(671a) Influence of Microwave Heating On Polyesterification Reaction

Polycondensation reactions in the industrial practice are usually performed at high temperature, in order to distill off the formed by-product and to shift the equilibrium to the product side. Formed by-product is usually successively vaporized, cooled, and then collected. Typically different monomers and additives are vaporizing with water, which results in diversified content of organic fraction in waste water. Hence the non-recovered part of the waste water is often treated in an incinerator. Therefore industrial polycondensation processes are characterized by high energy consumption. The importance of sustainability triggers the re-thinking of energy consumption and waste generation issues in the process. For that reason a fundamental research to explore the potential options of intensification of a polycondensation process has been performed. In recent years considerable research efforts have been directed towards applying microwave heating to the polymer synthesis. Microwave radiation has become an alternative to the conventional heating in chemical reactions.

In the present work the experimental research is focused on a selective microwave heating phenomena and its influence on a polycondensation process. As a model reaction the polyesterification reaction of dicarboxylic acid with diol has been selected. The influence of reaction temperature, catalyst addition and the type of heating i.e. electric heating or microwave heating is investigated. The impact of microwave heating on the formation of by-product (water) and its removal from the reactor is examined. The energy efficiency for both heating systems is compared.

Two types of heating have been tested in a non-catalyzed and catalyzed polyesterification reaction systems between neopentyl glycol and adipic acid. Special attention is given to applying comparable testing equipment and procedures in both microwave experiments and in conventional heating experiments. The first type of heating is microwave heating performed in a multimode cavity (MARS, CEM Corp.) with maximum power output of 1600 Watts and inboard temperature control system. The second type of heating is the conventional electric heating mantle. During both type of experiments the temperature has been measured by fiber-optic sensors. The chemical reaction has been carried out in a 250 ml glass reactor with mechanical stirrer under a nitrogen purge of 35-40 ml/min to help the removal of the by-product. Reaction has been carried out isothermally at two temperatures (140^oC and 165^oC) with maximum power generated of 800 Watts in the microwave cavity and 150 Watts in the heating mantle.

From experimental results can be concluded that besides a shorter melting time no specific microwave effects are observed. Whether microwaves enhance the melting of the solid reagents of the investigated polyesterification reaction remains an open question. Additional experiments are needed where the energy absorbed by the unit volume of the reaction mixture is equal in both heating systems.

The volumetrically absorbed microwave power is converted to heat in the exposed material. The efficiency coefficient which describes conversion of microwave energy into thermal energy is strongly dependent on the irradiated power and dielectric properties of the exposed sample. The issue of energy consumption in both heating systems will be compared and discussed. Estimated values of the volumetric absorption of the microwave energy by the irradiated samples will be evaluated.

ACKNOWLEDGEMENTS

The Dutch Ministry of Economic Affairs and SenterNovem are acknowledged for their financial support through the EOS-LT 04033 project grant. This research project is carried out in collaboration with DSM and CEM Corporation.