Application of microwave heating to a polyesterification plant

Abstract

Utilizing microwave irradiation, a fundamentally different method of the energy transfer, to the chemical process units can potentially be advantageous compared to the conventional heating, inter alia due to the selective nature of interaction of the microwaves with the matter. This doctoral dissertation addresses some of the aspects associated with the use of the microwave technology in a polyesterification plant. In this context, application of the microwave irradiation to a model polyestrerification reaction, using different types of the microwave applicators was studied. Influence of the microwave irradiation as an alternative form of heat for two selected separation techniques, which can potentially be used in a downstream recovery section in the polyesterification process (for the water purification and recovery unreacted dihydroxy alcohols), namely pervaporation and adsorption-desorption processes, were studied. The microwave-activated processes were compared with the conventionally heated ones focusing on possible reduction in the production time and energy consumption and increase in product quality. Study related to heating of the individual reagents of the polyesterification process in a multimode microwave oven showed that the heating time is several times shorter than with the conventional heating at the expense of a higher electric energy consumption. The results obtained for a model reaction system of adipic acid and neopentyl glycol under the conventional and microwave heating using the multimode microwave cavity did not show significant differences in terms of conversion and the end-product properties. Optimized (lower) usage of the microwave power during the process and effective scale up possibility compared to the multimode cavity were presented by applying the Internal Transmission Line (INTLI) microwave reactor to the studied reactor system (polyesterification reaction of maleic anhydride, phthalic anhydride and 1,2-propylene glycol). The INTLI allows for irradiation of the liquid phase from the inside of the reactor, thereby enabling better coupling of the microwave energy with the liquid mixture. For the selected process conditions, the total energy consumption of the INTLI reactor was up to a factor two lower. Comparison of desorption kinetics and desorption efficiencies for a model polar and non-polar molecules from 13X molecular sieves for two techniques, the microwave swing regeneration (MSR) and the temperature swing regeneration (TSR) showed that microwaves can help to overcome the heat transfer limitations by direct heating of the adsorbent. Significantly faster desorption processes were enabled under the microwave heating with the enhancement being more pronounced in the event of the polar compound. The MSR process runs faster even when the adsorbent temperature is quite lower than the gas temperature in TSR. It was verified that the microwaves do not affect the adsorption capacity of the molecular sieves after several consecutive adsorption-desorption cycles. Membrane pervaporation for dewatering of water/ethanol mixtures, using a hydrophilic membrane, were conducted under the microwave and conventional heating in a multimode microwave oven and a convection oven, respectively. Observations were made that at the conditions with higher water content in the feed, the water flux through the membrane was higher under the conventional heating. On contrary, with lower water in the feed, the opposite trend was found; the water flux through the membrane was higher under the microwave heating. Enhancement in the permeate flux under microwaves, compared to conventional heating, at higher ethanol concentrations in the feed can be explained on the basis of stronger coupling of microwaves with ethanol than water. Contrary to water, the dielectric loss of ethanol increases with increasing temperature; therefore, microwave dissipation is preponderant in the hot areas and can easily lead to local heating and the spatial temperature gradients.