Download - Steam Reforming - (ATM) Approach to Equilibrium

Gerard B. Hawkins Managing Director Kp

Temperature

The aim of this presentation is to • Give an understanding of equilibrium ◦ Methane Steam ◦ Water Gas Shift

• Explain what affects equilibrium • Explain concept of approach to equilibrium

• There is an equilibrium effect • Limits the reaction rate • Point at which forward and reverse reactions are

equal • Defined by

[ ] [ ]

[ ] [ ]CO.PHPOH.PCHPKp 3

2

24=

• Can rearrange into a more useful form

• So now we can relate partial pressure of methane to other parameters

• But can simplify again to

• This is the most useful form

[ ] [ ] [ ]O]P[H

CO.PHPKpCHP2

32

4 =

[ ] [ ] [ ]O][H

CO.HPKpCH2

32

2

4 =

• Equilibrium defined by

• We can see therefore ◦ Methane slip is proportional to Kp ◦ Methane slip is proportional to P² ◦ Methane is inversely proportional to steam fraction Reason why SC Ratio is high

• So we can now determine cause and effect by using this simple expression

[ ] [ ] [ ]O][H

CO.HPKpCH2

32

2

4 =

• Kp is inversely proportional to temperature • Therefore to achieve a good equilibrium position

need to use a high temperature

Kp

Temperature

• Water Gas Shift (WGS) is also equilibrium limited • Equilibrium defined by

• Which can be rearranged to

P[CO2] P[H2] Kp = ____________ P[CO] P[H2O]

[CO2] x [H2] [CO] = _________ Kp x [H2O]

• [CO] not dependant upon pressure

• [CO] α 1/[H2O] ◦ Higher steam to carbon ratio gives lower CO content

◦ Higher steam to carbon ratio gives higher CO2 content

• Kp is related to temperature

• High exit temperature gives more CO and conversely less CO2

Water Gas Shift (WGS) is at equilibrium at exit of the reformer

Reaches equilibrium very fast

Kp

Temperature

By using equation earlier can relate methane slip to temperature

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6

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Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490) Temperature °C (°F)

770 780 790 800 810 4

6

8

10

12

Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490) Temperature °C (°F)

Exit CH4

Equilibrium Temperature

Gas Exit Temperature

ATE

Approach to Equilibrium (ATE) is therefore defined by

ATE = Actual temperature - Equilibrium temperature

ATE is 0°F when gases at equilibrium This never happens Usually ATE in in range 5-20°F ATE rises as the catalyst ages

Can translate this graph to give

0 0.2 0.4 0.6 0.8 1 200

300

400

500

600

700

800

900

Fraction down tube

Tem

pera

ture

°C Gas Temp

Eq'm Temp

390

570

750

930

1110

1290

1470

1650

Can then superimpose the reaction path

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Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490) Temperature °C (°F)

Equilibrium Line

Reaction Path

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8

10

12

Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490) Temperature °C (°F)

ATE

770 780 790 800 810 4

6

8

10

12

Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490) Temperature °C (°F)

Equilibrium Line

Increasing pressure

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Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490) Temperature °C (°F)

Increasing pressure

ATE

• A reformer is essentially an equilibrium reactor ◦ Low ATE’s are achieved at the exit

• Any change that affects the equilibrium position will affect the performance of the catalyst/reformer ◦ Will affect the approach ◦ Will affect the methane slip

• Raising the outlet pressure will ◦ Increase kinetic rate which will reduce ATE ◦ But equilibrium position is worse

770 780 790 800 810 4

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8

10

12

Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490) Temperature °C (°F)

Equilibrium Line

Reaction Path

Effect of Raising Exit Temperature

• So raising the exit temperature ◦ Reduces the methane slip ◦ Tightens the approach to equilibrium

• But ◦ Will increase tube temperatures ◦ May reduce tube wall margin

• Classic trade off between maximizing production but sacrificing tube life

• Any increase in feed rate will ◦ Increase exit temperature or methane slip Depends on operating policy ◦ Increase pressure drop ◦ Increase ATE - lower residence time ◦ Increase maximum tube wall temperatures Hence reduce tube life ◦ Increase fluegas flow ◦ Increase temperature Reduced contact time

