WO1986002628A1

WO1986002628A1 - Process for removal of hydrogen sulfide from gases

Info

Publication number: WO1986002628A1
Application number: PCT/US1985/002179
Authority: WO
Inventors: Scott Lynn
Original assignee: The Regents Of The University Of California
Priority date: 1984-11-04
Filing date: 1985-11-01
Publication date: 1986-05-09
Also published as: EP0199815A1

Abstract

Two stage removal of hydrogen sulfide from a gas. In the first stage solutions of hydrogen sulfide and sulfur dioxide in a solvent react to produce sulfur and water. An excess of hydrogen sulfide is maintained in the first stage resulting in a solution containing sulfur, water and the excess hydrogen sulfide, or resulting in a gas containing the excess hydrogen sulfide. The excess hydrogen sulfide in the solution or in the gas is then treated in a second stage with a solution in the same solvent of sulfur dioxide in excess of or closely equal to that required to react with the hydrogen sulfide. Excess sulfur dioxide in the vapor phase from the second stage may be absorbed in neat solvent. The solvent is selected to have a high solvating power for sulfur dioxide, a lesser but substantial solvating power for hydrogen sulfide and to promote the reaction of hydrogen sulfide with sulfur dioxide. Such gases as natural gas, synthesis gas, and the tail gas of a claus plant may be so treated. By appropriate selection of solvents and/or conditions, accompanying minor components of the gas stream being treated may be removed and recovered if of value, e.g., propane from natural gas, carbon dioxide and water from synthesis gas, etc. The process avoids the need to maintain exact stoichiometric ratios of hydrogen sulfide and sulfur dioxide, it avoids the need to use high flow rates of solvent, a liquid phase capacitance for hydrogen sulfide is provided which dampens the effect of fluctuations in hydrogen sulfide content in the gas stream, etc.

Description

-I-

"PROCESS FOR REMOVAL OF HYDROGEN SULFIDE FROM GASES" ^: •

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the removal of hydrogen sulfide from gases.

2. Description of the Prior Art

One method of removing hydrogen sulfide from a gas, for example from natural gas or from a gas produced in an industrial process, is by absorbing it physically in an organic solvent and then reacting it with a solution of sulfur dioxide in the same solvent in accordance with the following equation:

(1) 2H S + S0₂ J 2H₂0 + 3S

The sulfur dioxide may be derived from an outside source, or it may be produced by burning some of the hydrogen sulfide in accordance with the following reaction

(2) 2H₂S + 30 ^■» 2H₂0 + 2S0₂

or by burning a part of the sulfur that is produced in Reaction (1) in accordance with the following equation

(3) S + 0₂ -> S0₂

A second, related, method of removing hydrogen sulfide from such a gas is by absorbing it chemically in an organic solvent that contains dissolved sulfur dioxide. Reaction (1) then occurs as the hydrogen sulfide is absorbed and may accelerate the absorption process.

Still -a third related method of removing hydrogen sulfide from such a gas is to add sulfur dioxide to the gas and then to contact the gas with an organic solvent. The hydrogen sulfide and sulfur dioxide are absorbed simultaneously and Reaction (1) occurs as absorption takes place.

In all three of these methods of removing hydrogen sulfide there is only one reaction zone. That zone may or may not correspond with the zone in which gas absorption takes place. Theoretically, if the sulfur dioxide entering the reaction zone is maintained at the exact stoichiometric ratio required by Reaction (1), there will be no excess hydrogen sulfide or sulfur dioxide in the liquid or gas streams leaving the reaction zone. However, it is not possible, as a practical matter, to do this. Among other things either the flow or the hydrogen sulfide content of the gas being treated may vary so that it is necessary to vary the feed of sulfur dioxide to the reaction zone. It is therefore difficult to avoid having residual, unreacted, hydrogen sulfide or sulfur dioxide in either the gas or liquid streams that leave a single reaction zone. The gas stream may then not meet specifications for sulfur species. The presence of hydrogen sulfide or sulfur dioxide may interfere with the recovery of elemental sufur from the liquid stream. Finally, it may be desired to co-absorb other gas components as well as hydrogen sulfide from the gas stream being treated, and it would then be desirable to recover such gases free of sulfur compounds. Thus all three of the processing methods described above encounter operating problems from unreacted hydrogen sulfide or sulfur dioxide. U. S . Patents Nos . 2 , 881 , 047 ; 2 , 987 , 379 ; 3 , 103 , 411 ; 3,363,989; 3,953,586 and 4,107,269 and British Patent Publication 2012806, and a paper by Bieder ann and Rossarie entitled, "The Solclaus Process: A Direct Sulfur Recovery From Sour Gas" -presented at the Cleveland Meeting of The American Institute of Chemical Engineers, August 29 - September 1, 1982, describe various procedures for accomplishing hydrogen sulfide removal by one of the above methods. However, the processes of each of these patents and this publication are lacking in one or more respects with regard to the solution of the problem of adequately removing hydrogen sulfide from gases and converting it to sulfur. For example, U.S. Patent 3,953,586 to Tani ura dissolves hydrogen sulfide- from a gas in a solvent such as N-methyl-2-pyrrolidone, then reacts the hydrogen sulfide with sulfur dioxide also dissolved in that solvent. The proportion of hydrogen sulfide and sulfur dioxide in the reactor are such that there is an excess of hydrogen sulfide which leaves in the exiting solution and is removed in a stripper. The hydrogen sulfide removed in the stripper is then burned to produce sulfur dioxide for Reaction (1) .

The Tanimura process requires that one-third of the hydrogen sulfide pass through the reaction zone unreacted, that this excess hydrogen sulfide be stripped from its solvent and burned to produce sulfur dioxide in accordance with Reaction (2), and that the resulting sulfur dioxide be absorbed in the same solvent. This is an energy-intensive process. It also results in the loss of any other gas that is co-absorbed with the hydrogen sulfide.

Other processes require a precise control over the proportions of hydrogen sulfide and sulfur dioxide in the reaction zone. This is difficult for reasons stated above. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of treating a gas containing hydrogen sulfide by a method involving Reaction (1) which avoids or diminishes difficulties which have been encountered heretofore.

It is a particular object of the invention to provide a method of removing hydrogen sulfide from a gas by means of Reaction (1) which does not depend upon maintaining a stoichiometric ratio of sulfur dioxide to hydrogen sulfide in the reaction zone, which provides a readily managed continuous control of the ratio of reactants and which does not require a large energy input to strip a gas or gases from solution.

