Vaporization Processes and Pressure Buildup during Dewatering of Dense Refractory Castables

This work describes an experimental investigation on the dewatering kinetics of high-alumina refractory bodies under several heating conditions. The drying stages involving the removal of unbound water have been correlated with the two consecutive thermal processes by which liquid water transforms into vapor during heating: evaporation and ebullition. Thermogravimetric data have been used as the basis for a discussion of the parameters that affect the performance of both vaporization processes and guide the design of suitable heating schedules aimed at minimizing the drying time and the risk of harmful pore pressurization.     

Osmotic drying of gelcast bodies in liquid desiccant

Osmotic drying of gelcast alumina bodies was carried out in water solutions of various polyethylene glycols (PEGs) with molecular weight ranging from 1000 to 80,000 g/mol. Up to 30% of the water content could be removed from the gelcast bodies immersed in a 43 wt% solution of PEG 80000. It was found that PEG 1000 was a less effective desiccant than the PEGs with higher molecular weights, even if the osmotic pressure was similar (4.9 MPa). The smaller PEG molecules penetrated the gelled bodies and reduced the dewatering. Moreover, the penetrated PEG molecules affected the pore structure of green bodies and in this way influenced the sintering behaviour. The time dependence of dewatering and the effect of the size of gelcast bodies were investigated and correlated with the green body structure and sintering behaviour in order to optimize the osmotic drying process.     

Triaxial mechanical behaviour of mortar: Effects of drying

The analysis of concrete structure durability is based on the investigation of the material long-term behaviour. Such behaviour is influenced by mechanical, hydrous and thermal actions applied to structures. The main purpose of this study concerns the characterisation of the coupled effects between drying shrinkage and damage for a cementitious material. An experimental study on a normalised mortar (European norm) is then presented to characterise the damage effect, induced by drying and desiccation shrinkage on the multiaxial compressive behaviour. Triaxial compression tests are carried out at different times of drying. The observed increase in deviatoric strength and decrease of Young's modulus and Poisson's ratio are related to the loss in mass of specimens. These results are commented through the damage processes of material because the drying phenomenon causes microcracking by exceeding tensile strength. This microcracking will have a strong influence on the damage process of the material and then on its failure behaviour. Furthermore, the effect of drying leads to an increase of the capillary suction into the mortar, hence, to an increase of the specimen strength. Such couplings have to be taken into account in a reliable modelling.     

A Study of the Power Density Distribution generated during the Combined Microwave and Convective Drying of Softwood

ABSTRACT

Investigations into new and innovative drying strategies can lead to the development of more efficient and effective drying processes. The commercialisation of these processes would prove invaluable to the drying industry as a whole and the associated technology would generate worldwide interest. Combined microwave and convective drying is one such process which offers great potential, with benefits that include : reduced drying times and increased drying rates; volumetric heating; higher fluxes of liquid to the drying surface; high temperature and internal pressure buildup within the material which enhances the overall moisture migration rate; and preferential heating of wetter areas. Numerical simulation can elucidate on the intricate details of the heat and mass transfer henomena that occur during the drying process, thus eliminating the need for performing numerous time consuming and expensive experiments. The simulations can predict the evolutionary behaviour of the moisture, temperature and pressure distributions, and can provide a detailed analysis of how microwaves interact with materials during drying and heating operations at a fundamental level. The research presented in this paper uses a comprehensive mathematical model to study the behaviour of the iniernal microwave power density distribution that is generated during the microwave enhanced convective drying of softwood. The configuration understudy concerns a plane wave microwave source irradiating the wood in the transverse direction.     

