5 THERMAL CONVERSION PROCESSES - Treccanitreccani.it/export/sites/default/Portale/sito/... · 5...

5

THERMAL CONVERSIONPROCESSES

5.1.1 Introduction

Coking is a thermal cracking process in which alow value residual oil, such as an atmospheric orvacuum residue, is converted into valuabledistillate products, off-gas and petroleum coke.Coking allows the refiner to significantly reducethe production of low value fuel oil.

Most modern units today are designed andoperated to maximize the yield of distillateproducts and produce fuel grade coke as a by-product. This type of coker represents themajority of coker installations. Some specializedplants, on the other hand, are designed to processspecial feeds and to produce high value anodegrade coke or needle coke. These units are normallyof small capacity, particularly the needle cokers.

Two different classes of coking processes areimplemented commercially: delayed coking andfluid coking. Delayed coking represents thelargest combined capacity and is the most widelyencountered process. Fluid Coking andFlexicoking, offered by ExxonMobil Researchand Engineering, consist of a class of cokingprocesses that are less widely practiced comparedto delayed coking. A discussion of Fluid Cokingand Flexicoking is provided in section 5.1.4.

Delayed coking is a semi-continuous process.Though the coking process is continuous, thecoke removal, handling and disposal are carriedout in a batch manner. The feed is heated to thereaction temperature in a direct fired heater, andsubsequently transferred to the coke drums. Thecoking reaction is delayed until the heated feed istransferred into the coke drums where theresidence time is long enough for the cokingreactions to go to completion. Coke is depositedin the drum and the cracked vapour product exits

the drum from the top, and enters the downstreamfractionator. Coke is removed from the drum bytaking the drum off-line. In order to achieve nearsteady state unit operation, the coke drumsoperate in pairs such that one drum is in fillingmode, while the other is off-line for decoking.

5.1.2 Evolution of the cokingprocess and its role in the refinery

The delayed coking process represents a naturalevolution from earlier thermal cracking processes.During the late Nineteenth century, refineriesemployed batch distillation techniques. Since thetemperature of the batch still and the residencetime were not well controlled, the oil oftenunderwent thermal decomposition. Cokeaccumulated in the vessel and was removedmanually. Later developments included use ofmultiple stills in series to produce different boilingrange products. In this arrangement, the first stillwas operated at the highest temperature to flash themajority of the crude oil. Coke accumulated in thefirst still and was removed using manualtechniques.

During the 1920s, development of continuousdistillation processes and improved thermalcracking processes, such as the Burton process,paved the way for the basic delayed coking process.The Burton process, developed by Standard Oil ofIndiana, was used to produce gasoline from gas oil.A by-product of this process was petroleum coke.As demand for gasoline in the United Statesincreased 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

these residue fractions. Coke drums downstream ofthe reaction furnace were employed to collect theincreased yield of petroleum coke. The delayedcoking process was commercially demonstrated byStandard Oil of Indiana at the Whiting refinery in1929. The term ‘delayed’ was attributed to the factthat the coking reaction is delayed until after theheated feed is transferred into the coke drumswhere adequate residence time is provided for thecoking reactions to reach completion. In the earlystages, manual decoking methods were employed.Development of hydraulic decoking methods beganin the 1930s and continues to the present day. Earlydevelopments included the use of drilling bits andhigh pressure hydraulic cutting nozzles to removecoke. Using a two-drum system, in which one wasfilled and the other emptied at the same time, itwas possible to operate in a semi-continuousfashion.

The growth in demand for motor gasoline fromthe 1950s through the 1970s saw an increase in the

number of delayed coking units constructed thatallowed the refiner to convert residual fuel oilstocks into gasoline and gas oil. The gas oilprovided an additional feed to the fluid catalyticcrackers which had become the predominantgasoline production units in the refinery.

Improvements to the delayed coking process arestill being made. These improvements are offeredby various process licensors and specializedequipment suppliers. Improvements of themechanical type generally address increasedfurnace run length, decreased decoking cycle time,improved operator safety and advances to allow forlarger diameter coke drums and increased capacity.Process improvements are offered to allow theprocessing of very heavy residues and to increaseliquid yields.

Residual conversion refineryThe economics of a refinery can be

considerably improved by the addition of a residual

214 ENCYCLOPAEDIA OF HYDROCARBONS

THERMAL CONVERSION PROCESSES

naphtha

reformate

gas oilalkylateC3-C4 olefins

C3-C4

gasoline

diesel

jet fuel

cokernaphtha

LCGO

LVGO

HVGO

HCGO

coke

light naphtha

crudeoil

light gasoline

heavy gasoline

light cycle oil

slurry oil

isomerate

heavy naphtha

kerosene kerosene/jet fuel

diesel

diesel

diesel

LPG

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

conversion unit, such as a delayed coker. Thedelayed coker converts the vacuum residue intomore valuable lighter products and petroleum coke.A schematic diagram of a residual conversionrefinery is shown in Fig. 1 (see also Chapter 1.1).The vacuum residue is processed in a delayedcoker rather than blending into fuel oil. The cokerproduces a wide range of products, which must beprocessed further in the refinery along with theother intermediate streams. The Heavy Coker GasOil (HCGO) is hydrotreated as feedstock to theFCC (Fluid Catalytic Cracking) unit while theLight Coker Gas Oil (LCGO) is hydrotreated andblended into the diesel pool. The off-gas and theun-stabilized naphtha are further processed in thevapour recovery unit to produce fuel gas, C3-C4LPG, and C5

