delayed coking by navid

8/3/2019 Delayed Coking by Navid

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Delayed CokingChapter 5

CHE 735Navid Omidbakhsh

02/11/06


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Purpose

Process heavy residuum to produce distillates (naphtha & gasoils) that may be catalytically upgraded

Hydrotreating, catalytic cracking, and/or hydrocracking

Attractive for heavy residuum not suitable for catalyticprocesses

Large concentrations of resins, asphaltenes, &

heteroatom compounds (sulfur, nitrogen, oxygen, metals)

Metals, sulfur, & other catalyst poisons generally end up incoke

Sold for fuel & other purposes

Carbon rejection process


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Development of Coking

Coking evolved from the Dubbs cracker

After World War II railroads shifted from steam todiesellocomotives

Demand for heavy fuel oil sharply declined Coking increases distillate production & minimizes heavy fuel

oil

Between 1950 and 1970 coking capacity increase five fold

More than twice the rate of increase in crude distillationcapacity


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Development of Coking

Unit Bbl/day Relative Capacity

Crude Units 17.6 MMbpd 13%

Vacuum Units 8.1 MMbpd 28%

Coking Units 2.3 MMbpd -

The increase in heavy high sulfur crude, together with thedecrease in fuel oil, accelerated the use of coking

Coking capacity is measured in terms of both coke production

in tons per day & residual oil feed rate in barrels per day2002 EIA database:


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Delayed Coking

Predominate coking technology is Delayed Coking

Fluid Bed Coking

Flexicoking

Delayed Coking technology is relatively inexpensive

Considered open art

Companies do license technology emphasizing cokefurnaces, special processing modes, & operations


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Coking Chemistry

Carbon rejection process

Also removes hydrotreating catalyst poisons

May be considered a cycle of cracking & combining

Side chains cracked from thermally stable polynucleararomatic cores

Cleavage of C-C bonds tend to make light hydrocarbons & polynucleararomatics

Liberated side chains can over crack to form light gases

Heteroatoms in side chains end up in light products

Polynuclear aromatics combine (condense) to formasphaltenes & ultimately coke

Metals & heteroatoms in polynuclear aromatic cores end up in coke


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Coking Chemistry

High temperatures & low pressures favor cracking

More distillate liquids

Lower yields of coke & hydrocarbon gas High residence time favor the combining reactions

Over conversion will reduce distillates & produce coke andhydrocarbon gases


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Coking Chemistry

Cracking reactions

Saturated paraffins crack to form lower molecular weightolefins & paraffins

Isomerization is insignificant Side chains cracked off small ring aromatics(SRAs),cycloparaffins (naphthenes), & polynuclear aromatics(PNAs)

Naphthenes may dehydrogenate to aromatics

SRAs may condense to larger polynuclear aromatics

(PNAs)

leave thermally stable PNAs


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Coking Chemistry

Combining reactions.

Low molecular weight olefins form higher molecular weightcompounds

Two ethylenes to butene

SRAs combine (condensation) to form resins Resins after cracking off side chains combine theirremaining polynuclear aromatics (PNAs) to form asphaltenes

Asphaltenes after cracking off side chains left with largePNAs

Partially saturated with short side chains & entangledwith heteroatoms

The large PNAs precipitate to form crystalline liquids &ultimately solidify to form coke

Metals & sulfur embedded


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Feed for the Delayed Coker

Delayed Coker can process a wide variety of feedstocks

Can have considerable metals (nickel & vanadium), sulfur,resins, & asphaltenes

Most contaminants exit with coke Typical feed is vacuum resid

Atmospheric resid occasionally used

Typical feed composition

6% sulfur

1,000 ppm(wt) metals Conradson Carbon Residue (CCR) of 20-30 wt%

Feed ultimately depends on type of coke desired


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Solid Products

Coke with large amounts of metals & sulfur may pose adisposal problem

Refiner may have to pay for disposal

Product grades Needle coke

Anode grade

Fuel grade

Shot coke


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Solid Products

Hydroprocessing upstream of delayed coker may be used tomake high quality coke

Needle coke

FCC cycle oils, gas oils, & resids with low sulfur & aromaticsused as feedstocks

Used for electrodes in steel manufacturing

Anode grade coke

Resids with small ring aromatics

Used for electrodes in aluminum production


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Solid Products

Fuel grade coke

About 85% carbon & only 4% hydrogen

11% heteroatoms sulfur, nitrogen, oxygen, vanadium &nickel

Resid high in polynuclear aromatics & sulfur used asfeedstock

Shot coke

From size of small ball bearings to golf ball

Nearly always try to avoid!

