DRYING OF LATEX FILMS OF POLY(VINYL ACETATE)

ABSTRACT

Drying of Poly(vinyl acetate) latex films has wide application in the industries of synthetic fibers, adhesives, coatings, paints, etc. In this investigation, drying of Poly(vinyl acetate) latex film was studied experimentally in a drying tunnel where the air velocity and temperature were controlled. The water evaporation rate was obtained by weighing the latex film during the drying process. The weight loss of latex was measured for different polymer concentrations, film thicknesses, drying areas, temperatures, and air velocities. Results emphasize the important role of these parameters on the relative water transport from the latex film. Increasing air velocity and temperature leads to a significant increase of the weight loss of latex and drying rate. Changing other parameters affects only the rate of weight loss but not the constant drying rate.     

DRYING OF LATEX FILMS OF POLY(VINYL ACETATE)

Drying of Poly(vinyl acetate) latex films has wide application in the industries of synthetic fibers, adhesives, coatings, paints, etc. In this investigation, drying of Poly(vinyl acetate) latex film was studied experimentally in a drying tunnel where the air velocity and temperature were controlled. The water evaporation rate was obtained by weighing the latex film during the drying process. The weight loss of latex was measured for different polymer concentrations, film thicknesses, drying areas, temperatures, and air velocities. Results emphasize the important role of these parameters on the relative water transport from the latex film. Increasing air velocity and temperature leads to a significant increase of the weight loss of latex and drying rate. Changing other parameters affects only the rate of weight loss but not the constant drying rate.     

PNEUMATIC DRYING OF MEALS: APPLICATION OF THE VARIABLE DIFFUSIV1TY MODEL

The superficial and bound moisture removal from soya meal was studied in a pneumatic dryer. The effect of external condition over the effective diffusivity in drying kinetics prediction has been demonstrated and a model that considers time variable diffusion coefficient was developed and correlated with gas temperature and velocities. A simple experimental technique using a pulse injection of humected solid was utilized to determine real drying kinetics under steady state operation with solids recycling. The model fitted the experimental data well over a gas temperature range between 100° C and 300° C and gas velocities of 3.5 m/ s and 20 m/ s.     

PNEUMATIC DRYING OF MEALS: APPLICATION OF THE VARIABLE DIFFUSIV1TY MODEL

ABSTRACT

The superficial and bound moisture removal from soya meal was studied in a pneumatic dryer. The effect of external condition over the effective diffusivity in drying kinetics prediction has been demonstrated and a model that considers time variable diffusion coefficient was developed and correlated with gas temperature and velocities. A simple experimental technique using a pulse injection of humected solid was utilized to determine real drying kinetics under steady state operation with solids recycling. The model fitted the experimental data well over a gas temperature range between 100° C and 300° C and gas velocities of 3.5 m/ s and 20 m/ s.     

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.     

Drying Kinetics of Sludge from a Wastewater Treatment Plant

Drying kinetics of sludge from a municipal sewage treatment plant were determined in a laboratory drying tunnel with parallel airflow at different temperatures and air velocities. The constant drying rate period was identified followed by two falling rate periods. Drying kinetics in these falling rate periods were then satisfactorily modeled using the modified quasi-stationary method and the Fick's second law. The two-period model was used, however, to describe drying kinetics over the entire drying process, and the equation for a generalized drying curve was proposed. The effective diffusivity was also determined from the integrated Fick's equation, and correlated with temperature by an Arrhenius type equation.     

An Artificial Neural Network Model for Prediction of Drying Rates

Drying rate data were generated for training of an ANN model using a liquid diffusion model for potato slices of different thicknesses using air at different velocities, humidities and temperatures. Moisture content and temperature dependence of the liquid diffusivity as well as the heat of wetting for bound moisture were included in the diffusion model making it a highly nonlinear system. An ANN model was developed for rapid prediction of the drying rates using the Page equation fitted to the drying rate curves. The ANN model is verified to provide accurate interpolation of the drying rates and times within the ranges of parameters investigated.     

Drying Characteristic of Porous Ceramic Microspheres in a Microwave Heated Fluidised Bed

This paper presents drying characteristics of both water and nitrate salt solution (simulated waste) impregnated ceramic microspheres in a pilot‐scale (10 kg), batch fluidised bed drier heated with microwave energy, under isothermal bed conditions. For comparison, some constant microwave power experiments with water saturated microspheres were also conducted. The effects of bed temperature, fluidising gas flow rates and initial moisture content on drying kinetics were examined. For a given initial moisture and gas flow rate, the total microwave energy consumption for both isothermal bed and constant power level experiments was found to be approximately the same. The decline in drying rates in the presence of nitrate salts was attributed to a decrease in the effective diffusivity resulting from changes in structural properties of salt solution impregnated powders. The use of fluidised bed coupled with microwave heating produced controlled drying of 10 kg batches of simulated waste loaded ceramic powders without any significant agglomeration or coating.     

