The Benefits and Disadvantages of Potash in Steam Reforming

The Benefits and Disadvantages of Potash In Steam Reforming

Gerard B. Hawkins Managing Director

www.GBHEnterprises.com

Introduction

Why do we include potash ? What are the benefits ? What are the disadvantages ?


Why do we include Potash ? Two reasons

• Prevents carbon formation within The catalyst On the inside wall of the tube

• If carbon is laid down, helps gasification of carbon

Potash has to be mobile so that it gets to the hottest point that the process gas sees • The inside tube wall


Why do we include Potash ? Catalyst Deactivation

Carbon Deposition : Thermodynamics & Kinetics

pH22

pCH4

10

1.0

0.1 550 600 650 700 750 800

1100 1200 1300 1400 (°F)

100

Temperature (°C)

High Methane Concentrations

Increasing Potential for Carbon Deposition




pH22

pCH4

10

1.0

0.1 550 600 650 700 750 800

1100 1200 1300 1400 (°F)

100

Temperature (°C)

Increasing Rate of Carbon Deposition




pH22

pCH4

10

1.0

0.1 550 600 650 700 750 800

1100

100

Temperature (°C)

Carbon Deposition Zone

1200 1300 1400 (°F)

Deposition possible but rate low

Deposition not favored




pH22

pCH4

10

1.0

0.1 550 600 650 700 750 800

1100

100

Temperature (°C)

CDZ

1200 1300 1400 (°F)

Composition - temperature profile along reformer tube

No carbon deposition




pH22

pCH4

10

1.0

0.1 550 600 650 700 750 800

1100

100

Temperature (°C)

CDZ

1200 1300 1400 (°F)

Zone of carbon deposition

30% of tube length



Carbon Deposition : Prevention • If carbon deposition occurs by :

CH4 C + 2 H2

• Then carbon deposition rate > carbon removal rate • Deposition rate is difficult to modify • Faster carbon removal is possible by leveraging an

additional removal reaction : C + H2O CO + H2

• Potash acts to increase the rate of this reaction



Carbon Deposition : Impact of Potash

pH22

pCH4

10

1.0

0.1 550 600 650 700 750 800

1100

100

Temperature (°C)

1200

Faster rate of carbon removal shrinks CDZ


CDZ

1300 1400 (°F)


Why do we include Potash ?

In terms of modelling we consider the margin to carbon formation

This is defined as the difference between the • Equilibrium temperature • Process gas temperature

Assumes GOM natural gas Care with gases that are heavier than this


Why do we include Potash ? Definition of carbon formation

margin

pH22

pCH4

10

1.0

0.1 550 600 650 700 750 800

1100

100

Temperature (°C)

CDZ

1200 1300 1400 (°F)


Margin to Carbon Formation



600

650

700

750

800

850

0 2 4 6

Distance Down Tube (m)

Tem

pera

ture

(°C

)

Inside TWTCarbon Equilibrium

Margin to Carbon Formation



600

650

700

750

800

850

0 2 4 6


Tem

pera

ture

(°C

)

Base Case Inside TWT

Base Case CarbonForming Equilibrium



600

650

700

750

800

850

0 2 4 6


Tem

pera

ture

(°C

)

Base Case Inside TWT

Base Case CarbonForming EquilibriumPotash CarbonFormation


Carbon Formation Margin

-50

0

50

100

150

200

250

0 2 4 6


Tem

pera

ture

(°C

)

Base Case Margin


Naphtha Steam Reforming Reaction chemistry (Tube inlet)

• Hydrocarbons undergo cracking reactions on hot surfaces at the tube inlet

• Products of catalytic cracking reactions can form polymeric carbon

• High strength catalysts required • Carbon resistant catalysts required

CxHy Cx + y/2H2

CxHy CH4 + H2 + Cx-1H2x-2

Thermal

Catalytic Polymers


Steam Reforming - Reaction Chemistry

Thermal cracking & carbon formation

Catalytic cracking and olefin polymerisation

Steam reforming reactions

Water gas shift reaction

Heavy Naphtha

Light Naphtha


Steam Reforming - Catalyst Design

High strength catalysts to tolerate carbon deposition

Carbon resistant catalysts

High activity catalysts

High activity catalysts

Heavy Naphtha

Light Naphtha

or

Mixed Feeds


So What are the Benefits ? Prevents carbon formation in applications

were carbon formation is an issue For example

• Highly stressed reformers High heat fluxes High throughput

• High levels of C2+ Even then can be insufficient

• Low steam to carbon ratios


What are the Disadvantages ? It is assumed that the potash will be

captured on the catalyst in the bottom of the tubes

This is not always the case In a number of situations there have been

problems Usually linked to potash loss

• In some cases fouls the WHB • Sometimes reaches the HTS • Can cause SCC of downstream heat

exchangers • Loss of catalyst strength


What are the Disadvantages ? Waste Heat Boiler Fouling For NG feeds main issue has been;

• H2 plants are more vulnerable as process gas temperatures are higher

• Rate of evolution of potash from catalyst is higher

Customer X WHB exit temperature increased at 2°F per day • Fouling in WHB was 23% potash


What are the Disadvantages ? Stress corrosion cracking On train at South American Methanol Plant

• 2nd BFW heater (3 in total) suffered from SCC

• Process gas on shell side Design flaw – should have been on tube

side as it is the dirty duty • Material specified as SS due to supply

issue Design flaw – should have been CS

• Huge amount of cracking – due to SCC Exchanger irreparable


What are the Disadvantages ? Effect on WHB

For Naphtha feeds – a variety of problems on plants • Again with fouling of WHB • Older plants have large fouling margin • Modern plants have had this cut and so have

insufficient surface area • Requires regular WHB cleaning • Sometimes bypass valve becomes coated and does

not operate correctly • Sometimes potash deposited in cold end of WHB

and needs digging out


What are the Disadvantages ? Effect on HTS

Most WHB have internal bypass for exit temperature control – automatic control

If tubes are fouled, tube exit temperature rises Bypass therefore closes to keep WHB exit

temperature constant Once bypass fully closes, exit WHB T rises Therefore inlet HTS temperature rises So CO slip rises If HTS catalyst or vessel temperature limit is

reached then plant rate has to be reduced


What are the Disadvantages ? Effect on HTS

Potash can also coat the surface of the HTS bed Deactivates leading edge of the bed Increases CO slip Increases DP


What are the Disadvantages ? Effect on Catalyst

As potash is removed then strength of catalyst is reduced

Causes excessive breakage



0 20

40

60

80

100

120

140 160

Z203 Z202 Z201 Z101

Catalyst type

kg Min

Avg Max



Carbon Laydown Test:

Feed cyclohexane + steam at 500 oC and S/C = 3.5

isolate steam

form carbon

remove/inspect sample after set times

2

9

15 Tim

e w

ithou

t ste

am (m

ins)

46-3 46-3Q

VSG-Z101 survives where Comp fails www.GBHEnterprises.com

Conclusions

Potash has many more advantages than disadvantages

We have options for the majority of plants to address carbon formation

Even the very worst cases On some plants there are problems were the

WHB is very sensitive to fouling

A Business Unit of GBH Enterprises, Ltd.

Technology

The Benefits and Disadvantages of Potash in Steam Reforming