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5

THERMAL CONVERSIONPROCESSES

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5.1.1 IntroductionCoking is a thermal cracking process in which a

low value residual oil, such as an atmospheric or

vacuum residue, is converted into valuable

distillate products, off-gas and petroleum coke.

Coking allows the refiner to significantly reduce

the production of low value fuel oil.

Most modern units today are designed and

operated to maximize the yield of distillate

products and produce fuel grade coke as

a by-product. This type of coker represents the

majority of coker installations. Some specialized plants, on the other hand, are designed to process

special feeds and to produce high value anode

grade coke or needle coke. These units are normally

of small capacity, particularly the needle cokers.

Two different classes of coking processes are

implemented commercially: delayed coking and

fluid coking. Delayed coking represents the

largest combined capacity and is the most widely

encountered process. Fluid Coking and

Flexicoking, offered by ExxonMobil Research

and Engineering, consist of a class of coking

processes that are less widely practiced compared to delayed coking. A discussion of Fluid Coking

and Flexicoking is provided in section 5.1.4.

Delayed coking is a semi-continuous process.

Though the coking process is continuous, the

coke removal, handling and disposal are carried

out in a batch manner. The feed is heated to the

reaction temperature in a direct fired heater, and

subsequently transferred to the coke drums. The

coking reaction is delayed until the heated feed is

transferred into the coke drums where the

residence time is long enough for the coking

reactions to go to completion. Coke is deposited in the drum and the cracked vapour product exits

the drum from the top, and enters the downstream

fractionator. Coke is removed from the drum by

taking the drum off-line. In order to achieve near

steady state unit operation, the coke drums

operate in pairs such that one drum is in filling

mode, while the other is off-line for decoking.

5.1.2 Evolution of the cokingprocess and its rolein the refinery

The delayed coking process represents a naturalevolution from earlier thermal cracking processes.

During the late Nineteenth century, refineries

employed batch distillation techniques. Since the

temperature of the batch still and the residence

time were not well controlled, the oil often

underwent thermal decomposition. Coke

accumulated in the vessel and was removed

manually. Later developments included use of

multiple stills in series to produce different boiling

range products. In this arrangement, the first still

was operated at the highest temperature to flash the

majority of the crude oil. Coke accumulated in thefirst still and was removed using manual

techniques.

During the 1920s, development of continuous

distillation processes and improved thermal

cracking processes, such as the Burton process,

paved the way for the basic delayed coking process.

The Burton process, developed by Standard Oil of

Indiana, was used to produce gasoline from gas oil.

A by-product of this process was petroleum coke.

As demand for gasoline in the United States

increased and demand for heavy fuel oil

(essentially atmospheric reduced crude) decreased,refiners began to utilize the thermal conversion of

213VOLUME II / REFINING AND PETROCHEMICALS

5.1

Coking

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these residue fractions. Coke drums downstream of

the reaction furnace were employed to collect the

increased yield of petroleum coke. The delayed

coking process was commercially demonstrated by

Standard Oil of Indiana at the Whiting refinery in

1929. The term ‘delayed’ was attributed to the fact

that the coking reaction is delayed until after theheated feed is transferred into the coke drums

where adequate residence time is provided for the

coking reactions to reach completion. In the early

stages, manual decoking methods were employed.

Development of hydraulic decoking methods began

in the 1930s and continues to the present day. Early

developments included the use of drilling bits and

high pressure hydraulic cutting nozzles to remove

coke. Using a two-drum system, in which one was

filled and the other emptied at the same time, it

was possible to operate in a semi-continuous

fashion.

The growth in demand for motor gasoline from

the 1950s through the 1970s saw an increase in the

number of delayed coking units constructed that

allowed the refiner to convert residual fuel oil

stocks into gasoline and gas oil. The gas oil

provided an additional feed to the fluid catalytic

crackers which had become the predominant

gasoline production units in the refinery.

