Ammonia Synthesis Converter

1. AMMONIA SYNTHESIS CONVERTER
Prem Baboo
Sr. Manager (Prod)
National Fertilizers Ltd. India
FIE, Institution of Engineers India,
Technical Advisor & an Expert for
www.ureaknowhow.com
Main Parts of Converter
Cylindrical Pressure Shell
Converter Basket for Catalyst
Inter bed Heat Exchanger
Internal Heat Exchanger as per Design
Pressure Shell Covers / Dome
Types of Ammonia Converter
AXIAL FLOW CONVERTER
• Gas Flow axially in the pressure shell through the catalyst
RADIAL FLOW CONVERTER
• Gas flow radially in the pressure shell through the catalyst
• Low Pressure Drop

2. HALDOR TOPSOE RADIAL FLOW AMMONIA CONVERTER
TYPES OF CONVERTERS
Type No. of Beds Inter bed Cooling By
S-100 2 Quench
S-200 2 Heat Exchange
S-50 1 None
S-300 3 Heat Exchange
Available in 2 versions, with or without lower heat exchanger
S-250 is a S-200 followed by a S-50 converter
SYNTHESIS CONVERTER S-200

3. S-200 with Lower HE

4. S-50 Convertor

5. S-300 Convertor

6. Converter flow difference
B/W Amm I & Amm II

7. IMPORTANT FEATURES OF AMMONIA CONVERTER PRESSURE SHEL
Top Cover for installation of the Basket in one piece
Top closure sealed by double conical gasket
Top cover bolt tightening by means of hydraulic bolt tensioner
Top portion and cylindrical shell are cooled by inlet gas, hence reducing the
required wall thickness
Bottom forging and its connections for high pressure piping are fully welded

8. Thermo wells for the basket are installed through top cover to avoid removal of cover
for the thermo wells removal
DESIGN OF PRESSURE SHELL
Cylindrical shell can be solid wall or a layered wall
Fabrication of solid wall alloy steel in higher thickness is difficult with controlled
properties of high strength
In the layered wall, the inner layer can be of material resistant to synthesis gas
( basically hydrogen) and outer layers material can be selected with emphasis on
strength
DESIGN CODES:
ASME SECTION VIII Division 2 (USA)
AD- Merkblatter (Germany)
Design Conditions
Cooled Zone
Outlet Zone
Startup gas inlet zone
Cooled Zone i.e. shell, top cover and bottom head where design conditions are
related to inlet gas
Outlet zone i.e. bottom forging where design conditions are related to exit gas
Bottom flange for cold shot gas during normal operation and hot gas during
start-up
Design Conditions
Design Pressure 245 kg/cm2
Design Temperatures:
Part Amm-I & II New S-50
Shell, Top head 370 0C 430 0C
Bottom Forging 480 0C 450 0C
Cold Shot Inlet 510/480 0C

9. Advantages of addition of S-50 Loop
• Lower compressor power due to less circulation
• Possible to achieve higher plant load with same equipment’s
• Low loop pressure drop due to less flow
• Higher conversion 35.5 % as compared to 28.3% in S-200
• Ammonia concentration at the outlet of S-50 = 24.35% as compare to 20.02% in
S-200
• Lower circulation rate as compared to S-200 for same load
• Higher steam generation 82 T/hr as compared to 70 T/hr in S-200
MATERIAL SELECTION
Hydrogen Attack: It is an attack on carbon & low alloy steels caused by diffusion
of hydrogen into the material which reacts with carbon to form methane gas
leading to internal decarburization & internal cracking of the material.
It is counteracted by alloying with Cr & Mo
Nitriding: It takes place in hot ammonia containing gases and is caused by
nitrogen reacting with the metal.
Carbon & low alloy steels are resistant if operating below 400 0C
In the pressure shell, this temp will exceed at the exit section, hence internal of
bottom forging is protected by a layer of Inconel material applied as a weld
overlay
Hydrogen Induced Cracking:
Medium & high strength steels are prone to cracking at lower or moderate temp
in presence of hydrogen
During operation at elevated temp, hydrogen dissolves in steel
When vessel is cooled, metal may be supersaturated with hydrogen as it may not
have sufficient time to escape in particular from thick sections and if rapid
cooling takes place
HIC is controlled by steel selection in which proper heat treatment is done to
achieve a max hardness of 250HV

10. Materials Used
Part Amm-I Amm-II New S-50
Shell(Inner layer) SA387Gr12 SA387Gr11 13CrMoV9-10
1Cr 0.5Mo 1.25Cr0.5Mo 2.25Cr1Mo
12 thk 32 thk 46 thk
Shell(Outer layers) SA724GrB SA533GrB 13CrMoV9-10
CS 0.5Mo 2.25Cr1Mo
15.8x10 41x3 46x3
Total thickness 173.2 155mm 184 mm
Weight 470 Te 402 Te 390 Te
Converter Basket:
The catalyst bed walls consist of special screens which work as gas distributor
and structural member
Catalyst beds are made with full diameter covers providing easy access for
catalyst loading & unloading
Covers provided with high temp resistant gaskets
The outer screen consists of a number of flow distribution panels for proper flow
distribution which are placed side by side along the outer periphery of the beds
Cylindrical screens are used in the centre for gas outlet
It consists of a perforated plate with 2 mm dia holes followed by screens with
0.6mm openings
This design results in lesser impact on catalyst from the basket due to very smooth
special screens than the conventional design of perforated plate with wire mesh.

11. Design Parameters:
Temperatures
Pressure
Catalyst weight
Metal weight
Flow Induced vibrations.
Loads from handling & transportation
Material Properties:
• High temp strength
• Resistance to Hydrogen
• Resistance to nitriding
Materials:
• SS 304 & 321 for major parts & inlet screen
• Inconel 600 for outlet screen (hottest part)

12. • Have sufficient mech strength at elevated temp & resistant to hydrogen attack
• Nitriding resistance.
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