DEEP LAYER MALT DRYING MODELLING

In malt production drying operation plays an important role in the total processing cost, however there are not many studies on malt drying modeling and optimization.

In this paper a deep layer malt drying mathematical model in the form of four partial differential equations is presented.

To determine drying constants, malt thin layer drying experiments at several air temperatures and relative humidities were made.

The model were validated at industrial scale. The greatest energy savings, approximately 5 5% in fuel and 7.5% in electric energy, were obtained by an additional (and increased) air recirculation, which is carried out during the last 6 hours of the drying process and a significant decrease of air flow-rate during the last 6 hours of the drying process.     

Improvement of Industrial Drying of Natural Rubber Through Analysis of Heat and Mass Transfers

An analysis of convective drying of natural rubber in granule form. Exploitation of the mathematical model developed from analysis of internal transfers requires determination of the 'drying rate' and the 'energy exchange coefficient' between the material and the drying air. This was performed using a portable laboratory drier at the production site, avoiding the problem of changes in the material during transport. Analysis of the drying kinetics of material in thin layers led to modelling the drying rate at the temperatures, rates and relative humidity values encountered in industrial driers. The energy exchange coefficient was found by measurement of air and granule temperatures. Thick layer trials showed the suitability of the model for simulation of changes in moisture content and temperature of air and granules in a granular environment. The study as a whole led to the development of a computer program for simulating a zone drier. This software can be used to optimise the management of an industrial natural rubber drying installation.     

Improvement of Industrial Drying of Natural Rubber Through Analysis of Heat and Mass Transfers

ABSTRACT

An analysis of convective drying of natural rubber in granule form. Exploitation of the mathematical model developed from analysis of internal transfers requires determination of the ‘drying rate’ and the ‘energy exchange coefficient’ between the material and the drying air. This was performed using a portable laboratory drier at the production site, avoiding the problem of changes in the material during transport. Analysis of the drying kinetics of material in thin layers led to modelling the drying rate at the temperatures, rates and relative humidity values encountered in industrial driers. The energy exchange coefficient was found by measurement of air and granule temperatures. Thick layer trials showed the suitability of the model for simulation of changes in moisture content and temperature of air and granules in a granular environment. The study as a whole led to the development of a computer program for simulating a zone drier. This software can be used to optimise the management of an industrial natural rubber drying installation.     

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.     

MODEL SELECTI0N IN AIR DRYING OF FOODS

The purpose of this investigation is to compare various drying models with respect to (a) the accuracy in calculating the material moisture content and temperature versus time and (b) the computation time required.

Mechanistic as well as phenomenological heat and mass transfer models are considered. The mechanistic models are formulated by considering different combinations of mechanisms between (1) moisture diffusion in the solid towards its external surface (2) vaporization and convective transfer of the vapor into the air stream (3) convective heat transfer from the air to the solid's surface (4) conductive heat transfer within the solid mass. The phenomenological model incorporates the drying constant while the mechanistic models incorporate the mass diffusivity, the mass transfer coefficient in the air boundary layer, the thermal conductivity, and the heat transfer coefficient in the air boundary layer.

The proposed methodology is applied to experimental data of four vegetables, namely, potato, onion, carrot, and green pepper. The experiments involve three thickness levels, five temperatures, three water activities, and three air velocities. The results obtained concern (a) the standard deviations between experimental and calculated values of material moisture content andtemperature, which, in combination with the computation time, are the necessary information for model selection for a special application, and (b) the model parameter estimates which are necessary to use the selected model.     

MASS TRANSFER MODELING FOR VIRGINIA TOBACCO CURING

Four mathematical models, describing the moisture transfer during Virginia tobacco curing under varying air-temperature and humidity were tested using experimental data from an industrial dryer.

The model, based on the assumption that the major resistance to moisture transfer is at the air boundary layer and that the driving force is the water activity difference, was found to be the most accurate. The appropriate mass transfer coefficient was determined.

An empirical equation, predicting the equilibrium moisture content of Virginia tobacco as a function of temperature and water activity of the surrounding air, was proposed and experimentally tested.     

IMPINGING JET DRYER

In coating and gravure printing, an impinging jet nozzle with high thermal efficiency for drying of coated film was developed.

Trial production 0f 40 kinds of nozzle enables to develop a high-performance impinging jet nozzle with heat transfer coefficient 1.5 times larger than that of current slit nozzle, through measurement of heat transfer coefficient, visualizations of air flow and heat transfer, and measuremenu of jet velocity and turbulence distribution. The purpose of the trial production was to expand a range of high heat transfer and promote turbulence compared with the current nozzle.