Improvements to the delayed coking process arestill being made. These improvements are offered

by various process licensors and specialized

equipment suppliers. Improvements of the

mechanical type generally address increased

furnace run length, decreased decoking cycle time,

improved operator safety and advances to allow for

larger diameter coke drums and increased capacity.

Process improvements are offered to allow the

processing of very heavy residues and to increase

liquid yields.

Residual conversion refinery

The economics of a refinery can be

considerably improved by the addition of a residual

214 ENCYCLOPAEDIA OF HYDROCARBONS

THERMAL CONVERSION PROCESSES

naphtha

re ormate

gas oa y ateC3-C4 e ns

C3-C4

gasoline

ese

et fue

coker naphtha

LCGO

LVGO

HVGO

HCGO

co e

g t nap t a

cru eoil

g t gaso ne

eavy gaso ne

g t cyc e o

slurry oi

somerate

eavy naphtha

kerosene kerosene/jet fuel

ese

ese

iese

LPG

Fig. 1. Residual conversion refinery based on delayed coking.LVGO, Light Vacuum Gas Oil; HVGO, Heavy Vacuum Gas Oil.

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conversion unit, such as a delayed coker. The

delayed coker converts the vacuum residue into

more valuable lighter products and petroleum coke.

A schematic diagram of a residual conversion

refinery is shown in Fig. 1 (see also Chapter 1.1).

The vacuum residue is processed in a delayed

coker rather than blending into fuel oil. The coker produces a wide range of products, which must be

processed further in the refinery along with the

other intermediate streams. The Heavy Coker Gas

Oil (HCGO) is hydrotreated as feedstock to the

FCC (Fluid Catalytic Cracking) unit while the

Light Coker Gas Oil (LCGO) is hydrotreated and

blended into the diesel pool. The off-gas and the

un-stabilized naphtha are further processed in the

vapour recovery unit to produce fuel gas, C3-C

4

LPG, and C5ϩ naphtha products. Petroleum coke is a

by-product from the coker unit. The overall yields

from a residual conversion refinery that processes

a blend of 50:50 Arabian Light and Arabian Heavy

compared to the yields from a refinery without a

coker are presented in Table 1.

Coker design

The design features of the coker vary depending

on the type of coke to be produced. Cokers

producing anode coke are usually subjected to

more severe temperature and pressure conditions.

They typically include smaller diameter coke

drums, high-pressure jet pumps, etc.Cokers producing needle coke operate at even

higher pressures and temperatures. In addition, the

recycle amount is also generally high (typically

greater than 50%). Current designs are mostly fuel

grade cokers, designed to operate at low pressures

and low recycle ratios in order to maximize liquid

product yields.

5.1.3 Process chemistry

Coking reactionsDelayed coking is a thermal cracking process,

with complete conversion of the vacuum residue to

solid petroleum coke and hydrocarbon products

that are lighter than the feed. Petroleum coke does

not consist of a single, simple chemical compound,

nor is it a form of pure elemental carbon, although

it approaches the latter (SRI Consulting, 1971;

Ballard et al ., 1981). It can be described as an

impure mixture of elemental carbon and hydrogen

compounds, in which the carbon to hydrogen ratio

is very high, often in excess of 20 by weight. The

ratio increases to well over 1,000 when the coke iscalcined.

During the coking process, many different

chemical reactions occur simultaneously. Thus, a

precise explanation of the reaction mechanism is

difficult.

The principal reactions can be summarized as

follows:

• Decomposition of large molecules into smaller

molecules, including free radicals.

• Free radicals, which are highly reactive and

short-lived species, react with other hydrocarbons, combine with other free radicals

resulting in termination, or decompose further

to olefins and smaller radicals, and so on.

• Thermal cracking of heavy stocks proceeds

stepwise through a series of progressively lower

molecular-weight products, for example, heavy

gas oil to light gas oil to gasoline to gas with

reactions occurring simultaneously.

• The other secondary reactions occurring in

coking are polymerization and condensation.