It is a further object of this invention to provide a solvent mixture in which Reaction (1) is very effectively catalyzed by a solvent component of such low volatility and such low concentration that volatile loss of that component is negligible.

In accordance with the present invention, the difficulties encountered in the prior art are solved by absorbing hydrogen sulfide from a gas in a solvent having the characteristics described below; providing a solution in the same solvent of sulfur dioxide; mixing the two solutions in proportions such that the hydrogen sulfide is in small excess and causing reaction to occur in a first reaction zone; providing a zone of substantial liquid capacitance for hydrogen sulfide to effect a method of damping the fluctuations in hydrogen sulfide concentration in the vapor or liquid streams leaving the first reaction zone; and providing a second reaction zone in which the hydrogen sulfide leaving the first reaction zone reacts substantially to completion according to Reaction (1) with sulfur dioxide solution that has been introduced thereinto. The zone of substantial liquid capacitance for hydrogen sulfide may coincide either with the first reaction zone or with the second reaction zone, or may be separate from both. The hydrogen sulfide leaving the first reaction zone may be transferred directly to the second reaction zone by a stream of vapor or liquid leaving the first reaction zone or, alternatively, may first be stripped from the liquid in which it is dissolved by a vapor stream and then be reabsorbed from that vapor into the liquid in the second reaction zone. The sulfur formed via Reaction (1) in either reaction zone may either remain dissolved in the solvent or precipitate, forming a slurry of crystals in the liquid in the reaction zone. Examples of these methods of employing this invention are illustrated in the following process descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the invention are illustrated by way of example in Figures 1, 2, 3, 4 and 5 in which:

Figure 1 is a flow diagram showing one embodiment of the invention;

Figure 2 is a similar flow diagram showing an alternative embodiment;

Figure 3 is a flow diagram of a third embodiment of the invention in which a gas at near-atmospheric pressure that contains hydrogen sulfide such as a Claus process tail 'gas, is treated;

Figure 4 is a flow diagram of an embodiment of the invention in which a gas at high pressure, such as synthesis gas, is treated to remove hydrogen sulfide and and also, where present as in synthesis gas, carbon dioxide;

Figure 5 is a flow diagram similar to Figure 4 but illustrating a-modification in which the reaction between hydrogen sulfide and sulfur dioxide is carried out under conditions to cause crystallization of sulfur.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to Figure 1 there are shown a Hydrogen sulfide absorber 10, a stripper-absorber 11, a solvent dehydrator 12, a sulfur crystallizer 13, a furnace 14, and a sulfur dioxide absorber 15.

Gas enters absorber 10 through line 16. In the process illustrated it is assumed that this feed gas is natural gas which contains typically about 0.01 to 10 percent of hydrogen sulfide and small amounts of water and hydrocarbons higher than methane, chiefly propane and butane. The bulk of the gas is methane and it is desired to remove hydrogen sulfide so that the amount remaining in the treated gas is one part per million or less. In the process illustrated it is also desired to remove water and to separate hydrocarbons higher than ethane. Other feed gases may, if desired, be treated, for example refinery gases from hydrotreating crude oil, synthesis gas obtained from petroleum or coal, and similar hydrogen-sulfide- containing gas streams. Also, the removal of one or more of the minor components such as ethane or higher hydrocarbons may be unnecessary or undesired, but in the case illustrated by Figure 1 it is desired to remove and recover higher hydrocarbons.

A suitable solvent selected from Table I below enters absorber 10 through line 17. This solvent is selected in accordance with the following criteria: It is a very strong solvent for sulfur dioxide and it has good solvating properties for hydrogen sulfide but less than its solvating property for sulfur dioxide. Solubility of various components of the feed gas in the solvent are as • follows: S0 >>5i2^{s>C0S c}3^H8_' Further, the solvent is selected to -promote Reaction (1). Criteria of the solvent are further elaborated below.

Suitable contacting equipment such as various commercially available trays or packing may be employed to promote contact between the feed gas and the solvent in absorber 10. The process is conducted so that the gas leaving through line 18 has an acceptably low content of hydrogen sulfide, e.g. one ppm or less.

The treatment in absorber 10 is at a low temperature, for example 0° to 30°C, in order to favor absorption of hydrogen sulfide in the solvent. The resulting solution leaving through line 19 is heated at 20 by indirect heating, for instance by using steam as the heating medium, e.g. to 80° to 120°C sufficient to maintain sulfur in solution. The steam used for this purpose may be generated in the process itself or it may come from an outside source or there may be a mix of the two sources of steam.

A solution of sulfur dioxide generated as described below joins line 19 through line 25 and the combined streams, suitably mixed as by means of a mixing tee (not shown), pass through a tube 26 which constitutes the first reaction zone where Reaction (1) occurs. In this reaction zone hydrogen sulfide is present in excess. Typical concentrations of hydrogen sulfide and the sulfur dioxide in the reacting stream at the point of mixing and ^" before reaction has occurred are about 0.1 to 10 percent sulfur dioxide and 0.1 to 10 percent hydrogen sulfide. Reaction (1) is exothermic and, since the solvent is selected to promote Reaction (1) the reaction is very rapid and goes to completion within a few seconds and a correspondingly short distance. The proportions in which the two streams are mixed are such that there is a small excess of hydrogen sulfide. This excess hydrogen sulfide is typically such that the solution entering the stripper section 11A of stripper-absorber 11 contains about 0.001 to 0.1 percent of hydrogen sulfide. A large fraction of this dissolved hydrogen sulfide is stripped from the solution in the stripper section 11A of the stripper-absorber 11.

A portion of the sulfur dioxide solution is diverted from stream 25 into line 27 and is introduced into the reactive absorber section 11B of stripper-absorber 11. The proportion so diverted is in excess of that required to react with the excess of hydrogen sulfide introduced into the stripper section 11A of stripper-absorber 11. The reactive absorber section 11B thus forms a second reaction zone, one in which sulfur dioxide is present in excess. This will carry Reaction (1) to completion so that no appreciable quantity of hydrogen sulfide remains and it results in a solution of sulfur in this solvent which also contains much of the sulfur dioxide that enters the stripper-absorber 11 through line 27. The resulting vapor in the top section 11C of stripper-absorber 11, which is completely free of hydrogen sulfide, contains some sulfur dioxide that is absorbed by fresh, neat solvent entering through line 30. The light gases leaving stripper-absorber 11 through line 35 are thus substantially free of both hydrogen sulfide and sulfur dioxide.

