An Introduction to Delayed Cokers
By A S Sahney
Topics of Discussion
Thermal Cracking - overview Delayed Coker ‘FIT’ in overall Refinery Feed stock Process Products Coke formation & types of Coke Coke Drum Cycle Operating Variables Safety Hazards Modern Delayed Coker design features
Topics of Discussion
Thermal Cracking - overview Delayed Coker ‘FIT’ in overall Refinery Feed stock Process Products Coke formation & types of Coke Coke Drum Cycle Operating Variables Safety Hazards Modern Delayed Coker design features
Delayed Cokers Thermal Cracking
Routes for upgradation
Essential requirement for upgradation - Improve properties e.g. increasing H/C ratio
Carbon rejection and hydrogen addition processes pr ocesses processes
Catalytic and non Catalytic processes
Hydrogen and Non-Hydrogen based processes
Thermal (conversion) and solvent based (separation processes)
Why Thermal Cracking ?
Residual fractions (Bottom of the barrel ) and heavy oils etc. are least valuable streams of a refinery Nearly 50% of the typical crude oils processed in INDIAN refineries contain 370 OC+ fraction
Worldwide limited reserves of sweet CRUDES
Disposal problems due to stringent environmental norms
Decreasing demand of fuel oils
Simultaneous increasing demand of middle distillates
THERMAL CRACKING AND ITS ADVANTAGES
THERMAL CRACKING
– SIMPLE AND COST EFFECTIVE PROCESS
CATALYTIC CRACKING – PRESENCE OF HIGH MOLECULAR WEIGHT COMPOUNDS e.g. RESINS, ASPHALTENES AND METALS IN RESIDUAL FEEDSTOCKS MAKE IT UNSUITABLE FEED FOR CAT CRACKING – METALS AND SULFUR COMPOUNDS ACT AS CATALYST POISONS
VARIOUS THERMAL CRACKING PROCESSES
VISBREAKING
DELAYED COKING
FLUID COKING
FLEXI COKING
Thermal Cracking Reactions
Thermal cracking reactions take place only as an effect of heat. During the cracking, large molecules decompose and form smaller (lighter) molecules
Generally, two types of reactions take place place » Primary reactions, in which decomposition of large molecules to smaller molecules takes place. » Secondary reactions by which active products from primary cracking reactions further crack or react to form other compounds, or polymerize to give heavy products
Delayed Cokers ‘FIT’ in a refinery
Where does ‘Delayed Coker’ fit in a refinery ?
Where does ‘Delayed Coker’ fit in a refinery ?
Primarily processes VR from primary units
Proven technology for converting heavier refinery streams into lighter products
Extremely flexible in processing a vide variety of crude oil slates
Increases distillate yield for the Refinery and minimizes or eliminates Heavy fuel oil production production
Simple and cost effective thermal cracking process process as compared to other upgrading options
Delayed Cokers in INDIA Operating Coke Drums Digboi
2
Guwahati
2
BRPL
2
NRL
2
Barauni
6
Panipat
4
RIL
8
total
26
Forthcoming Coke Drums IOCL, G,M,P 2,2,4 CPCL
2
BORL
2
Bhatinda
2
RIL
8
Essar
6
MRPL
4
HPCL/ BPCL
2
Total (Approx)
34
Delayed Cokers PROCESS
Delayed Cokers – Feedstock Feedstock
Delayed cokers can process a wide variety of feedstocks – Can have considerable metals (Ni and V), Sulfur, resins, and asphaltenes – Most contaminants exit with coke
Typical feedstock – VR, Refinery Slops, Sludge Atm. Residue occasionally used Typical Feed composition – 6 % Sulfur – 1,000 ppm (wt) metals – CCR of 20 – 30 wt.%
Feed ultimately decides the type of Coke obtained from the process
Delayed Cokers Process Only batch-continuous process
in a refinery Flow thru’ the furnace coils is continuous while the feed is switched between two coke drums On-line drum is getting filled with Coke while the off-line drum is going thru’ steam stripping, cooling, decoking, pressure testing, warm-up etc. Overhead vapors from the Coke drums flow to the fractionator Fresh feed combines with the condensed product vapors (recycle) in the fractionator bottom and forms furnace feed
DELAYED COKING
Delayed Coker - Products
GAS PRODUCTS – Fuel gas (H2, C1, C2) – LPG (C3, C4)
LIQUID PRODUCTS – Naphtha - highly olefinic – Light Gas Oil – Diesel component (requires hydro-treating) – Heavy Gas Oil – Hydro-cracker feed
Petroleum Coke
Delayed Cokers ‘COKE’
Coke Formation
Delayed Cokers – Chemistry of Coking
‘Carbon rejection’ Process – Coke has very little H 2 –– shifts to the lighter streams – Metals, Sulphur concentrate in coke
Cycle of cracking and combining – Side chains ‘crack-off’ of Polynuclear aromatic (PNA) cores and generally end up in light streams – PNAs combine (condense) to form coke. Metals and
hetroatoms in PNA core generally end up in coke