MOISTURE MOVEMENT IN SOFTWOOD TIMBER AT ELEVATED TEMPERATURES

The anatomy of a common plantation-grown softwood in New Zealand, Pinna radiata, is reviewed and the differences between the drying behaviour of softwoods and hardwoods are discussed. These differences have significant effects on the paths for moisture movement, and this review of the timber physiology has been used as the basis for a model of the moisture movement at elevated temperatures which distinguishes between the drying behaviour of sapwood and heartwood. It is postulated for the drying of heartwood that liquid flow does not occur within a board due to the aspirated state of the pits, so an evaporative plane at the boiling point of water sweeps through the timber until the centre of the board is reached. In the case of the much wetter sapwood where the pits are not initially aspirated, we assume that liquid flow initially maintains an evaporative plane just below the surface until the moisture is no longer funicular. Thereafter, drying proceeds in a similar way to heartwood. Independent experimental measurements of the temperature profiles at the centre and surface of heartwood and sapwood bkards (100 × 50 mm) have been compared with model predictions, and the close agreement gives some confidence in the model. The model predicts that the drying process for this timber can be divided into three periods, marked by transitions in the gradients of the drying rates when plotted against the average moisture contents. This behaviour has been observed experimentally for Pinus radiata, giving us further confidence that our drying model includes the important physical processes for moisture movement in this timber.     

Experimental investigation of the drying process of water-based paints used in automotive industry

The objective of the government-funded research project “SiLat” (Simulation der instationären Lacktrocknung bei dreidimensionalen Objekten—simulation of paint drying processes on three-dimensionally shaped geometries) is the development of a CFD-software simulation add-on for the drying process of paints [11]. The software is intended to be used in the automotive industry to predict the intermediate drying process of water based paints on car bodies in continuous convective dryers. To validate the software, several laboratory scale experiments, using industrial water-based paint systems and a model base paint system applied on a sheet of metal, were carried out. Conditioned air was used for the drying process. The experimental setup was varied in three different ways: paint on a flat sheet was dried in impinging jet and parallel flow conditions. Furthermore, a Z-shaped sheet was dried during parallel flow conditions. Additional experiments were performed varying the flow parameters. During all experiments, the sheet temperature and weight of the painted sheet were monitored with a rate of 1 Hz. The evaporation of the organic solvents could be monitored using FTIR-spectroscopy. This contribution deals with the experimental investigations and simulation results, whereas the paint drying model is presented by Domnick et al. [5] in detail.     

Defect-free drying of large fine-particle zirconia compacts prepared by gelcasting method

In this work, drying of bodies prepared by gelcasting fine submicrometre-sized zirconia particles was studied and a drying process for defect-free bodies with large cross-sections was proposed. It was found that the cracking of large bodies could be prevented by reducing the monomer content and using appropriate non-volatile cosolvents. Glycerol and polyethylene glycols with different molecular weights were used as non-volatile cosolvents in aqueous ceramic suspensions. The complex effects of the individual cosolvents on the gelcasting process and, in particular, on the drying step were investigated and explained. The applicability of individual cosolvents for the gelcasting process were discussed and their optimal use was indicated.     

Drying of Pharmaceuticals: The Applicability of Spouted Beds

This study evaluates the performance of conventional spouted beds (CSB) with inert bodies in the drying of pharmaceutical pastes, based on the dry product quality. The powder obtained by drying annatto extracts shows properties comparable to one of the commercial products, indicating that the spouting is competitive with other industrial and lab-scale processes. The contents of bixin and caffeine in annatto and guarana dry powders demonstrated a strong dependence on the inlet air temperature. Also, the properties of indomethacin-containing microcapsules dried in CSB and spray dryers revealed that spouting is a promising technique to obtain pharmaceutical forms with unique drug release qualities.     

Recycling the insoluble residue from titania slag dissolution (tionite) in clay bricks

