P7. Alkylation process
7.1 History of Alkylation
➢
Alkylation,
first
commercialized
in
1938,
experienced
tremendous growth during the 1940’s as a result of the
demand for high octane aviation fuel during World War II.
➢
During the mid 1950’s refiners’ interest in alkylation shifted
from the production of aviation fuel to the use of alkylate as a
blending component in automotive motor fuel.
➢The alkylation reaction combines isobutane with light olefins in
the presence of a strong acid catalyst.
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7.2 Definition
➢ An important refining process for the production of alkylate, a high-octane
gasoline blending component from olefins (propylene, butylene, amylene)
and isobutane in the presence of HF or H2SO4 as a catalyst
➢
Alkylate: a mixture of high-octane, branched- chain paraffinic hydrocarbons
(mostly iso-heptane and iso-octane), having MON of 90-95 and RON of 93-98
➢
Alkylate is a premium gasoline blending stock because it has exceptional
antiknock
properties
and
is
clean
burning.
Alkylate
has excellent
stability, low vapor pressure and high research octane number (RON).
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7.3 Source of Alkylation Feed
1. Olefins (C3=, nC4=, nC5=)
✓ FCC (15%)
✓ Coker (15%)
✓ Thermal Unit
2. Isobutane (iC4)
✓ FCC (6%)
✓ Hydrocracker (3%)
✓ Reformer (2%)
✓ Crude unit (0.5%)
✓ Coker (1%)
✓ Hydrotreater (1%)
✓ Isomerization (1%)
(Refinery gases produced from different units are collected and sent to the gas plant.
Olefins and isobutanes are separated and used as a feed to the alkylation unit)
7.4 Alkylation Process in Petroleum Industry
➢
Alkylation can take place at high temperatures without catalyst (t = 500oC, P = 200-
400 bars).
➢
The only processes of commercial importance today operate at low to moderate
temperatures (t < 50oC, P < 30 bars) using either sulfuric or hydrofluoric acid
catalysts.
➢
The reactions occurring in the alkylation process are complex and produce an
alkylate product that has a wide boiling range.
7.5 Side reactions
✓ Polymerization
✓ Reactions with the acid : ester formation
✓ Organic Flouride formation
✓ Cracking
✓ Hydrogen transfer
7.6 Pretreatment of Feed
✓ Removal of H2S by Amine treating
✓ Removal of residual H2S by dilute caustic wash
✓ Removal of Mercaptans with Merox extraction
✓ Removal of entrained caustic by water wash
✓ Removal of entrained water (coalescer)
✓ Drying of feed (activated alumina)
✓ Selective hydrogenation
Block diagram of alkylation process
7.7 Main types of alkylation process
1. Sulfuric Acid Alkylation Process (SAAU)
✓ Auto-refrigeration process (Exxon)
✓ Effluent refrigeration process (Stratford)
2. Hydrofluoric Acid Alkylation Process (HAAU)
✓Phillips Process
✓UOP Process
(Phillips and UOP are the two common types of hydrofluoric acid alkylation
processes in use)
Alkylation reaction is exothermic, cooling is needed: SAAU plants require
lower temperatures so the cooling medium needs to be chilled, for
HFAU normal refinery cooling water will suffice.
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7.7.1. Sulfuric Acid Alkylation Process
Auto-refrigerated sulphuric acid alkylation process
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7.7.1. Sulfuric Acid Alkylation Process
Block diagram for Stratco effluent refrigerated SAAU
Stratco reactor
A sulfuric acid alkylation unit can be divided into five major sections
1. Reaction Section: The reacting hydrocarbons are brought into contact with
sulfuric acid catalyst under controlled conditions.
2. Refrigeration Section: The heat of reaction is removed and light
hydrocarbons are purged from the unit.
3. Effluent Treating Section: The free acid, alkyl sulfates and di-alkyl sulfates
are removed from the net effluent stream to avoid downstream corrosion and
fouling.
4. Fractionation Section: Isobutane is recovered for recycle to the reaction
section and remaining hydrocarbons are separated into the desired products.
5. Blowdown Section: Spent acid is degassed, waste water pH is adjusted
and acid vent streams are neutralized before being sent off-site.
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7.7.2 Hydroflouric Acid Alkylation Process
✓The second main catalyst option is hydrofluoric acid. In
typical alkylation plants, rates of consumption for acid are
much lower than for sulfuric acid. These plants also
produce alkylate with better octane rating than do sulfuric
plants.