� naphtha products. Petroleum coke is aby-product from the coker unit. The overall yieldsfrom a residual conversion refinery that processesa blend of 50:50 Arabian Light and Arabian Heavycompared to the yields from a refinery without acoker are presented in Table 1.

Coker designThe design features of the coker vary depending

on the type of coke to be produced. Cokersproducing anode coke are usually subjected tomore severe temperature and pressure conditions.They typically include smaller diameter cokedrums, high-pressure jet pumps, etc.

Cokers producing needle coke operate at evenhigher pressures and temperatures. In addition, therecycle amount is also generally high (typicallygreater than 50%). Current designs are mostly fuelgrade cokers, designed to operate at low pressuresand low recycle ratios in order to maximize liquidproduct yields.

5.1.3 Process chemistry

Coking reactionsDelayed coking is a thermal cracking process,

with complete conversion of the vacuum residue tosolid petroleum coke and hydrocarbon productsthat are lighter than the feed. Petroleum coke doesnot consist of a single, simple chemical compound,nor is it a form of pure elemental carbon, althoughit approaches the latter (SRI Consulting, 1971;Ballard et al., 1981). It can be described as animpure mixture of elemental carbon and hydrogencompounds, in which the carbon to hydrogen ratiois very high, often in excess of 20 by weight. Theratio increases to well over 1,000 when the coke iscalcined.

During the coking process, many differentchemical reactions occur simultaneously. Thus, aprecise explanation of the reaction mechanism isdifficult.

The principal reactions can be summarized asfollows:• Decomposition of large molecules into smaller

molecules, including free radicals.• Free radicals, which are highly reactive and

short-lived species, react with otherhydrocarbons, combine with other free radicalsresulting in termination, or decompose furtherto olefins and smaller radicals, and so on.

• Thermal cracking of heavy stocks proceedsstepwise through a series of progressively lowermolecular-weight products, for example, heavygas oil to light gas oil to gasoline to gas withreactions occurring simultaneously.

• The other secondary reactions occurring incoking are polymerization and condensation.The decomposition and polymerizationreactions result in the formation ofpolycondensed aromatic compounds. Whenthese planar compounds rearrange andbecome stacked in a fixed direction, the stateis called the mesophase (or liquid crystalstate).

• With further heating and increased interfacialforces, mesophase spheres form and grow intodroplets dispersed in the oil. The spherescontinue to grow and coalesce into bulkmesophase.

• Further heating results either in ‘mosaic’ orfibrous coke formation. The coke structure canbe 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)

Overallrefinery yields(without coker)

Overall refinery yields

(with coker)

Product (liquid volume %) (liquid volume %)

C3components 3.3 4.9

C4components 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

composition of the feedstocks. The most criticaloperational factors are the temperature,residence time and gas-flow rate.In summary, the reaction mechanism of delayed

coking is complex, but three distinct steps occur inthe entire coking process: in the furnace, the feed ispartially vaporized and mildly cracked; in the cokedrum, the vapours crack as they pass through thedrum; and the heavy liquid feed, given thetemperature and residence time in the coke drum,is simultaneously cracked to vapour, polymerizedand condensed to coke.

Structure of petroleum cokeNumerous studies have been performed to

explain the formation and the structure of a coke.Petroleum cokes have an unordered crystallinestructure. In contrast to resin and asphaltenemolecules, the structural model of the carbenes andcarboids that constitute most of petroleum cokeprobably consist, not of individual memberscapable of being broken up on heating, but ofcondensed polycyclic nuclei having short methyland other side chains. Heterocompounds (with O,S, N, etc.) may be present in both the side chainsand the ring structures. The dimensions andordering of the coke crystals are the principalfactors in determining its physical properties(thermal conductivity, electrical conductivity,density, etc.), while the type of side chains(�CH3, �SH, �H) influences its chemical

reactivity. In addition, coke contains 2-10% ofadsorbed intermediate decomposition products,which have an effect on coke calcining technology.

Needle coke formation mainly involves thepolymerization and condensation of the aromaticsto such a degree that coke is eventually formed.The coke produced is more crystalline inappearance than asphaltic coke obtained fromconventional vacuum residues. Heterocompoundsare present in very small amounts and play only asmall role in the coking mechanism.