The stripper-absorber 11 is shown as having a tray 31 between stripper section 11A and absorber (second reaction) zone 11B. The tray is a chimney type of tray which permits vapor to pass from section 11A into section llB but prevents liquid from flowing from section 11B into section 11A. In the process just described it is relatively easy to maintain a net stoichiometric ratio of sulfur dioxide to hydrogen sulfide. In stripping zone 11A a small excess of hydrogen sulfide is maintained and the amount and fluctuation of this excess in the vapor phase is monitored and used to control the flow of sulfur dioxide solution through line 25 into reaction zone 26. If the concentration of hydrogen sulfide in the vapor in zone 11A becomes too small the rate of flow of sulfur dioxide solution will be diminished whereas if the concentration of hydrogen sulfide becomes too great the rate of flow of sulfur dioxide solution will be increased.

This procedure precludes the need to maintain a stoichiometric ratio of hydrogen sulfide to sulfur dioxide in reaction zone 26; the amount of hydrogen sulfide in the vapor in zone 11A is easily monitored and need not be maintained at a precise level but may fluctuate between limits; the management of flow of sulfur dioxide solution into reaction zone 26 is easy to accomplish; and the process lends itself to automated control by control equipment that is commercially available. The flow of sulfur dioxide solution through line 27 into reactive absorber section 11B is kept at a constant value. The capacitance for hydrogen sulfide of the solution inventory in this section of the stripper-absorber 11, provided by the excess sulfur dioxide in the solution, effectively dampens the fluctuations in the net flow of hydrogen sulfide.

Furthermore, since the excess of hydrogen sulfide in line 26 is small, and since the solubility of hydrogen sulfide in the solvent at the elevated temperature in stripper section 11A is relatively small, the flow of stripping vapor needed to strip the excess hydrogen sulfide from solution is not large. In the embodiment of Figure 1 propane is a constituent of the feed gas which it is desired to recover separately. The propane vapor generated in stripper section 11A will provide at least a portion of the gas needed to strip hydrogen sulfide from solution. Also the amount of sulfur dioxide in the vapor leaving zone 11B is small arid the solubility of sulfur dioxide at the temperature in -zone 11C is quite high, therefore very little lean solvent need be supplied through line 30. As a result the process is not energy-intensive.

The effluent gas and vapor from stripper-absorber 11 leaves through line 35, is cooled in cooler 36 and the condensate is collected in vessel 37. Gases, chiefly ethane, are vented through line 38. This ethane may be compressed and combined with the methane leaving absorber 10, or it may be burned as a fuel or it may be otherwise disposed of. A portion of the condensate is returned to the stripper-absorber 11 as reflux through line 39 and the remainder, chiefly propane, is withdrawn through line 40. This separation of propane provides a useful by-product of the invention.

The liquid phase is removed from the bottom of stripper-absorber 11 through line 41. This liquid phase may be a homogeneous solution of sulfur, water and higher hydrocarbons in the solvent or it may be a two-phase liquid mixture of such a solution and liquid sulfur. It is heated by steam in heater 42 and a portion of the heated liquid is returned through line 43 to the bottom of stripper-absorber 11. Therefore the heater 42 also serves as a reboiler. The liquid phase or phases passing forwardly through line 41 enters solvent dehydrator 12 at its mid-portion. Water is evaporated and leaves the solvent dehydrator along with hydrocarbon vapor, chiefly butane vapor, through line 44 and is condensed in cooler 45. The condensate passes by line 44 to vessel 50. Two phases are present in vessel 50, namely a lower aqueous phase and an upper hydrocarbon phase. The aqueous phase is removed from the system through line 51. A portion of the hydrocarbon phase is returned to the upper end of the solvent stripper 12 through line 52 to serve as reflux and the remainder of the hydrocarbon phase is recovered through line 53 as a by-product. ^: : ^' . The dehydrated solvent leaves the bottom of solvent dehydrator 12 through line 54. A portion of the liquid is vaporized in reboiler 55 and returned to the bottom of solvent dehydrator 12 through line 56 and the remainder of the solvent proceeds by line 57 to sulfur crystallizer 13. Sulfur crystallizer 13 may be of conventional design and mode of operation, for example the solvent may be cooled.by means of direct contact with liquid propane refrigerant. This results in the crystallization of sulfur. Suitable equipment (not shown) is employed to separate the solid sulfur from the solvent which is returned by way of line 17 to the top of absorber 10 and the top of stripper-absorber 11. One-third of the sulfur proceeds by way of line 60 to furnace 14 and two-thirds is withdrawn from the system through line 61.

Furnace 14 is supplied with air by compressor 62. The proportion of sulfur diverted to furnace 14 is sufficient to provide the sulfur dioxide needed for Reaction (1). The heat of Reaction (3) is used to generate steam in steam coil 64. This steam may be used in the system. The gas phase (sulfur dioxide, nitrogen, etc.) leaves the furnace by way of line 65 and enters sulfur dioxide absorber 15. This sulfur dioxide is absorbed in a stream of solvent diverted from line 17 through line 66. This step may be conducted so that the stack gas leaving through line 67 contains only a trace of sulfur dioxide. A solution of sulfur dioxide leaves the bottom of sulfur dioxide absorber 15 through line 25 and re-enters the system. Referring now to Figure 2 a variant is shown which is applicable to a situation in which it is not necessary to recover gases such as propane and other higher hydro- carbons. An example is the removal of hydrogen sulfide from a raw synthesis gas -- a mixture of hydrogen, carbon monoxide, carbon dioxide, methane and other, very minor components. In this instance the conditions in hydrogen sulfide absorber 10, such as the choice of solvent and the conditions of operation, will be such that only hydrogen sulfide and carbon dioxide are absorbed.

Most of the units employed in the variant of the Figure 2, namely the hydrogen sulfide absorber 10, the sulfur crystallizer 13, the furnace 14, and the sulfur dioxide absorber 15 remain^' the same although in certain instances, as will appear from the description below, the mode- of operation is different than in Figure 1. Lines and equipment in Figure 2 which are numbered as in Figure 1 are the same. Element 11 functions only as a hydrogen sulfide stripper.

