Download - Steam reforming - The Basics of Reforming
C2PT Catalyst Process Technology
By Gerard B Hawkins Managing Director
Steam Reforming Catalysis : ◦ Chemical reactions
◦ Catalyst shape design
◦ Catalyst chemistry
◦ Carbon formation and removal
The conversion of hydrocarbons to a mixture of CO, CO2 and H2 Two reactions: Reforming and Shift
Steam Reforming (very endothermic)
CH4 + H2O CO + 3H2
CnH2n+2 + nH2O nCO + (2n + 1)H2
Water gas shift (slightly exothermic)
CO + H2O CO2 + H2
Overall the reaction is highly endothermic
Both reforming and shift reactions are reversible
Rate of shift is fast compared to reforming
Methane conversion favored by: – low pressure – high temperature – high steam to carbon ratio
Steam
SecondaryReformer
Steam
Steam + Gas
SteamReformer
Air / Oxygen500°C
780°C
450°C
1200°C
950°C
10% CH4 0.5% CH4
The primary reformer is a heat exchanger Its function is to heat up process gas Catalyst and reaction in the tubes Combustion on the shell side Dominant heat transfer by radiation
0 0.2 0.4 0.6 0.8 1200
300
400
500
600
700
800
900
fraction down tube
tem
pera
ture
(°C
)
gas temp
Eq tempATE
Nickel on a ceramic support Three key factors in catalyst design:
– geometric surface area – heat transfer from tube to gas – pressure drop
Also of concern: – packing in the tube – breakage characteristics
Top Fired Reformer
0 0.2 0.4 0.6 0.8 1660680700720740760780800820840860
fraction down tube
tube
wal
l tem
pera
ture
(°C
)
base case
base case with twice GSA
base case with twice heat transfer
Outside tube wall temperature 830°C
Bulk ProcessGas Temp.715°C
1200°CFluegas
Inside tube wall temperature 775°C
Gas film
Tube Wall
Need to minimize thickness of gas film at tube wall
Smaller catalyst particles improve heat transfer from wall to bulk gas and reduce tube temperatures
Smaller particles increase pressure drop
Catalyst shape should be optimized for high heat transfer with low pressure drop
The traditional catalyst shape is a ring
Smaller rings give high activity and heat transfer but higher pressure drop
Optimized catalysts offer high surface area and heat transfer with low PD
Important that shape also provides good packing and breakage characteristics
Relative Pressure DropRelative HTC
Voidage
1 0.9 0.9 0.8
1 2 3 4
1 1.3 1.1 1.0 0.49 0.6 0.58 0.59
1 2 3 4
Design of catalyst shape is a complex optimization of:
– Higher surface area (needed for activity -
diffusion control) – Higher heat transfer (needed for cooler
reformer tubes) – Lower pressure drop (efficiency consideration)
Need also to consider breakage characteristics and loading
pattern inside the reformer tube
Catalyst loading can be improved using various dense loading techniques
Carbon formation is totally unwanted
Causes catalyst breakage and deactivation
Leads to overheating of the tubes
In extreme cases carbon formation causes a pressure drop increase
Carbon Formation and Prevention
Giraffe Necking
Hot Tube Hot Band
Reformer tube appearance - Carbon laydown
Cracking – CH4 C + 2H2 – C2H6 2C + 3H2 etc
Boudouard – C + CO2 2CO
Gasification
– C + H2O CO + H2
Under normal conditions carbon gasification by steam and CO2 is favored (gasification rate > C formation rate)
Problems of carbon formation occur when: – steam to carbon ratio is too low – catalyst is not active enough – higher hydrocarbons are present – tube walls are too hot – catalyst has poor heat transfer characteristics
Use of a potash doped catalyst reduces probability of carbon formation
Methods of preventing carbon formation:
– Use more active catalyst – Use better heat transfer catalyst – Reduce level of higher hydrocarbons – Increase the steam ratio – Use VSG-Z102 (3-7) -hole tailored catalysts
catalyst (potash-promoted)
Alkali greatly accelerates carbon removal
Addition of potash to the catalyst support reduces carbon formation in two ways: a increases the basicity of the support
b promotes carbon gasification
Potash is mobile on the catalyst surface
Potash doped catalyst is only needed in the top half of the reformer tube
C + H2O CO + H2 OH -
Increasing the content of alkali (potash)
– Higher heat flux possible for light feeds – Heavier hydrocarbons can be steam reformed – Lower steam to carbon ratios – Faster carbon removal during steaming
Fraction Down TubeTop Bottom
Non-AlkalisedCatalyst
Ring Catalyst
Optimised Shape(4-hole Catalyst)
Inside Tube WallTemperature
920 C(1688 F)
820 C(1508 F)
720 C(1328 F)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
AlkalisedCatalyst
Carbon FormingRegion
O
OO
OO
O
For light feeds and LPG etc using lightly alkalised catalyst VSG-Z101 – Potash is chemically locked into catalyst
support – Potash required only in the top 30-50% of the
reformer tube
– Catalyst life influenced by Poisoning Ni Sintering Process upsets etc
VSG-Z101
VSG-Z102
0
0.5
1
1.5
2
2.5
3
1.2m 3m 5m 6m 9mCatalyst samples at various depths down
reformer tube
Fresh1 year2 years4 years6 years
wt% of potash
VSG-Z102
VSG-Z102
Requirements : ◦ High and stable activity ◦ Low pressure drop ◦ Good heat transfer ◦ High resistance to carbon ◦ High strength ◦ Robust formulation/simple operation
Best achieved with VSG-Z101 (3-7) -hole tailored catalysts