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Module 3: Liquid Fossil Fuel (Petroleum)
Lecture 21: Secondary Processing
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Keywords: Petroleum coke, fluid coking, delayed coking
3.4.1.2 Coking
Coking is a refinery operation that upgrades low-valued bottoms like atmospheric or vacuum
residue into higher-value petroleum coke. Petroleum coke is a high carbon coal like material.
The process is actually a severe thermal cracking which completely converts the petroleum
residues into coke and lighter products. The heavy distillate fraction which is produced in the
process is recycled. The temperature used in this process ranges from 500 to 6500C.
There are two basic forms of coking operation, Fluid coking and Delayed coking. The first one is
a continuous process and utilizes a fluid bed. The second process is a semi-continuous and uses
coke drums for accumulation of coke. Delayed coking is more commonly used process in
refinery. The name of the process ‘Delayed’ is due to the reason that, coking reaction occurs in
the coke drum rather than in the heater. Coke drums are used to hold or delay the heated
feedstock while the cracking takes place. The cracking/carbonization reactions involve
dehydrogenation, rearrangement and polymerization.
Fluid coking
Fluid coking is a continuous process in which heated feedstocks are sprayed into a fluidized bed
of hot coke particles. The reactor is maintained at 20-40 psi and more than 500°C. The feed
vapors are cracked on the fine coke particles while forming a liquid film on the coke particles.
The particles grow by layers until they are removed and new seed coke particles are added. The
main unit of fluid coking process is the combined scrubber-reactor assembly, where the scrubber
is fixed above the coker reactor as shown in the Fig 1. The residue feed is introduced into the
scrubber at about 5700C. It is heated with the effluent stream of the reactor.
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Fig. 1. Fluid coking unit
In this scrubber, the high boiling hydrocarbons are condensed and scrubbed out with the reactor
effluent vapors at about 5250C. This stream is recycled to the reactor mixing with the fresh feed.
The lighter hydrocarbons from the overhead of scrubber are sent to the fractionators for recovery
of lighter products. In the reactor, the cracking reactions take place to finally produce the coke
and lighter products. Coke is sent to the burner and about 20% coke is burnt here with air to
satisfy the heat requirements of the reactor in the cracking reactions. A part of the coke from the
burner is recycled to the reactor and the rest is taken as a marketable product coke. The flue gas
from the burner is used for steam generation in CO boiler.
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Delayed coking
Delayed coking is the preferred choice of many refiners as it imparts the advantage of handling
very heavy residue. This process produces a significant amount of naphtha and diesel products.
Moreover, the yields of product can be tailored by adjusting the recycle and operating conditions.
Fig. 2 Delayed coker unit
Delayed coking is a semi batch process which uses alternative coke drums one of which is
switched off-line after filling. Except coke drum, a furnace, closed blow down, coke cutting and
handling and a water recovery system are included in the unit. Hot residual oil is mixed to the
bottom of the fractionators (heavy distillate from fractionators) and the combined stream is
heated in the furnace to initiate coke formation in the coke drums. Steam is injected in the
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furnace coil to avoid coking in the coil. Coke drum overhead vapour flows to the fractionator
where it is separated into wet gas, unstabilised naphtha, gas oil, heavy gas oil and recycle. The
Delayed coking unit is shown in Fig. 2. In the operating coke drum, the hot feed from the coker
furnace is injected into the bottom of the drum at high temperature and low pressure and is
cracked into gaseous products and coke. The gaseous products are sent to the fractionator for
product recovery and the solid coke is solidified in the coke drum. The other drum which is
offline and full of coke, is steamed, vented, and cooled prior to the drum being opened to
atmosphere. During the coke drum steam out and cooling period, all steam and hydrocarbon
vapours are directed to the blow down system where they are recovered. After this drum is
opened, the petroleum coke is cut from the drum using high pressure water jet. The coke is
dropped into a pit from the drum where water is separated from coke and recycled. Petroleum
coke or simply “coke” is similar to coal and is typically used for fuel in power plants.
There are two distinctive types of raw petroleum coke, one is calcined or green coke and another
one is fuel grade or Petcoke.
Fuel-grade coke is spongy in texture and contains high amounts of sulfur. It can withstand high
heat and contains little ash. This type of coke is primarily used as a fuel in power generators. As
it contains high sulfur, hence, burning of it produces sulphur dioxide gas. So, a sulfur capture
system should be there to reduce the amount of sulfur released into the air and meet clean-air
standards.
Calcined petroleum coke is made by calcining or roasting petroleum coke just below the melting
point. This coke is commonly used as electrodes in the smelting industry for the production of
metals such as titanium, aluminum and steel.
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Although, coke is a low value by-product compared to transportation fuel, there is a great
demand worldwide for even high sulphur petroleum coke as it is a very economical fuel.
Calcination grade raw petroleum coke (RPC) is produced at Barauni, BRPL, Digboi and
Guwahati refineries of Indian Oil Corporation (IOC). Fuel Grade Petcoke is product at Panipat
refinery of IOC.
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Reference:
1. The Chemistry and Technology of Petroleum, James G. Speight, 4th edition, CRC Press, 2010.
2. Modern petroleum technology, Downstream, ed. by Alan G, Lucas, Vol 2, 6th edition, IP, John
Wiley & Sons Ltd., 2001.
3. Modern Petroleum Refining Processes, B.K.B.Rao, 4th Ed.,Oxford & IBH Publishing Co. Ltd.
2002.
4. Indian Oil Corporation site, http://www.iocl.com/Products/RawPetroleumCoke.aspx
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