One drop at a time: a revolution in drying evaluation technology

The ability to test products and processes has always been an essential part of new product development. Until now, food processing companies have had to conduct exhaustive tests to establish the most appropriate formulations to optimise taste and the manufacturing process. But now a new particle analysis process, pioneered by GEA Niro, has the ability to revolutionise the testing of products requiring spray drying to enable greater flexibility during the development stage, bring products to market faster then previously possible and conduct the whole process much less expensively than has been possible to date. The process is called DRYNETICS^TM.

Whether food processing companies are manufacturing food products, additives or flavourings, it is always necessary to test any proposed product changes extensively. When products are to be spray-dried, the testing will be aimed at achieving the desired product characteristics in a commercially acceptable way and using the minimum amount of energy. The characteristics under question may include: solubility, colour, particle size, particle structure (hollow spheres, donuts, solid particles, etc.) stickiness, flowability and, of course, taste and nutritional value.

Until now, each product variant would need to be tested separately in a spray dryer to establish these properties and then re-tested to establish the effect of changes in feed or the drying environment. This process has been very time consuming and, therefore, expensive. Furthermore, even when using small capacity dryers specially designed for R&D work, the volume of product required to conduct these tests effectively is considerable: a real problem when products are expensive.

DRYNETICS^TM – drying for the 21st century

The process of DRYNETICS^TM, however, applies science to address this problem. Instead of drying a batch of feed material and analysing the result, DRYNETICS^TM dries just a single droplet of liquid, mimicking the drying conditions within the production dryer. The whole process is fast, reliable, uses virtually no raw material and the results can safely be extrapolated to indicate the result under full production.

The new technique uses an ultrasonic levitator to suspend a droplet of feed material in mid air as it dries under carefully controlled conditions. It allows GEA Niro to watch the changing structure of a single particle as it dries and extract a mathematical description of its drying kinetics for use in Computational Fluid Dynamics (CFD) simulations. This innovation allows the company to test and design powder properties to meet customers’ needs using just one drop of product rather than the several kilos that are required with traditional test equipment. It also makes it possible to test a wide range of feed properties and drying parameters much more quickly than previously.

Although ultrasonic levitation is not a new concept, using it in this way has not been done commercially before. “Before we would have to use several kilos of product, which would take a long time,” explained Michael Wahlberg, Test and Development Manager at GEA Niro. “Now we can screen a lot of different feeds very quickly, predict the morphology and work out the drying curve.”

“We can measure the drying kinetics of the material for use in CFD simulations,” said Michael. “This lets us predict when we will have deposits in the drying chamber and where they will form. We can change the feed, temperature and airflow within the dryer to minimise deposits and obtain the best possible particle properties for the customer.” It’s also possible to validate the technique by experimentation, thereby reducing the time and cost of bringing new products to market.

For customers, this analysis of how particles dry can help them make better products by adjusting drying parameters to create exactly the type of particles they require. For example, some customers might want a strong particle that is not easily damaged during transport; others might want a lightweight powder that dissolves easily. “Using the ultrasonic levitator, we can determine if a given feed dries into particles that are, for example, hollow spheres, donuts or shrivelled particles,” said Michael. “We can even coat particles for slow release or taste masking.”

As well as the benefits to product design, the new technique has the potential to completely change the way spray dryers are designed in the future. “We will no longer have to estimate how a powder will behave in the dryer,” said Michael. “New equipment can be designed using the mathematical description of how a particle dries to make dryers more efficient, kinder to the environment, safer, smaller and of higher capacity than has been possible in the past.”

Practical applications

The ease, speed and relative economy of DRYNETICS^TM makes the refining of designer products much easer than has ever been possible in the past. Where previously a development programme might test a handful of different recipes, now researchers have the ability to adjust formulations at will, until they achieve precisely the characteristics they desire.

This ability will prove to be most valuable when working with products that can be changed relatively easily. For example, drying a base product such as milk has limited opportunity for creativity. Drying a food additive or a composite product such as soup, however, might allow for many more adjustments to the feed recipe to achieve the desired result. The use of additives, for example, can have a dramatic effect on the hygroscopic properties of a powder and the way in which they behave during the drying process.

DRYNETICS^TM also allows different types of particle to be created to meet specific needs. For example, hollow spheres or donuts will produce a less dense powder than solid particles.

