J. J. Nijdam

An Investigation of Milk Powders Produced by a Laboratory-Scale Spray Dryer

ABSTRACT A mini spray dryer has been used to investigate morphological changes that occur to milk particles during the spray drying process. We have found that the mini spray dryer is ideal for such investigations, because phenomena such as skin and vacuole formation in particles can be analyzed without the added complication of particle agglomeration, which only occurs in much larger spray dryers where particle number concentrations are higher. We have confirmed observations made by various researchers that the bulk density of spray-dried milk powder is greatly affected by the drying temperature, due to the strong influence of the latter on the porosity of the particles. In addition, we have attempted to explain observations made by various workers that fat accumulates preferentially at the surface of a particle during drying by postulating that fluid fat is transported towards the surface, via a network of cracks and pores, by the development of a vacuole overpressure which is also responsible for the inflation of the particle. Finally, we have shown that milk powders can be spray dried a second time, by reconstitution with water, with no change to the thermodynamic characteristics of the resultant powder. Thus, milk concentrates for spray drying research can be prepared from already-spray-dried milk powders rather than using the more arduous evaporation method to concentrate unprocessed milk.

Challenges of Simulating Droplet Coalescence within a Spray

Abstract This article reports various challenges that have been encountered in the process of developing validated Lagrangian and Eulerian models for simulating particle agglomeration within a spray dryer. These have included the challenges of accurately measuring droplet coalescence rates within a spray, and modeling properly the gas–droplet and droplet-droplet turbulence interactions. We have demonstrated the relative versatility and ease of implementation of the Lagrangian model compared with the Eulerian model, and the accuracy of both models for predicting turbulent dispersion of droplets and the turbulent flow-field within a simple jet system. The Lagrangian and Eulerian droplet coalescence predictions are consistent with each other, which implies that the numerical aspects of each simulation are handled properly, suggesting that either approach can be used with confidence for future spray modeling. However, it is clear that considerable research must be done in the area of particle turbulence modeling and accurate measurement of particle agglomeration rates before any Computational Fluid Dynamics tool can be employed to accurately predict particle agglomeration within a spray dryer.

A high-temperature drying model for softwood timber

The high-temperature drying of a softwood board with relatively high permeability was modelled on a one-dimensional basis. The moisture-transport mechanism between fibre saturation and irreducible saturation was elucidated, and moisture flow within the thin layer of damaged cells at the sawn surfaces of the timber board was examined. The predicted temperature and average moisture-content profiles with respect to time were in good agreement with some experimental data from the literature. Comparison between the experimental and predicted within-board moisture-content profiles illustrated the importance of accounting for the orientation of the growth rings with respect to the drying surfaces and the effect of density variations on liquid permeability at high core moisture contents.

Intermittent and Continuous Drying of Red Beech Timber From the Green Condition

Intermittent and continuous schedules for drying New Zealands indigenous red beech timber (Nothofagus fusca) from the green condition have been compared. Both schedules can be used to dry red beech with good timber quality. Continuous schedules are appropriate for use in conventional kilns, where better humidity control can be used to minimize timber degrade and equalize moisture contents. Intermittent schedules can be adapted for use in dehumidifiers, where the lower achievable humidities—often a result of poor insulation and air leakages—can be compensated for by the relaxation periods, when the fans and heaters are switched off intermittently to relieve drying-induced stresses. However, equalization cannot be easily applied in a dehumidifier. In this case, a presort of the green timber into two classes—a heartwood heavy class and a mixed heartwood light and sapwood class—would then prove beneficial to address the issue of significant timber property and green moisture content variations that occur in red beech timber.

VALIDATION OF THE LAGRANGIAN APPROACH FOR PREDICTING TURBULENT DISPERSION AND EVAPORATION OF DROPLETS WITHIN A SPRAY

The accuracy of the Lagrangian approach for predicting droplet trajectories and evaporation rates within a simple spray has been addressed. The turbulent dispersion and overall evaporation rates of droplets are modeled reasonably well, although the downstream velocity decay of the larger droplets is underpredicted, which is attributed to a poor estimate of the radial fluctuating velocity of these droplets at the inlet boundary. Qualitative agreement is found between the predicted and experimental evolution of the droplet size distribution downstream of the nozzle. These results show that smaller droplets evaporate preferentially to larger droplets, because they disperse more quickly toward the edge of the jet, where the entrainment of fresh air from the surroundings produces a significant evaporative driving force. Droplet dispersion and evaporation rates are highly influenced by the rate of turbulence generation within the shear layer. This work demonstrates the potential of the Lagrangian approach for analyzing particle trajectories and drying within the more complex spray dryer system.

