DRYING WOOD: A REVIEW - PART I

This paper reviews the technology of wood drying and recent developments in the field. The importance of drying is reviewed first. Then the fundamentals of wood properties as they relate to drying are discussed, including how water is held in wood, how it moves through wood, and how drying stresses that lead to drying defects develop. Traditional drying technology for lumber, veneer, and heavy timbers is then presented. With this background, the discussion moves to recent developments in drying technology that have been implemented commercially, such as high-temperature drying, low-temperature drying, continuously rising temperature drying, vacuum drying, and advances in process control. Finally, a discussion is presented on drying technology of the future. This topic describes processes that offer potential improvements in drying technology, but are still in the research and development stage.     

THE INFLUENCE OF THE DRYING MEDIUM ON HIGH TEMPERATURE CONVECTIVE DRYING OF SINGLE WOOD CHIPS

ABSTRACT

High temperature convective drying of single wood chips with air and superheated steam respectively is studied theoretically. The two-dimensional model presented describes the coupled transport of water, vapour, air and heat. Transport mechanisms included are the convection of gas and liquid, intergas as well as bound water diffusion. In the initial part of the drying process, moisture is transported to the surface mainly due to capillary forces in the transversal direction where evaporation occurs, As the surface becomes dry, the drying front moves towards the centre of the particle and an overpressure is simultaneously built up which affects the drying process

The differences between drying in air and steam respectively can be assigned to the physical properties of the drying medium. The period of constant drying rate which does not exist (or is very short) in air drying becomes more significant with decreasing amounts of air in the drying medium and is clearly visible in Dure superheated steam drying. The maximal drying rate is larger in air drying, and shorter drying times are obtained since the heat flux to the wood chip particle increases with increasing amounts of air in the drying medium. The period of falling drying rate can be divided into two parts: in the first, the drying rate is dependent upon the humidity of the drying medium whereas in the second, there is no such correlation.     

Emerging food drying technologies with energy-saving characteristics: A review

Abstract

Drying is one of the most vital preservation techniques used in the food industry. It demands different levels of energy to produce commercially high-quality-dried food products. Novel drying technologies minimize deterioration of the food ingredients and produce novel products for consumers. In recent years, there have been many developments in the technology connected with the industrial drying of foods. Recent research has shown that novel food drying technologies could be utilized to improve the efficiency of drying by lowering the energy consumption and also to enhance the product quality. This article reviews selected energy- saving techniques in drying and discusses some novel combined drying technologies. These include solar-assisted, infrared-assisted, microwave-assisted and similar hybrid drying methods for food drying. Recommendations are also made for future research and development.     

Optimization of Solar Kiln for Drying Wood

Abstract

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.     

THE CURRENT SITUATION OF GRAIN DRYING IN CHINA

Abstract

Drying is an important technology widely used in the processing of such products as grain, food and other agricultural materials. In recent years the drying industry in China is in a fast developing period and the amount of grain dryers is increasing every year. The grain drying technology is also improving. In this paper the present situation of grain drying technology and grain drying equipment is presented. The type, structure and characteristics of most widely used grain dryers are described. Some recommendations for further development are given.     

HEAT PUMP DRYING: RECENT DEVELOPMENTS AND FUTURE TRENDS

ABSTRACT

This review paper focuses on the recent progress made in heat pump drying (HPD). An introduction is given on the operating and working principle of the heat pump drying system. Advantages and limitations of HPD are outlined. Two industrial applications of HPD, food and timber, are discussed. New developments in HPD are presented. A section discussing the potential of incorporating advanced heat pump cycles for drying application is also presented. Some possible new hybrid HPD technologies, e.g., radio frequency and infrared assisted HPD are discussed in terms of their industrial potential. Opportunities for further R&D to achieve better product quality and energy efficiency are identified. A new heat pump dryer design is also described to demonstrate the new trend in versatile heat pump design.     