Tionite is the insoluble residue from the titania slag dissolution process for TiO₂ manufacturing. It is a fine-grained sludge consisting of rutile, anatase, amorphous phase and bassanite. Chemical composition is TiO₂ (ca. 50%), SiO₂ (ca. 30%) and minor Al, Ca, Mg, and Fe, plus residual sulfur, implying an acidic pH of waste. Moisture is about 35% of dry weight. The potential of tionite as colouring agent in clay bricks was appraised by admixing (up to 9%) either as-produced or neutralized tionite to four industrial clay bodies. The effect on technological behaviour was assessed by laboratory simulation of the industrial brickmaking process and determining working moisture, drying sensitivity, shrinkage and bending strength, water absorption, bulk density, efflorescence, and colour. The use of tionite is technologically feasible, with little adjustment of industrial cycle, and resulting brick performances depend remarkably on the composition and properties of clay bodies. Carbonate-rich bodies seem to be affected by tionite more during drying than during firing; carbonate-poor bodies range from little changes to consistent worsening of brick performances. No relevant changes of process and product parameters were found up to 3% tionite. Additions over 5% induce significant variations, such as increase of working moisture and water absorption, decrease of bulk density and bending strength. A definite and consistent improvement of this technological behaviour is achieved by using neutralized tionite. The yearly output of tionite could be entirely recycled by approximately four average-size brickworks adding about 3% of residue (dry weight).     

Drying of Pharmaceuticals: The Applicability of Spouted Beds

ABSTRACT

This study evaluates the performance of conventional spouted beds (CSB) with inert bodies in the drying of pharmaceutical pastes, based on the dry product quality. The powder obtained by drying annatto extracts shows properties comparable to one of the commercial products, indicating that the spouting is competitive with other industrial and lab-scale processes. The contents of bixin and caffeine in annatto and guarana dry powders demonstrated a strong dependence on the inlet air temperature. Also, the properties of indomethacin-containing microcapsules dried in CSB and spray dryers revealed that spouting is a promising technique to obtain pharmaceutical forms with unique drug release qualities.     

Non-isothermal drying of fired-clay brick, an NMR study

Nuclear magnetic resonance (NMR) was used to measure moisture profiles during non-isothermal drying of fired-clay brick. Drying was induced by a temperature gradient resulting from one-sided heating. A saturated fired-clay brick sample which was closed and isolated on all sides except for the front surface was heated with a halogen lamp of variable intensity. By measuring the moisture profiles in time it was possible to identify two distinct drying stages. In the first drying stage homogeneous drying is observed. When the critical moisture content is reached a drying front enters the sample. The characteristic time t₀ after which this drying front develops depends on the applied heat flux.The total amount of energy needed for evaporation of the moisture in the first drying stage is found to be independent of the heat flux from the lamp. As a result the total amount of moisture as a function of time can be scaled to one master curve using the time t₀. The data from these measurements provided an interesting view on drying processes, not only under isothermal conditions but also under influence of a thermal gradient. By using the power input as a scaling parameter it is shown that both the total mass and the drying front positions as a function of time show the same behaviour in all experiments.     

SHRINKAGE OF POROUS MATERIALS DURING MICROWAVE DRYING

The dissipation of electromagnetic energy inside a material creates a thermal imbalance state producing some reactions different from those observed through slow classic drying processes. Important drying rates obtained by microwave application can be understood by taking into account induced pressure gradients which greatly accelerate the thermomigration mechanism and thereby modify the physical properties of the product. The shrinkage of porous materials during the drying stage is very sensitive to the internal vapour pressure.The quality of such products depends on the shrinkage behaviour, and it is therefore interesting to study this phenomenon in order to control the characteristics of the product. This study is an experimental approach leading to a theoretical model describing the shrinkage mechanism. This model is developed from results obtained by a computer controlled measurement system allowing to regulate the drying kinetics.     

SHRINKAGE OF POROUS MATERIALS DURING MICROWAVE DRYING

The dissipation of electromagnetic energy inside a material creates a thermal imbalance state producing some reactions different from those observed through slow classic drying processes. Important drying rates obtained by microwave application can be understood by taking into account induced pressure gradients which greatly accelerate the thermomigration mechanism and thereby modify the physical properties of the product. The shrinkage of porous materials during the drying stage is very sensitive to the internal vapour pressure.The quality of such products depends on the shrinkage behaviour, and it is therefore interesting to study this phenomenon in order to control the characteristics of the product. This study is an experimental approach leading to a theoretical model describing the shrinkage mechanism. This model is developed from results obtained by a computer controlled measurement system allowing to regulate the drying kinetics.     