A sticky problem

The stickiness of a product during the drying process can be a problem. Sticky products can adhere to the walls and atomisers in dryers resulting in product wastage, frequent stoppages for cleaning and, in extreme cases, a risk of explosion.

Over the years, there have been many methods of testing products for stickiness. They have all been expensive as they have to be done in purpose-built equipment as testing in pilot plant equipment is unreliable.

In general, equipment for measuring stickiness may be divided into two groups – one group involves the actual drying of a product, whereas the other requires the wetting of a powder.

One method, for example, is to place a dry powder in a glass tube and stir it with a motor-driven impeller. The temperature or the air humidity (or both) in the test tube is increased gradually. When the powder becomes adhesive, the individual particles stick together and the torque necessary to turn the impeller increases dramatically. The last recorded set of temperature and humidity is defined as the sticky point.

A similar method uses a fluid bed. The humidity of the fluidisation air is slowly increased until the particles become sticky and the fluidised layer collapses. In this way, the humidity and temperature of the sticky point are found.

An interesting technique for measuring the stickiness of dairy powders involves accelerating particles in a particle gun and impacting them against a stainless steel plate using air at different levels of humidity and temperature. The number of particles deposited on the steel plate is used as a measure of the stickiness of the powder at the given air temperature and humidity.

A different approach is to measure the stickiness using a feed material rather than moistening a dry powder. One of the methods is called a ‘probe tack test’ in which a large droplet is placed on a small platform and dried under well-defined conditions. At a predetermined time during drying, a steel probe is lowered to touch the particle surface. If sticky, the particle surface adheres to the probe; the force required to raise and thereby separate the probe from the particle is used as a measure of the stickiness. The advantage of this method is that the particle moisture content is known along with the drying conditions. However, the greatest advantage is that the method involves actual drying of a droplet and therefore resembles the spray drying process.

It is difficult to conduct stickiness measurements during conventional spray drying. Spray dryers have, however, been fitted with small steel plates in the drying chamber to measure the wall deposits. The plates are weighed before and after an experiment and, though tedious, the method gives useful data about the stickiness of a given product during spray drying.

Measuring stickiness using DRYNETICS^TM

Using DRYNETICS^TM, the particle is dried in a way that mimics the operation of the spray dryer using the feed material not by moistening a powder. The stickiness is measured on the surface of the particle by contacting the particle with a steel plate because this process occurs during the formation of unwanted wall deposits during spray drying. The method also allows the particle moisture content and temperature to be measured at any point in time during drying.

Measuring stickiness in this way makes the measurements more relevant to spray drying. The particle is dried under well-defined conditions and the drying temperature and moisture content are known throughout the entire drying process. The result of the measurement is a critical stickiness time i.e. the point in time during which the particle becomes non-sticky. Besides giving the specific stickiness properties of any given feed, the results may be used to obtain a deeper understanding of particle drying processes. In addition, the method can be used to quickly compare the dryability of different products and feed formulations.

The stickiness measurements are taken by suspending the droplet on a very thin glass filament rather than being suspended in the ultrasonic field. The droplet is dried in the drying gas stream and, at a predetermined time, a stainless steel platform is inserted under the droplet. The filament is lowered so that contact between the partially-dried particle and the platform is achieved. The filament is raised and the stickiness of the particle is determined visually. The processes of moving the platform as well as the filament are fully automated to ensure repeatability and to eliminate any operator dependency on the result.

If the particle is sticky, a new droplet is inserted and the drying time is increased. This is repeated until the time is found where the particle becomes non-sticky. This time is referred to as the ‘critical drying time’.

Results obtained from this stickiness measurement method can be implemented in CFD (Computational Fluid Dynamics) simulations of full-scale spray dryers.

Conclusion

The droplet drying process is an essential part of spray drying. Each product subjected to spray drying has unique drying properties and consequently the design of the dryer must depend on the drying properties of the feed in question.

To design dryers and product formulations for optimum performance, a detailed knowledge of drying kinetics, morphology formation and development in particle stickiness during drying is very valuable as a supplement to the conventional design methods. Coupling the ways in which single droplets dry with CFD simulations has the potential to further refine the process.

DRYNETICS^TM, developed by GEA Niro, is the modern way to identify these properties quickly and relatively cheaply to allow the maximum experimentation in product and equipment design to achieve desired products in the most commercially beneficial and environmentally sustainable way.