delayed coker fired heaters
TRANSCRIPT
© 2010 KBC Advanced Technologies plc. All Rights Reserved.
Delayed Coker Fired HeaterDesign and OperationsSim Romero
Rio Oil & Gas 201013-16 September 2010
Riocentro Convention CenterRio de Janeiro, Brazil
IBP2714_10
Como Conquistar aLiderança de Mercado
Na Nova Década
Heater Design and Operations
PROPRIETARY INFORMATION 2
The fired heater is the key piece of equipment in the delayed coker - delivering the correct thermal conditions to drive cracking and coking reactions
The objective is to keep the heater from coking or fouling as long as possible and still get the result needed Sufficient heat is needed to drive thermal
cracking and polymerization reactions in the coker
High heater outlet result in less coke and more liquid products – incremental gas oil is of very poor quality
Low heater outlet temperature result in several coke drum operating problems (foaming, hot spots etc…)
Heater Design and Operations
PROPRIETARY INFORMATION 3
Why Do Coker Heater Foul - ChemistryThermal Cracking Is Both Cracking And Polymerization
The polymerization or coking kinetics are a function of;
• Feed quality (i.e. asphaltenes, concarbon, sulfur etc…)• Feed contaminates (i.e. sodium, iron oxides/sulfides, general
inorganic solids)• Heater operating conditions – time at temperature and heat flux
0 500 1000 1500 2000 2500 3000Boiling Point, °F
CokeLiquids
Vacuum Resid or
other coker feeds
cracking polymerization
Delayed coker furnace fouling is a complex function of the thermal kinetics
Heater Design and Operations
PROPRIETARY INFORMATION 4
Why Do Coker Heater Foul - Feed Quality Issues
Feed quality is primary factor affecting heater run length
Asphaltene content increases exponentially as the API gravity decreases
Asphaltene and concarbon content are strong indicators of fouling rates
Heater Design and Operations
PROPRIETARY INFORMATION 5
Why Do Coker Heater Foul - Operating Conditions
The coke thickness acts as an insulation to heat transfer causing the tube wall temperature to increase.
Q =
Coke formation occurs at the boundary layer where the velocity is low and the temperature is high.
High Heat Flux and Low Velocities Increase Tube Fouling/Coking
Tube Skin Temperature
Heat Flux
∝
Surface Area
Heat Flux
x
Heater Design and Operations
PROPRIETARY INFORMATION 6
Why Do Coker Heater Foul - Operating Conditions
Clean ConditionsOutside Tube Wall Temperature Slightly Greater Than Boundary Layer Temperature - Thermal Resistance Due To Metal Wall
Fouled ConditionsOutside Tube Wall Temperature Significantly Greater Than Boundary Layer Temperature -Thermal Resistance Due To Metal Wall And More Importantly The Coke Deposited On The Tube
Heater Design and Operations
PROPRIETARY INFORMATION7
Why Do Coker Heater Foul - Contaminates Salts, iron oxides, oxygen and other contaminates can accelerate heater fouling – at times acting like a catalyst to coking in the heater tubes
Sample Date 3/24/2005 3/24/2005 3/24/2005 10/4/2005 10/4/2005
Moisture (as received, %) 10.4 7.05 7.3 1.66 1.8
Ash (%) 38.49 37.57 35.55 17.39 27.34
Analysis of AshSilicon (dry, ppm) 10,270 15,240 14,190 5,623 4,551
Iron (dry, ppm) 241,100 169,400 272,700 301,900 312,000
Vanadium (dry, ppm) 1,699 2,140 1,760 19,910 8,577
Nickel (dry, ppm) 1,023 1,607 1,393 15,880 3,037
Aluminum (dry, ppm) 251 111 2,385 2,645 2,506
Calcium (dry, ppm) 7,799 12,230 9,225 10,130 15,910
Sodium (dry, ppm) 5,439 7,227 3,954 7,004 19,800
Magnesium (dry, ppm) 2,764 3,196 2,107 842 3,519
Crude Unit Desalter Performance Significantly Affects The Delayed Coker Heater
Typical Coke In Furnace Tube Analysis
Heater Design and Operations
PROPRIETARY INFORMATION8
Design Parameters To Mitigate Coking In The Heater Tubes
SingleFired Tube
DoubleFired Tube
Uneven flux distributionpeak to average heat flux is about 1.8
Even flux distributionpeak to average heat flux is about 1.2
Single vs. Double Fired Heater Tubes
For an average heat flux of
10,000 BTU/Hr/SqFtthe peak flux on the
tube will be
18,000 BTU/Hr/SqFt
12,000 BTU/Hr/SqF
t
Double fired heater design reduces the peak flux and allows for higher average flux rates – the average flux should, in a new design, still be limited
Heater Design and Operations
PROPRIETARY INFORMATION 9
Higher velocities – velocity steam • Helps to reduce fouling by
removing coke as it form in the tubes
• Improves the heat transfer rate in the boundary layer
• Reduces the residence time in the heater
Higher velocities – velocity steam• Increased sour water• Increased pressure drop thru heater• Increased tower loading• Increased drum and flash zone velocities
