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Principle of the Microwave Dryer
In a microwave dryer, microwave radiation (energy) is passed through the solids to be dried. This energy is converted into internal heat by the interaction with the material. Therefore, the liquid inside the solids is vapourised and diffused to the surface. Hot air is used to rapidly sweep the moisture vapour from the drying chamber. Thus rapid drying is achieved by the theory of the pressure gradient principle.
Construction of the Microwave Dryer
The industrial microwave dryer is a static bed continuous type (Figure 1.1). Microwaves are produced by an electronic device known as a magnetron. Several such units are used.
Working on Microwave Dryer
The material to be dried is placed on the conveyor belt and allowed to move into the oven chamber. An oscillating electric field of 915 or 2450 MHz is applied. The microwave energy interacts with the polarized molecules and ions of the material. As the field reverses polarity, it relaxes and allows the molecules to return to their random orientation. In this process, the stored potential energy is released as kinetic energy or heat. This interaction of the alternating field with ions. causes billiard ball-like collisions with unionised molecules. This impact energy is also converted into heat.
The heat generated inside the material makes the moisture get vapourised and subsequently diffused to the surface of the solid. This process is quite rapid compared to liquid diffusion (slow). Thus, rapid mass transfer is achieved. Simultaneously a stream of hot air is also passed through the dryer so that the moisture vapour is swept away from the solid surface.
Uses: Microwave drying can be used for the drying of pharmaceuticals at low ambient temperature.
- Microwave treatment can also be used in the last stages of hot air drying, particularly during the second falling rate period. Thus, the last traces of moisture can be removed.
- The total drying rate period is reduced by 50%.
- At low pressure (7 to 140 kPa or 1 to 20 mmHg) and moderate temperature (30 to 40 °C), thermolabile substances can be dried. Examples are vitamins, enzymes, proteins and flavours.
- Energy utilisation is efficient (70% saving) because the walls of the dryer, the conveyor, and the trays do not get heated by microwave radiation.
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