5.1.4 Coking processes

Delayed coking

Delayed coking represents the largest numberof commercial coking units. A simplified flowdiagram of the delayed coking process is shown inFig. 2.

Main process sectionsThe delayed coking flow scheme can be divided

into the following sections: a) coking; b) fractionation; c) vapour recovery; d) closedblowdown; and e) coke removal.

Coking Fresh feed (residue) is delivered into the bottom

of the fractionator where it is mixed with the



frac

tion

atio

n to

wer

off-gas

LPG

naphtha

steam

LCGO

HCGO

steam

BFW

BFWsteam

coker heating

sour water

water

wat

er

coke

dru

ms

vapours torecovery

slop oil

make-upwater

coke

residuefeed

pumparound

Fig. 2.Simplified delayed coking flowscheme. BFW,Boiler Feed Water.

recycle material from the wash section. The totalfeed is then pumped from the fractionator bottomto the coker heater. In the heater, the feed is rapidlyheated to the coking temperature and sent to thecoke drums. Coking reactions occur in the cokedrum producing coke and light hydrocarbonvapours. Coke accumulates in the coke drums as itforms over the period of the coking cycle, while thehydrocarbon vapours exit the drum at the top andare sent to the bottom of the fractionator. Gas oil isinjected into the coke drum overhead line toquench the product vapours and minimize cokingin the line. High pressure steam or condensate isinjected into the heater tubes in order to maintain aminimum fluid velocity, reduce residence time andthus minimize coking in the heater tubes.

FractionationThe most conventional fractionator design has

shed decks above the feed zone with a trayed washsection immediately above the decks. The cokedrum vapours pass through the shed decks, washsection and enter the gas oil fractionation sectionwhere a circulating gas oil pumparound is used toremove heat and to condense the gas oil vapours.With low recycle designs, the internals near thebottom of the tower are minimized due to thepotential for coking deriving from the low wash oilrates. An open spray chamber design is commonlyemployed in such designs. Heavy Coker Gas Oil

(HCGO) is withdrawn as a total draw-off. A portionof the coker gas oil is pumped back to the washsection below as wash oil. The heavier portion of thecoke-drum vapours condenses in the wash section toform the recycle, which is mixed with the fresh feedand returned to the heater. The coker gas oilpumparound heat is typically used to preheat freshfeed, provide reboil heat in the vapour recoverytowers and to generate steam. The next side-drawproduct, Light Coker Gas Oil (LCGO), is steamstripped in a side stripper to remove the light ends,cooled and sent to storage. A portion of theunstripped LCGO is used as lean sponge oil in thesecondary absorber of the vapour recovery section.Rich sponge oil is returned to the fractionator forrecovery of the absorbed hydrocarbons.

The fractionator overhead vapours are partiallycondensed in an overhead condenser. Theuncondensed vapour is separated in the overheaddrum and sent to the vapour recovery section forLPG (Liquefied Petroleum Gas) recovery. A part ofthe condensed liquid is returned as reflux to the topof the fractionator and the remaining overheadliquid is sent to the vapour recovery section forstabilization. Sour water collected in the overheaddrum is sent off-site for treating.

Vapour recoveryA simplified flow scheme of the vapour

recovery is shown in Fig. 3. The fractionator


COKING

LPG to treating

naphtha to hydrotreating

fuel gas

rich light gas oil

light gas oil

fuel gas to treating

stri

pper

spon

geab

sorb

er

debu

tani

zerpr

imar

yab

sorb

er

fractionatoroverhead liquid

sour water

sourwater

washwater

wash water

fractionatorvapour

wet gascompressor

Fig. 3. Vapour recovery flow scheme.

overhead vapour is compressed and cooled, andthe resulting vapour and liquid streams are fed toan absorber-stripper. The vapour is fed to thebottom of the absorber while the liquid is fed tothe top of the stripper. The fractionator overheadliquid stream is introduced into the top of theabsorber as lean oil. Normally, this lean oil isinsufficient to achieve the desired LPG recovery,and therefore a portion of the stabilized naphthafrom the downstream debutanizer is cooled andrecycled to the top of the absorber assupplemental lean oil.

The bottoms from the stripper, containing C3sand heavier, flows to the debutanizer where LPG isrecovered as an overhead liquid product, and C5

� asbottom product. The C5

� product is cooled and sentto storage. The overhead C3-C4 LPG is furthertreated to remove sulphur compounds, includinghydrogen sulphide, mercaptans, etc., and sent forfurther processing.

The overhead gas from the absorber containingmostly C2 and lighter and some unrecovered C3s isfed to the bottom of the sponge absorber where itcomes into contact with lean sponge oil. Any C5 andheavier hydrocarbons present in the absorber off-gasare recovered in the sponge absorber and returned tothe fractionator as rich sponge oil. The spongeabsorber overhead off-gas is finally treated with anamine solution to remove hydrogen sulphide beforedischarging into the refinery fuel gas system.