Conditions – High temperature and low pressure favor cracking – High residence time favors the combining reactions
Delayed Cokers – Chemistry of Coking
Cracking Reactions – Saturated paraffins crack to form lower MW olefins & paraffins – Side chains cracked off small rings aromatics, cycloparaffins (naphthenes) & Polynuclear aromatics
Combining reactions – Low MW olefins form higher MW compounds – small rings aromatics combine to form resins – Resins after cracking off side chains combine their remaining remaining Polynuclear aromatics to form asphaltenes – Asphaltenes after cracking off side chains left with large PNAs
The large PNAs precipitate to form crystalline liquids and ultimately solidify to form coke embedded with Metals & Sulphur
Delayed Coker – Types of Petroleum Coke Sponge Coke Porous, irregular shaped Feed – moderate Asphaltene High ‘S’ - ‘fuel grade’ Low ‘S’ – Anode grade
Needle Coke
Shot Coke
Undesirable Needle like structures Feed – high Feed – low ‘S’, very Asphaltene low Asphaltene Difficult & unsafe Premium grade to handle Coke Very low ‘CTE’
Delayed Cokers Typical Product Yields
Delayed Coker – Product Yields
Coke and liquid yields from a typical Delayed Coker unit may be estimated by simple imperical equations as under Coke Yield (wt %) = 1.6 x (wt % CCR) Gas (C4 - ) (wt %) = 7.8 + 0.144 x (wt % CCR) Gasoline ( wt % ) = 11.29 + 0.343 x (wt % CCR) Gas Oil (wt %)
= 100 – (wt % Coke) – (wt % Gas) – (wt % Gasoline)
e aye o ers yp ca Product Yields Case 1A Yield % Case1B Yield % – –
Arab Mix Crude No CFO
ME Mix Crude No CFO
Unit Feed
kg/hr % m3/hr kg/hr % 101 300000 100.00 322.83 300000
Sweet Fuel Gas LPG Light Naphtha Heavy Naphtha LCGO HCGO CFO Coke H2S & Misc
149 144 155 156 117 124 185 -
10715 9555 20865 4761 99272 61890 88980 3962
3.57 3.19 6.96 1.59 33.09 20.63 29.66 1.32 100.00
2916.6 9411 18.65 10595 30.44 22479 6.41 5615 118.73 115390 68.66 67563 66330 2617
m3/hr 334.06 3.14 2645.05 3.53 20.68 7.49 32.81 1.87 7.56 38.46 138.01 22.52 74.95 22.11 0.87 -
100.00
Delayed Cokers Coke Drum Cycle
Delayed Cokers: Typical Drum Cycle Drum Cycle - Activities
Time (Hours)
Steam to Fractionator
0.5
Steam to Blow down
1.5
Low-range cooling & Water quench
4.5
Depressurization & Drain
2.0
Un-head Top and Bottom
0.5
Drilling/ Cutting
3.5
Re-head / Steam Purge/ Pressure Test
1.0
Drum Warm-up (Vapor Heating)
4.0
Delayed Cokers – Drum Cycle
Steam Stripping – A ‘critical’ activity before, during and after Coke Drum switch – If Steam is not flowing in the Coke Drum feed line after the switch; » The un-solidified material (pitch) from bottom of Coke Drum will flow back in the feed line line and restrict the flow of cooling water » The unconverted feedstock on the top of coke bed will run down the channels in the coke bed and plug them. This results in isolation of the plugged sections of Coke Coke bed from steam and cooling water. The hot-spots thus formed may result in potentially dangerous steam eruption during drilling/ cutting operation
– Adequate Steam stripping helps to » Prepares the Coke bed for water cooling by bringing down Bed temperature to ‘moderate’ level and by keeping the channels ‘live’ » Eliminates the pitch left in the bottom section of the coke drum » Increases the amount of gas oil yield (middle distillates) Reduce the amount of volatile matter in coke (quality + loss)
Delayed Cokers – Drum Cycle – Low range cooling & Water quench » Low range cooling follows the steam stripping. ‘Small’ water is injected in to the Coke Drum without fully cutting-off the steam
Intermediate step – approx. 1 hr
Precaution – avoid excessive excessive ‘hammering’
» Water quench is ‘Big’ water cooling wherein water is filled in the Coke drum for complete Bed cooling
Duration 3-4 hrs
Drum skin temperature are watched for for confirmation
– Depressurizing & drain » After adequate cooling, the Coke drum is depressurized and residual water is drained-out
Delayed Cokers – Drum Cycle – Un-head Top & Bottom Covers
– Drilling/ Cutting
– Rehead/ Steam Purge/ Pressure Test
Delayed Cokers – Drum Cycle
Drum Warm-up/ Vapor Heating – To prepare the cold empty coke drum for receiving fresh feedstock, hot vapors from online drum are routed into the empty drum – The hot vapors at approx. 425 oC condense in the cold drum, heating the drum to a target temperature of around 340 oC – Condensed vapors are continuously drained out of the drum – After approx. 4-5 hrs of vapor heating, drum is ready to receive feed
Delayed Cokers – Drum Cycle
