In hydrotreater, what is the difference between direct desulphurization reaction route and indirect desulphurization reaction route for HDS reactions?
Is it type of catalyst(NiMo/CoMo) or the nature of the sulphur molecule that decides the reaction route of HDS reactions?Feb-2023
Marcio Wagner da Silva, Petrobras, email@example.com
I believe that the question is regarding the desulfurization of heavy oils. The desulfurization of heavy fractions can be divided in two routes:
1 - Direct Desulfurization - The whole atmospheric residue (or the hydrotreating feed) is fed to a hydrodesulphurisation unit and the sulphur compounds are treated according to hydrodesulphurization reactions.
2 - Indirect Desulfurization - The heavier fraction is separated from the atmospheric residue (or another stream which is the goal of the desulfurization process) from a separation process like vacuum distillation unit or through carbon rejection routes like Solvent Deasphalting (SDA). Once the sulphur and other heteroatoms tend to concentrate in the heavier fractions of the crude oil, this process indirectly reduces the sulphur content of the light fractions.
The chemical characteristics of the sulfur compounds have a direct effect on its removal performance. Desulfurization of compounds that contain aliphatic sulphur, i.e. thiols and sulfides, is easier than desulfurization of compounds that contain aromatic sulphur, i.e. thiophenics. Due to this fact, the hydrodesulphurisation of heavier fractions requires higher operating severity than the process units operating with lighter fractions.
The characteristics of the catalysts affects the performance of the desulfurization process, Ni-Mo catalysts are more chemically active then the Co-Mo catalysts. For this reason, the Ni-Mo catalysts are employed for hydrotreating feeds with high nitrogen and refractory sulphur compounds (Thiophenics) content while the Co-Mo catalysts are employed to treating less refractory feeds.