Predicting Thermal Efficiency in Timber Radio Frequency Vacuum Drying

Abstract

In this work, the efficiency of transforming dielectric energy into evaporated water is analyzed for the case of timber radio frequency vacuum drying. Based on well-known heat and mass transfer equations, a simplified mathematical model is proposed that estimates the drying efficacy in regards to the thermo-physical properties of wood. Although not exact, the theoretical results are close to the experimental observations and elucidate some phenomena like the tendency of the timber to dry from inside to outside, and the drying rate increase with the rise of the timber gas permeability. The theoretical efficiency model also predicts a range of wood permeability values for which the drying efficiency changes from 100 to 0%, thus providing a quantitative scale for classifying the spectrum of “difficult-to-dry” all the way to “easy-to-dry” wood species when using radio frequency vacuum technology.     

THE RELATIVE INFLUENCE OF DIELECTRIC AND OTHER DRYING TECHNIQUES ON THE PHYSICO - MECHANICAL PROPERTIES OF A PHARMACEUTICAL TABLET EXCIPIENT. PARTI: COMPACTION CHARACTERISTICS.

The relative Influence of nine techniques for drying wet granulated microcrystal-lline cellulose (MCC) on the subsequent compaction characteristics was studied In terms of the tensile strength and corrected work of failure of the tablets. Wet granulation resulted in a substantial decrease in compatibility. However, the drying technique used was found to affect the degree of loss in compatibility. In general, microwave-vacuum drying using the “high” process type resulted in the production of granules with the highest compatibility followed by freeze drying and fluidized bed drying. Granules dried under ambient conditions, and granules tray dried to “just dry” or “over dried” conditions resulted In tablets possessing approximately comparable compatibilites, with the poorest compaction characteristics being exhibited by vacuum dried granules, in addition It was found that use of a “low” drying process type during microwave- vacuum drying yielded granules with inferior compaction characteristics to those dried by the “high” process type. Radio frequency drying was found to yield granules which produced tablets having slightly inferior tensile strength to tray dried material, although the tablet work of failure values were comparable. The effect of drying technique on the subsequent compaction characteristics was not found to be directly related to the moisture content of the granules.     

ANALYSIS AND OPTIMIZATION OF THE CONDITIONING PHASE IN TIMBER DRYING

The conditioning phase of the conventional timber drying process has been investigated. Due to wood shrinkage, tensile stress will develop in the surface layer of the board during drying. This stress combined with the change in MC causes mechano-sorptive creep. If the process is abruptly terminated when the target MC is reached, then there will be a considerable internal MC gradient and the creep deformation will cause distortions in subsequent machining. A conditioning phase is often introduced at the end, in order to reduce these problems. The conditioning phase has been analyzed with a wood drying simulation model. It is shown that traditional conditioning cannot completely reverse the deformations, even at very long conditioning times. An optimizing procedure has thus been used to create a new conditioning “schedule” which gives the best possible result within a given time. Features of such optimized schedules are presented. As the surface layer is elongated during drying, there is a risk that considerable stress develops as the moisture profile levels out. In some cases internal checking will be the result. The optimization procedure is thus modified to avoid such situations and the impact on the conditioning schedule is discussed.     

Radio Frequency Vacuum Drying of Wood. III. Two-Dimensional Model, Optimization, and Validation

A simple two-dimensional model is presented for the radio frequency vacuum drying of thick timbers. The developed code is linked to powerful optimization methods to account for the variability of the permeability and the internal pressure deviation from the saturated vapor pressure above the fiber saturation point. The model is compared with recently obtained experimental results where the radio frequency heating source is an amplifier. Despite the simplicity of the model, good agreement with experimental data was obtained for a variety of drying conditions, especially in predicting the average moisture evolution and the drying time as well as the development of the temperature profiles in two types of softwood timbers.     

Moisture Distribution Changes and Wetwood Behavior in Subalpine Fir Wood during Drying Using High X-Ray Energy Industrial CT Scanner

In some species, such as subalpine fir (Abies lasiocarpa [Hook] Nutt), the water content of the confined zones in heartwood is as high as or greater than that of sapwood. Such wet zones of heartwood are referred to as “wetpocket” or “wetwood.” Wood products from subalpine fir forests are adversely affected by the wetwood-associated problems, particularly during the drying process. The objectives of the study were as follows: (1) to investigate feasibility of a high X-ray energy industrial computed tomography (ICT) scanner for imaging wetwood; and (2) to determine changes of the 2-D and 3-D moisture profiles (from core to shell) at different drying times.

