Spray drying is the most commonly applied process for drying food products. The main reasons for using spray drying are the relatively mild heat treatment of the product, the good powder properties that can be obtained (like solubility and bulk density) and the large scale at which it can be applied.
However, a spray dryer is a complex piece of equipment. Many process and product parameters can be altered and aspects like the particle size and residence time distribution influence the process significantly. My Nizo colleagues and myself often visit sites with issues like too low a capacity or insufficient product quality. Looking at the issues, these are short- and long-term issues for which solutions should be identified.
When troubleshooting a spray drying process to solve short-term issues, it starts with good analysis of the issue. This is often underestimated and it should include the interaction between the product and process. For example, when looking at cone fouling, at Nizo we usually start with the determination of the sorption isotherms of a powder, which describe the equilibrium powder moisture content at a given temperature and relative humidity. Next, a sticky curve is determined, which tells you at which temperature and relative humidity it gets sticky (and can attach to the wall of the dryer). This way it can be analysed whether the dryer is running at safe conditions, or whether dryer conditions need to be altered to avoid fouling. For other issues like too low a solubility, too low a bulk density or bad flowability, a thorough product and process analysis should always be the starting point in order to really identify the root cause, instead of applying a tentative solution which may result in additional problems.
When looking at the improvement of the spray drying process in the longer term, understanding the drying process and process control are of key importance. A good example to illustrate this is measurements on the atomisation process. In a spray dryer, a feed is atomised into small droplets, which can coalesce into bigger droplets, agglomerate (together with fines) or dry as a single particle. In addition, it influences the time/temperature/moisture content history of the particle. The primary particle size distribution (ie the droplet size distribution after atomisation) is therefore of key importance to the final product properties. However, the atomisation process is one of the less well understood parts of the spray drying process. The final powder size distribution is often not showing the primary particle size distribution, since the first particles are agglomerated towards larger particles.
At Nizo we have developed a setup to measure the primary particle size distribution with a laser inside a spray dryer. With this setup, we can test different nozzles, different orifices or different product properties. This information can be used to increase the dry solids content of the feed (resulting in a higher dryer capacity), improve agglomeration behaviour, or avoid fouling (by avoiding the generation of very large primary particles).
Another aspect for the longer term is process control. We are quite often investigating issues where the direct cause cannot immediately be found due to large variation in process conditions in the process. In the ideal case, a process should always run the same with similar heat treatment, similar concentration steps, similar holding times etc. This is usually not the case, but companies should understand the most important factors which affect final product quality and take care that those process conditions are kept constant. When looking at a spray dryer, it is therefore intriguing to see that usually relatively little is measured. Feed flow and temperatures are measured and often also the inlet air humidity, but usually not much more. The outlet air humidity, for example, has a large influence on the final powder moisture content, but is often not measured. And when it is measured, it is often done at the high outlet temperature with an RH meter. This, however, can result in inaccuracy levels that are not suitable for process control purposes. A dewpoint measurement will result in a more accurate value, but is relatively expensive.
Recently, Hobré Instruments has developed a novel CO detection system which can also measure the absolute humidity (ie grams of water per kilogram of dry air). This system is primarily a safety system for early warning of smouldering material inside the dryer, but the water measurement is very precise and offers the possibility to use it for process control. At Nizo we are currently proving how this reading can be combined with model-based process visualisation and monitoring, so optimal conditions can be chosen with, for example, varying inlet air humidity. This offers the possibility to run a dryer at optimal capacity with varying weather conditions, like rainy or dry days, hot or cool days and with a constant product quality such as moisture content.
Summarising, spray dryers are very common, but also complex equipment. Troubleshooting with a good problem analysis will remain necessary, as well as a focus on product quality, capacity and energy efficiency. To improve product quality, the drying behaviour of all the various products as well as the processing should be better understood. Therefore, companies should especially focus on understanding atomisation and improved process control. Process control can be achieved by a combination of advanced sensors for a variety of product properties in combination with dedicated control. With regard to capacity and energy efficiency, especially air dehumification in combination with winning back energy from the outlet air looks promising.
© FoodBev Media Ltd 2020