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Encapsulation via spray drying – Part 1

Prof. Dr.-Ing. Erich J. Windhab (ETH Zürich)

Encapsulation via spray drying I - structure formation for encapsulation in spray drying /-chilling
Hello, and welcome, everybody on this course lecture for spray drying encapsulation, the course part number one, which will be entitled with subtitles as introduction, spray filament and drop breakup in spray processing, structure morphology changes along drop particle tracks in spray drying and freezing, drop/capsule drying kinetics, and finally, summary and conclusions will be given.
Let me start with giving you an overview on nomenclature abbreviations, which you can use when studying the scriptum and with my introduction, again, by the expert square scheme, because it gives us a nice overview what’s addressed from a process structure and properties side because process make structure. Structure codes properties. So these relationships are in principle of major impact. On the property side, to start backwards in a reverse engineering manner, so we have the consumer’s interest in providing the best properties, satisfying preference, exemptions, and needs. Technofunctional properties, but also be of economic interest to produce products accordingly. The structure is always a hierarchical scheme from micro to macro scale.
In this first course part, we will focus on filament drops, capsules, and the glomerates as they can be generated from the spray in order to be suitable for encapsulation. And the processing aspect will address drop formation and spray filament breakup accordingly, followed by solidification upon drying or crystallisation of emulsional suspension systems. Encapsulation in simple multiple emulsions or in spray particle agglomerates will be named. Spray drying process is probably well-known. So we have as the main part a drying chamber. In the drying chamber we have the spray.
We have a circulation of hot gas for drying that can be a recirculation also, backwards in order to use, let’s say, the fines as well as the hot gas from an energy efficiency perspective and material efficiency perspective again, and forming some agglomerates by the finds, and using the energy which is in the hot gas, which is exhausted again. Key relationships– effect on feed composition will have an impact on spray filament and drop breakup on the drying kinetics on particle surface properties and a shell formation. Agglomeration in the spray dryer and wall deposition is a key issue. Product particle stickiness can be quite a hindrance in such a process. The size of the chamber will accordingly have to be dimensioned.
And the positioning of fines, the returns, will have also a strong impact on the shell formation of these type of products. The functional product properties are the goal. And what I showed here with the special marks is of major encapsulation relevance, because in these first course part I will look a bit more closely to the boundary conditions from the spraying technology. Looking at the spray tower, we have different types of nozzles, which can be two phase nozzles with gas and liquid sprayed, or only one phase nozzles only with a liquid.
We are mostly in favour of two phase nozzles because this gives higher flexibility in generating smaller droplets, and also tailor them in the more narrow size distribution within the spray tower. Here you see a spray tower which is equipped for spray freezing, but similar let’s say design is also suitable for the spray drying as we will see later on. Now let’s start. Whenever we run through the nozzle with the filament, the filament will break up. And depending on the velocity and particularly the relative velocity between the filament and the surrounding air. So we will have different break up patterns. As you can see here, the so-called Rayleigh breakup, first wind, second wind breakup, and the atomization.
We can, in principle, visualise this easily. So this is through a flat nozzle. One can nicely see how filaments are forming, then breaking up and into single droplets, which are then, during the separation of the spray, then also having larger distances in the early section. And so there might still be some re-coalescence. But as soon as the spray has developed, this should not happen any longer. The so-called Ohnesorge number versus Reynolds diagram is a typical diagram for looking at sprays. Ohnesorge is a combination of the capillary number and the vapour number. And so we have the Reynolds number here on the x-axis.
And this type of Ohnesorge versus Reynolds allows us to have a nice distinction between different zones of breakup. So as we saw the breakup patterns before, A, B, C, and D. So they are nicely sorted between these lines shown here. So this is why the Ohnesorge Reynolds diagram has become quite famous for checking the spray type and breakup of the filament and formation of droplets. The question for us is very much what type of filament, breakup, or are there limitations with respect to capsule structure formation and stability. Can we keep a certain– can we adjust a certain structure? That’s key for us.
If you look in further detail to such a filament breakup we can subdivide between let’s say, so-called spray filament core. Then we have a multi-phase mixing zone where we have the breakup in larger chunks. And finally, the larger chunks or larger drops will break up into smaller drops depending on the so-called vapour number, which is the characteristic dimension number allowing us to estimate what an equilibrium droplet size or mean droplet size will be at the end. We have different instabilities in the system, which are supporting the breakup. The Rayleigh instability is making of wavy surface. And the Kelvin-Helmholtz instability is kind of wave generation by the relative motion of the airflow relative to the surface of the droplet.
