Reduction and Start-up of Steam Reforming

Catalyst

By:

Gerard B. Hawkins Managing Director, CEO

Contents

Introduction Start-up procedures

• Warm-up • Catalyst reduction • Feed introduction

Case study

Introduction Steam reformer is complex

• heat exchanger • chemical reaction over catalyst • combustion, leading to steam generation

Common symptoms of poor performance • high exit methane slip • high approach to equilibrium • high tube wall temperature • high pressure drop

Need properly active catalyst

As supplied - NiO on support

Active species - Ni crystallites

Reduction process needed:

NiO + H2 Ni + H2O

Steam Reforming Catalyst

400 500 600 700 800 100

200

300

500

700

Temperature oC (oF)

Partial Pressure of H2O / Partial Pressure H2

Equi

libriu

m C

onst

ant

Reducing Conditions

Oxidizing Conditions

(752) (932) (1112) (1292) (1472)

Reduction of Bulk Nickel Oxide

NiO Reduction

Faster at high temperature

Slower in presence of steam

Thermodynamically, very little hydrogen needed

Support can affect ease of reduction

Catalyst Reduction Requires high temperature

• fire steam reformer Requires hydrogen

• supply H2 or reduce gas • re-circulation or once-through

Since little or no steam reforming is taking place, • less heat is required to warm up gas:

50% steam rate, with 5:1 steam: H2 ratio requires 1/7 fuel of normal operation

Extreme danger of local overheating!

Contents

Introduction Start-up Procedure

• Warm-up • Catalyst Reduction • Feed Introduction

Case Study

Warm-Up 1. Purge plant of air with N2 - must be free of hydrocarbons and carbon oxides 2. Heat reformer above condensation temperature 2. Add steam when exit header temperature 50oC (90oF) above condensation temperature - low pressure favours good distribution and

lowers this temperature 4. Increase steam rate to 40 - 50 % of design rate - min 30 % 5. Stop N2 circulation

Air warm-up possible, but not for previously reduced catalyst (possible carbon)

Warm-Up - Feedstock Isolation • Before a flow of steam is established in the

steam reformer, hydrocarbons must not be present – Carbon formation!

• Ensure that hydrocarbon feed lines are fully isolated – Double-block and bleed – Do not rely on block or control valves

• Or keep the pressure of the hydrocarbon feed supply below hydrogen plant start-up pressure

Traditionally 50oC (90oF) /hr Modern materials 100oC (180oF) /hr Catalyst 150 - 170oC (270 - 350oF) /hr

Warm-Up rates Rapid warm-up minimises energy usage/time Limited by mechanical considerations of steam

reformer Assess effect on plant equipment

• thermal expansion of inlet/exit pipes • steam reforming tensioners • steam reformer tubes • refractory linings

• Water can damage the steam reforming catalyst

• Temperature “shock” • Rapid drying of “wet” catalyst

•The expansion of water to steam in the catalyst pores causes catalyst break-up

• Pre-reforming catalyst much more sensitive to water

• Essential to avoid condensation

Warm-up - Avoiding Condensation

Steam Reformer

Cold Pipework

Steam

If upstream pipe work is cold, then it is good practice to warm up by steam flow with vent to prevent carry-over of water

Warm-up - Avoiding Condensation

To Vent

Warm-up - Condensation

Ensure that all drain points are operational

To steam reformer

Steam

Condensate to drain

Temperatures Temperatures referred to are true catalyst

temperatures at exit of tube Measured temperatures during normal operation

are 10 - 100oC (18 - 180oF) cooler due to heat losses Most catastrophic failures of tubes in top-fired

furnaces occur during start-up Cannot rely on plant instrumentation during start-

up • lower flows than normal • higher heat losses than normal • fewer burners can give severe local effects

Frequent visual inspection of reformer tubes and refractory is essential during start-up

Effect of Pressure and Temperature

800 900 1000 1,100 1,200 1

10

100

1,000

10,000

100,000

1,000,000

10,000,000

Tube Wall Temperature oC (oF)

( 1500 ) ( 1650 ) ( 1830 ) ( 2010 ) ( 2200 )

30 bar 5 bar

Steam Reformer Tube Life

Contents

Introduction Start-up procedure

• Warm-up • Catalyst reduction • Feed introduction

Case study

Reduction Procedures Reduction with hydrogen Reduction with natural gas

Reduction with other sources of hydrogen

• higher hydrocarbons • ammonia (not discussed) • methanol (not discussed)

