Multiscale dynamic Monte Carlo/continuum model of drying and nonideal polycondensation in sol‐gel silica films

The process of forming sol‐gel silica thin films involves multiple length and time scales ranging from molecular to macroscopic, and it is challenging to fully model because the polymerization is nonideal. A multiscale model is described to link macroscopic flow and drying (controlled by process parameters) to film microstructure (which dictates the properties of the films). In this modeling strategy, dynamic Monte Carlo (DMC) polymerization simulations are coupled to a continuum model of drying. The entire DMC simulation is treated as a particle of sol whose position and composition are tracked using a diffusion/evaporation finite difference method. By simulating swarms of particles starting from different positions in the film, the multiscale model predicts different drying/gelation phenomena, and predicts the occurrence of gradients of concentration and gelation in the films which can lead to the formation of a gel skin near the top surface of the film. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Intelligent Wood Drying Control: Problems and Decisions

Common drying schedules are intended for different species with average properties and frequently they are too conservative. The algorithm allowing the drying optimization of a board batch in real-time scale using computer simulation is described in this article. The method of calculation of simulation model factors using interrelation of parameters of wood properties is described. Allowable reduction of residual wood strength was used as a criterion to find temperature levels at optimization of drying of a board batch. The proposed algorithm for the computation of the optimal drying parameters was tested experimentally in an industrial kiln. Results of tests have confirmed the reliability of the algorithm and the opportunity of its use for the creation of intelligent control systems (ICS).     

A Numerical Study of Transient Deformation and Stress Behavior of a Clay Slab During Drying

A transient three-dimensional analysis was carried out on internal strain-stress as well as heat and the moisture transfer in a ceramic slab during drying. A model was developed to analyze viscoelastic behavior, heat conduction and moisture diffusion. The basic equations were solved by the finite element method. The effects of several dimensionless parameters are discussed to find an optimum drying process and a precise design of molds in ceramic production. The stress and the gradient of moisture content were influenced significantly by the Biot or Lewis number. When the moisture diffusion is enhanced or the drying is controlled well so as to form only gentle gradients of moisture content in the slab, the maximum tensile stress can be reduced. Nonuniform drying results in the develoment of warp and increase in the maximum tensile stress. The drying characteristics were not appreciably influenced by shrinkage.     

Intelligent Wood Drying Control: Problems and Decisions

Common drying schedules are intended for different species with average properties and frequently they are too conservative. The algorithm allowing the drying optimization of a board batch in real-time scale using computer simulation is described in this article. The method of calculation of simulation model factors using interrelation of parameters of wood properties is described. Allowable reduction of residual wood strength was used as a criterion to find temperature levels at optimization of drying of a board batch. The proposed algorithm for the computation of the optimal drying parameters was tested experimentally in an industrial kiln. Results of tests have confirmed the reliability of the algorithm and the opportunity of its use for the creation of intelligent control systems (ICS).     

Moisture Profiles Determination During Convective Drying Using X‐Ray Microtomography

The knowledge of internal moisture profiles which develop during drying is essential for model validation purposes, but they are difficult to determine experimentally. This paper shows that X‐ray microtomography, together with image analysis, provides an accurate, non destructive and easy to use technique to determine moisture profiles. Results reported concern the drying of wastewater sludges whose management is becoming a real environmental challenge. An analysis of the development of moisture gradients at the sample external wall shows an influence of drying operating conditions. Finally, mass diffusion coefficients are estimated from the knowledge of both the moisture gradients and the drying flux.     

Shrinkage cracking in Roman cement pastes and mortars

Roman cements were key materials used in the architecture of the nineteenth and early twentieth centuries. Fine cracks, caused by restrained shrinkage during drying, are a distinct characteristic of all Roman cement stuccoes. Today, cracking has become an important barrier preventing broader acceptance of Roman cement as a material by the restoration and construction sector. Drying shrinkage and tensile properties of Roman cement pastes and mortars submitted to various curing and drying regimes were determined as key parameters controlling cracking. A higher volume of aggregate in the mortar mix and a moderate curing time produce optimum Roman cement mortars from the standpoint of reducing the risk of cracking. Fast drying produced significant microcracking due to moisture gradients and differential shrinkage across the specimens. Stress relaxation observed during the long-time loading of the materials reduced their vulnerability to cracking.     

FINITE ELEMENT SIMULATION OF THE THERMO-HYDRO STRESSES IN A VISCOELASTIC SPHERE DURING DRYING

The objective of this study is the formulation of a finite element model that could be used to analyze the stress crack formation in a viscoelastic sphere resulting from temperature and moisture gradients during the drying process. Numerical solutions to the simultaneous moisture and heat diffusion equations describing moisture removal and heat intake process for the sphere are obtained. The distribution and gradients of temperature and moisture developed inside the sphere during drying are established. The calculated temperature and moisture gradients are used in a finite element analysis of the thermo-hydro viscoelastic boundary value problem to simulate the stresses in the body.

