DETERMINATION OF THE EVAPORATION OF WATER DURING WOOD DRYING BY MEANS OF HEAT FLUX MEASUREMENT

ABSTRACT

Since the only measured value that is derived from the wood for controlling the kiln drying process is the mean moisture content, it is essential to develop new techniques for the measurement of additional process parameters. When the drying rate, which could be such an additional parameter, is seen in conjunction with other process variables, conclusions on the instantaneous drying behaviour of the wood are possible. A simple and practical way for determining the drying rate is based on the relationship between the drying rate and the heat-flux for evaporation. A measurement of the heat-flux by means of a heat-flux sensor allows the calculation of the drying rate.     

INTERRELATION BETWEEN HEAT AND MASS TRANSFER DURING DRYING OF WOOD

ABSTRACT

Continuous measurement and recording of the core temperature in wood samples during convective drying in an airstream reveals that the temperature difference between the airstream and the core of the wood reflects almost quantitatively the influence of the external conditions and the characteristic features of the drying wood material on the course of the drying velocity. Conclusions concerning the drying practice of timber kiln-drying which arise from the experimental findings are discussed.     

INTERRELATION BETWEEN HEAT AND MASS TRANSFER DURING DRYING OF WOOD

Continuous measurement and recording of the core temperature in wood samples during convective drying in an airstream reveals that the temperature difference between the airstream and the core of the wood reflects almost quantitatively the influence of the external conditions and the characteristic features of the drying wood material on the course of the drying velocity. Conclusions concerning the drying practice of timber kiln-drying which arise from the experimental findings are discussed.     

THE RELEASE OF MONOTERPENES DURING CONVECTIVE DRYING OF WOOD CHIPS

ABSTRACT

The release of volatile organic components (VOC) during high temperature convective drying of wood chips was studied experimentally and theoretically. The drying medium was superheated steam with a pressure of two bar. Two different temperature levels of the drying medium, 160 and 180 °C, and two different materials, Scots Pine and Norway Spruce, were investigated. It was found that the main components released consist of various types of monoterpenes, with α-pinene dominating in each of the two materials. The amount released is dependent on the drying temperature as well as the time of the drying process.

In order to describe the release rate of monoterpenes during drying, two separate models, called the communicating and the non-communicating model respectively, were developed. The mechanisms included for the transport of monoterpenes are, in the communicating model, transport by diffusion and with the advective gas and liquid flow within the tracheids and, in the non-communicating model, diffusion within the resin canal system.

The results obtained using the communicating model largely overpredict the experimental results. To avoid this rapid release, additional mass transfer resistance for the transport of monoterpenes between the two canal systems could be introduced. The non-communicating model was found to be useful in explaining the release rate when drying spruce. This model, however, can not distinguish between the two temperature levels studied.     

MODELING VACUUM-CONTACT DRYING OF WOOD: THE WATER POTENTIAL APPROACH

A two-dimensional mathematical model for vacuum-contact drying of wood is presented. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady state conservation equation of dry air. Most of the model parameters were determined during independent experiments. The set of equations is then solved in a coupled form using the finite element method. The validation of the model is performed using experimental results obtained during vacuum-contact drying of sugar maple sapwood. The experimental and calculated data are in good agreement. Nevertheless, some discrepancies are observed which can be attributed to the boundary conditions used and to the fact that heat transfer by convection was neglected.     

MODELING VACUUM-CONTACT DRYING OF WOOD: THE WATER POTENTIAL APPROACH

ABSTRACT

A two-dimensional mathematical model for vacuum-contact drying of wood is presented. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady state conservation equation of dry air. Most of the model parameters were determined during independent experiments. The set of equations is then solved in a coupled form using the finite element method. The validation of the model is performed using experimental results obtained during vacuum-contact drying of sugar maple sapwood. The experimental and calculated data are in good agreement. Nevertheless, some discrepancies are observed which can be attributed to the boundary conditions used and to the fact that heat transfer by convection was neglected.     

