This invention is concerned with the conversion of H.sub.2 S to sulfur. H.sub.2 S is found in large quantities as a component of natural gas and in many other situations such as its formation when sulfur-containing petroleum fractions are hydrogenated and when synthetic gas is made from sulfur-containing coal. In each of these and similar cases whenever there are more than minor amounts of H.sub.2 S present, the H.sub.2 S must be separated from the mixture of which it had been a part before the remaining material can be used. By carrying out the separation, not only is the H.sub.2 S separated from most of the other gases, it is recovered in a more concentrated form. In nearly all cases, the H.sub.2 S concentration exceeds about 35%. After being separated, the H.sub.2 S is processed to recover elemental sulfur.
Another source of H.sub.2 S is in the treatment of sulfur dioxide-containing flue gas. Such treatment consists of absorbing the SO.sub.2 in an absorbent of one type or another and by further procedures eventually forming H.sub.2 S. Such a scheme is described in U.S. Pat. No. 4,141,961.
The present widely practiced method used to convert H.sub.2 S to sulfur is the classical Claus process and some of its more ingenious modifications. A brief description of this process is contained in Hydrocarbon Processing vol. 61, No. 4 p. 109 (1982). This process is used in hundreds of installations all over the world. There is a voluminous literature describing its various aspects. The use of the Claus process to convert appreciable amounts of H.sub.2 S to sulfur is so prevalent and so fixed in the minds of those involved in making sulfur from H.sub.2 S that the decision to employ this method is automatic. No other possibility is ever considered. Because of this knee-jerk reaction, the disadvantages and limitations of the process are automatically acceptd and automatically provisions are made to overcome them. Providing these provisions is expensive but tolerated.
The disadvantages and limitations of the process are inherent in its chemistry and its use of air as the other reactant. The overall reaction can be summarized: EQU (1) 2H.sub.2 S+Air[O.sub.2 +3.76 N.sub.2 ]=2S+2H.sub.2 O+3.76 N.sub.2
At the temperature required to carry out the reaction over the usual alumina catalyst, the equilibrium mixture formed still contains an appreciable amount of H.sub.2 S. To convert more of the H.sub.2 S to sulfur, the gas mixture is cooled below the sulfur condensation temperature. The condensed sulfur is separated from the gas. The separated gas is reheated and contacted with a second catalyst bed to reestablish the equilibrium. These steps are repeated several times until 95% to 96% of the H.sub.2 S has been converted to sulfur. The remaining gas still contains so much H.sub.2 S that it cannot be incinerated to sulfur dioxide and vented to the atomosphere. Consequently, Claus plant tail gas must be processed in a second gas treating installation.
From the preceding paragraph, it is evident that the use of the Claus process to convert H.sub.2 S to sulfur requires an expensive installation. Moreover, only about 95% or so of the H.sub.2 S is converted. To this must be added the cost of the equipment to separate the H.sub.2 S, the cost of operating the separation equipment plus the capital and operating cost to process the tail gas.
The tail gas of most Claus process installations is treated by contact with a solution containing an air regenerable oxidizing agent. The H.sub.2 S is absorbed by the solution and quickly oxidized to form elemental solid sulfur and water. The resulting slurry is separated from the H.sub.2 S contacting step and then contacted with air to regenerate the oxidant. The sulfur in the slurry is separated from the solution either before or after the air contacting. The choice depends upon the characteristics of the system that is used, local circumstances, and the likes and dislikes of the process designer.
A large number of air regenerable systems have been devised. One of the first to enjoy widespread use is the Stretford process. It employs an aqueous solution containing sodium carbonate, sodium vanadate and 1,4-anthraquinone disulfonic acid. A second process widely used is the Giammarco-Vetrocoke Process. This process uses a solution containing sodium carbonate, sodium arsenate and arsenite. A recently described process is the Sulfint process. This process employs a ferric chelate as the oxidant. The iron is reduced from the trivalent state to a ferrous complex. Subsequently, by contacting the aqueous solution containing the ferrous iron complex with air, the iron is oxidized back to the trivalent state and recycled. This process is described in some detail in Hydrocarbon Processing vol. 61 No. 3 pages 169-172 (1982). Another widely used process is the Takahax Process which uses sodium 1,4-naphthoquinone 2-sulfonate as the air regenerable oxidant.
The ability of these symptoms to remove practically all of the H.sub.2 S from an H.sub.2 S-containing gas mixture is due to the conversion of the H.sub.2 S to sulfur. Since the sulfur is not soluble, as such, in any of these aqueous solutions, no equilibrium is established. Because the H.sub.2 S is removed from the solution almost as quickly as it dissolves, the ability of the solution to continue to dissolve H.sub.2 S is unimpaired. It is this property of these systems that enables them to achieve any degree of H.sub.2 S removal required or desirable. The necessary condition is sufficient gas-liquid contact.
