alkylate. Acid consumption can be as low as 16 lb per barrel alkylate for diene-free,
MTBE raffinates run at optimal process conditions. Acid consumption can exceed
40 lb/bbl of alkylate when running high levels of dienes, isobutylene, pentenes, and
propylene at high temperatures. The effects of some typical contaminants are
summarized below:
• Dienes – butadiene and pentadienes are major contaminants affecting acid
consumption. Dienes can account for 10–30 % of the acid consumed. Dienes
can dilute 5–20 lb of 99 % acid down to 90 % per lb of diene. Some refiners use
diolefin selective hydrogenation units (such as UOP-H€ uls SHP) to eliminate this
source of acid consumption.
• Water and oxygenates – water can also account for 10–20 % of acid consumption. Wet feeds, wet isobutane recycle, and high reactor temperatures are the
main sources of water. Oxygenates including acetone from the FCC and
methanol and dimethyl ether from MTBE and TAME units also consume
acid. Water and oxygenates dilute 10 lb of 99 % acid to 90 % per lb of
contaminant.
• Sulfur and nitrogen compounds dilute about 20 lb of 99 % acid to 90 % per lb of
sulfur or nitrogen.
• C 6 + dilutes 2 lb of 99 % acid to 90 % per lb of contaminant.
This can be a significant contributor to acid consumption when alkylating
pentenes.
• Ethylene dilutes 20 lb of 99 % acid to 90 % per lb of contaminant by forming
stable sulfates that stay in the acid.
• Ethane and lighter – while inert, light ends increase compressor discharge
pressure for the total condensation. These indirectly increase acid consumption
when limited by horsepower by increasing reactor temperature.
H 2 SO 4 Regeneration
Sulfuric acid alkylation uses about 100 times as much acid as HF alkylation because
H 2 SO 4 cannot be stripped from the conjunct polymer, water, and contaminants.
Sulfuric acid is typically spent at 88–91 wt% H 2 SO 4 , below which it is too weak to
carry out alkylation.
The spent sulfuric acid is not a waste. It is regenerated by burning the organics
(usually along with H 2 S) which reduces the H 2 SO 4 to SO 2 and water vapor. The
SO 2 is then dried with oleum oxidized to SO 3 and finally scrubbed with water to
produce oleum and 98.5–99.5 % H 2 SO 4 .
For sulfuric alkylation, aside from the direct hazards of contacting the acid, there
are transportation safety issues around the need to ship and regenerate 20–500 tons
of H 2 SO 4 daily. Tomitigate transportation risk, several refiners have built dedicated
acid plants to regenerate spent acid from the alkylation unit or pipelines to and from
an acid manufacturer.
Alkylation in Petroleum Processing
451
MTBE raffinates run at optimal process conditions. Acid consumption can exceed
40 lb/bbl of alkylate when running high levels of dienes, isobutylene, pentenes, and
propylene at high temperatures. The effects of some typical contaminants are
summarized below:
• Dienes – butadiene and pentadienes are major contaminants affecting acid
consumption. Dienes can account for 10–30 % of the acid consumed. Dienes
can dilute 5–20 lb of 99 % acid down to 90 % per lb of diene. Some refiners use
diolefin selective hydrogenation units (such as UOP-H€ uls SHP) to eliminate this
source of acid consumption.
• Water and oxygenates – water can also account for 10–20 % of acid consumption. Wet feeds, wet isobutane recycle, and high reactor temperatures are the
main sources of water. Oxygenates including acetone from the FCC and
methanol and dimethyl ether from MTBE and TAME units also consume
acid. Water and oxygenates dilute 10 lb of 99 % acid to 90 % per lb of
contaminant.
• Sulfur and nitrogen compounds dilute about 20 lb of 99 % acid to 90 % per lb of
sulfur or nitrogen.
• C 6 + dilutes 2 lb of 99 % acid to 90 % per lb of contaminant.
This can be a significant contributor to acid consumption when alkylating
pentenes.
• Ethylene dilutes 20 lb of 99 % acid to 90 % per lb of contaminant by forming
stable sulfates that stay in the acid.
• Ethane and lighter – while inert, light ends increase compressor discharge
pressure for the total condensation. These indirectly increase acid consumption
when limited by horsepower by increasing reactor temperature.
H 2 SO 4 Regeneration
Sulfuric acid alkylation uses about 100 times as much acid as HF alkylation because
H 2 SO 4 cannot be stripped from the conjunct polymer, water, and contaminants.
Sulfuric acid is typically spent at 88–91 wt% H 2 SO 4 , below which it is too weak to
carry out alkylation.
The spent sulfuric acid is not a waste. It is regenerated by burning the organics
(usually along with H 2 S) which reduces the H 2 SO 4 to SO 2 and water vapor. The
SO 2 is then dried with oleum oxidized to SO 3 and finally scrubbed with water to
produce oleum and 98.5–99.5 % H 2 SO 4 .
For sulfuric alkylation, aside from the direct hazards of contacting the acid, there
are transportation safety issues around the need to ship and regenerate 20–500 tons
of H 2 SO 4 daily. Tomitigate transportation risk, several refiners have built dedicated
acid plants to regenerate spent acid from the alkylation unit or pipelines to and from
an acid manufacturer.
Alkylation in Petroleum Processing
451