HEAT AND MASS TRANSFER WITH POLYCONDENSATION IN RESIN FILM DURING DRYING

ABSTRACT

In a drying process of dielectric resin films coated on electric conductive substances, phenomena such as polymerization of monomers, by-products yield, shrinkage and stress generation lake place simultaneously in addition to heat and mass transfer. For the enhancement of the drying with high efficiency and high quality, it is important to understand the drying mechanism. In this paper, the characteristics of heat and mass transfer in the resin film including polycondensation reaction are presented. The apparent drying rate of polyamideimide varnish films was measured in two different heating modes of radiation and convection. The reaction rate of polycondensation was analyzed both by the thermogravimetry and the differential scanning calorimetry. The apparent drying rate began to drop remarkably when the reaction rate became significant. It implies that the diffusion of the solvent is inhibited by skinning at the surface. Applying the Vrentas/Duda free-volume diffusion model to the prediction of diffusivity, the heat and mass transfer in the resin film were analyzed theoretically with a reasonable accuracy.     

HEAT AND MASS TRANSFER IN IMPINGEMENT DRYING

ABSTRACT

In industrial drying applications, efficient transfer of heat and mass between a drying medium and the material being dried is very critical for the overall economics of the operation. Impinging jets are therefore widely used for their enhanced tmnsport characteristics, especially for drying of continuous sheets of materials such as paper and textiles. In such applications, a thin sheet of material, as wide as 6m in cross machine direction, speeds at velocities as high as 90 km/hr under high velocity jets emerging from a confining surface parallel to the material surface. Many variables and effects need to k considered for proper design of such impinging jet systems: nozzle geometry and size, nozzle configuration, location of exhaust pons, nozzle-to-surface separation, jet-to-jet separation, cross flow, jet exit velocity and surface motion. For permeable materials, additional enhancement of heat and mass transfer that occur when some of the impinging gas is removed through the material makes this option an atmctive one.

Here, we review the above effects and offer predictive correlations from literature which may be used in the design of high velocity impinging jet systems.     

ANALYSIS OF HEAT AND MASS TRANSFER DURING DRYING OF PAPER/BOARD

ABSTRACT

A mathematical model has been successfully developed to study the heat and mass transfer process during paper drying. This model takes into account the consective transfer of vapor and liquid apart from the known transport mechanisms of capillary flow of liquid, diffusion, vaporization-condensation, and heat conduction. The partial differential equations describing temperature, saturation and pressure change within the web during drying with associated boimdary conditions and initial conditions were solved using finite difference method. The model predictions show that during the drying process the web can be conveniently divided into three different zones, namely dry zone, wet zone and an intermediate zone. The movement of liquid and vapor in opposite directions in the intermediate zone is similar to the action of a heat pipe. Also, as drying proceeds the location of the intermediate zone and hence the heat pipe advances progressively through the thickness of the web.     

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.     

HEAT AND MASS TRANSFER DURING SAPWOOD DRYING ABOVE THE FIBRE SATURATION POINT

ABSTRACT

Pine sapwood was dried in an air convection kiln at temperatures between 60-80 °C. Temperature and weight measurements were used to calculate the position of the evaporation front beneath the surface. It was assumed that the drying during a first regime is controlled by the heat transfer to the evaporation front until irreducible saturation occurs. Comparisons were made with CT-scanned density pictures of the dry shell formation during initial stages of drying of boards.

The results indicate a receding evaporation front behaviour for sapwood above approximately 40-50% MC when the moisture flux is heat transfer controlled. After that we finally reach a period where bound water diffusion is assumed to control the drying rate.

The heat transfer from the circulating air to the evaporation front controls the migration flux. In many industrial kilns the heating coils therefore have too small heat transfer rates for batches of thin boards and boards with high sapwood content.     

TRANSFER OF HEAT AND MASS IN FALLING LIQUID FILM ON A SCREW GROOVED SURFACE TUBE

Mass and heat transfer of a falling liquid film on an external screw grooved surface of perpendicular tubes were studied and correlations by similarity parameters were obtained. The mechanism for enhancement of the transfer rate by screw grooved surface was analysed. Applying Danckwerts surface renewal model to mass transfer and heat transfer, the frequencies of renewal were evaluated and compared.     

TRANSFER OF HEAT AND MASS IN FALLING LIQUID FILM ON A SCREW GROOVED SURFACE TUBE

Mass and heat transfer of a falling liquid film on an external screw grooved surface of perpendicular tubes were studied and correlations by similarity parameters were obtained. The mechanism for enhancement of the transfer rate by screw grooved surface was analysed. Applying Danckwerts surface renewal model to mass transfer and heat transfer, the frequencies of renewal were evaluated and compared.     

ANALOGY BETWEEN HEAT AND MASS TRANSFER IN THREE SPOUTED BED ZONES DURING THE DRYING OF LIQUID MATERIALS

This work, presents an experimental study of simultaneous heat and mass transfer in spouted bed zones during the drying of liquid materials. Distilled water was used as the liquid material and glass beads as inert material. The effect of the inlet gas temperature (100 to 140 °C), the feed air flow rate relative to air flow at minimum spouting (1.1 to 1.3), and liquid feed flow rate to the bed were investigated. The similarity between heat and mass transfer was analyzed using the Lewis and Chitton-Colburn analogies. Results showed that the ratio between the heat and mass transfer Chilton-Colbum factors lay between 1,31 and 1.74.     