Closed blowdownThe closed blowdown system, shown in

Fig. 4, is used to separate and recover hydrocarbonand steam vapours generated during the cokedrum steaming and cooling operations. The cokedrum blowdown vapours are condensed in theblowdown scrubber by contact with circulating oildrawn from the bottom of the blowdown scrubber.The uncondensed vapour, mostly steam and lighthydrocarbon vapours, is condensed in theoverhead condenser before entering the blowdowndrum. In the blowdown drum, light oil isseparated from the steam condensate and pumpedto the refinery slops system, while the recoveredwater is pumped off-site, initially for furthertreating in a sour water stripper and later sent tothe clear water tank for reuse in coke cutting. Thevent gas from the blowdown water separator isreturned to the wet gas compressor or othersuitable refinery hydrocarbon recovery systems.The net bottoms from the blowdown scrubber,containing wax tailings, are removed and returnedeither to the fractionator or sent to the refineryslops system.

Coke removalThe coke drum filled with coke is taken off-

line, steam stripped, and quenched by water. Thevapours generated during steaming and quenchingare routed to the blowdown scrubber for



quench watertank

slop oil tank

coke handlingreturn water

to sour waterflash 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

watermake-up

condenser

blow

dow

n sc

rubb

er

blowdowndrum

Fig. 4. Closedblowdown system.

hydrocarbon and steam recovery. Coke is removedfrom the drum by hydraulic decoking.

Various coke handling methods are in use,including coke pit loading, coke pad loading, direct railcar and hydraulic coke handling. The most commonlyused methods are coke pit and coke pad handling.

Major design considerations

Coker heaterThe coker heater provides the necessary heat to the

feed in order to reach the coking reaction temperature.There are two principal types of heater design in usetoday: single-fired heaters or a double-fired heaters(Fig. 5). In modern cokers the double-fired heaterdesigns are mainly used, in which the heat input isfrom both sides of the tube. This arrangement allowshigher average heat flux, resulting in lower peaktemperatures and shorter residence time.

Heater tube metallurgy is also being enhanced.New designs employ steel alloys containing 9% Cr and347 SS tubes, which permit higher skin temperaturesand 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 approximately43,000 W/m2. Steam injection in the radiantsection, particularly when processing heavy feeds,is common in modern cokers. Steam is injected inorder to increase the fluid velocity, thereby greatlyreducing the residence time and the potential forcoking in the heater tubes.

Coke drumCoke 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 ofthe vapour density and the foaming tendency of thefeedstock. Typical drum vapour velocities are in therange of 0.1 to 0.2 m/s, although some units run atvelocities higher than 0.2 m/s. The drum outage,which is the disengaging height between the toptangent line and the maximum coke level in thedrum, is typically in the range of 4 to 6 m. The actualoutage is determined based on the type and origin offeedstock, its foaming tendency and the operatingconditions. The foaming in the drum is controlled bythe addition of anti-foam chemicals (generally as amixture with a distillate fluid) during the last fewhours of the fill cycle. The coke drum level, which isindicative of the progress of coking in the drum andpreparation for the drum switching, is monitored by anuclear backscatter level instrument mounted on theoutside of the coke drums. These are also used todetect the foam levels as the drum fills up.

In the past, cokers were designed with cokingcycle times of 20 to 24 hours (overall drum cycle of40-48 hours). Modern designs and retrofits useshorter cycle time of the order of 14-18 hours. Thecycle time schedule sets the total volume required forthe coke drum, which, for a given drum diameter,essentially sets the overall dimensions for the cokedrum. Coke drums of about 9 m diameter arecurrently in commercial use. The coke drum shell isfabricated 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 feedbetween the drums. The switch valve also allowsthe drum to be bypassed during unit start-up andshutdown.


COKING

outletoutletoutlet

old burnersbushy flames

single-fired type double-fired type

modern low NOx burnerslong, thin flames

peak flux

average

outlet

�1.2peak flux

average�1.8

Fig. 5. Single-firedvs. double-firedcoker heater.

Coker fractionatorThe coker fractionator separates the coke drum

vapours into various products, including wet gas,gasoline, LCGO, and HCGO. The bottom sectionof the fractionator, up to the HCGO draw-off pan,is highly prone to coking due to the entrained cokeparticles as well as the high-temperature vapour-phase coking. Modern designs therefore minimizetower internals at the bottom of the fractionatingtower, with some designs using an open spraychamber below the HCGO pan. A slotted standpipein the bottom of the fractionator is used forcollecting the coke particles and to provide passageto the heater charge pump. Also, a separate cokeremoval system, consisting of a circulating pumpand coke filters, is employed in order to remove theaccumulated coke from the bottom of thefractionator to minimize unit downtime.

Coker recycleCoker recycle is one of the key operating

variables in a coker to control the HCGO end pointas well as to reduce the coking propensity of theheavy feed in the heater tubes.

