Furnace OperationsTutorial

Ken BeasleyConsultant

Ethylene Global Consulting Services

Introduction to Ethylene Cracking Furnace Operation Fundamentals

The modern cracking furnace is the heart and soul of any ethylene plant.When operated and maintained properly it can have a long life and provide consistent production of product. Conversely improperly operated and/or maintained, it can be unreliable and very expensive to keep running.Today we will offer some fundamentals in the proper operation of modern cracking furnaces.

DisclaimerAlways follow your licensor design recommendations for your furnace.Since all modern furnace designs are tailor made for each application, the best general guidelines will not work for every furnace, even ones that are the most typical. Any values provided today, while they may be typical of many furnaces, are only used to illustrate general trends and tendencies and are not to be used without consulting your operations manual.If anything presented here conflicts with your furnaces design and operation manual, follow your manual.

Parameters for Controlling the Pyrolysis Reaction

Conversion/Severity – disappearance of feed-stock on a once through basisSelectivity – a measure of olefins production at a set conversion/severity Coil geometry - fixed by designCoil pressure - limited by design

Maintaining Proper Furnace Balance

Dilution steam rate well controlledHomogenous feed composition and steady composition will result in best operation Balanced Feed Flow to each pass • Adjust Feed flow +/- five percent maximum to

balance Coil Outlet Temperatures• Critical Flow Elements to divide flows equally

Adequate and Controlled DraftSufficient Excess Air - 2 to 3% oxygen minimum leaving the radiant sectionBalanced Coil Outlet Temperatures: for equalseverities (assuming equal feed and dilution steam flows and even heat flux) +/- 2 degrees C (~4 deg F)

Coking

A naturally occurring byproduct of steam cracking that results inHeater radiant coil inlet or outlet pressure increaseCoil tube wall temperature increaseIncrease in arch (bridgewall) temperatureTransfer Line Exchanger fouling – gradual increase in outlet temperature and/or increase in pressure drop

Sulfur Content in Feed

Sulfur reduces catalytic effect of the metals in the radiant coils, producing less CO/CO2Gaseous feedstock: 80 to 100 PPM (typical)Liquid feedstock: 100 PPM + (typical)

Typical Conversion LevelsEthane: 65 to 68 percent (majority of ethane

furnaces)Up to 75 percent & higher for some

Propane: 80 to 95 percent

Butane: 80 to 95 percent

Liquid Feed: Composition and desired distribution of products often measured by the ratio of Ethylene to Propylene or Methane to Propylene

Dilution SteamControls the partial pressure for reacting hydrocarbonsIncreases yield of olefinic product components and reduces coking at the same severityTypical dilution steam mass ratio to feed:• Ethane 0.30-0.35• Propane 0.30-0.40• Butane 0.40-0.45

Dilution steam is also used for final vaporization of liquid feedsMinimize contaminants such as Sodium

• Naphtha 0.50-0.80• Gas oil 0.60-1.2• HGO;HVGO 0.75-1.2

Typical Operating Temperature Limitations for Radiant Coils

Knowing your specific design metallurgical limits is the key

HK-40: 1040 degrees C (1905 degrees F)

HP-40 modified:1125 degrees C (2055 degrees F)

XTM: 1150 degrees C (2100 degrees F)

These are only typical examples of radiant coil metallurgy

Typical Operating Temperature Limitations (Cont‘d)

Typicals for many modern designs:• Crossover process: 815 deg C (1500 deg F)

[decoking maximum]• Radiant coil process: 900 deg C (1650 deg F)

[decoking maximum]• TLE (Carbon Steel): less than 343 deg C

(650 deg F) • TLE(Carbon Steel with ½ MO): less than 400 deg C

(750 deg F)

Typical Operating Temperature Limitations (Cont‘d)

Typicals for many modern designs (Continued):

• Convection section tube support: 1155 deg C (2110 deg F) (maximum)

• Insulating Fire Brick: >1370 deg C (2500 deg F)