✓However, due to its hazardous nature, HF acid is
produced at very few locations and transportation must
be managed rigorously.
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a. Phillips Process
✓ This process uses a riser type reactor ,in which hydrocarbon mixture is introduced
through nozzles.
✓ High velocities are achieved due to pressure drop ,resulting in emulsions of liquid HF
and liquid hydrocarbons that rise in the uncooled reactor.
✓ In vertical reactor riser perforated trays provide dispersion of the hydrocarbon phase
from acid phase.
✓
Recycle HF is continuously leave the bottoms of the settler because of gravity
Hydrocarbon mixture in the settler which contains mainly isobutane and alkylate and
small amount of propane ,n butane dissolved HF and isoalkyl flourides is passed to
the main fractionators.
✓
The Mixture is into top stream and side stream top stream is directed to an HF
stripper to separate HF and propane.
✓ All the streams leaving the unit are treated with KOH to eliminate residual acidity.
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b. UOP process
✓
Isobutene olefin feed mixture enters the shell of a vertical reactor heat exchanger.
✓
HF is introduced at the bottom of the shell.
✓
Heat of reaction is removed by cooling water in a cooling coil.
✓
Effluent mixture is directed to the settler.
✓
Hydrocarbon phase passed through stripper and acid is recycled back to reactor
heat exchanger.
✓
Isobutane is withdrawn as a side stream and recycled to the reactor.
✓
Isostripper overhead is sent to HF stripper to strip acid.
✓
Overhead of hf stripper is sent to top of the stripper for recovery of the acid and
isobutane and bottom is sent to depropanizer.
✓
An internal depropanizer is required between isostripper and the HF stripper, if
amount of propane in C4 fraction is too high.
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7.8 Comparision of SA and HA Alkylation process
✓
Cost of catalysts is much higher (about 30 percent ) than HA (~ 5 percent).
✓
HA is more dangerous when using in alkylation process than SA.
✓
Octane number:
Olefin
Octane number of alkylate
SA
HA
Propylene
88 -89
91 - 92
Isobutylene
91- 92
93 - 95
1 butene
95.7 - 97.7
87 - 89
2 butene
96 -98
91 - 93
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7.9 HF Challenges and Opportunities
Modify HF based on HF/Additive system
Replace HF with solid alkylation catalyst
New alternatives
7.10 Process variables
1. Reactor temperature low temp requires higher refrigeration and
investment costs.
2. Isobutane recycle rate high isobutane recycle rate increases the
investment requirement for the deisobutanizer and increases the
operating cost of tower because of high steam requirement.
3. Space velocity high space velocity increases the formation of acid
sulphates corrosive to the downstream fractionation facilities.
4. Spent acid strength low spent acid strength tends to decrease
make up acid requirements ,it increases the risk of acid runaway.
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7.11 Properties of Alkylate gasoline products
7.12 Ideal quality of Alkylate
1. High Research Clear Octane.
2. Low sensitivity – High road octane.
3. Good Lead susceptibility.
4. No olefins or aromatics.
5. Clean burning.
6. Little or no sulfur.
7. Low volatility.
8. High front end octane quality.
9. Excellent stability
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7.13 Health and safety considerations
✓ Sulphuric acid and hydrofluoric acid are dangerous chemicals, and care during delivery and unloading of acid
is essential.
✓
There is a need to maintain sulphuric acid concentrations of 85 to 95% for good operation and to minimize
corrosion.
✓ To prevent corrosion from hydrofluoric acid, acid concentrations inside the process unit
must be maintained above 65% and moisture below 4%.
✓
Vents can be routed to soda ash scrubbers to neutralize hydrogen fluoride gas or hydrofluoric acid vapours
before release.
✓ There is a potential for serious hazardous and toxic exposures should leaks, spills or releases occur.
✓
Direct contact with sulphuric or hydrofluoric acid will cause severe skin and eye damage, and inhalation of
acid mists or hydrocarbon vapours containing acid will cause severe irritation and damage to the respiratory
system.
✓
Safe work practices and appropriate skin and respiratory personal protective equipment are needed where
potential exposures to hydrofluoric and sulphuric acids during normal operations exist, such as reading
gauges, inspecting and process sampling, as well as during emergency response, maintenance and
turnaround activities
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