CLIAMS:We claim,
1. A method for removing hydrogen sulfide from a gaseous mixture, the method comprising:
- co-injecting the gaseous mixture with a scrubbing
liquid at a predetermined pressure to obtain a liquid-
gaseous mixture solution;
- allowing the liquid-gaseous mixture solution to react for a predetermined time to obtain a hydrogen sulfide free gas and a sulfur containing scrubbed liquid;
- oxidizing the sulfur containing scrubbed liquid to regenerate a sulfur containing scrubbing liquid; and
- filtering the regenerated sulfur containing scrubbing liquid to obtain a sulfur free scrubbing liquid.
2. A method for removing hydrogen sulfide from a gaseous mixture, the method comprising:
- injecting the gaseous mixture into a scrubbing liquid filled chamber at a predetermined pressure to obtain a liquid-gaseous mixture solution;
- allowing the liquid-gaseous mixture solution to react for a predetermined time to obtain a hydrogen sulfide free gas and a sulfur containing scrubbed liquid;
- oxidizing the sulfur containing scrubbed liquid to regenerate a sulfur containing scrubbing liquid; and
- filtering the regenerated sulfur containing scrubbing liquid to obtain a sulfur free scrubbing liquid.
3. The method of claim 1, wherein co-injection is achieved by injection through a ejector further wherein the ejector is selected from a group comprising single stage ejectors, multi-stage non-conditioning ejectors, multi-stage conditioning ejectors and multi-stage with both condensing and non-condensing stages.
4. The method of claim 1, wherein the scrubbing liquid comprises of a near equi-molar concentration of co-ordination complex of iron and a chelating agent.
5. The method of claim 4, wherein the scrubbing liquid further comprises of a stabilizing agent to the extent of less than one tenth of the chelating agent.
6. The method of claim 1, wherein the sulfur containing liquid comprises of co-ordination complex of iron in reduced state and elemental sulfur.
7. The method of claim 1, wherein oxidizing the sulfur containing liquid comprises of reacting the sulfur containing liquid with oxygen to convert the co-ordination complex of iron in reduced state to oxidized state.
8. The method of claim 7, wherein reacting the sulfur containing liquid with oxygen is achieved by injection of oxygen through a ejector further wherein the ejector is selected from a group comprising single stage ejectors, multi-stage non-conditioning ejectors, multi-stage conditioning ejectors and multi-stage with both condensing and non-condensing stages.
9. The method of claim 1, wherein filtering the sulfur containing regenerated scrubbing liquid comprises of passing through at least one filtration stage to obtain a sulfur free scrubbing liquid and elemental sulfur.
10. A system for removal of hydrogen sulfide from a gaseous mixture, the system comprising:
- a scrubbing chamber;
- a regeneration chamber downstream of the scrubbing chamber; and
- at least one filtration unit downstream of the regeneration chamber.
11. The system of claim 10, wherein the scrubbing chamber and regeneration chamber further comprises of ejectors.
,TagSPECI:A METHOD FOR REMOVING HYDROGEN SULFIDE FROM A GASEOUS MIXTURE

FIELD OF INVENTION
The present invention relates to the field of chemistry and particularly to a liquid-redox method for removing hydrogen sulfide from a gaseous mixture.

BACKGROUND
Hydrogen sulfide is commonly found in natural gas, biogas and liquefied petroleum gas. Hydrogen sulfide is also found in emissions from coke ovens, paper mills, tanneries and distilleries. Hydrogen sulfide being a highly toxic and corrosive gas needs to be removed. There are various methods available in the art for removal of hydrogen sulfide including but not limited to liquid adsorption, solid phase adsorption, gas phase reactions with a gas oxidant, high temperature dissociations and liquid phase reactions.
Liquid adsorption process commonly uses amines, alkaline solutions or metal ion containing organic compositions for removal of hydrogen sulfide. The solid phase adsorption process involves physical adsorption of hydrogen sulfide onto a solid adsorbent. The disadvantage of abovementioned process is inefficient in removing hydrogen sulfide from the gases containing high concentration of hydrogen sulfide.
The gas phase process uses a solid reactant and converts hydrogen sulfide to elemental sulfur in the presence of gas oxidant such as O2 and SO2. The major limitation of the process is that it cannot be applied to a hydrogen sulfide containing gas mixture; first hydrogen sulfide gas has to be separated from the gas mixture and then converted to elemental sulfur.
High temperature dissociation requires high temperature to decompose hydrogen sulfide into sulfur and H+. The temperature for decomposition usually exceeds 900oC thus limiting practical applications.
Liquid phase reaction or liquid redox reaction uses catalytic polyvalent metal redox solution for removal of hydrogen sulfide. One significant disadvantage encountered is in removal of elemental sulfur.
