DELAYED COKING EKB4303 PETROLEUM REFINERY ENGINEERING

Type of thermal cracking

Heat is required- supplied by furnace

Takes place in drums operating continuously

Carbon rejection process

Minimizes residence time in furnace, while sufficient time allowed in drums where coking takes place

Coke is rejected in the drums, increasing H/C ratio in rest of the products (unstable & unsaturated) requires hydrogenation

Feed to coker: Vacuum reside (high on asphaltenes, resins, aromatics, Sulphur, metals)

Deposited coke contains asphaltenes, Sulphur, metals present in feed and products are unsaturated gases (olefins) and highly aromatic liquids

ROLE OF DELAYED COKER

Feed: any undesirable heavy stream containing high metal content

Common feed: Vacuum residue

Other feeds: Fluid catalytic cracking slurry & visbreaking tar (residues)

Products from coker: unsaturated gases, olefins, isobutane

Olefins: very desirable feedstocks to petrochemical industry

Coker: Only unit in the refinery to produce coke

Overall refining yield of light products increases as a result of coke removal

*Role of delayed coker: Handles very heavy undesirable stream and produce desirable refinery products

DELAYED COKER

PROCESS DESCRIPTION

Delayed Coker Feed & Furnace

Vacuum residue enters bottom of flash zone in distillation column (below gas oil tray)

Total feed (fresh feed + recycle) to coker drums are heated >482°C(900°F)

Outlet temperature: 925°F (endothermic reactions)

Cracking starts: about 800°F

Fractions lighter than heavy gas oil flashed off

Remaining oil fed to coking furnace

Steam injected to prevent premature coking Reduce oil partial pressure & increase vaporization Maintain high fluid velocities

Delayed Coker Coke Drum

“Filling” drum Liquid-vapour mixture passes to one of the coking

drum- coke deposited for 24 hour

“Cutting” drum: decoking & cleaning

Flow up from bottom

Vapours (gas, naphtha, gas oils) out top of drum to fractionator

Number of coke drums Even numbers: typically 2/4 Operates as pairs: 1 filling, 1 decoked

Delayed Coker Fractionator

Above fresh feed entry, 2-3 trays below gas oil drawoff tray: Reflux with partially cooled gas oil providing fine trim control of gas oil

end point Minimize entrainment of any fresh feed/recycle liquid into gas oil product

Gas oil side draw, a conventional configuration using a 6-to-8-tray stripper with steam under the bottom tray for vaporization of light ends to control initial boiling point (IBP) of gas oil

Steam & vaporized light ends returned from top of gas oil stripper to fractionator, 1 or 2 trays above draw tray

Pump around reflux system at the draw tray provided: Recover heat at a high temperature level Minimize low temperature level heat (cannot be recovered by heat

exchange & rejected to atmosphere thru cooling water tower) removed by overhead condenser

8-10 trays used between gas oil draw and naphtha draw or column top. Additional trays required above naphtha draw tray if naphtha side draw is employed.

Vapors (Fuels Gas & LPG) compressed & sent to gas plant

Naphtha condensed & sent to other refinery unit for gasoline production

Gas oils (Light & Heavy) are side streams draws sent for hydrotreating into diesel & other products

Flash Zone Gas internally recycled to coke drums/recovered as additional liquid

Delayed Coking Variables

Process Variables

Feedstock Variables

Engineering Variables

Cycle time Characterization factor

Mode of operation

Temperature Conradson carbon

Pressure Sulphur content Capacity

Recycle ratio Metal content, characterization

Equipment used for coke removal & handling

Temperature controls quality of coke produced: High temperature

removes more volatile materials coke yield decreases coke formation in furnace

Low temperature Incomplete coking if low inlet furnace Short cycle time increase capacity but lower amount of liquid

products & shortens drum lifetime

High Pressure Increase coke formation Increases gas yield

New units: 1 bar gauge (15psig) Current units: 2.4 bar gauge (35psig) Needle coke production: 150psig

Recycle ratio (3%): controls the endpoint of the coker gas oil

Feedstock variables: Characterization factor & Conradson carbon- affect yield

production Sulphur & metal content- retained in coke production

Engineering variables: affect process performance

Types of Coke

Type of coke

Operating condition Feed characterization

Coke property End use as calcinated coke

Sponge Reflux ratio >35% Operating pressure: 2-4bar

Low metal Low STar residue FCC heavy dist Low to moderate asphaltene

M< 200 S < 2.5%High density >780 HGI a~ 100

Anodes for aluminum industry

Shot Low pressure Low reflux ratio Large drums

High SHigh metal Low asphaltene

High S and metal Low HGI<50 Low surface area

Fuel (green)

Needle Pressure >4bar Reflux ratio: 60-100% to max. coke yield High temperature to reduce volatile material

High aromatics content Tars, FCC decant Low S <0.5wt% Low ash <0.1wt% No asphaltene

Crystalline structure Small needles of high conductivity

Graphite electrodes

Sponge Coke Sponge-like appearance Produced from feeds with low to moderate asphaltene

content

Needle coke Needle-like structure Made from feed: no asphaltene contents (decent oils

from FCC) Used to make expensive graphite electrodes- steel

industry

Shot cake Undesirable product Produced when feedstock asphaltene content is

high/when drum temperature is too high Discrete mini balls Methods to eliminate: Adding aromatic feed (FCC decant oil) Decreasing temperature Increasing pressure & recycle ratio

Coking and Decoking Operation

Coke formation takes the “channel branching theory” Allows further gas flow while coke is formed progressively

Decoking: Each coke drum has a drilling rig that raises & lowers a rotating cutting head

High pressure (4,000psig) water Drilling a vertical hole in coke after cooling using a mechanical boring tool

(Figure 6.5A) Further coke removal: Hydraulic cutting tool requires a great amount of jet

of water which has to be treated (Figure 6.5B) Switching the drums, cooling by steam, draining in the coke, warming up the

drum & leaving spare time for contingency

Time cycle of Delayed Coking

Decoking to rail car Decoking to pit

Decoking

Drum cooled & displaced with water to remove volatiles

Pilot hole is drilled thru coke to bottom head

Pilot drill bit replaced with a much larger high-pressure water bit

Cut-direction- predominantly top to bottom Bottom up cutting risks stuck drill if bed collapses

The coke falls coke drum into a collection system

REFERENCES

1. Fahim, M. A. Fundamentals Of Petroleum Refining. Amsterdam: Elsevier Science, 2010. Print.

2. Adams, Jack. Coking 101 An Introduction To Delayed Coking. 1st ed. Process Engineering Associates, LLC, 2015. Web. 2015.http://www.processengr.com/ppt_presentations/coking_101.pdf

3. Jechura, John. Delayed Coking Chapter 5. 1st ed. Colorado: Colorado School of Mines, 2013. Web. http://inside.mines.edu/~jjechura/Refining/06_Delayed_Coking.pdf

THANK YOU Prepared by, SASWINY SANGGARI A/P RICHARD RAVI