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

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

S-200 with Lower HE

S-50 Convertor

S-300 Convertor

Converter flow difference

B/W Amm I & Amm II

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

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

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

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.

� 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)

• Have sufficient mech strength at elevated temp & resistant to hydrogen attack

• Nitriding resistance.

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