delayed coking by navid
TRANSCRIPT
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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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Fractionator Products
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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Fractionator Products
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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