The decomposition and polymerization

reactions result in the formation of polycondensed aromatic compounds. When

these planar compounds rearrange and

become stacked in a f ixed direction, the state

is called the mesophase (or liquid crystal

state).

• With further heating and increased interfacial

forces, mesophase spheres form and grow into

droplets dispersed in the oil. The spheres

continue to grow and coalesce into bulk

mesophase.

• Further heating results either in ‘mosaic’ or

fibrous coke formation. The coke structure can be related to the chemical and molecular


COKING

Table 1. Overall refinery yields based

on 50:50 Light Arabian and Heavy Arabian crude mix

(100,000 bbl/d crude rate)

Overall

refinery yields(without coker)

Overall refinery

yields(with coker)

Product (liquid volume %) (liquid volume %)

C3

components 3.3 4.9

C4

components 1.8 2.5

Gasoline 44.0 53.4

Jet fuel 4.9 5.2

Diesel 26.3 34.1

Fuel oil 19.6 0.0

Coke (t/d) – 1,075

Sulphur (t/d) 142 224

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recycle material from the wash section. The total

feed is then pumped from the fractionator bottom

to the coker heater. In the heater, the feed is rapidly

heated to the coking temperature and sent to the

coke drums. Coking reactions occur in the coke

drum producing coke and light hydrocarbon

vapours. Coke accumulates in the coke drums as itforms over the period of the coking cycle, while the

hydrocarbon vapours exit the drum at the top and

are sent to the bottom of the fractionator. Gas oil is

injected into the coke drum overhead line to

quench the product vapours and minimize coking

in the line. High pressure steam or condensate is

injected into the heater tubes in order to maintain a

minimum fluid velocity, reduce residence time and

thus minimize coking in the heater tubes.

Fractionation

The most conventional fractionator design has

shed decks above the feed zone with a trayed wash

section immediately above the decks. The coke

drum vapours pass through the shed decks, wash

section and enter the gas oil fractionation section

where a circulating gas oil pumparound is used to

remove heat and to condense the gas oil vapours.

With low recycle designs, the internals near the

bottom of the tower are minimized due to the

potential for coking deriving from the low wash oil

rates. An open spray chamber design is commonly

employed in such designs. Heavy Coker Gas Oil

(HCGO) is withdrawn as a total draw-off. A portion

of the coker gas oil is pumped back to the wash

section below as wash oil. The heavier portion of the

coke-drum vapours condenses in the wash section to

form the recycle, which is mixed with the fresh feed

and returned to the heater. The coker gas oil

pumparound heat is typically used to preheat freshfeed, provide reboil heat in the vapour recovery

towers and to generate steam. The next side-draw

product, Light Coker Gas Oil (LCGO), is steam

stripped in a side stripper to remove the light ends,

cooled and sent to storage. A portion of the

unstripped LCGO is used as lean sponge oil in the

secondary absorber of the vapour recovery section.

Rich sponge oil is returned to the fractionator for

recovery of the absorbed hydrocarbons.

The fractionator overhead vapours are partially

condensed in an overhead condenser. The

uncondensed vapour is separated in the overhead

drum and sent to the vapour recovery section for

LPG (Liquefied Petroleum Gas) recovery. A part of

the condensed liquid is returned as reflux to the top

of the fractionator and the remaining overhead

liquid is sent to the vapour recovery section for

stabilization. Sour water collected in the overhead

drum is sent off-site for treating.

Vapour recovery

A simplified flow scheme of the vapour

recovery is shown in Fig. 3. The fractionator


COKING

LPG to treating

naphtha to hydrotreating

ue gas

rich light gas oil

g t gas o

uel gas to treating

s t r p p e r

a s o r e r

e u t a n i z e r p

r i m

a r y

a b s o r b e r

ract onator over ea qu

sour wa er

sour wa er

washwater

wash water

ractionator

a our

wet gascom resso

Fig. 3. Vapour recovery flow scheme.

