A METHOD FOR REMOVING HYDROGEN SULFIDE FROM GAS STREAMS

Abstract: The invention disclosed relates to a process for removing hydrogen sulfide from a gas stream, such as sour natural gas, with the formation of elemental sulfur as a by-product. By controlling the reaction conditions, the conversion of hydrogen sulfide is maximized, and the sulfur dioxide selectivity is controlled. Specifically, the sulfuric acid concentration and the reaction temperature may be balanced, depending on the desired product mix.

Description: A METHOD FOR REMOVING HYDROGEN SULFIDE FROM GAS STREAMS This invention relates to a method for removing hydrogen sulfide(H 2 S) from a gas stream such as sour natural gas and acid gases, with the formation of elemental sulfur as a by-product. BACKGROUND OF THE INVENTION It is well-known that numerous large untapped reserves of natural gas containing H 2 S currently exist, rendering such gas unsuitable for most uses. H 2 S-containing gas streams also originate in petroleum refineries and natural gas processing plants. Moreover, some natural gas reservoirs containing only a few percent of H 2 S have been kept shut-in because the high cost of removal of H 2 S and its conversion to sulfur exceeds the market value of the gas. Removal of H 2 S from such gas streams is necessitated by the high demand for such clean energy sources, by the value of sulfur as a by-product and by the need to regulate emmissions to meet air pollution standards. DESCRIPTION OF THE PRIOR ART It is known that H 2 S can be removed from a gas stream by absorption using an amine such as an alkanolamine. The two alkanolamines which are most commonly used are monoethanolamine and diethanolamine. After desorption of the H 2 S, the modified Claus process is typically used to convert H 2 S to sulfur. A furnace, two catalytic reactors and a clean-up unit may be operated in series to achieve sulfur recovery. However, this process is both capital and energy intensive. Further, the Claus process requires moderate to high concentrations of H 2 S, i.e above 30%, in the feed. In US Patent no. 3,849,540, a process for removing H 2 S from natural gas is disclosed in which the H 2 S is removed by catalytic means. Specifically, the natural gas is treated with an aqueous solution SUBSTITUTESHEET containing dissolved oxygen and a transition metal catalyst. In US Patent no. 3,761,569, a method for removing H 2 S from a gas stream is disclosed in which the H 2 S-containing gas is passed through a solution of cupric ions in a strong acid in the presence of oxygen. One of the acids disclosed is sulfuric acid, although the only examples are directed to the use of hydrochloric acid. This method fails if both oxygen and cupric ions are absent. It will also be appreciated by those skilled in the art that the presence of substantial amount of oxygen in such an environment is detrimental from both a corrosion and safety standpoint. SUMMARY OF THE INVENTION It is an object of the invention to provide a method for the removal of H 2 S from a gas stream, such as sour natural gas or H 2 S- containing acid gases. It is another object of the invention to provide such a method in which elemental sulfur is preferentially formed. Accordingly, a method for the removal of H 2 S from a gas stream is provided, comprising contacting the gas stream with sulfuric acid under conditions which preferentially favor the formation of elemental sulfur. The method according to the invention is suitable for the removal of H 2 S from many gas streams, such as sour natural gas, petroleum refinery gas and inert gases, such as nitrogen gas, but is particularly well-suited to the removal of H 2 S from low-level H 2 S- containing natural gas streams i.e. those containing less than 5 mol % of H 2 S. However, the process has also been found to be applicable to gas streams containing concentrations of H 2 S of up to about 80 %/w. SUBSTITUTESHEET For example, for the production of elemental sulfur the following chemical reaction (1) is favored: 3H 2 S + H 2 S0 4 -± 4S + 4H 2 0 (1) However, when flexibility in the product mix is of interest, the following chemical reaction (2) is favored: H 2 S0 4 + H 2 S <—> S + S0 2 + 2H 2 0 (2) That