In all of these processes, while the conversion of the H.sub.2 S to sulfur takes place rapidly, the rate of regeneration of the oxidant is slow. Usually, only 10% to 20% of the oxygen in the air supplied to the regeneration step reacts. While air is free, it cannot be moved without cost. Moreover, low air utilization requires large volume equipment so that the cost of moving the air does not become excessive.
Very often H.sub.2 S-containing mixtures also contain appreciable amounts of CO.sub.2 as well as not insignificant amounts of hydrocarbons. In the combustion step of the Claus process, the hydrocarbons burn to water vapor and CO.sub.2. Substantial amounts of CO.sub.2 cause operating difficulties when operating the Claus process. The high temperatures employed in the combustion chamber cause the CO.sub.2 to react to some extent with H.sub.2 S to form carbonyl sulfide. The COS is objectionable and the subsequent operations of the process must be carefully controlled to bring to a minimum its concentration in the Claus plant tail gas. As the CO.sub.2 content of the gas increases, the heating value of the gas goes down. As a result it becomes necessary to add fuel gas to the H.sub.2 S-containing gas so that the final gas mixture reaches the necessary reaction temperature in the combustion chamber. This fuel addition is necessary in addition to providing extensive preheat of the H.sub.2 S containing gas and the combustion air.
Claus plant tail gas treating processes which use an alkali carbonate as the H.sub.2 S absorbent are also deleteriously affected if required to handle gas streams whose concentration of CO.sub.2 approaches 30%.
Two other processes have been proposed for treating gas containing very small concentrations of H.sub.2 S. One is described in German Pat. No. 2,819,130 November, 1978. In this disclosure the H.sub.2 S-containing gas is contacted with aqueous solutions containing an appreciable concentration of sulfuric acid, minor amounts of both copper sulfate and ferric sulfate as well as trace amounts of nitrate or nitrite. During the contacting most of the ferric sulfate is reduced to ferrous sulfate, part of the copper is precipitated as cupric sulfide particles along with elemental sulfur particles. The resulting slurry is treated with elemental oxygen in an autoclave at a temperature in excess of the melting point of sulfur, i.e. 120.degree. C., at a pressure in excess of the vapor pressure of the solution. Under these conditions the ferrous ion is slowly oxidized to ferric ion, the copper sulfide is oxidized to copper sulfate and molten sulfur is formed. Under pressure, the molten sulfur can be separated from the aqueous solution. The regenerated ferric ion-containing solution can then be reused.
The other process which has been proposed for removing the sulfur from a gas in the form of very diluted hydrogen sulfide is de Loisy, U.S. Pat. No. 1,516,915. In this process the gas containing the H.sub.2 S is contacted with an acidic ferric sulfate solution. Elemental solid sulfur is formed along with sulfuric acid and ferrous sulfate. Ferric sulfate is consumed: EQU (2) H.sub.2 S+Fe.sub.2 (SO.sub.4).sub.3 =S+2FeSO.sub.4 +H.sub.2 SO.sub.4
The sulfur is then separated from the solution in which it had been formed.
The separated solution is processed to regenerate the ferric sulfate from the sulfuric acid and ferrous sulfate. To accomplish this regeneration de Loisy employs two oxidation reactions as shown in the patent page 1, column, 2, lines 51-62 EQU (3) 6 FeSO.sub.4 +2 HNO.sub.3 +3H.sub.2 SO.sub.4 =3 Fe.sub.2 (SO.sub.4).sub.3 +2NO+4H.sub.2 O EQU (4) 2 FeSO.sub.4 +H.sub.2 SO.sub.4 +2HNO.sub.2 =Fe.sub.2 (SO.sub.4).sub.3 +2NO+2H.sub.2 O
De Loisy requires that these reactions be carried out at an elevated temperature from the explanation provided in the section of the patent cited above.
The de Loisy invention is described succinctly on page 2 of the patent column 1, lines 3 through 18 which reads:
"For this operation the ferrous liquid to be regenerated first serves to absorb the mixture 2NO+O.sub.2 evolved in a previous treatment. For that purpose the ferrous liquid is caused to trickle in an absorption tower, at the bottom of which enters the mixture of air and oxide of nitrogen. In proportion as it passes down, the mixture becomes laden with nitric acid and nitrous acid constituting the oxidizing agent. This liquid collected at the bottom is then suitably heated so that it is thereby converted into ferric salt and gives up in the form of NO the nitric reagent it has absorbed and which serves for the treatment of a new quantity of ferrous liquid" PA0 "In all cases it will be understood that with an absorption column sufficiently high, by suitably controlling, on the one hand, the stream of air and nitrous oxide and, on the other hand, the afflux of the acid ferrous sulfate, it is possible to completely reoxidize the latter, without allowing the escape of the catalyzer which moves to and fro from the top to the bottom of the absorption apparatus and which does not issue either with the ferric liquid from which it has been expelled by the heat."
