Methanol Steam Reforming in Pd-Ag Membrane Reactor for High Purity Hydrogen Generation
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Transcript of Methanol Steam Reforming in Pd-Ag Membrane Reactor for High Purity Hydrogen Generation
Methanol Steam Reforming in a Pd-Ag Membrane Reactor:
Experiments & Modeling
Sameer H. Israni
(Advisor : Prof. Michael P. Harold)
Acknowledgement: Support by NSF CTS-0521977
Palladium based membranes
Dense Palladium & Palladium-Alloys have very high selectivity for H2
Non-porous Gas Mixture Pd Membrane Pure H2
Porous Substrate
α-Al2O3
Conventional or Top LayerMembrane
ELP
α-Al2O3
Pd nuclei
α-Al2O3
Pd nucleiPd or Pd-Alloy
Sensitization & Activation
α-Al2O3
Pd nuclei
-Al2O3
α-Al2O3
-Al2O3
α-Al2O3
Pd nuclei
- Al2O3
NanoporeMembrane
Sol-Gel Slip Casting
Sensitization & Activation
Sol-Gel Slip Casting
ELP
Pd
Pd / Pd-Alloy Membrane SynthesisConventional & Nanopore Membranes
B.R.K. Nair et al., J. Membrane Sci., 290 (2007) 182
• Current commercial membranes are ~ 50 - 100 m thick Too costly
• Thinner Pd membranes required (5-20 m)– High flux and low cost– More defects
Porous Substrate
-Al2O3 hollow fibers
Characterization of Membranes
Membrane characterization
• SEM – defects, grain size
• EDS / XPS – bulk composition, composition profile along depth of membrane
• XRD – bulk composition, alloy formation
Membrane Reactors for H2 Generation
Advantages – Process intensification– Overcome equilibrium limitations
or kinetic inhibitions
Disadvantages – Cost– Durability (leaks)– Productivity vs. Utilization issue
H2 generation from various fuels (CH4, ethanol, methanol, NH3 etc.)– Stationary H2 generation
– On-vehicle H2 generation
Single Fiber Packed Bed Membrane Reactor Multi Fiber Packed Bed Membrane Reactor
Objectives
1. Membrane flux under reaction conditions• Effect of reactant & product species
2. Methanol steam reforming: experiments & 2-D modeling
• Packed Bed Reactor (PBR)
• Single-fiber Packed Bed Membrane Reactor (PBMR)
3. 3-D model to simulate larger scale multi-fiber membrane reactor
Part 1
Membrane H2 Flux
In
Reaction Conditions
Pd-Ag (23 wt % Ag) Nanopore membrane synthesized for methanol steam reforming study
Total Pd-Ag thickness – 3.7 to 4.0 m
-Al2O3
(50 nm pore size)
-Al2O3
(5 nm pore size, 50% porosity)
Pd-Ag2.5 m
1.2 m
Hollow Fibre Membrane - α-Al2O3 (3.7 mm OD)
Supplier - Media & Process Tech Inc.© , PA, USA
Permeation Characteristics 3.4 micron Pd-(23 wt. %)Ag membrane
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 50 100 150 200 250 300 350 400 450
[Pret0.5 - Pperm
0.5)] (Pa0.5)
H2
flu
x (m
ol/
m2 /s)
225 C
250 C
300 C
Pd-Ag Membrane Pure Gas Permeation Results
•Infinite H2/He selectivity
•No degradation in performance after exposure to reforming conditions – 12 days
)( 5.0,
5.0, 222 permHretHH PPQJ
Inhibition of H2 Flux in Reaction Conditions
Presence of CH3OH, H2O, CO2, CO reduces the flux of H2
1. Decrease in H2 partial pressure
2. Concentration polarization
3. Surface coverage – reduction in surface area
H2
Other Gas
Conc.
