PROCESSES RELATED TO DRYING: PART II USE OF THE SAME MODEL TO SOLVE TRANSFERS BOTH IN SATURATED AND UNSATURATED POROUS MEDIA

From the mathematical formulation presented in part I, a numerical code is developed to simulate heat and: mass transfers in porous media. The aim of this· tool is to understand and to improve each process related to drying. The association of a comprehensive set of equations with a efficient 2-D computer code allows us to predict the comportment of several porous media even if submitted to severe drying conditions. A few runs have been selected with special attention paid to the effect of internal gaseous pressure:

Convective drying of softwood at high temperature illustrate the typical two-dimensional transfers that occur in an anisotropic medium.

Microwave drying of light concrete pinpoints liquid expulsion of water which is driven by the pressure due to internal heating.

Finally, appropriate physical behaviours of a bed of glass spheres allows one to deal with simple processes for which full saturation occurs.     

VACUUM DRYING OF OAKWOOD :MOISTURE STRAINS AND DRYING PROCESS

After presenting the characteristics and the data acquired in an industrial evacuated kiln, a simplified analysis of heat and mass transfers is proposed. This analysis is based on the existence of a evaporization front determining two zones in the longitudinal direction :

-a dried zone in which moisture is less than 30 %

-a wet zone in which moisture is still at its initial value.

Such a hypothesis allows to study transfers transversally first, then longitudinally. Both equation systems ore linked by conditions of continuity for mass and energy.

This study allows to determine the shapes of the temperature and pressure curves in the longitudinal direction. The linearity of the variation of the average drying velocity versus the average moisture content of the board is also proved. Finally, the modeling of mechanical phenomena thanks to a finite element program shows the rupture zones appearing during the drying process.     

Food Drying and Dewatering

Food drying and dewatering raises a growing interest because of increasing requirements in quality, specially in the production of ingredients and additives for food formulation. Heat and mass transfers, as well as mechanical phenomena and reactions kinetics induced by these transfers must be more and more carefully controlled during drying and storage.

This chapter relates recent advances in

- drying of solids

- spray-drying

- drum-dryine

- superheateded steam drying

- osmotice dehydration

- hot oil immersion drying     

A Control-Volume procedure compared with the Finite-Element method for calculating Stress and Strain during Wood Drying

This paper proposes a suitable method for calculating drying stresses. This is an application of the finite element method (FE) and the so-called control volume (CV) method. The latter has been chosen because most of coupled heat and mass transfer codes are based on CV.

A parallel between the resulting formulation and a particular case of FE method (quadrilateral four-nodes element and Galerkin's method) has been established Both analytical expressions and numerical results were compared. Furthermore, it is shown how to choose interpolation coefficients in CV procedure in order to get exactly the results obtained with FE.

Such a stress calculation has been added to the code TRANSPORE in order to obtain a complete drying code which, in addition to heat and mass transfers, solves stress and strain due to shrinkage, Interesting and promising simulations of non-symmetric convective drying are presented. Indeed, for this drying configuration, the stresses induce a global curvature of the section.     

PROCESSES RELATED TO DRYING: PART I, THEORETICAL MODEL

The aim of this paper is to summarize ten years of work in order to model the drying of solids. Nowadays there is a general agreement amongst the various researchers worldwide. If the porous medium is viewed at the so-called “macroscopic” scale (that is to say when it is viewed as a continuous and homogeneous equivalent medium) a set of three partial derivative equations is derived, three state variables being necessary to thermodynamically describe the medium (temperature, moisture content and total pressure in the gaseous phase for example).

After a short historical survey of the modeling of drying, we want to obtain this set of equations drawing the readers attention to three particular points : the importance of the air mass balance, the right way to write the energy equation (and the associated boundary conditions) and the taking into account of the adsorbed water.

A second part of this article [I] will be devoted to the numerical solution of this problem and will illustrate these points.     

HEAT AND MOISTURE TRANSFER IN CAPILLARY-POROUS BODIES SOME EXPERIMENTAL METHODS OF INVESTIGATION

This paper presents a synthesis of our scientific activity in the area of heat and moisture transfers in capillary-porous bodies. The materials which have been under investigation are essentially in the field of Civil Engineering but our ways of experimentation may be generalized.

Transfers may be characterized by two independent variables : temperature and water content. We have been concerned by the achievement of measurement technics of these quantities and of perturbation technics of materials at equilibrium.

When comparing a fitting model and the temperature or water content response to such perturbations, it is possible to infer the values of parameters which are significant. Then, the experimental results must be compared with results obtained by other authors or by means of different methods.

In the future, the assumption of an unstrained porous skeleton will not be kept any longer and the coupling of transfers with mechanical phenomena will be considered.     

DRYING OF BIOLOGICAL PRODUCTS WITH HIGH WATER CONTENT IN SOLAR REGIME. APPLICATION TO THE POTATO DRYING.