Little market

Operational problems


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Light Products

Light ends processed in coker gas plant.

Liquids

Reduced aromatics & heteroatoms concentrated in coke, butstill contain large amounts of sulfur & olefins

Naphtha fraction may be used as catalytic reformer feedafter hydroprocessing

Only small fraction of gasoline pool

Gas oil fed to catalytic cracker or hydrocracker

Hydrocracker preferred does better job of processingaromatic rings


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Feedstock Selection

Amount of coke formed related to the carbon residue of thefeed

Since this parameter correlates well with hydrogen/carbonatomic ratios and indicates coking tendencies, this parameter

is especially useful for screening feeds

Three main tests

Ramsbottom method (ASTM D-524)

Conradson Carbon (ASTM D-189)

Microcarbon Residue Test


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Yields

Low yields of liquids relative to hydrocracking Mass conversion of vacuum resids to liquids about 55% toabout 90% for hydrocracking

Coke & liquid yields may be estimated by simple equations(misprint pg. 75 see pg. 89)

Coke Yield (wt%) = 1.6 (wt% CCR)Gas (C4-) (wt%) = 7.8 + 0.144 (wt% CCR)

Gasoline (wt%) = 11.29 + 0.343 (wt% CCR)

Gas Oil (wt%) = 100 - (wt% Coke) - (wt% Gas) - (wt%Gasoline)

Gasoline (vol%)=186.5/(131.5+API) x (wt% Gasoline)

Gas Oil (vol%)=155.5/(131.5 +API )x (wt% Gas Oil)


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Effect of Operating

Variables Pressure and Temperature

Trend to lower pressures

Coke drums normally 25 psig but 15 psig common

Pressure dictated by the suction pressure of gascompressor removing vapors from fractionatoroverhead accumulator drum

Lower pressures & higher temperatures increase gas oilyields by favoring cracking reactions

Increase the load on the gas compressor

Increase probability of shot coke formation, foaming, &coke carryover


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Effect of Operating

Variables Recycle Ratio

Low recycle ratios common increase gas oil yields &decrease coke yields

Contaminants produced with the gas oil, loweringquality

Contaminates increase probability of shot cokeformation, foaming, & coke carryover


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Equipment & Operating Modes

Typical equipment

Heater (furnace)

Coke drum vessels

Fractionator Downstream vapor processing vessels

Coke drums run in two batch modes

Filling

Decoking

Both modes of operation concurrently feed to the fractionator


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Operating Modes

Filling of coke drums

Feed heated in the furnace to cracking temperature

Hot feed routed up from bottom of coke drums cokingreaction occur in drums & solid coke deposited

Gas from top of coke drum fed to fractionator Continue for full cycle time until coke drum is full Decoking

At end of cycle empty drum switched on-line & fillingoperations continue

Off-line drum filled with coke undergoes operation

Quench step hot coke quenched with water givingoff steam &volatile hydrocarbons

Vapors fed to the fractionator

Coke drilled out with water drills


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Operating Modes

Fractionator

Vapors compressed & sent to gas plant Naphtha is condensed from fractionator overhead

