Microwave drying coupled with convective air and steam for efficient dehydration of extruded zeolite honeycomb monolith

Zeolite honeycomb monoliths (ZHMs) as key adsorbents or catalysts are widely applied in industries, for which, however, the popularization is limited by its low production efficiency due to the long and careful drying process of the fragile ZHMs green body. Here we address this challenge by developing a microwave drying strategy coupled with convective air and steam. The individual microwave, microwave-air, and microwave-air-steam drying characteristics on the honeycomb and slab monoliths of 13X zeolite were systematically studied, and the drying quality-microwave power/air temperature/humidity relationship was correlated. The results showed that microwave achieves rapid dehydration of ZHMs, eliminating 29.94 % relative mass of water within only 660 s. However, the uncontrollable microwave adsorbing inhomogeneity and thermal accumulation over the hydrophilic ZHM resulted in overburning, cracking and deformation issues. Introducing convective air at room temperature eliminates overburning and cracking phenomena, and after coupled with 110 g/m³ steam, the irregular deformation of ZHMs was further mitigated (shrinkage rate reduced from 2.10 % to 1.11 %) with even increased drying efficiency by 4.47 %. The advantageous mechanism of microwave-air-steam coupling drying was revealed as the combined effect of the heat and mass transfer through the honeycomb hole enhanced by convective air and the heating and dehydration homogeneity improved by uniform microwave absorbing on surface-condensable steam. The efficacy of the optimized drying strategy was demonstrated by significantly shortened time for scale-up production of 1 m³ ZHMs from nearly 1 month to 2 days. This work affords new insights for improving production efficiency of high-quality ZHMs and presents microwave-air-steam drying process as a platform to advancing structured material preparation.