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ntroduction

Liquid sulphur dioxide (SO2) is a versatile chemical with many uses, both in liquid form or as a source of gaseous SO2.  Liquid SO2 is used the pulp and paper industry, mining industry, and in the food industry as a preservative.  It can function as a reducing agent, an oxidizing agent, a pH controller, purifying agent, preservative, a germicide and bleaching agent.  SO2 can also be used as a refrigerant, heat transfer fluid and selective solvent.

Liquid SO2 can be produced from gas containing SO2 concentration in the range of 1% to 100% using different processes.

Production

There are several different processes for the production of liquid SO2:

• Compression and Condensing
• Partial Condensation
• Absorption and Acidification
• Sulphur Trioxide and Sulphur

Compression and Condensing

At atmospheric pressure, pure SO2 will begin to condense at –10.1°C (13.9°F).  If the gas is compressed to 388 kPa(g) (56.3 psig), SO2 will begin to condense at 32.2°C (90°F).  This temperature is high enough that normal cooling water can be used to condense SO2. 

When the concentration of SO2 is less than 100%, the gas must be compressed to higher pressures to obtain a high enough condensing temperature to use cooling water as the condensing medium.

The tail gas leaving the system may be further cooled in a refrigeration unit to achieve nearly 100% or full condensation of the SO2.

Partial Condensation

When the concentration of SO2 in the gas is low (typically 7-14%), it becomes impractical to attempt to fully condense all the SO2 contained in the gas.  Extremely high pressures are required in order to use cooling water to condense SO2 from the gas.  The alternative to full condensation is partial condensation of the SO2 is using refrigeration only.   Refrigeration systems can achieve temperatures as low as –55°C (-67°F).  Typically, only 50% of the SO2 can be condensed from the gas.  The tail gas from the refrigeration process in used to pre-cool the incoming gas prior to being directed to some other process, such as a sulphuric acid plant, for further treatment. 

Absorption and Acidification

Gas containing low concentrations of SO2 (typically 1-2% vol) is scrubbed using an ammonia solution to form ammonium bisulphite according to the following reaction:

SO2(g) + NH4OH = NH4HSO3

The ammonium bisulphite solution is reacted to sulphuric acid to form ammonium sulphate, water and SO2.

2 NH4HSO3 + H2SO4 à (NH4)2SO4 + 2 H2O + 2 SO2(g)

The SO2 is stripped from the ammonium sulphate solution and is directed to liquid SO2 production.   The gas containing essentially 100% SO2 and moisture is first dried by condensing water from the gas and then drying the gas using concentrated sulphuric acid.  The dried SO2 gas is compressed and then condensed using cooling water. 

An absorption and acidification process has been operating at Cominco’s metallurgical facility since 1931.  The production of liquid SO2 is unfortunately tied stoichiometrically to the production of ammonium sulphate.

Sulphur Trioxide and Sulphur

Pure sulphur trioxide (SO3) will react with solid sulphur to produce SO2.  SO3 can be obtained by distilling oleum to drive off SO3 gas.  The SO3 passes through a column packed with solid sulphur to produce SO2 out the top of the column.

Product Specification

A typical product specification for liquid SO2 is as follows:

Liquid SO2 produced from sulphur burning acid plants is generally able to meet the above criteria.   It will be more difficult but not impossible to meet the above requirements in a metallurgical acid plants.  An efficient gas cleaning system is required to achieve the specification.

Shipping and Storage

Liquid SO2 can be shipped in a number of different ways.  Small quantities are available in 68 kg (150 lb) cylinders similar to cylinders for welding gases.  The rate of delivery will depend in the temperature of the liquid SO2 in the cylinder.  In order for gas to continue to be produced some liquid SO2 must vapourize but vapourization of liquid SO2 will cool the remaining liquid SO2 reducing the pressure in the cylinder.  The overall rate of SO2 gas discharge will depend on the heat transfer rate from the surroundings through the walls of the cylinder.  At room temperature a discharge rate of about 0.9 kg/h (2.0 lb/h) of gaseous SO2 is possible.

Larger quantities of liquid SO2 are available in one tonne containers.

Bulk storage of liquid SO2 can be done in large horizontal cylindrical pressure vessel specifically design for the service.  These storage tanks can be designed to hold several hundred tonnes to thousands of tonnes of liquid SO2.

Bulk shipments of liquid SO2 are done using either rail tank cars or tank trucks.

Piping and Fittings

Normally, heavy walled (i.e. schedule 80) seamless carbon steel pipe will be used to handle liquid SO2.  If SO2 is produced using the refrigeration method, extremely low operating temperatures are involved.  In this case the material must have adequate notch toughness to eliminate the risk of low temperature embrittlement. If the piping is installed where the ambient temperature can get very low, the requirement for adequate notch toughness is also required.

Flanges should be 300 lb. ANSI rated.

Screwed joints should be avoided but if it is necessary to have screwed connections, they should be back welded.

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