Coke Furnace

Advances in Advances in CrackingCracking Furnace Technology Furnace Technology

Karl [email protected]

John [email protected]

Jeff [email protected]

Cheah Phaik [email protected]

Cyron Anthony [email protected]

Refining Technology ConferenceDubai Crown Plaza Hotel

14-18 December 2002

Advances in Cracking FurnaceAdvances in Cracking FurnaceTechnologyTechnology

Outline

1. Introduction2. Historical Development3. Design Constraints4. Comparison of Current Designs5. Furnace Run lengths6. Anti Coking7. Future Opportunities8. Conclusions

IntroductionIntroduction

• Furnace technology is an area of activeresearch. The high-energy consumption,capital and maintenance cost of thecurrent cracking furnace are a drivingforce to develop improved conversionroutes.


• The pyrolysis ofhydrocarbons for theproduction ofpetrochemicals isalmost exclusivelycarried out intubular coils locatedin fired heaters.


• Steam is added tothe feedstock toreduce the partialpressure of thehydrocarbon in thecoil.


• The reactions that result in thetransformation of saturatedhydrocarbons to olefins are highlyendothermic and require temperaturesin the range of 750 to 900 degrees Cdepending on the feedstock and designof the pyrolysis coil.

HistoryHistory

• The first commercial unit for pyrolysiscracking of hydrocarbons wascommissioned at Esso’s facility in BatonRouge, Louisiana during 1943.

• The feedstock was Gas Oil and theButadiene extracted from the crackedproducts was used to make Butylrubber.

HistoryHistory

• Ethylene, propylene and other productswere flared.

• Shortly thereafter, two morecommercial pyrolysis-cracking unitswere constructed along with ethanoland ethylene glycol plants to utilize theethylene.

HistoryHistory

• All pyrolysis cracking furnacesconstructed during the 40’s and 50’shad horizontal radiant tubes andresidence times in excess of 0.5 seconds.

• Radiant tube materials were 310stainless (wrought 25 Chrome 20 Nickel)or incoloy. Cast tubes were not yetinvented.

HistoryHistory• Prior to the mid

60’s all furnaceswere fired with avery large numberof wall burnersspaced on aboutsix foot centers inthe horizontalwalls and facingthe row of radianttubes.

HistoryHistory

• Thereafter most designers working withburner manufacturers developed thecapability of firing cracking furnacesmainly or exclusively with a muchsmaller number of floor burners.

HistoryHistory

• Some Technologies still utilizes wallburners for a small portion of the heatfired. This change made possiblebecause of much better control of theexcess air within the firebox.

HistoryHistory

• During 1960 the first vertical radianttube pyrolysis cracking furnace wascommissioned at Esso’s plant in Koln,Germany.

• Shortly thereafter essentially all newcracking furnaces were designed withvertical tubes.

HistoryHistory

• The driving force was the much lowerinvestment cost required for verticaltube furnaces.

• The residence time for these furnaceswas about 0.3 seconds. Typical tube IDwas about 4 to 5 inches.

HistoryHistory

• By 1965 manufacturers (initiallyDuraloy in the US) started to producecast tubes.

• A number of plants tried them withlittle success.

HistoryHistory

• Finally it was realized that the dross atthe tube ID was causing very rapidcoking and very poor tube life.

• Then the manufacturers found a way tomachine the tube ID to a smooth,imperfection free, surface and theirperformance greatly improved.

HistoryHistory

• During the late 70’s and 80’s the radianttube diameters used in crackingfurnaces decreased.

• The smaller diameter tubes had ahigher surface to volume ratio, whichallowed the heat necessary for crackingto enter the tubes in a much shortertube length.

HistoryHistory

• This allowed the cracking to take placein a much shorter residence time whichgave much better yields of the desiredproducts (mainly ethylene, propyleneand butadiene).

HistoryHistory

• Ultimately, about 1979 both Kellogg andEsso (Exxon) developed furnaces withmultiple parallel radiant tubes eachabout 40 feet long and 1 to 1.5 inches ID.

Design ConstraintsDesign Constraints

• The typical modern pyrolysis furnacesconsist of a rectangular (important)firebox with a single or double row ofvertical tubes located in the center planebetween two radiating refractory walls.