Thus, there is a need in the art for a method for removal of hydrogen sulfide from a gaseous mixture which is easy, cost effective, less time consuming and easy to operate and maintain.

BRIEF DESCRIPTION OF DRAWINGS
So that the manner in which the recited features of the invention can be understood in detail, some of the embodiments are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
FIG. 1 is a schematic representation of a system for removing hydrogen sulfide from a gaseous stream, according to an embodiment of the invention.
FIG. 2 is a schematic representation of an ejector assembly, according to an embodiment of the invention.

SUMMARY OF THE INVENTION
One aspect of the invention provides a method for removing hydrogen sulfide from a gaseous mixture. The method includes co-injecting the gaseous mixture with a scrubbing liquid at a predetermined pressure to obtain a liquid-gaseous mixture solution, allowing the liquid-gaseous mixture solution to react for a predetermined time to obtain a hydrogen sulfide free gas and a sulfur containing scrubbed liquid, oxidizing the sulfur containing scrubbed liquid to regenerate a sulfur containing scrubbing liquid and filtering the sulfur containing regenerated scrubbing liquid to obtain a sulfur free scrubbing liquid.
Another aspect of the invention provides a method for removing hydrogen sulfide from a gaseous mixture. The method includes injecting the gaseous mixture into a scrubbing liquid filled chamber at a predetermined pressure to obtain a liquid-gaseous mixture solution, allowing the liquid-gaseous mixture solution to react for a predetermined time to obtain a hydrogen sulfide free gas and a sulfur containing liquid, oxidizing the sulfur containing liquid to regenerate a sulfur containing scrubbing liquid and filtering the sulfur containing regenerated scrubbing liquid to obtain a sulfur free scrubbing liquid.
Yet another aspect of the invention provides a system for removing hydrogen sulfide from a gaseous mixture. The system includes a scrubbing chamber, a regeneration chamber downstream of the scrubbing chamber and atleast one filtration unit downstream of the regeneration chamber.

DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a method for removing hydrogen sulfide from a gaseous mixture. All the terms mentioned in the description herein shall be interpreted in their usual and standard meaning unless otherwise specified.
The method for removing hydrogen sulfide from a gaseous mixture includes co-injecting the gaseous mixture with a scrubbing liquid at a predetermined pressure to obtain a liquid-gaseous mixture solution, allowing the liquid-gaseous mixture solution to react for a predetermined time to obtain a hydrogen sulfide free gas and a sulfur containing scrubbed liquid, oxidizing the sulfur containing scrubbed liquid to regenerate a sulfur containing scrubbing liquid and filtering the sulfur containing regenerated scrubbing liquid to obtain a sulfur free scrubbing liquid.
The method also includes injecting the gaseous mixture into a scrubbing liquid filled chamber at a predetermined pressure to obtain a liquid-gaseous mixture solution, allowing the liquid-gaseous mixture solution to react for a predetermined time to obtain a hydrogen sulfide free gas and a sulfur containing scrubbed liquid, oxidizing the sulfur containing scrubbed liquid to regenerate a sulfur containing scrubbing liquid and filtering the sulfur containing regenerated scrubbing liquid to obtain a sulfur free scrubbing liquid.
The method described herein above in brief shall be described in detail. The gaseous mixture includes a mixture of hydrogen sulfide with gases including but not limited to methane, carbon dioxide, nitrogen, hydrogen, ammonia and oxygen.
The gaseous mixture is co-injected with a scrubbing liquid at a predetermined pressure to obtain a liquid-gaseous mixture solution. In one embodiment of the invention, the scrubbing liquid is prepared by mixing 16 g of FeCl3, 40 g of ethylene diamine tetra acetic acid (EDTA), 20 g of Na2CO3 and 1 g of hydroquinone per litre of solution. In another embodiment of the invention, the scrubbing liquid is prepared by mixing 16 g of FeCl3, 40 g of ethylene diamine tetra acetic acid (EDTA), 20 g of Na2CO3 and 10 g of sandalwood powder per litre of solution. In the scrubbing liquid, co-ordination complex of iron is in oxidized (Fe3+) state. The co-injection is achieved by ejecting the gaseous mixture and the scrubbing liquid by means of an ejector. The ejector is selected from a group comprising of single stage ejectors, multi-stage non-conditioning ejectors, multi-stage conditioning ejectors and multi-stage with both condensing and non-condensing stages. The scrubbing liquid is injected at a flow rate of 13-24 m3/hr and at a pressure ranging from 1.5 kg/cm2 to 10 kg/cm2. Injection of the scrubbing liquid with pressure results in formation of a negative pressure gradient. The negative pressure gradient results in suction of the gaseous mixture. During co-injection, the gaseous mixture combines with the scrubbing liquid in the ejector to form a liquid-gaseous mixture. With the formation of liquid-gaseous mixture, conversion of hydrogen sulfide to elemental sulfur starts. The liquid-gaseous mixture is allowed to react for a predetermined time to obtain a hydrogen sulfide free gas and a sulfur containing scrubbed liquid. During the reaction, the hydrogen sulfide is converted to sulfur and the co-ordination complex of iron is reduced from Fe3+ state to Fe2+ state. The gaseous mixture free of hydrogen sulfide is collected and the sulfur containing scrubbed liquid having co-ordination complex of iron in reduced Fe2+ state is passed to the next step of regeneration.