And these surface waves are then stripped off in little droplets. This is a bit more demonstrated here in further detail. We call this also a so-called cascade breakup. Cascade, this starts with a so-called bag breakup at a certain vapour number range. The vapour number defined as the gas density times the diameter of the droplets, the non-deformed droplet times the relative velocity between gas and droplets to the square, and divided by the individual tension. So if the so-defined vapour number has numbers between 12 and 150, we have the bag breakup, between 150 and 350, the stripping breakup, and finally a catastrophic breakup for larger vapour numbers than 350. This can also be modelled, this cascade of breakup.
And there are some assumptions made and the breakup frequency defined depending on these ranges of vapour numbers as we have just discussed. And then we can, in principle, describe the radius, which is generated divided by the initial radius by a kind of exponential function for this breakup in the different domains. So a turbulent breakup is possible to be simulated. You see a bag breakup assimilated here together with Franz Tanner, Kathy Feigl, and we can identify these bag formation and the breakup into a certain distribution of droplet sizes. This shows the spray as it develops from the nozzle. At the nozzle outlet, we have the diameter of the filament, which is small is equal to the nozzle diameter.
And then we have this breakup zone, first of the filament, and then of larger chunks and droplets into the fine ones. We have a scale on the right-hand side. So this would be droplet size distributions up to 150 microns. That’s what it tells us here. And so the small ones in the bluish colours, they are around, let’s say 20, 30 microns. This can also be done for simulation– or the simulation is also possible for the transient development of the spray, as you can see here from 0.2 up to 1.2 milliseconds, we can see the spray development versus time.
In case we want to spray also under, let’s say heated or freezing conditions in order to preserve some internal structure– so this is just showing that besides the drying, we can also have that send some cold gas by liquid nitrogen evaporated and then solidifying. I’m mentioning this here because it has a big advantage. We are not losing mass. And as a consequence, we can better identify and also forward test sprays. We can easily measure the resulting droplet and particle size distributions. This can be approximated by like a Chi-squared distribution as shown here.
So this is from the model, but also approved or validated in the work of Tim Althaus, where you can see the experimental data shown here with the closed curves and the dotted curves are giving us, where the full symbols are giving us the simulation. So there is quite a good agreement between these experiments and the simulations. In order to also simulate the impact of the temperature– so in this case now again, in the freeze tower. So for cooling down and freezing the particles, you can see that also the temperature distribution and the cooling down of the particles is also accessible by the simulation for a certain set of parameters like exemplary shown here.
Now when we look at the spray performance, so we have to take care about really generating a narrow size distribution– this is our interest– of the droplets and the final particles. And accordingly, as the droplets, when they are very fresh, they may coalesce. So from two separated droplets, we may get again one. And in a further drying state, when we have already seen these solid type of particles, they may have sticky surfaces and stick together. Or in a later state they may be non-sticky, and then it’s no problem. So this can be a bit sorted according to the Ohnesorge number square because we have the square root in the definition of the Ohnesorge number.
This is why we take the square. Risk is forces divided by inertia and surface tension. Forces is the meaning. And we can see Ohnesorge square is smaller than one, we still have the very liquid droplets and everything is dominated let’s say by inertia and surface tension properties. And then this is where, when it’s so liquid, so the coalescence may dominate if it gets higher risks. So we have an increase in the Ohnesorge number. Then the temperature, and particularly the difference between the surface temperature and the glass transition temperature, give us an idea about the surface stickiness and this probability to stick together or to keep separated.
In drawing experiments, one can prove this. And you can see here, when you look at the radius change of a little drop which hangs on a wire, so in the drying chamber now for a drying experiment. So to simulate the spray drying we have coalescence and stickiness in a very early stage here. And as soon as the skin is formed, which is no longer sticky, so then it’s no longer a problem. So we can keep the sizes, which have been adjusted at that point. From assimilation, which Loredana Malafronte has done in her PhD work, so we can see regions for coalescence where the drops are still very liquid and fulfil these conditions for coalescence and where they are sticky.
So let’s say agglomeration regions where the stickiness may lead to this type of coalescence, but agglomerate type of formation.
We might be interested to even form agglomerates sometimes. So in freeze spray systems, we may be interested to centre together the powders. We can do this by slight increase of temperature and make it tabulating or centering type of single particles which have been generated. And in spray drying, using the stickiness characteristics and agglomerating, or we might even add some liquid components afterwards in order to generate such agglomerates. This is certainly also a good way for encapsulation in a not completely sealed capsule, because the interfaces are not completely sealed. We have no closed shells but nevertheless, in the voids of such a system, encapsulation of particulate or even molecule layer components is possible.
