ON THE DEVELOPMENT OF MULTIDIMENSIONAL TWO-FLUID MODELS FOR VAPOR/LIQUID TWO-PHASE FLOWS

The status of multidimensional two-fluid modeling of vapor/liquid two-phase flows is reviewed. In particular, a three dimensional two-fluid model is derived using ensemble averaging. It is proposed that realistic interfacial closure laws should:

• Be mathematically objective

• Lead to well-posed models

• Satisfy the second law of thermodynamics

• Agree with all relevant experimental data.

Unfortunately, most two-fluid models in use today do not satisfy these fundamental constraints.     

INFERENTIAL SENSORS FOR ON-LINE MONITORING OF TISSUE MACHINE QUALITY PROPERTIES

There are many processes in a pulp and paper mill where an on-line parameter analyzer cannot be used due to several reasons

•The analyzer is very expensive

•It cannot survive in the environment we want to use it

•It is not operational due to hardware problems, maintenance etc

•Such an analyzer does not exist in the market

In all these situations it would be great for the mill to have an alternative way of measuring those parameters in real-time. Neural network models can serve as virtual sensors that infer process parameters from other variables, which can be measured on-line. One excellent application of inferential sensors in the pulp and paper industry is the on-line prediction of paper properties, like tensile, stretch, brightness, opacity etc. In tissue machines, the most important quality parameter is softness, which is usually measured in a very subjective manner by the touch of a human finger. In this work we examine how neural networks can be deployed in order to build online virtual sensors for softness and other tissue quality properties. The results are promising and show that neural network technology can improve productivity and minimize out of specs production in a tissue machine, by providing accurate real time monitoring ofquality parameters.     

THE ELECTRODEWATERING OF SEWAGE SLUDGES

Electrodewatering (EDW), the enhancement of conventional pressure filtration by an electric field, is an emerging technology with the potential to improve dewatering especially for difficult materials. CSIRO has many years of experience in EDW, ranging from bench scale tests to demonstration trials. A recent programme has investigated the applicability of EDW to aerobic wastewater treatment sludges which are particularly difficult to dewater using conventional equipment.

The bench scale filtration experiments produced cakes with solids contents of 35-46 wt% using EDW, compared with 24-30 wt% using pressure filtration alone. This paper : • describes how the dewatering results were achieved;

•identifies a relationship between moisture removal limits by EDW and the forms of water within the sludge;

•shows the results of preliminary attempts to mathematically model the EDW process     

AN EXPLORATORY STUDY OF A COMBINED SONIC AGGLOMERATION AND CROSSFLOW FILTRATION SYSTEM FOR HOT GAS CLEANUP

The Institute or Gas Technology has investigated a combined sonic agglomeration/crossflow filtration system to remove particles smaller than 10 microns from high-temperature, high-pressure gas streams. Sonic energy induces agglomeration so that particles can be removed in a continuously operating cross-flow filler element. Cold-model and preliminary high-temperature, high-pressure results are promising.

The objective of this investigation was to explore the potential effectiveness of sonic agglomeration, crossflow filtration, and a combination of these techniques to remove particles from high-temperature, high-pressure (HTHP) gas streams. The technique of sonic agglomeration has been known since the 1930's, and crossflow filtration has been used successfully in liquid filtration. This investigation is unique in that these two techniques were combined. Sonic energy was used to agglomerate particles to sizes large enough to be separated from the gas stream in a crossflow filter. The crossflow filter has advantages over conventional filters as a paniculate agglomerate removal system because it (1) operates continuously, (2) does not subject the fragile agglomerates to the high stress typical of inertial capture devices, and (3) can control the buildup of a filter cake when properly combined with a sonic agglomerator.

This investigation was supported by the Gas Research Institute and the Institute of Gas Technology Internal Research and Development Fund.

In this preliminary study, we found that—

• A 2-micron porosity filter must be used to achieve 98% paniculate removal from 95% of a dust-laden stream. (In the crossflow operating mode, 5% of the stream bypasses the filter.)