The model is used to solve a sample problem of drying a soybean kernel. The simulated drying curve for the soybean model is obtained and compared favorably with the experimental results reported in the literature. Tangential stress, as a criteria for failure, is shown to change from compressive to tensile stress as it approaches the surface. It reaches its peak value at the surface in one hour and then decays slowly. The effect of different drying conditions is studied and the results are discussed.     

FINITE ELEMENT SIMULATION OF THE THERMO-HYDRO STRESSES IN A VISCOELASTIC SPHERE DURING DRYING

The objective of this study is the formulation of a finite element model that could be used to analyze the stress crack formation in a viscoelastic sphere resulting from temperature and moisture gradients during the drying process. Numerical solutions to the simultaneous moisture and heat diffusion equations describing moisture removal and heat intake process for the sphere are obtained. The distribution and gradients of temperature and moisture developed inside the sphere during drying are established. The calculated temperature and moisture gradients are used in a finite element analysis of the thermo-hydro viscoelastic boundary value problem to simulate the stresses in the body.

The model is used to solve a sample problem of drying a soybean kernel. The simulated drying curve for the soybean model is obtained and compared favorably with the experimental results reported in the literature. Tangential stress, as a criteria for failure, is shown to change from compressive to tensile stress as it approaches the surface. It reaches its peak value at the surface in one hour and then decays slowly. The effect of different drying conditions is studied and the results are discussed.     

a rview of: UNIFIED ANALYSIS AND SOLUTIONS OF HEAT AND MASS DIFFUSION

ABSTRACT

The objective of this study is the formulation of a finite element model that could be used to analyze the stress crack formation in a viscoelastic sphere resulting from temperature and moisture gradients during the drying process. Numerical solutions to the simultaneous moisture and heat diffusion equations describing moisture removal and heat intake process for the sphere are obtained. The distribution and gradients of temperature and moisture developed inside the sphere during drying are established. The calculated temperature and moisture gradients are used in a finite element analysis of the thermo-hydro viscoelastic boundary value problem to simulate the stresses in the body.

The model is used to solve a sample problem of drying a soybean kernel. The simulated drying curve for the soybean model is obtained and compared favorably with the experimental results reported in the literature. Tangential stress, as a criteria for failure, is shown to change from compressive to tensile stress as it approaches the surface. It reaches its peak value at the surface in one hour and then decays slowly. The effect of different drying conditions is studied and the results are discussed.     

Kinetics of Drying of Fibre Materials and Fibre-Forming Polymers with Warming

Drying with heating of fibres, fibre materials, and fibre-forming polymers takes place with low-intensity energy feed, with an increase in the thickness of the material dried, or with a high moisture content of the drying agent. Heating of the material in the initial drying period is analyzed and the corresponding equation for the drying kinetics is obtained. The dependences for describing the kinetics of drying with heating are obtained for different materials and conditions. The general kinetic curve describes periods of heating and a constant and diminishing drying rate. Drying with first and second periods (instantaneous heating of the material) is obtained from the overall dependence as a special case. The dependences obtained are compared with the experimental data for different types of drying of fibres, fabrics, and fibre-forming polymers. The calculated drying parameters are found. The calculated curves are in good agreement with the experimental data in the working range. A method for calculating drying using the dependences obtained is described.     

Experimental Investigation of Drying in a Model Porous Medium: Influence of Thermal Gradients

In this study, isothermal and non-isothermal drying experiments with imposed constant temperature gradients have been conducted in a two-dimensional square pore network of borosilicate glass (SiO₂) with interconnected etched channels. In experiments with temperature decreasing from the open surface of the pore network, a travelling stabilized gas-liquid region could be established, while in isothermal experiments with uniform temperature faster breakthrough and an extended two-phase zone were observed. Both findings are in good agreement with pore network simulations. However, numerical underestimation of drying rates (especially in the second period of drying) indicates that liquid films might play a major role in the experimental pore networks, even in the presence of thermal gradients.     

A spray-dried α-olefin sulfonate from concept to marketplace

This paper examines the spray drying process, particularly the spray drying of α-olefin sulfonates. Topics covered center on the spray drying process for α-olefin sulfonates, important parameters and considerations in that spray drying process, and an examination of the product from the spray tower. Product development from laboratory to pilot plant is described with attendant translation to full-scale facilities. At least 4 different cocurrent spray-drier units are illustrated and key parameters are discussed. Presented at the AOCS annual meeting, May 1981.     