Inasmuch as lines 27 and 30 are eliminated, the unit 11 will not function as a hydrogen sulfide absorber or reactor; therefore the gas leaving this unit through line 35 will contain a small amount of hydrogen sulfide which, along with other uncondensable gases, principally carbon dioxide, will be separated from water by cooler 36 and the water phase will be removed through line 40. The noncondensable gases will leave through line 38 and go to sulfur dioxide absorber 15 which will result in reaction of this small excess of hydrogen sulfide with the sulfur dioxide in the absorber 15. It will therefore be apparent that the sulfur dioxide absorber 15 performs the function of zone 11B in unit 11 in Figure 1, providing both a zone of substantial liquid capacitance for hydrogen sulfide and the second reaction zone for the process. Referring now to Figure 3 the application of the present invention to the treatment of a gas at near- atmospheric pressure is illustrated. When hydrogen sulfide is present in such a gas, especially when its concentration is also relatively low, it may not be practical to absorb the hydrogen sulfide physically as an initial step. The solvent flow required might then be excessively high. Alternatively, when hydrogen sulfide is the only component that one wishes to remove from a gas stream, it may be desired to keep the flow of solvent as low as possible to minimize co-absorption of other components. For both of these design conditions the process configuration shown in Figure 3 is a preferred embodiment of this invention. This process configuration is also suitable for treating the tail gas from a Claus process sulfur plant. Such a gas contains sulfur dioxide as well as hydrogen sulfide.

The Claus process carries out Reaction (1) in the vapor phase by contacting a mixture of hydrogen sulfide and sulfur dioxide in the vapor phase with a solid catalyst. The hydrogen sulfide and sulfur dioxide are employed in stoichiometric ratio; the reaction is carried out at an elevated temperature and results in an equilibrium in which about five percent of the H₂S/S0₂ mixture is unreacted; and this gaseous mixture (the tail gas) requires treatment before it can be vented to the atmosphere. Typically the hydrogen sulfide results from absorbing it from a gas, for example natural gas or a process gas, into an alkaline solution such as an aqueous solution of ethanolamine or sodium carbonate from which it is then stripped by steam and is used in the Claus process.

The gas to be treated (whether from a Claus .plant or from some other source) enters reactor-absorber 84 through line 85. The gas stream is usually kept well above the dewpoint of water because of the corrosive nature of the sulfoxy acid compounds that form in liquid water when ,__ both hydrogen sulfide and sulfur dioxide are present (Wackenroder's liquid). The gas enters the bottom section 84a of absorber 84 which has a reactor section 84b and an upper section 84c. A solution of sulfur dioxide enters the top of section^'84b through line 86. Within the reactor section of column 84 hydrogen sulfide reacts with both the sulfur dioxide entering with it through line 85 and with the sulfur dioxide entering with the solvent in line 86. Fresh, neat solvent enters the top of the absorber section 84c of column 84 through line 87. Within the absorber section virtually all of the ^'sulfur dioxide remaining in the gas stream is absorbed, reacting with the hydrogen -sulfide that is co-absorbed with it. The concentration of hydrogen sulfide in the gas stream leaving the top of column 84 through line 88 is monitored and is kept at a small, approximately constant value by varying the flow of sulfur dioxide solution entering through line 89.

The liquid stream leaving the reactor section of column 84 contains a small amount of dissolved hydrogen sulfide, which is stripped from the liquid by steam that is introduced through line 90 at the bottom of the stripper section 84a either by direct injection or by boiling water out of the solvent mixture by indirect heat exchange. The liquid stream leaving the bottom of column 84 through line 91 is thus substantially free of dissolved hydrogen sulfide and sulfur dioxide, but contains in solution the sulfur that has been formed by reaction. This stream is then sent to a crystallizer for recovery of sulfur as has been previously described.

The gas stream in line 88 enters the bottom of reactor-absorber column 92. A fixed flow of sulfur dioxide solution enters the top of the reactor section 92a of column 92 through line 93. The sulfur dioxide contained in this stream is in small excess over the hydrogen sulfide contained in the gas stream from line 88, so that substantially all of the hydrogen sulfide is removed from the gas stream as it passes through section 92a of column 92. Fresh, neat solvent entering the absorber section 92b of column 92 through line 94 serves to reabsorb any sulfur dioxide that may have been stripped from solution by the gas stream as it passed through the reactor section of column 92. The gas stream leaving column 92 through line 95 is thus substantilly free of both hydrogen sulfide and sulfur dioxide and may be discharged to the atmosphere. The liquid stream leaving column 92 through line 96 contains unreacted sulfur dioxide and is pumped by pump 97 to column 84 as previously noted.

The sulfur dioxide used to form the sulfur dioxide solution that enters through lines 89 and 93 can be produced by burning either hydrogen sulfide or part of the sulfur made in this process. As before, the heat released in the combustion can be used to produce steam.

As in the previous embodiments of this invention, the process configuration in Figure 3 makes use of two reaction zones, one in which hydrogen sulfide is in excess and one in which sulfur dioxide is in excess. The control of the exact stoichiometry of the reaction is made relatively simple by the capacitance for hydrogen sulfide of- solvent containing sulfur dioxide, which allows moderate variations in the hydrogen sulfide content of the tail gas from the Claus plant to occur without upsetting the control of the process.

In operating the^' process shown in Figure 3 •the gas stream entering through line 85 must contain a ratio of hydrogen sulfide to sulfur dioxide that is substantially greater than 2. This excess hydrogen sulfide can be provided by bypassing part of the hydrogen sulfide around the Claus plant if it proves undesireable to operate the Claus plant with such an excess. However, the process shown in Figure 3 is not limited to the treatment of the tail gas from a Claus plant nor to the treatment of a gas stream containing both hydrogen sulfide and sulfur dioxide. It is suitable as an alternative to the process shown in Figure 1 for treating any gas stream that contains hydrogen sulfide, but is particularly suitable for treating low-pressure gas streams because of the reduced flow of solvent that is required. Furthermore, it will be apparent to those skilled in the art that heating or cooling of some of the streams in Figure 3 may be required in some instances, and that columns 84 and 92 could be consolidated if desired.