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overhead vapour is compressed and cooled, and

the resulting vapour and liquid streams are fed to

an absorber-stripper. The vapour is fed to the

bottom of the absorber while the liquid is fed to

the top of the stripper. The fractionator overhead

liquid stream is introduced into the top of the

absorber as lean oil. Normally, this lean oil isinsufficient to achieve the desired LPG recovery,

and therefore a portion of the stabilized naphtha

from the downstream debutanizer is cooled and

recycled to the top of the absorber as

supplemental lean oil.

The bottoms from the stripper, containing C3s

and heavier, flows to the debutanizer where LPG is

recovered as an overhead liquid product, and C5ϩ as

bottom product. The C5ϩ product is cooled and sent

to storage. The overhead C3-C

4LPG is further

treated to remove sulphur compounds, including

hydrogen sulphide, mercaptans, etc., and sent for

further processing.

The overhead gas from the absorber containing

mostly C2

and lighter and some unrecovered C3s is

fed to the bottom of the sponge absorber where it

comes into contact with lean sponge oil. Any C5 and

heavier hydrocarbons present in the absorber off-gas

are recovered in the sponge absorber and returned to

the fractionator as rich sponge oil. The sponge

absorber overhead off-gas is finally treated with an

amine solution to remove hydrogen sulphide before

discharging into the refinery fuel gas system.

Closed blowdown

The closed blowdown system, shown in

Fig. 4, is used to separate and recover hydrocarbon

and steam vapours generated during the coke

drum steaming and cooling operations. The coke

drum blowdown vapours are condensed in the

blowdown scrubber by contact with circulating oildrawn from the bottom of the blowdown scrubber.

The uncondensed vapour, mostly steam and light

hydrocarbon vapours, is condensed in the

overhead condenser before entering the blowdown

drum. In the blowdown drum, light oil is

separated from the steam condensate and pumped

to the ref inery slops system, while the recovered

water is pumped off-site, initially for further

treating in a sour water stripper and later sent to

the clear water tank for reuse in coke cutting. The

vent gas from the blowdown water separator is

returned to the wet gas compressor or other

suitable refinery hydrocarbon recovery systems.

The net bottoms from the blowdown scrubber,

containing wax tailings, are removed and returned

either to the fractionator or sent to the ref inery

slops system.

Coke removal

The coke drum filled with coke is taken off-

line, steam stripped, and quenched by water. The

vapours generated during steaming and quenching

are routed to the blowdown scrubber for



quench water tank

slop oil tank

coke handlingreturn water

to sour water flash drum

to vapour recovery

slop oil toreprocessing

cokedrum

blowdown

LCGOmake-up

heavy oil to fractionator

jet water to coke drums

quench water to coke drums

water make-up

condenser

b l o w d o w n s c r u b b e r

blowdowndrum

Fig. 4. Closed

blowdown system.

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hydrocarbon and steam recovery. Coke is removed

from the drum by hydraulic decoking.

Various coke handling methods are in use,

including coke pit loading, coke pad loading, direct rail

car and hydraulic coke handling. The most commonly

used methods are coke pit and coke pad handling.

Major design considerations

Coker heater

The coker heater provides the necessary heat to thefeed in order to reach the coking reaction temperature.

There are two principal types of heater design in use

today: single-fired heaters or a double-fired heaters

(Fig.5). In modern cokers the double-fired heater

designs are mainly used, in which the heat input is

from both sides of the tube. This arrangement allows

higher average heat flux, resulting in lower peak

temperatures and shorter residence time.

Heater tube metallurgy is also being enhanced.

New designs employ steel alloys containing 9% Cr and

347 SS tubes, which permit higher skin temperatures

and allow longer run lengths to be achieved.The cold-oil velocities vary from 1.8 to 2.4 m/s,

and the average radiant heat flux is approximately

43,000 W/m2. Steam injection in the radiant

section, particularly when processing heavy feeds,

is common in modern cokers. Steam is injected in

order to increase the fluid velocity, thereby greatly

reducing the residence time and the potential for

coking in the heater tubes.

