Technical evaluation of a modular dryer for medicinal and aromatic plants in practical German conditions

Abstract

A modular batch dryer with partial recirculation of outlet air to save thermal energy was developed for small-scale medicinal plants producers in Germany. Different operational modes were tested for energy consumption and the quality of the dried product using lemon balm leaves. Fresh air mode, partial recirculation-controlled flap mode and full recirculation-controlled flap mode alternated depending on the progress of drying and the set relative humidity at the inlet. Experiments consisted of comparing two modes of controlled flap modes with relative humidity varying from 80 to 30 % for one mode and fixed at 40 % for the other mode. A total mass of 500 kg of lemon balm leaves were dried in an average of 23 hours to reduce moisture content from 72 % to 7 %. The throughput ranged from 19.7 to 23.7 kg∙h-1 for lemon balm drying. Air recirculation significantly reduced the thermal energy consumption where a specific thermal energy consumption of 3540 kJ·kg-1 H₂O was achieved by controlling the inlet air humidity to 40 % when drying lemon balm compared to the 4075 kJ·kg-1 H₂O achieved under the 80-30 % mode. No significant difference in essential oils content was observed between the two humidity control modes. This research confirmed the energy-efficient attributes of the dryer and recommends the implementation of air recirculation as one of the methods to reduce energy consumption in medicinal plants drying.