Technical Manual - Section 3 (Safety Hazards)

HEALTH AND SAFETY CONSIDERATIONS
Fire Protection and Prevention
Because thermal cracking is a closed process, the primary
potential for fire is from leaks or releases of liquids, gases, or
vapors reaching an ignition source such as a heater. The
potential for fire is present in coking operations due to vapor
or product leaks. Should coking temperatures get out of
control, an exothermic reaction could occur within the coker.
Safety
In thermal cracking when sour crudes are processed,
corrosion can occur where metal temperatures are between
450º and 900º F. Above 900º F coke forms a protective layer
on the metal. The furnace, soaking drums, lower part of the
tower, and high-temperature exchangers are usually subject
to corrosion. Hydrogen sulfide corrosion in coking can also
occur when temperatures are not properly controlled above
900º F.
Continuous Coking
Continuous (contact or fluid) coking is a moving-bed process
that operates at temperatures higher than delayed coking. In
continuous coking, thermal cracking occurs by using heat
transferred from hot, recycled coke particles to feedstock in
a radial mixer, called a reactor, at a pressure of 50 psi. Gases
and vapors are taken from the reactor, quenched to stop any
further reaction, and fractionated. The reacted coke enters a
surge drum and is lifted to a feeder and classifier where the
larger coke particles are removed as product. The remaining
coke is dropped into the preheater for recycling with
feedstock. Coking occurs both in the reactor and in the surge
drum. The process is automatic in that there is a continuous
flow of coke and feedstock.
Continuous thermal changes can lead to bulging and cracking
of coke drum shells. In coking, temperature control must
often be held within a 10-20º F range, as high temperatures
will produce coke that is too hard to cut out of the drum.
Conversely, temperatures that are too low will result in a high
asphaltic-content slurry. Water or steam injection may be
used to prevent buildup of coke in delayed coker furnace
tubes. Water must be completely drained from the coker, so
as not to cause an explosion upon recharging with hot coke.
Provisions for alternate means of egress from the working
platform on top of coke drums are important in the event of
an emergency.
Health
The potential exists for exposure to hazardous gases such as
hydrogen sulfide and carbon monoxide, and trace polynuclear
aromatics (PNAs) associated with coking operations. When
coke is moved as a slurry, oxygen depletion may occur within
confined spaces such as storage silos, since wet carbon will
adsorb oxygen. Wastewater may be highly alkaline and
contain
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