Coke drum

Coke drum sizing is governed by the superficial

vapour velocity, cycle time and the allowable outage.The vapour velocity typically determines the drum

diameter, while the cycle time sets the drum volume.

The allowable drum vapour velocity is a function of

the vapour density and the foaming tendency of the

feedstock. Typical drum vapour velocities are in the

range of 0.1 to 0.2 m/s, although some units run at

velocities higher than 0.2 m/s. The drum outage,

which is the disengaging height between the top

tangent line and the maximum coke level in the

drum, is typically in the range of 4 to 6 m. The actual

outage is determined based on the type and origin of

feedstock, its foaming tendency and the operatingconditions. The foaming in the drum is controlled by

the addition of anti-foam chemicals (generally as a

mixture with a distillate fluid) during the last few

hours of the fill cycle. The coke drum level, which is

indicative of the progress of coking in the drum and

preparation for the drum switching, is monitored by a

nuclear backscatter level instrument mounted on the

outside of the coke drums. These are also used to

detect the foam levels as the drum fills up.

In the past, cokers were designed with coking

cycle times of 20 to 24 hours (overall drum cycle of

40-48 hours). Modern designs and retrofits useshorter cycle time of the order of 14-18 hours. The

cycle time schedule sets the total volume required for

the coke drum, which, for a given drum diameter,

essentially sets the overall dimensions for the coke

drum. Coke drums of about 9 m diameter are

currently in commercial use. The coke drum shell is

fabricated from alloy steel (typically 1-Cr and 0.5-

Mo) with a stainless steel (410S, 11-13 Cr) cladding.

A switch valve, usually a four-way ball valve,

located at the drum inlet is used to switch the feed

between the drums. The switch valve also allows

the drum to be bypassed during unit start-up and shutdown.


COKING

outletoutletoutlet

old burners bushy flames

single-fired type double-fired type

modern low NOx burnerslong, thin flames

peak flux

average

outlet

ϭ1.2 peak flux

averageϭ1.8

Fig. 5. Single-fired

vs. double-fired

coker heater.

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Coker fractionator

The coker fractionator separates the coke drum

vapours into various products, including wet gas,

gasoline, LCGO, and HCGO. The bottom section

of the fractionator, up to the HCGO draw-off pan,

is highly prone to coking due to the entrained coke

particles as well as the high-temperature vapour- phase coking. Modern designs therefore minimize

tower internals at the bottom of the fractionating

tower, with some designs using an open spray

chamber below the HCGO pan. A slotted standpipe

in the bottom of the fractionator is used for

collecting the coke particles and to provide passage

to the heater charge pump. Also, a separate coke

removal system, consisting of a circulating pump

and coke filters, is employed in order to remove the

accumulated coke from the bottom of the

fractionator to minimize unit downtime.

Coker recycle

Coker recycle is one of the key operating

variables in a coker to control the HCGO end point

as well as to reduce the coking propensity of the

heavy feed in the heater tubes.

Recycle is produced at the fractionator bottom

by condensing the heavier portion of the coker gas

oil, which is then mixed with the fresh feed and

sent to the heater. Higher recycle produces more

coke at the expense of gas oil yields, however the

HCGO end point decreases, and other impuritieslike Conradson Carbon Residue (CCR) and metals

are also reduced. In fuel grade cokers, where

maximizing liquid yields is the primary objective,

low to ultra-low recycles are used. Cokers with

recycle less than 5% are considered ultra-low

recycle operations.

Blowdown scrubber

The blowdown scrubber recovers and provides

primary separation of the hydrocarbons and the

steam that are generated during the coke drum

steaming and cooling operations. The blowdownsystem includes a blowdown scrubber, overhead

condenser, water separator, circulating oil cooler,

bottom heater and the associated pumps. The drum

blowdown temperature varies from a maximum of

450°C at the start of the cooling cycle to about 150°C

near the end of the cycle. Below 150°C, the drum

effluent bypasses the scrubber and is sent directly to

the blowdown overhead condenser. A demulsifying

agent is added to the blowdown overhead water

separator to aid the oil/water separation.

