(79c) Energy Efficient Spray Drying By the Use of Superheated Steam with a Focus on Process Evaluation and Powder Properties of Food Products

The major part of energy for spray drying is used to evaporate water, without the possibility for further use of the applied energy. A coupling of processes acting as energy source and energy sink is a way to overcome this situation. Prerequisite is, that the right amount of energy is available with the required temperature at a certain time. With conventional air spray drying it is not possible to reuse this energy as the dew point is around 40-45°C. Most of the related processes require energy input at higher temperatures. As an alternative, it is possible to use superheated steam instead of hot air for drying. In this case, the excess steam can be used for other processes at temperatures of about 100°C. Thus, the coupling with a pre-concentration step e.g. a multi stage thin film evaporator can reduce the energy requirement significantly.

Heat recovery of the heat of evaporation allows also the efficient drying of low concentrated feeds, which would be very energy intense in conventional air spray drying. This applies especially to products that are difficult to atomize due to high viscosity, like xanthan gum. These materials can be produced more cost efficient by superheated steam drying, provided that the condensed excess steam can be used elsewhere.

Particle size distributions and powder structures resulting from different atomizing techniques will be shown in comparison to air drying results. Bulk density i.e. particle porosity, powder flow and surface wetting of the produced powders will be discussed in detail. Key learnings of the study are related to process design: Due to the high product temperatures, it is essential to remove the dry product from the steam atmosphere as fast as possible. A special rotary valve solution was designed and characterized. It is equally important to keep the temperature impact on the feed stream at an acceptable level. This was achieved by defined cooling of the nozzle system. In a final step, the process was characterized by determining the residence time distribution of the powder in dependence of the particle size.

Pilot plant trials were done with dairy powders and a couple of other food powders and compared to air spray drying. The dairy applications were chosen because of the high production volume worldwide with the greatest potential for a reduction in energy consumption. Other products are interesting as well, because the process itself has an influence on particle properties. This results in an improved solubility of the powder in water based applications. The high temperature of the steam atmosphere has a sterilizing effect which gives the product a benefit In terms of product safety.

Advantages regarding the product quality are related to the absence of oxygen and a short residence time, which is caused by the smaller possible specific volume of the drier due to higher heat capacity of steam. Furthermore, the inert atmosphere is beneficial in terms of process safety, because dust combustion in such systems is not possible. Disadvantages are a more complex instrumentation and process control and a longer start up and shut down process. A big challenge in food production is the high product temperature. This is not the case for available applications in chemical and detergent industry as those products can withstand temperatures of 120 to 140°C. Therefore, in food applications, residence time for sensitive products needs to be as short as possible.