NONDESTRUCTIVE MEASUREMENT OF MOISTURE DIFFUSION COEFFICIENT IN WOOD DRYING

ABSTRACT

The computer tomography(CT image of wood was analyzed to estimate the moisture content from CT number. Comparing moisture contents, densities and CT numbers, moisture distributions were nondestructively determined by CT numbers detected with the scanner.

By periodically measuring CT images of the same cross section of specimens in drying, change of moisture distribution was computed. The coefficients of moisture movement at given positions inside the specimen were directly calculated from Fick's Law with known values of moisture content change, distance and drying time.

The diffiion coeficient was described as a function of various driving forces of moisture content, partial vapor pressure and chemical potential of moisture in wood. The results were comparable to data in the literature. Furthermore, it is shown that diffusion coefficients based on moisture content varies as a curve with the maximum value at about 15% m.c. during drying, and that the moisture content is actually adaptable to the driving force for moisture movement in wood because of capability over wide moisture content range.     

NONDESTRUCTIVE MEASUREMENT OF MOISTURE DIFFUSION COEFFICIENT IN WOOD DRYING

The computer tomography(CT image of wood was analyzed to estimate the moisture content from CT number. Comparing moisture contents, densities and CT numbers, moisture distributions were nondestructively determined by CT numbers detected with the scanner.

By periodically measuring CT images of the same cross section of specimens in drying, change of moisture distribution was computed. The coefficients of moisture movement at given positions inside the specimen were directly calculated from Fick's Law with known values of moisture content change, distance and drying time.

The diffiion coeficient was described as a function of various driving forces of moisture content, partial vapor pressure and chemical potential of moisture in wood. The results were comparable to data in the literature. Furthermore, it is shown that diffusion coefficients based on moisture content varies as a curve with the maximum value at about 15% m.c. during drying, and that the moisture content is actually adaptable to the driving force for moisture movement in wood because of capability over wide moisture content range.     

TEMPERATURE,DISTRIBUTIONS AND HEAT TRANSFER DURING THE DRYING OF LUMBER

ABSTRACT

This paper describes techniques that have been developed for accurately measuring the surface temperature of drying lumber using a radiation thermometer, and interior temperatures at various depths using 30 gauge thermocouples. Methods for calculating heat transfer coefficients during the drying of lumber are also described. Experimental results, showing surface and interior temperatures, and moisture content, as functions of drying time are presented.     

THE DRYING OF SMALL WOOD PARTICLES

Small particles of Douglas fir of characteristic size 0.2 to 1cm have been dried by flowing the wood particles in vitiated air in a long vertical tube. Drying rate, dM/dt, was determined by measuring the increasing humidity of the transporting gas flow, and by measuring the absolute velocity of the particles by a radioactive isotope tagging method. Sawdusts of three sizes and flakes were examined. Moisture content (dry basis) was varied from approximately 2 to 125%. Most of the results were obtained for gas temperatures between 350 and 550 K.

Upon injection into the flowing hot gases, the wood particles initially dried rapidly as they encountered high relative temperature and velocity, and released their surface moisture. Following· this transient regime, the particles dried more gradually accord-     

DRYING OF WATER GELS: DETERMINATION OF THE CHARACTERISTIC CURVE OF AGAR-AGAR

Agar-agar is a polysaccharide extracted as a hydrocolloid from red seaweed, whose gels are homogeneous, stable and transparent. The characterization of ternary equilibrium and mass transfer kinetics in the agar-water-air system is essential for designing operations in the extractive process as well as for ascertaining the behaviour of these gels and sols during evaporation. humectation and swelling.

In this work, the convective thermal drying of agar gels using a laboratory scale discontinuous drier has been studied, the raw data being interfaced to an ancillary microprocessor and generalized according to the characteristic curve formalism useful in the design of full scale driers.     