Sensitivity Analysis of the Reaction Engineering Approach to Modeling Spray Drying of Whey Proteins Concentrate

Whey proteins concentrate (WPC) powder is an important protein source for humans and is commonly produced from whey using a spray-drying technique. Predicting several drying parameters as well as the parameters that govern the quality of the product is essential before manufacturing WPC in industries. Drying kinetics is an essential tool for predicting the drying rate and various parameters that are rate dependent. However, there have been only a few studies published previously on both modeling WPC drying and dryer-wide simulations using different computational tools. In this article we review the application of a reaction engineering approach (REA) to model the droplet drying process. Results based on dryer-wide simulations using the REA are presented. More importantly, a sensitivity analysis of the REA using drying of WPC and skim milk droplets is reported. This analysis will be helpful to select an appropriate drying kinetics model and forms a benchmark for the future WPC drying modeling work.     

Measurement of Shrinkage and Cracks Associated to Convective Drying of Soft Materials by X-ray Microtomography

Traditionally, the measurement of shrinkage occurring during drying is performed by destructive or poorly accurate techniques such as volume displacement methods. Cracks detection and quantification are realised either by destructive techniques or sophisticated but expensive nondestructive ones (NMR imaging). X-ray microtomography in combination with image analysis provides an accurate, nondestructive and easy to use technique to determine simultaneously shrinkage and crack extent. Results reported in this article concern drying of wastewater sludges whose management will become a real challenge in the years to come. These results show a clear relation between drying kinetics and crack development. This could be related to the development of internal diffusional limitations inducing moisture gradients and mechanical stresses leading to cracks formation.     

Optimizing Drying Processes Using a Structural Transition Model and Entropy Change Maximization

We describe a new approach for drying optimization based on a structural transition model involving species with distinct degrees of hydration α, the ratio between the water content and the sample mass. The raw material used consisted of jackfruit (Artocarpus heterophyllus) seeds that were cooked and then dried at five temperatures (50, 60, 70, 80, and 110°C). The drying process could be described as the material possessing two predominant structures concerning the drying behavior. The resulting curves furnished thermodynamic parameters as Gibbs free energy, enthalpy and entropy of the drying process, which allowed optimization of the energy and time required for drying.     

PHYSICAL PROPERTIES OF RICE RELATED TO DRYING THE GRAIN

Stresses in the rice grain can be produced by mechanical equipment such as the combine, elevator, auger and the rice mill. There are, however, other and more insidious means of producing severe stresses in the grain with moisture and thermal gradients. Moisture gradients can be found 1) in the field on a humid night before harvest, 2) in a hopper of freshly harvested rice containing high-, low- and intermediate-moisture grains, and 3) in certain types of dryers ahead of the drying front. A single high humidity exposure of rough rice at storage moisture can be very detrimental to the quality of the grain. Other steep moisture gradients can be produced by rapidly drying high-moisture rice, With time, after drying, the gradients cause moisture to diffuse from the center to the surface thus causing 1) the moisture gradient to decrease, 2) the grain surface to gain moisture and expand, 3) the grain interior to lose moisture and contract, and 4) the grain to fissure several hours after it has been dried. Fissured rice will usually break when it is milled.     

Production of Crispy Granules of Fish: A Comparative Study of Alternate Drying Techniques

Three alternate dying methods—vacuum drying (VD), vacuum microwave drying (VMD), and pulse-spouted vacuum–microwave drying (PSVMD)—were examined experimentally for their potential as an industrial scale technology to produce granules of fish. Drying kinetics as well as key quality parameters such as expansion ratio, texture, color, sensory characteristics, and microstructure of crispy granules produced by these drying techniques were examined. Results showed that the drying time is reduced with an increase in microwave power level and with reduced pressure as expected. The quality of the granules is affected by the drying method and the operating parameters employed. Granules obtained using PSVMD at a microwave power density of 6.0 W/g displayed optimal quality required for the commercial product accepted by consumers.     

OPTIMAL DESIGN OF WELL-MLXED FLUIDIZED BED DRYERS

In a recent article, Baker described a novel technique for estimating the energy consumption of well-mixed fluidized bed dryers based on the use of experimental drying curves. An integrated approach to performing sizing and energy consumption calculations for such dryers using this technique is described in the present paper. A computer program, which includes not only the dryer simulator but also a heat recovery module and an exhaust air recycle option, is used to evaluate the viability of different energy-conservation strategies. The effects of operating parameters such as bed temperature, solids loading and air velocity on energy consumption and bed area are also explored. The observed drying kinetics are shown to have a major effect on the optimal design and operation of the dryer.