The experimental results on the drying of biological porous media (potato) are modeled using the Luikov equations, under simulate insolation conditions. The temperature of the environment was regulated, within the typical range corresponding to hot-temperature and rather wet climate (Argentina's Humid Pampa region).

The comparison between measurements and calculations gave good agreement. The present study confirms independently the identification of several characteristic parameters of the product obtained under constant conditions in a previous work [1].     

Drying Characteristics of Porous Materials in a Fluidized Bed under Reduced Pressure

The drying of porous materials immersed in the fluidized bed under reduced pressure was performed, and the results were compared with those of hot air drying. The pressure in drying chamber was changed (5.0-101.3 kPa) and the effect of it was examined.

The temperature of the sample center becomes lower as the pressure in drying chamber decreases, and the temperature in fluidized-bed drying is higher than that in hot air drying at the same pressure. The effect of pressure in drying chamber on the sample temperature is significant for different temperatures of drying gas.     

COMPARISON OF SUPERHEATED STEAM AND AIR OPERATED SPRAY DRYERS USING COMPUTATIONAL FLUID DYNAMICS.

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase.

The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air Considering that there is no risk of powder ignition in superheated steam, we choosed a rather high inlet temperature (973 K). For the simulation, drop size spectrum is represented by 6 discrete droplets diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice.

It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Using superheated steam or air as a drying medium shows only slight differences in flow patterns. Except for the recirculation which is tighter in steam.

The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a “cool” zone which is narrower in steam than in air. Finally, the panicle deposit problem seems to be more pronounced in air than in steam.

Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular. under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

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.     

Steam Drying - Modelling and Applications

The concept of steam drying originates from the mid of the last century. However, a broad industrial acceptance of the technique has so far not taken place. The paper deals with modelling the steam drying process and applications of steam drying with in certain industrial sectors where the technique has been deemed to hove special opponunities.

In the modelling scction the mass and heat transfer proceases are described along with equilibrium, capillarity and sorption phenomena occurring in porous materials during the steam drying process. In addition existing models in the literslure are presented.

The applications discussed involve drying of fuels with high moisture contcna, cattle feed exemplified by sugar beet pulp. lumber. paper pulp. paper and sludges.

Steam drying is compared to flue gas drying of biofuels prior to combustion in a boiler. With reference to a current insrallation in Sweden. the exergy losses. as manifested by loss of co-generation cupacity. are discussed. The energy saving potentid when using steam drying of sugar beet pulp as compared to other possible plant configurations is demonstrated.

Mechanical vapour recompression applied to steam drying is analysed with reference to reponed dau from industriul plsnts. Finally. environmcntul advantages when using steam drying are presented.     

Drying of Transformer Board

The internal insulation in shell type power transforms is usually ensured by the stacking of plates of board impregnated with oil. A board is in equilibrium with the atmosphere at a moisture content varying from seven to eight percent by weight, but, when the insulating function is required, the water content must be lower than 0.5 %. The aim of the present work is the understanding and the modelling of transformer boards drying.

The first part of this work presents experimental studies showing that the drying of transform board follows two regimes :

- a fast regime in the fim period

- a slow regime, up to 99 percent of dryness.

The effect of heating and the influence of the board thickness on the drying rate also studied. In the second part of this paper, we presnt a model capable to represent the observed kinetics of transformer board drying. This model is based on the diffusion of water vapour in the gaseous phases combined with the transfer of water vapor from the fibres to the -us phase. The due of the average global transfer coefficient can be deduced from the slope of the curve 1 f(drying time), where x     

CRAPE DRYING : FROM SAMPLE BEHAVIOUR TO THE DRIER PROJECT

A drier project necessitates the efficient formulation of the behaviour of product samples. When the temperature gap is great, the drying rate must make explicit the influence of the thnc air parameters : temperature, moisture, velocity. The case of the grape demonsnates that the adimensional expression formulated by Van Meel and Keey. must be completed in order to be adjusted to experiments.

The hot air drier model composed of the equations of conservation and thin layer drying rate can sometimes be simplified into a form of analytic integral equation around the drying rate. W e drying can be considered generally as adiabatic.

Two examples of grape driers are presented and show the value of simplified tools for the project. In a tunnel drier with a high air temperature one can speak of the celerity of a drying front which progress along the trolleys. In a short drier connected to an agricultural solar collector, the integration of the drying rate takes into account the variation of the meteorological data.     

Effect of Fluidizing Particle on Drying Characteristics of Porous Materials in Superheated Steam Fluidized Bed under Reduced Pressure

The hygroscopic porous particle was used as the fluidizing particle for the superheated steam fluidized bed drying under reduced pressure. A relatively large material was immersed in the fluidized bed as the drying sample. The drying characteristics of the sample were examined experimentally and the results were compared with those in the case of inert particle fluidized bed.

