steam reforming - a comprehensive review
DESCRIPTION
Equilibrium, Approach & Kinetics Carbon Laydown Potash Doping Catalyst Loading Pigtail Nippers GHR & AGHR Secondary Reforming Metal DustingTRANSCRIPT
www.gbhenterprises.com
Gerard B. Hawkins Managing Director
Equilibrium, Approach & Kinetics Carbon Laydown ◦ Potash Doping
Catalyst Loading Pigtail Nippers GHR & AGHR Secondary Reforming Metal Dusting
Equilibrium, Approach and Kinetics
Steam Reforming limited by Heat transfer Catalyst activity Kinetic rate Equilibrium
Approach to Equilibrium
770 780 790 800 810 820 2
4
6
8
10
12 M
etha
ne s
lip (%
) Gas Exit T Eq'm T
ATE
(1418) (1454) (1436) (1472) (1490) Temperature Deg C (Deg F)
Exit CH 4
Approach to Equilibrium CH4 + H2O <=> CO + 3H2
Approach Tms = Actual T gas - EquilibriumT gas (A.T.E.)
measured calculated •Measure of catalyst activity
–If ATE = O, system at equilibrium
–As catalyst activity decreases, ATE increases
Kinetics
•Reversible action, zero rate at equilibrium
•Reforming reaction very fast
•Limited to pellet surface only-diffusion limit
•Depends upon catalyst GSA
•Depends upon gas composition
•Exponential with temperature
•C2, C3 & C4 considered as not reversible
Diffusion Limitation inactive active
Pore
Catalyst pellet
CHH OHCOCO
422
2
CO H2 CO2
nickel crystal
Steam Reforming Catalysis Key Reaction Steps
1. Fast - Diffusion of the molecules in the bulk gas phase
2. Slow - Diffusion of the molecules through the gas film
3. Slow - Diffusion through catalyst pores
4. Fast - Absorption of the molecules onto the Ni sites
5. Fast - Chemical reaction to produce CO and H2
Kinetics - Natural Gas
•Kinetic model used for natural gas feeds
•Methane kinetics well validated
•C2 ,C3, & C4 kinetics ok for natural gas tail only
•NOT to be used for LPG feeds
•Kinetics for steam reforming reactions
•Shift reaction taken to be at equilibrium
•Shift very fast compared to reforming
Kinetics - Natural Gas
•Form of equation as below:
R[CH4] = K.GSA.Ract.exp(T).P[CH4].(Kp’-Kp)
P[H2O]
R[CH ] = Rate of methane reaction T = Temperature K = Constant P[X] = Partial pressure Ract = Catalyst relative activity Kp' = Equilibrium constant of gas GSA = GSA Kp = Equilibrium constant at T
Geometric Surface Area
• GSA for short –Area per unit volume –Typically 200-500 m2/m3
• Important as apparent activity is a very strong function
• Function of –shape –size –number
Kinetics
•Major points of interest
–Steam is a poison
–Kp’- Kp can be changed to approach
–GSA term is included
–Relative activity term is included
Kinetics •For higher hydrocarbons
•R[C2H6] = K.GSA.Ract.exp(T).P[C2H6]
•R[C3H8] = K.GSA.Ract.exp(T).P[C3H8]
•R[C4H10]= K.GSA.Ract.exp(T).P[C4H10]
•Simple first order kinetics-non reversible
•Hence limit to natural gas tail only
Catalyst Types
VSG-Z101: Gas reforming
Catalyst Applications
• VSG-Z101
–Other light feeds - refinery offgas –Light duties e.g. side fired reformers –Use up to butane at 4.0 S:C ratio –Use in top fired reformers at tube top
Steam Ratios for Catalyst/Feedstock Combinations
Feedstock Natural Gas Reforming
Non- alkalised
Associated Gas Ref
Lightly alkalised
Dual Feedstock Reforming Moderately alkalised
Naphtha Reforming
Heavily alkalised
Non-alkalised Low alkali Moderate alkali High alkali Naphtha 3.0-3.5
Light Naphtha 6.0-8.0 3.0-4.0 2.5-3.0 Butane 4.0-5.0 2.5-3.5 2.0-3.0
Propane, LPG 3.0-4.0 2.5-3.0 2.0-2.5
Refinery Gas 6.0-10.0 3.0-4.0 2.0-3.0 2.0-2.5
Associated Gas
5.0-7.0 2.0-3.0 2.0-2.5
Natural Gas 2.5-4.0 1.5-2.0 1.0-2.0
Pre-reformed Gas
2.0-3.0 1.0-2.0 1.0-2.0
Pre - reduction
• This will maximize the activity of a catalyst • The start up will be easier and quicker • Catalyst should remain more active at
tube top • Useful for low inlet temperature reformers
Catalyst Support - Reduction Temperatures
Alp
ha A
lum
ina
Cal
cium
Alu
min
ate
Mag
nesi
um A
lum
inat
e Sp
inel Temperature
(Deg F)
Temperature (Deg C)
800 1000 1200 1400 1600
400 500 600 700 800 900
Magnesium Aluminate spinel material usually supplied pre-reduced
Carbon Laydown
Carbon Formation
•Carbon formation formed by side reactions
•Totally unwanted due to damage caused
•Catalyst break up and deactivation
•Catalyst tubes overheated - hot bands
–Premature tube failure
–Catalyst activity reduction
–Pressure drop increases
Carbon Formation
•Carbon forms when
–Steam ratio is too low
–Catalyst has too little activity
–Higher hydrocarbons are present
–Tube walls are hot - high flux duties
–Catalyst has poor heat transfer coefficient
Carbon Formation - Types
•Carbon cracking
•Boudouard
•CO Reduction
Carbon Formation
•Cracking
–CH4 <=> C + 2H2
