Properties - Sulphur Trioxide
November 9, 2003

Introduction Properties Materials of Construction Storage Spills

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Introduction

Sulphur trioxide is produced in a sulphuric acid plant converter from the conversion of sulphur dioxide and oxygen over a vanadium catalyst.   In this case it is an intermediate product since the SO3 produced is absorbed in concentrated sulphuric acid to produce sulphuric acid.  If SO3 itself is required, it is generally produced from oleum (i.e. fuming sulphuric acid).   Oleum is heated which will drive off the free SO3 which can then be used in the process or condensed to form pure liquid SO3.  

Sulphur trioxide (SO3) reacts readily with the moisture in the air to form submicron sulphuric acid particles that has the appearance of a dense white smoke or fog.  Sulphur trioxide will react violently with water evolving large amounts of heat and sulphuric acid.

A fire may result if sulphur trioxide comes in contact with combustible materials.

Both liquid and vapour are corrosive to eyes, skin, mucous membranes and respiratory tract.  Inhalation of sulphur trioxide may cause damage to the upper respiratory tract and even to lung tissue.

Properties

Physical State	Liquid
Specific Gravity @ 35°C (90°F)	1.85
Freezing Point	32.5°C (beta form) 16.8°C (gamma form)
Boiling Point	45.0°C
Viscosity @ 35°C (90°F)	1.28
Specific Heat @ 35°C (90°F)	0.77 cal/g/°C
Colour	Colourless
Odour	Sharp, irritating

Solid sulphur trioxide can exist in one of three forms; alpha (a), beta (b) and gamma (?).  All three forms may be present at the same time depending on conditions. 

Physical State	Solid
Form	Alpha	Beta	Gamma
Appearance	Fibrous, Stringy	Feathery, silk-like	Ice-like
Melting Point	80°C (176°F)	32.5°C (90.5°F)	16.8°C (62.2°F)

Unstabilized liquid SO3 maintained above 32.5°C will not freeze.  When cooled below this temperature, traces of moisture will cause the feathery beta form to develop.  Given time the beta form will polymerize into the alpha form.  A stabilizer can be added to liquid SO3 to prevent the formation of the beta form and hence the alpha form.

If equipment containing sulphur trioxide is not heated properly, the alpha form of solid may form.   Because the melting point of the alpha form (approximately 80^oC) is higher that the boiling point (45^oC), rapid localized vapourization will occur.  If melting of solid sulphur trioxide is not done properly, over pressurization of the equipment can occur.

Materials of Construction

Carbon steel is a suitable material of construction for handling and storing pure SO3.  A corrosion allowance of 3 mm (1/8") is usually specified.  If used for piping, velocites should be kept below 0.5 m/s (1.6 ft/s).

Flake graphite cast iron has good corrosion and erosion resistance but has poor mechanical strength which limits its use to pumps.  Nodular cast iron has poor erosion resistance and should not be used.

Stainless steel and special nickel alloys are suitable materials of construction.

Storage

Liquid SO3 must be stored in a heated storage tank to prevent freezing of the liquid.  A minimum temperature of 35°C (95°F) is recommended.  Inside heating coils should never be used because of the possibility of leaks into the tank contents.  Outside coils are better but should not be in direct contact with the tank to avoid hot spots which can lead to localized accelerated corrosion.  The preferred method of heating is by electric blankets which cover the entire surface of the storage tank.  As well, locating the storage vessel in a heated building of enclosure is also acceptable. 

All storage tanks containing SO3 need to vented to scrubbing system.  A release of SO3 directly will result in the formation of dense white cloud of sulphuric acid.

To prevent condensation of sulphur trioxide in vent lines and possible blockage, these lines should be heated to 90^oC. 

Spills

Even small spills of sulphur trioxide will produce a dense fog.  The large size of the cloud will seem to be out of proportion to the amount of liquid spilled.  Fuming is proportional to the surface area so a reduction in the surface area of the spill will reduce the amount of fuming.