• For Diesel hydrotreaters. How to finalize the catalyst from only CoMo & only NiMo and combination of NiMo/CoMo? Why is it said to be that NiMo catalyst consumes more H2 than CoMo catalyst?



  • Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

    The catalyst grading of the diesel hydrotreater reactors relies on the feed stream quality, especially related to the contaminants content like sulfur and nitrogen as well as the participation of cracked streams like LCO, Coker Gas oil, etc. which are harder feeds to hydrotreating process. For feed streams with high content of these compounds it's applied a catalyst grading in the hydrotreating reactors with increased presence of high active catalysts like NiMo over alumina.

    Once the CoMo is less active than NiMo catalysts, the first is applied to improve sulphur removal and olefins saturation while the NiMo catalyst is responsible for promoting nitrogen removal and aromatics saturation. The filling of the reactor (downflow reactors) normally starts with guard beds to protect the active catalysts against contaminants like metals (Ni and V) followed by the heteroatoms and unstable compounds saturation in the following beds in order to ensure an adequate temperature control in the catalyst beds. A relatively common configuration is to use a wide pore NiMo catalyst in the guard bed followed by a blending of CoMo and NiMo in the first catalytic bed aiming to promote sulfur removal and aromatics saturation followed by a NiMo bed aiming to promote the hydrodenitrogenation reactions followed by a last catalytic bed with a catalyst with high dehydrogenation performance (CoMo). Again, the catalyst grading configuration relies on the feed stream quality, design characteristics of the processing unit, and hydrotreating goals (specifications of the hydrotreated stream).

    Regarding the higher hydrogen consumption of NiMo catalysts, as described above these catalysts are more chemically active than CoMo and are responsible for nitrogen removal and aromatics saturation which are more refractory contaminants, leading to a higher hydrogen consumption to achieve hydrotreating goals.