Non-oxidative methane aromatization
in a catalytic membrane reactor
Olivier RIVAL, Bernard GRANDJEAN, Abdelhamid
SAYARI, Faïçal LARACHI
Department of Chemical Engineering and CERPIC
Université Laval, Ste-Foy, Québec
and
Christophe GUY
Department of Chemical Engineering,
Ecole Polytechnique de Montreal
Environmentally Friendly Gas Technologies
2nd Canadian-Korean joint WORKSHOP Feb. 28 to Mar. 2, 2000
Montreal / Boucherville / Varennes / Bells Corners Canada
1
Production of H2 : growing interests
§ Demand in ecofriendly fuels and processes
§ Fuel-cell technology development
F
ideal energy carrier
2
Major source of H2
§ Natural gas
- Abundance of methane reserves
- Canada is the 3rd world largest producer
the 2nd world exporter
F
Economic significance of CH4 conversion into H2
3
H2 production from CH4
Main industrial processes:
§
Gas steam reforming
§
Catalytic methane decomposition
§
Methane pyrolysis
Disadvantages :
energy intensive and costly
low H2 purity
greenhouse gas emissions
F
non-oxidative process
4
Non-oxidative methane conversion
into H2 and hydrocarbons
x CH4
y H2 + z CnHm
CnHm = alkanes, alcenes (C2 to C8)
1- or 2- step processes : Garnier et al.,1997, Smith et al.,1995,
Cheikhi et al.,1994
CnHm = aromatics (benzene, toluene,...)
1- or 2-step processes:
al., 1999, Weckhuysen et al.,1998
F
Iglesia et al., 1999, Shu et
low yield , complexity
5
Aromatization:
Equilibrium-limited conversion
6 CH4
9 H2 +C 6H6
CH4 CONVERSION
% mol
35
30
25
20
15
10
5
0
373
473
573
673
773
873
973
1073
1173
TEMPERATURE (K)
F
Limitation: 11.3% at 973K
6
Shift of the thermodynamic equilibrium
using permselective membrane
k1
6 CH4
9 H2 +C 6H6
k2
Permselective
membrane
H2 withdrawal
F
lower temperature, higher yield separation in situ
7
Objective of this study
Investigation of methane aromatization
in a catalytic membrane reactor
8
Experimental set-up
Membrane reactor “REB Research”
(with Palladium-Coated Tantalum and/or Niobium
membrane http://www.rebresearch.com)
GC
CH4, Ar, H2 , C6H6
(reaction side)
permselective
Membrane
CH4, Ar
(feed in)
H2 (permeation side)
under vacuum (10-2 Pa)
T
Furnace
Catalyst fixed bed
Ru~Mo-HZSM5
9
prepared by wet impregnation
Test on H2 permeation through REBResearch Membrane
Sievert’s type permeation equation
H2 permeation rate
(mLSTP min -1 )
J = Jo e(-Ep / RT) (PH2,r0.5-PH2,p0.5)
100
90
80
70
524 K
Jo= 10-5 m.s-1.Pa-0.5
60
599 K
Ep= 18 kJ.mol-1
50
686 K
40
870 K
30
20
10
0
0
50
100
150
200
250
Preaction0.5 - Ppermeation0.5 (Pa 0.5)
300
350
10
Test on catalytic activity
without hydrogen permeation
1) Catalyst is 100% selective in benzene
2) Catalyst activity: complex pattern
Formation of
molybden carbides
which are the actives
species for
aromatization
CH4 CONVERSION TO C6H6
o
-1
[CH4]o=100% FCH4 =12,2mL.mIn T=600C)
1.75
CH4 to C6H6 ( %mol)
1.5
1.25
1
quasi steady
state reactivity
0.75
0.5
… and slight
occurrence of catalyst
deactivation
0.25
0
0
60
120
180
240
300
360
420
480
540
600
Time (min)
methane adsorption and
decomposition and catalyst reduction
11
Residence time effect on methane conversion
(without hydrogen permeation)
CH4 conversion into C6H6 (%mol)
Experimental Conditions
No permeation
Temperature
= 873K
P reaction side
CH4 feed conc.
= 101 kPa
= 100%
3
GHSV = 400 h-1
GHSV = 710 h-1
2,5
GHSV = 1100 h-1
2
1,5
1
0,5
0
0
60
120
180
240
300
360
420
480
540
600
TIME (min)
F
still under kinetics control
12
Effect of hydrogen permeation on conversion
CH4 conversion into C6H6 (%mol)
1) hydrogen permeation
increases conversion
3.0
2.8
2.5
2.3
2.0
1.8
1.5
1.3
1.0
0.8
0.5
0.3
0.0
2) ..but it promotes
coke-laydown catalyst
deactivation
No permeation
T = 873K
[CH4]o = 100%
GHSV = 710h-1
With permeation after
360min on stream
0
60
120 180 240 300 360 420 480
Time on stream (min)
13
Results on CH4 conversion:
H2 withdrawal Temperature
(permeation)
K
CH4 feed
dilution
G.H.S.