THE INFLUENCE OF KILN GEOMETRY ON FLOW MALDISTRIBUTION ACROSS TIMBER STACKS IN KILNS

ABSTRACT A one-dimensional mathematical model is evaluated for flow through a timber-drying kiln. This model is used to develop recommendations for the design of the plenum chambers on either side of the timber stack. The variation of the vertical air velocity was explored for a sticker spacing of 20 mm and a board thickness of 50 mm. When the width of the plenum chambers is at least equal to the sum of the thickness of the stickers, flow maldistribution is substantially reduced in single-track kilns. The width of the plenum chambers of double-track kilns may be reduced to three-quarters of the sum of the thickness of the stickers, because the increase in timber-stack resistance to airflow mitigates the effect of pressure variations down the length of the plenum chambers.

AIRFLOW BEHAVIOUR IN TIMBER (LUMBER) KILNS

ABSTRACT Even drying of stacked timber boards in a kiln depends, amongst other things, on having a uniform airflow through the pile. A study of possible airflow maldistribution in existing kilns has been carried out using a Perspex model of a kiln, with water as the fluid, which has been designed to be geometrically similar to an actual kiln and have dynamically similar flows. Measurements of local velocities were possible by tracing the flow of fine gas bubbles. The standard kiln arrangements result in significant recirculation zones before and after the stack, with a non-uniform incident velocity profile. Different design alternatives for the geometry of the plenum spaces on either side of the stack have been investigated.

MOISTURE MOVEMENT ON DRYING SOFTWOOD BOARDS AND KILN DESIGN

ABSTRACT This paper provides an overview of present understanding of how moisture can move through softwood boards, as a basis for determining kiln-seasoning strategies. Moisture in green wood is held essentially unbound, whereas below fibre saturation it is bound to a variable extent to the fibre walls. Sapwood, which is that part of the timber used for the transport of liquid nutrients, contains more moisture than physiologically inactive heartwood. Sawing the felled log creates a moisture-denuded layer at the damaged exposed surfaces. These features have a profound influence on the way that moisture can be removed on drying. Superimposed are differences arising from seasonal variations in the growth of wood between earlywood and latewood, which have different moisture permeabilities. When the width of the annual growth ring is relatively large compared with the board dimensions, moisture movement and the development of drying stresses depend markedly upon the sawing orientation relative to the grain direction. Quarter-sawn boards dry more uniformly (in the direction normal to the drying surfaces), but more slowly than flat-sawn boards. Most timber boards are stacked and then dried in box-shaped kilns. The uniformity of drying depends on the goodness of this stacking and on a uniform airflow being presented to the inlet face of the stack. Some non-uniformities can be mitigated by periodic reversals of the airflow direction through the stack and by overdrying the majority of boards to reduce wet spots, but there are limits, while overdrying reduces kiln capacity. Attention to aspects of the kiln geometry can reduce the fan-energy requirements and shorten the drying time, with a more uniform moisture content through out the kiln load.

Effect of Diffusion on Component Segregation During Drying of Aqueous Solutions Containing Protein and Sugar

Water content profiles within thin aqueous films of lactose and bovine serum albumin (BSA) were tracked during drying using inverse microscope Raman spectroscopy (IMRS). These film drying experiments provide useful insights into component diffusion within droplets during spray drying in the food industry. Virtually no segregation of lactose and BSA occurred in the films during drying at 30°C. Thus, in film drying simulations at this temperature, lactose and BSA can be assumed to be a single homogenous solute, and binary diffusion of water and this single solute can be modeled. A solute-fixed coordinate system drying model properly predicted the water content profiles and shrinkage of the films during the constant rate period. This represents a partial validation of the model. The drying model predicted significant gradients in water content in films dried at a higher temperature of 80°C, which have the potential to drive segregation of lactose and BSA. However, IMRS analysis of films dried at 80°C revealed no segregation of lactose and BSA. We propose that lactose and BSA are relatively immobile in the bulk of the film during drying, forming a homogenous gel that allows the significantly smaller water molecules to diffuse through. The observed accumulation of BSA as a monolayer at the surface of the dried films appears to be a local phenomenon most likely driven by the surface activity of BSA.

An improved drying model for highly-impermeable hardwoods.

Abstract Previously reported drying experiments on highly impermeable hardwoods have shown that bell-shaped within-board moisture content profiles develop as drying proceeds, with a change in concavity around the fibre saturation point. However, drying models based on Ficks second law of diffusion with an Arrhenius temperature dependency for the diffusion coefficient predict a parabolic form. The present work showed that the influence of moisture content on the diffusion coefficient must also be considered to improve these drying predictions. A drying model that includes both temperature and moisture content effects on the diffusion coefficient was developed. This model is able to accurately predict the drying rates and within-board moisture content profiles of New Zealands indigenous red and hard beech timbers for various timber thicknesses over a range of temperatures, relative humidities and air velocities used in practice.