Recycle is produced at the fractionator bottomby condensing the heavier portion of the coker gasoil, which is then mixed with the fresh feed andsent to the heater. Higher recycle produces morecoke at the expense of gas oil yields, however theHCGO end point decreases, and other impuritieslike Conradson Carbon Residue (CCR) and metalsare also reduced. In fuel grade cokers, wheremaximizing liquid yields is the primary objective,low to ultra-low recycles are used. Cokers withrecycle less than 5% are considered ultra-lowrecycle operations.

Blowdown scrubber The blowdown scrubber recovers and provides

primary separation of the hydrocarbons and thesteam that are generated during the coke drumsteaming and cooling operations. The blowdownsystem includes a blowdown scrubber, overheadcondenser, water separator, circulating oil cooler,bottom heater and the associated pumps. The drumblowdown temperature varies from a maximum of450°C at the start of the cooling cycle to about 150°Cnear the end of the cycle. Below 150°C, the drumeffluent bypasses the scrubber and is sent directly tothe blowdown overhead condenser. A demulsifyingagent is added to the blowdown overhead waterseparator to aid the oil/water separation.

Another significant function of the blowdownsystem is to handle the coke drum emergency reliefdischarge during any drum overpressure event.

Other coking processes

Delayed coking represents the largest number ofcommercially practised coking units. Fluidizedcoking processes are a specialized class of cokingprocesses that consume part of the coke produced tosupply the necessary endothermic heat of reactionfor thermal cracking. The Fluid Coking andFlexicoking processes, licensed by ExxonMobilResearch and Engineering (EMRE), are incommercial use. Detailed information on thesetechnologies and their applications can be found inan article by EMRE (Hammond et al., 2003).

Fluid CokingA generic flow diagram of the Fluid Coking

process is shown in Fig. 6. This figure and thedescription that follows apply to the Fluid Cokingreactor system only. The fractionation, vapourrecovery and coke-handling systems are similar tothose used in the delayed coking process.

The reaction section entails two primary vessels:a coking reactor and a heater. A scrubber, located ontop of the reactor, preheats the fresh feed, cools thereactor effluent vapours, removes coke particlesentrained by the vapours, and condenses the heavyrecycle stream. The hydrocarbon conversionreactions occur in the reactor.

Feed enters the reactor into the fluidized bed ofcoke. Stripping steam is injected at the bottom of thereactor, and reaction product vapours fluidize the bedas they rise toward the reactor cyclone and scrubber.New coke produced from the cracking reactions isdeposited on the coke particles in the reactor bed. In



net coke

flue gas toCO boiler

hotcoke

cold coke

feed

steam

air

product tofractionator

Fig. 6. Simplified Fluid Coking scheme.

order to supply heat and maintain reactortemperature, hot fluidized coke is circulated from theheater to the top of the reactor through the hot cokeline. To provide bed level control in the reactor, coldcoke from the bottom of the reactor stripper sectionis circulated back to the heater through the cold cokeline. Each of the transfer lines consists of astandpipe, a sharp angle bend, an angle riser, and avertical riser. The heater normally operates at about500-600ºC and slightly above atmospheric pressure.The cold coke from the reactor is heated by directcontact with hot gas. Heat required to support thecoking reaction is obtained by partially combusting aportion of the gross coke produced in the reactor. Theflue gas, which contains mostly carbon monoxide(CO), sulphur oxides (SOx) and inerts, can be utilizedin a CO-boiler. The net coke produced by the processexits the heater.

FlexicokingThe objective of the Flexicoking process is to

further reduce the amount of net coke produced inthe reactor by utilizing a gasifier to convert the netcoke to a synthetic gas. The gasifier is also used toheat the circulating coke and supply the heat requiredfor the coking reaction. A flow diagram showing theFlexicoking process is provided in Fig. 7.

The Flexicoking process produces a largequantity of low heating-value gas. This gas isnormally cooled, treated to remove coke fines andprocessed to remove H2S. The gas can be utilizedin burners designed to handle low heating-valuegas (approx. 3.5-4.8 MJ/m3).

All coking processes are severe thermal conversionprocesses and accomplish that conversion using muchthe same reaction mechanisms. The gross cokequantities produced from delayed coking, Fluid Cokingand Flexicoking processes are similar. The heat inputrequired to achieve thermal conversion is also similaramong the processes. Fluid Coking consumes about20% of the gross coke produced to supply heatrequired for the coking reaction. The Flexicokingprocess consumes additional coke to produce asynthetic gas product. About 90-97% of the gross cokeproduced is consumed by the Flexicoking process.

Typical yields for processing an Arabianvacuum residue provided by the process licensor(ExxonMobil Research and Engineering) areshown in Table 2. The yield of liquid product andgross coke production are the same. The net coke


COKING

purgecoke

feed

steam

product tofractionator

dry cokefines

air steam

wet cokefines

steam

tertiarycyclones

Venturiscrubber

low BTUgas

ejector

Fig. 7. SimplifiedFlexicoking scheme.