Another significant function of the blowdown

system is to handle the coke drum emergency relief discharge during any drum overpressure event.

Other coking processes

Delayed coking represents the largest number of

commercially practised coking units. Fluidized

coking processes are a specialized class of coking

processes that consume part of the coke produced to

supply the necessary endothermic heat of reactionfor thermal cracking. The Fluid Coking and

Flexicoking processes, licensed by ExxonMobil

Research and Engineering (EMRE), are in

commercial use. Detailed information on these

technologies and their applications can be found in

an article by EMRE (Hammond et al ., 2003).

Fluid Coking

A generic flow diagram of the Fluid Coking

process is shown in Fig. 6. This figure and the

description that follows apply to the Fluid Coking

reactor system only. The fractionation, vapour

recovery and coke-handling systems are similar to

those used in the delayed coking process.

The reaction section entails two primary vessels:

a coking reactor and a heater. A scrubber, located on

top of the reactor, preheats the fresh feed, cools the

reactor effluent vapours, removes coke particles

entrained by the vapours, and condenses the heavy

recycle stream. The hydrocarbon conversion

reactions occur in the reactor.

Feed enters the reactor into the fluidized bed of

coke. Stripping steam is injected at the bottom of thereactor, and reaction product vapours fluidize the bed

as they rise toward the reactor cyclone and scrubber.

New coke produced from the cracking reactions is

deposited on the coke particles in the reactor bed. In



net co e

lue gas toboiler

oto e

co co e

f ee

steam

a r

pro uct toract onator

Fig. 6. Simplified Fluid Coking scheme.

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produced using the Flexicoking process for the

same feedstock is substantially reduced compared

to the Fluid Coking process.

Capital costs for a fluid coker are

approximately the same as those for a delayed

coker. On the other hand, the capital costs for a

flexicoker are greater (30-40%) than for a fluid coker because of the need for an additional gasifier

vessel, gas clean-up and the requirement for a

larger air blower. The major utility cost for a fluid

coker or flexicoker is associated with the air

blower.

5.1.5 Process variables

This section provides a description of coker

feedstocks, product yields and quality of the

various coker products and the variables that affect

the yields and product qualities.

Feedstocks

Delayed cokers can process practically any heavy

oil material in the refinery. While the typical feedstock

to a coker is a straight-run vacuum residue, a variety

of other refinery residual feedstocks and intermediate

products can also be processed in the delayed coker.

The ability of a coker to handle a variety of feedstocks

is demonstrated by the range of the gravity

(Ϫ

5 toϩ

15°API) and carbon residue (4 to 40 wt%)of the materials it can process.

The feedstocks to a coker can be classified into

the following main categories:

• Straight-run residual material, such as the

atmospheric and vacuum tower bottoms, from

the distillation processes and asphaltenes

produced from deasphalting.

• Heavy aromatic stocks such as the decant or

slurry oil produced from FCC units, thermal

tars from thermal cracking units, aromatic

extracts from lube operations, and pyrolysis tars

from ethylene plants.

• Other materials such as visbroken tars, slop oils,

tank sludge bottoms, and coal tar pitches, etc.The above feedstock classification leads to

different qualities of the by-product coke produced.

In addition to the origin and upstream

treatment, the feedstock properties that affect the

yields and product quality are: the specific gravity,

CCR, and the content of sulphur, metals and

asphaltenes. These properties determine the quality

of coke produced as well as the entire slate of the

coker products. The properties of some typical

feedstocks are summarized in Table 3.

Operating variables

The three primary operating variables that

affect product yield and quality are: coke drum

pressure, recycle ratio, and coke drum temperature.

The operating conditions are selected

depending on the feedstock quality and the process

objectives. The conditions vary significantly

between the three types of coking operations,

depending on the overall economic objectives.