For regeneration the sulfur containing scrubbed liquid having co-ordination complex of iron in reduced Fe2+ state is reacted with oxygen present in the air. The sulfur containing scrubbed liquid is injected at a flow rate of 13 m3/hr to 24 m3/hr and at a pressure of 1.5 kg/cm2 to 10 kg/cm2 through an ejector. The ejector is selected from a group comprising of single stage ejectors, multi-stage non-conditioning ejectors, multi-stage conditioning ejectors and multi-stage with both condensing and non-condensing stages. A negative pressure gradient is formed resulting in suction of the air through the ejector. The sulfur containing scrubbed liquid having co-ordination complex of iron in reduced Fe2+ state reacts with oxygen present in the air. The oxygen converts the co-ordination complex of iron in reduced Fe2+ state to oxidized Fe3+ state, thus regenerating the scrubbing liquid. The scrubbing liquid still contains elemental sulfur and is passed to next step of filtration.
The scrubbing liquid containing sulfur is passed through at least one filtration stage to obtain a sulfur free scrubbing liquid and elemental sulfur. The sulfur free scrubbing liquid having co-ordination complex of iron in oxidized Fe3+ state is used again for co-injection with the gaseous mixture, thus forming a closed loop system with zero wastage of the scrubbing liquid.
In an alternate embodiment of the invention, initially the scrubbing liquid is filled in a chamber. Subsequent to the filling of the chamber with the scrubbing liquid, the gaseous mixture is injected into the scrubbing liquid. The injection of gaseous mixture is achieved by a sparger, an ejector or other such systems which injects the gas into a liquid at a regulated temperature, pressure and flow rate. Injection of gaseous mixture in the scrubbing liquid results in formation of a liquid- gaseous mixture.
With the formation of liquid-gaseous mixture, conversion of hydrogen sulfide to elemental sulfur starts. The liquid-gaseous mixture is allowed to react for a predetermined time to obtain a hydrogen sulfide free gas and a sulfur containing scrubbed liquid. During the reaction, the hydrogen sulfide is converted to sulfur and the co-ordination complex of iron is reduced from Fe3+ state to Fe2+ state. The gaseous mixture free of hydrogen sulfide is collected and the sulfur containing scrubbed liquid having co-ordination complex of iron in reduced Fe2+ state is passed to the next step of regeneration.
For regeneration the sulfur containing scrubbed liquid having co-ordination complex of iron in reduced Fe2+ state is reacted with oxygen present in the air. Initially, the sulfur containing scrubbed solution is filled in a regeneration chamber. Subsequent to the filling of the regeneration chamber with the sulfur containing scrubbed liquid, air is injected into the regeneration chamber. The injection of air is achieved by a sparger, an ejector or other such systems which injects the gas into a liquid at a regulated temperature, pressure and flow rate. The sulfur containing scrubbed liquid having co-ordination complex of iron in reduced Fe2+ state reacts with oxygen present in the air. The oxygen converts the co-ordination complex of iron in reduced Fe2+ state to oxidized Fe3+ state, thus regenerating the scrubbing liquid. The scrubbing liquid still contains elemental sulfur and is passed to next step of filtration.
The scrubbing liquid containing sulfur is passed through at least one filtration stage to obtain a sulfur free scrubbing liquid and elemental sulfur. The sulfur free scrubbing liquid having co-ordination complex of iron in oxidized Fe3+ state is used again for co-injection with the gaseous mixture, thus forming a closed loop system with zero wastage of the scrubbing liquid.
Various embodiments of the invention also provide a system for removing hydrogen sulfide from a gaseous mixture. The system includes a scrubbing chamber, a regeneration chamber downstream the scrubbing chamber and at least one filtration unit downstream of the regeneration chamber. Fig. 1 is a schematic representation of a system for removing hydrogen sulfide from a gaseous mixture, according to an embodiment of the invention.
