Normal Operation of Steam Reformers on Hydrogen Plants

Normal Operation of Steam Reformers on Hydrogen

Plants By:

Gerard B. Hawkins

Managing Director, CEO

Contents

Typical controlled variables Plant data analysis Approach to equilibrium Prediction of remaining catalyst life Tube wall temperature measurement

Typical Controlled Variables

Process gas exit temperature Process gas and steam inlet temperature Steam/carbon ratio Process pressure Furnace parameters

• Air preheat temperature • Excess air

Catalyst Activity

Plant Rate

Exit Pressure

-1 bar +1 bar

Exit Temp(oC)

-10 -20

+20 +10

Steam Ratio

-10% -8%

+8% +10%

Reformer Optimization : Hydrogen Reformer (Top-Fired) Exit Temperature 856oC (1573oF)

Note relatively small changes in exit temperature or steam to carbon ratio can have significant effect on exit Methane slip Catalyst activity has relatively less impact

Catalyst Activity 40%

150% 200 %

Exit Temp(oC)

+10 -20

Steam Ratio

Exit Pressure

Plant Rate

90% 80%

Reformer Optimization : Hydrogen Reformer

(Top-Fired) Exit Temperature 856oC (1573oF)

Catalyst activity has relatively more impact on methane-steam approach to equilibrium temperature

o C (o

Catalyst Activity 40%

200% 60%

Plant Rate

110% 90% 80%

Exit Temp(oC)

Steam Ratio

(Small effect)

890 (1634)

880 (1616)

870 (1598)

860 (1580)

850 (1562)

Exit Pressure

(Small effect)

Reformer Optimization : Hydrogen Reformer

(Top-Fired) Exit Temperature 856oC (1573oF)

If exit temperature remains constant, then catalyst activity has relatively more impact on maximum tube wall temperatures

Monitoring Operations Furnace Inspection

• tube appearance • refractory condition

external hot-spots • burns

flame characteristics Steam reformer exit temperature measurement

• subheader/pigtail temp, measurements burner trimming

Feedstock purification performance sulfur/chlorides etc

Hot Band Hot Tube Settling Giraffe Necking

Tiger Tailing

Reformer Tube Appearance

Contents

Plant Data Analysis Important to cross-check measured data

• gas compositions inlet steam reformer exit steam reformer exit shift reactors(s)

• pressures/temperatures at these points • flowrates

recycle hydrogen hydrocarbon feedstocks steam (need also steam/BFW HTS feed

quench) fuel & air

Plant Data Analysis

Match measured plant data with heat/mass balance • if good match, then data accurate • if poor match, then errors in plant data

Total plant data computer fitting program • can use product rates and compositions etc for

cross-checking of data • can suggest likely sources of measurement error

Plant Data Analysis Total plant data fitting

• CO conversion across shift converter(s) temperature increase very accurate due to

multiple thermocouples cross-checks CO analysis AND steam rate

• Product rate/composition (methanator exit or PSA product and offgas) cross-checks feed rate, steam rate and

methane in reformer exit analysis • Methanator temperature rise

cross-checks CO slip from LTS and CO2 slip from CO2 removal system

Steam Reformer Feed flow (Nm3/hr) Steam flow (tonne/hr) Exit gas temperature (oC) Exit gas composition (mol % dry)

H2 N2 CH4 CO CO2

Exit gas flow (Nm3/hr) Steam : dry gas ratio Equilibrium temperature (oC) Approach to M/S equilb.(oC) Steam : carbon ratio

Measured Value 1975

65.27 -

4.65 9.02 21.05

Best Fit Value 2459

71.37 0

3.23 8.63 16.77 8634

1.1745 755.5 9.5

Percentage Error 24.5

-9.3 -

30.5 4.3 20.3

Plant data Verification - Poor Fit

Plant Data Verification - Poor Fit

Poor fit Areas to check

• feed flowrate • exit methane • exit CO/CO2

Feed flowrate originally quoted as 1.156 tonne/hr naphtha - Revised to be 1.59 te/hr naphtha

Plant Data Verification - Revised Fit

H2 N2 CH4 CO CO2

Measured Value 2644

65.27 -

4.65 9.02 21.05

Best Fit Value 2554

71.33 0

3.23 8.68 16.76 8954

1.1384 758.1 0

Percentage Error -3.4

-9.3 -

30.4 3.8 20.4

Plant Data Verification - Revised Fit Better fit for flowrate Significant error still on reformer exit gas

analysis CH4 CO/CO2

Methane slip originally quoted as 4.65 mol %(dry) - Revised to 3.56 mol % (dry)

Plant Data Verification - Final Fit

H2 N2 CH4 CO CO2

Measured Value 2644

69.86 -

3.56 8.24 18.34

Best Fit Value 2554

71.33 0

3.23 8.68 16.76 8954

1.1384 758.1 0

Percentage Error -3.4

-2.1 -

9.4 5.3 8.6

Plant Data Measurement - Problem Areas

Sampling/analysing exit gas compositions Exit temperature from reformer Flow measurement

Exit Gas Composition

CO shift reaction can occur if not quench cooled quickly

CO2 may dissolve in water • dry gas analysis!