Coke drum pressure

The reference pressure at which coking reactionstake place is generally considered to be the operating

pressure at the top of the coke drum. The pressure is

actually controlled at the reflux drum near the top of

the coker fractionator. Increasing coke drum (coking)

pressure increases coke yields, reduces liquid yields

and reduces the gas oil end point. Increasing coke

drum pressure also increases gas and gasoline yields.



Table 3. Typical coker feedstock characteristics

Anode coke Fuel grade coke Needle cokeFeedstock Vacuum residue Vacuum residue Slurry oil Thermal tar

Feedstock source African crude50:50 Light/Heavy

Arabian MixFCC Thermal cracker

Specific gravity at 15°C 1.01 1.041 1.052 1.21

API gravity 9.2 4.5 3.5 – 1.1

Conradson carbon

(wt%)18.9 25.0 5.0 8.6

Sulphur (wt%) 0.9 5.0 0.22 0.37

Vanadium (ppm) 39 161 – –

Nickel (ppm) 89 46 – –

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Product yieldsTypical coker yields for conventional residual oils

as well as those for needle coker feedstocks are

summarized in Table 6. Included in the table are the

following yield cases: low sulphur and low CCR

residue feedstocks suitable for anode grade

production; high sulphur and high CCR residue feed

with high metals that produce a fuel grade coke; and

highly aromatic feedstocks that produce needle coke.

These yield estimates are developed using the

KBR (Kellogg Brown and Root) yield models.

Coker product propertiesCoker products are set primarily by the ref inery

product slate, product specif ications, and theability of the refinery process units to handle their

further processing.

The estimated properties for various coker

products are summarized in Table 7 for the Arabian

crude feed blend. The product treatment steps and

the end usage are summarized in Table 8. In general,

all coker products are highly olefinic. The bromine

number, which is indicative of the degree of

olefinicity, ranges between 10 and 70. The sulphur

and nitrogen are distributed among the various

products with coke retaining a major portion of the

feed sulphur and nitrogen. Essentially all feed metals are retained by the coke.



Table 7. Delayed coker product properties based on 50:50 light Arabian

and heavy Arabian crude mix

Coker naphtha LCGO HCGO

Specific gravity at 15°C 0.740 0.857 0.946

API gravity 59.5 33.5 18.0

Sulphur (wt%) 0.65 2.2 3.8

Nitrogen (wt%) 0.09 0.14 0.40

Bromine number 60 30 12

Cetane index – 40 – RON 80 – –

Carbon residue (wt%) – 0.3

Paraffins 45.0 – –

Olefins 30.0 – –

Naphthenes 10.0 – –

Aromatics 8.0 – –

RON, Research Octane Number; PONAs, Paraffin Olefin Naphtenes Aromatics

Table 6. Typical delayed coker yields

Anode coke Fuel grade coke

Feedstock Vacuum Residue Vacuum Residue Slurry Oil Thermal Tar

Feedstock source African crude Arabian mix FCC Thermal Cracker

Dry gas 4.3 6.08.8 9.5

C3-C

4components 4.0 4.1

Gasoline (C5-205°C) 15.6 15.6 7.5 7.7

LCGO 18.9 20.839.3 35.9

HCGO 31.2 20.9

Coke 26.0 32.6 44.4 46.9

Total 100.0 100.0 100.0 100.0

Needle coke

Yields (wt%)

PONAs (vol%)

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Needle coke

This form of coke is the most valuable of all the

various petroleum cokes produced. It is used

primarily for the production of electrodes for the

steel industry (electric arc furnaces). The coke is

uniquely characterized by properties such as its low

sulphur and metals content, low Coefficient of Thermal Expansion (CTE), its needle like

crystalline structure and high electrical

conductivity. All premium needle cokes have a low

CTE. Typical needle coke specifications are shown

in Table 11. Needle coke production requires a

special feedstock, which is typically high in

aromatics, and low in asphaltenes, sulphur and

metals. In addition, the coker unit must be operated

at conditions that will provide the best premium

needle coke quality. Typical feedstocks include

slurry or decant oil from FCC units or thermal tars

from gas oil thermal cracking units. In addition,

aromatic extracts from lube operations, pyrolysis

tars from ethylene plants and some coal tar pitches

are considered potential coker feeds for needle

coke production.