On-Line Filling – After vapor heating the drum, hot oil from furnace at approx. 485oC is switched into the drum gradually – in steps – Most of the ‘initial’ hot vapors condense on the colder wall of the drum, thereby heating the coke drum walls – Once the coke drum is heated up properly, the drum ‘top’ temperature starts increasing to achieve ‘normal’ temperature of approx. 445- 450 oC. – Until the drum top temperature reaches ‘normal’ value, the vapor load on the fractionator is also less than ‘normal’
Delayed Cokers Effect of Operating Variables
Delayed Coker – Operating Variables
Transfer / coke drum temperature Pressure in coke drum
Recycle ratio
BFW Injection
Delayed Coker – Temperature
Higher the Coke higher is the coke and recycle ratio
Drum inlet temperature, yield at constant pressure
When the temperature is too high the coke formed generally is very hard and difficult to remove from the coke drum with hydraulic decoking equipment
Higher Temperature reduces the volatile combustible (VCM) content of the coke
Delayed Coker – Pressure
Coke yield decreases by reducing coke drum pressure. Each 0.5 kg/cm 2 reduction in drum pressure increases liquid yield by 1.3% vol and cut in coke yield by 1.0% wt. of fresh RESID feed
Higher the operating pressure, higher the coke make
Higher pressure improves the coke quality primarily by lowering CTE and electrical RESISTIVITY but at the expense of liquid products
Delayed Coker – Recycle Ratio
Higher the recycle ratio, higher the coke make
Increasing the recycle rate lowers the combined feed resin ASPHALTENE content and in turn also provides a higher concentration of aromatics in coke drum
Trend is to go for lower recycle ratio so as to increase throughput
Low recycle ratio HCGO quality and more coke deposition in coils is a concern – –
Delayed Coker – BFW Injection
Used as a turbulising water in Heater Coils
Reduces partial pressure of hydrocarbon in the coke drum
Leads to formation of loose coke
Delayed Coker – Variables
Delayed Cokers Safety Hazards & Their Mitigation
Delayed Cokers – Safety Hazards
Coke Drum Switching Severe accidents have been experienced due to faulty valve operation during Coke drum switching operation
Coke Drum Head removal – Eruptions – due to inadequate coke bed cooling – Incomplete draining – Avalanche of shot coke
Coke cutting operation – Cutting tool raised out of coke drum w/o isolation – Water hoze bursting – Failure of the wire rope supporting the drill stem
Others – Dust irritants, Toxic exposure, Coke handling by O/H cranes, conveyors, crushers, pay-loaders etc
Delayed Cokers – Mitigation of Safety Hazards
Coke Drum Switching – Comprehensive remote operated system combining Interlocks & permissive for reducing faulty operation – Colour coding of valve appropriately – Employees working in pairs – ‘buddy system’
Coke Drum Head removal – Remote opening/ closing of Coke drum head – Restrict movement of ppl on coke drum top & bottom platforms
Coke cutting – Interlocks to prevent drill stem to come out of the Coke drum w/o isolation – Interlocks for ‘slack cable’ – always ON
Modern Delayed Cokers Design Features
Modern Delayed Cokers Process Design Objectives
Maximize The Production Of Liquid Products
Minimize The Production Of Coke
Produce Heavy Gas Oil suitable for downstream Cat Processing
Optimize Number And Size Of Coke Drums
Optimize Energy Recovery
Optimize Feed Preheat System And Heater Duty
Maximize Air Cooling And Minimize Water Cooling
Modern Delayed Cokers Typical Process Features
Low Pressure Operation: 1.05 kg/cm2
Operating Conditions & Recycle Ratio Optimized to needs while maximizing Distillate Yield
Low Recycle Operation: ~10%
High CCR Feed Processing
Modern Delayed Cokers Typical Process Features
Shorter Coke Drum Cycle
No sloping from blowdown
Total Water Reuse System
Refinery Slop Processing
Refinery Sludge Processing
~18hrs
No Quench Column/RFO Generation
No CFO Generation
Modern Delayed Cokers Typical Process Features
Double Fired Heaters with Online Spalling & Decoking Facility
Provision for Shot Coke Control & Handling
Automatic Heading and Deheading System
Efficient & Cost Effective Coke Handling
Hydrocyclones for Recovery of water
In NUTSHELL
Delayed Coking is an attractive option for upgradation of residual fractions
Coking is a severe thermal cracking process aimed at distillate and lighter production, in which coke is also formed.
Trend is to go for low recycle, low pressure cokers so as to maximize liquid yield
SHOT COKE
SPONGE COKE
