3,431,070 March 4, 1969
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March 4, 1969 3,431,070 METHOD OF TREATING AMMON IA AND HYDROGEN SULFIDE GASES To 6 INVENTOR. 274,725 ZA2ZZafar 17724ay 3,431,070 3 4 thiosulfate and sulfur. The sulfur dioxide reacts with the Where the initial feed gas contains hydrogen sulfide, but absorbed ammonia to form ammonium bisulfite according no ammonia, all of the thiosulfate formed by reaction (3) to the reaction: will be reduced to sulfur by reaction (4), resulting in no net yield of thiosulfate. If, on the other hand, ammonia is in excess, the mecha The remainder of the sulfur dioxide, in excess of that re nism of reacting the excess is believed somewhat different. quired to neutralize the ammonia, forms sulfurous acid In this case, reduction of the thiosulfate according to re as follows: action (4) is retarded as the ammonia raises the solution SO--HO->H+--HSO (2) pH. Since insufficient sulfur dioxide has been added to Complete reaction of the ammonia and hydrogen sul 10 convert all of the ammonia to ammonium bisulfite, the fide content of the feed gas to ammonium thiosulfate, to ammonia is probably in solution as a mixture of ammo sulfur, or to a mixture of ammonium thiosulfate and sul nium sulfite and ammonium bisulfite. The sulfite and bi fur requires 0.5 mole of sulfur dioxide per mole of the sulfite are reacted with the excess hydrogen sulfide to form total ammonia plus hydrogen sulfide reactant content of thiosulfate according to the reaction: the feed gas. Addition of the proper quantity of sulfur 15 dioxide is desirable, as the proportion of sulfur dioxide As previously mentioned, product distribution is af in part determines the completeness of reaction and ulti fected not only by feed gas composition, but by the amount mate yield of ammonium thiosulfate and sulfur. Insuffi of sulfur dioxide added. Although 0.5 mole of sulfur diox cient sulfur dioxide results in an incomplete reaction leav ide per mol of total ammonia plus hydrogen sulfide ing unreacted hydrogen sulfide, or, if ammonia is in excess 20 reactants are usually added to achieve complete reaction in the feed gas mixture, yields a product contaminated with of ammonia and hydrogen sulfide to ammonium thiosul ammonium hydrosulfide and other undesirable contami fate and/or sulfur, it is within the scope of my invention to nants. Excess sulfur dioxide merely forms additional am add more, or less, sulfur dioxide. monium bisulfite, sulfurous acid, or thiosulfuric acid, the Although the primary control of the chemical reactions particular byproduct depending on the proportions of am 25 and product yields is by the sulfur dioxide addition rate, monia and hydrogen sulfide in the initial feed gas. Thio pH may be used to monitor this control. The pH varies Sulfuric acid may further decompose to polythionates, sul from point to point within my process, depending on the fur, or sulfates, thereby adding additional impurities to particular reactions taking place and the proportion of the product. reactants. Ammonium thiosulfate in aqueous solution is The ammonium bisulfite and sulfurous acid content of 30 mildly acid, with a pH of about 4, the exact value depend the aqueous ammonium thiosulfate solution can be reacted ing on concentration and temperature. This pH is raised with hydrogen sulfide in the substantially ammonia-free by absorption of ammonia and lowered by absorption of gaseous product from the ammonia scrubbing step accord sulfur dioxide, the magnitude of the changes depending on ing to the reaction: the concentration of these reactants. For example, absorp 35 tion of equimolar quantities of ammonia and sulfur diox ide will first raise the pH of the solution as the ammonia Thiosulfate-forming reaction (3) proceeds moderately is absorbed, and then lower it on addition of the sulfur rapidly and is accompanied by a reduction in pH. If a dioxide, the pH of the resulting solution being about the proper quantity of sulfur dioxide has been added, either same as that of the original solution. On reaction with bisulfite or hydrogen sulfide will be in excess, depending 40 hydrogen sulfide, the solution pH is reduced as the am on the initial proportions of ammonia and hydrogen sul monium bisulfite and/or sulfurous acid are converted to fide in the feed gas. Subsequent reactions then occur stronger thiosulfuric acid and its ammonium salts. As the whereby additional ammonium thiosulfate and/or sulfur is lower thiosulfate reduction reaction (4) is completed, the formed, the exact mechanisms of these reactions again pH again increases to about the original level. depending on the proportions of ammonia and hydrogen 45 All of the reactions noted above occur in an aqueous sulfide in the feed gas. ammonium thiosulfate solution, a portion of which also Unless the feed gas initially contained at least 3.0 moles usually serves as an