MEASUREMENT OF ENAMEL-SLIP CONSISTENCY BY MEANS OF THE BROOKFIELD VISCOSIMETER*

An instrument commonly employed to measure the viscosity of truly viscous liquids was tested to determine its applicability for evaluating consistencies of enamel slips which are solid suspensions in a water medium. Within wide limits of specific gravity differences and variations in set resulting from electrolyte additions, the Brookfield vis-cosimeter afforded comparable information with the pick-up and slump tests employed as references.     

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

ABSTRACT

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

HEAT AND MASS TRANSFER DURING DRYING OF GRANULAR PRODUCTS BY COMBINED CONVECTION AND CONDUCTION

Numerical simulation of grain drying in a vertical cylindrical bed has been carried out with an imposed hot air flow and a conductive heat flux at the wall.

The model equations are numerically solved using a finite volume method. The numerical simulation gives the time and space evolution of temperature when the lateral area of the cylinder is heated by a constant density flux and a constant temperature. The influence of different parameters (essentially the ratio of heat flux to the heat capacity of flow, and the dryer geometry) on the relative moisture content and the drying time is examined.     

HEAT AND MASS TRANSFER DURING DRYING OF GRANULAR PRODUCTS BY COMBINED CONVECTION AND CONDUCTION

Abstract

Numerical simulation of grain drying in a vertical cylindrical bed has been carried out with an imposed hot air flow and a conductive heat flux at the wall.

The model equations are numerically solved using a finite volume method. The numerical simulation gives the time and space evolution of temperature when the lateral area of the cylinder is heated by a constant density flux and a constant temperature. The influence of different parameters (essentially the ratio of heat flux to the heat capacity of flow, and the dryer geometry) on the relative moisture content and the drying time is examined.     

ANALYSIS OF HEAT AND MASS FLUXES DURING MICROWAVE DRYING

Abstract

The kinetics of combined convective and micro-wave drying presents universally more than the two habitual drying phases. Superimposed to a general heating of the product, a brutal and short acceleration of the drying rate raises all flux densities at a very important level. This is particularly evident with the class of product we used in this study : non porous polymeric gels.

Moreover, an unusual correlation between the kinetics and the surface temperature around the micro-wave specific phase might appear : unexpectedly, the evaporation flux density decreases when the surface temperature increases noticeably.

In order to elucidate this apparent complexity and understand the underlying physical phenomena, we analysed term by term all fluxes according to their origin. The splitting was achieved through mathematical modelling and partially validated by the recording of temperatures and mass fluxes.

Usually with convective drying, all rate changes are governed by the state o f the product : hygroscopicity and porosity development. Here in the case of combined micro-wave drying, all accidents on the drying curves are determined by the heat generation and the heat transfer situation. The constant rate period is also governed by the balance of heat fluxes. The accelerated period corresponds to a heat wave arriving to the surface. Most of the time, the product gives out heat to the surrounding air : this heat is lost for the drying but this cooling effect may he used for the protection of the sample.     

THE DETERMINATION OF POROSITY OF CHINA BY THE WATER SOAKING METHOD1

Comparative tests show that prolonged soaking is more effective than boiling in attaining saturation of nearly vitrified ware, and a period of 4 days' immersion in cold water is recommended.     

STUDY OF HEAT AND MASS TRANSFER DURING IR DRYING OF PAPER

ABSTRACT

A mathematical model has been developed to study the drying of paper using a gas-fired IR dryer. The model accounts for various phenomena : water and vapour mass transfer, conduction, convection and radiation heat transfer. The phenomenological equations are solved with a finite difference scheme, including a modified upwind differencing scheme to account for water migration within the paper sheet. The simulation results illustrate the basic underlying phenomena involved in IR paper drying and can be instrumental to the engineer to make the detailed analyses of such a drying process. A sensitivity analysis has shown that the drying rate is most sensitive to parameters governing the IR beat transfer process whereas the paper sheet temperature is most sensitive to parameters governing the mass transfer process with the surroundings.