Process Variables in HF Alkylation
Key process variables are reactor temperature, isobutane to olefin ratio, acid
strength, and acid-to-hydrocarbon ratio.
• Reaction temperature is one of the more important process variables as it has a
significant influence on the octane number of the product. Almost all HF
alkylation reactors are operated below 110
F. At higher temperatures, a
decrease in alkylate octane number will occur. Above 120
F polymerization
and cracking side reactions become excessive, reducing alkylate quality, and in
many cases, acid regeneration capacity of the HF alkylation unit would not be
able to maintain proper control of the acid strength.
• Isobutane to olefin ratio is also an important variable in the reactor. As the
isobutane to olefin ratio is increased, octane increases and the polymer production decreases. Therefore, the flow of isobutane recycle is usually kept at a
practical maximum at all times, up to the capacity of the isostripper. Possible
reasons to reduce the recycle ratio are for the conservation of fractionation
energy or for a reduction in the hydride transfer reaction.
• Acid strength is usually kept between 85 and 95 mass% HF. The fresh acid is
supplied by acid manufacturers at 99.97+ wt% HF. This purity is too high for the
optimum performance of the alkylation process. The water content of the acid is
typically controlled between 0.5 and 1.5 wt%. Water levels below 0.5 wt% cause
a reduction in octane. Levels above 1.5 % lead to higher corrosion rates on the
carbon steel piping and vessels. The optimum acid strength is set by a balance
between the performance of the unit feed treating systems for sulfur and water
removal and acid regeneration operation. In some cases, oxygenate removal or
diene removal systems are also used on the feed where there are known to be
high oxygenates (such as downstream of an MTBE unit) or high diolefins (from
severe FCC conditions). The action of the acid on reactions is a complex
phenomenon and is dependent on the type as well as the amount of diluents.
• The acid-to-hydrocarbon volume ratio used in the HF alkylation process is
generally 1:1 or higher. At some point, below 0.8:1.0, excess polymerization
occurs. In the most extreme cases, alkylate production could stop.
• Pressure does not have an effect on the process as long as it is kept high enough
to prevent the vaporization of hydrocarbon and acid in the reactor section.
HF Feed Contaminants
As with many refining processes, the control of contaminants coming into the unit
with the normal feedstocks is critical for the long and dependable operation of the HF
alkylation unit. Above the recommended maximum levels of feedstock contaminants,
acid consumption, acid regeneration requirements, and in some cases unit corrosion
and product quality are all measurably affected. Generally all contaminants are kept
Alkylation in Petroleum Processing
445
Key process variables are reactor temperature, isobutane to olefin ratio, acid
strength, and acid-to-hydrocarbon ratio.
• Reaction temperature is one of the more important process variables as it has a
significant influence on the octane number of the product. Almost all HF
alkylation reactors are operated below 110
F. At higher temperatures, a
decrease in alkylate octane number will occur. Above 120
F polymerization
and cracking side reactions become excessive, reducing alkylate quality, and in
many cases, acid regeneration capacity of the HF alkylation unit would not be
able to maintain proper control of the acid strength.
• Isobutane to olefin ratio is also an important variable in the reactor. As the
isobutane to olefin ratio is increased, octane increases and the polymer production decreases. Therefore, the flow of isobutane recycle is usually kept at a
practical maximum at all times, up to the capacity of the isostripper. Possible
reasons to reduce the recycle ratio are for the conservation of fractionation
energy or for a reduction in the hydride transfer reaction.
• Acid strength is usually kept between 85 and 95 mass% HF. The fresh acid is
supplied by acid manufacturers at 99.97+ wt% HF. This purity is too high for the
optimum performance of the alkylation process. The water content of the acid is
typically controlled between 0.5 and 1.5 wt%. Water levels below 0.5 wt% cause
a reduction in octane. Levels above 1.5 % lead to higher corrosion rates on the
carbon steel piping and vessels. The optimum acid strength is set by a balance
between the performance of the unit feed treating systems for sulfur and water
removal and acid regeneration operation. In some cases, oxygenate removal or
diene removal systems are also used on the feed where there are known to be
high oxygenates (such as downstream of an MTBE unit) or high diolefins (from
severe FCC conditions). The action of the acid on reactions is a complex
phenomenon and is dependent on the type as well as the amount of diluents.
• The acid-to-hydrocarbon volume ratio used in the HF alkylation process is
generally 1:1 or higher. At some point, below 0.8:1.0, excess polymerization
occurs. In the most extreme cases, alkylate production could stop.
• Pressure does not have an effect on the process as long as it is kept high enough
to prevent the vaporization of hydrocarbon and acid in the reactor section.
HF Feed Contaminants
As with many refining processes, the control of contaminants coming into the unit
with the normal feedstocks is critical for the long and dependable operation of the HF
alkylation unit. Above the recommended maximum levels of feedstock contaminants,
acid consumption, acid regeneration requirements, and in some cases unit corrosion
and product quality are all measurably affected. Generally all contaminants are kept
Alkylation in Petroleum Processing
445