is, high conversion to elemental sulfur, accompanied by little or no conversion to S0 2 is desired when sulfur production is paramount. To provide a feed for sulfuric acid production, complete conversion to S0 2 would be optimal. Applicants have found that higher concentrations of sulfuric acid and lower reaction temperatures favor formation of elemental ]91jfur, whereas looer concentrations of sulfuric acid and higher te}peradmr%s ,d Λ Nction (2) may be varied in S0 2 selectivity from 0 to 100%. Preferably, the concentration of acid should be maintained between 82 and 96 wt%. To further optimize the method, a reaction temperature of 20 to 150°C, preferably 120 to 130°C and most preferably 125 to 127°C is provided. The flow rate/contact time may also be controlled to further optimize the method. Flow rates of between 26 and 260 SCCM per 500 g of H 2 S0 4 have been found useful, with rates nearer to the lower limit being preferred. SUBSTITUTESHEET Preferably,, the concentration of acid should be maintained between 82 and 96 wt%. To further optimize the method, a reaction temperature of 20 to 150°C, preferably 120 to 130°C and most preferably 125 to 127°C is provided. The flow rate/contact time may also be controlled to further optimize the method. Flow rates of between 26 and 260 SCCM per 500 g of H j ,S0 4 have been found useful, with rates nearer to the lower limit being preferred. BRIEF DESCRIPTION OF THE DRAWING Figure l is a schematic flow drawing of one embodiment of an apparatus for carrying out the method according to the invention. Figure 2 is a schematic flow drawing of another embodiment of an apparatus for carrying out the method according to the invention. Figure 3 is a schematic drawing illustrating a laboratory scale apparatus used to illustrate the performance of the method according to the invention. Figures 4 and 5 are graphs illustrating the performance of the method according to the invention using the apparatus of figure 3. DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Two embodiments of an appropriate apparatus for carrying out the method according to the invention are now described. In the apparatus schematically illustrated in figure 1, two SUBSTITUTESHEET autoclave reactors 3 and 3 7 are provided, containing sulfuric acid. In a first stage of operation, a H 2 S-containing gas(natural gas) is fed to reactor 3 via feed inlet 1 and valve 2. The gas stream is bubbled through the acid under agitation to effect the conversion of H 2 S to elemental sulfur. The clean gas stream comprising natural gas which is essentially free of H 2 S(generally less than 4 ppm) , exits the system via outlet 4. With time, the by-product elemental sulfur accumulates in the reactor, while the concentration of the sulfuric acid decreases. After a predetermined operating time, the operation is shifted to reactor 3 y by closing valve 2 and opening valve 2 7 . By the time that reactor 3 / is in operation, the acid medium in reactor 3 is heated to remove the elemental sulfur and the concentration of the sulfuric acid is increased to the appropriate level e.g. by adding fresh acid. Accordingly, a continuous procees is achieved by alternately operating the two reactors. The figure 2 apparatus includes a packed column reactor 8 containing suitable conventional packing materials to provide sufficient gas-liquid contact area, resist acid-corrosion and operating at a temperature above 120°C, i.e. above the melting point of sulfur. This minimum temperature is required to avoid plugging of the column materials by solid sulfur. A sulfur-acid separator 9 and acid-enriching equipment 10 are also provided. The reactor 8 has a gas feed inlet 1, a clean gas outlet 2, an acid medium inlet 3 and a downstream acid medium outlet 4. The outlet 4 is connected to the sulfur-acid separator 9, which has a sulfur product outlet 5 and an acid outlet 6 connected to the acid enriching equipment 10. The enriched acid medium is re-circulated through outlet 7 to the reactor 8 via acid medium inlet 3. In operation, the H 2 S-containing gas stream is fed into the reactor 8 via feed inlet 1. The sulfuric acid medium is sprayed into the reactor through acid medium inlet 3. Inside the reactor, the