Hydrocracking refers to a process in which hydrocarbons crack in the presence of hydrogen and catalyst to lower molecular weight hydrocarbons. Depending on the desired output, the hydrocracking unit may contain one or more beds of the same or different catalyst. Hydrocracking is a process used to crack hydrocarbon feeds such as vacuum gas oil (VGO) to diesel including kerosene and gasoline motor fuels.
Mild hydrocracking (MHC) is generally used upstream of a fluid catalytic cracking (FCC) or other process unit to improve the quality of an unconverted oil that can be fed to the downstream conversion unit, while converting part of the feed to lighter products such as diesel. As world demand for diesel motor fuel is growing relative to gasoline motor fuel, mild hydrocracking is being considered for biasing the product slate in favor of diesel at the expense of gasoline. Mild hydrocracking may be operated with less severity than partial or full conversion hydrocracking to balance production of diesel with the FCC unit, which primarily is used to make naphtha. Partial or full conversion hydrocracking is used to produce diesel with less yield of the unconverted oil which can be fed to a downstream conversion unit.
Due to environmental concerns and newly enacted rules and regulations, saleable diesel must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra low sulfur diesel (ULSD) requirement is typically less than 10 wppm sulfur.
Hydrotreating refers to a process in which olefins and aromatics are saturated and heteroatoms, such as sulfur, nitrogen and metals are removed from the hydrocarbon feedstock over catalyst in the presence of hydrogen. Hydrotreating is an essential step in the production of ULSD.
Hydroprocessed streams are typically stripped with an inert gas such as steam to remove volatile ammonia and hydrogen sulfide to reduce sulfur and nitrogen concentration in the product fuel stream. Stripped hydroprocessed streams are then fractionated to provide product fuel streams.
MHC reactors are typically operated at low to moderate conversion and lower pressures than higher conversion hydrocrackers, so the distillate produced from MHC units can be high in sulfur, such as 20 to 150 wppm sulfur, because the environment in the MHC reactor has a high concentration of hydrogen sulfide. In addition, the high concentration of ammonia in the MHC reactor reduces hydrocracking activity requiring higher operating temperatures further limiting organosulfur conversion. As a result, diesel from the MHC reactor must be also treated in a hydrotreating unit to achieve ULSD.
There is a continuing need for improved methods of producing more diesel from hydrocarbon feedstocks than gasoline. Such methods must ensure that the diesel product meets increasingly stringent product requirements.