570
The Chemistry and Technology of Petroleum
21.2.3 reACtIon temPerAture
A higher reaction temperature increases the rate of desulfurization at constant feed rate and the
start-of-run temperature is set by the design desulfurization level, space velocity, and hydrogen
partial pressure. The capability to increase temperature as the catalyst deactivates is built into the
most process or unit designs. Temperatures 415°C (780°F) and above result in producing excessive
coking reactions and higher than normal catalyst aging rates. Therefore, units are designed to avoid
the use of such temperatures for any significant part of the cycle life.
21.2.4 CAtAlyst lIFe
Catalyst life depends on the charge stock properties and the degree of desulfurization desired. The
only permanent poisons to the catalyst are metals in the feedstock that deposit on the catalyst, usually quantitatively, causing permanent deactivation as they accumulate. However, this is usually of
little concern except when DAOs are used as feedstocks since most distillate feedstocks contain
low amounts of metals. Nitrogen compounds are a temporary poison to the catalyst but there is
essentially no effect on catalyst aging except that caused by a higher temperature requirement to
achieve the desired desulfurization. Hydrogen sulfide can be a temporary poison in the reactor gas
and recycle gas scrubbing is employed to counteract this condition.
Providing that pressure drop buildup is avoided, cycles of 1 year or more and ultimate catalyst
life of 3 years or more can be expected. The catalyst employed can be regenerated by normal
steam-air or recycle combustion gas-air procedures. The catalyst is restored to near fresh activity by regeneration during the early part of its ultimate life. However, permanent deactivation
of the catalyst occurs slowly during usage and repeated regenerations, so replacement becomes
necessary.
21.2.5 FeedstoCk eFFeCts
The different types of streams that can undergo hydroprocessing range from heavy feedstocks
of resid and vacuum gas oil to lighter feedstocks of naphtha and distillate. Naphtha, or gasoline, is hydroprocessed to remove contaminants such as sulfur, which is harmful to downstream operations (such as precious metal reforming catalyst). Diesel hydroprocessing removes
sulfur to meet fuel requirements, and saturates aromatics. The purpose of resid and vacuum
gas oil hydroprocessing is to remove metals, sulfur, and nitrogen (e.g., hydrotreating), as well
as to convert high molecular weight hydrocarbons into lower molecular weight hydrocarbons
(e.g., hydrocracking).
Thus it is not surprising that the character of the feedstock properties, especially the feed boiling
range, has a definite effect on the ultimate design of the desulfurization unit and process flow. In
agreement, there is a definite relationship between the percent by weight sulfur in the feedstock and
the hydrogen requirements.
In addition, the reaction rate constant in the kinetic relationships decreases rapidly with increasing average boiling point in the kerosene and light gas oil range but much more slowly in the heavy
gas oil range. This is attributed to the difficulty in removing sulfur from ring structures present in
the entire heavy gas oil boding range.
The hydrodesulfurization of light (low-boiling) distillate (naphtha or kerosene) is one of the
more common catalytic hydrodesulfurization processes since it is usually used as a pretreatment
of such feedstocks prior to deep hydrodesulfurization or prior to catalytic reforming. This is similar to the concept of pretreating residua prior to cracking to improve the quality of the products
(Chapter 21). Hydrodesulfurization of such feedstocks is required because sulfur compounds
poison the precious-metal catalysts used in reforming and desulfurization can be achieved under
relatively mild conditions and is near quantitative (Table 21.2). If the feedstock arises from a
The Chemistry and Technology of Petroleum
21.2.3 reACtIon temPerAture
A higher reaction temperature increases the rate of desulfurization at constant feed rate and the
start-of-run temperature is set by the design desulfurization level, space velocity, and hydrogen
partial pressure. The capability to increase temperature as the catalyst deactivates is built into the
most process or unit designs. Temperatures 415°C (780°F) and above result in producing excessive
coking reactions and higher than normal catalyst aging rates. Therefore, units are designed to avoid
the use of such temperatures for any significant part of the cycle life.
21.2.4 CAtAlyst lIFe
Catalyst life depends on the charge stock properties and the degree of desulfurization desired. The
only permanent poisons to the catalyst are metals in the feedstock that deposit on the catalyst, usually quantitatively, causing permanent deactivation as they accumulate. However, this is usually of
little concern except when DAOs are used as feedstocks since most distillate feedstocks contain
low amounts of metals. Nitrogen compounds are a temporary poison to the catalyst but there is
essentially no effect on catalyst aging except that caused by a higher temperature requirement to
achieve the desired desulfurization. Hydrogen sulfide can be a temporary poison in the reactor gas
and recycle gas scrubbing is employed to counteract this condition.
Providing that pressure drop buildup is avoided, cycles of 1 year or more and ultimate catalyst
life of 3 years or more can be expected. The catalyst employed can be regenerated by normal
steam-air or recycle combustion gas-air procedures. The catalyst is restored to near fresh activity by regeneration during the early part of its ultimate life. However, permanent deactivation
of the catalyst occurs slowly during usage and repeated regenerations, so replacement becomes
necessary.
21.2.5 FeedstoCk eFFeCts
The different types of streams that can undergo hydroprocessing range from heavy feedstocks
of resid and vacuum gas oil to lighter feedstocks of naphtha and distillate. Naphtha, or gasoline, is hydroprocessed to remove contaminants such as sulfur, which is harmful to downstream operations (such as precious metal reforming catalyst). Diesel hydroprocessing removes
sulfur to meet fuel requirements, and saturates aromatics. The purpose of resid and vacuum
gas oil hydroprocessing is to remove metals, sulfur, and nitrogen (e.g., hydrotreating), as well
as to convert high molecular weight hydrocarbons into lower molecular weight hydrocarbons
(e.g., hydrocracking).
Thus it is not surprising that the character of the feedstock properties, especially the feed boiling
range, has a definite effect on the ultimate design of the desulfurization unit and process flow. In
agreement, there is a definite relationship between the percent by weight sulfur in the feedstock and
the hydrogen requirements.
In addition, the reaction rate constant in the kinetic relationships decreases rapidly with increasing average boiling point in the kerosene and light gas oil range but much more slowly in the heavy
gas oil range. This is attributed to the difficulty in removing sulfur from ring structures present in
the entire heavy gas oil boding range.
The hydrodesulfurization of light (low-boiling) distillate (naphtha or kerosene) is one of the
more common catalytic hydrodesulfurization processes since it is usually used as a pretreatment
of such feedstocks prior to deep hydrodesulfurization or prior to catalytic reforming. This is similar to the concept of pretreating residua prior to cracking to improve the quality of the products
(Chapter 21). Hydrodesulfurization of such feedstocks is required because sulfur compounds
poison the precious-metal catalysts used in reforming and desulfurization can be achieved under
relatively mild conditions and is near quantitative (Table 21.2). If the feedstock arises from a