• The heat transfer to the tube is effectedlargely by radiation and only to a smalldegree by convection.

• The firebox temperature is typically inthe range of 1200 degrees Centigrade.


1. Process Chemistry

2. Heat of Reaction

3. Metallurgy

4. Flame pattern / Fire Box


Process Chemistry

• To fully understand the furnace designconstraints a review of processchemistry is required.


• When a hydrocarbon feedstock isundergoing pyrolysis a multitude ofreactions are happening simultaneously,but for practical purposes a simplifiedoutlook will explain many of the endresults in which are of primary interest.


• Hydrocracking - Decomposition byfree radical chain mechanisms intothe primary products: hydrogen,methane, ethylene, propylene andlarger olefins.


• Hydrogenation anddehydrogenation – reactions whereparaffins, di-olefins, and acetylenesare produced from olefins.


• Condensation – reactions wheretwo or more small fragmentscombine to produce larger stablestructures such as cyclo-di-olefinsand aromatics.


Hydrocracking of Ethane

C2H6 = CH3* + CH3* (1)

CH3* + C2H6 = CH4 + C2H5* (2)

C2H5* = C2H4 + H* (3)

H* + C2H4 = H2 + C2H5* (4)

C2H6 = C2H4 + H2 (5)


The process chemistry review revealsat least three design requirements:

1. Low Pressure

2. Low Hydrogen Partial Pressures

3. Short Residence Time


1. Low Pressure

• The predominately desired reactionis: C2H6 = C2H4 + H2

• Any time the moles of products arelarger than the moles of reactantsthe equilibrium favors lowpressures.


2. Low Hydrogen Partial Pressure

• To reduce the unwantedhydrogenation reaction, lowerhydrogen partial pressures wouldproduce more of the desiredproducts.


3. Short Residence Time

• To reduce the unwantedcondensation reaction, shorterresidence times would producemore of the desired products.


Heat of Reaction

• The reaction is endothermic andrequires high temperatures.

• Couple the heat of reaction with thelow pressure requirement - size andlength of coils are dictated.

Design ConstraintsDesign ConstraintsDiameter To Length Ratio

Q = U A DT

1. If the pressure drop is fixed Q is set by thediameter of the coil.

2. U and DT are fixed.

3. A determines the length of the coil.


Diameter To Length Ratio

1. 1 inch diameter is approximately 40 feet.





Typical Metallurgy Constraints

Trade Name Composition Temperaturelimits

Developed Carbon PickUp

HK 40 25/20 : Cr/Ni 1830 F1000 C

Late1960’s

1% at1055 C

HP Modified 25/35 : Cr/Ni 2,060 F1125 C

Early1970’s

1% at1125 C

35/45 35/45 : Cr/Ni 2100 F1150 C

Mid1980’s

1% at1155 C


Flame Pattern Constraints



• The pyrolysis reaction are endothermic andtime dependant reactions.

• The flame pattern and the resulting heat fluxcan have the net effect of changing theeffective length of the coil.



• If the heat flux is not uniform the coileffective length can be reduced.

• If heat flux is not uniform hot areas can causeover-cracking and shorting coil life.

Design ConstraintsDesign ConstraintsFlame Pattern Constraints

HEAT FLUX DISTRIBUTION

0

1

2

3

4

5

6

7

8

9

20 40 60 80 100

RELATIVE HEAT FLUX (% OF MAX)

RA

DIA

NT

SE

CTI

ON

HE

IGH

T (M

ETR

ES

)

Existing

Improved



The shape of the flameis determined byburner designersaccording to heatinput requirements bythe technologyprovider.



• The shape of the fire box will effect the flamepattern and heat flux.

• Deviation from rectangular have not provedto be successful.

Design ConstraintsDesign ConstraintsFlame Pattern ConstraintsFire Box Shape CONVECTION

SECTION

SHP STEAMDRUM

CROSSOVERLINES

PRIMARYQUENCH

EXCHANGERS

DOUBLEIN-LINE

TUBEROWS

BOTTOMFIREDEACHSIDEINLET

MANIFOLD



• Steam may be added to the fuel gas to reducethe NOx emissions.