• When sonic agglomeration is combined with crossflow filtration, the same removal efficiency can be achieved with a 10-micron porosity filter.

• Combined sonic agglomeration/crossflow filtration removed particulates smaller than 10 microns in experiments at 265° C and 7000 kPa.

• The pressure drop across a 10-micron filter is about one-half that of a 2-micron filter, which could reduce the energy requirements for filtration.

This method of particulate removal should be applicable to many different coal reactor effluent streams, especially because it can operate at elevated temperatures and avoid gas cooling, liquid condensation, and subsequent liquid-solids separation. Preliminary estimates show that the power requirements of a combined sonic agglomerator/crossflow filter are lower than those of a crossflow filter alone, and that they are lower than or comparable with other particulate removal techniques. Additional tests are needed to establish the degree to which these benefits can be realized.     

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

Abstract

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.     

SIMULATION AND EXPERIMENTAL STUDY ON THE AIR FLOW AND HEAT LOADS OF DIFFERENT REFRIGERATOR CABINET DESIGNS

As the economy has been growing in the past decade, consumers in Taiwan start to consider how to raise living quality. As people tend to adopt the westerners' living style of purchasing food once every week or two weeks. Large grocery stores, hence, have been built both in urban and suburban areas. This change results in a growing need of better-designed refrigerator-freezers (RFs). Consequently, consumers need RF models with better or more cooling functions, larger volumetric capacity, and efficient energy use

A 1997 top-mount refrigerator with a volume of about 460 liters was selected as the baseline model. This study used computer aided finite element methods (FEM) to analyse and establish designs for RFs. These designs included different insulation properties and various cool air flow rates and outlets in the freezer and fresh food compartments. The following outcomes were investigated in the simulating work:

•The thermal responses of the insulation material on RF cabinets

•The effect of a modified cool air outlet on the air flow distribution and heat loads

The simulation results were then compared with experimental data. Agreement was found between simulation and experimental results. This suggested that computer-aided simulation could serve as a reliable tool for other designs similar to cases practiced in this RF study.     

Polymer—Aluminium Adhesion IV. Kinetic Aspect of the Effect of a Liquid Environment

The effect of a liquid in the failure of an assembly depends strongly on the ability of the liquid to follow the fracture front. Thus, the existence of a critical rate of separation above which the liquid has no effect on the strength of the assembly, has been clearly demonstrated.

From a crack model, parameters determining the penetration rate of the liquid in the growing fracture have been established and are:

—the viscosity of the liquid

—its surface energy

—and the solid/liquid interactions.

This analysis also shows that the dimension of the crack opening is of the order of a micron.     

Uneven Ink Absorption and Its Relation to Drying of Coated Papers

The aim of this work was to study mottling in offset printing. The papers studied were coated and princed LWC-papers.The base paper was obtained from a commercial source. Both macroscopic and microscopic analyses were performed.The conventional macroscopic methods take into account variations in nm-cm scale while the microscopic methods consider the nm-pm scale.

The used conventional methods could not emplain mottling. Microscopic studies, however, revealed that mottling could be a problem caused by varying coat weight and surface structure.

The influence of different drying strategies was also studied. The beet print result was obtained on paper where the drying was relativeiy intense in the beginning of the process and mild during the coating color immobilization stage. Uneven binder migration was not found in these samples.     

Effects of Surface Concentration on Drying Behavior of Carbohydrate Solutions

As the surface properties of the drying materials are very important not only for the drying rate but also for the quality change during drying, the effects of surface concentration on the drying behavior of liquid foods (sugar solutions) were investigated by isothermal drying experiments and by numerical calculation experiments. The isothermal drying experiments with gelled sugar solution systems (sucrose and maltodextrin) were carried out at various relative humidity (RH) values (RH = 0 to 84%). Separate experiments were carried out for determination of the desorption isotherms.

The isothermal drying curves of sugar solutions at RH = 0 to 51% were very similar. Numerical simulations also showed that the drying curves of these sugars at the surface concentration = 0 and 0.1 are almost the same, although the concentration distributions are different.