ANALOGY BETWEEN HEAT AND MASS TRANSFER IN THREE SPOUTED BED ZONES DURING THE DRYING OF LIQUID MATERIALS

ABSTRACT

This work, presents an experimental study of simultaneous heat and mass transfer in spouted bed zones during the drying of liquid materials. Distilled water was used as the liquid material and glass beads as inert material. The effect of the inlet gas temperature (100 to 140 °C), the feed air flow rate relative to air flow at minimum spouting (1.1 to 1.3), and liquid feed flow rate to the bed were investigated. The similarity between heat and mass transfer was analyzed using the Lewis and Chitton-Colburn analogies. Results showed that the ratio between the heat and mass transfer Chilton-Colbum factors lay between 1,31 and 1.74.     

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.     

ADIABATIC SORPTION OF BULK SINGLE ADSORBATE FROM AN INERT GAS-EFFECT OF GAS-SOLID MASS AND HEAT TRANSFER COEFFICIENTS

The effects of the gas-solid mass and heat transfer coefficients on the dynamics of adiabatic adsorption of a single adsorbate exhibiting Type I behavior were analyzed by numerically solving the conservation equations for a column. It was assumed that the adsorbate obeyed Langmuir equilibra and the local mass transfer rate could be described by the linear driving force model. The properties of the front zones were found to be controlled by mass transfer, while those of the rear zones were primarily determined by heat transfer. The separation between the two zones might vanish under certain conditions resulting in overlap of the two zones. The front zones were found to be constant pattern. The rear zones can be constant or proportionate pattern depending on the equilibrium properties and the feed condition of the adsorbate.     

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.     

ADIABATIC SORPTION OF BULK SINGLE ADSORBATE FROM AN INERT GAS-EFFECT OF GAS-SOLID MASS AND HEAT TRANSFER COEFFICIENTS

The effects of the gas-solid mass and heat transfer coefficients on the dynamics of adiabatic adsorption of a single adsorbate exhibiting Type I behavior were analyzed by numerically solving the conservation equations for a column. It was assumed that the adsorbate obeyed Langmuir equilibra and the local mass transfer rate could be described by the linear driving force model. The properties of the front zones were found to be controlled by mass transfer, while those of the rear zones were primarily determined by heat transfer. The separation between the two zones might vanish under certain conditions resulting in overlap of the two zones. The front zones were found to be constant pattern. The rear zones can be constant or proportionate pattern depending on the equilibrium properties and the feed condition of the adsorbate.     

STUDY OF HEAT AND MASS TRANSFER DURING IR DRYING OF PAPER

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.     

ANALYSIS OF HEAT AND MASS TRANSFER DURING DRYING OF PAPER/BOARD

A mathematical model has been successfully developed to study the heat and mass transfer process during paper drying. This model takes into account the consective transfer of vapor and liquid apart from the known transport mechanisms of capillary flow of liquid, diffusion, vaporization-condensation, and heat conduction. The partial differential equations describing temperature, saturation and pressure change within the web during drying with associated boimdary conditions and initial conditions were solved using finite difference method. The model predictions show that during the drying process the web can be conveniently divided into three different zones, namely dry zone, wet zone and an intermediate zone. The movement of liquid and vapor in opposite directions in the intermediate zone is similar to the action of a heat pipe. Also, as drying proceeds the location of the intermediate zone and hence the heat pipe advances progressively through the thickness of the web.     

初始饱和度对微波冷冻干燥过程传热传质的影响

以非饱和含湿牛肉为例,进行了微波冷冻干燥实验研究。获得了干燥时间与物料初始饱和度近似成正比的结论。与升华面模型的计算结果比较表明,升华冷凝模型更符合实验规律,证实了非饱和含湿多孔介质微波冷冻干燥时升华冷凝区的存在。     

MATHEMATICAL MODELING FOR FIXED-BED DRYING CONSIDERING HEAT AND MASS TRANSFER AND INTERFACIAL PHENOMENA

A mathematical model for fixed-bed drying with air cross-flow was obtained to simulate the drying process. Special attention was placed on the interfacial conditions. A system of four couple nonlineal partial differential equations in conjunction with three nonlineal algebraic equations was applied and solved numerically by both finite differences and Runge-Kutta methods. Simulations were compared with experimental data from carrot slabs in deep fixed-bed drying. Slab thicknesses were 1.0 and 0.1 cm, and air drying temperatures were 50-60°C. Simulation predicted that water transport was controlled by internal diffusion in slabs with 1.0 cm of thickness; therefore, the interfacial conditions may be considered in steady state. Nevertheless, the predominant phenomenon in slabs with 0.1 cm of thickness was by convection; therefore, the interfacial conditions varied with respect to space and time. In the proposed phenomenological representation of fixed-bed drying, the properties and variables of the system defined the type of mechanism controlling of the drying process without previous assumptions.