Thermodynamic Analysis of Coking in the Steam Reforming of

Thermodynamic Analysis of Coking in the Steam Reforming of Hydrocarbons

Ju Ho Lee and Chul Soo LeeDept. of Chemical Engineering

Korea University

Thermodynamics and Properties Lab.

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Coke formation in steam reforming and steam cracking

1. The high-temperature pyrolysis of HC Acetylenic intermediate stage

2. Aromatics intermediate stage(Low T)

CH2=CH2 CH2=CH CH≡CH CH≡C C≡C Cn


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Suppression of Coke DepositionEffect of Carbon Deposition

Pressure drop in the coilCorrosion of the coil

Method of suppression of Coke depositionBy diluent or gasifying agents(He, H2, CO,steam)By Changing surface conditions of coilBy Additives like Sulfur, Phosphate


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Gasification of coke mechanism

C + H2O = CO + H2

CO + H2O = CO2 + H2

C+CO2 = 2COC + 2H2 = CH4


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Why steam is used in reforming and cracking process for gasification ?

Gibbs Energy of components in pyrolysis

T(K)

600 800 1000 1200 1400

G/R

T

-40

-30

-20

-10

0

10

20Carbon(s) CH4C2H2 C2H4 C2H6 CO CO2

CO2

CO

Carbon(s)

Gf of CO and CO2has lower value than carbon(graphite)

Gf of CO and CO2has lower value than carbon(graphite)


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Critical Steam Ratio At equilibrium states,

There is steam/feed ratio which makes no carbon deposition in process

Critical steam ratioSteam/feed ratio which which makes no carbon deposition in process


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The Equilibrium Model Minimization of the total Gibbs energy

Minimization routine : DLCONF(IMSL)

(dGsystem)T,P = 0Gsystem = ∑njGj

Gj = Gjo + RTln(aj)

for ideal gas phase. aj=yiPfor solid phase(C), aj=1


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Species Feed : CH4,C2H6,C3H8,C4H10,C5H12 Product :

Calculation Conditions:T = 800~1300P = 0.1, 1, 10bar

H2O, H2, CO2, CO, C(s)

CH4,C2H2,C2H4,C2H6,C3H4,C3H6,C3H8(For CH4)

C4H6,C4H8,C4H10,C6H6 (For higher alkane)


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Methane

Temperature(oK)

700 800 900 1000 1100 1200 1300 1400

Crit

ical

ste

am ra

tio

1

2

0.1 bar1.0 bar10 bar


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Ethane

Temperature(oK)

700 800 900 1000 1100 1200 1300 1400

Crit

ical

ste

am ra

tio

2

3

4



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Propane

Temperature(oK)

700 800 900 1000 1100 1200 1300 1400

Crit

ical

ste

am ra

tio

3

4

5



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Butane

Temperature(oK)

700 800 900 1000 1100 1200 1300 1400

Crit

ical

ste

am ra

tio

4

5

6

7



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Pentane

Temperature(oK)

700 800 900 1000 1100 1200 1300 1400

Crit

ical

ste

am ra

tio

5

6

7

8

9



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How this result be used in cracking process?

At equilibrium conditions,(High residence time)The amount of carbon deposited has maximumvalue

Yields of later products from the pyrolysis of methane in static system at 1181oC and 307 torr(Chen and Back, 1979)


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How this result be used in cracking process?

By reducing the coke in equilibrium state, the coke in the entire reactorlength would not be precipitated

Steam reforming is regarded as the state of steam cracking when residence time prolongs

Effect of steam to Coke deposition

Residence time

Car

bon

depo

site

d

Equilibrium

Equilibrium with steam


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The Kinetic ModelReaction Model – Ethane Pyrolysis

Coke formation Model

Caron-steam reaction Model

Radical mechanisms of Sundaram and Froment(1978)

22 species and 49 radical reactions

→ 17 species and 28 radical reactions

C(s)+2H2O(g) →CO2+2H2

C2H2 →2C(s)+H2 (Sundaram and Froment(1978))


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Simulation results.

Tinlet=900K

Pinlet=304kPa

Steam/ethane = 0.67(mol)

Feed rate = 0.63kg/s

Heat flux=70kW/m2

Length = 80m

Inside diameter=0.108 m

mt

jtjrj

iii

jjij

i

GDf

dzdP

DzQHrSdzdTCpn

raSdzdn

ρ

π

22

0)()()(

0

=−

=−Δ−+

=+


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Residence time & Reactor Length

Reactor Length(m)

0 20 40 60 80

Res

iden

ce ti

me(

S)

0.0

.1

.2

.3

.4

Effect of carbon to carbon deposition

Reactor Length(m)

0 20 40 60 80

Car

bon

depo

site

d(w

eigh

t %)

1e-241e-231e-221e-211e-201e-191e-181e-171e-161e-151e-141e-131e-121e-111e-101e-91e-81e-71e-61e-51e-41e-3

No Steam Steam ratio : 0.1 (molar)Steam ratio : 1.4 (molar)


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Conclusion Coking in steam-reforming reactions was studied to

find the critical steam ratio above which coking doesnot occur as a function of temperature and pressureat the reaction equilibrium. And this results is shownto used for carbon-free deposition conditions of steamcracking process

The critical steam ratio is strongly dependent onpressure at lower temperatures.

By simulation based on kinetic modeling, depositedcarbon is affected by steam ratio to HC

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Thermodynamic Analysis of Coking in the Steam Reforming of