Although medical CT scanning has been used for attaining signal intensity profiles of typical wood at different drying times, the technology has not, to date, been used for the study of wetwood phenomenon. This study presents, for the first time, results from the ICT imaging of the wetwood phenomenon. The results indicate that the ICT imaging system provides a powerful technique for imaging wetwood at different drying times. In addition, the results show that during the initial phase of drying, almost flat moisture profiles were observed in all wood types except for the wetwood, which showed a relatively higher moisture profile. A much slower (sluggish) drying development pattern at each increment from core to shell was found within the wetwood zone than normal wood regions along the width, thickness, and length of the board.     

Drying of Porous Material During the Constant and the Falling Rate Period: A Critical Review of Existing Hypotheses

During drying of porous material a so-called “falling rate” period is observed, where the drying rate decreases as the moisture content decreases. This behavior is usually described by the well known “shrinking core model.” This model, however, contradicts experimental findings and violates basic laws of multiphase mass transfer in porous media as well. A new model, named “wet surface model,” is suggested which eliminates those discrepancies.     

COMPARISON OF EXPERIMENTAL AND MODELLING STUDIES FOR THE MICROWAVE DRYING OF IRONBARK TIMBER

Temperature profiles through boards during the microwave-assisted drying of Australian Ironbark timber have been investigated in this work in order to explore evidence for timber degrade via “charring” at internal temperatures below those required for pyrolysis (Brooke et al., 1998). A previously published model (Turner and Jolly, 1990a) describing one-dimensional microwave power absorption, based on the solution of Maxwell's equations, has been reviewed and significant limitations identified and overcome. Improvements included the use of a linear-mixing approach for the dielectric constants, the incorporation of temperature and moisture dependencies for these dielectric properties and the inclusion of diffusion within an overall system model. A control-volume technique has been used for predicting both moisture and temperature distributions within the timber, with a second-order finite-difference method being used to solve Maxwell's equations. The final model showed     

COMPARISON OF CONVECTIVE, VACUUM, AND MICROWAVE DRYING CHLORPROPAMIDE

Drying kinetics of convective, vacuum, and microwave drying of a pharmaceutical product, chlorpropamide, has been investigated on a laboratory scale, in the temperature interval from 40°C to 60°C, and the range of microwave heating power from 154 W/kg_dm to 385 W/kg_dm.

The experimental data obtained were approximated with the “thin-layer” equation and a two parameter exponential model. In order to compare convective, vacuum, and microwave drying, effective diffusion coefficients and specific heat consumption were calculated for each drying method.

Higher rates and shorter drying times were achieved at a higher temperature and microwave heating power. The highest drying rates and the lowest specific heat consumption were achieved with microwave drying. This leads to the conclusion that microwave heating is the most appropriate method for drying of chlorpropamide. The quality of product was not changed for all applied methods.     

ESTIMATION OF FUNCTIONS IN DRYING EQUATIONS

In drying problem, particularly for drying foodstuff, modelling is very difficult. Many physical effects have to be taken into account for mass transfer ; then mass transfer coefficient varies

In different models unknown functions must be estimated. It is particularly the case in simple models of drying using average values of water content, where the mass transfer varies versus mean water content in falling rate period. On the other hand in the “diffusion model” we have the same problem concerning the diffusion coefficient which must be also estimated

The method we propose in this paper for these two models : simple and “diffusion model” of drying consists from measurements of temperature and water content of the product to search a numerical approach of the unknown function. This method uses optimization techniques on computer and least squares criterion between model values and experimental data

Results are given for the “diffusion model” applied to shelled corn drying to find the diffusion coefficient and for a simple 11107 del applied to plum drying to find the mass transfer coefficient.     

Radio Frequency Vacuum Drying of Wood. III. Two-Dimensional Model,Optimization, and Validation

Abstract

A simple two-dimensional model is presented for the radio frequency vacuum drying of thick timbers. The developed code is linked to powerful optimization methods to account for the variability of the permeability and the internal pressure deviation from the saturated vapor pressure above the fiber saturation point. The model is compared with recently obtained experimental results where the radio frequency heating source is an amplifier. Despite the simplicity of the model, good agreement with experimental data was obtained for a variety of drying conditions, especially in predicting the average moisture evolution and the drying time as well as the development of the temperature profiles in two types of softwood timbers.     