• Ceramic Fiber Blanket/Modules: >1370 deg C (2500 deg F)

• Castable Refractory: 1100 to 1370 deg C (2010 to 2500 deg F)

Coil Movement

All furnace coils but particularly the radiant coils must be able to move freely.Radiant coils have a short life if not allowed to move freely due to high stresses and deformation that results in• bowed tubes• uneven heat flux

Preoperational InspectionSpring hanger/counterweight• Proper setting• Proper weight• No interference

Coil guides and anchors• Proper clearance• Seal installed• Proper anchor installed

Preoperational Inspection (Cont‘d)Coil (inside heater)• Free to move as they expand or contract• All clearances meet tolerance• No tubes bowed

Coil (outside heater)• Proper crossover pipe support with sufficient

clearance• Crossover insulation installed• Proper spring hanger/counterweight installed

Preoperational Inspection (Cont‘d)Burner and piping• Burner properly installed and clean• Fuel gas piping cleaned (Initial Commissioning)• Check if all burner block valves are accessible from

platform (Initial Commissioning)

Instrumentation• Thermocouple/thermowell properly installed in the

Coil outlet, crossover, firebox, TLE outlet• Verify calibration

Preoperational Inspection (Cont‘d)Structure• Casing stiffening installed (Initial Commissioning)• Convection section tube support bolting tightened

(Initial Commissioning)• No bolts/nuts missing (Initial Commissioning)

Refractory/expansion joints• Check if all brick, castable, and ceramic fiber

insulation are in place• Check setting drawings for expansion joint size and

location (Initial Commissioning)• Make sure all debris has been removed

Preoperational Inspection (Cont‘d)Platform and ladders (Initial Commissioning)• Check accessibility to instruments• No interference with piping or instruments/electrical• Unobstructed emergency exit available in two

directions

Peep and access doors • Can be opened easily and closed tightly• Internal insulation installed fills peep door• Access openings closed with brick/ceramic fiber

blocks

Preoperational Inspection (Cont‘d)Convection section tubes (Initial Commissioning)• Check for cleanliness• Check for damaged fins• Check crossover piping for proper setting and

guides• Check manifold/jumpover anchor points

Preoperational Inspection (Cont‘d)

TLEs and steam drum (Initial Commissioning)• Check TLE support beams for level and square• Check top guides for proper length and

clearance• Check risers/downcomers are free to expand

vertically and laterally• Check if steam drum supports have clearance

to expand

Preoperational Inspection (Cont‘d)Stack damper/induce draft fan• Check for construction materials left (Initial

Commissioning)• Check insulation for damage• Expansion clearance between damper plate and

insulation (Initial Commissioning)• Damper shaft alignment• Check linkage, minimum stop, and air failure

position• Check instruments for proper location• Verify ID fan rotation (Initial Commissioning)

Furnace OperationFurnace Strategy (Startup)• Check instrumentation and interlocks• Check blind installation• Decoking air is blinded or disconnected• Check stack damper for proper operation• Open steam drum and super heater vent valve• Check all fuel gas block valves are closed• Check transfer line valves in proper position• Open decoking valve• Fill steam drum to 50 percent, open blowdown to

keep flow through Boiler Feed Water coil

Furnace Operation (Cont‘d)Furnace Strategy (Startup) (Cont’d)• Adjust burner dampers for start• Start I.D. fan, check draft, and purge box for fifteen

minutes• Check combustibles with portable analyzer• Light pilots with portable torch or igniters• When permissives are satisfied, begin to light

burners• Use an even pattern to keep firebox uniform

temperature• Follow furnace vendor recommendation

Furnace Operation (Cont‘d)Hydrocarbon injection• Coil outlet temperature should be below target

temperature - 15 to 20 deg C (60 to 70 deg F) typical

• Dilution steam flow above normal (around 160% of design is typical)

• Make sure furnace effluent is lined up to gasoline fractionation or quench tower

• Light as many burners as possible and still have control

• The board operator observes the Coil Outlet Temperatures and as soon as they begin increasing, start feed flow to the furnace

