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Clever absorption process for drying herbs

Researchers at the Institute of Energy Systems and Fluid Engineering (IEFE) at the ZHAW have developed a new drying process for food based on the absorption principle. Until now, herbs, tea and spices have been preserved using conventional condensation dryers, which are energy-intensive and require considerable amounts of electricity. The absorption process developed by the ZHAW is a sustainable alternative that requires 75% less energy and is also economically viable.

We have been preserving herbs, tea and spices with air for thousands of years - whether directly in the sun or in a professional environment with warm, dry air. In the food industry, the warm drying air is usually generated using fossil fuels or electricity (directly or with a heat pump). This process is energy-intensive, pollutes the climate (CO2) and causes high energy costs. As part of a research project co-financed by the Swiss Federal Office of Energy (SFOE), the Institute of Energy Systems and Fluid Engineering (IEFE) at the ZHAW has developed a new drying system. This utilises the absorption principle, operates at a low temperature and can therefore dry the herbs using environmental heat or waste heat. The closed absorption cycle is based on a solution of sodium hydroxide (caustic soda, also known as baker's lye or pretzel lye), which does not come into contact with outside air.

Divided processes of absorption and desorption

The dried material - the ZHAW worked with nettles, apple mint and peppermint - is placed in a drying chamber developed by the IEFE researchers. The chamber is sealed tightly and a fan blows in dry air. This extracts water from the dried material (drying process). The moist air from the chamber is channelled into the absorber. In the absorber, the air is sprinkled with a concentrated solution of caustic soda (NaoH). The caustic soda binds the water and the air is thus dehumidified. The originally concentrated solution becomes a diluted solution due to the absorption of water. It is channelled to a heat exchanger, where it is heated with the heat from a solar thermal system or with process waste heat. The warm solution then enters the desorber. Dry outside air, which is blown into the desorber, removes some of the water from the caustic soda and thus increases the concentration. The humid outside air is blown out - and with it the water that has been removed from the drying of the herbs.

Caustic soda stores solar energy

The whole process also works for a certain time without heat supply (e.g. lack of sun in winter), which acts as "driving energy" for desorption. In this case, the concentration of the soda solution slowly decreases because less water can be removed during desorption. Nevertheless, with correctly dimensioned storage tanks for the caustic soda solution, the drying system can continue to be operated without the addition of heat because the energy density of the caustic soda solution is very high at 1800 MJ/m3 - compared to the energy density of 100 MJ/m3 of water at 100 °C. When the heat is added again, some of the water can be removed from the diluted solution. The concentrated solution charged with "driving energy" can now be stored for any length of time and without loss. Another advantage: the desorption process for concentrating the caustic soda does not have to take place on site. Desorption could take place in a sunny location with "solar collectors" or with unutilised waste heat and the concentrated solution could then be transported to the drying plant in tankers or pipelines.

Just as good in terms of quality and flavour

After drying, the herbs dried using the new process were analysed by the Institute for Food and Beverage Innovation (ILGI) at the ZHAW in Wädenswil for undesirable ingredients and flavour content. The dried nettles were crushed and compared with store-bought nettle tea. When comparing the colour of the herbs and the pH value, as well as when tasting the brewed tea, no differences in quality or taste were found.

Lower costs, 75 per cent less energy

Based on several comparative drying processes (nettles, apple mint, peppermint), it has been proven in practice that the sorption drying process can achieve and even fall below the prescribed final product moisture content of 12%. In addition, the sorption process can more than halve the drying time compared to conventional processes. And sorption drying also shines when it comes to the energy balance. It requires around 75 per cent less energy than conventional drying with condensation dryers. Even if the investment costs of the sorption drying system are currently still almost 10 per cent higher than those for a conventional drying system, the new process pays off twice over. The total costs over the 20-year service life of the system are 30 per cent lower than those of a conventional drying system. In addition, the new sorption drying system only requires half the drying area - which is particularly important in regions with high land and property prices. Sorptive low-temperature drying therefore has advantages in all areas.

The concept developed by IEFE researchers is characterised by its modular and scalable design and simple control system. Sorptive low-temperature dryers can be easily expanded with several small absorbers, thereby increasing energy independence or drying capacity. In addition to applications in the food industry (herb drying), the scientists also see other areas of application, for example in paint shops or for drying building materials. The next step is likely to be the development of peripheral components that are not yet available, such as storage tanks, regeneration systems and transport lines. Further pilot plants can then be constructed.

Project name

SONITRO - Sorptive low temperature drying


Project manager: Thomas Bergmann

Project team: Irene Chetschik, Serena Danesi, Claudio Koller, Lukas Vontobel

Funded by: Swiss Federal Office of Energy (SFOE)

Project partners: Karl Barth AG, Matthias Barth

Project duration

March 2020 to September 2023

Project publications

Final report SONITRO:

SFOE technical article: