Title of Invention

A PROCESS OF REMOVING HYDROGEN SULFIDE FROM A GASEOUS STREAM CONTAINING HYDROGEN SULFIDE

Abstract

(57) Abstract: 'This invention relates co a process of removing hydrogen sulfide from a gaseous stream containing hydrogen sulfide comprising: contacting the said gas stream in a contactor with an aqueoits reaciani solution containing co-ordination complex of f'e (III) with a chelating agent in near equi-molar proportions and a stabilizing agent to the extern of less than one tenth the chelating agent as herein described to produce a purified gas stream, removing the sulfur precipitated in the contactor from the spent aqueous reaciani solution having an increased content of co-ordination complex of Fe (11) with the chelating agent, and regenerating the spent aqueous reactant solution by contacting with air and recycling the regenerated solution for further purification of the gas stream. PRICE: THIRTY RUPEES

Full Text

This invention relates to a process of removing hydrogen sulfide from a gaseous stream containing hydrogen sulfide. BACKGROUND Modem society by virtue of its urbanization and the need to serve a large population in a given area generates wastes in large quantities. Urban and municipal wastes which need to be disposed of in an environmentally benign manner are handled by recycling, land-filling, or bio-treatment of the bio-degradable organic matter. The wastes from urban living are sewage, vegetable and market wastes, garden and kitchen wastes. The wastes from industrial operations are press-mud from sugar factories, and effluents from distilleries, dairies, and odier food processing industries, which produce solid, semi-solid or liquid wastes. Urban sewage has approximately 1 to 3% solid content, mostly degradable organics. Market vegetable wastes, which are green and therefore, have a high water content, and kitchen and garden wastes can be combined into a liquid slurry having a solid loading of 10 to 12 %. These effluents have a high degree of organic matter as measured by Bacterial Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) and therefore are unsuitable for being discharged into rivers or 2icultural lands. Values of BOD and COD for many typical industrial effluents are 30,000 to 50,000 mg/litre and 100,000 mg/litre respectively. Acceptable values for discharge are 30 to 100 mg/litre. The anaerobic means of reducing BOD/COD is the most common technique and results in the generation of biogas having 45 to 65% methane (CHA), 30 to 50 % carbon dioxide (CO2) and some amount of hydrogen sulfide (H2S). Landfill gas varies in composition with time (changing over years) with CH4 as low as 35% in some cases. Most usually it is about 40 to 50%. Landfill gas has also very small amount of H2S, usually less than 0.05%. Sewage treatment plants produce H2S firom 0.05 to 0.1%. Distillery effluents produce bio-gas with H2S content firom 2 to 6% , the gas is so highly corrosive that the engine components such as the turbo-supercharger will disintegrate in less a few hundred hours of operation. In addition, the removal of hydrogen sulfide from gaseous streams has become increasingly important because of the limitations on the environment liberation and/or burning of gaseous streams containing hydrogen sulfide. Hydrogen sulfide contributes to air pollution (when released to the atmosphere or burned), is hazardous to human health, and creates an odor nuisance in very low concentrations. Regulations exist in many countries restricting the amount of H2S, as well as SO2, the combustion product of H2S, that can be released to the atmosphere. Therefore, it is usually necessary to remove H2S from these gases before end use, sale, or even emission to the atmosphere. The upper limit of H2S content in the gas, specified by different engine manufacturers, varies from 0.05 to 0.1%. Though these engines are tolerant to H2S at these levels, reduced amount of this pollutant implies longer life and longer time between overhauls. Hence there is motivation to develop a technology that can reduce the level of H2S in the gas to very low levels, say less than 0.01%. This invention claims to a relatively inexpensive system with low running costs to reduce H2S from 6% or more to 0.01% or less. There are many processes known in the art for removing hydrogen sulfide from gaseous sfreams including caustic scrubbing, as exemplified by the disclosure in US Pat. No. 2,747,962 to Heitz et al., scrubbing with aqueous solution of a water soluble nitrite such as sodium nitrite as disclosed in US Pat. No. 4,515,759 to Bumes et al., and numerous other processes. The use of a polyvalent metal chelate catalyst in aqueous solution to oxidize hydrogen sulfide in a gaseous stream to produce elemental sulfiir in the solution is well known in the art and can be illustrated by the following representative disclosures which are incorporated herein by reference: US Pat. No. 3,068,065 to Hartley et al., US Pat. No. 3,097,925 to Pitts et al., US Pat. No. 3,199,946 to Fujita et al, US Pat. No. 3,676,356 to et al., US Pat. No. 3,933,993 to Salemme, US Pat. No. 4,009,251 to Meuly, US Pat. No. 4,011,304 to Mancini et al., US Pat. No. 4,036,942 to Sibeud et al., US Pat. No. 4,189,462 to Thompson, US Pat. No. 4,238,462 to Hardison, US Pat. No. 4,356,155 to Blytas et al, US Pat. Nos. 4,368,178; 4,382,918; 4,400,368 & 4,515,764 to Diaz, US Pat. No. 4,374,104 to Primick, US Pat. Nos. 4,390,516 & 4,414,194 to Blytas, US Pat No. 4,431,616 to Chou, US Pat No. 4,499,059 to Jones et al, US Pat. No. 4,525,338 to Klee, US Pat. No. 4,532,118 to Tajiri et al., US Pat. No. 4,534,955 to Rosenbaum, US Pat. No. 4,649,032 to Snavely et al., and "Shell Redox Desulfiirization Process Stresses Versatility" by Fong et al.. Oil and Gas Journal (OGL Report), May 5, 1987, pp. 54-62. U.S. Pat. No. 4,871,520 discloses a method for removing hydrogen sulfide from a gas mixture. In the contacting stage of this method, hydrogen sulfide is oxidized to elemental sulfur by Fe (111) which in tum is reduced to Fe (II). The oxidation is believed to take place according to the following reaction: The spent aqueous reactant solution is regenerated for further use by contacting the solution with a free oxygen containing gas so that Fe(n) is oxidized to Fe(III) to produce a regenerated aqueous reactant solution which can be used in the contacting stage. The regeneration is believed to take place according to the following reactions: 2Fe(II)(L) + O2 + 2H ,► 2Fe(lII)(L) + H2O2 Fe (II)(L) + H2O2 2 Fe(III)(L) + OH" + OH" Fe(II)(L) + OH- ► Fe(m)(L) + OH" As the reaction proceeds, the chelating agent (L) is degraded. The degradation of the chelating agent (L) is believed to take place according to the following reaction: L + OH' »- degradation products hi the above reaction equations OH' is a free hydroxyl radical, Fe(IIl)(L) is the coordination complex of Fe(UI) with a chelating agent L, and Fe(II)(L) is the coordination complex of Fe(II) with a chelating agent L. In European Patent Specification Publication No. 215 505 it is proposed to maintain a certain concentration of Fe(II) in the regenerated aqueous reactant solution, larger than about 0.15 mol Fe(II) per mol Fe in order to reduce degradation of the chelating agent, suitably the amount is larger than about 0.15 mol Fe(n) per mol Fe. European Patent specification No. 186 235 discloses the regeneration of a spent aqueous solution wherein the solution and free oxygen containing gas are forced to flow co-currently through a contact vessel, so that in the direction of flow through the contact vessel the concentration of OH' simultaneously decreases with the concentration of Fe(II) which is oxidized to Fe(III). The US patent No. 5,233,173 proposed a method in which Fe is entirely in the state of Fe(II) and H2S is absorbed, but not reacted, in the absorption column. The precipitation of sulfur takes place in the regeneration vessel, where the regenerated Fe(in) reacts with S " ion and precipitates sulfur. A selective absorbent is required in the process, since no reaction takes place in the first column where H2S is absorbed. The US patent no. 4,891,205 has described a number of stabilizing agents for reducing the degradation of the chelating agents used in the H2S scrubbing solution. However even after adding these reagents in equimolar proportions to the chelating agent, degradation of the chelating agent is not completely overcome. The object of the present invention is to provide a process wherein degradation of the chelating agent is reduced, and at the same time almost the entire iron is maintained in the Fe(ni) form using a stabilizing agent in trace amounts and concentration of H2S in the gas is reduced to less than 100 ppm. i o achieve the said objective this invention provides a process of removing hydrogen sulfide from a gaseous stream containing hydrogen sulfide comprising: contacting the said gas stream in a contactor with an aqueous reactant solution containing co-ordination complex of Fe (III) with a chelating agent in near equi-molar proportions and a stabilizing agent to the extent of less than one tenth the chelating agent as herein described to produce a purified gas stream, removing the sulfur precipitated in the contactor from the spent aqueous reactant solution having an increased content of co-ordination complex of Fe (II) with the chelating agent, and regenerating the spent aqueous reactant solution by contacting with air and recycling the regenerated solution for further purification of the gas stream The said chelating 2ent is ethylene diamine tetra acetic acid (EDTA). The gas stream is contacted in the contactor with the said aqueous reactant solution at ambient temperature. The stabilizing agent is hydroquinone or water extract of sandal wood powder. The invention will now be described with reference to the accompanying drawings. Figure shows a flow diagram of H2S cleaning system. Referring to the drawings, the system consists of two different stages, (absorption and regeneration stage) as can be seen from the accompanying figure. In the first stage the absorption of and corresponding conversion of hydrogen sulfide absorbed by the solution takes place in absorption column (A). In the process Fe3+gets reduced to Fe'2+ and sulfur is precipitated. The elemental sulfur that is precipitated during the process is filtered at (D). The filtered solution at (E) is then pumped to the next stage by means of pump (P), which is the regeneration stage, where the reduced iron is oxidized to Fe22. The oxidized Fe3+solution is pumped back to the absorption column (B) for further absorption of hydrogen sulfide, thus making it a closed loop operation. Item (A) shows an absorption column in which H2S gas stream is absorbed and converted to elemental sulfur. The liquid is taken to a buffer tank (C) and then pumped into a filter (D) to remove sulfur. The second absorption column (B) is the regeneration column. The invention will now be described by way of examples given below: EXAMPLE 1: The basic solution for absorption and subsequent oxidation of H2S to sulfur had the following composition per litre of the solution. FeCl3 16 g EDTA 30 g NazCOg 20 g To this basic solution was added 1 g of hydroquinone per litre, 800 ml of this solution was sparged with 5% H2S added to air at a rate of 12 1/min. 4.8 g of sulfur was precipitated. The amount of EDTA degraded per gram of sulfur precipitated was 0.025 g. This is very small when compared with 0.3-0.7 g of the chelating agent degraded as reported in Patent No. 4,891,205 at much larger levels of other stabilizing agents proposed in that patent. EXAMPLE II: Basic solution as in Example I was prepared and to this was added 100 ml of water extract of sandal wood powder obtained by adding 100 g of sandal wood powder to one litre of water, keeping for 10 hour and filtering out the extract. 800 ml of this solution was sparged with 5% H2S added to air at a rate of 10 1/min. 5.0 g of sulfur was precipitated. The amount of EDTA degraded per gram of sulfiir precipitated was 0.028 g. This is very small when compared with 0.3-0.7 g of the chelating agent degraded as reported in Patent No. 4,891,205 at much larger levels of other stabilizing agents proposed in that patent. EXAMPLE III: Refer to the flow diagram. The pilot plant was designed for a gas flow rate of 8 m2/hr (about 2 g/s). For 5% of H2S in the gas flowing at a rate of 8 m2/hr, the flow rate of the liquid was found to be 0.016 mVmin. The working rate of the fluid was taken as 0.025 m2/min. The absorption tower (A) was made of PVC pipe of 110 mm diameter and 4 m high. The towers were filled with 12 mm rings cut from 12 mm diameter PVC pipes. A total of 3.5 m in each pipe was filled with rings in each pipe. A buffer tank of 0.3 m2 was included in the pilot plant to provide reactive residence time for the solution. A 750 mm cartridge filter of 0.05 m2 was added to the set-up for the purpose of filtering sulfur out of the solution. Two polypropylene pumps (PI, P2) were used to circulate the solutions. The solution used in the pilot plant was prepared as in Example II. For the experiments a mixture of H2S, nitrogen and carbon dioxide having a composition 5-7% H2S, 50% N2, and 43-45% CO2 was used to simulate the biogas. Only difference between biogas and the gas used in the experiments is that methane is replaced by nitrogen. H2S was produced by the reaction of FeS with H2SO4 (2M) and was stored in a floating inverted vessel under pressure a few mm of water above atmospheric. The gas composition was measured both at the inlet and at the exit of the absorption tower using two independent gas chromatographs. The presence of H2S in the gas at the outlet was also tested using lead acetate paper, which can detect H2S at levels above 100 ppm. All the measurements confirm that the gas at the exit is less than 100 ppm. The ratio of CO2 in the exit to that at inlet is 0.98 to 0.99, showing that there is no absorption of CO2 in the system. The advantage of the regeneration process in the present invention can be explained as follows. In most other process patents describing removal of H2S using polyvalent metal chelates, it is essential to restrict the amount of Fe(II) regenerated to Fe(III). This makes the operation of the plant at conditions other than the design operating point difficult. Since the addition of the stabilizing agent in the present case allows the complete regeneration of the Fe(II) ions to Fe(III) ions, over-exposure of the solution with the regenerating air does not pose any serious problem. Hence plant designed for one particular capacity can be operated at any lower gas flow rate or H2S content. We claim: 1. A process of removing hydrogen sulfide from a gaseous stream containing hydrogen sulfide comprising: contacting the said gas stream in a contactor with an aqueous reactant solution containing co-ordination complex of Fe (III) with a chelating agent in near equi-molar proportions and a stabilizing agent to the extent of less than one tenth the chelating agent as herein described to produce a purified gas stream, removing the sulfur precipitated in the contactor from the spent aqueous reactant solution having an increased content of co-ordination complex of Fe (11) with the chelating agent, and regenerating the spent aqueous reactant solution by contacting with air and recycling the regenerated solution for further purification of the gas stream A process as claimed in claim 1 wherein said chelating agent is ethylene diamine tetra acetic acid (EDTA). A process as claimed in claim I wherein the gas stream is contacted in the contactor with the said aqueous reactant solution at ambient temperature A process as claimed in claim 1 wherein the stabilizing agent is hydroquinone or water extract of sandal wood powder A process for removing hydrogen sulfide from a gaseous stream containing hydrogen sulfide substantially as herein described with reference to the examples and the accompanying drawings.

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