• Converse is also true

• Reducing steam to carbon ratio ◦ If methane slip is held constant will require an increase in

exit temperature ATE will reduce Maximum tube wall temperature will rise ◦ If exit temperature is held constant will cause methane

slip to rise ATE will increase ◦ Pressure drop will be reduced

• Converse is also true

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Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490) Temperature °C (°F)

Cold Zone

Hot Zone

Actual Operating Point

Effect of such a spread is • Tight approaches in both hot and cold zones • Methane slip is average of both zones • Temperature is average of both zones • Approach for ‘mixed’ gas is high • Appears as if catalyst is not working well

From the above, there are conflicts as defined below

Temperature Pressure Steam to

Carbon

Kinetics Equilibrium Overall

Conflict ?

High High

High

No

High Low

?

Yes

Low High

?

Yes

• Pressure - defined by other issues ◦ CAPEX of reformer High pressure = thicker tubes ◦ Size of synthesis gas compressor High pressure reduces size of synthesis gas machine But increases the size of the air compressor ◦ Methane slip is increased But secondary reduces the effect Does represent an inefficiency

• Classic balance between CAPEX and OPEX

• Steam to Carbon - defined by other issues ◦ Must raise HP steam for synthesis gas machine Do get MP steam from extraction from turbine Try to minimise capacity to reduce CAPEX - more steam

raising requires more coils/heat exchangers ◦ HTS operation - over reduction ◦ CO2 removal - reboiler heat load ◦ Metal dusting is a problem at low steam to carbon ratio’s

508

203

102

Equi

libriu

m %

CH

4 (d

ry b

asis

)

Pres

sure

(psi

g)

Pres

sure

(bar

g)

Steam Ratio

2.0

3.0

4.0

5.0

35

14

7

1.0

2.0

5.0

10

20

50

Equilibrium exit CH4 at these conditions ?

P= 30bar (435psi) T = 850°C (1562°F) Steam/Carbon = 3.5

508

203

102

Equi

libriu

m %

CH

4 (d

ry b

asis

)

Pres

sure

(psi

g)

Pres

sure

(bar

g)

Steam Ratio

2.0

3.0

4.0

5.0

35

14

7

Equilibrium CH4 = ~5.6% 1.0

2.0

5.0

10

20

50

CH4 slip is a function of ◦ Catalyst activity ◦ Reformer exit temperature ◦ Reformer exit pressure ◦ S:C Ratio ◦ varies with operating conditions

Assessment of catalyst performance ◦ CH4 slip alone is not a good measure of performance ◦ ATE is a better guide ◦ will not change dramatically with operating conditions

Take inlet and exit gas samples for analysis Measure steam reformer exit T & P Measure reformer inlet steam and feed flows Need to calculate reformer exit dry gas flow ◦ all carbon in the feed ends up in the dry gas ◦ hence we can calculate exit dry gas rate by carbon

balance

Need to calculate the reformer exit steam flowrate ◦ can be done by Hydrogen balance across the

reformer Can also do a balance on O2 as a cross check Calculate exit wet gas composition and then Kp Calculate Equilibrium Temperature Calculate Approach to Equilibrium

Assumes measured data is correct In practise always errors in measured data ◦ feed flow ◦ feed analysis ◦ steam flow ◦ exit analysis ◦ exit pressure ◦ exit temperature

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Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490)

Temperature °C (°F)

Exit CH4

Equilibrium Temperature


ATE

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Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490)


Exit CH4


Pressure

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Met

hane

slip

(%)

(1418) (1454) (1436) (1472) (1490)


Worst ATE CH4


Actual ATE

Best ATE

Pressure

Need to do a consistency check on the data Check data for a consistent H & M balance ◦ GBHE VULCAN CERES H & M Balance reconciliation

WGS at reformer exit temperatures ◦ At high temps, WGS reaction should be at Equilibrium

◦ We can similarly calculate approach to WGS equilibrium

Often WGS appears to not be at equilibrium ◦ Suggests errors in exit analysis and exit T

• Well ◦ Use VULCAN CERES

◦ Use VULCAN TP3

◦ Use VULCAN REFSIM

◦ And most of all Think !