Table 2. Typical yields from Fluid Coking and Flexicoking (4.4°API and 24.4 wt%

carbon residue feedstock)

FOE, Fuel Oil Equivalent

Product yields Fluid Coking Flexicoking

Butanes and lighter(wt%)

13.1 13.1

C5 - 510 °C (vol%) 65.1 65.1

Gross coke (wt%) 30.7 30.7

Net coke (wt%) 24.9 0.6

Low BTU gas(FOE-vol%) – 18.8

produced using the Flexicoking process for thesame feedstock is substantially reduced comparedto the Fluid Coking process.

Capital costs for a fluid coker areapproximately the same as those for a delayedcoker. On the other hand, the capital costs for aflexicoker are greater (30-40%) than for a fluidcoker because of the need for an additional gasifiervessel, gas clean-up and the requirement for alarger air blower. The major utility cost for a fluidcoker or flexicoker is associated with the airblower.

5.1.5 Process variables

This section provides a description of cokerfeedstocks, product yields and quality of thevarious coker products and the variables that affectthe yields and product qualities.

FeedstocksDelayed cokers can process practically any heavy

oil material in the refinery. While the typical feedstockto a coker is a straight-run vacuum residue, a varietyof other refinery residual feedstocks and intermediateproducts can also be processed in the delayed coker.The ability of a coker to handle a variety of feedstocksis demonstrated by the range of the gravity (�5 to �15°API) and carbon residue (4 to 40 wt%) of the materials it can process.

The feedstocks to a coker can be classified intothe following main categories:• Straight-run residual material, such as the

atmospheric and vacuum tower bottoms, fromthe distillation processes and asphaltenesproduced from deasphalting.

• Heavy aromatic stocks such as the decant orslurry oil produced from FCC units, thermaltars from thermal cracking units, aromaticextracts from lube operations, and pyrolysis tarsfrom 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 theyields and product quality are: the specific gravity,CCR, and the content of sulphur, metals andasphaltenes. These properties determine the qualityof coke produced as well as the entire slate of thecoker products. The properties of some typicalfeedstocks are summarized in Table 3.

Operating variablesThe three primary operating variables that

affect product yield and quality are: coke drumpressure, recycle ratio, and coke drum temperature.

The operating conditions are selecteddepending on the feedstock quality and the processobjectives. The conditions vary significantlybetween the three types of coking operations,depending on the overall economic objectives.

Coke drum pressureThe reference pressure at which coking reactions

take place is generally considered to be the operatingpressure at the top of the coke drum. The pressure isactually controlled at the reflux drum near the top ofthe coker fractionator. Increasing coke drum (coking)pressure increases coke yields, reduces liquid yieldsand reduces the gas oil end point. Increasing cokedrum pressure also increases gas and gasoline yields.



Table 3. Typical coker feedstock characteristics

Anode coke Fuel grade coke Needle coke

Feedstock Vacuum residue Vacuum residue Slurry oil Thermal tar

Feedstock source African crude50:50 Light/Heavy

Arabian Mix FCC 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 – –

In old designs, drum operating pressures of 2 bar werecommon for sponge coke production, while today,cokers are being designed and revamped to operate atpressures as low as 1 bar. Table 4 shows the effects ofdrum operating pressure and recycle on delayed cokeryields at a given drum temperature (Sloan et al., 1992;Bansal et al., 1993). With heavier feeds containinghigh CCR and asphaltenes, reducing the drumpressure and recycle can potentially contribute to shotcoke formation.

To produce anode grade or needle coke, higheroperating pressures are employed and generallyjustified because of the higher value coke producedfrom those units. While anode cokers are typicallylimited to 2 to 3 bar pressures, it is not uncommon to operate needle cokers at much higher pressures, 4 to 6 bar.

Recycle ratioThe recycle ratio represents the amount of recycle

material (typically 540°C plus) produced at the bottomof the coker fractionator and recycled back (along withthe fresh feed) to the heater and coke drum foradditional cracking. As the recycle ratio is increased,the resulting effects are an increased coke yield,reduced liquid yield, and a lower gas oil end point. Thehigher coke yield leads to more gas and gasolineyields also. Higher recycle rates produce cleanerHCGO with lower end point, carbon residue andmetals. In the past, conventional cokers were designedwith recycle ratios of 10 to 15% in order to producecleaner gas oils, as limited by the capability of the

downstream units to handle contaminants. Today, withadvances in hydrotreating, hydrocracking andfluidized catalytic cracking technologies, greateramounts of HCGO contaminants can be tolerated andcokers are designed with recycle ratios of 5% or lower,with many operated at recycle rates of 2 to 3%.