Gas oils are two sidestream draws from the fractionator


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Fresh Feed & Furnace

Fresh feed to bottom of fractionator before charging to thefurnace

Feed heated in furnace

Outlet temperature about 925F

Cracking starts about 800F

Endothermic reactions

Superheat allows cracking reactions to continue in cokedrums

Velocities in the furnace kept high

Prevent coking in the furnace tubes

Delay coking reaction until the coke drums

Hence name Delayed Coking


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Fresh Feed & Furnace

Steam injected into furnace

Reduce oil partial pressure & increase vaporization

Maintains proper fluid velocities

Provides sufficient wash on tube walls to minimize coking inthe tubes

Heater design trends due to heavier feeds & longer runlengths

Lower heat fluxes, typically 9,000 Btu/hr-ft2

More flexible residence times in tubes


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Coke Drums & Coking

Flow direction

Furnace outlet enters from bottom of the coke drum

Vapor flows up the coke drum

Coke solidifies & precipitates in the drum Coking reaction takes about three hours for the coke to fully solidify

Lower molecular weight product exit top of drum as vapors Sent to bottom of fractionator

Number of coke drums

Even number of coke drums, typically two or four

One set on-line in a coking step while the other set is beingdecoked


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Coke Drums & Coking

Increase of throughput capacity

Large coke drums

More coke drums

Good control of the coke fill height

Cycle Time

Trend is to decrease cycle time

Once typically 24 hour cycle

16 hour cycles now common & preferable

14 hour & 12 hour cycles being designed

Primary purpose for reducing cycle time to increasethroughput

Emphasis for decreased cycle time

Automated deheading operation

Advanced control for heating & cooling


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Coke Drums & Coking

Deheading

Removal of injection equipment & opening of the bottomplate

Automatic deheading may take 15 minutes vs. 45 minutes

for manual deheading Impact of heating & cooling cycles

Shorting the cooling, heating, draining, or coke cuttingoperations may affect fractionator operation & coke sizequality

Short cycle times may impose greater heating-cooling cycles

on the rum & reduce drum life Thermal stresses of these repeated cycles calls for careful

attention to vessel design

For example, drum & its supporting skirt are atdifferent differential temperatures during the cycle


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Coke Drums & Coking

Decoking steps

Filled drum removed from service & replaced with emptycoke drum

Full drum steamed to remove hydrocarbon vapors

Drum is filled with water until coke bed submerged Controlled cooling rate to minimize thermal effects of

temperature swing

Coke cooled to 200F

Drum drained & deheaded

Coke drilled from the drum

Drum reheaded

Testing & warmed up

Replaces filled drum & switched back into service


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Coke Drums & Coking

Number of Coke Drums

Before 1930, only one coke drum was utilized

Generally devoted to producing specialty coke

Anodes Completely batch

Currently in sets of 2, 4 or 6

4 drums most common

Number is function of throughput & tradeoff of coke drumdiameter & coke cycle time


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Coke Drums & Coking

Size of Coke Drums

Normally 25 feet or more in diameter & three times as high

In 1930, typiclly 10 feet in diameter

27 foot drum appeared in 1985, 28 foot in 1995, & 29foot in 1998

Diameter set by several factors

Ability to cut coke hydraulically

Transportation

Fabrication capacity

Vessel mechanical design


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Coke Drums & Coking

Materials of construction

Low alloy steel clad with stainless

Combat hydrogen blistering

Sulfide attack

Thermal stress

Stresses of cutting operation


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Coke Drums & Coking

Coke Drum Fill Height

Filling to maximum height increases capacity of coke drums

At end of coking cycle drums are not completely full

Volume allowance for foaming

Coke drum instrumentation is used to optimize fill heights


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Coke Drums & Coking

Foaming

Caused by waxy liquid state molecules in top of vessel

Upper portion of coke not hard at end of coking cycle

Amorphous mass large amount volatile material

Steam-out step strips out volatiles & cools agglomerates coke mass into rigid structure

Antifoams commonly used

Excessive use increases amount of silicon in productliquids degradation of hydroprocessing catalysts

Carryover Foaming

High vapor velocities

High capacity boosted by shortened coke cycle times

Low operating pressures


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Coke Product

Coke cut out of drum called green coke

Varies in size from fines, to particulates, to chunks

Shot coke

Hard spheres resembles shot

Sizes from small ball bearings to golf balls

High asphaltene content feed requiring very high cokingtemperatures


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Automated Drum Switching

Provides many benefits

Greater safety

Diminished operational upsets

Reduced hydrocarbons releases to atmosphere

Reduced numbers of personnel required

Scheme

Correct set of valves opened & closed automatically as unitsrun through the coke cycle