Fuel grade coke

With the current trend in refineries to process

heavy crude oils, the industry continues to see a

major shift in terms of coke quality. Many new and

existing cokers have switched to processing heavy

crude oils in order to achieve improved refiningeconomic margins. This change results in the

production of poorer quality coke that is not

suitable for anode production. Due to very high

sulphur, metals and other impurities present in the

heavy feedstocks, the coke produced is only suitable

for fuel purposes, either in power plants or the

cement industry. This coke is thus referred to as fuel

grade.

5.1.6 Support process operations

The support (auxiliary) operations include some of

the key mechanical operations associated with the

delayed cokers and the coke calcining processes:

• Decoking operation (removal of coke from coke

drums).

• Automated drum unheading (removal of the top

and bottom heads).

• Hydraulic decoking by high pressure water jets.

• Coke receiving and water drainage.

• Quench water management.

• Coke calcining.

In the following section we will briefly describethe coke calcining process. The other operations

are essentially mechanical and are too specific for

the purposes of this work.

Coke calcining

Petroleum coke (green coke), either anode or

needle coke, is calcined in rotary kilns; such

processes are frequently performed outside the petroleum refinery. The characteristics of the

calcined coke depend primarily on the properties of

the green coke fed to the calciner as well as the

major operating variables, such as rate of heating,

hot zone (calcining) temperature, residence time

and rate of cooling. Calcined anode coke is used

mostly in the manufacture of anodes for the

aluminium industry. Consumption of calcined coke

in specific user industries varies considerably as

shown in Table 12.

Typical specifications of green coke vs.

calcined coke are presented in Table 10

and Table 11.

In a typical coke calcining plant, the green

coke is calcined in a rotary kiln.

Process heat is supplied through a fuel burner.

Another source of the process heat is the volatiles

which are released in the kiln.

Cokes with varying amounts of the volatile matter

can be burned in the kiln. From the kiln, the



Table 11. Typical needle coke specifications

Specification

(wt%)Green coke Calcined coke

Moisture 6-10 0.1

Volatile Combustible

Material (VCM)4-7 0.5

Sulphur 0.2-0.5 0.2-0.5

Ash 0.1 0.1

Bulk density (kg/m3) 720-800 670-720

CTE (°C) – 1-5и10Ϫ7

Real density (g/cm3) – 2.11

Table 12. Usage of calcined coke

User industryCalcined coke usage

(kg/kg)

Aluminium 0.5

Silicone carbide 1.4

Phosphorous 1.8

Calcium carbide 0.69

Graphite 1.25

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calcined coke is discharged into a rotary cooler

where it is quenched with direct water sprays at

the cooler inlet. Additional cooling is

accomplished by pulling a stream of ambient air

through the cooler. From the discharge of the

cooler, the calcined coke is conveyed

to storage silos.

References

Ballard W.P. et al. (1981) Thermal cracking , in: McKetta

J.J. (editor in chief) Encyclopaedia of chemical processing

and design, New York, Marcel Dekker, 1976- ; v. XIII.

Bansal B.B. et al . (1993) Design and economics for low

pressure delayed coking , in: Proceedings of the National

Petroleum Refiners Association annual meeting , San Antonio

(TX), 21-23 March.

Hammond D.G. et al. (2003) Review of fluid coking and

flexicoking technologies, in: Proceedings of the American

Institute of Chemical Engineers Spring national meeting ,

New Orleans (LA), 30 March-3 April.

Sloan H.D. et al. (1992) Delayed coking has a role in clean fuels environment , «Fuels Reformulation», July-August.

SRI Consulting (1971) Petroleum coke, Process Economics

Program Report 72.

Bharat B. BansalJoseph A. Fruchtbaum

Aldrich H. NorthupRao Uppala

Kellogg, Brown & RootHouston, Texas, USA


COKING

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