)( 5.0,
5.0, 222 permHretHH PPQJ
)()1( 5.0,
5.0, 222 permHretHH PPQJ
the fraction of membrane surface sites covered by
species other than hydrogen
Experimental Setup
2 “
H2 + Impurity
Retentate to GC
3/8” OD
Pd77Ag23 Membrane
Permeate H2 to GC (@ 1 atm)
Feed (varying concentrations)
1. Single Impurities
• H2 + CO
• H2 + CO2
• H2 + H2O
• H2 + CH3OH
2. Mixture of impurities
• H2 + CO + CO2 + H2O + CH3OH
Temperatures
• 225 oC
• 250 oC
• 300 oC
Retentate pressures
• 3 – 5 bars
Measured
• Decrease in H2 flux through membrane (compared to pure H2 feed)
Model Details
• Momentum balance (Navier-Stokes – weakly compressible)
• Mass balance (Stefan-Maxwell)
• Membrane poisoning (Langmuir adsorption)
i ii
T
MxT
p
UUUpUU
U
)].(32))(([).(
0).(
0)))((.(
p
pxxDU ijj
iijii
)()1( 5.0,
5.0, 222 permHretHH PPQJ
2-D, isothermal model
is the fraction of membrane surface sites covered by species other than hydrogen
JH2 - H2 flux thru the membrane Q – permeability of membrane P i – partial pressure of species i
Ki – adsorption coefficients of species I KI, KII – intermediate reaction constants -fraction of surface covered by non-H2 species
)(
)()()()()()(1
1)1(
2
33
2
33
3322222
2
21
21
31
HH
OHCHOHCHIII
HH
OHCHOHCHIOHCHOHCHCOCOCOCOOHOHHH
HH
PK
PKKK
PK
PKKPKPKPKPKPK
PK
Assumptions
• Steps (5), (10) & (12) are rate determining steps
• All other steps are in equilibrium
• Adsorption energies independent of surface coverage
• Multi-site adsorption model (Martinez & Basmadjian, Chem. Eng. Sci., 7
(1996) 1043)
1) H2 + 2S 2H-S
2) H2O + S H2O-S
3) CO + 3S CO-S3
4) CO2 + 2S CO2-S2
5) CO2-S2 + 2S CO-S3 + O-S
6) O-S + 2H-S H2O-S + 2S
7) CH3OH + 2S CH3OH-S2
8) CH3OH-S2 CH3O-S + H-S
9) CH3O-S + S CH2O-S + H-S
10) CH2O-S + S CHO-S + H-S
11) CHO-S +3S CO-S3 + H-S
12) 2CH3OH-S2 CH3O-S + CH3-S + H2O-S + S
13) CH3-S + H-S CH4-S + S
14) CH4-S CH4 + S
Surface mechanisms based on experimental results and previous studies reported in literature
S – surface site on Pd-Ag membrane
Proposed Surface Mechanism
Results – Experimental vs. Simulation
• Main cause of decrease in H2 flux is surface coverage / poisoning
– CO causes the largest decrease
• As temperature ↑ H2 flux ↑
• Drop in flux is reversible
Single impurity studies300 C, 5 bars
Results – Experimental vs. Simulation
• As temperature ↑ H2 flux ↑– except for CH3OH; maximum H2 flux at ~ 250 oC
• During CO2 experiments CO & H2O detected at outlet
• During CH3OH experiments CO detected at outlet (H2O also detected for certain conditions)
225 C, 3 bars
Single impurity studies
250 C, 3 bars
250 C, 5 bars
Results – Parameters Estimated
)exp(RT
HKK o
K0 H (kJ/mol)
KH2 3.33e-10 -58.46
KH2O 1.54e-10 -49.12
KCO2 3.67e-15 -106.2
KCO 6.38e-11 -88.42
KCH3OH 1.69e-16 -123.3
