Fixed beds of Rh/Al_2O_3-based catalysts for syngas production in methane SCT-CPO reactors

摘要

The present experimental work deals with methane short contact time (SCT) CPO in a fixed bed reactor considering CH_4 conversion and H_2 and CO selectivity in a wide range of weight hourly space velocity (WHSV). Two different Rh/Al_2O_3-based catalysts both loaded with 0.5% by weight of Rh were developed: one catalyst carrying Rh on the external support surface (Egg-Shell configuration), and the other one with Rh embedded into the porous support (Egg-Yolk configuration). The goal was the design of the optimal fixed bed structure (not only considering beds made of egg-shell or egg-yolk catalysts alone, but also their various combinations), able to either attain the best performance or maintain a reaction temperature along the bed without excessive variations with WHSV. The highest CH_4 conversion (>90%) and H_2 selectivity (>98%), moreover stable despite the WHSV variation of about 3.6 times, and reactor working temperature with not too large variations (maximum of about 16%) by increasing WHSV were obtained with the fixed bed of Egg-Yolk catalyst alone. Instead, the fixed bed of Egg-Shell catalyst alone showed the worst performance: CH_4 conversion and H_2 selectivity were lower of about 15% and 10%, respectively, and decreasing with the increase of WHSV; on the contrary, the CO selectivity remained practically the same, only a slightly decrease being observed. Suitable combinations of the two catalysts in the fixed bed produced intermediate performance between those of the catalysts alone. The different performance of the two catalyst types was probably due to the different structure of the particles and to the Rh position on the carrier itself. Finally, thermal and performance durability tests up to 16 working hours showed that the Egg-Yolk catalyst employed alone in the fixed bed was able to maintain the CH_4 partial oxidation activity with practically disregardable decrease.