Model based analysis of the drying of a single solution droplet in an ultrasonic levitator

A model for the drying of a single solution droplet into a solid, dense particle is presented and simulations are made to achieve a more fundamental understanding of the single droplet drying process relevant in connection with spray drying processes. Model predictions of drying behaviour are compared to data for the drying of aqueous solutions of maltodextrin DE 15 and trehalose from experiments conducted using an ultrasonic levitator. Model predictions are in good agreement with the experimental data, indicating that the model describes the most important physical phenomena of the process.     

AN AUTOMATIC DEVICE FOR PRODUCING A CURVE COÖRDINATING SHRINKAGE AND DRYING LOSS1

An apparatus is described which automatically plots a curve and coordinates the shrinkage associated with the drying loss at any point on the curve. From such curves correct dryir.g schedules may be developed. The apparatus may be used in a drier to indicate correct drying performance and the curves furnish valuable information as to the drying peculiarities of bodies not usually available.     

A STUDY OF TEMPERATURE AND SAMPLE DIMENSION IN THE DRYING OF POTATOES

It is common knowledge that sample size and drying temperature are two of the most important factors affecting the drying rate and quality of food. However, what is less commonly known are how these two factors interact relative to each other and their effects on the drying behaviour of potato. These effects were evaluated under controlled drying conditions using a thermogravimetric analyser. The least-squares method was used to fit a model to the experimental data and the response variables (slope of the first drying stage, transition moisture content and slope of the second drying stage) were determined systematically. Central composite design was applied for response surface modelling of the factors and regression methodology was used to analyse the results. The results showed that both temperature and sample dimension have significant (P=0.01) effect on drying behaviour of potato. Sample dimension significantly affected the transition moisture content, whilst both temperature and sample dimension significantly affected the slopes of the first and second stage of drying.     

Modeling Power Transformer Field Drying Processes

Power transformer field drying is becoming a habitual practice because water is damaging for transformer insulation, and its presence increases the probability of unexpected failures. Different drying methods are currently being used by electrical companies but sometimes without a profound knowledge of the drying processes involved, and consequently drying is not as effective as would be desirable.

Physical models have been developed by the authors in order to study power transformer drying processes. The use of the models will help to plan more effective drying processes, tailoring the drying times and drying conditions for each particular case. The models have been tested in a test transformer fitted with sensors. In order to monitor the process, insulation samples were extracted from the transformer before and after the process.

In this article, several transformer drying procedures are described. Theoretical models and their experimental validation processes are reported. Finally, some general recommendations about transformer drying in the field are provided.     

Drying powdered coal with the aid of ultrasound

The results of introducing ultrasound into three different drying processes has been discussed. The processes included: (1) static drying of a layer of powdered coal with airborne ultrasound, (2) rotary kiln drying with airborne ultrasound, and (3) drying of powdered coal on a sloping, ultrasonically vibrated metal trough. In all three processes, the introduction of ultrasound increased the drying rate.

The frequency of ultrasound used was 20 kHz with sound intensities up to 165 dB. The pulverized coal used had a size range from 400 μm to less than 2 smm, with 65% being less than 150 μm in size. Air temperatures used varied from 65 to 300 °C.

The most energy-efficient drying process with ultrasound was found to be the sloping, ultrasonically vibrated trough. However, this process was limited to the use of an initial coal powder feed containing less than 10% moisture. For moisture contents above 10%, a dual drying system could be used. For example, the rotary kiln drier could be used ahead of the trough drier.

Both the static airborne ultrasound drying process and the ultrasonically vibrated trough drying process work best with a relatively thin layer of powdered coal. However, the residence time for the powdered coal being dried is measured in seconds for the sloping trough process in contrast to minutes for the static drying process. The static drying of a layer of powdered coal is also a batch-drying process while the rotary kiln and sloping trough are continuous processes. The advantages of the static airborne ultrasonic drying process would be: (1) closer control of the drying process, and (2) smaller quantities of powder can be used.     

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.