Energy efficient microwave hot-air drying of porous materials
Special Symposium - Innovations in Food Technology (LMC Congress)
Hygiene, Hygienic Design & Unit Operations (Food-5b)
Keywords: microwave convective drying, energy efficiency, porous material
New drying processes, including combined-methods, have been developed to increase dehydration rates and simultaneously improve the quality of dehydrated products. Microwave assisted air-drying is a promising technique for plant foods, due to the shorter drying time and better quality of the resulting products. These advantages are mostly due to the fast, instantaneous, volumetric heating caused by microwave energy. Air flow on the other hand can contribute to a more homogeneous heating and effective removal of evaporated moisture. The development of an energy efficient process requires that air and microwave energy are balanced taking into account the heat and mass transfer mechanisms during drying.
In order to clarify such mechanisms, the effect of process parameters on drying kinetics are studied in a small-scale drying equipment, specially developed at SIK, for combined microwave – hot air drying. A porous material (aerated concrete) was chosen as a food model to avoid deformation and chemical changes of the sample during heating/drying. Further studies will be performed on food products (bread crumb, potato) for comparison. The equipment was designed to create a well defined drying situation, and in this way simplify the description of the heat and mass transfer mechanisms. The rectangular sample is placed inside a standard waveguide and hot air is convected over the same side that the microwave energy impinges on. The microwave power can be held at low constant power, without pulsing. The sample is weighed at intervals to calculate moisture content, and temperature is monitored by infrared pictures of samples that have been split in two, or by optic temperature fibres. The model material, aerated concrete, is characterized by measuring dielectric data and sorption isotherms. The process parameters studied are microwave power, air temperature and air velocity. These parameters strongly affect energy efficiency which is evaluated as total energy consumption for drying and includes microwave energy as well as energy consumed to produce hot air for convection. Microwave power has a large influence on drying time and consequently on energy efficiency, as short drying time reduces energy consumption. However, the microwave power output has to be chosen to avoid detrimental quality changes due to high temperatures. Product quality is, therefore, a limiting factor for short drying time.
Presented Thursday 20, 10:10 to 10:15, in session Hygiene, Hygienic Design & Unit Operations (Food-5b).