The water transfer from the sample to the fluidizing particle bed in the case of hygroscopic porous particle facilitated the drying regardless of pressure and temperature in the drying chamber. The increment degree of the sample temperature at the earlier period of drying was smaller in the case of hygroscopic porous particle than in the case of inert particle, and the phenomenon was more remarkable in the case of superheated steam than in the case of hot air.     

THE USE OF LOCAL THERMODYNAMIC PATH FOR QUALITY PROBLEMS DURING CONVECTIVE DRYING

The pharmaceutical drug under investigation in this paper has two hydration states with several polymorphic forms. The two hydration states are tetrahydrate and monohydrate. Two polymorphs of monohydrate are in competition. Process must lead to the metastable therapeutic form (TF) although the non therapeutic form (NTF) is the stable one. The sensitive stage is drying

From desorption isotherms coupled with infrared spectroscopy, an equilibrium diagram of the different forms versus the external conditions (temperature and relative humidity) is performed. A macroscopic study achieved on tangential convective drying shows that the equilibrium diagram does not insure to obtain the metastable form after drying. It is deduced from an experimental analysis of internal temperature and water activity that the final form depends on the local thermodynamic path. Indeed, if NTF favourable conditions occur within the product during the process, this irreversible form remains although the final form predicted by the diagram is TF

It is concluded that NTF is promoted by a slowing down of transfers and an enhanced water vapour removing (greater exchange surface).     

VACUUM CONTACT DRYING OF SELECTED BIOTECHNOLOGY PRODUCTS

The paper is mainly concentrated on investigations of the vacuum contact drying of the following biomaterials: (1) beer yeast, (2) whey and its components: proteinaceous liquid, lactose, post-lactose liquid, (3) saturated solution of the citric acid, (4) post-fermentation broth of the fodder antibiotic bacitracin. The investigations of vacuum contact drying process in which dried material adheres closely to a hot surface were performed. In order to make possible the determination of optimal process parameters the measurements were performed for various constant temperatures of the hot surface, total pressures in vacuum chamber and layer thickness of biomaterials.

The effective thermal properties for dry layers of selected biomaterials depending on temperature and total pressure were also experimentally determined.

A mathematical model of the vacuum contact drying for most intensive regime of its performance i.e. boiling one was presented. It will enable the process simulation and estimation of the dryers efficiency depending on process parameters.

In addition, investigations of beer yeast viability depending on the hot surface temperature and the total pressure were performed.     

HIGH TEMPERATURE DRYING OF WOOD SEMI-INDUSTRIAL KILN EXPERIMENTS

We briefly introduce the main results obtained in our laboratory about high temperature drying kinetic of softwood and hardwood. We point out during drying the acceleration effect of pressure gradient in vapour phase which develops in the wood.

Then we give the main experimental results carried out on a small industrial kiln which can dry wood boards with moist air and superheated steam at high temperature till 180°C and velocity above to 6 m/s. The drying process is completely automated. Tested species are : beech, poplar, fir and maritime pine.

We show that the drying kinetics are almost the same using the industrial kiln or the wind tunnel and we define for different board thicknesses, temperatures and velocities the optimal sequences of the process. The influence on the quality of the pretreatment, the various drying periods and the post-steaming is of importance.     

Image Analysis, Homogenization, Numerical Simulation and Experiment as Complementary Tools to Enlighten the Relationship between Wood Anatomy and Drying Behavior

This paper has been written for a special issue devoted to the works done in France in the field of drying. Therefore, it aims, through numerous examples, to show the spirit which initiates and guides the research work of the Forest Products Unit at ENGREF : how more and more microscopic information can be included in the study of the drying process ?

Two steps are involved :

•microscopic observation and experiments to predict the macroscopic properties

•use of the macroscopic properties (the previous step + experiments carried our at the macroscopic scale) to predict the drying behavior.

In this approach, each prediction comes from a model, that means assumptions, formulations, calculations and then validation.

Several tools (image analysis, homogenization, measurement of microscopic and macroscopic properties, numerical simulation of heat and mass transfer, refined drying experiments) are used to study the drying of wood.     

THE INFLUENCE OF VACUUM ON DRYING OF PAPER

It is well known in the drying of paper that it is possible reduce the size of the dryer section and/or increase the drying capacity by using vacuum. Furthermore a smaller dryer section contributes to a decrease in the energy losses. However, the use of Minton vacuum dryers in the late 20's was never really successful. Especially with increasing machine speeds maintenance became a problem.

Vacuum drying leads to an improvement in the optical pro- perties of papers made from mechanical pulps. Some physical properties such as softness and porosity may also be improved. When the paper is pressed towards the hot surface under me- chanical pressure during vacuum drying a gain in mechanical properties can be achieved.

In the present investigation, the influence of heat transfer between the web and the hot surface as well as mechanical com-pression of the sheet during vacuum drying have been avoided by using an IR heat source. The results show that the main effect of vacuum is a reduced evaporation temperature. This allows the drying to reach its maximum rate faster. The lower temperature level during vacuum drying also makes cheaper energy sources avilable.