Problems Sulphur dioxide

Impermeable timbers have a good resistance to polluted atmospheres where acid fumes rapidly attack steel. Wood has given excellent service in the buildings of chemical works and railway stations. Permeable wood species and sapwood can suffer defibration problems caused by the sulphur dioxide of industrial atmospheres. Tile battens are particularly vulnerable. The heartwood of Douglas fir, pitch pine, larch, Scots pine/European redwood and many tropical hardwoods give good service in these conditions. 

In the 1960s there were attempts to use a moving bed of carbon to remove sulphur dioxide from flue gas on a pilot scale. As described by Katell(46) and Cartelyou(47), this Reinluft process was abandoned because of the problems caused by the carbon igniting in the presence of oxygen. 

Lead sulphide (galena - PbS) is another likely candidate for hydrometallurgical processing particularly in the United States where, apart from the problems of the sulphur dioxide emissions, the lead toxicity problem is making it very difficult for the lead smelters to operate their conventional pyrometallurgical process and comply with EPA and OSHA standards. The total amount of lead mined in the United States is about 600,000 tons per year which, if fully converted, would yield about 100,000 tons per year of by-product sulphur. The Bureau of Mines in Reno, Nevada, have an active pilot plant study to produce lead via a hydrometal-lurigal process (2). In this process the common lead mineral galena is dissolved in an acid brine solution of ferric chloride. 

Because of the rapid developments in the field of heterogeneous catalysis, the material reviewed here is exclusively dedicated to selective oxidations. No attention is given to total oxidations or combustion processes (including the problem of automotive exhaust gases). There is one exception, however the oxidation of sulphur dioxide to trioxide. Work on vanadate catalysts for this reaction is close to research on selective catalysts and therefore included. 

Although there is only one oxidation reaction possible with sulphur dioxide and hence a selectivity problem does not exist, recent results from kinetic research are included in this chapter, since there is a close analogy with other oxidations, especially on V2Os-based catalysts. 

With inorganic compounds, there can also be a selectivity problem, as illustrated by the oxidation of ammonia to nitrogen. Deep oxidation leads to nitrogen oxides. With sulphur dioxide, no selectivity problem rises. 

Addition of the ingredients in the correct order is essential to avoid production problems. The normal order starts with the presence of around 30-50% of final product volume of process water to which preservatives other than sulphur dioxide are first added. This volume should be as large as possible to allow the addition of carbohydrates and fruit components, which follow in that order. At this point, the volume should be approaching 90% of final volume to allow the dilution of preservatives. Acidulant is then added, followed by colourings, flavourings and all other components. 

There are particular problems in the manufacture of non-carbonated RTD beverages that are not aseptically packed. These relate to microbial contamination. Products that have no carbon dioxide in their head space are particularly vulnerable to contamination by moulds and certain types of bacterial infection. For many years it was possible to control such potential contamination by the use of low levels of sulphur dioxide (50 ppm). Changes in European Preservative Regulations now make the use of this preservative in RTD formulations (but not dilutables) illegal unless it is carried over from a fruit component, when up to 20 ppm SO2 may be present. Even at this level, the gaseous preservative is rapidly lost and is quickly ineffective. 

The Oxidation of Sulphur Dioxide. - This very important reaction is probably the oldest one in which vanadium catalysts have been used in practice. It is generally assumed that in these catalysts the vanadium is present dissolved in a liquid mixture of alkali metal meta- and/or pyro-sulphates. Villadsen and Livbjerg3 recently reviewed the properties of these supported liquid phase catalysts and showed that a number of questions still remain unanswered. Urbanek et al.la and Kenney75 presented reviews of the catalytic oxidation of S02, considering both the kinetics and problems of industrial reaction design. 

Suspensions of hematite have also been used and studied for other aims than photooxidation of water, e.g. catalytic oxidation of sulphur dioxide in aqueous solutions [52]. Aqueous dispersion of semiconductor particles could be an easy and attractive way to photooxidise water, but they have the drawback that dihydrogen and dioxygen are produced simultaneously in the same suspension. Apart from the separation problem the two produced gases may create a pathway for back reactions that reduces the yield of the overall photo-oxidation process. The latter obstacle can partly be avoided by addition of Na2C03, which was successfully shown by Arakawa et al [115]. 

When sulphur dioxide dissolves, the disulphide chemical bonds which result destroy the vitamin B or thiamine in foods by breaking up the protein molecules. Sulphurous acid, produced when sulphur dioxide is dissolved, may cause gastric irritation. Healthy people have no problem metabolising sulphur dioxide the kidneys and liver both produce enzymes which oxidise sulphites, but those with impaired kidney and liver may need to avoid sulphites. Foods containing sulphites may precipitate an asthmatic attack in asthma sufferers, who are very sensitive to the irritant effects of sulphur dioxide gas which may be liberated from the foods containing it and inhaled as the food is swallowed. It is one of the additives which the Hyperactive Children s Support Group recommends is eliminated from the diets of the children it represents. ... 

Carbon monoxide is very toxic by inhalation TLV 50ppm. Prepare by slow addition of anhydrous formic acid to concentrated sulphuric acid at 90-100°C (frothing tends to be a problem). One millilitre of formic acid generates 26.6 mmoles of gas. The CO is contaminated with small amounts of carbon dioxide and sulphur dioxide which may be removed by passage over potassium hydroxide or the commercial product Ascarite (sodium hydroxide on silica). Dispose of carbon monoxide by slow venting in an efficient hood. 

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