Technical Manual - Section 3 (Safety Hazards)

chemicals and other hazards such as noise and heat; during
process sampling, inspection, maintenance, and turnaround
activities; and when handling amine or exposed to catalyst.
ISOMERIZATION
Isomerization converts n-butane, n-pentane and n-hexane into
their respective isoparaffins of substantially higher octane
number. The straight-chain paraffins are converted to their
branched-chain counterparts whose component atoms are the
same but are arranged in a different geometric structure.
Isomerization is important for the conversion of n-butane into
isobutane, to provide additional feedstock for alkylation units,
and the conversion of normal pentanes and hexanes into
higher branched isomers for gasoline blending. Isomerization
is similar to catalytic reforming in that the hydrocarbon
molecules are rearranged, but unlike catalytic reforming,
isomerization just converts normal paraffins to isoparaffins.
There are two distinct isomerization processes, butane (C 4 )
and pentane/hexane (C 5 /C 6 ). Butane isomerization produces
feedstock for alkylation. Aluminum chloride catalyst plus
hydrogen chloride are universally used for the
low-temperature processes. Platinum or another metal
catalyst is used for the higher-temperature processes. In a
typical low-temperature process, the feed to the isomerization
plant is n-butane or mixed butanes mixed with hydrogen (to
inhibit olefin formation) and passed to the reactor at 230-340º
F and 200-300 psi. Hydrogen is flashed off in a
high-pressure separator and the hydrogen chloride removed
in a stripper column. The resultant butane
mixture is sent to a fractionator (deisobutanizer) to separate
n-butane from the isobutane product.
Pentane/hexane isomerization increases the octane number
of the light gasoline components n-pentane and n-hexane,
which are found in abundance in straight-run gasoline. In a
typical C5/C6 isomerization process, dried and desulfurized
feedstock is mixed with a small amount of organic chloride
and recycled hydrogen, and then heated to reactor
temperature. It is then passed over supported-metal catalyst
in the first reactor where benzene and olefins are
hydrogenated. The feed next goes to the isomerization reactor
where the paraffins are catalytically isomerized to
isoparaffins. The reactor effluent is then cooled and
subsequently separated in the product separator into two
streams: a liquid product (isomerate) and a recycle
hydrogen-gas stream. The isomerate is washed (caustic and
water), acid stripped, and stabilized before going to storage.
SAFETY AND HEALTH CONSIDERATIONS
Fire Protection and Prevention
Although this is a closed process, the potential for a fire
exists should a release or leak contact a source of ignition
such as the heater.
Safety
If the feedstock is not completely dried and desulfurized, the
potential exists for acid formation leading to catalyst
poisoning and metal corrosion. Water or steam must not be
allowed to enter areas where hydrogen chloride is present.
Precautions are
Table III:2-15 ISOMERIZATION PROCESSES
Feedstock From Process Typical products................To
n-Butane Various Rearrangement Isobutane.........................Alkylation
n-Pentane processes Isopentane........................Blending
n-Hexane Isohexane.........................Blending
Gas.................................Gas Plant
III:2-35