Safety interlocks preventing inappropriate switching


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Decoking

Each coke drum has a drilling rig that raises & lowers arotating cutting head

Uses high-pressure (4,000 psig) water

Steps

Drum cooled & displaced with water to remove volatiles

Pilot hole is drilled through the coke to bottom head

Pilot drill bit replaced with a much larger high-pressurewater bit

Cut either from top to bottom or bottom to top

The coke falls from coke drum into a collection system


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Coke Handling

Collection systems

Direct discharge to hopper car

Coke drums straddle railroad track

Water drains from cars, collected in pit beneath track, & sentto reprocessing

Pros

Lowest cost option

Disadvantages

Only used for two drum cokers

Logistics of moving cars under the drums

Slow cutting rate to avoid damage to cars & give evendistribution in cars

Personnel required to move cars & distribute load duringcutting

Fines & dirty water control

Dusting controlled by water spray in the cars


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Coke Handling

Collection systems

Pad loading

Coke dropped down inclined slide from the coke drumbottom to a pile sitting beside but not under the drum

Water drainage from pile on the pad Ports packed with sized coke to retain the fines

Coke moved to stockpiling by a front-end loader

Pit & Crane loading

Provides for both storage & dewatering of coke

Coke loading crane system transfers coke to railwaycars

Pit minimizes elevation of drums

Pit makes dust control easier


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Coke Handling

Collection systems

Slurry loading

Most elegant of coke handling options

Scheme

Coke drops from drum during decoking Passes through crusher

Conveyed by a sluiceway to a slurry sump

Slurry pump sends coke slurry up into dewateringbins

Water from bins drain back to sump Drained coke sent to silos

Must handle slurry water & process discarded water beforeit goes to refinery water disposal system


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Coke Handling

Other considerations

Method used establishes height coke vessels & drillingderricks above them

Method used determines nature of environmental measures

Coke is handled wet but it dries, leading to dusting & dirtywater management

Switching coke drums imposes an incremental cyclic steam& vapor load on the refinery system

Steam is consumed in only parts of the decoking cycle

Vapor recovery system must handle vapor fromflushing a drum before decoking


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Fractionator

Fractionator feeds

Coke drum overhead

Early designs

Refluxed by cooled fractionator bottom recycle stream

Now sent to bottom of fractionator above liquid level Fresh resid feed

Early designs

Routed directly to furnace

Above hot vapor feed from the coke drum

Provided refluxing of hot vapors

Now typically sent to bottom of fractionator below the liquidlevel

Strips light ends from the feed

Preheats feed

Condenses heavy ends from coke drum vapor

Suppresses coke formation by cooling and reducingthe concentrations of resins in the hot coke drum

vapor.


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Fractionator

Flash Zone

Space above the feed to the bottom of the heavy gas oildraw Height of the flash zone about one columndiameter(typically 25 feet)

Necessary for liquid disengaging & wash zone equipment

Feeds introduced at liquid pool at bottom of flash zone

Quench oil from the gas oil stream injected in the lineat the top of the coke drum

Suppress coking in line to the fractionator


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Fractionator

Wash Zone

Upper part of the flash zone just below the heavy gas oil draw

Washes vapors coming up the column to remove coke fines, resins& asphaltenes

Could cause coking in upper sections of fractionator

Washing is done by a series of spray nozzles across the column areausing cooled gas oil

Low cost & low pressure drop

Current spray designs minimize amount of spray liquid &hence recycle

May use a grid section washed with cooled gas oil

Higher pressure drop & prone to clogging Sieve trays, shed trays, and decks used before grids