Analysis of sample must be taken in the same time frame as the process data recording

Exit Reforming Catalyst

(mol % dry)

"Shifted" Sample Analysis

(mol % dry) CH4 4.4 4.2 CO 13.8 10.3 CO2 8.6 11.4 H2 71.9 72.8 N2 1.3 1.3

CO>CO2 CO<CO2

“Shifting” in Gas Sample

Note also reduction in CH4

Exit Temperature

Heat/mass balance requires temperature exit catalyst

Plant temperature measurement often at inlet to waste heat boiler • large heat losses possible

outlet pigtails, headers, transfer mains

Top-fired : 10-20oC (18-36oF) heat loss

Side-fired : 25-35oC (45-63oF) heat loss (Air ingress at base of steam reformer can lead to further cooling)

Note that hydrocarbon composition variations may effect the metered accuracy and also the

steam/carbon ratio calculation

Flow Measurement

Hydrocarbon feedstock generally high accuracy • “costing” meter • multiple feed streams may be less accurate

Steam flow often less accurate • error in steam/carbon ratio can have a

significant effect on heat/mass balance

Plant Data Analysis

Best to record trends • relative changes partially remove

measurement errors Monitor monthly/quarterly

• measures of catalyst activity methane slip assuming constant operating conditions

• approach to equilibrium • tube wall temperature

Plant Data Analysis

0 10 20 30 40

Months on line

Plant Data Analysis N

atural Gas R

ate (x1000 N

m3/hr) 8

Contents

Approach Tms = Actual T gas - Equilibrium T gas (A.T.E.)

Measured Calculated

• Measure of catalyst activity • If ATE = O, system at equilibrium • As catalyst activity decreases, ATE increases

Approach to Equilibrium CH4 + H2O CO + 3H2 ⇔

Calculation of Approach to Equilibrium

1. Take gas samples and record steam reformer exit temperature 2. Calculate wet reformer exit composition - Hydrogen atom molar balance (inlet/exit) - Calculate steam in exit gas - Convert exit dry gas to wet gas composition 3. Calculate equilibrium temperature corresponding to this exit composition - Use tables or equations 4. Calculate approach to equilibrium

Contents

Case Study

Terraced wall reformer How much longer will catalyst last (from

Jan’08) Change-out when?

• September ‘08 • April ‘09 • September ‘09

11/Apr/06 03/Oct/06 27/Mar/07 18/Sep/07 12/Mar/08

Outlet Temperature Methane Slip

Steam Reformer Performance

GBH Enterprises Ltd.

10/Feb/04 22/Dec/05 02/Nov/05 12/Sep/06 24/Jul/07 04/Jun/08 0

Design EOR

Design SOR

Catalyst On- line: Oct ‘02

01/Apr/03

Catalyst Performance Monitoring

01/Apr/05 26/May/06 20/Jul/07 12/Sep/08 06/Nov/09

Design temperature

Tube wall temperatures (Top)

Tube wall temperatures (Bottom)

Tube Wall Temperatures Tu

0 0.2 0.4 0.6 0.8

Fraction down Tube (%)

Tube Wall Temperature Process Gas

Delta T

Tube Wall Temperatures

Bottom minus Top

01/Apr/06 26/May/07 20/Jul/08 12/Sep/08 06/Nov/09

-9oF/year

Tube wall Temperatures

June 06 June 07 June 08 Sep 08 Sep 09

Exit CH4 (mol% dry) 7 7 7 7 7

Exit Temp oC (oF)

787 (1432)

789 (1452)

795 (1463)

Max Tube Temp oC

(oF) 829

(1524) 831

(1528) 838

(1540) 838

(1540)

M/S Equilib. Approach oC (oF)

10 (18)

12 (22)

13 (23)

14 (25)

15 (27)

Steam Reformer Data

Looks OK to September ‘09 BUT……..….