• This steam has been shown to even the heatflux distribution resulting in higher yields andrun lengths.

Com parison of Current DesignsCom parison of Current Designs

• It is the diameter and length of thetubes and the manner in which they areinterconnected to and from thepyrolysis coil which determine to whatextent a particular design will becharacterized by an optimumcombination of pyrolysis parameters.


• The design calculations applied topyrolysis coils are necessarily complexsince heat transfer and chemicalreaction are involved.


• Various options exist for the specificdesign of a pyrolysis coil, and thisaccounts for the variety of industrialpyrolysis furnaces presently inoperation.


Types of Furnace Coils

SINGLEPASS

“U”- COIL orTWO PASS

“W”- COIL or“M” - COIL

HYBRID COIL


• If short residence time is considered thesingle most important objective, then ashort coil with tubes of small diameterwill be considered.


• If a combination of high capacity,medium resident time and lowhydrocarbon partial pressure is judgedto be most beneficial, then a relativelylarger tube will result.


• For this presentation the comparisonwill be limited to four designs:

1. Short Residence Time - One pass of uniform size- Fire box floor to roof orientation


Typical End ViewSingle Pass

CONVECTIONSECTION

SHP STEAMDRUM

CROSSOVERLINES

PRIMARYQUENCH

EXCHANGERS

DOUBLE IN-LINE TUBE

ROWS

BOTTOMFIRED

EACH SIDE

INLETMANIFOLD


2. U Sweep Bends - one or more passes of uniform size - U configuration - fire box roof to roof orientation


Typical End ViewTwo Pass CONVECTION

SECTION

SHP STEAMDRUM

PRIMARYQUENCH

EXCHANGERS

DOUBLEIN-LINE

TUBEROWS

BOTTOMFIRED

EACH SIDE

INLETMANIFOLD


3. W Sweep Bends - one or more pass of increasing size - W configuration -fire box roof to roof orientation


Coil Configuration


4. Hybrids - one or more passes of increasing size - fire box roof to roof orientation -


FEEDDISTRIBUTORS

MANIFOLDS

INLET TUBES

OUTLETTUBES

QUENCHERS

Coil Configuration

Hybrid Coil


Size ResidenceTime

(seconds)

Design runLength onNaphtha

DesignDecokeTime

(hours)Coil One 1 inch by

forty feet0.08 - 0.12 30-35 Days 18-24

Coil Two 2 inch byeighty feet

0.20 – 0.25 35-45 Days 24-30

Coil Three 4 inch by120 feet

0.35 – 0.45 45-60 Days 30-36

Coil Four 2 inch to 6 inch by

80 feet

0.20 - 0.25 35-45 Days 24-30

Com parison of Current DesignsCom parison of Current DesignsTypical Residence Times Yields for Light Naphtha

ResidenceTime

(seconds)0.10 0.20 0.50

Methane 15.48 15.78 16.16

Ethylene 34.16 32.16 29.37

Propylene 17.02 17.35 17.78

Butadiene 5.2 5.1 5Benzene 5.89 6 5.75Toluene 2.59 2.65 2.52Fuel Oil 3.12 3.35 3.61


Advantages of Coil 1

1. Highest Olefin Conversion due to short residence time

2. Down Stream Separation Section can be smaller.

Comparison of Current DesignsComparison of Current Designs

Advantages Of Coil 2

1. Moderate Olefin Conversion

2. Operations Friendly

3. Dilution Steam and Feed tolerant

4. Moderate coil life

4. Moderate Thermal shock

Radiant Coil Therm al ShockRadiant Coil Therm al Shock-theory-theory

• The coke layercan reach > 10mm thicknessdepending on thetype of feedstockand severity