Increased velocity steam will help reduce coke fouling but at a cost (drum solids carry over, tower flooding, sour water etc…)
Heater Design and Operations
PROPRIETARY INFORMATION 10
Design & Operating Parameters – Firebox Flame impingement will rapidly foul the
affected area Ultralow NOx burners have very small fuel
orifices at the burner tip and will plug with time The fuel should be filtered with a fuel gas
coalescer The fuel gas line from the coalescer to the
burners should SS Steam trace the fuel gas line – especially
in cold climates
In a retrofit the box height needs to be reviewed - ultralow NOx burner extend the flame and can cause flame impingement
Flame impingement can rapidly foul the heater coil
Heater Design and Operations
PROPRIETARY INFORMATION 11
Design & Operating Parameters – Tube Metallurgy Tube metallurgy – 9 Chrome vs. SS 347 SS Sch 80 tubes design temperature limit is much higher ~1400ºF The higher temperature limit may not be possible if you spall because o
f the coke thickness at temperature higher than 1300ºF The coefficient of expansion is much greater than 9 Chrome, which can
be good for spalling but can cause problems with uneven tube growth or shrinkage and keeping the tubes from moving off their supports
SS can significantly reduce scale on the outside of the tube External tube ceramic coating
Effective in reducing scale Can shift the heat load away from high heat flux and high tube wall
temperature zones Will slightly increase firing rates SS tubes are a good replacement for 9 Chrome but some of the perceived
benefits of longer runs may not be possible due to excessively thick coke in the coil and the difficulty this presents for spalling
Heater Design and Operations
PROPRIETARY INFORMATION 12
Design & Operating Parameters – Firebox Oxygen Control
O2 levels can be controlled too closely (less than 3%) – run higher O2(greater than 5%) will help reduce fouling by lowering the tube wall temperature Higher O2 will shifts heat to convection section and reduces radiant
flux rates Higher O2 will lower peak by lowering the tube wall temperature Increasing the O2 from ~3% to ~8% will lower the tube wall
temperature by ~75ºF Multiple O2 analyzers are needed in a typical fire box
Air preheat systems Good way to improve efficiency but are costly Startup procedures need to be well thought out with air preheat
systems – generally start with the on natural draft 1st
Because of the severe coking issue in a delayed coker heater the O2 levels should be relaxed to 5% to 8%
Heater Design and Operations
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Design & Operating Parameters – Temperature Of The Heater Outlet
Location of Thermowell Perpendicular to pipe location results in a short thermowell and can
lead to errors in measurements Poor insulation around the TW can cause poor measurements Return bend location gives better performance Decoking methods need to be considered with the location of the
thermal wells Metallurgy or special hardening should be required to prevent erosion Some locations are using the process temperature two to four tubes
back in the process
Badly installed thermowells can significantly effect heater performance short thermowell longer thermowell
Straight run out of heater
First 90º bend out of heater
Heater Design and Operations
PROPRIETARY INFORMATION 14
Operating Parameters – Heater Outlet Temperature
The objective is to deliver sufficient heat to the coke drums – the drum inlet should be about 890ºF to 900ºF
The outlet temperature can vary depending on: Feedstock – paraffinic feeds require more heat due to increased cracking Lighter boiling point distribution in feed will vaporize in the transfer line
and enter the drum cooler High pressure drops in the transfer line will increase vaporization in the
transfer line and enter the drum cooler – also create high backpressure and lower velocities in the heater coil
Heat loss in the transfer line and coke drums will require added heater outlet temperatures
What should the outlet be set to Enough to avoid problems in the drum – foaming, excessively soft coke
and hot spots Enough to meet coke quality specifications i.e. anode coke VCM specs
Heater Design and Operations
PROPRIETARY INFORMATION
Steam-Air Decoking Difficult and labor intensive – must watch air/steam ratio to prevent overheating the
