Phase controller, microwave drying

The measured electric field wilt rise sharply as free solvent becomes exhausted and the microwaves attempt to couple with the ever-decreasing solvent load (or product load). At this time the temperature may also sharply rise. Therefore, it is imperative to cut back on the amount of microwave energy going into the chamber when the end of the steady state phase is reached. By proper instrumentation of the dryer careful monitoring of the drying process should allow the operator to prevent overheating of the product. The relationship between the electric field and temperature can be determined experimentally on small batches and ultimately be used to control the drying process. 

The best temperature control is achieved with a phase controller, which can continuously and automatically adjust the variable microwave power, based on predefined power profiles (Li et al., 2010). It is worth emphasizing here that maximum power levels must be selected, based on trial and error, for a designated temperature. If the power was too high, the temperature fluctuations were too large, but if the power was too low, then during certain drying stages the temperature could not reach the preset temperature. 

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