscoelasticity of Wood After Various Drying Processes

In this study, specimens of heartwood from Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) plantation trees were dried by high-temperature drying (HTD), low-temperature drying (LTD), and freeze vacuum drying (FVD), respectively. The dynamic viscoelastic properties of dried and untreated wood specimens with various moisture contents were investigated in the temperature range between ? 120 and 40°C at 1 Hz using a dynamic mechanical analysis (DMA). The results indicated that the relative storage modulus and relative loss modulus were both the highest for HTD wood and the lowest for FVD wood, and that two mechanical relaxation processes developed. The α relaxation process in the higher temperature range was presumably a result of surpassing the glass transition of hemicelluloses with low molecular weight, whereas the β relaxation process occurring in the lower temperature range was most probably due to the motions of both methyl groups in the amorphous region of wood cell wall and adsorbed water molecules in wood. As moisture content increased, the decrease of relative storage modulus with increasing temperature became more dramatic, and the loss peak temperatures of the relaxation processes shifted to lower temperature range. The difference of dynamic mechanical behavior among untreated and dried specimens reduced with the increase of moisture content.     

Visual Method to Assess Lumber Sorting Before Drying

This article explores the possibility of using a simplified but intuitive method to quickly assess the potential benefits of sorting lumber before industrial kiln drying. The method consists of using scatter plots to visualize the probability of obtaining a certain drying result, such as final moisture content, as a function of a property of the green lumber that can be measured in practice. The method was first validated with four drying runs of 116 mm × 52 mm hemlock lumber: one run contained unsorted lumber and the others contained the same type of lumber but sorted into low, medium, and high groups depending on the electrical capacitance of the green wood. After validation, the scatter plots were used to assess the benefits of two typical industrial sorting strategies, namely, sorting by electric capacitance and sorting by weight. It was found that both methods have the potential to increase lumber production and reduce over dried lumber in approximately the same magnitude. For a typical industrial schedule, sorting into three groups reduced the drying time by approximately 10% and over dried lumber to practically zero.     

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.     

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.     

Analysis of the Disintegration of Distiller's Spent Grain Compacts as Affected by Drying in Superheated Steam

Breaking and disintegrating of biomass compacts while they are being dried in superheated steam (SS) is a common problem observed during the initial stage of SS drying. The present work investigated the moisture and temperature changes, volume and density variation, crushing resistance, and tensile strength of single cylindrical compacts produced from wet distiller's spent grain (WDG) under SS drying conditions. The wet compacts were dried in SS at 110, 130, and 150°C with SS velocities of 0.9, 1.1, and 1.4 m/s. For a specific temperature and velocity, the compacts were exposed to SS for time periods of 5, 120, 300, and 600 s and the changes in physical properties were analyzed. An increase in a percentage increase in volume by 78–130% and a decrease in density by 51–61% were observed as a result of drying the compact in SS. The results obtained from the study were compared with hot-air dried compacts for 600 s, indicating that SS drying had a substantial role in stimulating the relaxation of stresses stored in the compacts as compared with a convection hot-air drying process.     

Considerations on Drying Frozen Spruce Wood and Effects upon Its Properties

The aricle presents the results of a research performed in order to establish whether drying lumber from frozen state (in winter) has repercussions upon its properties, compared to drying under the same conditions lumber parts originating from the same log and position within the log section but that were not frozen prior to drying. To this purpose, spruce (Picea abies L.) lumber specimens, 35 mm and 55 mm thick, cut from the same log, half frozen at ? 30°C and half unfrozen, were dried under the same conditions in a climate chamber. Some mechanical properties (hardness, bending strength, and modulus elasticity in static bending), as well as workability (expressed by means of the absorbed power and specific resistance to cutting during milling), were determined The results revealed slight differences between the frozen and the unfrozen samples both during the drying process and afterwards. It was noticed that a significant amount of water was removed from wood during the very beginning of the heating phase (thawing). With regards to wood properties after drying, a slight lowering of the mechanical properties and better workability could be established for the initially frozen samples.     

