Comparison of Two Drying Schedules for European Hophornbeam (Ostrya carpinifolia Scop.) Lumber

Two types of conventional kiln-drying schedules (mild and harsh) based on moisture content (MC) were compared with regard to time, drying quality, and energy cost. The results were evaluated according to the classification of the European Drying Group. Proper drying periods of mild and harsh schedules were found to be 550 and 514 h, respectively. Evaluations in terms of drying quality indicated that better results were achieved with the mild schedule, especially when comparing drying defects and final MC. From an energy efficiency point of view, the harsh schedule, by saving 36 h of drying time, reduced electricity by 594 KWh and was therefore found to be $65 more profitable in this trial.     

A Simulation Tool for the Optimization of Lumber Drying Schedules

Abstract

A two-dimensional wood drying model based on the water potential concept is used to simulate the convection batch drying of lumber at conventional temperature. The model computes the average drying curve, the internal temperature and moisture content profiles, and the maximum effective moisture content gradient through board thickness. Various scenarios of conventional kiln-drying schedules are tested and their effects on drying time, maximum effective moisture content gradient, final moisture content distribution within and between boards, and energy consumption are analyzed. Simulations are performed for two softwood species, black spruce (Picea mariana (Mill.) B.S.P.) and balsam fir (Abies balsamea (L.) Mill.). The simulation results indicate that the predictive model can be a very useful tool to optimize kiln schedules in terms of drying time, energy consumption, and wood quality. Such a model could be readily combined with intelligent adaptive kiln controllers for on-line optimization of the drying schedules.     

A Simulation Tool for the Optimization of Lumber Drying Schedules

A two-dimensional wood drying model based on the water potential concept is used to simulate the convection batch drying of lumber at conventional temperature. The model computes the average drying curve, the internal temperature and moisture content profiles, and the maximum effective moisture content gradient through board thickness. Various scenarios of conventional kiln-drying schedules are tested and their effects on drying time, maximum effective moisture content gradient, final moisture content distribution within and between boards, and energy consumption are analyzed. Simulations are performed for two softwood species, black spruce (Picea mariana (Mill.) B.S.P.) and balsam fir (Abies balsamea (L.) Mill.). The simulation results indicate that the predictive model can be a very useful tool to optimize kiln schedules in terms of drying time, energy consumption, and wood quality. Such a model could be readily combined with intelligent adaptive kiln controllers for on-line optimization of the drying schedules.     

Mechanical Characterization of Lumber of Small-Diameter Hardwood Species after Different Drying Schedules

The influence on the drying behavior as well as the mechanical properties of lumber of small-diameter hardwoods at different low-temperature convection kiln dryings was investigated. Standard test methods in three-point-bending and the modulus of elasticity in three-point-bending, compression tests parallel and perpendicular to the grain, as well as Brinell hardness tests were performed in order to assess the drying quality. Furthermore, non-standard tensile tests perpendicular to the grain and fracture energy tests in radial/longitudinal and also in tangential/longitudinal crack propagation systems were carried out as a most sensitive test method. The results showed significant changes of the specific fracture energy, bending strength, compression strength parallel and perpendicular to the grain, and the Brinell hardness with good drying performance at the same time.     

Application of Wood Drying Simulation Models in Commercial Lumber Manufacturing

Drying models are created both to develop a better understanding of the governing heat and mass transfer phenomena and to assist drying practitioners in achieving commercial goals. This article is a review of the application of wood drying and kiln simulation models to commercial lumber drying challenges. Examples from the literature are briefly reviewed and four selected applications in commercial lumber manufacturing are described. They include development of equalization schedules, assessment of green lumber sorting, development of alternative drying schedules, and assessment of fan reversal frequency and timing.     