12 mm8 mm

Radiant Coil Therm al ShockRadiant Coil Therm al Shock-theory-theory

• The thickness ofthe coke is afunction of theTMT andconversion

Com parison of Current DesignsCom parison of Current Designs Thermal Shock

Com parison of Current DesignsCom parison of Current Designs Thermal Shock



1. Good run length between decokes

2. Good coil life

3. Moderate Thermal shock



1. Good run length between decokes

3. Good coil life

4. Good Thermal shock

5. Dilution Steam and Feed tolerant


Coil 1 Quench Exchanger

1. Established reliable system

2. Many units in operation

3. Metallurgy of Quench Exchanger is very important - Should be sodium stress corrosion resistant

Comparison of Current DesignsComparison of Current Designs

Typical ElevationSingle Pass

ID FAN

PRIMARYQUENCHERS

RADIANTTUBE

BANKS

CONVECTIONSECTION

STACK

STEAMDRUM

OUTLETMANIFOLDS

CROSSOVERLINES

RADIANTBOX

INLET MANIFOLDS













1. Recently changed system to reduce residence time


3. Previous design required hydro blasting every 6 months


Quencher

Furnace Run LengthsFurnace Run Lengths

• Many factors influence furnace run lengths.A partial list includes

1. Decoke Procedure

2. Sulfiding Procedure

3. Tube size and metallurgy

4. Feed and Steam impurities

5. The ability to utilize steam only decokes

6. Use of coke mitigating additives


Design runLength onNaphtha

Actual runlength onEthane

Actual runlength onNaphtha

Cause of Deviation

Coil One 30-35 Days 10-20 30 1. Feed Impurities2. Dilution Steam Impurities3. Flame / Fire Box Issues

Coil Two 35-45 Days 20-25 30-35 1. Over sulfiding2. Low air during decokes

Coil Three 45-60 Days 35-40 40

Coil Four 35-45 Days 30-40 45-50 1. Sulfiding impurities


Mixing Element Radiant Tube (MERT)

• Developed by KUBOTA

• Aims to create additional turbulence resulting inefficient and homogeneous heating of gas

• Efficient heating allows lower Tube Skin Temperatures and Higher yield


• Properties include:-1. Heat transfer Co-efficient increased by 20-50%

compared to bare metal

2. 2% larger surface area

3. 3% weight gain

4. 2.0-3.5 times higher pressure drop

Mixing Element Radiant Tube (MERT)


PEP Cast Finned tubes

Anti-Coking TechnologiesAnti-Coking Technologies

1. GE Betz - Pycoat

2. Nalco - Cokeless

3. Chevron / Phillips

- CCA 500

4. Westaim

5. Nova / Kubota

Coking in Ethylene Furnace

Reduce product yields

Restrain cracking severity Increase

Loss of production capacity

Increase energy consumption

Shorten coil service life

Increase maintenance cost

Add operator load

Furnace Utilization, 90~95%

Worldwide Production Loss 5% Improvement = $1 Billion/yr

10% Improvement = $2 Billion/yr

Coking Im pact in EthyleneCoking Im pact in Ethylene

Metal catalysisMetal catalyze dehydrogenation -->Carburized / metal boosted

Radical reaction

-->Filamentous coke growth

Acetylene / Butadiene etc. , growth on radical active site

C

Alloy tube

Linear growth

Lateral growth

Blocked metal cluster

Still active metal cluster

M echanism of Coke Form ationM echanism of Coke Form ation

Types of CokeTypes of Coke


1. GE Betz - Pycoat

Advantages

• Treated off line - no unit contamination

• Silicon Based

• Diffusion barrier

- Adherent film : enhance binding film with m etal surface

- Barrier film : prevent carbon & oxygen from penetrating

Tube material

Coke Precursor

Diffusion Barrier

No Diffusion No Catalytic

Coke+ Minimize Pyrolytic Coke

H2, CO, CO2

+ No Pyrolytic Coke

Decoking Film

•Decoking Film : Gasify the remained coke (In Development)

GE Betz PY-COAT ConceptGE Betz PY-COAT Concept

Coated Film

Tube Metal

Coating CharacteristicsCoating Characteristics

Ethane Furnace Designed by KBR

DescriptionLocated in North AmericaMillisecond Furnace27 T/Hr and 70 % Conversion160 1.3” tubesMain Criteria for Decoking : CoilInlet PressureOriginal R/L : 13-15 days

Com m ercial ReferenceCom m ercial Reference

PY-COAT Perform ancePY-COAT Perform ance

PY-COAT Performance Vs. Baseline

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10

Run #

Ru

n le

ng

th (

day

s)