absorbent for the ammonia, although of ammonia per mole of hydrogen sulfide, a portion of the other solutions may also be used as absorbents. The prod thiosulfate formed according to reaction (3) will be re lucts of the hydrogen sulfide reactions are, except at the duced to sulfur by reaction with unreacted hydrogen 50 extremes of concentration range, an aqueous ammonium sulfide as follows: thiosulfate solution containing elemental sulfur crystals. Although these crystals are extremely finely divided, they can be removed by air flotation, filtration, or other sep Since hydrogen ions produced in reaction (3) are con aratory means. Thus, the final products of my process sumed in reduction reaction (4), the reaction is pH de 55 are finely divided elemental sulfur and a concentrated pendent and will not proceed where the solution pH is ammonium thiosulfate solution. As will be hereinafter above about 6.3, which condition can occur with insuffi described, the finely divided elemental sulfur can be fur cient sulfur dioxide addition or excess ammonia. ther processed to molten liquid sulfur, all or a portion of Where the initial feed gas contains equal molar propor which can then be oxidized to supply the sulfur dioxide tions of ammonia and hydrogen sulfide, an equivalent 60 requirement of the process. Minor amounts of polythionic quantity of sulfur dioxide is required and the reactions of acids can also be formed, but these react further with hy Equations 3 and 4 can be expressed by the net reaction: drogen sulfide to form thiosulfate and/or sulfur by reac tions such as: If the initial feed gas contains other than equal molar 65 H2SOs--H2S->2H2SO (8) quantities of ammonia and hydrogen sulfide, the excess In any event, no appreciable quantities of polythio reactant is consumed by further reactions, the particular nates are contained in the final product of my process, mechanism and final products depending on whether am thus constituting a marked advantage over some of the monia or hydrogen sulfide is in excess. In the case where prior art processes, such as that of U.S. Patent No. the initial feed contains hydrogen sulfide in excess of am 70 1,868,843 to Overdick, which yield considerable quanti monia, the excess hydrogen Sulfide reacts with a portion ties of polythionates. of the thiosulfate produced in the reaction of Equation 3 The above reactions are believed the mechanisms to form sulfur according to Equation 4. The net reaction whereby ammonia and hydrogen sulfide are converted to for the excess hydrogen sulfide can be expressed as: ammonium thiosulfate and sulfur in my process. Even 75 though the actual mechanism may not be completely 3,431,070 5 6 understood, or may be otherwise than as set forth above, any dissolved hydrogen sulfide from the ammonium thio I have conclusively demonstrated that ammonia and hy sulfate solution containing the absorbed ammonia pass drogen sulfide can be reacted to ammonium thiosulfate ing out of the bottom of absorber 1. Although carbon and/or sulfur by the method set forth herein. dioxide stripping is not absolutely necessary, it is ad Since the net reaction of my process is exothermic, heat vantageous particularly where inerts contained in the will build up which must be removed, either by radiation 5 sulfur dioxide are vented to the atmosphere. Thus, the losses to the atmosphere or by interchange of the hot section of the contacting zone of ammonia absorber 1 reaction media with a coolant. A desirable temperature below the point of feed gas entry is a stripping section range is from about 35° C. to about 70° C., although serving the sole purpose of preventing hydrogen sulfide temperatures up to about 100° C. or higher can be em from being retained in the ammonia thiosulfate solution ployed depending upon the system pressure, the upper 10 leaving absorber 1. Displacement of substantially all of limit being established by the boiling point of the solu the absorbed hydrogen sulfide is particularly important tion. The lower limit of the operable temperature range where the sulfur dioxide is obtained from combustion of is established by the temperature at which precipitation sulfur or byproduct recovery as absorbed hydrogen sulfide of salts commences. A higher temperature favors in 15 would be stripped from the solution on contact with the creased reaction rates, but reduces the absorptivity of the sulfur dioxide and vented with the inerts from the sulfur gases in the ammonium thiosulfate solution. Thus, the dioxide contacting step. effect of a temperature change on reaction rate depends The ammonia content of the feed gas is substantially on whether the reaction rate is, under those conditions, lowered by contact with the ammonium thiosulfate solu controlled by chemical reaction rate or by gas absorption 20 tion and if proper contacting is obtained can be substan TateS. tially completely removed.