A Study of the Power Density Distribution generated during the Combined Microwave and Convective Drying of Softwood

ABSTRACT

Investigations into new and innovative drying strategies can lead to the development of more efficient and effective drying processes. The commercialisation of these processes would prove invaluable to the drying industry as a whole and the associated technology would generate worldwide interest. Combined microwave and convective drying is one such process which offers great potential, with benefits that include : reduced drying times and increased drying rates; volumetric heating; higher fluxes of liquid to the drying surface; high temperature and internal pressure buildup within the material which enhances the overall moisture migration rate; and preferential heating of wetter areas. Numerical simulation can elucidate on the intricate details of the heat and mass transfer henomena that occur during the drying process, thus eliminating the need for performing numerous time consuming and expensive experiments. The simulations can predict the evolutionary behaviour of the moisture, temperature and pressure distributions, and can provide a detailed analysis of how microwaves interact with materials during drying and heating operations at a fundamental level. The research presented in this paper uses a comprehensive mathematical model to study the behaviour of the iniernal microwave power density distribution that is generated during the microwave enhanced convective drying of softwood. The configuration understudy concerns a plane wave microwave source irradiating the wood in the transverse direction.     

THE INFLUENCE OF COMPRESSION WOOD ON THE DRYING CURVES OF PINUS RADIATA DRIED IN DEHUMIDIFIER CONDITIONS

ABSTRACT

Measurements of drying rate of Pinus radiata at 55°C and 30% RH are presented. The data, which has been used to establish empirical models for P. radiata under dehumidifier drying conditions, was obtained in four drying runs in a drying tunnel, each yielding detailed drying curves for twelve sample sapwood boards of size 350 × 100 × 50 mm. Compression wood was found to have a significant effect on the drying rate curve, giving lower drying rates at 40–100% MC. This effect is interpreted through the use of a numerical multiple-mechanism two-zone model and quantified by using best-fit diffusion parameters from an isothermal diffusion model. A positive correlation was discovered between the moisture diffusion coefficient and initial moisture content, a strong indicator for the presence of compression wood. In the two-zone model, the compression wood effect was replicated by using a tenfold decrease in permeability to liquid flow. Attributes of compression wood that could cause reduced permeability include an increased proportion of latewood, narrower lumen, and a scarcity of bordered pits on the radial walls of longitudinal tracheids.     

THE EFFECT OF HETEROGENEITY ON WOOD DRYING,PART I: MODEL DEVELOPMENT AND PREDICTIONS

ABSTRACT

A two dimensional model which can predict the effects of the anisotropy and heterogeneity on the transport phenomena which occur in wood during drying is developed. It is shown that the appropriate driving potential for moisture transport is the ratio of the moisture content to the driving potential. In its one dimensional form, die model results compare favorably with experimental measurements for drying in the radial direction. In its two dimensional form the model is used to predict drying in a direction midway between the radial and the tangential. In this case free water moves in a diagonal direction because the low density earlywood dries faster than the latewood during the early stages of drying. The result is significant gradients in moisture content, not only in the drying direction, but also in the direction parallel to the drying surface.     

Industrial Experience in Pressurized Steam Drying of Beet Pulp,Sewage Sludge and Wood Chips.

ABSTRACT

Drying in superheated steam under pressure gives possibilities for pollution free drying, energy recovery. reduced drying time and a very compact drying equipment.

Products are dried i n a pressurized cellular fluid bed by super heated steam blown through the cells. Above the fluid bed dust is separated before the steam is reheated and recirculated to the cells by a fan. The evaporated water leaves the dryer as usable steam at e.g. 3 bar g, and full energy recovery is obtained. Today 14 plants have been builtor are under construction, with capacities from 2 to 40t / h water evaporation.

This technology is suited for drying of beet pulp, residues from starch production of corn or wheat. sludge. spent grain. brown coal. wood chips and bark.

The technology shows great options for integration in steam systems in various industries, whereby drying is possible without using primary energy and pollution is avoided.     

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.     

