Sulfur dioxide equilibrium considerations

Many industrial reactions are not carried to equilibrium. In this circumstance the reactor design is based primarily on reaction rate. However, the choice of operating conditions may still be determined by equilibrium considerations as already illustrated with respect to the oxidation of sulfur dioxide. In addition, the equilibrium conversion of a reaction provides a goal by which to measure improvements in the process. Similarly, it may determine whether or not an experimental investigation of a new process is worthwhile. For example, if the thermodynamic analysis indicates that a yield of only 20 percent is possible at equilibrium and a 50 percent yield is necessary for the process to be economically attractive, there is no purpose to an experimental study. On the other hand, if the equilibrium yield is 80 percent, an experimental program to determine the reaction rate for various conditions of operation (catalyst, temperature, pressure, etc.) may be warranted. 

Briston and Dainton 16), who investigated the interpolymerization of sulfur dioxide with cfs-butene-2 and benzoyl peroxide, observed a considerable geometrical isomerization above 250°. Prolonged heating with sulfur dioxide resulted eventually in the attainment of cis-trans equilibrium, while in the absence of sulfur dioxide no isomerization occurred. At 100° and high catalyst concentrations, a slow double-bond migration leading to the formation of butene-1 was also detected. 

As well as the sulfur cyde, there are other cycles in nature water, nitrogen, phosphorus and carbon. Man has influenced them all, but the sulfur cycle is the most disturbed among aU these cydic processes. Fossil fuels are burned in order to warm up houses and to provide cars, power stations and industries with energy. Then the sulfur that was built into coal, oil and natural gas, millions of years ago, is set free. It is transported as sulfur dioxide to the atmosphere, the SO content of which has increased considerably compared to the equilibrium content of the natural cyde. The acid gas is transported by the winds over the continents, and, long distances from a specific source, people experience a rain of sulfuric acid . This acid rain not only attacks the needles and leaves of trees but also damages the roots of plants. In acidified lakes the ability of fish to reproduce is disturbed. If acidification is not neutralized by liming, metal ions may be leached from rocks and damage animal life. 

It is this key second stage (known as the Contact step) that is reversible and requires the consideration of kinetic and equilibrium ideas. To improve the yield of sulfur trioxide the equilibrium is displaced to the right by mixing the sulfur dioxide with an excess of air - about three times more than is necessary from the equation. 

Sulfur Dioxide and Sulfites. Sulfur dioxide dissolves readily in aqueous formaldehyde Avith evolution of considerable heat. According to Reinking and co-workers , an approximately 22 per cent formaldehyde solution dissolves about 2.6 times as much sulfur dioxide as an equivalent amount of water. Commercial 37 per cent formaldehyde dissolves even a larger ratio, giving a solution having a density of 1.4 . These solutions smell of both formaldehyde and sulfur dioxide. It is possible that they contain equilibrium concentrations of methylolsulfonic acid. 

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