Effects of Wood Drying Schedules on Fluid Flow in Paulownia Wood

Effects of three drying schedules on fluid flow were studied in the sapwood and heartwood of Paulownia wood (Paulownia fortunei). Boards with a commercial thickness of 5 cm were randomly dried to a final moisture content of 8 ± 2% using a mild (T₆E₃), a moderate (T₆E₄), and a severe (T₇E₄) drying schedule. Permeability measurement was carried out when specimens reached the final moisture content. Results showed a significant difference in the specific gas permeability as well as liquid permeability of the boards dried under the three drying schedules. Furthermore, a significant difference was observed in the gas permeability of sapwood and heartwood, dried under the different schedules, but not much significant difference was seen in the liquid permeability between sapwood and heartwood. T₆E₃ had the highest liquid permeability; furthermore, it was reported to have resulted in the lowest warping and most homogeneous moisture profile. This mild schedule is therefore recommended for commercial drying of Paulownia wood when further preservation and impregnation processes are planned for the dried boards. In the meantime, it is concluded that the age and drying schedule have significant effects on the formation of tyloses, significantly affecting gas and liquid permeability in Paulownia wood.     

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.     

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.     

Impact of High-Temperature Schedules on Drying of Spruce and Pine

The impact of high-temperature drying (HTD) for spruce and pine lumber of 5 × 10 × 244 cm was investigated in this study. Four drying schedules with dry-bulb temperatures from 60 to 110°C were evaluated including Control 1 (conservative), Control 2 (accelerated), HTD1 (104°C) and HTD2 (110°C). In each of HTD1 and HTD2 schedules, two runs were performed, one humidified with steam and the other applied without steam. Drying rates, internal stresses (prong test), warp (bow, crook, and twist), modulus of elasticity (MOE), and modulus of rupture (MOR) were assessed for the dried lumber.

Six drying runs were carried out in a laboratory kiln and the results of this research indicated: (1) The drying rates in HTD1 and HTD2 increased by 2.2 to 3.5 times in comparison to the conservative schedule. (2) Both HTD1 and HTD2 schedules with steam resulted in greater internal stresses in the dried lumber compared to Control 1, but these stresses were smaller than those in Control 2. The standard deviations of final moisture content from HTD1 and HTD2 with steam were higher than those of Control 1 but similar to the values obtained for Control 2. (3) Warp was, in general, reduced by using the HTD schedules except for HTD2 without steam. Less crook was observed for HTD1 and HTD2. (4) Reductions in MOE and MOR were found for both HTD1 and HTD2 schedules but were not statistically significant.     

Freeze Drying of Saffron (Crocus sativus L.)

Saffron obtained from the dried stigmas of a flower scientifically known as Crocus sativus L. is considered to be the most precious and expensive agricultural product due to its labor-intensive harvest and post-harvest processing. The post-harvest processing such as dehydration and storage conditions determine stability, quality, and economical value of the final product. The contents of crocin (degraded carotenoids) and safranal (carotenoid oxidation products) are the key components that characterize color, taste, and aroma characteristics of saffron. In this work, the quality parameters such as crocin and safranal contents of commercial saffron that were obtained by using the freeze-drying method and natural sun drying were studied. The sarfanal contents of the samples dried in a freeze dryer were found to be five times higher than the safranal contents of the samples dried naturally under the sun, while crocin contents of the samples dried in a freeze dryer were about 40% higher than the crocin contents of the samples dried naturally under the sun. These encouraging results indicate that the freeze-drying process can be used effectively for dehydration of saffron by minimal loss of safranal and crocin contents.     

A Vacuum Drying System for Green Hardwood Parts

A vacuum drying system was designed and fabricated and that system was used to dry green rough hardwood dimension. The red oak samples, 76.2 (long) × 7.62 (wide) × 2.54 (thick) cm, were dried from green moisture content (MC) to 7% MC in this system. They were dried at a pressure of 12 mm Hg and temperatures ranging from 30 to 50°C within 25 to 70 h. Drying quality tested included warp, internal checking, and surface checking. Moisture gradients along the length and thickness were measured. The standard prong test was used to assess the drying stresses. Vacuum drying was fast and the drying rate increased as the temperature increased. It was found that the general drying quality was good with no color change. Drying stresses including longitudinal and transverse stresses were small. There were no internal checks.     

