Presulfiding of hydrotreating catalyst by elemental sulfur outside of the reactor

Abstract

The ecological safety of petroleum products is the most important factor in the steady development of the oil processing industry. To lower the content of sulfur, nitrogen, and aromatic hydrocarbons in petroleum fuels the capacities of hydrogenating processes have been stepped up by reconstruction and modernization of existing units for hydrotreating and hydrocracking, on the one hand, and construction of new ones, on the other. At the same time great consideration has been given to developing and bringing into use new catalytic systems of high effectiveness because even small improvements in catalyst characteristics increase the total profitability of the process. The most widespread industrial catalysts for petroleum distillate hydrotreating are alumina-cobalt-molybdenum and alumina-nickel-molybdenum systems in the form of oxides. Their optimal activity is provided by total sulfiding. Sulfiding insufficiency results in a decrease in the activity and selectivity of the catalyst, reduction of its service life, increase in the initial temperature of the process and therefore reduction of the interregeneration cycle. There are five industrial processes of sulfiding: using a gas mixture of hydrogen sulfide and hydrogen; a feedstock; a mixture of feed and sulfating agents; elemental sulfur; preimpregnation with organosulfur compounds (presulfiding) outside the technological plant. The most well investigated method is sulfiding by hydrogen sulfide. With this sulfiding agent, the catalyst coking is excluded. Long processing in the atmosphere of hydrogen, however, results in overreduction of its active components and decreases its hydrodesulfurization activity. Sulfiding in the gas phase proceeds much more rapidly than in the liquid phase but is accompanied by local overheating due to the low heat capacity of the gas and high exothermicity of the reaction. Local overheating and deficit of hydrogen sulfide may result in catalyst spoiling. Moreover, hydrogen sulfide is remarkable for its high toxicity and pungent unpleasant smell. In industry this method is generally used in the USA. Sulfiding in the liquid phase by a feedstock is more widespread in Japan, and that by a mixture o f a feed and sulfuring agents in the West and the USA. There is no possibility to achieve total sulfiding by nonrefined oil distillates; in this case the catalyst is coked. The process of sulfiding by a feedstock may be recommended at low temperatures only. It takes a long time to realize this process in industry. Experience in hydrotreating plants has shown that catalysts sulfided by a feedstock with addition of a sulfuring agent are more active than those sulfided by a feed or a mixture of hydrogen sulfide and hydrogen. In the first case sulfiding is mostly provided by sulfur of a sulfuring agent added at a concentration in a feedstock of I wt. %. Cheap organic compounds decomposing at low temperatures have been chosen as sulfuring agents (Table 1). Carbon disulfide was used for presulfiding before. It is quite dangerous, however, for the environment, so its application is rather limited despite of its high sulfur content. Mercaptans are good sulfuring agents, but their pungent unpleasant smell has forced users to look for other variants. At present dimethylsulfide and dimethyldisulfide are successfully used as sulfuring agents. Dimethylsulfide, however, is difficult to transport and pipe due to its high volatility, inflammability, and pungent smell. The advantages ofdimethyldisulfide are its lower vapor pressure, which reduces its concentration in the atmosphere and decreases the risk of poisoning to personnel, and its lower decomposition temperature and higher flash point (18~ Organic polysulfides, specifically di-tert-nonylpolysulfide, are effective presulfiding agents, but it is very