Sulfur dioxide oxidation reactor design

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. 

The conversion of hydrogen sulfide to elemental sulfur in the Claus process is limited by a combination of equilibrium and kinetic factors. Over the past decade, the pressures of air pollution control requirements have resulted in major improvements in the design and operation of Claus plants, with consequent increases in conversion and reduction of sulfur oxides emissions (74-79). Nevertheless, emissions still commonly exceed the permissible limits coming into force both in the United States and abroad. Sulfur dioxide reduction plants present similar problems. Apart from the initial furnace or reactor, they are essentially Claus plants. 

A reactor is to be designed for the oxidation of sulfur dioxide, with excess oxygen from air, to sulfur trioxide. The entering feed, at 550 K and 1.1 bar, consists of 0.219 kmol/s of nitrogen, 0.058 kmol/s of oxygen, and 0.028 kmol/s of sulfur dioxide. The fractional conversion of sulfur dioxide is 50%. The reaction is very exothermic. Three cases are to be considered ... 

The modem two-step process converts hydrogen sulfide mixed with a stoichiometric volume of air to sulfur. In theory, one-third of the hydrogen sulfide is oxidized to sulfur dioxide in a carefully designed furnace, while the remaining hydrogen sulfide reacts with the sulfur dioxide to produce sulfur in two or more reactors containing a suitable catalyst. 

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