Thermal-Microwave Hybrid Reactor Configuration for Energy Efficient Production of Aromatics from Natural Gas

This paper presents a unique reactor and process design where natural gas dehydroaromatization was carried out in a thermally heated fixed-bed reactor, followed by regeneration under microwave irradiation in the presence of either oxygen or CO2.The process was operated in cyclic reaction-regeneration mode. The performance of the two regeneration approaches are compared. Different from oxidative regeneration using air, in CO2 regeneration, coke removal was conducted via Boudouard reaction: CO2 + C=CO. The advantage of the CO2 regeneration is the endothermic nature of the regeneration reaction, avoiding metal sintering that normally takes place in oxidative regeneration. The microwave - assisted regeneration of a zeolite supported metal catalyst was tested on 4%Mo/ZSM-5 for the dehydroaromatization reaction of ethane. The effects of microwave catalytic regeneration were tested by conducting cycles of reactions in a conventional fixed bed followed by regeneration in a 2.45 GHz microwave fixed bed reactor system under a 10% oxygen or 10% carbon dioxide flow. The microwave was effectively able to regenerate the catalyst at lower temperatures than the conventional fixed bed reactor in both the exothermic oxidation reaction of oxygen and carbon, as well as the endothermic reverse Boudouard reaction involving carbon dioxide and carbon. Conventional fixed bed coke removal temperatures were determined by oxygen or carbon dioxide temperature programed oxidation techniques. The catalyst had an overall initial loses in ethane conversion between each cycle. TGA analysis found that in both oxygen and carbon dioxide environments not all the coke was able to be removed. The residual coke deposits are believed to have contributed to the decrease in ethane conversion between each cycle. These coke deposits are thought to be due to the microwaves heating mechanism in a cylindrical catalyst bed. Temperature programed reduction techniques determined that carbon dioxide regeneration resulted in the formation of less molybdenum oxide species and more of the active molybdenum carbides species remained. X-Ray diffraction and Transmission electron microscopy suggested that little to no structural change occurred in the zeolite after microwave regeneration in oxygen or carbon dioxide. In conclusion, we have demonstrated a hybrid reactor type system that could effectively remove coke deposits from a supported metal catalyst using oxygen or carbon dioxide at lower temperatures then the conventional fixed bed.