–C2H6 => 2C + 3H2 etc
•Boudouard
–2CO <=> C + CO2
•CO Reduction
–CO + H2 <=> C + H2O
Heavier Feedstocks
•Steam reforming
–Not practical to increase steam to carbon ratio using gas reforming catalysts
–Carbon formation more problematic
Need promoters to limit carbon and/or increase its removal once formed
Carbon formed not just by cracking but also by polymerisation of intermediates
Naphtha Carbon Formation
Alkalized Catalysts
Heavier Feedstocks •Carbon removal (heavy feed reforming)
–Potash (alkali) incorporated into catalyst support
Inhibits cracking rate
Accelerates carbon gasification
Needs to be “mobile” to remove carbon on the inside tube wall surface - Potash released by complex chemical
Release reaction controlled by temperature
Required only in the top section of the reformer tube - Lower section of catalyst absorbs liberated potash
Heavier Feedstocks •Potash Addition (Heavy feed reforming)
–Reduces catalyst activity
Need extra Ni
–MgO/NiO solid solution
Low Polymerization activity - reduces carbon formation
MgO must be “fixed” so as to avoid hydration of the “free” MgO to magnesium hydroxide - weakens catalyst pellet severely (cannot steam catalyst!)
Carbon Formation
•Use of potash to prevent carbon formation
•Increases the rate of carbon removal
•Does not stop the formation from cracking
•Potash catalyzes the rate of steam gasification
•Balance of kinetics altered to favour removal
•Can steam with care
Carbon Formation and Prevention
Increasing potash addition
Methane feed/Low heat flux
Methane feed/High heat flux Propane, Butane feeds (S/C >4)
Propane, Butane feed (S/C >2.5) Light naphtha feed (FBP < 120°C)
Heavy naphtha feed (FBP < 180°C)
K2O wt% 0
2-3
4-5
6-7
Role of Alkali - Lightly Alkalized For light feeds and LPG etc using lightly alkalized catalyst
–Potash is chemically locked into catalyst support
–Potash required only in top 30-50% of the reformer tube
Catalyst life influenced by
–Poisoning
–Ni sintering
– Process upsets etc
Lightly alkalized
Non-alkalized
Carbon Formation and Prevention
Top Fraction Down Tube Bottom
Non-AlkalisedCatalyst
Ring Catalyst
Optimised Shape(4-hole Catalyst)
Inside Tube WallTemperature
920(1688)
820(1508)
720(1328)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
AlkalisedCatalyst
Carbon FormingRegion
Optimized Shape VSG-Z101
Potash and Activity
•However, potash is a catalyst poison
–Potash does reduce activity
•At low levels 2%, the effect is minimal
•Use a gas type catalyst in bottom of tube
•Use naphtha catalyst in top of tube for carbon
•Therefore no carbon and low exit approach
Potash Levels in Heavy Feed Steam Reforming Catalyst
0 20 40 60 80 1000
1
2
3
4
5
6
7
% Down Reformer Tube
wt% Potash
2 year
1 year
0 years
Potash Promoted Non-akalised
Must watch the interface temperature At >650°C Potash leaching too high Leads to ◦ Fouling of WHB etc ◦ Loss of carbon resistance ◦ Hot banding etc
Naphtha Loading
02468
10121416
180 220Final boiling point Deg C
% R
educ
tion
in n
apht
ha
load
ing
% reduction
200 210 190
Naphtha Loading
0
5
10
15
20
25
30
10 20Aromatic content (%)
Red
uctio
n in
nap
htha
load
ing
12 14 16 18
When to Use and Why
• Used when major feedstock variations – NG to LPG
• Feedstock flexibility – When 650oC limit is reached
• Problem of Potash Leaching
Catalyst Loading
Sock Loading - Measurements
• Key part of charging procedure • Aim to pack catalyst to uniform voidage • Measure pd
– Not outage in tube at any one time – Not weight per tube – Not catalyst density – After 50% – After full loading
• Use defined and consistent procedure throughout
DP Measurement
• Use VSC Pressure Drop Rig • If too high then
– suck out catalyst and recharge • If too low then
– Vibrate tube – Top up if outage too great
Pressure Drop Measurement
• Fixed flow of air (choked flow through orifice) • Mass flow rate through orifice function of
– Upstream pressure – Orifice diameter (known) – Temperature (known)
• Downstream pressure is measure of pd
Measurement of Pressure Drops
PD rig
Inlet pigtail
Exit pigtail
4a. Exit pigtail
Empty tube
PD rig
4b. Catalyst
catalyst
PD rig
4c. Inlet pigtail
catalyst
Digital Pressure Drop Instrument
Sock Loading - Vibration
• Electric or pneumatic vibrators – rotating cams are noisy
• Soft-faced shot-filled hammer –need consistent blows
“UNIDENSE” Method • Developed by Norsk Hydro
– licensed to a number of organisations • Tried in a number of plants
– ammonia, hydrogen and DRI • Leads to “denser” packing
– less pd variation • more uniform gas flows
– easier procedure • shorter loading time (70%)
– slightly higher pd • effect on throughput?