Evaluation of Thin-Layer Models for Mushroom (Agaricus bisporus) Drying

In the present research, seven well-known mathematical thin-layer drying models were fitted to mushroom (Agaricus bisporus) drying experimental data, implementing nonlinear regression analysis techniques. The experiments were conducted in two laboratory-scale dryers. A range of temperatures 50–65°C and air velocities 1.0–5.0 m/s were tested. The statistical analysis concluded that the best model in terms of fitting performance was the logarithmic model. Correlations expressing this model parameter dependence with the drying air coefficients are also reported.     

Evaluation of Thin-Layer Models for Mushroom (Agaricus bisporus) Drying

In the present research, seven well-known mathematical thin-layer drying models were fitted to mushroom (Agaricus bisporus) drying experimental data, implementing nonlinear regression analysis techniques. The experiments were conducted in two laboratory-scale dryers. A range of temperatures 50-65°C and air velocities 1.0-5.0 m/s were tested. The statistical analysis concluded that the best model in terms of fitting performance was the logarithmic model. Correlations expressing this model parameter dependence with the drying air coefficients are also reported.     

The Comparison of Two Models of Convective Drying of Shrinking Materials Using Apple Tissue as an Example

Modeling of drying of capillary-porous materials is a mathematically complex problem. It takes into consideration simultaneous heat and mass transfer inside the material with physicochemical properties changing during the drying process. Modeling of the process mentioned above consists of describing the heat and mass transfer balances by means of differential equations. Moisture diffusion coefficient as a function of moisture content and temperature of the material is a crucial parameter that controls the process. An additional problem occurs when moving boundary of the shrinking material is taken into account. In the present work, the identification of diffusion coefficient as a function of moisture content and temperature on the basis of two different models is shown. The two models include the Pakowski model (defined in the stationary coordinates) and the Kechaou model (defined in moving coordinates). Experimental data necessary to verify the models were obtained on the basis of series of tests for different boundary conditions performed on an apple tissue. During the drying process, samples of apple undergo significant volumetric shrinkage. In this article, the comparison of the two models describing the convective drying process of shrinking material is presented together with the comparison of the identified moisture diffusion coefficient.     

The Comparison of Two Models of Convective Drying of Shrinking Materials Using Apple Tissue as an Example

Modeling of drying of capillary-porous materials is a mathematically complex problem. It takes into consideration simultaneous heat and mass transfer inside the material with physicochemical properties changing during the drying process. Modeling of the process mentioned above consists of describing the heat and mass transfer balances by means of differential equations. Moisture diffusion coefficient as a function of moisture content and temperature of the material is a crucial parameter that controls the process. An additional problem occurs when moving boundary of the shrinking material is taken into account. In the present work, the identification of diffusion coefficient as a function of moisture content and temperature on the basis of two different models is shown. The two models include the Pakowski model (defined in the stationary coordinates) and the Kechaou model (defined in moving coordinates). Experimental data necessary to verify the models were obtained on the basis of series of tests for different boundary conditions performed on an apple tissue. During the drying process, samples of apple undergo significant volumetric shrinkage. In this article, the comparison of the two models describing the convective drying process of shrinking material is presented together with the comparison of the identified moisture diffusion coefficient.     

Management of Surface Drying Temperature to Increase Antioxidant Capacity of Thyme Leaf Extracts (Thymus vulgaris L.)

Thyme leaves are an important source of essential oils with antioxidant activity; these compounds are located in trichomes on the leaf surface. The drying conditions affect not only the drying time but also the antioxidant activity. In the literature, a drying temperature of 70°C appears to be the best for drying thyme leaves according to their antioxidant capacity. Considering drying periods at different temperatures also could be beneficial. With these considerations, the goal of this work was to establish a drying strategy with which to manage a drying temperature on the leaf surface that will enable the drying time to be shortened and improve the antioxidant capacity (AC) of the extract of dried thyme leaves. The drying strategy consisted of two consecutive drying periods in order to manage the drying temperature on the leaf surface. The first drying period was carried out at 80°C (T _a1) until the sample surface reached a temperature of 70°C, and the temperature was then immediately set to 70, 60, 50, and 40°C (second drying period, T _a2) at different air velocities (v; 1 and 2 m s^?1). Compared to constant drying conditions, two consecutive drying periods were found to improve the drying kinetics: the AC increased from 10.5 to 27.4% while reducing the drying time by 14.5 to 39.2%. The use of this drying strategy was found to be an interesting means of intensifying the convective drying of thyme leaves and its application should be considered when drying similar materials with bioactive compounds on the surface.     