When producing anode or needle grade coke,higher recycle ratios are commonly used andjustified because of the higher value cokeproduced. While anode cokers are typically limitedto recycle ratios of 25 to 30%, needle cokers oftenrun with recycle ratios of 50 to 80%.

Coke drum temperatureCoke drum temperature is a key operating variable

in a delayed coker. Although the drum temperature isnot directly controlled, the narrow range of furnaceoutlet temperatures at which a given feed must be runis critical for the smooth operation of the unit and themaintenance of reasonable furnace run lengths. Toolow a furnace outlet temperature leads to incompletecoking reactions in the drum, which results in theproduction of soft coke. A very high temperature, onthe other hand, produces a hard coke that would bedifficult to remove from the coke drum. A higher drumtemperature can only be achieved through a higherfurnace outlet temperature, which could result inexcessive furnace tube coking and the need forfrequent tube cleaning. Also, the drum overheadsystem (overhead piping, valves up to the quenchpoint) may be subject to excessive coking which couldlead to increased unit downtime. The optimumtemperature at which the drum must be operated for agiven feed is a compromise between the yield benefits,plant operability and hardware limitations.

With anode or needle grade coke production,higher drum temperatures are required to produce acoke with the required properties. While drumtemperatures for anode cokers are only slightly higher(5°C), needle cokers typically run at much higherdrum temperatures, in the 450 to 460°C range. Itshould be recalled that fuel grade cokers typically runat a drum temperature of about 435-440°C.

With these key operating variables in mind, therange of coker operating conditions can varysignificantly, as shown in Table 5.


COKING

Table 4. Effect of low pressure and low recycle on coking yields (20.5% carbon

residue feedstock)

Pastdesigns

Currenttrend

Drum pressure (bar) 2.1 1.0

Recycle ratio (vol%) 10 5

Coke yield (wt%) 32.1 29.7

C5 plus liquid yield (vol%)

69.7 72.6

Table 5. Range of coker operating conditions

Fuel grade Anode coke Needle coke

Drum pressure (bar) 1.0-1.5 1.5-3.0 4.0-7.0

Recycle ratio (vol%) 5-10 25-30 50-80

Drum temperature ( °C) 435-440 440-445 450-455

Product yieldsTypical coker yields for conventional residual oils

as well as those for needle coker feedstocks aresummarized in Table 6. Included in the table are thefollowing yield cases: low sulphur and low CCRresidue feedstocks suitable for anode gradeproduction; high sulphur and high CCR residue feedwith high metals that produce a fuel grade coke; andhighly aromatic feedstocks that produce needle coke.

These yield estimates are developed using theKBR (Kellogg Brown and Root) yield models.

Coker product propertiesCoker products are set primarily by the refinery

product slate, product specifications, and theability of the refinery process units to handle theirfurther processing.

The estimated properties for various cokerproducts are summarized in Table 7 for the Arabiancrude feed blend. The product treatment steps andthe end usage are summarized in Table 8. In general,all coker products are highly olefinic. The brominenumber, which is indicative of the degree ofolefinicity, ranges between 10 and 70. The sulphurand nitrogen are distributed among the variousproducts with coke retaining a major portion of thefeed sulphur and nitrogen. Essentially all feedmetals 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-C4 components 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%)

Coker gas includes hydrocarbons such as methane,ethane, and ethylene. Also, a small amount of hydrogenproduced is present in the dry gas. This gas is usuallyproduced as ‘wet’ gas from the gas separator, containingmost of the coker LPG and some heavier hydrocarbonsand must be processed in a Vapour Recovery Unit(VRU) to recover LPG and a gasoline product.

Coker gasoline is recovered and stabilized in theVRU and sent for hydrotreating and, ultimately, forcatalytic reforming for octane improvement beforeblending with the finished gasoline product.

Light coker gas oil is typically blended with otherdiesel range materials from other refinery units and sentfor further hydrotreating for low sulphur (�500 ppm) orultra-low sulphur (�10 ppm) diesel production.

Heavy coker gas oil is also sent for hydrotreatingor hydrocracking along with the virgin vacuum gas oiland is ultimately sent as feed to the FCC unit.

Coke qualityThe coke quality not only determines the unit

economics but also influences its operability,operations, reliability, maintenance and safety. Typicalcoke properties are summarized in Table 9.

There are essentially three grades of coke currentlybeing produced in the petroleum industry: regulargrade sponge coke, widely used in the aluminiumindustry for the manufacture of electrodes (also calledanode coke); high grade needle coke, a premium cokeused to manufacture electrodes for the steel industry;and fuel grade coke, used primarily in power andcement plants as fuel.

Regular (anode) cokeMany delayed cokers produce a regular grade coke

also known as anode coke. This coke has a sponge-likestructure, is porous and exhibits structural consistency.Typically, anode grade coke has less than 3 wt%sulphur and no more than 350 ppm by weight totalmetals. The anode coke is generally produced from

paraffinic or asphaltic materials. The quality of cokeproduced can vary significantly depending on theresidue feed being processed. Generally, the feedsulphur and metals content need to be sufficiently lowso that the produced coke can meet the desiredspecifications. Typical specifications for the anodecoke are shown in Table 10.