Pigtail Nippers
Pigtail Nipper
This hydraulic device designed by ICI is operated at a safe distance from the leaking tube and squeezes the pigtail flat with the plant still operating.
Allows furnace to stay on line.
No thermal cycle.
Pigtail Nipper
Pinched Pigtail with Clamp in Place
Pinched Tube in Steam Reformer
Row of steam reformer tubes
Pinched tube
Gas Heated Reformer (GHR) Advanced Gas Heated Reformer (AGHR)
GHR Based Reforming
Secondary Reformer
Steam + Gas
Air / Oxygen
GHR
LCM Flowsheet
Purifier
Saturator GHR Secondar
y Converter
Preheater
Purge to fuel
Topping Column
Refining Column
Process condensate
water
Fusel oil
Natural gas
Oxygen Steam
Refined methanol
Purge
Crude methanol
GHR History
•Developed for ammonia process - LCA •Early 1980's - Paper exercise •Mid 1980's - Sidestream unit at Billingham •Mid 1980's - LCA design developed •Late 1980's - ICI Severnside plants start up •1991 - BHPP LCM plant designed •1994 - BHPP plant start up •1998 - AGHR Start Up •1998 - MCC Start Up
GHR Shellside Design
•Shellside heat transfer usually poor •Minimize tube count with expensive alloys •Tubes are externally finned •Designed as double tubes - Sheath tube •Produces much smaller tube bundle •Allows scale up to higher capacities
GHR Tubesheets Gas/Steam Hot
gas Twin tubesheets
Refractory
Syngas
GHR and Secondary Arrangement
Normal Operating Conditions
SecondaryReformer
GHR
Syngas
Gas/steam425`C
701`C
975`C
515`C
742`C
21,000 Nm3/Hr
Oxygen30`C
1200`C
2,590 Nm3/Hr
43.7 Barg 39.2 Barg
38.6 Barg
37.9 Barg
22.0% Methane
16.6% Methane
0.4% Methane
40.6 Barg
LCA GHR
LCM GHR Internals
Advanced GHR
-Shellside heat transfer enhancement
-Non bayonet design
-Hot end tubesheet
-Sliding seal system
Uprate Capabilities
•GHRs can be used in parallel to existing primary reformer •Potential to uprate capacity by 40% •Severely impacts steam system •Most applicable to hydrogen plants •No changes to radiant or convection sections of reformer / fans burner etc. •New WHB may be required •Rest of plant must be uprated
Fluegas Heated Reformers
FHR
Combustion chamber
Natural gas & Steam
Fuel
Air
Fluegas to stack
Combustion air compressor
Fluegas recycle compressor
Fluegas power recovery turbine
Syngas
GHR
Secondary Reforming VSG-Z201/202/203
Keys to Good Performance
•Burner Design •Mixing Space •Catalyst
Poor Mixing Performance
•Good mixing is absolutely essential
•Poor mixing in combustion zone gives high approach and high methane slip
•Problem is poor mixing can not be differentiated from poor catalyst performance UNLESS thermocouples are in the bed.
•Bed temps will show divergence •Bed temps will show odd behaviour
Catalyst Bed Sizing
•Based on a space velocity technique •Wet Gas Space Velocity (WGSV) •Uses exit flow with steam included •See attached graph •Modify space velocity by catalyst GSA •See table for relative catalyst GSA
WGSV Chart
0
5
10
15
20
25
0 2000 4000 6000 8000 10000Wet Gas Space Velocity (Nm³/m³)
App
roac
h (°
C)
Two types LCM style ◦ Single ‘pipe’
Ring burner
Metal Dusting
Metal Dusting Key features
•Catastrophic carburization •Occurs at "low" temperatures 700 - 450°C •Induction period sometimes required •Often local pitting - pits coalesce •Can have general corrosion •Can be very rapid 3mm/year •Carbon formation occurs
Metal Dusting
Mechanism of Metal Dusting Initiation
•Gas has propensity to deposit carbon 2 CO => CO2 + C (Boudouard) CO + H2 => H2O + C (CO Reduction) •Oxide film breakdown exposes active Fe, Ni, Co sites •Carbon deposits at active sites
Metal Dusting and GBHE
•GBHE have great experience •GBHE have a solution
•Proven to work in operation