gas and liquid streams contact each other either co-currently or SUBSTITUTESHEET counter-currently, to effect the gas-liquid reaction converting H 2 to elemental sulfur. The resultant gas stream flows out of th reactor via clean-gas outlet 2, ready for use or ventilation. Th liquid phase including the by-product elemental sulfur enters th sulfur-acid separator 9, wherein the sulfur is separated by heatin to above the melting point of sulfur i.e. above 120°C. Th separated sulfuric acid medium is then transported to the acid enriching equipment wherein the concentration of the sulfuric aci is adjusted to the required level e.g. by adding fresh sulfuri acid, and re-circulated to the reactor via acid medium inlet 3 Alternatively, the entire operation is carried out above 120°C s that sulfur does not precipitate as a solid within the equipment EXAMPLES The invention will now be further described by reference to th following examples, tabulated results and figures 3 to 5. A laboratory scale apparatus (figure 3) , was used. The H 2 S-containing gas stream was fed to the reactor at a flow rat of 260 standard cubic centimetre per minute (SCCM) for Tables 1 t 4, and 26 SCCM for Tables 5 and 6. The results in Tables 1 to 4 are for a gas feed composition of H 2 = 4.61 mol% and N 2 = 95.39 mol%; and the results in Tables 5 and are for a gas feed composition of H 2 S = 1.49 mol% and CH 4 = 98.5 mol%. The gases exiting the reactor were passed through a ga chromatograph (GC) (see figure 3) sampling valve by which the fee or product gases may be sampled for GC separation of constituent for analysis. SUBSTITUTESHEET Each run was conducted over five hours at various temperatures an acid concentrations. The results are shown in Tables 1 to 6 whic follow, and in figures 4 and 5. Referring again to figure 3, the mixed gases of H 2 S and either N 2 or CH 4 provided in storage cylinder 11 are fed to the reactor 3 via a mass flow controller (MF) (Side Trak <R , Sierra Instrument) . The reactor was made of glass, and was 70 mm I.D. and 145 mm in height. A tube of 10 mm O.D. with a sparger(not shown) at the end inserted into the reactor from the top functioned as the gas inlet. To prevent the back-up of sulfuric acid medium to the mass flo controller if the down stream line was plugged up with product sulfur, a buffer gas bottle 13 was placed in the line between the mass flow controller and the reactor. Prior to each run, about 500 grams of sulfuric acid solution of varied concentration (from 82 to 96 wt%) was loaded into the reactor. Following a 10 minute purging of the system by N 2 , the H 2 S-containing gas was introduced and reacted with the sulfuric acid solution. The experiments were carried out at temperatures r,anging from 22 to 125°C. A hot oil bath was used to maintain the higher temperatures. The effluent gas exits the reactor for GC analysis, and the results compared to pre-calibration with the H 2 S containing gases. Specifically, Table 1 was conducted at constant sulfuric aci concentration of 96 wt%. Over 97% conversion of H 2 S is achieved. However, an undesirably high SO z selectivity of 66% was observed. It will be appreciated by those skilled in the art that S0 production is undesirable because of its harmful corrosion, tarnishing and air pollution effects. SUBSTITUTE SHEET Table 2 was conducted at constant sulfuric acid concentration of 88.6 wt%. In the range of temperatures tested of 22 to 127°C, the level of H 2 S conversion is below 66.4%. However, a good S0 2 selectivity of below 0.6% is observed. It is noted that with decreasing temperature, I^S conversion decreases and the S0 2 decreases to zero. Tables 1 and 2 thus indicate the significant effect of the concentration of sulfuric acid on H 2 S conversion and S0 2 selectivity in the reaction. Run 7 represents a good balance between H 2 S coversion and S0 2 selectivity. Tables 3 and 4 ( and figure 4) , conducted at varied sulfuric acid concentrations, show the effect of sulfuric acid concentration on H 2 S conversion at 22 and 125°C, respectively. Note that with decreasing sulfuric acid concentration, H 2 S conversion decreases with the disappearance of S0 2 . Tables 5 and 6 (and figure 5) further indicate that at a lower gas flow rate (higher contact time) , the reaction process more effectively purifies a natural gas stream by converting more H 2 S to elemental sulfur. The level of H 2 S conversion of over 94 to virtually 100% was achieved, without S0 2 being produced. Similar results for S0 2 selectivity were later found. SUBSTITUTESHEET Table 1 Gas Feed Composition: H2S=4.61 mol %, N2=95.39 mol % Run Temp.