thermodynamic analysis of coking in the steam reforming of
TRANSCRIPT
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.
Korea University
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
Thermodynamics and Properties Lab.
Korea University
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
Thermodynamics and Properties Lab.
Korea University
Gasification of coke mechanism
C + H2O = CO + H2
CO + H2O = CO2 + H2
C+CO2 = 2COC + 2H2 = CH4
Thermodynamics and Properties Lab.
Korea University
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)
Thermodynamics and Properties Lab.
Korea University
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
Thermodynamics and Properties Lab.
Korea University
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
Thermodynamics and Properties Lab.
Korea University
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)
Thermodynamics and Properties Lab.
Korea University
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
Thermodynamics and Properties Lab.
Korea University
Ethane
Temperature(oK)
700 800 900 1000 1100 1200 1300 1400
Crit
ical
ste
am ra
tio
2
3
4
0.1 bar1.0 bar10 bar
Thermodynamics and Properties Lab.
Korea University
Propane
Temperature(oK)
700 800 900 1000 1100 1200 1300 1400
Crit
ical
ste
am ra
tio
3
4
5
0.1 bar1.0 bar10 bar
Thermodynamics and Properties Lab.
Korea University
Butane
Temperature(oK)
700 800 900 1000 1100 1200 1300 1400
Crit
ical
ste
am ra
tio
4
5
6
7
0.1 bar1.0 bar10 bar
Thermodynamics and Properties Lab.
Korea University
Pentane
Temperature(oK)
700 800 900 1000 1100 1200 1300 1400
Crit
ical
ste
am ra
tio
5
6
7
8
9
0.1 bar1.0 bar10 bar
Thermodynamics and Properties Lab.
Korea University
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)
Thermodynamics and Properties Lab.
Korea University
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
Thermodynamics and Properties Lab.
Korea University
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))
Thermodynamics and Properties Lab.
Korea University
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−+
=+
Thermodynamics and Properties Lab.
Korea University
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)
Thermodynamics and Properties Lab.
Korea University
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