What is the primary purpose of maintaining a specific steam-to-carbon ratio in the SMR process?

The primary purpose of maintaining a specific steam-to-carbon ratio in the Steam Methane Reforming (SMR) process is to prevent carbon formation, also known as coking, on the surface of the catalyst. Coking deactivates the catalyst, reducing its efficiency in converting methane and steam into hydrogen and carbon oxides. The steam-to-carbon ratio is the molar ratio of steam (H2O) to carbon atoms in the hydrocarbon feed, which is primarily methane (CH4) in SMR. A higher steam-to-carbon ratio favors the water-gas shift reaction, which converts carbon monoxide (CO) and water into carbon dioxide (CO2) and hydrogen (H2). This helps to remove CO, a precursor to carbon formation. Insufficient steam leads to increased CO concentration, which can then undergo reactions like the Boudouard reaction (2CO -> C + CO2), depositing solid carbon on the catalyst. Carbon deposition blocks the active sites on the catalyst, reducing its activity and leading to decreased hydrogen production and increased pressure drop across the catalyst bed. Therefore, maintaining the appropriate steam-to-carbon ratio ensures optimal catalyst performance, prolongs catalyst life, and maximizes hydrogen yield.