Baseline Treated

Plant Upset

Gas Furnace Designed by KBRGas Furnace Designed by KBR

Test started Mid OctoberTest started Mid October

PY-COAT PERFORMANCE @ NORTH AMERICACOIL PRESSURE DROP_MAX

0

5

10

15

20

25

30

35

40

45

50

55

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

R un days

PY-COAT RUN 1

PY-COAT RUN 2

BASE 1

BASE 2

BASE 3

Still operating


2. Nalco - Cokeless

Advantages

• First to develop on line coating - Severalgenerations of development

• Present generation has shown some success

• Applied on line - high furnace utilization

• Phosphorus based

TMT Profile ComparisonTMT Profile ComparisonTube Metal Temperature Coil 3

1010

1020

1030

1040

1050

1060

1070

1080

1090

1100

1110

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86

Day of Run

De

gre

es

, C

Lummus

Naphtha cracker

Lummus

Naphtha cracker

4 Coke-Lessruns average

4 Coke-Lessruns average

3basecaserunsaverage

3basecaserunsaverage


3. Chevron Phillips CCA 500

Advantages

• Treated off line

• Tin / Silicon Based

50

55

60

65

70

75

80

85

90

0 2.5 6 11 16 21 26 31 36 41 46

Time of Stream (days)

% M

ax

imu

m C

oil

Pre

ss

ure

Dro

p

Hydrogen Sulfide

CCA 500

Coil Pressure Drop (S-Coil, Ethane Feed)


3. Westaim COATALLOY™

Advantages

1. Coating applied on new tubes - Furnace Utilization

2. Carbonization Resistant

M ICROGRAPH OF COATALLOY™ ENGINEEREDM ICROGRAPH OF COATALLOY™ ENGINEEREDCOATING SYSTEM - BEFORE SERVICECOATING SYSTEM - BEFORE SERVICE

EngineeredSurface

EnrichmentPool

DiffusionBarriers

Bulk HighTemperatureAlloy

50 microns

COATALLOY™-1100 CASE HISTORY - FORMOSACOATALLOY™-1100 CASE HISTORY - FORMOSA

0.0

0.5

1.0

1.5

2.0

2.5

Typical Runs forUncoated Furnaces

Runs 3-9 forCoatAlloy™-1100

coated Coil

Days on Line

Max

imu

m P

ress

ure

Dro

p R

atio

96

HIGHLIGHTS OF OTHER CASE HISTORIESHIGHLIGHTS OF OTHER CASE HISTORIES

•Short Residence Time• 2% increase in ethylene yield• 3 x run length• 5% increase in conversion

•Long Residence Time• 4% increase in ethylene yield• Min. Feed rate increase ofapprox.. 8%

• 9% increase in conversion

•Medium Residence Time• 4.5% increase in ethylene

yield• 2 x run length• 8% increase in conversion

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Uncoated-BaseConversion

Coated-BaseConversion

Coated-10%Conversion Increase

ShortResidence

MediumResidence

LongResidence

Co

kin

g r

ate

(psi

ris

e/d

ay)


4. Nova / Kubota - ANK 400

Advantages

• Coating applied on new tubes -Furnace Utilization

• 400 + Run lengths

Future OpportunitiesFuture Opportunities

1. Better Coil Metallurgy

• Coil manufactures will continue todevelop new designs with highertemperature limits


2. Better Coil Coatings

• Research is presently ongoing inceramics and other coke resistancematerials.

• Betz / SK is developing a coke filmingagent that will keep the coke in gasphase reducing accumulation on thetube.


3. Catalyst Development

• Research is presently ongoing foroxidative coupling of methane toethane and ethylene by Li/MgOcatalyst at 700 degrees C.

ConclusionsConclusions

1. Introduction

2. Historical Development

3. Design Constraints

4. Comparison of Current Designs

5. Furnace Run lengths

6. Anti Coking

7. Future Opportunities

8. Conclusions

ConclusionsConclusions

• Thanks for your time

• Our goal was to give an overview of thecurrent advancements pyrolysiscracking furnaces

• Please contact us for additionalinformation or questions

Documents

Coke Furnace