(629c) Role of Microwave Heating Strategies In Enhancing the Progress of a First Order Endothermic Reactions

Basak, T., Indian Institute of Technology Madras
Bhattacharya, M., Independent Researcher

This work presents a detailed analysis on the role of microwave heating strategies in optimizing the progress of a first order endothermic reaction occurring in a semiinfinite vertical column. Here, microwave heating strategies are analyzed in terms of different intensities of incident radiations from left and right sides of the reactor. Four different configurations of incident radiations are considered such that that they represent the entire spectrum of possible arrangement of microwave sources spanning uniform intensities of radiations from both the sides to highly localized radiations from one side. In all the cases, total intensities of incident radiations from left and right sides are kept constant in order to keep input power constant. The analysis has been performed by considering detail description of microwave propagation within the reactor in presence of varying intensities of incident radiations from left and right sides via Helmoltz equation.

Progress of reactor in presence of heat and mass transfer limitations have been simulated by solving detailed energy and mass balance equations coupled with Helmoltz equation for evaluation of microwave induced heating pattern for various configurations of microwave sources. Efficiency of various strategies of microwave heating are evaluated by compairing the simulated progress of microwave heated reactors with their conventional counterparts, where total heat generated by microwave power absorption in each configuration has been supplied as surface heat flux in its corresponding conventional reactor. The comparison is done for different reactor dimensions corresponding to {\it three} possible regimes of microwave induced heating patterns, namely (1) uniform, (2) oscillatory and (3) exponentially decayed heating from the surface in {\it thin}, {\it intermediate} and {\it thick} regimes, respectively. It has been shown that although all the configurations of microwave heating lead to faster progress of reactor compared to their conventional counterparts, extent of enhancement depend on microwave heating strategy especially in presence of strong diffusion limitations. Progress of reactor in {\it thin} and {\it thick} regimes of microwave heating can be greatly enhanced by configuring equal intensity of radiations on both sides compared to one sided incidences. On the other hand, effect of microwave heating strategies on reactor progress are nontrivial in {\it intermediate} regime of microwave heating, where either one sided or distributed incidences may enhance reactor progress depending on the dimension of the reactor with respect to wavelength and penetration depth of microwave within it. The enhancement of reactor progress in presence of microwave heating has further been quantified in terms of percentage savings of reaction time [$\eta_{\mbox{MW}} = (1 - \tau_{f_{\mbox{\small MW}}}/ \tau_{f_{\mbox{\small Conv}}})\times 100$]. Variations of $\eta_{\mbox{MW}}$ for all the four heating strategies are determined and shown over the entire range of {\it thin} to {\it thick} regimes. These master curves are extremely essential for selection of the appropriate heating strategy for a given reactor dimension in various regimes of microwave heating especially in {\it intermediate} regime. In this work, the master curves are provided for varying degree of mass transfer limitations showing its effect on the efficiency of various strategies of microwave heating.

Efficiencies of various strategies of microwave heating are also analyzed for different packing materials measured in terms of $f_p$ and $f_w$. It may be noted that dielectric properties of packing materials as well as packing arrangement can significantly alter $f_w$ and $f_p$. Effect of $f_p$ and $f_w$ on $\eta_{\mbox{MW}}$ for each configuration of microwave sources are shown over the entire range of {\it thin} to {\it thick} regimes. These master curves reveal nontrivial variation of optimum heating strategy with $f_w$ and $f_p$ especially in {\it intermediate} regime and show their utmost importance for design of optimum heating strategy to maximize the enhancement of reactor progress for different packing materials.