tubes with accelerated combustion Not practices as much Requires a heater/unit shut down Can cause damage to the tubes if the tubes are overheated – carburization of tubes Requires some spalling to remove the bulk of the coke before the actual air burn
Pigging or mechanical coke removal Very easy for operations – contracted work Requires heater/unit shut down Can work inside heater box simultaneously (but not common) Can damage the tube if the pig metal studs are improperly used
o Tungsten carbide has a Brinell hardness of 600-800o Most furnace tube materials, will have a Brinell hardness of 150-225
Online Spalling Can be difficult initially – operation needs to walk through the process carefully –
detailed MOC Does not require unit shutdown Every effective in removing coke in the lower radiant section of the heater – not
effective for removing inorganic solids in the convection section of the heater Risk of plugging the coil if the spall is done too aggressively and/or if there is too
much coke in the tubes – ¼ “ is a good maximum thickness Return bend in the heater and 90º bend directly outside the heater need to be thicker
to prevent erosion from spalling coke
General practice is to online spall and pig decoke when the opportunity arises
Heater Design and Operations
PROPRIETARY INFORMATION
Fouling rates and monitoring heater operations Design should be for less than 1.5ºF/day Greater than 3ºF/day implies an operational
problem or excessively high heat flux 3ºF/days = 3 month run 1.5 º/day = 6 month run
Use a linear regression to filter out variables Infrared scans should be done to verify or check
tube metal skin temperatures
Operating Practices - Heater Tubes And Unit Monitoring
Provides a way to estimate decoking schedule Shows abnormal operations or feed quality
Sudden changes in sodium content Fire box problems Measure the effectiveness of increased
steam velocity Measure the effectiveness of shifting O2
levels
General practice is to online spall and pig decoke when the opportunity arises
Heater Design and Operations
PROPRIETARY INFORMATION
Fire box startup problems Auto ignition systems - keep the operator safely away from the box on startup Forced draft systems – go to natural draft 1st then latter switch to forced draft O2 level controls – avoid O2 level optimization until after startup
Circulation or putting the unit into by-pass requires lowering the outlet temperature significantly Burners will need to be cut out and sometimes pilots The outlet temperature must be kept below 700ºF or lower to prevent polymerization
Frequent (per shift min.) visual inspection of the heater is required regardless of the degree of instrumentation
Loss of flow requires immediate steam purging Automate the purge system on loss of flow After a loss of flow event, operate with a higher than normal velocity steam rates to
remove newly deposited coker. This should not be done on a full drum especially if the coke drum was filled cold
The coke drum can not be filled with low heater outlet temperatures for extended periods of time – this will cause foaming and a possible foam over.
Operating Practices – Safety Issues
Heater Design and Operations
PROPRIETARY INFORMATION
Acoustic pyrometry is a relatively new technology for measuring gas temperature in a furnace. This method involves determining the temperature of flue gas by measuring the speed of sound waves as they pass through the gas.A detailed mapping of the gas temperature is possible with a matrix of sound transmitters and receivers.
DCS
Acoustic pyrometry provides a continuous monitoring of the heat flux in the fire box
Recent Innovations In Coker Fired Heaters – Acoustic Pyrometry
Heater Design and Operations
PROPRIETARY INFORMATION
Recent Innovations In Coker Fired Heaters – Flow MetersWedge Meter
Better reliability - large diaphragm pressure taps Similar accuracy to an orifice plate Fewer solid plugging issues
Sonic Meter New technology very low maintenance and good reliability No obstruction in flow path Pressure drop equal to an equivalent length of straight pipe Unaffected by changes in temperature, density or viscosity Corrosion/erosion -resistant Accuracy about 1% of flow rate
Coriolis Meter New technology some maintenance and startup issues Good reliability Excellent accuracy- better than +/-0.1% with an turndown rate more than 100:1.
The Coriolis meter can also be used to measure the fluid density.
A great man once said “if you can’t measure it, you can’t manage it”
PROPRIETARY INFORMATION
Muito ObrigadoSim Romero
KBC Advanced Technologies, Inc.+1 832 494 0441
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