°C H2SO Cone, wt % H2S Conv. % SO2 Sel. % 1 21.5 96 97.4 66.05 2 95.0 96 98.8 67.20 3 121.8 96 100.0 67.39 Table 2 Gas Feed Composition: H2S=4.61 mol %, N2=95.39 mol % Run Temp.°C H2SO4 Cone. wt % H2S Conv. % SO2 Sel. % 4 22 88.6 28.2 0.0 5 54 88.6 44.2 0.0 6 91 88.6 56.2 0.02 7 127 88.6 66.4 0.60 Table 3 Gas Feed Composition: H2S=4.61 mol , N2 =95.39 mol % Run Temp.°C H2SO4 Cone, wt % H2S Conv. % SO2 8 22 96 97.4 66.1 9 22 94 " 70.1 18.7 10 22 92 32.8 12.5 11 22 88.6. 28.2 0.0 12 22 91.9 25.3 0.0 SUBSTITUTE SHEET Table 4 Gas Feed Composition: H 2 S=4.61 mol %, N2=95.39 mol % Run Temp.°C H 2 S0 4 Cone. wt % H 2 S Conv. % SO2 Sel. % i3 125 96 100 67.4 14 125 94 97.8 62.7 15 125 91 85.2 19.7 16 125 ββ 66.4 0.6 17 125 81 34.6 0.0 Table 5 Gas Feed Composition: H2S=1.49 mol %, CH4=98.51 mol % Run Temp.°C H 2 S0 4 Cone. wt % Η.2 S Conv- % 18 22 96 94.4 19 22 94 93.0 20 22 92 73,5 21 22 88 28.9 22 22 82 11.4 Table 6 Gas Feed Composition: H2S=1.49 mol %, CH4=98.51 mol % Run Temp. °C H 2 S0 Cone. wt % H 2 S Conv. % 23 125 96 > 99.9 24 125 94 > 99.9 25 125 92 97.4 26 125 88 81.4 27 125 82 25.5 SUBSTITUTE SHEET

Claims: WE CLAIM: 1. A process for removing H 2 S from a gas stream, comprising contacting the gas stream with concentrated sulfuric acid under conditions which preferentially favor the formation of elemental sulfur. 2. A process according to Claim 1, wherein said conditions include a conversion of H 2 S to elemental sulfur of > 65% and a S0 2 selectivity of < 5%. 3. A process according to Claim 1, wherein the concentration of sulfuric acid is 80 to 100 wt%. 4. A process according to Claim 3, wherein the reaction temperature is 20 to 150°C. 5. A process according to Claim 4, wherein the H 2 S-containing gas stream is a natural gas stream. 6. A process according to Claim 5, wherein the natural gas stream contains less than 5 mol% of H 2 S. 7. A process according to Claim 6, wherein the flow rate of the natural gas stream is about 26 SCCM per 500 g of sulfuric acid. 8. A process according to Claim 5, wherein the contact between the gas stream and the sulfuric acid is effected concurrently or counter-currently. 9. A process acording to Claim 3, wherein the concentration of sulfuric acid is 82 to 96 wt%. 10. A process according to Claim 9, wherein the reaction SUBSTITUTESHEET temperature is 120 to 130°C. 11. A process according to Claim 10, wherein the flow rate of the gas stream is 26 to 260 SCCM per 500 g of sulfuric acid. 12. A process according to Claim 9, wherein the concentration of sulfuric acid is 94 to 96 wt%. 13. A process according to Claim 12, wherein the reaction temperature is 125 to 127°C. 14. A process according to Claim 1, wherein the process is continuous. 15. A process according to Claim 14, wherein as the H 2 S is coverted to elemental sulfur and is removed, the sulfuric acid is replenished. 16. A process according to Claim 1, wherein the process is conducted in a semi-batch reactor holding sulfuric acid solution. 17. A process according to Claim 8, wherein the process is conducted in a packed column reactor. 18. A process for removing H 2 S from a gas stream, comprising contacting the gas stream with concentrated liquid sulfuric acid of a concentration of 88-94 wt%, according to the chemical reaction H j SO .! + H j S → S + S0 2 + 2H 2 0 performed at a temperature of 120 to 130 °C. 19. A process according to Claim 18, wherein the reaction provides for a conversion of H 2 S to elemental sulfur of > 65% and a S0 2 selectivity of < 5%. SUBSTITUTE SHEET

Inventors: CHUANG KARL TZE-TANG, DALLA LANA IVO GIOVANNI, CHUANG, KARL, TZE-TANG, DALLA LANA, IVO, GIOVANNI
Applicants: CHUANG KARL TZE TANG, DALLA LANA IVO GIOVANNI, CHUANG, KARL, TZE-TANG, DALLA LANA, IVO, GIOVANNI
Application year: 1995
Main classification: B01D53
IPC codes: B01D53, C01B17