An Artificial Neural Network Model for Prediction of Drying Rates

Abstract

Drying rate data were generated for training of an ANN model using a liquid diffusion model for potato slices of different thicknesses using air at different velocities, humidities and temperatures. Moisture content and temperature dependence of the liquid diffusivity as well as the heat of wetting for bound moisture were included in the diffusion model making it a highly nonlinear system. An ANN model was developed for rapid prediction of the drying rates using the Page equation fitted to the drying rate curves. The ANN model is verified to provide accurate interpolation of the drying rates and times within the ranges of parameters investigated.     

The effect of molecular diffusion on mixing in fluidized beds

Gas mixing in a 6·5 in fluidized bed of sand was studied using frequency response techniques with He and CO₂ tracers. Samples taken in the bed, where the probe saw mainly dense phase gas, had a lower amplitude ratio than samples at the top of the bed where the bubble contribution predominated. The gas interchange coefficients showed a small dependence on the molecular diffusivity at about twice the minimum fluidization velocity but no effect of diffusivity at high velocities.     

Convective air drying characteristics of sweet potato cube (Ipomoea batatas L.)

The effects of drying conditions on the drying behavior of sweet potato (Ipomoea batatas L.) were investigated in a cabinet dryer. The convective air drying was carried out under five air temperatures; 50, 60, 70, 80 and 90 °C, five air velocities of 1.5, 2.5, 3.5, 4.5 and 5.5 m/s and three sweet potato cubes of 5, 8 and 12 mm thickness. Data were analyzed to obtain diffusivity values from the period of falling drying rate. Results indicated that drying took place in the falling rate period. Moisture transfer from sweet potato cubes was described by applying the Fick's diffusion model, and effective moisture diffusion coefficients were calculated. Effective diffusivity increased with increasing temperature. An Arrhenius relation with an activation energy value of 11.38 kJ/mol expressed effect of temperature on the diffusivity. Two mathematical models available in the literature were fitted to the experimental data. The page model gave better prediction than the first order kinetics of Henderson and Pabis model and satisfactorily described drying characteristics of sweet potato cubes.     

Computer model of drying behaviour of ceramic green bodies with particular reference to moisture content dependent properties

A numerical model was developed to predict the drying behavior of ceramic green bodies. Resolution of the simultaneous heat and mass transfer equations involved finite elements and the Backward Euler method. Based on experimental data, the model uses equivalent moisture diffusivity, water activity, thermal conductivity and heat capacity as input parameters which depend on moisture content. In particular, the equivalent moisture diffusivity is a key parameter controlling water transport from the body interior to the surface. A simple method was used to estimate the effect of shrinkage on drying rate during the initial drying stage. Predictions of the internal moisture distribution, drying rate and surface temperature as a function of time gave good agreement to experiment for green bodies of alumina paste. External conditions of convection coefficient and relative humidity are shown to sensitively control drying rate and surface temperature evolution during the constant rate period.     

构建微波冷冻干燥质扩散系数的一种新方法

针对现有微波冷冻干燥模型中必须依靠实验以确定扩散系数的问题．提出一种构建扩散系的新方法。根据毛细管低压气体输运理论，利用物料本身的细微结构、气体分子平均自由程、气体状态参数和气体物性参数来构建扩散系数，在此基础上建立微波冷冻干燥模型。并结合牛肉的微波冷冻干燥行模型可靠性分析。结果表明，新建模型能够很好地描述微波冷冻干燥过程温度场的变化及其特征．升过程干燥曲线的理论值与实验值之间的相对误差小于10％。     

A model for the evaporation of water from large glass particles in pneumatic transport

A model is presented for the evaporation of water from large particles in pneumatic transport. The work is relevant for estimating the coating solution feed rate and the length of the draft tube in Wurster‐type particle coaters. Specifically, the rate of evaporation of water from 1 mm glass particles in a 28.45 mm tube was calculated from the model. The rate increased with solids mass flow rate, inlet air temperature and inlet particle temperature. The heat was more rapidly removed from the particle phase than from the air phase and high inlet air temperatures are tolerated. The model presupposes that the gas and particle velocities, and voidage are known and that the water film on a particle is thin and uniformly distributed. Hydrodynamic considerations that impact on the calculations are discussed.     

Influence of High-Intensity Ultrasound on Drying Kinetics of Persimmon

Drying persimmon pieces is recognized as a way to preserve and add value to the excess production of the fruit in Spain. To this end, air drying kinetics of persimmon cylinders (30 mm height and 13 mm diameter) were determined under different drying conditions: 8 air drying velocities (0.5, 1, 2, 4, 6, 8, 10, and 12 m/s) with and without application of high-intensity ultrasound (21.8 kHz and 154.3 dB). The drying process was modeled using two diffusion models with and without the influence of external resistance to drying. From the effective diffusivity and the mass transfer coefficient identified from the data it was concluded that high-intensity ultrasound increased the drying rate at the lowest air velocities tested, affecting both external and internal resistances.     

