Method for making sulfur dioxide-containing gas stream

#Combustion of elemental sulfur in air or in an oxygen-containing gas stream is a convenient method for generating sulfur dioxide. However, combustion of liquid sulfur effected within a body of the same results in a stream of gaseous product heavily laden with vaporized elemental sulfur. High elemental sulfur content of such gases makes them unsuitable for bleaching or other processes wherein substantially sulfur-free sulfur dioxide is desired or required. Moreover, the vaporized sulfur tends to deposit as liquid or solid upon surfaces of equipment, creating or aggravating operational and design difficulties.

#U.S. Pat. No. 2,726,933 to Merriam et al. describes a method for making sulfur dioxide-containing gas streams substantially free of oxygen and sulfur which involves contacting an oxygen-containing gas stream with liquid sulfur maintained at auto-ignition temperature (at about 500.degree. F. or above) within a reaction zone comprising a pool of molten sulfur. The resultant gases comprising sulfur dioxide and vaporous sulfur are contacted further with liquid sulfur having temperature below auto-ignition, for time and under conditions adequate to substantially reduce the temperature of those gases to obtain a sulfur dioxide-containing gas stream substantially free of oxygen and free sulfur. Merriam et al. found that if in the second step the gases are cooled to temperature below minimum auto-ignition temperature of the sulfur, by about 200.degree. F. or more, then their sulfur content may be nil for practical purposes. The sulfur dioxide-containing gas stream obtained by the method of Merriam et al. is sufficiently free of elemental sulfur for use in bleaching or reduction of paper or other cellulose material, or for exothermic reaction with hydrogen sulfide to produce elemental sulfur.

#However, we have found that when a sulfur dioxide-containing gas stream made in accordance with Merriam et al.'s method is used for making sodium sulfite by contacting an aqueous sodium carbonate or hydroxide solution with the sulfur dioxide-containing gas stream, then the sodium sulfite product always contains as impurity elemental sulfur or significant amounts of sodium thiosulfate as a result of reaction of initially formed sodium sulfite with traces of residual elemental sulfur contained in the gas stream. Further investigation showed that Merriam et al.'s method is incapable of consistently producing a sulfur dioxide-containing gas stream containing less than about 0.0002 lb. of sulfur per cubic foot of gas, measured at standard conditions. While that sulfur level is sufficiently low for most applications wherein a sulfur dioxide-containing gas stream is required having low oxygen and sulfur contents, that already low sulfur content is still not sufficiently low if the sulfur dioxide is to be used for making photographic grade sodium sulfite and sodium metabisulfite.

#The following examples further illustrate the improvement of the present invention and set forth the best mode presently contemplated for practicing it.

#Apparatus employed was as illustrated in the drawing. The temperature of pool of molten sulfur 5 in sulfur dioxide generator 4 was 738.degree. F. Air was fed to sulfur dioxide generator 4 at feed rate of 0.20 cubic feet per minute through dip tube extending about 2 inches into the pool of molten sulfur. The sulfur laden sulfur dioxide gas stream generated in sulfur dioxide generator 4 was cooled in double pass gas cooler 7 to temperature of about 260.degree. F., and was introduced into sulfur equilibrator 10 containing pool of molten sulfur 11 at temperature of 252.degree. F. Pool of molten sulfur 11 in sulfur equilibrator 10 was about 16 inches deep, and the sulfur dioxide-containing gas stream was introduced near the bottom thereof. From sulfur equilibrator 10 the gas stream was passed through a venturi scrubber using water as scrubbing fluid, from whence it was passed through a bubbler containing deionized water at room temperature via a dip tube at about 8 inch depth. From the amount of sulfur collected in the scrubbing fluid it was estimated that the sulfur dioxide-containing gas stream fed to the venturi scrubber contained about 0.0001 pound of sulfur per cubic foot. The gas exiting the venturi scrubber contained no detectable sulfur, with limit of detectability by our method at 1.times.10.sup.-.sup.7 lb. of sulfur per cubic foot of gas, measured at standard conditions.

#In apparatus similar to that illustrated by the drawing, air was introduced into sulfur dioxide generator 4 at a rate of 650 cubic feet per minute. Pool of molten sulfur 5 in sulfur dioxide generator 4 was at about 730.degree. F. Gas exiting from sulfur dioxide generator 4 was passed through a single pass gas cooler which was a vertically arranged shell and tube heat exchanger. The bulk of the sulfur evaporated in sulfur dioxide generator 4 was condensed in the gas cooler and returned by gravity to sulfur dioxide generator 4. The liquid coolant employed on the shell side of the gas cooler was water, the temperature of which was controlled at 255.degree. F. by regulating the pressure of the water in the shell at 18 psig. Cooled sulfur dioxide-containing gas from the gas cooler was bubbled through sulfur equilibrator 10 having pool of molten sulfur 11 of 16 inch depth, maintained at temperature of 280.degree. F. The sulfur dioxide-containing gas exiting from sulfur equilibrator 10 contained about 0.0002 lb. of sulfur per cubic foot of gas, measured at standard conditions. The sulfur dioxide-containing gas from sulfur equilibrator 10 was passed into a venturi scrubber wherein it was contacted under conditions of turbulent flow with an aqueous solution of ammonium bisulfite containing about 5% by weight of ammonium bisulfite. The scrubbed sulfur dioxide-containing product gas was used in the manufacture of sodium bisulfite which was then transformed into anhydrous sodium metabisulfite which contained less than 0.02% by weight of thiosulfate, expressed as S.sub.2 O.sub.3, fully meeting the requirements of the American Standards Specifications published by the American National Standards Institute for sodium metabisulfite, anhydrous, photographic grade.