OPTIMIZATION OF A WOOD DRYER KILN USING THE MIXED INTEGER PROGRAMMING TECHNIQUE: A CASE STUDY

Abstract

When wood is to be utilized as a raw material for furniture, buildings etc., it must be dried from approximately 100% to 6% moisture content. This is achieved at least partly in a drying kiln. Heat for this purpose is provided by electrical means, or by steam from boilers fired with wood chips or oil. By making a close examination of monitored values from an actual drying kiln it has been possible to optimize the use of steam and electricity using the so called mixed integer programming technique Owing to the operating schedule for the drying kiln it has been necessary to divide the drying process in very short lime intervals i.e., a number of minutes. Since a drying cycle takes about two or three weeks, a considerable mathematical problem is presented and this has to be solved.     

A Method for Studying Dynamic Sorption Behavior of Small Wood Samples

A method for determining mass changes (～ 0.0001 g) in small (～ 4-mm cubes) wood samples using the resonance of small mechanical springs mounted on piezoelectric sensors is presented. The size of the sample allows individual heartwood or sapwood, earlywood, latewood, and compression wood samples to be measured in an oscillating relative humidity environment and compared in terms of time constant and moisture gain. Since the apparatus is cost effective, many replicates can be achieved in one experiment. The use of small wood samples such that all tracheids are exposed removes the effect of structure on bound water transport. This, in combination with the ability to measure the response to small perturbations in relative humidity, presents the possibility to determine gaseous diffusion coefficients as a function of moisture content for individual wood types.

Preliminary, representative results of a study comparing the effect of drying temperature on sorption behavior of wood dried at 20, 90, and 150°C are presented.     

MOISTURE MOVEMENT ON DRYING SOFTWOOD BOARDS AND KILN DESIGN

ABSTRACT

This paper provides an overview of present understanding of how moisture can move through softwood boards, as a basis for determining kiln-seasoning strategies. Moisture in green wood is held essentially unbound, whereas below fibre saturation it is bound to a variable extent to the fibre walls. Sapwood, which is that part of the timber used for the transport of liquid nutrients, contains more moisture than physiologically inactive heartwood. Sawing the felled log creates a moisture-denuded layer at the damaged exposed surfaces. These features have a profound influence on the way that moisture can be removed on drying. Superimposed are differences arising from seasonal variations in the growth of wood between earlywood and latewood, which have different moisture permeabilities. When the width of the annual growth ring is relatively large compared with the board dimensions, moisture movement and the development of drying stresses depend markedly upon the sawing orientation relative to the grain direction. Quarter-sawn boards dry more uniformly (in the direction normal to the drying surfaces), but more slowly than flat-sawn boards. Most timber boards are stacked and then dried in box-shaped kilns. The uniformity of drying depends on the goodness of this stacking and on a uniform airflow being presented to the inlet face of the stack. Some non-uniformities can be mitigated by periodic reversals of the airflow direction through the stack and by overdrying the majority of boards to reduce wet spots, but there are limits, while overdrying reduces kiln capacity. Attention to aspects of the kiln geometry can reduce the fan-energy requirements and shorten the drying time, with a more uniform moisture content through out the kiln load.     

A Comparison of the Drying Simulation Codes Transpore and Wood2D which are used for the Modelling of Two-Dimensional Wood Drying Processes

ABSTRACT

This study is devoted to investigating the heat and mass transfer phenomena that occur during the convective drying of wood at high temperatures. A comparison will be made between an existing two-dimensional computer code known as Transpore. which was developed by Perre in France, and another two-dimensional code which was developed independently by Turner in Australia. Both numerical codes use a comprehensive set of macroscopic equations to describe the drying process, and most importantly treat the anisotropic behaviour of the wood. The porous medium is defined by three state variables: temperature, moisture content and gaseous pressure and the numerical simulation codes allow the evolution of the distributions of these state variables to be analysed throughout the drying process. The numerical investigation presented in this research work will compare the results obtained from both simulation codes and comments will be made on their consistencies. The influence that the drying air characteristics (moist air and super-heated stream) have on the overall drying kinetics, together with the effect that varying the mesh structure or changing the relative permeability curves have on the results will be throughly scrutinised.     