Paying attention to mass transfer within gravure ink coated film, drying characteristic of the film was analyzed by numerical solution of a set of equations governing the drying process in which concentration dependencies 0f the diffusion coefficient and the equilibrium vapor pressure were considered.

Applying these analyses. an industrial scale dryer with excellent drying efficiency has finally been developed.     

DEVELOPMENT AND VALIDATION OF A PROCESS SIMULATOR FOR DRYING SUBBITUMINOUS COAL

Drying subbituminous coal has never been practiced commercially. The commercial dryers built to date have been designed for drying surface moisture in conjunction with upstream coal preparation facilities. This type of drying is mainly controlled by input energy and the basis of the design is an energy balance. In drying inherent moisture from subbituminous coal, the thermal conductivity of the coal and the diffusion of molecular water within coal particles impose limitations on the process conditions. Energy input and solids residence time in the dryer have to be controlled properly for simultaneously balancing the heat and mass transfer within the coal particles. Improper control of either parameter can cause fires and explosions during the key steps of the drying process—drying and cooling

In parallel to the Anaconda coal drying pilot plant program, a cross-flow, fluid-bed coal drying/cooling process simulator was developed for: (1) understanding the drying phenomena on an individual particle basis; (2) analyzing potential risks and safety limits, and (3) designing the Anaconda pilot plant program

The development of the process simulator was based on both first principles and laboratory data and can be divided into two phases:

1 Development of a semi-mechanistic drying model for Powder River Basin subbituminous coal employing an analytical solution of the diffusion equation

2.Formulation of a fluid-bed cross-bed cross-flow dryer/cooler simulator employing simultaneous heat and mass transfer

This model was validated against process variables data taken on a 4 tph pilot plant. An operable range, or process envelope, has been developed through the pilot plant experience and the process simulation study. Based on the model predictions, an uncertainly range was defined in the design recommendations of a pioneer coal drying plant in scale-up.     

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.     

INDUSTRIAL THROUGH-AIR DRYING OF NONWOVENS AND PAPER BASIC PRINCIPLES AND APPLICATIONS

The requirements for drying and thermobonding equipment working on the basis of the hot air flow-through system vary considerably due to the large variety of nonwovens qualities, production methods and required production speeds. It is therefore necessary to construct diverse drying systems in order to be able to offer appropriate machines for the desired purposes. Optimum heat and mass transfer from the penetrating air to the permeable material to be dried is ensured by through-air drying

Following Hans Fleissner's invention of the through-air system for drying of textiles and its practical application in the perforated drum dryer for more than 50 years a second engineering breakthrough for the paper and nonwovens industry has been achieved by Gerold Fleissner with the high-tech through-air drum

This paper presents various possibilities of air flow through the material and provide criteria to facilitate selection of the proper drying system     

RELATIONSHIP BETWEEN A SOLAR DRYING MODEL OF RED PEPPER AND THE KINETICS OF PURE WATER EVAPORATION (II)

In part one, a simple drying model of red pepper related to water evaporation was developed. In this second part the drying model is applied by means of related experiments.

Both laboratory and open air drying experiments were carried out to validate the model and simulation results are presented.     

DRYING OF GRANULAR PARTICLES IN A MULTISTAGE INCLINED FLUIDIZED BED WITH MECHANICAL VIBRATION

A mathematical model for the drying rate of granular particles in a multistage inclined fluidized bed(IFB) is presented from the standpoint of simultaneous heat and mass transfer, with taking the effect of mechanical vibration added vertically into consideration.

Steady-state distributions for the temperatures and concentrations of the particles and the heating gas, and for the moisture content of the particles are numerically calculated based on the present model. The calculated results show fairly good agreement with the experimental data, which were obtained from the drying experiments of brick particles in a three-stage IFB using comparatively low temperature air(40-60°C) as the heating gas.

It has been found within the range of the experimental conditions employed that, the mechanical vibration added vertically enhances the over-all drying rate of the particles and its effect can be considered equivalent to an increase in the air velocity.     

A DISTRIBUTED PARAMETER APPROACH TO THE DYNAMICS OF ROTARY DRYING PROCESSES

A nonequilibrium distributed parameter model for rotary drying and cooling processes described by a set of partial differitial equations with nonlinear algebraic constraints is developed in this work. These equations arise from the multi-phase heat and mass balances on a typical rotary dryer. A computational algorithm is devekped by employing a polynonial approximation ( orthogonal collocation) with a glotal splinc technique leading to a differential-algebraic equation ( DAE) system. The numerical solution is carried out by using a standard DAE solver.