Still another example of a process configuration that utilizes the principles of this invention is shown in Figure 4. The gas to be treated in this example is raw "synthesis gas", a mixture of hydrogen and carbon monoxide that contains carbon dioxide and hydrogen sulfide as undesireable components. The pressure of the gas stream is relatively high, typically at about 40 atmospheres. The gas to be treated enters high-pressure absorber 97 through line 98. Lean solvent enters through line 99 and contacts the gas in absorber 97 countercurrently, absorbing most of the carbon dioxide and substantially all of the hydrogen sulfide from the gas, which leaves absorber 97 through line 100. Small amounts of hydrogen and carbon monoxide are co-absorbed in the solvent stream, which leaves absorber 97 through line 101. This hydrogen and carbon dioxide are stripped from the solvent in the operation described below and are returned, together with some carbon dioxide and hydrogen sulfide, through line 102.

stream of sulfur dioxide dissolved in the solvent is added to the fat solvent in line 101 through line 104. Reaction (1) occurs in line 105 before the combined streams enter stripper 106, where hydrogen and carbon monoxide are stripped from the solvent. The temperature of the stream in line 105 is monitored and heater 103 supplies any heat that is needed to ensure that sulfur does not precipitate. The quantity of sulfur dioxide entering through line 104 is regulated to be about 90 to 99 per cent of that required to react with all of the hydrogen sulfide in the stream in line 101. This regulation is accomplished by monitoring the hydrogen sulfide content of the gas leaving stripper 106 through line 102 and keeping it at a relatively low, constant value.

The stripping vapor in stripper 106 is predominantly carbon dioxide, which is boiled out of solution in line 107 as a result of heat supplied by heater 108. The purpose of this stripping vapor is not only to recover hydrogen and carbon monoxide from the solvent but also to homogenize the hydrogen sulfide concentration in the solvent. As the solvent flows through the column, countercurrently to the vapor stream, small fluctuations in the hydrogen sulfide concentration tend to disappear because of the mass transfer and back-mixing that is characteristic in such devices. As a result, the concentration of hydrogen sulfide in the solvent leaving stripper 106 through line 109 will have a relatively constant value that is about one-tenth to one-hundredth that of the stream leaving absorber 97 through line 101. This remaining hydrogen sulfide is eliminated by adding a second regulated stream of sulfur dioxide through line 110. Reaction (1) occurs in line 111 before the solvent mixture reaches cooler 112. The sulfur content (either hydrogen sulfide or sulfur dioxide) of the carbon dioxide stream leaving crystallizer 113 through line 114 is negligible. The flow of sulfur dioxide solution through line 110 is regulated to keep the sulfur compound level in line 114 in the low parts-per-million range. It is advantageous to maintain a slight excess of sulfur dioxide in the solvent leaving line 111. Because of its higher solubility, a given excess of sulfur dioxide in the solvent results in substantially less sulfur content of the gas in line 114 than does a similar excess of hydrogen sulfide. However, for this process configuration operation in either mode is÷feasible. Cooler 112 reduces the solvent temperature to-the point of incipient sulfur precipitation. Additional cooling is produced when the dissolved carbon dioxide flashes from solution at the pressure of the crystallizer. The operations of the crystallizer 113, the sulfur furnace and boiler 116, and the sulfur dioxide absorber 117 are similar to those described in the previous examples. Solvent leaves crystallizer 113 through line 118 and a portion is diverted to sulfur dioxide absorber 117 where it absorbs sulfur dioxide from furnace 116 entering through line 119. A portion of the sulfur leaving crystallizer 113 is diverted through line 115a to furnace 116.

As in the previous three examples of Figures 1, 2 and 3, the process configuration of Figure 4 depends upon having two reaction zones and a method of homogenizing the concentration of the liquid stream passing from the first zone to the second by utilizing the capacitance of the solution for dissolved hydrogen sulfide. The zone of high capacitance in this example is located between the two reaction zones. Again in this example it is essential to have excess hydrogen sulfide in the first reaction zone and advantageous to have excess sulfur dioxide in the second reaction zone. However, in this example it is feasible to operate with a small excess of hydrogen sulfide in the second reaction zone as well.

Under some circumstances it may be preferable to carry out the reaction between hydrogen sulfide and sulfur dioxide without heating the solution to ensure that the sulfur formed remains dissolved. In this case crystallization will occur simultaneously with reaction. It may also be desirable to carry out the reaction and crystallization with simultaneous cooling, by flashing a dissolved gas. An example of this method of practicing the invention is shown in Figure 5, a modified version of Figure 4. : - ^• , In Figure 5, the high-pressure absorber 97 is operated as described with reference to Figure 4, and flow lines and pieces of equipment that remain unchanged bear the same reference numerals as in Figure 4. The rich solvent leaving absorber 97 through line 101 is cooled in exchanger 103 to near the temperature of the cooling water before it enters flash cha ber-crystallizer 120. In 120 the pressure is reduced to flash carbon dioxide from solution in sufficient quantity to obtain the desired temperature. Sulfur dioxide solution enters 120 through line 104 at a metered rate that is regulated to maintain a constant, small fraction of unreacted hydrogen sulfide in the gas leaving 120 through line 102. This gas is compressed to the pressure of absorber 97 in compressor 121 and returned to the bottom of the absorber. The liquid stream leaving flash chamber-crystallizer 120 through line 109 contains a slurry of sulfur crystals and a small, nearly constant concentration of unreacted hydrogen sulfide. As in the flowsheet of Figure 4, sulfur dioxide solution is added to this stream through line 110 in nearly exact stoichiometric ratio, as determined by monitoring for traces of unreacted hydrogen sulfide or sulfur dioxide in the sweet carbon dioxide in line 114 that has flashed from solution in crystallizer 113.

The remainder of the process -- sulfur separation, sulfur burning, and sulfur dioxide absorption -- is unchanged from Figure 4.

As will be apparent from a comparison of Figures 4 and 5, combining flashing, reaction and crystallization in a single operation can reduce energy consumption for this process under some conditions. Operation in this manner is within the scope of this invention since one is still utilizing the high rate of reaction that can be realized even at ambient temperature by the reaction catalysts disclosed herein and is still utilizing the two reaction zones that facilitate control of the overall reaction stoichiometry. The zone of high liquid capacitance for hydrogen sulfide in this example coincides with the first reaction zone. Reactor-crystallizer 120 is a stirred-tank reactor, with well-defined capacitance characteristics.

Choice of Solvent Components

The organic solution or solvent should, as stated above, be one in which sulfur dioxide has a high solubility and which has a solvating power for hydrogen sulfide and other concomitants in accordance with the following descending scale: S0₂>>H₂S>COS and mercaptans >C0₂, C3H₈.

The solvating power of the solvent for the major constituents of the gas, for example methane in the case of natural gas, should of course be quite low. Further, the solvent should promote Reaction (1) and it should not form strong chemical complexes with sulfur dioxide or with constituents of the feed gas. Where it is desired to remove water from the feed gas a more polar solvent is indicated. The solvent should have moderate miscibility with water and should be a moderately good solvent for sulfur, for example, 'capable of dissolving at 25°C one gram per liter or more.

