Production of liquid sulfur trioxide

#Sulfur trioxide is generally commercially produced from sulfur-containing substances such as free sulfur, hydrogen sulfide, sludge acid, pyrites, and other materials, by combusting these in the presence of air or oxygen to produce a gas containing predominantly sulfur dioxide. The sulfur dioxide is then subjected to several stages of catalytic conversion, usually utilizing a platinum or vanadium catalyst, to produce a gas containing sulfur trioxide. Sulfur trioxide is then recovered either by absorption with sulfuric acid to produce oleum or by absorption of a portion of the sulfur trioxide with sulfuric acid, and condensing the remainder of the sulfur trioxide in a cooled condensation apparatus to produce sulfur trioxide in liquid form. It has also been found that the production of sulfur trioxide is enhanced by conducting the process at elevated pressures, generally 3 atmospheres and above.

#For example, U.S. Pat. No. 3,803,297 to Guth et al. discloses the production of sulfur trioxide by combustion of sulfur with oxygen in at least two stages, each of which is preferably subdivided into sub-stages, followed by the cooling of the combustion gases, catalytic conversion of the gases to sulfur trioxide and recovery of sulfur trioxide by absorption with sulfuric acid to produce oleum. The temperature in the combustion stages is controlled by (a) utilizing less than a stoichiometric amount of oxygen and (b) passing some of the feed oxygen through the sulfuric acid absorption stage to pick up some sulfur trioxide, thereby recycling this sulfur trioxide to the combustion stage to act as a heat sink (the decomposition of sulfur trioxide is an endothermic reaction). The patent requires a careful control of combustion by dividing it into two main stages, with from about 5-40% of the sulfur being combusted in the first stage together with at most a stoichiometric amount of oxygen, and recycled sulfur trioxide, and the remaining 50-95% sulfur being oxidized in a second stage using a stoichiometric amount of oxygen. The second stage is additionally divided into a series of sub-stages for better control of the operating temperature. Some sulfur trioxide is also introduced into the gases entering the sulfur dioxide catalytic conversion steps.

#Another such process is disclosed in U.S. Pat. No. 3,455,652 of James. A sulfur-containing feed is combusted with air or oxygen-enriched air, the oxygen being present in excess, the sulfur dioxide being converted to sulfur trioxide in a conventional catalytic conversion step or steps, and the sulfur trioxide being recovered by condensation of one portion and absorption of the remaining portion with sulfuric acid to produce oleum. The process is conducted at elevated pressures, between 3 and 50 kg/cm.sup.2.

#The invention is more clearly described with reference to the Figure, which shows a typical flow sheet for conducting the process.

#Referring to the Figure, sulfur in line 8, oxygen in line 9 and recycled process gases in line 10 (as hereinafter described) are introduced into combustion furnace 1. Preferably, relatively pure oxygen and sulfur are used in order to minimize the amount of impurities contained in the resulting process gas; the oxygen feed will preferably contain about 0.5% nitrogen. However, the nitrogen content of the oxygen may be higher, up to about 10%. Variations in the nitrogen content of the oxygen feed can be compensated for by varying the amount of gas vented from the system as hereinafter described.

#The sulfur is preferably as pure as possible. However, depending on its source, it may contain small amounts of impurities such as dissolved H.sub.2 S, inorganic ash and/or traces of hydrocarbons. During combustion, these will be converted to SO.sub.2, CO.sub.2 and water. CO.sub.2 can be removed by a small increase in the rate or amount of venting gas from the system, as hereinafter described.