Experimental modeling of solvent-casting thin polymer films

An apparatus was designed and assembled to study the solvent removal from solution-cast thin polymer films. The computer interfacing of a thermogravimetric analyzer, spectrophotometer, electronic flowmeters, and control valves for the apparatus enabled the preprogramming of the carrier gas velocity, carrier gas solvent content, and temperature profiles to simulate the environment experienced in large parallel flow industrial driers. The apparatus has also been designed and operated to enable the visual observation of the drying film with an optical microscope. Initial experimental studies conducted with the apparatus involved the effect of temperature on solvent removal. The results indicate that high dryer gas temperatures can apparently cause skinning of the film surface resulting in slower solvent removal rates. The skin formation can be suppressed by higher solvent concentration in the carrier gas. The visual observations revealed the formation of standing waves in the film surface during drying at high gas velocities (>2OO cm/min). The wave formation at least partially overcomes the effect of skinning by increasing the surface area of the film, and may be the manifestation of flow instabilities involving circulation within the film.     

Drying of Suspensions in a Fluidized Bed of Inert Particles Under Reduced Pressure

A drying method in which a heat-sensitive material is immersed in a fluidized bed under reduced pressure was used to continuously obtain dispersed, dry, fine powders of that material from a dilute suspension or solution at a low temperature with a high drying rate. The mass velocities of the drying gas, sample flow rate, and chamber pressure were varied, and the effects of these variations on the corresponding drying characteristics were examined.

The fluidization state of a fluidized bed of inert particles strongly affects the drying rate in the bed and has a greater effect than the chamber pressure on the corresponding drying characteristics. In other words, it is important to maintain a vigorous fluidization state to achieve a high drying rate. Although the maximum drying rate is independent of the chamber pressure, it can be achieved at a low mass velocity of the drying gas under reduced pressure. That is, at a low chamber pressure a high drying rate can be achieved, even at a relatively low mass velocity of the drying gas. The bed temperature at the time of drying is strongly influenced by the drying rate and decreases linearly with an increase in the maximum drying rate when the sample flow rate is equal to the drying rate.     

Effective Diffusivities and Energy Consumption of Whole Fruit Chinese Jujube (Zizyphus jujuba Miller) in Microwave Drying

Microwave drying of whole fruit Chinese jujube was performed at 45, 90, and 135 W. Ten commonly used mathematical models of thin-layer drying were compared. The Midilli model was best in describing drying time dependency of product moisture ratios. The initial drying rate and drying rate constant are linearly proportional to microwave power level. An effective diffusivity model was presented and validated with the Renka-Cline algorithm. The model has very high predictive precision, suggested by the relative percentage error of 3.734% on average between the model and the Renka-Cline algorithm. The effective diffusivity was proved to be a linear function of microwave power level and a quadratic function of moisture content. Energy consumption in microwave drying of Chinese jujube decreased as microwave power increased from 45 to 135 W, but 90 W was adequate for high-quality products with less energy consumption.     

Mathematical modeling and simulation of shrunk cylindrical material's drying kinetics—Approximation and application to banana

In the present work, a diffusion-based model was adopted to represent the convective drying behavior of cylindrical banana samples, taking into consideration the shrinkage along drying. The developed model simulated a significant number of situations resulting from the variations of some operating conditions. The temperatures tested were 50, 60, 67 and 70 °C, the air velocities were 3, 4 and 4.5 m/s and the relative humidity range of the drying air was from 3.5 to 11.5%. The calculated drying curves were compared to the experimental ones in order to determine apparent moisture diffusivity. An empirical equation was suggested, describing the apparent moisture diffusivity within the banana versus product temperature and local moisture content. A good agreement was found between experimental and calculated drying kinetics.     

湿污泥颗粒的流化床干燥实验及模型

在鼓泡流化床内以河砂为干燥介质,对单颗粒湿污泥的流态化干燥特性进行实验研究,得到了流化床温度、污泥初始水分、污泥粒径及流化速度对干燥速率的影响规律：流化床温度及污泥粒径对干燥速率的影响都呈指数规律;污泥的水分越大,干燥速率越大;在鼓泡流化床流化速度达到2倍临界流化速度以上时,充分流化,流化速度再增大(2~5倍临界流化速度)对干燥速率没有明显影响. 在基本的扩散传质理论的基础上,利用实验数据回归得到湿污泥在鼓泡流化床内干燥的半经验模型,为流化床污泥干燥器的设计提供了基础数据和依据.