The two- phase-flow heat transfer coelficient is computed by introducing a correction factor to the commonly accepted correlations. Since interaction between the falling particles are considered in the correction factor,the results are more reliable than those computed by assuming that heat transfer between a single falling particle and the drying air is unaffected by other particles. The heat transfer computations can be further justified via a study on the analogies between heat and mass transfer.

The general model devloped in this work is mathematically more ritorous yet more flexible that the lumped parameter models established by one of the authors (Douglas et al., (1993)). The three major assumptions of an equilibrium operation, perfect mixing and constant drying raic, are removed in the distributed parameter model.

The simulation results are compared with the operational data from an industrial sugar dryer and predictions from earlier models. The model and algorithm successfully predict the steady state behaviour of rotary dryers and collers. The generalized model can be applied to fertilizer drying processes in which the assumption of constant drying rate is no longer valid and the existing dynamic models are not applicable.     

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.     

CHANGES OF RHEOLOGICAL PROPERTIES OF APPLE TISSUE UNDERGOING CONVECTIVE DRYING

The aim of this work was to investigate rheological properties of apple cubes undergoing convective drying. Compression-relaxation test was used to follow changes of properties under investigation.

Raw apple appeared to be very heterogeneous material from the rheological point of view. The relationship between stress and strain was concave downwards and the concavity increased until water content of 2·5 g/g d.m. was reached. Then the relationship begun to straighten and for dry material developed stress was linearly dependent on strain. The resistance of deformation decreased with decreasing water content. Analysis of relaxation showed that the rate of relaxation increased with decreasing water content and the unrelaxed stress was smaller the lower was the water content.

Apple cubes undergoing drying are pictured as material composed of three compartments with different rheological properties. The outer layer is dry, inelastic and difficult to deform. Underneath is moist material with no turgor and relatively easy to compress, and the core with properties of raw apples. During drying the second layer grows in expense of the core and porous structure with many voids filled with air is formed. Share of each compartment in the volume of apple cube undergoing drying affects average rheological properties of the material.     

MODELISATION PRINCIPLES FOR DRYING OF GELS

When one wants to understand and model the drying of high water content gels, one needs to gather all thermal physical parameters and to derive a physical model based on independently measured parameters. We chose to work experimentally and theoretically on water diffusion and drying of spheres of polyacrylamide gel.

We have demonstrated the strong influence of vitreous transition on the relationship of water diffusivity with moisture content. Values of the diffusion coefficient and temperature shift factor for diffusion have been proposed.

Convective and micro-wave drying kinetics show the presence of a drying phase purely specific to radiation absorption. Finally, we have proposed a mathematical model containing independent physical parameters, which' not only predicts mass fluxes but also analyses critical physical phenomena.     

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.     

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.     

DRYING OF OPTICALLY SEMITRANSPARENT MATERIALS BY COMBINED RADIATIVE-CONVECTIVE HEATING

The objective of this work has been to basically elucidate the drying characteristics of an optically semitransparent material by combined radiative and convective heating. The experiments were conducted for a graphite suspension, a slurry of surplus activated sludge and a wet silica sand. The time-change of the drying rate as well as of the surface temperature of the brimfully wet material layer were measured under the step heating conditions using an infrared lamp bundle and a blast of hot air heated by an electric heater.

The experimental data obtained show satisfactory agreement with the calculated results from unsteady heat and mass transfer equations derived on the basis of a semitransparent drying model during the preheating and the constant drying rate periods.     

ATMOSPHERIC CONTACT DRYING OF GRANULAR BEDS WETTED WITH A BINARY MIXTURE

When regarding the atmospheric contact drying of granular beds wetted with a liquid mixture, both the drying rate and the selectivity of the process, i.e. the change of moisture composition, are of interest. The batch drying of a free flowing ceramic substance, wetted with a 2-propanol-water mixture, is investigated in a rotary dryer with heated wall and air flow.

The theoretical analysis is based on physical models for heat and mass transfer, moisture migration and particle transport, which are presented in examples.

The experimental and theoretical results show that higher selectivities can be achieved by reducing the particle size because of the lower liquid-phase mass-transfer resistance. An increase of the rotational speed leads to a higher drying rate with slightly decreased selectivity if the particles are sufficiently small, since contact heat transfer is enhanced.