0 0.2 0.4 0.6 0.8 1 500

Fraction from inlet of tube

Carbon Formation Catalyst ageing

New catalyst

Carbon Formation

Activity Decay Factor Need to consider carbon formation

• Accurate model of catalyst activities needed to correctly simulate catalyst ageing

Take data at different times and calculate relative activity • for terraced wall reformer

(i) top 30% slowly poisoned (ii) middle 30% very slowly poisoned (iii) bottom 40% sinters very slowly

(i) and (ii) account for delta T (iii) accounts for increased approach

Jan 02

May Sep Jan 03

May Sep Jan 04

May Sep Jan 05

May Sep 0

Today September ‘04

September ‘05

Carbon margin

Carbon Margin with Time

Activity (arbitrary)

Time (years)

Carbon forming region

Initial sintering

"Stable" activity

Margin

Period where carbon can be formed at anytime due to variation in process conditions

Catalyst Deactivation (Schematic)

Conclusions #1

In terms of M/S Approach and Tube Wall Temperatures, can run till September ‘05

Concern about carbon margin from April ‘05 onwards • options

change April ‘05 - CHOSEN OPTION OR run with spare on site and change

September ‘05

Conclusions #2

• Sometimes difficult for operator to predict change-out requirement – Couldn’t rely on M/S Equilibrium Approach

and Tube Wall Temperature trending – Needed complex reformer simulation

• HOWEVER, recording of historic data from start-of-run conditions allowed accurate assessment by the catalyst vendor – Take data from SOR!

Contents

Importance of Tube Wall Temperature Measurement

Need accurate information • Tube life ! • Artificial limitation on plant rate

850 (1560)

900 (1650)

950 (1740)

1000 (1830)

20 Design

Effect of Tube Wall Temperature on Tube Life

Temperature oC (oF)

+ 20oC + 36oF

Tube Wall Temperature Measurement

Contact • surface Thermocouple

“Pseudo-contact” • Gold Cup Pyrometer

Non-contact • disappearing filament • infra-red optical pyrometer • laser pyrometer

Surface Thermocouples

Continuous measurement, by condution “Slotting” can weaken tube wall Spray-welding leads to high readings Short, unpredictable lives (6-12 months)

Not commonly used for steam reformer tubes

Disappearing Filament Hand held instrument Tungsten filament superimposed on

image of target Current through filament altered until it

“disappears” Current calibrated to temperature Range 800-3000oC (1470 - 5430oF)

Very operator sensitive Largely displaced by IR

Infra-red Pyrometer

Easy to use Need to correct for

emissivity and reflected radiation

Inexpensive

Radiation Methods

Measure emitted energy at given wavelength Use Planck’s Law to give temperature Correction factors needed

• target emissivity real versus black body

• reflected radiation

"e" is the emissivity of the tube

Target Tube T t

Refractory Wall

Measured Temperature

Flame T f

The Effect of Reflected Radiation from Target Surroundings

Measured True Averaged target target background temperature temperature temperature

e = emissivity r = reflectance = (1-e)

Temperature Correction

E (Tm) = e E (Tt) + r E (T’w)

0.7 0.75 0.8 0.85 0.9 0.95 1

Difference in wall and target temperature oC (oF)

Deg C Deg F (540 F)

(360 F)

(180 F)

Target Emissivity

Error in measured tube temperature

Theoretical Effect of Wall Temperature (0.9 micron pyrometer)

Laser Pyrometers

Laser pulse fired at target and return signal detected

Can determine target emissivity Must correct for background radiation High speed selectivity Very accurate for flat surfaces

Laser Pyrometer

Laser Pyrometer - Angle of Incidence

Scattered laser pulse

Gold Cup Pyrometer

Excludes all reflected radiation Approximates to black body conditions High accuracy/reproducibility But…..

• limited access • awkward to use

Tube Furnace Wall

Water Cooling

To Recorder

Gold Cup

Lance *

Gold Cup Pyrometer

Accurate Temperature Measurement

Combination of IR pyrometer and Gold Cup • Gold Cup allows us to calculate “e” • Full accurate survey of reformer

possible with IR

• Measure Tt using Gold Cup • Measure Tm and Tw using Infra Red Pyrometer • Calculate e

Calculate "e" Use IR to give Tt with measured T’w and Tm and calculated e

Accurate Temperature Measurement

E (Tm) = e E (Tt) + (1-e) E (T’w)

a (Nearby tubes) 2

Background Temperature Measurement

Background Measurement for Tube A

1 Refractory Wall

0 0.2 0.4 0.6 0.8 1

Uncorrected Pyrometer

Corrected Pyrometer

Calculated = Gold Cup Measurements

Fraction down tube

Comparison of IR pyrometer and Calculated Tube Wall Temperature

Measurements

Tube Wall Temperature Measurement - Conclusions

IR typically reads high • top-fired reformer 32oC (58oF) • side-fired reformer 50oC (90oF)

IR with Gold Cup “calibration” • top-fired reformer 2oC (4oF) • side-fired reformer 16oC (29oF)

Summary Effect of operating variables on performance Plant data analysis

• fitting plant data • problem areas

reformer exit temperature flow errors sample analysis shifting

Approach to equilibrium Prediction of remaining catalyst life Tube wall temperature measurement

Normal Operation of Steam Reformers on Hydrogen Plants

Technology

Transcript of Normal Operation of Steam Reformers on Hydrogen Plants

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