Zoned microwave drying apparatus and process

Abstract

An apparatus for drying nodular, granular and other products includes a vessel containing an evacuated chamber and an endless belt conveyor that passes through the chamber. Microwave energy is supplied to the chamber by several microwave generators, with each generator supplying energy for a different zone along the belt. The moisture-laden product is introduced into the chamber without affecting the pressure within the chamber, and is then distributed onto the conveyor belt at uniform thickness and width. The belt transports the product through the zones of microwave energy, and within each zone the strength of the field is below the breakdown point of the field for the particular pressure of the chamber, so the destructive effects of ionization are avoided. By reason of the greater moisture content in the product at the first zone, more energy can be supplied to the product in that zone, and indeed the energy supplied diminishes with each successive zone. Beyond the last microwave zone the product is transferred to another endless belt conveyor which moves the product beneath the microwave zones. Here it may be heated with radiant energy to drive off still more moisture and perhaps cooled to crystallize sugar that may be within the product. A dry purge gas is passed over the product where it is radiantly heated to carry away the vapor from the additional moisture, and more of the gas is passed over the product where it is cooled to prevent condensation on cooling panels.