KI 3.77e+38 418.9
KII 4.44e+29 328.8
Estimated binding energies correspond well with literature
reported values
)(
)()()()()()(1
1)1(
2
33
2
33
3322222
2
21
21
31
HH
OHCHOHCHIII
HH
OHCHOHCHIOHCHOHCHCOCOCOCOOHOHHH
HH
PK
PKKK
PK
PKKPKPKPKPKPK
PK
Results – Gas mixtures
No major interactions between species
Temperature
(oC)
Pressure(bars)
CH3OH(mole
%)
CO(mole
%)
CO2
(mole %)
H2O(mole
%)
Experimental % drop in H2
flux
Simulated % drop in
H2 flux
225 3 20 3 10 20 89 ± 3 92.2
250 3 20 3 10 20 84 ± 3 90.2
250 5 20 3 10 20 76 ± 3 83.7
300 5 20 3 10 20 77 ± 3 84.4
225 3 5 10 25 5 85 ± 3 87.4
250 3 5 10 25 5 80 ± 3 86.4
250 5 5 10 25 5 78 ± 3 81.3
300 5 5 10 25 5 75 ± 3 79.8
Part 2
Methanol Steam Reforming in
PBR & Single-Fiber PBMR
Experimental Details – PBR & PBMR
• CH3OH : H2O 1:1 molar basis
• Catalyst BASF V1765
(CuO/ZnO/Al2O3 modified with ZrO2)
(avg. particle diameter 0.45 mm)
• Temperatures 225, 250, 300 oC
• Retentate pressures 3, 5 bars
Performance Metrics
1. CH3OH Conversion
2. Permeate H2 Purity
3. H2 Utilization %
4. H2 Productivity
flowrateOHCHInlet
flowrateHPermeate
3
2
3
32
ms
mol
volumereactor
flowratemolarHpermeate
4 “
CH3OH + H2O(1:1 Molar basis)
CuO/ZnO/Al2O3
Catalyst Bed
Retentate to GC
3/8” OD
Pd78Ag22 Membrane
Permeate to GC (@ 1 atm)
• Mass balance : Stefan Maxwell equation
– Incorporating membrane permeability inhibition factor (1-)
– Radial mass diffusivities : D.J. Gunn, Chem. Eng. Sci., 42(1987) 363
Model Details
2-D, non-isothermal, pseudo-homogeneous
y
yijji
ijii Rp
pxxDU )))((.(
• Heat balance : Convection + conduction
– Effective catalyst bed thermal conductivity + Wall to bed heat transfer
coefficient : Specchia et al, Chem. Eng. Commun., 4 (1980) 361
y
yypeff RHTUCTk .).(
• Momentum balance : Darcy-Brinkman equation
)( 2UPk
U
Model Details
))(1())()(1(
)1()(5.02
5.0*)1(
5.022
*)1(
5.022
*)1(
5.023
*)1(3
112323
25.023
*)1(3
HHHOHOHHCOHCOOHOHCHOCH
assOHOHCHSRCOHHOHCHOCHSRSR pKppKppKppK
CCppKeqppppKkR
))(1())()(1(
)1()(5.02
5.0*)2(
5.022
*)2(
5.023
*)2(3
2232
25.023
*)2(3
HHHOHOHHOHCHOCH
assOHCHMDCOHHOHCHOCHMDMD pKppKppK
CCpKeqppppKkR
))()(1(
)1()(5.022
*)1(
5.022
*)1(
5.023
*)1(3
21222
5.022
*)1(
HOHOHHCOHCOOHOHCHOCH
sOHCOWGSCOHHOHCOOHwWGS ppKppKppK
CppKeqpppppKkR
• Reaction Kinetics : Peppley et al, Appl. Cat. A: Gen., 179 (1999) 31
i) Methanol steam reforming
CH3OH + H2O 3H2 + CO2 dH= +49.4 kJ/mol
ii) Methanol Decomposition
CH3OH 2H2 + CO dH= +90.5 kJ/mol
iii) Water-gas shift
CO + H2O 3H2 + CO2 dH= -41.1 kJ/mol
• Reaction Kinetics : Peppley et al, Appl. Cat. A: Gen., 179 (1999) 31
i) Methanol steam reforming
CH3OH + H2O 3H2 + CO2 dH= +49.4 kJ/mol
Cs,i – concentration of active catalyst sites
Kinetic Parameters
obtained from PBR data
