Comparison of the performance of upflow and downflow small-reactors in hydrodesulfurization reactions

An experimental study for comparing the behavior of a small-reactor in the hydrodesulfurization of straight-run gas oil with two modes of operation, u pflow and downflow, is presented. The reactor has 525 min length and 14.3 m in internal diameter. Experiments were conducted at commercial HDS operatin g conditions (5.3 MPa total pressure, 356.2 mL/mL hydrogen-to-oil ratio, re action temperature of 613-653 K, and LHSV of 1.0-2.5 h(-1)). All tests were carried out over a commercial NiMo/gamma -Al2O3 catalyst. The effect of di fferent catalyst-to-diluent ratios on the sulfur removal was also studied. The results of SRGO hydrodesulfurization showed that upflow with a 50/50 vo l % catalyst-to-diluent ratio is the best system to avoid external gradient s and other effects such as poor wetting of catalyst, axial dispersion, wal l effects, and flow maldistribution. Apparent activation energies were dete rmined for upflow and downflow modes of operation with and without diluent.