Thin-layer drying of porous materials: Selection of the appropriate mathematical model and relationships between thin-layer models parameters

Experimental investigation of the drying kinetics of various types of materials was carried out in laboratory-scale dryers under different conditions of temperature, microwave heating power and pressure. Leather samples (mechanically and vacuum-dewatered bull napa and wet blue cutting), paperboards (grafopack, testliner), wood (alder, birch, willow) and two pharmaceutical powders (chlorpropamide and hydrochlorotiazide) were dried in a microwave dryer. Thin clay slabs, Al–Ni catalyst and chlorpropamide were dried in a convection dryer, while chlorpropamide and ketoprofen were dried in a vacuum dryer. In order to compare drying kinetics, experimentally obtained data, X = f(t), were correlated with the Lewis “thin-layer” equation, the modified Page equation and Fick's second law. The drying constant, effective diffusion coefficient, mass transfer coefficient and modified Page model parameters were estimated by fitting the selected mathematical models to experimental data. High levels of correlation between measured and calculated data were obtained for all materials and dryers using modified Page model. The application of the Lewis and Fick's equation is justified only for drying of clay, catalyst and leather. Mass transfer coefficient depends linearly on the drying constant. The relation between the modified Page model parameter and the drying constant can be represented by a unique power function.     

Variation in Drying Behavior and Final Moisture Content of Wood during Conventional Low Temperature Drying and Vacuum Drying of Betula pendula Timber

Conventional and vacuum drying experiments were conducted on Betula pendula timber, which was sawn from trees felled during three different seasons. The influence of the wood procurement season on drying behavior differed, on the one hand, between the drying phases above and below 30% moisture content in the conventional drying, and, on the other hand, between the conventional and vacuum drying methods. During the first steps of the conventional drying process, relative humidity in the kiln, as well as drying time and drying rate, varied according to the felling season. Variations in environmental conditions outside the kiln and the seasonal variation in the physical properties of the wood were presumed to be the reasons for differences in drying behavior. The difference in moisture content gradient, i.e., the difference in final moisture content between the inner wood and the surface layer of boards, was greater in conventionally dried timber than in vacuum-dried timber. In conventionally dried timber there was a clear seasonal variation in the gradient of final moisture content, which was greatest for winter-felled wood. The premature drying of the surface layer during the first steps of the conventional drying process of winter-felled wood was the reason for the higher gradient of moisture content. Storage of wood as logs decreased the standard deviation of the final moisture content.     

Analysis of Moisture Content and Density of Pears During Drying

Solar dried pears of the “S. Bartolomeu” variety are a very much appreciated and preferred dried food product in Portugal. Nevertheless, the traditional solar drying is carried out at open air during the months of 07 and 08, and this nowadays is a disadvantage for larger productions. This work is to evaluate the possibility of producing dried pears from this and other varieties, maintaining the characteristics of the traditional dried pears. In this study four different types of pears were studied, including “S. Bartolomeu” as a basis for comparison and the drying method employed was the traditional one. From the results it was concluded that, although the behaviour of the four varieties do not vary significantly, one particular variety (“D. Joaquina”) is a good alternative to the “S. Bartolomeu” pear.     

Variation in Drying Behavior and Final Moisture Content of Wood during Conventional Low Temperature Drying and Vacuum Drying of Betula pendula Timber

Conventional and vacuum drying experiments were conducted on Betula pendula timber, which was sawn from trees felled during three different seasons. The influence of the wood procurement season on drying behavior differed, on the one hand, between the drying phases above and below 30% moisture content in the conventional drying, and, on the other hand, between the conventional and vacuum drying methods. During the first steps of the conventional drying process, relative humidity in the kiln, as well as drying time and drying rate, varied according to the felling season. Variations in environmental conditions outside the kiln and the seasonal variation in the physical properties of the wood were presumed to be the reasons for differences in drying behavior. The difference in moisture content gradient, i.e., the difference in final moisture content between the inner wood and the surface layer of boards, was greater in conventionally dried timber than in vacuum-dried timber. In conventionally dried timber there was a clear seasonal variation in the gradient of final moisture content, which was greatest for winter-felled wood. The premature drying of the surface layer during the first steps of the conventional drying process of winter-felled wood was the reason for the higher gradient of moisture content. Storage of wood as logs decreased the standard deviation of the final moisture content.     

