Sulphitation is the practice of adding sulphur dioxide (SO2) or derivatives to process streams in a sugar factory. This is done for one of three reasons :
1. pH control - the SO2 in aqueous solution forms sulphurous acid H2SO3(aq) which reduces the pH of the process stream. An example of this would the control of diffusion water pH in a beet factory, where keeping the pH below 5.5 reduces the extraction of pectin from the beet cell walls which helps pulp pressing.
2. Biocide - used in sufficient quantities the SO2 inhibits the life cycle of bacteria, reducing the quantity of sugar lost by bacterial degradation to lactic acid. This is similar to the use of sodium metabisulphite for sterilising home brewing equipment. The efficiency of SO2 as a biocide is sometimes challenged in the literature.
3. Colour blocking - SO2 reacts with the carboxyl groups of invert sugars (glucose and fructose) to inhibit their participation in the colour forming Maillard reaction with amino compounds. By adding SO2 to juice before evaporation the increase in colour through the evaporators is kept to a minimum, protecting the juice from excessive colour formation at high temperature in the evaporators.
SO2 can be purchased as a liquefied gas, either in 1 tonne containers or in bulk tankers of 20 - 36 tonnes. The containers are pressure vessels usually capable of holding 10 bar g pressure, which covers the vapour pressure of SO2 up to about 50 °C.
All liquefied gas systems need pressure relief facilities, however the consequences of a discharge of toxic SO2 to the atmosphere can be serious in a populated area. For this reason special transport regulations apply to the movement of SO2 containers in most countries, and fixed storage installations are covered by Major Hazard legislation usually requiring evacuation plans etc. Liquid SO2 can also be quite expensive, perhaps 4 times the price of the equivalent amount of solid sulphur.
Solid sulphur is the most common material used for sulphitation, although liquid or molten sulphur is also available but solidifies below 120 °C. Solid sulphur comes from a variety of sources including sulphur mines and oil or gas desulphurisation plants. It also comes in a variety of physical forms, from dust through formed prills to cast roll sulphur of ~50mm diameter.
Sulphur is burned to form SO2 in many beet factories, cane mills and cane refineries.
The basic reaction is S + O2 = SO2 where the oxygen is supplied by atmospheric air.
The undesirable side reaction SO2 + ½ O2 = SO3 is minimised by the design of the burning equipment, although it would not be a problem where SO2 is used purely for acidification. For biocide action and colour blocking it is SO2 that is required, SO3 will form sulphuric acid on dissolving in water which will leave the sugar process in molasses, increasing the sugar loss.
Sulphitation equipment including burners and absorbers is supplied by Cocksedge Engineering who have a marketing / sales agreement with Fletcher Smith.
For a sulphur data sheet see Sultran's site.
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