Contact Section - Converters
May 3, 2009

Introduction Conventional Converter Stainless Steel Converter Single Bed Converters

Associated Links Materials of Construction Converter Maintenance/Problems Catalysts Quartz Converter Fabrication and Transportation

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Introduction

The acid plant converter is the heart of the acid plant.   In the converter, sulphur dioxide (SO2) reacts with oxygen (O2) in the presence of a catalyst to form sulphur trioxide (SO3) according to the following reaction:

        SO2 + 1/2 O2 <=> SO3            DH = Btu/lb

The converter serves the following functions:

• Provides single or multiple beds in which the catalyst is placed
• Provides for the distribution of gas across the catalyst bed.
• Provides for the collection of gas to exit the converter

The design of converters has evolved over time as plants have increased in size and materials of construction have changed.

Conventional Converter

The conventional converter consists of a carbon steel shell with cast iron support grids and division plates, cast iron posts, insulating brick lining of Bed 1 and metallizing of carbon steel.  The disadvantages of the conventional post and beam converter design are:

a)       Poor gas distribution

Acceptable gas distribution with side inlet nozzle requires gas inlet velocities substantially lower than designers are willing to use - either for cost or mechanical strength considerations.  Also, high velocity inlet gas at one point can blow catalyst around (digging holes in bed) worsening gas distribution.  The net result is a higher catalyst loading to achieve the same conversion.

b)       Poor access to beds

The first bed which is screened the most often is usually located on the top due to poor strength of carbon steel shell at high temperatures.   Posts in the beds make access difficult for catalyst loading and screening.

c)       Bypassing of gas

Asbestos rope is jammed between grids and shell to give a tight seal.  Poor installation or differential expansion between the shell and the grids can cause gaps to open up and provide a path for gas down the shell.

d)       Bed collapse

Bed collapse can occur due to either creep of the shell causing bulging, creep of the catalyst support grids, failure of the metallized lining or corrosion of angle iron welded to shell.

e)       Leaks at gas nozzle

Rectangular nozzles are often used to minimize the height of the converter.  This results in high stress corners and less resistance to stresses imposed by ducting leading to failures and gas leaks.

Stainless Steel Converter

Many of the disadvantages of the conventional converter have been eliminated by employing stainless steel as a material of construction.  The advantages of the stainless steel converter are:

• all stainless steel construction (304 SS)
• radial symmetry.
• all welded construction
• core tube for gas distribution and internal heat exchanger(s)
• all ducts connected to converter via circular nozzles
• flexibility in design to position catalyst beds in any order to ensure optimum ducting arrangements and access to Bed 1 which is screened the most often.

The design of the stainless steel converter is generally not governed by ASME Code due to the low operating pressure (< 15 psig).  The advantages of all stainless steel construction are apparent from the following graph which shows the allowable stresses versus design temperature.   Allowable stresses for 516/70 carbon steel are only quoted to a temperature 570°C (1058°F) whereas the operating temperature in Bed 1 can exceed 610°C (1130°F).  The sudden drop in stress values for carbon steel at approximately 400°C (752°F) is the cause of bulging of conventional converter shells.   Type 304 stainless steel is the 'ideal' material of construction having acceptable stress values at the normal operating temperatures of the converter.

Single Bed Converters

Single bed converters are typically installed as part or an acid plant retrofit.  A single bed converter can be a new first pass where the plant is handling a higher gas strength and the existing converter is not suitable for the higher operating temperatures.  Single bed converters can also be used as the final pass in a single to double absorption retrofit.  The existing converter passes become the passes before the intermediate absorption tower.  After passing through the intermediate absorption tower, the gas is heated back up prior to entering the single bed converter holding the final pass.

Single pass converters have been constructed in the traditional carbon steel/cast iron design as well as the newer stainless steel design.  The first pass single bed converters are usually stainless steel since these operate at higher gas strengths and temperatures.