COKING

Table 8. Coker product treating steps and end use

Product Treating step End use

C3-C4 olefins Mercaptan extraction LPG Alkylation feed

Light naphtha Mercaptan extractionGasoline blendingIsomerization feed

Heavy naphtha HydrodesulphurizationReforming feedGasoline blending

LCGO Hydrodesulphurization Diesel blending

HCGOHydrodesulphurization Hydrocracking

FCC feed

Table 9. Typical coke properties

Property Value

Sulphur (wt%) 7.0

Nitrogen (ppm) 6,000

Volatile material (wt%) 10-12

Vanadium (ppm) 141

Nickel (ppm) 489

Bulk density (kg/m3) 880

Table 10. Typical anode coke specifications

Specification(wt%)

Green coke Calcined coke

Moisture 8-12 0.3

Volatile CombustibleMaterial (VCM)

8-12 0.5

Sulphur 1.0-3.5 1.0-3.5

Silicon 0.02 0.02

Iron 0.02 0.02

Nickel 0.02 0.03

Ash 0.25 0.04

Vanadium 0.02 0.03

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

Real density (g/cm3) – 2.06

Needle cokeThis form of coke is the most valuable of all the

various petroleum cokes produced. It is usedprimarily for the production of electrodes for thesteel industry (electric arc furnaces). The coke isuniquely characterized by properties such as its lowsulphur and metals content, low Coefficient ofThermal Expansion (CTE), its needle likecrystalline structure and high electricalconductivity. All premium needle cokes have a lowCTE. Typical needle coke specifications are shownin Table 11. Needle coke production requires aspecial feedstock, which is typically high inaromatics, and low in asphaltenes, sulphur andmetals. In addition, the coker unit must be operatedat conditions that will provide the best premiumneedle coke quality. Typical feedstocks includeslurry or decant oil from FCC units or thermal tarsfrom gas oil thermal cracking units. In addition,aromatic extracts from lube operations, pyrolysistars from ethylene plants and some coal tar pitchesare considered potential coker feeds for needlecoke production.

Fuel grade cokeWith the current trend in refineries to process

heavy crude oils, the industry continues to see amajor shift in terms of coke quality. Many new andexisting cokers have switched to processing heavycrude oils in order to achieve improved refiningeconomic margins. This change results in theproduction of poorer quality coke that is notsuitable for anode production. Due to very highsulphur, metals and other impurities present in theheavy feedstocks, the coke produced is only suitablefor fuel purposes, either in power plants or thecement industry. This coke is thus referred to as fuelgrade.

5.1.6 Support process operations

The support (auxiliary) operations include some ofthe key mechanical operations associated with thedelayed 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 forthe purposes of this work.

Coke calciningPetroleum coke (green coke), either anode or

needle coke, is calcined in rotary kilns; suchprocesses are frequently performed outside thepetroleum refinery. The characteristics of thecalcined coke depend primarily on the properties ofthe green coke fed to the calciner as well as themajor operating variables, such as rate of heating,hot zone (calcining) temperature, residence timeand rate of cooling. Calcined anode coke is usedmostly in the manufacture of anodes for thealuminium industry. Consumption of calcined cokein specific user industries varies considerably asshown 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 greencoke is calcined in a rotary kiln. Process heat is supplied through a fuel burner.Another source of the process heat is the volatileswhich are released in the kiln. Cokes with varying amounts of the volatile mattercan 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 CombustibleMaterial (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) – 1-5�10�7

Real density (g/cm3) – 2.11

Table 12. Usage of calcined coke

User industry Calcined coke usage (kg/kg)

Aluminium 0.5

Silicone carbide 1.4

Phosphorous 1.8

Calcium carbide 0.69

Graphite 1.25

calcined coke is discharged into a rotary coolerwhere it is quenched with direct water sprays atthe cooler inlet. Additional cooling isaccomplished by pulling a stream of ambient airthrough the cooler. From the discharge of thecooler, the calcined coke is conveyedto storage silos.

References

Ballard W.P. et al. (1981) Thermal cracking, in: McKettaJ.J. (editor in chief) Encyclopaedia of chemical processingand design, New York, Marcel Dekker, 1976- ; v. XIII.

Bansal B.B. et al. (1993) Design and economics for lowpressure 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 andflexicoking technologies, in: Proceedings of the AmericanInstitute 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 cleanfuels environment, «Fuels Reformulation», July-August.

SRI Consulting (1971) Petroleum coke, Process EconomicsProgram Report 72.

Bharat B. BansalJoseph A. Fruchtbaum

Aldrich H. NorthupRao Uppala

Kellogg, Brown & RootHouston, Texas, USA


COKING

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