DRYING WITH SUPERHEATED STEAM

Abstract

The principle of drying with superheated steam is known for a long time already, and different designs have been worked out and partly realised. The principle of the steam drying described in this paper starts with superheated steam that is blown on to the products to be dried. The superheated steam transfers its sensible heat to the product and the water to be evaporated. The superheated steam acts both as heat source and as drying medium to take away the evaporated water. The majority of the superheated steam has to be recirculated and reheated. The excess steam from this process, corresponding to the evaporated water, can be used elsewhere in the process or plant, thus making an efficient energy recovery possible. Possible emissions from the drying process can be effectively removed by condensation. Drying by means of superheated steam impingement is one of the possible designs that are in progress now for the paper industry. Work is going on with respect to this type of drying for other materials, especially foodstuffs. At present experimental research at the laboratory steam dryer of TNO-MEP is carried out for vegetables, fries, herbs, cacao nuts, wheat, flour, etc. Besides the energy savings and environmental benefits, the oxygen free steam atmosphere and the higher product temperatures in the dryer appear to have positive effects on the product qualities. For foodstuffs combinations of steam drying with blanching, pasteurisation, sterilisation, etc. are possible and make the steam drying very attractive for food producers. Modelling of the processes in foodstuffs during steam drying is established, both on macro and micro scale. In this paper examples and results of research on steam drying for specific products will be presented.