When a small amount of gelatin was added to sugar solutions, the drying rate decreased remarkably as the gelatin might form a thin film (skin) near the surface, and consequently the retention of ethanol increased.     

Stress and Failure Prediction for the Drying, Tempering, and Cooling of Extruded Durum Semolina

This work presents a method to predict the stress and breakage that is caused by the drying of hygros-copic materials. Stresses were predicted for a viscoclasic cylinder with the properties of extruded durum semolina, or pasta noodles. The stresses were calculated as functions of the transient moisture and tem-perature gradients in the material which were predicted for the combined processes of drying, tempering,and cooling. The time and radial position of failure were predicted based on failure data for extruded semolina.

Isotherm data for extruded durum semolina were obtained for temperatures from 40 to 60°C and for relative humidities from 75 to 95%. The results were fit with a modified form of Henderson's equation.Thermal conductivities were measured for temperatures from 30 to 50°C and a moisture range of 12 to 27% (dry basis).

A drying model based on the principles of irreversible thermodynamics; (Fortes, 1978; Fortes and Okos, 1981a, 1981b) was used to successfully predict drying curves for a range of experimental conditions. Transient moisture and temperature profiles were calculated numerically, and a receding evapora-tion front was predicted to exist. Drying was predicted to be a coupled liquid, vapor, and heat transport phenomena.

The drying data were used in a stress analysis of a Maxwell viscoelastic cylinder to predict trends in stress development under various contiitions of combined drying, tempering, and cooling. High temperature-high humidity drying, HTHH, (lOO°C, 65% RH) was compared with low temperature-low humidity drying, LTLH, (53°C, 13% RH). The HTHH drying offered definite advantages in terms of reduced product breakage susceptibility. The reasons for those advantages were increased failure strength and a decreased moisture gradient at the end of drying. In a five-stage drying process, the cooling stage was shown to have a significant impact on the predicted levels of stress and on the strength of the extruded material. Analysis of the model suggested that gradual temperature and humidity transitions from stage to stage in multistage processes were important to product quality.     

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     

RADIO FREQUENCY ENHANCED DRAUGHT TUBE SPOUTED BED DRYER

A study of the operation of a draught tube spouted bed in conjunction with dielectric heat addition was carried out. The addition of radio frequency allowed a greater amount of energy to be input into the particulate material in any given time, increasing the drying rate: however, the drying pattern was unaffected by the ratio of radio frequency to convective heat addition. The rate of moisture loss was proportional only to the total energy input. The synergy which often occurs with low levels of radio frequency addition does not occur with rf enhanced spouted beds.

The properties of spouted beds seem to be improved only for moisture contents at which packed bed drying can perform effectively.

Although the properties of spouted beds can be improved by radio frequency heat addition, their qualities although useful for specific operations, are unsuitable for more general use.     

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.     

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.     

KINETICS OF DRYING IN CYCLICALLY SHIFTED SPOUTED BED

This paper presents kinetics of batchwise drying in a spouted bed apparatus with cyclically shifted gas stream, i.e. drying medium flowrate is changed cyclically along inlet cross-section of a bed during operation time.

Such an apparatus provides advantages of both spouted and oscillating beds which include: good hydrodynamic conditions within the bed and energy savings. Two types of particulate materials, i.e. beet and wheat seeds, which differ largely in physical properties were used in experiments.

Based on the results of this investigation formulae for reduced moisture content (X^red) in dependence on drying time and air temperature were developed.     

A Simulation of Wet Pocket Lumber Drying

In general, wood containing wet pockets is difficult to dry and to ensure uniformity of moisture content at the end of the drying process. Large variations of final moisture content and severe case hardening are common problems associated with the drying of wet wood. In order to devise optimal strategies for drying wood containing wet pockets, it is necessary to understand its complex moisture movement mechanisms and therefore predict drying times and final moisture content. Sub-alpine fir dimension lumber was used in this research because of its inherent issues related to wet pockets.