Dielectric Drying

Dielectric heating is a term which covers both radio frequency (RF) and microwave systems. It has been used for industrial drying for many years because it has a number of advantages over other drying processes. These advantages include the volumetric dissipation of energy throughout a product, and the ability to automatically level any moisture variation within it. It is generally true to say that these techniques can only be justified in processes involving material of “high” value being processed in modest quantities but when combination with other fuel fired techniques there may be opportunities to broaden the range of applications. In addition to a desciption of current industrial applications, the theory of dielectric heating, the similarities and differences between RF and microwave are discussed together the effects of the dielectric roperties of a product on the design of the equipment     

VACUUM-PRESS DRYING OF THICK SOFTWOOD LUMBERS

In vacuum-press drying of softwood species, wood lemperaiure exhibited a low temperature gradient, and plateau temperature of core lasied during all stages of drying. The drying curves were close to “linear”. The drying rates of the short lumbers were higher for red pine and western hemlock, lower for white pine and similar for larch lumber compared to the long ones. Transverse and longitudinal moisture gradients were small for western hemlock and red pine, and great for larch and white pine lumber. Energy consumption curves were split into three sections: increasing moderately, fairly constant and increasing rapidly. Casehardening stress of dried lumber occurred very slightly. Dried lumbers exhibited strong tendency for fine end checking, slight surface checking and no internal checking. Shrinkage appeared to be low.     

A GRAPHICAL DESIGN METHOD FOR A PARTIAL SOLAR DRYER FOR CORN

This paper presents the application of a design method for a partial solar heating system of polyvalent modular dryers called “GJ-ABAQUE” to the drying of thick layers of grains.

This method is based on the use of charts or polynomial correlations. In the actual case where the drying air is not recycled, we only need one chart which allows one to determine the fraction of the monthly heating load supply by solar energy as a function of two dimensionless parameters. The latter implies the use of monthly average radiation data, the collector surface and estimates of drying loads.

The “GJ-ABAQUE” method was applied for drying 777 kg of corn, corresponding to 1 m³ of fresh product, in a thick layer in each modular dryer.     

Research and Development in Drying: Recent Trends and Future Prospects

Drying R&D has seen nearly exponential growth over the past three decades. Initially driven by the need to conserve energy in this highly energy-intensive operation found in almost all industrial sectors, now the focus is on product quality, environmental impact, safety issues, new products, and processes etc. Drying provides challenging areas for multi- and cross-disciplinary research of fundamental as well as applied nature coupling transport phenomena with material science. An attempt will be made to summarize the new developments in drying technologies, identify recent trends and make predictions about the future trends that may be expected. Also, personal perspectives on models of research appropriate for drying technology will be presented. The need for industry-academia interaction and for a stake of industry in academic research is noted as a key step towards successful transfer of innovative drying technologies to industry. Finally, the weaknesses of the currently popular “closed loop” approach of research in academia, by academia and for academia, resulting on over-emphasis on “high impact-factor” rather than “high impact research” will be discussed. It is hoped that future IDS meetings will increase industry participation by enhancing their impact on industrial adoption of newer more efficient drying technologies. It is essential to make it a worthwhile—even essential—exercise for industry to participate in these meetings.     

COMBINED MICROWAVE AND CONVECTIVE DRYING OF A POROUS MATERIAL

A model is formulated to describe the drying of a slab of porous material in a combined microwave and convective environment. The model describes the evolution of temperature, pressure, moisture and power distributions that occur during the drying process. The microwave internal heat source is calculated from electromagnetic theory with varying dielectric properties. The inclusion of pressure in the model allows the physical phenomena of “water pumping”, often observed in microwave drying systems, to be accounted for. The influence of sample size; on the drying kinetics 1s examined and found to be an important parameter during the drying process. In particular the effect of resonance on the moisture and temperature profiles and the need for careful consideration of surface mass transfer coefficients are investigated. Simulation results are presented for the combined microwave and convective drying of a homogeneous, isotropic porous material.