Where it is desired to absorb propane or other higher hydrocarbons the solvent may be modified by including a less polar component. Solvent mixtures (i.e., "mixtures" in the sense of two or more liquid components which are in solution as a homogeneous phase) may be employed to advantage to achieve such objects.

To minimize loss of volatile solvent components in the gas streams being treated it is desirable to use components of low volatility. In most cases liquids with normal boiling points greater than 180°C are preferred.

in Table I below, there is a list of solvent components by category. Such solvents may be employed individually or in combination of two or more solvents.

Table I

(1) Solvent components which have very high solvating power for SO_.

Dialkyl ethers of polyethylene glycols such as triethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, etc.

Dialkyl ethers of polypropylene glycols such as tripropylene glycol dimethyl ether, tetrapropylene glycol diethyl ether, etc.

Monoalkyl ethers of polyethylene glycols such as diethylene glycol monomethyl ether, triethylene glycol monoethyl ether, etc.

Monoalkyl ethers of polypropylene glycols such as dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether.

These glycol ethers have the general formula R_0-f—R-0-_—_nR where R is -CH -CH - or -CH -CH(CH3 ) -, n represents the number of alkylene oxide units, e.g. 3 or 4, R]_ is alkyl (e.g. methyl or ethyl) and R is hydrogen or alkyl.

Tertiary aromatic amines such as N,N-dimethγl aniline, N-phenyl diethanolamine, etc. ^{' :} Trialkyl phosphates such as tributyl phosphate, tripropyl phosphate, etc.

Tetrahydrothiophene oxide (sulfolane).

(2) Solvents which are good catalysts for Reaction (1)

High boiling aromatic compounds containing nitrogen within a ring, such as quinoline, aσrolein, the benzyl pyridines and similar compounds.

Dimethyl aniline.

bis-Methylene-4,4'-dimethylaminobenzene.

Trialkyl phosphates such as those in Section (1) above.

(3) Solvents which are good solvents for sulfur

N,N-Dimethyl aniline.

bis-Methylene-4,4' -dimethylaminobenzene.

Other tertiary aromatic amines such as N,N-diethγl aniline, quinoline and isoquinoline.

Further considerations in the choice of a solvent are as follows: Reaction (1) is a first order reaction with respect to each of the reaσtants. Therefore its rate is described by the following equation:

in which R = rate of disappearance of S0 , moles/liter-sec. k₂ = reaction rate constant, liter/mole-sec. [^{' ■}]. = concentration in moles/liter ^' . and k₂ is assumed to be a function only of temperature. For use in the process of the present invention a solvent preferably has a value of k₂ at room temperature (25°C) of at least 1.0 and preferably of 10 liter/mole-sec. or higher.

The kinetics of Reaction (1) was determined for a specific solvent composition by carrying out the reaction in the following way: A sample of solvent containing hydrogen sulfide was placed in a calorimeter, together with a thermocouple and a magnetic stirring bar. A sample of solvent containing sulfur dioxide was then added rapidly to the calorimeter while stirring vigorously. The temperature rise that resulted from reaction was followed by recording the potential of the thermocouple as a function of time during the experiment. The change in temperature was used to calculate the change in concentrations of both hydrogen sulfide and sulfur dioxide as the reaction progressed, and this information was used to calculate the value of k₂ in the equation above.

Experiments of this sort show that quinoline and similar aromatic ring-nitrogen compounds are exceptionally effective catalysts for Reaction (1) and are particularly advantageous in the practice of this invention. As an example, the value of k₂ was determined for mixtures of N,N-dimethyl aniline (DMA) and triethylene glycol dimethyl ether (Triglyme) at 25°C as a function of composition. The values were about 1.0 at 1% DMA, 4.0 at 10% DMA, and 8.0 at 100% DMA. For a mixture of 1% quinoline in Triglyme the value of k₂ was about 20. In more concentrated solutions of quinoline in Triglyme the values of k were too high to be estimated accurately by this technique. Because of their low volatility and the low concentrations at which they are effective catalysts, quinoline, substituted pyridiries such as 4-benzyl pyridine and 3-pyridyl carbinol, and similar compounds can be used in the practice of this invention at low cost and with little volatile loss. Their use is thus preferred to the use of DMA, as taught by Urban (U.S. Patent No. 2,987,379), or the use of N-methyl-2-pyrrolidone as taught by Fuchs (U.S. Patent No. 3,103,411) and Tanimura (U.S. Patent No. 3,953,586).

A combination of solvents may be preferred, e.g. a mixture of a solvent from Section 1 of Table I for high solvating power for sulfur dioxide and dimethyl aniline for its catalytic and sulfur solvating properties.

It will therefore be apparent that a novel and advantageous method of removing hydrogen sulfide from gases has been provided.

Claims

WHAT IS CLAIMED IS:

- 1. In a method of continuously treating a gas stream containing fluctuating amounts of hydrogen sulfide to remove the hydrogen sulfide by reacting it with sulfur dioxide in accordance with the reaction

(1) 2H₂S + S0₂ -> 2 H₂0 + 3S

the improvement which comprises

(a) providing a solvent having a high solvating power for sulfur dioxide and a lesser but substantial solvating power- for hydrogen sulfide, such solvent being one which promotes the reaction of hydrogen sulfide with sulfur dioxide in accordance with the Reaction (1) and which does not form a strong chemical complex with or react chemically with sulfur dioxide

(b) absorbing hydrogen sulfide from such gas stream in such solvent

(c) reacting the absorbed hydrogen sulfide with sulfur dioxide, also dissolved in such solvent, in a first reaction zone in accordance with the Reaction (1) and maintaining in said first reaction zone an excess of hydrogen sulfide over that required for Reaction (1)

(d) reacting excess hydrogen sulfide from step (c) with sulfur dioxide in a second reaction zone in solution in such solvent so as to react substantially all of the excess hydrogen sulfide in accordance with the Reaction (1) (e) controlling the input of sulfur dioxide solution to step (c) to maintain the excess of hydrogen sulfide within desired limits and

^' ; ^'(f) providing a liquid phase capacitance for hydrogen sulfide which dampens the effect of fluctuations in hydrogen sulfide content in the gas stream.

2. The improvement of Claim 1 wherein control step (e) includes monitoring the hydrogen sulfide concentration in a gas phase or in a liquid phase separated from the first reaction zone.

