Microwave vacuum drying of porous media: experimental study and qualitative considerations of internal transfers

This paper deals with experimental results obtained on a laboratory scale dryer. Two porous media were dried, a packing of initially water saturated glass beads and a packing of initially unsaturated pharmaceutical granules. For the first one, the main parameter is permeability of the bed varying the beads diameter and for the second one it is the porosity of the bed varying the amount of product. These experiments were carried out at fixed incident power and pressure level. In the first period of the drying, it was observed that permeability was a limiting parameter to internal mass transfers when free liquid water is expulsed, revealing high gas pressure gradients inside the product. They occur when we have a massive vaporisation with ebullition. As for the porosity of the granules bed, it has no effect on mass transfers. In the second period, we have no influence of both parameters on the drying kinetics. In the last period, so in the hygroscopic field or below the irreducible saturation, the internal mass transfers are governed by the electromagnetic energy absorption.