In this process the absorption apparatus is open to the atmosphere. Consequently, the gas leaving its contact with the ferrous liquid is at atmospheric pressure. The maximum pressure employed in the process is equal to the pressure in the system at the location where air enters the system. This maximum pressure cannot exceed the sum of the prevailing atmospheric pressure plus the hydrostatic pressure created by the total height of liquid between the location at which the air enters the system and the location at which the nitrogen plus any excess air leaves its contact with the ferrous liquid. In other words, the maximum superimposed pressure in the system is equal to the pressure created by the hydrostatic head just described. Moreover, as the nitrogen plus any excess air moves from the location at which air enters the system to the ferrous liquid-atmosphere interface, the pressure of the undissolved gas continually diminishes.
The reaction between ferrous sulfate and nitric acid requires a temperature that cannot be substantially below 80.degree. C. as explained by de Loisy--page 1, lines 51-55.
According to de Loisy, the NO oxygen-carrier, or oxygen transfer agent termed catalyzer by de Loisy, once entering the system in which the ferrous is regenerated to ferric sulfate, except for insignificant make-up quantities, never leaves the system. In other words, when the nitrogen separates from its contact with the ferrous liquid at atmospheric pressure, the nitrogen is accompanied by an insignificant amount of NO.
In the words of the patent, page 2, column 1 lines 59 through page 2 column 2 line 70:
The relatively low pressure at which the de Loisy process is carried out necessitates the installation of quite elaborate gas-liquid contacting means to prevent the escape of NO. This requirement is necessary because the reaction at atmospheric pressure of NO and O.sub.2 is notoriously slow. To provide the required gas-liquid contacting means requires a very high capital investment. This very high capital investment causes the de Loisy process to be economically unattractive. In addition to this economically unattractive feature of de Loisy, it suffers from a second draw-back. According to de Loisy the ferrous liquid to be regenerated first serves to absorb the mixture 2NO+O.sub.2 and becomes laden with nitric and nitrous acid constituting the oxidizing reagent. De Loisy describes on page 1, column 2, lines 69-76 the reaction of NO and oxygen to form nitrogen dioxide and its further reaction with water in which the NO.sub.2 is converted to a mixture of nitric and nitrous acid: EQU (5) 2NO+O.sub.2 =2NO.sub.2 EQU (6) 2NO.sub.2 +H.sub.2 O=HNO.sub.3 +HNO.sub.2
The patent is silent with regard to the possibility of a reaction between NO.sub.2 or nitrous acid and ferrous ion at low temperatures. Why this possibility is not mentioned can only be conjected. Without being concerned about the reason for this omission, it can be stated as a fact, that at a temperature as low as 0.degree. C., NO.sub.2 or nitrous acid and ferrous ion react at an appreciable rate to oxidize the ferrous ion to ferric ion concomitantly with the reformation of NO. Not only does this reaction take place at low temperatures it takes place at low concentrations of the reactants.
For example, at a temperature of 0.degree. C., employing a ferrous ion concentration of 1 millimole per liter, i.e. 0.056 grams of ferrous ion per liter; and a nitrous acid concentration of 4 millimoles per liter, i.e. 0.252 grams per liter; more than 20% of the ferrous ion is oxidized to ferric ion in three (3) minutes. It has been found that the reaction goes much more rapidly at:
(a) ambient and higher temperatures; PA1 (b) higher concentrations of NO.sub.2 or nitrous acid; and PA1 (c) higher concentrations of ferrous ion.
By employing ferrous sulfate-sulfuric acid solutions of any practical concentrations at temperatures that might be economically feasible, it becomes substantially impossible to obtain, except momentarily, a ferrous ion-containing solution in which there is more than a trace of nitrous acid.
Because NO.sub.2 or nitrous acid reacts quickly with ferrous ion to form ferric ion and NO, it is impossible to keep NO completely out of the gas phase. To avoid having more than insignificant quantities of NO leave with the nitrogen in the de Loisy process, an exceedingly elaborate gasliquid contacting apparatus must be used. Under any practical set of conditions within the purview of the de Loisy disclosure, it is substantially impossible to avoid the loss of appreciable amounts of NO when the nitrogen is vented to the atmosphere.
Because of these two reactions, the oxidation of NO with O.sub.2 to form NO.sub.2 which is very slow, at low pressures, and the oxidation of ferrous ion to ferric ion by nitrous acid or NO.sub.2 which is rapid avoiding significant losses of NO is neither technically nor economically feasible when operating in accordance with the teachings of de Loisy.
If the limitations of the de Loisy process described above did not exist, and if, in actuality, it could be carried out precisely in accord with the disclosure in the patent, it is clear from reactions (2) and (3) above that for every 4 moles of NO which are evolved in the oxidation step, 4 moles of ferric sulfate are formed. The four (4) moles of NO which are evolved are converted to two moles of HNO.sub.3 and two moles of HNO.sub.2 in the absorption apparatus. That is, a mole of NO moves, in the language of the patent, "to and fro from the top to the bottom of the absorption apparatus" for each mole of ferric sulfate regenerated.
The fundamental concepts of this invention are fundamentally different from that of de Loisy as will be evident from the following description and explanation.