3. The improvement of Claim 2 wherein the monitoring step is applied to a gas phase resulting from vaporizing excess hydrogen sulfide from the first reaction zone.

4. The improvement of Claim 2 wherein the monitoring step is applied to the liquid reaction mixture resulting from step (c).

5. The improvement of Claim 1 wherein the capacitance step (f) is provided by the sulfur dioxide solution employed in step (d) .

6. The improvement of Claim 1 wherein the capacitance step (f) is provided by continuously mixing the product of step (c) in a tank or other vessel before carrying out step (d) .

7. A method of reacting hydrogen sulfide with sulfur dioxide in accordance with the following reaction

(1) 2 H₂S + S0₂ > 2 H₂0 + 3S

comprising providing a solvent [defined as in Claim 1] including at least a catalytic amount of an aromatic ring nitrogen compound.

8. A continuous process of treating a gas to remove hydrogen sulfide comprising:

(a) continuously contacting the gas with a solvent having a high solvating power for sulfur dioxide and a lesser but substantial solvating power for hydrogen sulfide, such solvent being one which promotes the reaction of hydrogen sulfide with sulfur dioxide to produce water and sulfur and which does not form a strong chemical complex with or react chemically with sulfur dioxide, thereby absorbing the hydrogen sulfide in such solvent

(b) continuously reacting the hydrogen sulfide in the solution resulting from step (a) with sulfur dioxide also dissolved in such solvent

(c) maintaining in step (b) a small excess of hydrogen sulfide, resulting in a solution of sulfur, water and at least a portion of such excess hydrogen sulfide in such solvent

(d) continuously stripping the solution resulting from step (c) of hydrogen sulfide, resulting in a gas phase containing hydrogen sulfide

(e) continuously reacting the gaseous hydrogen sulfide resulting from step (d) with sulfur dioxide in solution in the aforesaid solvent and (f) monitoring the quantity of hydrogen sulfide gas going from step (d) to step (e) and adjusting the input of sulfur dioxide to the process to maintain such excess within certain limits.

9. In the method of treating a gas to remove hydrogen sulfide wherein the gas is treated with a liquid solvent to absorb hydrogen sulfide and the resulting solution is contacted with sulfur dioxide to convert the hydrogen sulfide to sulfur and water in accordance with the following reaction

(1) 2H S + S0 ϊ» 2H₂0 + 3S

the improvement which comprises:

(a) employing as a solvent for absorbing hydrogen sulfide and as a solvent medium for Reaction (1) a liquid having a high solvating power for sulfur dioxide and a lesser but substantial solvating power for hydrogen sulfide, such solvent being one which promotes Reaction (1) and which does not form a strong chemical complex with or react chemically with sulfur dioxide,

(b) absorbing hydrogen sulfide from such gas in such solvent

(c) mixing the resulting solution with a solution of sulfur dioxide in the same solvent, said hydrogen sulfide solution and sulfur dioxide solution being mixed in proportions such that there is a small excess of hydrogen sulfide thereby resulting in a liquid reaction mixture containing such small amount of hydrogen sulfide,

(d) separating from the reaction mixture a gas phase containing most of the excess hydrogen sulfide, (e) contacting such gas phase with a solution in the same solvent of sulfur dioxide in excess of that required for Reaction (1) , thereby removing the hydrogen sulfide from the gas phase by Reaction (1) and resulting in a-gas phase containing a small amount of sulfur dioxide' and

(f) removing such small amount of sulfur dioxide from such gas phase by absorption in a quantity of the same solvent.

10. A method of treating a gas to remove hydrogen sulfide which comprises:

(a) providing a solvent having a high solvating power for sulfur dioxide and a lesser but substantial solvating power for hydrogen sulfide which ' solvent also serves to promote the following reaction

(1) 2H₂S + S0₂ » 2H₂0 + 3S

and which does not form a strong chemical complex with nor react chemically with sulfur dioxide,

(b) contacting such gas with such solvent under conditions to absorb substantially all of the hydrogen sulfide from the gas thereby resulting in a treated gas which is substantially free from hydrogen sulfide and a solution of hydrogen sulfide in the solvent,

^* (c) providing also a solution of sulfur dioxide in the same solvent, (d) mixing the aforesaid solutions of hydrogen sulfide and sulfur dioxide in proportions such that the hydrogen sulfide is in small excess of that required by Reaction (1), thereby resulting in a liquid reaction mixture containing dissolved sulfur, and a gas phase containing a small quantity of hydrogen sulfide,

(e) contacting such gas phase with a solution of sulfur dioxide in such solvent in excess of the amount required by Reaction (1) , thereby converting substantially all of the hydrogen sulfide to sulfur and water in accordance with Reaction (1).

11. The method of Claim 10 in which the gas phase resulting from step (e) contains sulfur dioxide and is contacted with fresh solvent to absorb the sulfur dioxide.

12. A method of treating a solution of hydrogen sulfide dissolved in a solvent which has a higher solvating power for sulfur dioxide than for hydrogen sulfide, which promotes the reaction of hydrogen sulfide with sulfur dioxide to produce sulfur and water and which does not form a strong chemical complex with nor react chemically with sulfur dioxide, said method comprising

(a) continuously mixing said solution with a solution in the same solvent of sulfur dioxide to cause such reaction to occur and maintaining an excess of hydrogen sulfide in the mixture;

(b) separating from the product of step (a) a vapor phase containing hydrogen sulfide and one or more liquid phases including a solution of sulfur in said solvent; and (c) continuously contacting the separated vapor phase with a solution of sulfur dioxide in the same solvent in excess of that required stoichiometrically for reaction of the hydrogen sulfide with sulfur dioxide to produce sulfur -and water thereby converting the hydrogen sulfide to sulfur and water.

13. The method of Claim 12 wherein a vapor phase is separated from the reaction mixture resulting from step (c), and is continuously contacted with the same solvent to absorb excess sulfur dioxide resulting from step (c).