V.
CH4
conversion b,c
Thermodynamic
CH4 conversion
% mol
h-1
% mol
% mol
No
773
100
350
0.20
1.8
Yes a
773
100
350
0.36
1.8
No
823
100
380
0.76
3.2
No
873
100
400
2.5
5.2
Yes a
873
100
400
5.8
5.2
No
873
100
710
1.6
5.2
Yes a
873
100
710
2.8
5.2
No
873
100
1100
1.0
5.2
No
873
43
800
2.2
7.2
No
873
24
1500
2.8
8.9
a = PTOTAL permeation = 0.2 Pa
b = calculated on a benzene basis
c without permeation, quasi steady state conversion is reported
with permeation, initial conversion is reported
14
Permeation effect on methane conversion
CH4 conversion into C6H6 (%mol)
Experimental Conditions
P permeation side = 0,2 Pa
P reaction side
G.H.S.V.
= 350-400 h -1 CH4 feed conc.
6,0
Thermodynamic equilibrium
5,0
= 101 kPa
= 100%
5,8
Permeation
x 2.3
No permeation
4,0
3,0
2,5
2,0
1,0
x 1.8
0,0
723
0,36
0,2
773
823
873
923
TEMPERATURE (K)
15
Literature comparison
‹ Best yield in C6H6at 873K:
§ 2.6 %mol
(G.H.S.V.= 50h-1, Pt-HZSM-5, Marczewski et al., 1994)
‹ Permeation effect:
§ Conversion of propane into aromatics is increased
by a factor 2 in a membrane reactor
(Uemiya et al., 1990)
16
Membrane Reactor Modelling:
Schematic of the membrane reactor (plug-flow)
Reaction
Fi,r
side
Fi,r+ Fi,r
H
Permeation
side
Fi,p+ Fi,p
Fi,p
H
Membrane
Fi,r+ Fi,r
Fi,r
Shell
Simplified reaction rate
rCH = k1
4
PCH4 α
PC6H6 β PH2,r γ
- k2
R.T
R.T
R.T
17
Membrane Reactor Modelling:
d XCH4
rCH4 =
d FW
CH
CH4 balance:
Permeation rate:
4
d FH2,p= J dA
J = Jo
e(-Ep / RT)
(PH2,r
0.5-P
H2,p
0.5)
H2 balance:
3/
PH2,r = Pt,r
2
XCH4 FCH4- FH2,p
(1+ 2/3) XCH4 FCH4 + FoAr - FH2,p
Fitting of kinetic parameters using Powell algorithm:
α = 0.41
β = 0.41
γ = 0.31
k1 = 6.10-5
k2 =4.10-4
18
Modelling results (at 873 K)
EXPERIMENTAL or equilibrium
CH4 conversion (%mol)
0,10
Experimental Conditions:
P permeation side
= 0,2 and 101 kPa
P reaction side
= 101 kPa
CH4 feed conc.
= 24 - 100%
Temperature
= 873 K
G.H.S.V.
= 350 - 1500 h -1
Active reactor lentgh = 0.035 m
0,09
0,08
0,07
0,06
E
E
0,05
E
0,04
0,03
0,02
E: Equilibrium conversion simulation
with an hypothetical 1m-length reactor
0,01
0,00
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,1
PREDICTED CH4 conversion (%mol)
19
Conclusions
•Ru~Mo-HZSM5 catalyst was prepared
•Methane aromatization with 100% benzene selectivity was observed
•REB-Research membrane reactor has been tested:
Hydrogen permeation:
J improves the conversion rate (by a factor 2)
at 600 oC, conversion of 5.8 % vs 2.5% without permeation
L contributes to catalyst deactivation
•Membrane reactor model has been proposed and validated
Future work
§ Enhancement of catalyst performances:
Æ XCH , coking, Æ stability
4
adding small amounts of CO, CO2 (Ichikawa et al.,1999)
20
Boudouart:
CO
⇔
[C]active + CO2
carbide species formation (promoting effect)
[C]actif + x/2 H2
⇔
CHx
« Decokefaction» and regeneration (stabilizing effect)
[C]inert + CO2 ⇔
CO
(coke)
ex:
{+1.8% CO during 100h}
2x 4% = 8% conv. of CH 4
with a selectivity in C6H6 alm. cste (# 67%)
(Ichikawa et al., 1999)
21