Attrition of Victorian brown coal during drying in a fluidized bed

Analyzing the attrition of Victorian brown coal during air and steam fluidized bed drying, the change in particle size distribution over a range of initial moisture contents (60% to 0%) and residence times (0 to 60 minutes) was determined. Dried at a temperature of 130°C with a fluidization velocity 0.55 m/s and an initial particle size of 0.5–1.2 mm, both fluidization mediums show a shift in the particle size distribution between three and four minutes of fluidization, with a decrease in mean particle size from 665 µm to around 560 µm. Using differential scanning calorimetry (DSC), the change in particle size has been attributed to the transition between bulk and non-freezable water (approximately 55% moisture loss) and can be linked to the removal of adhesion water, but not to fluidization effects. This is proved through the comparison of air fluidized bed drying, steam fluidized bed drying, and fixed bed drying—the fixed bed drying is being used to determine the particle size distribution as a function of drying. The results show the three drying methods produce similar particle size distributions, indicating that both fluidization and fluidization medium have no impact upon the particle size distribution at short residence times around ten minutes. The cumulative particle size distribution for air and steam fluidized bed dried coal has been modeled using the equation P_d = A₂ + (A₁ ? A₂)/(1 + (d/x₀)^p), with the resultant equations predicting the effects of moisture content on the particle size distribution. Analyzing the effect of longer residence times of 30 and 60 minutes, the particle size distribution for steam fluidized bed dried coal remains the same, while air fluidized bed dried coal has a greater proportion of smaller particles.     

THE EFFECT OF DRYING TEMPERATURE ON THE MECHANO-SORPTIVE BEHAVIOR OF RED OAK LUMBER

ABSTRACT

Matched units of 3.2 cm thick red oak lumber were dried simultaneously in a steam heated and dehumidification kiln. The two pairs of runs are designated Set I and Set II. The objective was to compare the mechano-sorptive behavior and board shrinkages while using the recommended U.S. Forest Products Laboratory schedule with the steam kiln and comparatively low temperature drying in the dehumidification kiln.

Drying rates in Set I and Set II were comparable for the two kilns up to approximately 450 hours, which illustrated the dependence of the drying rate on the relative humidity of the kiln air rather than its temperature. Subsequently, the stepwise increases in dry bulb temperature for the steam kiln were accompanied by accelerated drying.

Less compression set developed in the interior mechano-sorptive slices for the dehumidification kiln runs. On an average, the maximum compression set for the core slices from the dehumidification kiln was about S0% of that for core slices from the steam heated kiln. Simultaneously the surface slices from the dehumidification kiln developed more tension set than those from the steam heated kiln. Board width shrinkage at the end of drying, at the same average moisture content, was greatest for the steam kiln. The greater shrinkage is attributed to greater compression set due to the higher drying temperatures. These results support McMillen)s explanation for the effect of drying temperatures upon sets and the shrinkage of red oak lumber.     

Effects of Drying Schedules on Physical and Mechanical Properties in Paulownia Wood

The effects of six drying schedules on physical and mechanical properties of Paulownia wood (Paulownia fortune Seem.) were studied. Three schedules were based on the recommendations by Forest Product Laboratory (FPL), while the other three were established based on diffusion theory. FPL schedules consisted of a mild (T₆E₃), a moderate (T₆E₄), and a severe (T₇E₄) drying schedule; diffusion schedules consisted of three initial moisture contents (MC) of 113% (Dif-1), 75.5% (Dif-2), and 53.5% (Dif-3). Boards with a commercial thickness of 5 cm were randomly dried to the final moisture content of 8 ± 2% in all six schedules. Results indicated that drying under diffusion schedules had the most improved properties due to the beneficial effect of heat upon wood plasticity. However, lower plasticity effect in FPL schedules, due to lower temperatures, resulted in lower properties in most cases.     