Optimizing Drying Conditions for Microwave-Vacuum (MIVAC®) Drying of Russet Potatoes (Solanum tuberosum)

MIVAC® combines microwave heating with vacuum drying. Microwave power is modulated based upon product temperature and can limit overheating compared to other microwave-vacuum methods. Blanched potatoes were dried at 50, 60, and 70°C for 0 to 150 min. Potatoes dried at 70°C had a lower moisture content in less time compared to potatoes dried at 50 and 60°C, but the color of the dehydrated potatoes was affected due to overheating. Drying at 60°C for 150 min resulted in dried potatoes with acceptable color. Drying at 50°C resulted in dehydrated potatoes of acceptable color; however, it required more time.     

Optimization of Solar Kiln for Drying Wood

Theoretical investigation of the physical process of solar drying of timber based on conventional heat and mass transfer equations is presented. The governing equations and boundary conditions of the mass diffusion through the wood timbers are derived; also the governing equations of the components of the solar kiln are presented. The finite difference technique is used to solve the set of these equations by means of a simulation program that is based on object-oriented approach. The simulation program is used to investigate the effect of several design parameters on the drying rate and duration of the wood timbers in order to accomplish the drying process with minimal drying defects. These parameters include the ventilation conditions that control the drying schedule inside the solar kiln, wood volume as a ratio to the solar kiln absorber area, wood timber thickness, season of drying, the drying air velocity, and the stresses that formed on the timber boards due to drying with these several parameters, leading to derive the limit of damage for a selected local wood type. The selected local wood type is Casuarina, which is common in Egypt, and it is commonly used in many simple industries.     

Drying Characteristics of Purslane (Portulaca oleraceae L.)

Abstract

Thin layer drying rates of purslane were determined experimentally as a function of temperature with air velocity kept constant at 1.1 m/s and relative humidity below 5%. Thin layer drying data were obtained for purslane at four drying air temperatures (35, 70, 95, and 120°C). Five thin layer-drying models (Henderson and Pabis, exponential, Page, two-term exponential, and Thompson models) were fitted to the drying data. The color of purslane was determined after drying using a spectro-colorimeter (Hunter Lab) in terms of Hunter L, a, and b values. The Page model was found to be most suitable in describing the drying characteristics of purslane. New parameters developed for the model resulted in a good fit at different temperatures. Color measurement indicated that greenness decreased with an increase in drying air temperature. Typical drying times were 88.41, 138.53, 416.38, and 1371.85 min at 120, 95, 70, and 35°C, respectively.     

Drying Characteristics of Purslane (Portulaca oleraceae L.)

Thin layer drying rates of purslane were determined experimentally as a function of temperature with air velocity kept constant at 1.1 m/s and relative humidity below 5%. Thin layer drying data were obtained for purslane at four drying air temperatures (35, 70, 95, and 120°C). Five thin layer-drying models (Henderson and Pabis, exponential, Page, two-term exponential, and Thompson models) were fitted to the drying data. The color of purslane was determined after drying using a spectro-colorimeter (Hunter Lab) in terms of Hunter L, a, and b values. The Page model was found to be most suitable in describing the drying characteristics of purslane. New parameters developed for the model resulted in a good fit at different temperatures. Color measurement indicated that greenness decreased with an increase in drying air temperature. Typical drying times were 88.41, 138.53, 416.38, and 1371.85 min at 120, 95, 70, and 35°C, respectively.