Modeling energy transfer in a focused microwave digestor

A physical model has been developed for studying microwave energy transfer 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 balance equations were written taking into account conduction, convection and radiation heat transfer. The resolution of the differential equation set obtained gave the predicted temperatures. Experimental measurements of temperature were carried out with a thermocouple probe for several types of reagents classically used in wet ashing techniques. They were compared to predicted temperatures, and a Chi-squared parameter was used to estimate the fit of the model. The good results obtained demonstrated that this model can predict correctly the temperature of reagent heating in the presence of a microwave field. The addition of organic material did not significantly modify the results. It also produces abaci for predicting the time necessary to heat a reagent up to a given temperature. The model also gives a numerical estimation of the microwave energy loss during the transfer from the magnetron to the sample. It is then possible to explain some major differences observed between microwave ashing technique and conventional heating ashing technique, mainly the important reduction of time.