Effects of High-Temperature Fluidized Bed Drying and Tempering on Kernel Cracking and Milling Quality of Vietnamese Rice Varieties

Studies on the effects of high-temperature fluidized bed drying and tempering on physical properties and milling quality of two long-grain freshly harvested Vietnamese rice varieties, A10 (32±1% wet basis moisture) and OM2717 (24.5±0.5% wet basis moisture), were undertaken. Rice samples were fluidized bed dried at 80 and 90°C for 2.5 and 3.0 min, then tempered at 75 and 86°C for up to 1 h, followed by final drying to below 14% moisture (wet basis) at 35°C by thin-layer drying method. Head rice yield significantly improved with extended tempering time to 40 min. Head rice yield tended to increase with decreasing cracked (fissured) kernels. The hardness and stiffness of sound fluidized bed dried rice kernels (in the range of 30–55 N and 162–168 N/mm, respectively) were higher than that of conventionally dried ones (thin layer dried at 35°C). The color of milled rice was significantly (P < 0.05) affected by high-temperature fluidized bed drying, but the absolute change in the value was very small.     

Modeling Shrinkage Response to Tensile Stresses in Wood Drying III. Stress—Tensile Set Correlation in Short Pieces of Lumber

This article reports on the correlation between tensile stresses, temperature, and target moisture content in short pieces of lumber based on relationships developed on experiments made on small wood strips subjected to different types of restraints. Linear motion position sensors were placed around a tested specimen using a frame connected to a support body. A wireless moisture and temperature monitoring system was used to record wood moisture and temperature changes. All measurements were performed perpendicular to fiber grain while drying at 40, 60, and 80°C in an environment set for a target moisture content of 5%; the experiments were stopped when the average moisture content of the lumber was around 10%. High stress values were obtained for specimens dried at 60°C and a low relative humidity, whereas a high reduction in stress level could be obtained for 80°C. The findings are intended to be used in further studies of the shrinkage process as an indicator of the tensile stresses generated in the early stages of wood drying.     

A Formaldehyde-Free Soy-Based Adhesive for Making Oriented Strandboard

A formaldehyde-free adhesive consisting of soy flour, polyethylenimine, maleic anhydride, and sodium hydroxide was investigated for making randomly oriented strandboard (R-OSB) and oriented strandboard (OSB). The hot-press conditions and the adhesive usage rate were optimized in terms of enhancing internal bond strength (IB), modulus of rupture (MOR), and modulus of elasticity (MOE) of the resulting R-OSB and OSB. The IB, MOR, and MOE were the highest at a hot-press temperature of 170°C, a hot-press time of 4–5 min, and an adhesive usage rate of 7%. The strengths of the OSB panels made with this formaldehyde-free adhesive were compared with those of commercial OSB panels purchased at a local Home Depot store.     

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.     

THE EFFECT OF DRYING TEMPERATURE ON THE MECHANO-SORPTIVE BEHAVIOR OF RED OAK LUMBER

Matched units of 3.2 cm thick red oak lumber were dried simultaneously in a steam heated and dehumidification kiln. The two pairs of runs are designated Set I and Set II. The objective was to compare the mechano-sorptive behavior and board shrinkages while using the recommended U.S. Forest Products Laboratory schedule with the steam kiln and comparatively low temperature drying in the dehumidification kiln.

Drying rates in Set I and Set II were comparable for the two kilns up to approximately 450 hours, which illustrated the dependence of the drying rate on the relative humidity of the kiln air rather than its temperature. Subsequently, the stepwise increases in dry bulb temperature for the steam kiln were accompanied by accelerated drying.

Less compression set developed in the interior mechano-sorptive slices for the dehumidification kiln runs. On an average, the maximum compression set for the core slices from the dehumidification kiln was about S0% of that for core slices from the steam heated kiln. Simultaneously the surface slices from the dehumidification kiln developed more tension set than those from the steam heated kiln. Board width shrinkage at the end of drying, at the same average moisture content, was greatest for the steam kiln. The greater shrinkage is attributed to greater compression set due to the higher drying temperatures. These results support McMillen)s explanation for the effect of drying temperatures upon sets and the shrinkage of red oak lumber.