Hydrotreating of petroleum feedstocks to remove heteroatoms, particularly sulfur, is critical to modem refiners in order to meet ever more demanding environmental regulations. A major source of air pollution worldwide is the exhaust from hundreds of millions of motor vehicles due to fuel combustion. Fuels containing sulfur produce sulfur dioxide and other pollutants leading to a host of environmental concerns, such as smog and related health issues, acid rain leading to deforestation, water pollution, as well as other environmental problems. To help reduce or eliminate these environmental problems, the sulfur content of fuels has been, and will continue to be, restricted to increasingly smaller concentrations.
Hydrodesulfurization is one of the fundamental hydrotreating processes of the refining and petrochemical industries. The removal of feed sulfur by conversion to hydrogen sulfide is typically achieved by reaction with hydrogen over non-noble metal sulfides, especially those of Co/Mo and Ni/Mo, at fairly severe temperatures and pressures to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur sensitive process.
Some naphtha fractions contain olefins, such as, for example, cracked naphthas or coker naphthas, which typically contain over about 20 wt. % olefins. At least a portion of the olefins are hydrogenated to saturated hydrocarbons during the hydrodesulfurization operation. Since the olefins are high octane components, for some motor fuel use, it is desirable to retain the olefins rather than to convert them to saturated compounds. Conventional fresh hydrodesulfurization catalyst have both hydrogenation and desulfurization activity. Hydrodesulfurization of cracked naphthas using conventional naphtha desulfurization catalysts under conventional startup procedures and under conditions required for sulfur removal, produces a significant loss of olefins through hydrogenation. This results in a lower grade fuel product which needs more refining, such as isomerization, blending, or other refining, to produce higher octane fuel, adding significantly to production expenses.
Selective hydrodesulfurization to remove sulfur while minimizing hydrogenation of olefins and octane reduction by various techniques, such as selective catalysts, have been described. For example, U.S. Pat. Nos. 4,132,632 and 4,140,626 disclose selective desulfurizaton of cracked naphthas by using specific catalysts having particular amounts of Group VI and VIII metals on magnesia support. U.S. Pat. No. 4,149,965 discloses a process for starting-up naphtha hydrodesulfurizaton using partially deactivated hydrotreating catalyst under relatively low pressure of less than 200 psig. The catalyst is partially deactivated using a substantially non-metals containing hydrocarbonaceous oil for a time ranging from about 10 hrs to about 20 days. U.S. Pat. No. 2,983,669 discloses a processes for treating petroleum having a high sulfur content using fractionation and hydrodesulfurization. It is suggested in this '669 patent that the hydrodesulfurization catalyst be one that does not readily promote hydrogenation, such as a partially spent catalyst.
Hydrodesulfurization catalysts age, losing activity during use by collecting deposits of carbonaceous material and/or impurities, such as metals, from the treated feedstock. Eventually, with increased deposition, the catalyst is no longer able to provide effective hydrodesulfurization. The deactivated catalyst may be regenerated and reused, but is generally less effective than fresh catalyst by requiring higher temperature to give the desired activity and becoming deactivated more quickly than fresh catalyst. Although hydrodesulfurization catalysts can usually be repetitively regenerated, they eventually become irreversibly deactivated, or spent, essentially losing their intended hydrodesulfurization utility.
It is known to hydrodesulftirize an olefinic naphtha by adding a nitrogen compound to the feed in order to deactivate the catalyst for the hydrodesulftirization reaction. See U.S. Pat. No. 2,913,405.
It is also known to hydrotreat a straight run fuel oil at a pressure not greater than 150 psig and at a temperature from 400.degree. to 500.degree. F. in the presence of a catalyst that had been employed in a prior hydrotreating process operated at a higher pressure than the pressure of the fuel oil hydrotreating step until the catalyst had been substantially permanently deactivated for the purpose of said high pressure process, as shown in U.S. Pat. No. 3,870,626.
Spent hydrodesulfurization catalysts have been used in hydrodesulfurization. For example, U.S. Pat. No. 3,876,532 discloses a process for hydrodesulfurizing middle distillate, virgin oils using spent hydrotreating catalysts under extremely mild conditions to reduce acid and mercaptan content, to remove sulfur below 0.2 wt. %, or 2,000 ppm. U.S. Pat. No. 4,414,102 discloses the use of spent hydrodesulfurization catalyst to transform nitrogen- or oxygen-containing compounds to sulfur-containing compounds followed by mild hydrodesulfurization treatment. Also, U.S. Pat. No. 5,286,373 discloses a process for selectively hydrodesulfurizing naphtha by contacting the naphtha, which contains olefins and thiohydrocarbons, with hydrogen under vigorous hydrodesulfurization conditions in the presence of essentially deactivated hydrodesulfurization catalyst which selectively produces hydrogen sulfide and desulfurized hydrocarbons and resulting in a relatively high olefin content.
Although some of the above processes have met with commercial success, there still remains a need in the art for improved activation process for cat naphtha desulfurization catalysts that do not require the use of an additional deactivation step.