Reduction with Hydrogen

H2 or H2-rich gas can be added at any time to the steam when plant is free of O2

Steam: hydrogen ratio normally 6:1 - 8:1 Get tube inlet temperature as high as

possible Increase exit temperature to design value

(>700oC/1292oF) Hold for 2-3 hours

Hydrogen Source

Hydrogen must be • free of poisons (S, Cl)

Special consideration must be given to the

presence in impure hydrogen sources of • carbon oxides • hydrocarbons

Also applies to nitrogen (or inert) source used for purge/warm-up

Carbon Oxides Re-circulation loop may include HDS unit

(at temperature) Carbon oxides above 250oC (480oF)

methanate over unsulfided CoMo catalyst • temperature rise 74oC (133oF) per 1% CO

converted • temperature rise 60oC (108oF) per 1%

CO2 converted

If H2 contains >3% CO or >13 %CO2 or a mixture corresponding to this, then by-pass the HDS system

Hydrocarbons

May be converted to carbon oxides in reformer

May crack thermally to give carbon

Reduction with Natural Gas

1. Warm-up as before 2. Introduce natural gas at 5% of design rate 3. Slowly increase gas rate to give 7:1 steam: carbon over 2-3 hours 4. Simultaneously increase reformer exit temperature to design level (>700oC/1292oF) 5. Increase inlet temperature as much as possible (to crack natural gas to hive H2) 6. Monitor exit methane hourly: reduction complete when methane reaches low, steady value (4 to 8 hours)

Reduction with Higher Hydrocarbons

Increased possibility of carbon formation Great care needed Longer time periods needed More precision in all measurements

needed Hydrogen addition recommended Purification issues

Only use if absolutely necessary

Contents

Introduction Start-up procedure

• Warm-up • Catalyst reduction • Feed introduction

Case study

Feedstock Introduction 1. Introduce feedstock at high steam: carbon

ratio (5:1 for natural gas; 10:1 for higher

hydrocarbons) 2. Steam reforming will give small increase in

inlet pressure, cooling of tubes, and lower exit temperature

3. Need to increase firing to maintain exit temperature 4. Then increase feedstock flow 5. Increase pressure to operating pressure 6. Adjust steam: carbon ratio to design

Feedstock Introduction Increase flow of natural gas to design steam:

carbon ratio (2 hours) Maintain exit temperature Check that exit methane stays low

• (reducing steam: carbon ratio will increase methane slip and heat load) if not, hold at 7:1 steam : carbon for 2 hours

Increase throughput to design level Increase pressure to design level

Always increase steam rate before feed rate

Steam Reformer Re-starts

Shorter re-reduction recommended • typically 4-6 hours for heavy feeds

Not essential to carry-out reduction with Natural gas or light off-gas feedstock

• start-up at 50% design rate, high steam: carbon ratio

Contents

Introduction Start-up procedure

• Warm-up • Catalyst reduction • Feed introduction

Case study

Case Study: Overfiring

Large modern top-fired steam reformer Significant tube failures during start-up Caused by over-firing at start-up due to a

number of coincident factors

Case Study: Background

Site steam shortages requiring conservation of steam

Pressure to avoid a shut-down (due to low product stocks)

Burner fuel usually from two sources, mixed: • one low-calorific value • one high-calorific value

At time of incident, all high-calorific value (unexpectedly) fuel received

Operators had seen many shut-down/start-ups during past two years

Case Study: Events

Plant trip (loss of feedstock to steam reformer) due to valve failure

Feedstock to steam reformer not isolated adequately by valve

Setpoint on reformed gas pressure not reduced Steam introduced for plant restart at reduced

rate All burners lit (deviation from procedure)

Reformer tubes remained at normal operating pressure of 16 barg (232 psig)

Case Study: Events (Contd.) Steam reformer tubes “looked normal” Nearly 3x as much fuel going to burners than there should

have been High calorific value fuel added an extra 15% heat release First tubes rupture High furnace pressure (trip bypassed) Oxygen in flue gas dropped to zero Flames seen from peep holes Normal furnace pressure Visual inspection revealed “white hot furnace” and tubes

peeling open

30 m

inut

es

Emergency Shutdown Activated!

Case Study: Conclusions Reformer exit gas temperature on panel never

exceeded 700oC (1290oF) • cannot use this instrument as a guide to tube

temperature Reformer start-up at normal operating pressure

• tube failure temperature 250oC (450oF) lower than normal for start-up

All burners lit • far too much heat input resulted in excessive

temperatures

Summary

Start-up procedures • Warm-up

Feedstock isolation Rate Condensation True temperatures/Tube temperatures

• Catalyst reduction Using hydrogen Using hydrocarbon Feed introduction

• Case Study