MODELING ENERGY-TRANSFER IN A FOCUSED MICROWAVE DIGESTER

A physical model has been developed for studying microwave energy tran sfer and to predict temperature evolution of a liquid sample placed in an open microwave heating digestor. The thermal system was considered as involving three different media having different properties, i.e., the liquid sample, the glass flask and the microwave cavity. Energy b alance equations were written taking into account conduction, convecti on and radiation heat transfer. The resolution of the differential equ ation set obtained gave the predicted temperatures. Experimental measu rements of temperature were carried out with a thermocouple probe for several types of reagents classically used in wet ashing techniques. T hey were compared to predicted temperatures, and a Chi-squared paramet er was used to estimate the fit of the model. The good results obtaine d demonstrated that this model can predict correctly the temperature o f reagent heating in the presence of a microwave field. The addition o f organic material did not significantly modify the results. It also p roduces abaci for predicting the time necessary to heat a reagent up t o a given temperature. The model also gives a numerical estimation of the microwave energy loss during the transfer from the magnetron to th e sample. It is then possible to explain some major differences observ ed between microwave ashing technique and conventional heating ashing technique, mainly the important reduction of time.