Hydrotreatment Catalysts and Reactions

Catalysts used in hydrotreatment (hydrodesulfurization, HDS) processes are the same as those developed in Germany for coal hydrogenation during World War II. The catalysts should be sulfur-resistant.

The cobalt-molybdenum system supported on alumina was found to be an effective catalyst.  The catalyst should be reduced and sulfided during the initial stages of operation before use. Other catalyst systems used in HDS are NiO/MoO₃ and NiO/WO₃. Because mass transfer has a significant influence on the reaction rates, catalyst performance is significantly affected by the particle size and pore diameter.

Figure 3-9. Flow diagram of an Exxon hydrotreating unit: (1) filter, (2) guard vessel to protect reactor, (3) main reactor, (4) gas treatment, (5) fractionator.

Reactions occurring in hydrotreatment units are mainly hydrodesulfurization and hydrodenitrogenation of sulfur and nitrogen compounds. In the first case H₂S is produced along with the hydrocarbon. In the latter case, ammonia is released. The following examples are hydrodesulfurization reactions of some representative sulfur compounds present in petroleum fractions and coal liquids.

Examples of hydrodenitrogenation of two types of nitrogen compounds normally present in some light and middle crude distillates are shown as follows:

More complex sulfur and nitrogen compounds are present in heavy residues. These are hyrodesulfurized and hydrodenitrogenated, but under more severe conditions than normally used for lighter distillates. For example, for light petroleum distillates the approximate temperature and pressure ranges of 300–400°C and 35–70 atm. are used, versus 340–425°C and 55–170 atm. for heavy petroleum residua. Liquid hourly space velocities (LHSV) in the range of 2–10 hr^–1 are used for light products, while it is 0.2–10 hr^–1 for heavy residues.