COMPARISON OF EXPERIMENTAL RESULTS WITH ANALYTICAL SOLUTIONS AND TWO DIHENSIONAL MODEL OF OAK DRYING IN AN EVACUATED KILN

ABSTRACT

In continuation of a series of tests, the original results of oak drying in an evacuated kiln are presented here for different plate temperatures and for various pressures in the kiln. These results include more particularly the drying curves, the evolution of temperature, of moisture and of pressure in and on the wood. They evidence the pressure and the levels of temperature occurring in the wood during the drying period.

These results also allow the development of two types of drying models a simple monodimensional model of drying curves from the analytical solutions of the equations of water diffusion in the wood and, moreover, a model, in two dimensions, of temperature, moisture and pressure fields in the wood by applying the finite element method. The boundary conditions of the second model can be fixed with precision thanks to the results of the first model. In both cases, the proposed solutions are justified by experimental results.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

ABSTRACT

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

Design of Solar–Dehumidification Wood Drying Kiln Characterized by High Capacity and Temperature

ABSTRACT

Solar–;dehumidification wood drying kilns attract more and more interest due to their energy saving characteristics. However. such dryers are only available for small effective drying volume capacities. less than 30 m³, and for low drying temperature under 65 °C, for Chinese domestic products. In fact, wood drying kilns over 60 m³and high drying temperatures upto 95 °C or even higher are commonly desired from economic and process technology points o f view. For instance, if the drying temperature is below 60 °C. a long drying period will result in high operational costs and may also cause mould as well colour changes on the lumber surface. It is also known that the anti- decay ability of the seasoned lumber is lhus reduced. As such. the design of the solar–dehumidification wood drying kiln with an effective drying volume capacity of 60 m³ and a drying temperature upto 95 ⁰C was made. Two compression processes for dehumidification and heat pump systems were adopted. The refrigerant of R1⁽²⁾. which has low RODP and RGE, was utilized as the actuating medium. The values of COP. PER and SMER were 4.42. 1.19 and 3.08 kg ( H20) /( kWh) for the dehumidification system. The valucs of COP and PER were 3.05 and 0.824 for the heat pump (heat supply) system. The averaged total COP and PER over the both were 3.74 and 1.01 respectively.     

FAST AND SLOW GENERATION OF A MULTILEVEL CONTROL FOR THE WOOD DRYING PROCESS

ABSTRACT

The dynamic characteristics of the wood drying process were represented by an actuator network and an internal model. These two subsystems are composed of a set of ordinary differential, partial differential and algebraic equations with adjusting operating functions. The formulation of the operating functions is based on interaction between air and the lumber board in a semi-industrial drying kiln. Using this internal model, it was possible to establish a state observer, a state predictor, and a reference model for which the boundary conditions were generated by the actuator network. Simulation results were obtained with a C++ software package. In order to implement the optimization techniques to the wood drying process, the two-level control structure was proposed and tested for a minimum drying time as a performance criterion. Numerical results confirm that the internal model and its operating functions were properly chosen.     

Evaluation of the usefulness of serial combination processes for drying of apples

Abstract

Serial combination drying processes are currently studied as alternatives to conventional drying processes. Compared to freeze-drying (FD), serial combination drying appears to be faster and less expensive, while providing better product quality than hot-air drying (HAD) and microwave-vacuum drying (MVD) as previously demonstrated for carrots. Using the example of carrots, it has also been shown that the drying front moves radially outwards over the course of FD. This unexpected behavior was suggested to originate from the carrots’ heterogeneous structure. It was hypothesized that apple pieces behave differently. Here, this hypothesis was proven by using micro-computed tomography measurements of partly freeze-dried apple pieces. In order to improve the drying process of apple pieces, several single and combination drying processes were carried out. Processes were evaluated by using drying time and sample quality as relevant parameters. Sample quality was determined by analyzing the 3D-structure, rehydration behavior, color, and ingredient retention. Results showed that single MVD is a well suitable drying technique for apple pieces, producing dried products of equal quality to FD. Different from carrots, serial combinations are not necessary to improve the quality of dried apple pieces. Nonetheless, especially a combination of HAD and MVD was useful to obtain specific structures such as puffed pores that did not result from single MVD.