Membrane permeability
obtained from pure gas permeation results
Membrane Inhibition Factor
Obtained from experiments & modeling
PBMR Model
PBR ResultsExit Compositions
250 C, 5 bars
CH3OH+
H2O
CuO/ZnO/Al2O3
Catalyst
PBR ResultsTemperature profile at center of PBR
250 C, 5 bars
Kinetic Parameters
obtained from PBR data
Membrane permeability
obtained from pure gas permeation results
Membrane Inhibition Factor
Obtained from experiments & modeling
PBMR Model
PBMR ResultsExit Retentate Compositions
250 C, 5 bars
PBMR Results
)()2( 23
22 flowrateOHInletflowrateOHCHInlet
flowrateHPermeatenUtilizatioH
250 C, 3 bars
250 C, 5 bars
300 C, 5 bars Temp ↑, Utilization ↑
Press ↑, Utilization ↑
W/Fao ↑, Utilization ↑
H2 Utilization in PBMR
PBMR Results
32
ms
mol
volumereactor
flowratemolarHpermeatetyproductivi
250 C, 3 bars
250 C, 5 bars
300 C, 5 barsTemp ↑, Productivity ↑
Press ↑, Productivity ↑
Optimum W/Fao
Permeate H2 Productivity of PBMR
PBR vs. PBMRMethanol Conversion
PBR vs. PBMR undiluted catalyst3/8" OD
20
40
60
80
100
0 5 10 15 20 25
W/Fao (g. cat. hr/ gmoles)
Met
han
ol C
on
vers
ion
%
250 C, 3 bars
250 C, 5 bars
300 C, 5 bars
Solid lines – PBR
Dashed lines - PBMR
~ 5 - 15 % increase
Methanol Conversion
)()1( 5.0,
5.0, 222 permHretHH PPQJ
Fraction of membrane surface sites covered by species other than hydrogen300 C, 5 bars
PBMR – Simulation Results
Rate limiting step H2 flux through membrane Surface Poisoning
= 0.5) implies that 50 % of membrane surface
poisoned
300 C, 5 bars
PBMR – Simulation Results
Permeate H2 ProductivityOvercoming surface poisoning effects
Permeate Purity
• 100 % pure H2 obtained
• GC TCD detection limit ~10 ppm
• If separation factor is not infinite
Concentration Polarization also
severely affects purity
H2
Other Gas
Conc.
Part 3
3-D model to Simulate Large-Scale Multi-Fiber
PBMR
Large scale multi-fiber PBMR model
PBR & single-fiber PBMR model
Model for membrane inhibition factor
Multi-Fiber PBMR
z
y
x
x
y
H2 mole fraction
x
y
z
H2 mole fraction
Multi-Fiber PBMR Simulation Results
Comsol © software used for simulations
Reactor ID – 5”
Length – 6 m
Membrane OD -3.7mm
No. of fibers - 85
Inlet velocity – 1 m/s
Inlet Temp – 250 C
Wall Temp – 265 C
Temperature (K)
Conclusions
1. Membrane flux in reaction conditions
2. Experiments & 2-D model for single-fiber PBMR– Rate limiting step Membrane H2 flux
– Surface poisoning is the main cause of low membrane flux
3. 3-D model developed for large scale multi-fiber PBMRs– For understanding effects of design & operating parameters on productivity
)(*)1( 5.0,2
5.0,22 permHretHH PPQJ
))(
1)(()()()()()(1
)(1)1(
25.022
3315.0
333
13
1
22225.0
22
5.022
KPK
PKKPKPKPKPKPK
PK
HHOHCHOHCHOHCHOHCHCOCOCOCOOHOHHH
HH