Hydrotreating processes aim at the removal of impurities such as sulfur and nitrogen from distillate fuels—naphtha, kerosene, and diesel—by treating the feed with hydrogen at elevated temperature and pressure in the presence of a catalyst. Hydrotreating has been extended in recent years to atmospheric resids to reduce the sulfur and metal content of resids for producing low-sulfur fuel oils. The operating conditions of treatment are a function of type of feed and the desulfur-ization levels desired in the treated product. The feed types considered here are
Atmospheric resids or reduced crudes. The principal impurities to be removed are
The basic reactions involved are outlined in Figure 2-1. Sulfur
The sulfur-containing compounds are mainly mercaptans, sulfides, disulfides, polysulfides, and thiophenes. The thiophenes are more difficult to eliminate than most other types of sulfur.
Methyl thiophene n pentane
CH3 CH2 CH2 CH2 CH2 SH + H2 Amyl mercaptan
C5H12 + H2S n pentane
CH3— CH2— CH2—S — S—CH2—CH2— CH3 + 3H2 Dipropyl disulfide
Figure 2-1. Basic reactions.
The nitrogen compounds inhibit the acidic function of the catalyst considerably. These are transformed into ammonia by reaction with hydrogen.
The oxygen dissolved or present in the form of compounds such as phenols or peroxides are eliminated in the form of water after reacting with hydrogen.
The olefinic hydrocarbons at high temperature can cause formation of coke deposits on the catalyst or in the furnaces. These are easily transformed into stable paraffinic hydrocarbons. Such reactions are highly exothermic. Straight run feeds from the crude unit usually contain no olefins. If, however, the feed contains a significant amount of olefins, a liquid quench stream is used in the reactor to control the reactor outlet temperature within the design operating range.
The metals contained in the naphtha feed are arsenic, lead, and to a lesser degree copper and nickel, which damage the reforming catalyst permanently. Vacuum gas oils and resid feeds can contain a significant amount of vanadium and nickel. During the hydrotreating process, the compounds that contain these metals are destroyed and the metals get deposited on the hydrotreating catalyst.
The principal variables for hydrodesulfurization (HDS) reactions are temperature, the total reactor pressure and partial pressure (PPH2) of hydrogen, the hydrogen recycle rate, and the space velocity (VVH).
The HDS reactions are favored by an increase in temperature, but at the same time, high temperature causes coking reactions, diminishing the activity of the catalyst. The desulfurization reactions are exothermic and the heat of reaction is approximately 22-30 Btu/mole hydrogen. It is necessary to find a compromise between the reaction rate and the overall catalyst life. The operating temperature (start of run/end of run) is approximately 625-698°F according to the nature of the charge. During the course of a run, the temperature of the catalyst is gradually raised to compensate for the fall in activity due to coke deposits until the maximum permissible temperature limit (EOR) for the HDS catalyst is reached. At this stage, the catalyst must be regenerated or discarded.
The increase in partial pressure of hydrogen increases the HDS rate and diminishes the coke deposits on the catalyst, thereby reducing the catalyst fouling rate and increasing the catalyst life. Also, many unstable compounds are converted to stable compounds. Operation at higher pressure increases the hydrodesulfurization rate because of higher hydrogen partial pressure in the reactor, requiring a smaller quantity of catalyst for a given desulfurization service. In an operating unit, higher-pressure operation can increase the feed throughput of the unit while maintaining the given desulfurization rate.
The liquid hourly space velocity (LHSV) is defined as
T iTci; Per hour feed rate of the charge (ft3/hr)
volume of the catalyst bed (in ft )
Hydrodesulfurization reactions are favored by a reduction in VVH. The rate of desulfurization is a function of (PPH2/VVH) or the ratio of partial pressure of hydrogen in the reactor to liquid hourly space velocity. For a given desulfurization rate (at constant temperature), the ratio PPH2/ VVH is fixed. Fixing the total reactor pressure automatically fixes the partial pressure and the required hydrogen recycle rate. In general, the total reactor pressure is fixed from the available hydrogen pressure, the hydrogen partial pressure, and other variables such as VVH are adjusted until these fall within the acceptable limits.
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