Delayed Coker Fired Heaters

Delayed Coker

Fired Heater

Design and Operations

Sim Romero

Rio Oil & Gas 2010

13-16 September 2010

Riocentro Convention Center Rio de Janeiro, Brazil

IBP2714_10

Como Conquistar a

Liderança de Mercado

Heater Design and Operations



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

Why Do Coker Heater Foul - Chemistry

Thermal 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 3000 Boiling Point, °F Coke Liquids Vacuum Resid or other coker feeds cracking _{polymerization}

Heater Design and Operations

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

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

Why Do Coker Heater Foul - Operating Conditions

Clean Conditions

Outside Tube Wall Temperature Slightly Greater Than Boundary Layer Temperature - Thermal Resistance Due To Metal Wall

Fouled Conditions

Outside 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

7

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 Ash Silicon (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

Heater Design and Operations

Design Parameters To Mitigate Coking In The Heater Tubes

Single Fired Tube Double Fired Tube Uneven flux distribution

peak to average heat flux is about 1.8

Even flux distribution peak to average heat flux is about 1.2

Single vs. Double Fired Heater Tubes

For an average heat flux of

10,000 BTU/Hr/SqFt the 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

Heater Design and Operations

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

Design & Operating Parameters – Firebox

 Flame impingement will rapidly foul the affected area

 Ultralow NO_x 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 NO_x burner extend the flame and can cause flame impingement

Heater Design and Operations

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

Heater Design and Operations

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

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

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



 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

Heater Design and Operations

 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

oTungsten carbide has a Brinell hardness of 600-800

o 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

Heater Design and Operations

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

 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}

 O₂level 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.

Heater Design and Operations

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

Heater Design and Operations

Recent Innovations In Coker Fired Heaters – Flow Meters

Wedge 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.

Muito Obrigado

Sim Romero

KBC Advanced Technologies, Inc.

+1 832 494 0441

www.kbcat.com [email protected]