Furnace Operation (Cont‘d)Hydrocarbon injection (Cont’d)• The feed rate can be increased at whatever rate is

required to keep Coil Outlet Temperatures reasonably uniform

• Maximum variation in Coil Outlet Temperatures in the range of 10 to 30 deg C (20 to 55 deg F)

• When feed rate reaches 50 percent of design start reducing dilution steam so when you reach design feed flow you have design steam to hydrocarbon ratio

• Place controllers on automatic• Check furnace for stable operation. If OK, increase

Coil Outlet Temperatures to target temperature

Furnace Operation (Cont‘d)Furnace alignment and adjustment• Draft and excess air• Burners firing pattern• Coil Outlet Temperature adjustment• Tube Metal Temperature pyrometer survey• Transfer Line Exchangers outlet temperature • Transfer Line Exchangers pressure drop• Check firebox oxygen, NOX, and CO

Furnace Operation (Cont‘d)

Pyrometer Survey of the radiant coils once furnace is in stable operationOnce experience with this furnace is established, the intervals and locations should be adjusted

Furnace DecokingDecoking is required:• Whenever the radiant Tube Metal

Temperatures reach their limit• Whenever there is a significant increase in the

coil inlet or outlet (10 psi typical)• When the Transfer Line Exchangers outlet

reaches the design temperature• When a furnace which has been in operation

for more than a few days and has a severe upset that has caused spalling often indicated by increased pressure differential across the coils and/or TLEs

Furnace Decoking (Cont‘d)

Methods of decoking• Steam• Steam and air

Furnace Decoking (Cont‘d)

Typical decoking limitations(follow licensor design)• COT’s 830 - 900 deg C (1525 - 1650 deg F)• Crossover processes 750 to 815 deg C (1380 to

1500 deg F)• COT’s 1000 - 1065 deg C (1830 - 1950 deg F)

(steam decoking only)• TMT’s 950 deg C (1740 deg F) (steam/air

decoking)• Transfer Line Exchangers outlet 425 deg C

(800 deg F)

Furnace Decoking (Cont‘d)Decoking parameters• COTs should be balanced at end of run temperature

to insure maximum burn and complete decoke

• Cross Over temperatures should start at the maximum allowable to insure a good start of burn at the inlet.

• Using an on-line CO2 analyzer is good measure of the burn

• Choke flow is required at all times for passes with critical flow elements.

Furnace Decoking (Cont‘d)Elements of a Good Decoke Procedure• Firing

is operated on manualis only increased

• Dilution Steam Flowis held constant or decreasedsufficient flow to ensure diffusion does not limit burning and to fluidize spalled cokenot enough to cause erosion

• Air FlowAir is only added

• Radiant tubes are visually inspected during decoke

Furnace Decoking (Cont‘d)

Characteristics of a complete decoke• Coil passes have a clean coil pressure

drop• There are no hot spots on the coils• CO2 at the end of the decoke is < 1%

Furnace Decoking (Cont‘d)Steam-air decoking – plugged tube• Restricted tube has a partial loss of flow

Will clear during decokingAdd time

• Plugged tube has no flowWill not clear during decokingIf a shutdown to replace the tube is not possible, then operate with the tube plugged, but monitor to make sure it doesn’t bow and touch adjacent tubes

Furnace Decoking (Cont‘d)Transfer Line Exchangers Cleaning• Typical experience is to have 3 to 4

decoke cycles between Transfer Line Exchanger cleaning - furnace shutdown and hydroblasting of the Transfer Line Exchanger

• Proprietary processes to steam/air decoke Transfer Line Exchangers

Glossary of Terms

TLE (TLX) - Transfer Line ExchangerCOT - Coil Outlet TemperatureTMT - Tube Metal TemperatureHC - Hydrocarbon (usually as feed to the furnace)DS - Dilution SteamHGO - Heavy Gas OilHVGO - Heavy Vacuum Gas Oil