14. The method of treating a feed gas containing a relatively small but objectionable amount of hydrogen sulfide comprising

(a) providing a solvent which has a high solvating power for sulfur dioxide and a lesser but substantial solvating power for hydrogen sulfide, which promotes the reaction of hydrogen sulfide with sulfur dioxide to produce sulfur and water and which does not form a strong chemical complex or react chemically with sulfur dioxide;

(b) providing a solution of sulfur dioxide in such solvent;

(c) treating the feed gas with such solvent to absorb the hydrogen sulfide and reduce its quantity in the treated gas to the desired amount;

(d) separating the resulting solution of hydrogen sulfide; (e) mixing solutions (b) and (d) in proportions such that the hydrogen sulfide is in excess of that required for reaction with sulfur dioxide to produce sulfur and water, and causing such reaction to go to completion thereby resulting in a solution of sulfur and water in the solvent and a gas phase containing excess hydrogen sulfide;

( f ) treating such gas phase ^wifch some of solution (b) in excess of that required for reaction with hydrogen sulfide to produce sulfur and water, thereby resulting in a solution containing excess sulfur dioxide and a gas phase containing sulfur dioxide; and

(g) recycling the solution resulting from step (f) to step (e).

15. The method of Claim 14 wherein the gas phase resulting from step (f) is contacted with such solvent to absorb the unabsorbed sulfur dioxide.

16. The method of Claim 15 wherein the unabsorbed gas is cooled to condense a portion thereof while leaving the remainder as a gas phase.

17. The method of Claim 16 wherein the feed gas treated in step (c) is natural gas containing at least one of the minor components, water, carbon dioxide and hydrocarbons higher than methane, and the condensation step of Claim 8 effects a separation of one or more of such minor components from.one or more of the other of such components.

18. The method of Claim 17 wherein the condensate contains propane and such higher hydrocarbons as are present in the feed gas.

19. A method of treating a gas containing a small concentration of hydrogen sulfide which comprises:

(a) contacting the gas in a^* reaction zone with a solution of sulfur dioxide in a solvent which has a high solvating 'power for sulfur dioxide and which promotes the reaction

(1) 2H₂S and S0₂ » 2H 0 + 3S

(b) maintaining in such reaction zone a small excess of hydrogen sulfide over that required for Reaction (1), thereby resulting in a solution of sulfur and water in the solvent and a gas phase containing hydrogen sulfide,

(c) removing such solution from the reaction zone,

(d) removing such gas phase from the reaction zone,

(e) treating the gas phase so removed with a solution in the same solvent of sulfur dioxide in excess of that required for Reaction (1), resulting in a gas phase substantially free of hydrogen sulfide.

20. The method of Claim 19 wherein heat is supplied to the solution resulting from step (b) to evaporate dissolved hydrogen sulfide and to add it to the gas phase resulting from step (b).

21. The method of Claim 19 wherein the gas introduced into the reaction zone in step (a) contains both sulfur dioxide and hydrogen sulfide.

22. The method of Claim 21 wherein the gas introduced into the reaction zone in step (a) is the tail gas of a Claus plant.

23. The method of Claim 19 wherein the gas phase resulting from 'step (e) is treated with fresh solvent to absorb sulfur dioxide.

24. A process of treating a gas at high pressure containing carbon monoxide and hydrogen as major components and hydrogen sulfide and carbon dioxide as minor components, said method comprising:

(a) treating the gas at high pressure with a solvent having substantial solvating power for hydrogen sulfide and carbon dioxide and a higher solvating power for sulfur dioxide and which'promotes the reaction

(1) 2H₂S + S0 . 2H₂ + 3S

such step resulting in a gas substantially free of hydrogen sulfide and carbon dioxide and a solution in such solvent of hydrogen sulfide and carbon dioxide,

(b) mixing said solution with a solution in^'the same solvent of sulfur dioxide and maintaining a small excess of hydrogen sulfide over that required in

Reaction (1), resulting in a solution containing most of such excess hydrogen sulfide,

(c) mixing the solution resulting from step (b) with a solution of sulfur dioxide in the same solvent, thereby reacting the excess hydrogen sulfide according to Reaction (1) and resulting in a solution of water and sulfur in such solvent, (d) separating sulfur, carbon dioxide and water from the product of step (c), and

(e) recycling the solution resulting from step (d). •

25. The process of Claim 24 in which the high pressure gas treated in step (a) is synthesis gas and wherein in step (b) heat is supplied to assist in the evaporation of carbon dioxide and to promote, thereby, the removal of carbon monoxide and hydrogen and to homogenize the hydrogen sulfide in solution.

26. The method of Claim 8 wherein the solvent contains an aromatic component containing nitrogen in an aromatic ring.

27. The method of Claim 26 wherein said component is selected from the group consisting of quinoline, phenanthridine and substituted pyridines.

28. The method of Claim 9 wherein the solvent contains an aromatic component containing nitrogen in an aromatic ring.

29. The method of Claim 10 wherein the solvent contains an aromatic component containing nitrogen in an aromatic ring.

30. The method of Claim 12 wherein the solvent contains an aromatic component containing nitrogen in an aromatic ring.

31. The method of Claim 14 wherein the solvent contains an aromatic component containing nitrogen in an aromatic ring.

32. The method of Claim 19 wherein the solvent contains an aromatic component containing nitrogen in an aromatic ring.

33. ^' The method of Claim 24 wherein the solvent contains an aromatic component containing nitrogen in an aromatic ring.

34. In the treatment of a gas containing hydrogen sulfide and another volatile gaseous component to remove the same from the gas, the improvement which comprises:

(a) treating the gas with a solvent having substantial solvating power for hydrogen sulfide and said other gaseous component and a higher solvating power for sulfur dioxide and which promotes the reaction

(1) 2H S + S0 > 2H₂ + 3S

such step resulting in a gas substantially free of hydrogen sulfide and said other gaseous component and a solution in such solvent of hydrogen sulfide and said other gaseous component,

(b) mixing the solution resulting from step (a) in a reaction zone with a solution of sulfur dioxide in the same solvent to carry out Reaction (1) ,

(c) maintaining during step (b) a temperature such that sulfur formed by Reaction (1) crystallizes and

(d) separating the resulting mixture of solution of sulfur in the solvent and sulfur crystals from the reaction zone.

35. The improvement of Claim 34 wherein step (c) is accomplished at least in part by flashing said other gaseous component from the solution in the reaction zone.

36. ^* The improvement of Claim 35 wherein said other gaseous' component is carbon dioxide.

37. The improvement of Claim 34 wherein the solution resulting from step (a) is cooled before it is introduced into the reaction zone.

38. The method of Claim 34 wherein a small excess of. hydrogen sulfide is maintained in the reaction zone whereby the solution withdrawn in step (d) contains hydrogen sulfide, and sulfur dioxide in said solvent is mixed with the solution to react with the excess hydrogen sulfide.

39. The improvement of Claim 34 wherein the gas treated in step (a) is synthesis gas containing hydrogen and carbon monoxide as the major components.