A two-dimensional mathematical drying model for wood containing wet pockets was developed. An effective diffusion coefficient (D_eff) was utilized in the model and heat and mass transfer equations were solved using a control volume approach. The difficulties involved in the simulation of the drying process of wet pocket lumber are due to the differences in moisture content and physical properties between wet and normal wood. Thus, an adjustable D_eff based on the moisture content (for both below and above fiber saturation point) was used during the simulation.

Four drying runs involving green unsorted sub-alpine fir lumber were carried out in a 3-ft laboratory kiln and in an 8-ft pilot kiln. The results of the simulations were in agreement with the results obtained through the drying experiments.     

ANALYSIS OF OPEN SUN DRYING EXPERIMENTS

Open sun drying has lost its previous importance due to the fact that different factors affect its reliability and the quality of the products obtained.

One of the set-backs for the analysis of solar drying exoeriments is their deoendence on a non-controlled source of energy, i.e. solar radiation depends on climatic conditions and experiments are difficult to compare. It is thus necessary to investigate the advantages of a particular set up as well as the climatic influences. Open sun drying could constitute the natural reference, allowing the comparison of different drying strategies.

A new way of standardizing drying times, based on solar radiation inout, is proposed, to allow better evaluation of the experiments. An equivalent time is defined, allowing comparison of experiments carried out under different circumstances. Carrots and potatoes were used in these experiments.

The use of the average daily solar radiation 15.28 MJ m^-2.d^-1 in Palma de Mallorca (39.33°N, 2.37°E), is proposed for comparison purposes. An improvement of more than 12 % in the explained variance was observed, the unexplained variance being lower than 1 %.     

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

Drying of red pepper under solar radiation was investigated, and a simple model related to water evaporation was developed. Drying experiments at constant laboratory conditions were undertaken where solar radiation was simulated by a 1000 W lamp.

In this first part of the work, water evaporation under radiation is studied and laboratory experiments are presented with two objectives: to verify Penman's model of evaporation under radiation, and to validate the laboratory experiments. Modifying Penman's model of evaporation by introducing two drying conductances as a function of water content, allows the development of a drying model under Eolar radiation.

In the second part of this paper, the model is validated by applying it to red pepper open air solar drying experiments.     

MULTI-STAGE SPRAY DRYING METHOD IN MANUFACTURING HEAVY DUTY DETERGENTS

Collision of droplets; counter-current spray dryer; drying rate; heat transfer; nonphosphated detergent; spray drying

The spray drying method of non-phosphated granular detergents is studied to decrease the amount of agglomerate particles. The formation of agglomerates is mainly influenced by the concentration of droplets in spray cloud and the water content of droplets at the time of collision. The overlaps of different spray clouds should be de- creased.

The drying rate near the nozzle zone is considerably faster than that calculated by Ranz-Marshal's equation. According to these phenomena, “Multi-stage spray drying” is developed, which is characterized by in stalling plural spraying stages in a spray dryer.

Consequently, non-phosphated detergents are manufactured with the same powder properties and productivity as phosphated detergents.     

Theoretical and Experimental Analysis of the Scarf Joint Bonded Structure: Influence of the Adhesive Thickness on the Micro-mechanical Behavior

In this work, we have studied the influence of the adhesive thickness on the micro-mechanical behavior of a scarf joint bonded structure loaded in uniaxial tension. Adherends are made of mild steel containing 0.18% Carbon (French Standard XC18), the adhesive is a two-component epoxy resin with a 5800 MPa elastic modulus. The experimental method is based on strain gauge measurements and acoustic emission. It makes it possible to determine the following zones:

—the areas of the joint where the start of microcracks occurs (threshold Fd);

—the areas where crack propagation occurs (threshold Fg) up to the failure (threshold Fr). The experimental results confirm the good correlation between the different thresholds. They also show that there is an optimal adhesive thickness close to 0.1 mm, which confers to the scarf joint the greatest resistance to microcrack initiation and crack extension. We have compared our experimental measurements with the main theories in this domain to determine their limits and their fields of application, particularly in the angular singularities regions near the ends of the lap.