Now let me finally get in to some observations of drying kinetics, again from simulations. But also there could be measurements of particle tracks with traces or particle imaging velocimetry type of measuring can give an insight into particle tracks. Here it’s from simulation done by the group of Mezhericher and co-workers. And you can see four different particle sizes, the tracks. And you can also see that as soon as you reach let’s say a colour which is in the bluish zone, so it should no longer be sticky.
So that means the stickiness or the coalescence probability is more or less in the very close to the nozzle domain if you have the temperatures high enough adjusted and the drying kinetics is going fast enough. Now a bit to the theory of the drying kinetics when we have a pure liquid drop. So we have the heat transfer to the drop and the vapour transfers or the water transfer out of the drop. So heat and mass transfer can be modelled in a dimensionless manner by the Ranz and Marshall equations as shown here and well known. They are used by many authors.
If you want to go a bit more into detail, energy conservation and mass conservation can easily be formulated for this case. And then the droplet mass can be calculated depending on this balance– or on these balances. And also the drop radius evolution is depending on this. If you want to translate this to an encapsulated type of system, or to what has also been modelled formally by, again from the group of Mezhericher and co-workers. So the shell formation. So you can look at the red core combined with the shell.
And this is similar to having an encapsulation done, like with an encapsulated skin which you may have generated according to the formulation of the system and having the red core inside during the drying. So the shell has pores through which we can have the mass flow, the water vapour flow. And the heat flow certainly has to cross the shell which may have different heat conductivity than the wet core. So again energy conservation for the shell. And the core can be separated under certain boundary conditions and the vapour transport re-formulated.
At the same time– so when in terms of if there’s crust formation or if the shell thickness also for encapsulates may increase, so you may have a receding rate of the crust wet core interface. So this will move to the inside to smaller radii and be described by the equations here shown below. Just to substantiate this a bit with real systems, when we look at the drawing kinetics of various milk products, because this is a nice series where the fat content is increasing from the skim milk to the heavy cream. So we can see when we have the normalised weight of these components versus time.
So a normalised drying kinetics, we can see that we have the fastest drying kinetics for the skim milk and the slowest for the heavy cream and also the coffee creamer. So the reason is we have dispersed fat in the system, which is a hurdle for the water transport. So we can see this in the slide here where the effective diffusivity through these materials has been evaluated experimentally, again by Loredana Malafronte in her PhD work. And you can see with increasing fat content, here the nice dispersed fat droplets are shown in these micrograph. So we have a slowing down of the kinetics as a consequence of these hurdles for the water transport.
What is superimposed certainly is the shrinkage due to some water loss due to the water transport and the heat transfer to the core, to the shell, and to the core is certainly determining the kinetics from a processing parameter side. With this I would like to summarise. So we have shown the type and intensity of spray filament and drop breakup are crucial for what is formed out of the filament. And this is a basic boundary condition when we want to encapsulate, that we do not too finely atomize, because then structures which we may bring in may be destroyed. Atomization does, as a consequence, seems not to be preferred compared to a Rayleigh filament breakup, which is the most gentle breakup.
We will come to this in a second course part. The spray simulation can be done. There is already advanced 3D programmes which can, by computational fluid dynamics, enable quite good estimates of spray drop and resulting powder particle size based on the models during spray processing. We have some hurdles due to collisions of drops and particles. The drops may coalesce. The particles, semi-dried droplets may still be sticky at the surface stick together. The drying kinetics can experimentally determine, but also be simulated, even though the simulations are still in the development phase, at least on a 3D base in order to fit optimally, in particular when we have multiphase drop systems as they are of interest for encapsulation.
And the models for dried crust formation and a skin of capsule type of powders can be compared to the same type of models with a bit of modification of the characteristics of the shell that can be applied. And last but not least, the multiphase composition of the droplets sprayed have a strong impact on heat and mass transfer to be taken into account. This has been demonstrated for the milk systems, which we have briefly addressed. With this, I have another table of nomenclature and abbreviations, particularly for the particle drying modelling that you can follow this easily, the equations. And with this, I would certainly like my co-workers and the funding bodies for the support of these type of work.
And I hope you have enjoyed that and do some rework of what you have heard. In case of questions I’m certainly also ready to interact and correspond with you. So thanks a lot for your attention.
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Drying and Encapsulation of Food Ingredients in Powder Form

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