The Euro V standard for the level of sulfur in gasoline is 10 wt ppm sulfur. Special processing is need to obtain these sulfur levels.
Gasoline from fluid catalytic cracking (FCC) processes comprises up to 50 vol % of a refinery's motor gasoline pool, and up to 90% of the motor gasoline pool's sulfur content. Consequently, it is important that the treatment of this stream not significantly reduce its octane contribution to the pool.
In order to obtain the needed sulfur levels, the majority of gasoline worldwide obtained from fluid catalytic cracking (FCC) processes is selectively hydrodesulfurized which generally preserves the alkenes and aromatics. Typical processing conditions for hydrodesulfurization include a temperature of about 250° C. to about 315° C. and a pressure of about 1.7 MPa(g) to about 17-26 bar(g) with a supported CoMo catalyst.
However, selective hydrodesulfurization cannot bring down the sulfur level down sufficiently to meet the 10 wt ppm due to formation of recombinant mercaptans. The H2S produced during the selective hydrodesulfurization reaction stage reacts with olefins present in the effluent to form mercaptans, predominantly butyl mercaptans. In addition, the current selective hydrodesulfurization catalytic system and the operating conditions are not optimized to target the reduction of the recombinant mercaptans in the selective hydrodesulfurization reaction stage.
Consequently, in order meet this limit, some refiners have added a polishing reactor downstream of the selective hydrodesulfurization reactor. Typically, the polishing reactor uses a Ni based catalyst with LHSV of about 1 hr−1 and a temperature of about 280° C. to about 380° C. The polishing reactor reduces the mercaptans especially by saturating the olefins and thereby reducing the equilibrium mercaptans in the reactor effluent. However, saturating the olefins reduces the octane content.
Therefore, there is a need for improved processes for desulfurizing gasoline.