DELAYED
COKING
Delayed
coking can be described as a semi-continuous process in which the feedstock to
the process is heated to a high temperature at high velocity, and then
transferred to a large drum which allows for the long residence time required
for the cracking reactions to proceed to completion, thereby depositing the coke
in the drum.
FIGURE 1 shows a
simplified flow diagram of a delayed coking unit operating on a straight-run
atmospheric residue feedstock. The feed is charged to the base of the main
product fractionators after heat exchange with coker distillate products. The
feed, and a recycle stream of heavy coker distillate, are then heated in a
specially designed furnace to a temperature in the range of 480 to 515°C at the
furnace outlet. The heated oil then passes to one of a pair of coking drums.
Then cracked products leave from the top of the drum, whilst the coke formed is
deposited in the drum. To maintain continuous operation, two or more coke drum
is emptied. As the cracking reactions are endothermic the temperatures in the
coke drum are lower than at the furnace outlet and usually are in the range 415
to 460°C. Drum pressure varies from 1 bar (g) up to about 7 bars (g).
The cracked
products pass to a large fractionation tower, where gas, naphtha and gas oil
fractions are removed as products. Heavy distillate products may be recycled
with fresh feed for recracking.
The heart of
the coking process is the coker furnace, and the design has been modified in
recent years to give longer run lengths before decoking of the furnace tubes
becomes necessary. Cold oil velocities are now about 2 m/sec, and multiple
injection of steam into the heater coil is practiced to adjust coil residence
time and velocity. Heat flux rates are being reduced to 25000 kcal/m2/h from the
traditional values of 28-32000 kcal/m2/h.
The main process variable in delayed coking
are:
1) Feedstock
type. Feedstock quality as measured by the UOP K-factor, can affect the yield of
coke and the quality of the distillate products. The lower the UOP K-factor, the
more aromatic the final products. The coke yield is dependent on the Condrason
carbon content of the feedstock, high values giving higher yields of
coke.
2) Recycle
ratio, which is defined as the (fresh feed + recycle feed) / fresh feed, can
vary from 1.1 to nearly 2.0. The higher the recycle ratio, the greater the coke
yields from a given feedstock. Furthermore, more naphtha and less heavy gas oil
are produced, and the overall liquid yield decreases.
3) Drum
pressure. Increase in coke drum operating pressure retains more of the heavy
hydrocarbons in the coke drum. This increases the coke yield. It also results in
slightly increased gas yield, whilst decreasing the overall liquid produced. The
effect of pressure is shown in
TABLE 1.
The coke drum is usually sized to be on stream for 24 h
before becoming full of 'green' coke. After switching feed to the second drum,
the following procedure is used to remove coke from the drum:
a) The coke
drum is first depressured and steamed out to remove uncondensed vapours, which
are sent to a blowdown drum via the fractionators.
b) The drum
is then filled with cold water to cool the coke down. Steam generated, plus
hydrocarbon vapours, pass to the blowdown drum.
c) The water
is drained down and the top and bottom heads of the coke drum are removed to
permit emptying.
d) A hole is
drilled through the centre of the coke deposit using a high pressure water
jet.
e) The coke
is then removed by cutting out the deposits with multiple high pressure water
jets spraying radially from the centre of the drum.
The coke
dropping out of the base of the drum is accompanied by large volumes of drilling
water. Many different coke dewatering/ handling systems are available, the most
common being:
- Direct rail-car loading
- Pad or apron loading
- Pit loading
- And dewatering bins
Direct
rail-car loading is the cheapest, but has the disadvantage that decoking is
dependent on the time for rail-car movement.
Pad loading
allows the coke water to flow from the drum through a chute on to concrete
apron. The water drains to the periphery of the pad into a settling maze where
the coke fines settle out before the clear water is recycled to the decoking
water surge tank for re-use. The coke is removed from the pad by a front-end
loader or an overhead crane.
Pit loading
is very similar to pad loading, except that the coke empties into a concrete
apron. An overhead crane is required for coke handling.
Dewatering
bins have evolved to provide totally enclosed systems to meet exceptional
environmental requirements or to prevent coke contamination in areas where sand
storms may be a problem.
Delayed
coker design has been modified over the years to meet mounting environmental
concern. One example is the need for the blowdown facilities mentioned above.
Blowdown systems did exist for the recovery of wax tailings and the condensation
of steam. However, non-condensable vapours were water scrubbed and vented to
atmosphere. A typical modern system is shown in
FIGURE 2. The vent vapours pass
to the flare or may be recovered using a vent gas compressor.
Typical
yields and product properties for a range of different feedstock are shown in
TABLE 2.
Cokes
produced from delayed coking can be used for a number of different applications.
The limiting quality factors for each of the major coke markets are summarized
in TABLE 3. Premium quality 'green' coke (low sulphur and low metals) is
traditionally calcined for the production of amorphous carbon anodes used in the
manufacture of aluminium. The critical parameters for this application are given
in TABLE 4. It is evident from
TABLE 2 that cokes produced from
selected crude can be used for anode
production. 