Often washed with fresh feed slipstream

Rarely used

Coking, mechanical damage, fluid distribution, highrecycle ratios


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Fractionator

Recycle

Ratio of furnace charge (fresh feed plus recycle) to fresh feed

Throughput ratio of one indicates no recycle

Used to decrease contaminants (metals, sulfur, Conradson carbonresidues) in the heavy gas oil

Recycle ultimately cracked & contaminants contained with thecoke

Types

Internal condensed liquids to fractionator from coke drumoverhead feed

External quench flows to the top of coke drum & gas oil

flows to the wash section Current designs minimize recycle

Increased resins & asphaltenes in coke decreases gas oilyields

Equipment is larger

Anode & needle grade cokes usually made with higher recycles thanfuel grade coke


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Fractionator

Avoiding Coking in Fractionator

Major design consideration

Black oil passing down to the surge section of the fractionator isheavy with resins & solublized asphaltenes

Produces more coke than gas oil in the coke drum

Also likely to coke in bottom of the fractionator

Several methods

Surge time limited to several minutes since coking is afunction of time

Slipstream from the bottoms cooled & recycled back to thesurge section

Remove some oil from bottom of fractionator & dispose of asNo. 6 heavy fuel oil or mix with HGO

Historical option

Common when the demand for heavy fuel oil washigh

This reduces the amount of coke made


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Fractionator Products

Gas Product

During coking cycle vapors from the drum go to thefractionator

Fractionator overhead produces liquid naphtha & gas light

ends Light ends compressed and sent to gas plant

The gas plant is usually built as integral part of thecoker unit

It has the same configuration as the saturates gasplant or catalytic light ends gas plant

Absorber Deethanizer

Sponge oil absorber

Fractionator


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Naphtha Product

Naphtha fractions high in sulfur & olefins but low inaromatics

If naphtha products processed further in a catalytic reformer

they are hydrotreated first

Sulfur hydrogen sulfide

Olefins saturates

Aromatics naphthenes


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Gas Oil Side Draw

At least one gas oil sidestream but two are common

Steam stripped to narrow end point

Part of side draw cooled & pumped back for reflux

Cooling also helps to prevent coking Two pump around configurations

Cooled liquid is fed back to tray above the drawtray

Ensure sufficient liquid from the draw tray

Mainly for heat transfer Fed to the tray below the draw tray

Fractionation is better & number of trays may be reduced

Amount of oil to be recycled back to coker controlled byvarying end point of the lower gas oil draw


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Coke Products & Calcining

Green Coke coke produced directly by a refinery

Sub-classifications

Needle coke

Produced from low sulfur & highly aromatic feeds

Sponge coke

Largest production from the delayed coker

Term comes from its porosity

Shot coke

Less desirable coke Shot coke is a spheroid with diameters from 1 to 300

millimeters


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Coke Products &Calcining

Calcining

Green coke heated to finish carbonizing coke & reduce volatilematter to very low levels

Heated 1800 2400F

Calcining done in rotary kiln or rotary hearth Drained & crushed sponge coke fed to wet end of kiln

Opposite end fired by oil or gas & hot gases flow upthrough the rotating cylinder

Coke tumbles down the rotating barrel of the kilnmoisture & evolved gases swept counter currently to wet

end of kiln Calcined coke discharged at lower end to a rotary cooler

Cooled with water spray followed by air cooling


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Coke Products & Calcining

Calcining does not remove metals

About of green coke is calcined

Uncalcined sponge coke has heating values of 14,000 Btu/lb

Primarily used for fuel

Crushed & drained of free water contains 10% moisture,10% volatiles, & the rest coke

Anode grade coke has low sulfur (< 2%) & low metalsconcentration to prevent metal contaminant

Needle coke used for anodes in the steel industry

Low sulfur & low metals sponge used for anodes inaluminum industry


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Fluid Bed Coking &

Flexicoking Fluid Coking & Flexicoking are expensive processes that have

only a small portion of the coking market

Continuous fluidized bed technology

Coke particles used as the continuous particulate phase with

a reactor and burner

Exxon Research and Engineering licensor of Flexicokingprocess

Third gasifier vessel converts excess coke to low Btu fuelgas


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delayed coking by navid

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