The system of Fig. 1 includes a scrubbing chamber 1. The scrubbing chamber 1 is made up of materials including but not limited to fiberglass reinforced plastic coated with polypropylene or poly vinyl chloride and other such materials which are resistant to the hydrogen sulfide gas and the scrubbing liquid. The dimension of the scrubbing chamber depends on the flow rate of the gaseous mixture, hydrogen sulfide concentration in the gaseous mixture and the required concentration of hydrogen sulfide in the outlet gas. In one example of the invention, the scrubbing chamber 1 is of about 2 m height and with a volume of 1500 liters. The scrubbing chamber 1 includes an ejector 2 placed at one end of the scrubbing chamber 1, an inlet 3, an outlet for gas 4 and an outlet for liquid 5. The ejector 2 has an inlet 2a for injection of the scrubbing liquid, an inlet 2b for injection of the gaseous mixture and an outlet 2c for ejection of the liquid-gaseous mixture (Fig.2). The ejector 2 is selected from a group comprising of single stage ejectors, multi-stage non-conditioning ejectors, multi-stage conditioning ejectors and multi-stage with both condensing and non-condensing stages. In one embodiment of the invention, single stage ejector is selected. The outlet 2c of the ejector 2 corresponds with the inlet 3 of the scrubbing chamber 1. The outlet for gas 4 is connected via an outlet line 6 to a gas wash column 7. The outlet for liquid 5 of the scrubbing chamber 1 is connected via an outlet line 8 to a regeneration chamber 9.
The regeneration chamber 9 is made up of materials including but not limited to fiberglass reinforced plastic coated with polypropylene or poly vinyl chloride and other such materials which are resistant to the hydrogen sulfide gas and the scrubbing liquid. The dimension of the regeneration chamber depends on air flow rate and the amount of the sulfur containing scrubbed liquid to be regenerated. In one embodiment of the invention, the regeneration chamber 9 is of about 2 m height and with a volume of 1500 liters. The regeneration chamber 9 includes an ejector 10, an inlet 11, an outlet for air 12 and an outlet for liquid 13. The ejector 10 has an inlet 10a for liquid, an inlet 10b for gases and an outlet 10c. The ejector 10 is selected from a group comprising of single stage ejectors, multi-stage non-conditioning ejectors, multi-stage conditioning ejectors and multi-stage with both condensing and non-condensing stages. In one embodiment of the invention, single stage ejector is selected. The outlet 10c of the ejector 10 corresponds with the inlet 11 of the regeneration chamber 9. The outlet for liquid 13 of the regeneration chamber 9 is connected via an outlet line 14 to a regeneration tank 15. From the regeneration tank 15, an outlet line 16 bifurcates into two lines 17 and 18. The line 17 goes to a first filtration unit 19 while the line 18 goes to a second filtration unit 20. The first filtration unit 19 and the second filtration unit 20 can be a filter press, a rotary vacuum drum filter, a centrifugal filter or a combination thereof. The first filtration unit 19 and the second filtration unit 20 are connected to a filter tank 21 via outlet line 22. The filter tank 21 is connected to the scrubbing chamber 1 via a line 23.
Example1:
The gaseous mixture is co-injected with the scrubbing liquid in the scrubbing chamber 1. The co-injection is achieved first by injecting the scrubbing liquid into the scrubbing chamber 1 with the help of ejector 2. Through the inlet 2a of the ejector 2, the scrubbing liquid is injected into the scrubbing chamber 1. The scrubbing liquid is injected at a flow rate of 13-24 m3/hr and at a pressure ranging from 1.5 kg/cm2 to 10 kg/cm2. A negative pressure gradient is formed resulting in suction of the gaseous mixture through inlet 2b of the ejector 2. The liquid-gaseous mixture thus formed is ejected via outlet 2c of the ejector 2 into the inlet 3 of the scrubbing chamber 1. In the scrubbing chamber 1, hydrogen sulfide in the gaseous mixture is converted to sulfur and the scrubbing liquid having co-ordination complex of iron in oxidized Fe3+ state is converted to reduced Fe2+ state. The hydrogen sulfide free gas is collected via the outlet 4, washed in the gas wash column 7 and used for further operations. The sulfur containing scrubbed liquid is passed through the outlet 5 via the outlet line 8 to the regeneration chamber 9. For regeneration, the sulfur containing scrubbed liquid is injected through the inlet 10a of the ejector 10 into the regeneration chamber 9. The sulfur containing scrubbed liquid is injected at a flow rate of 13 m3/hr to 24 m3/hr and at a pressure of 1.5 kg/cm2 to 10 kg/cm2. A negative pressure gradient is formed resulting in suction of the air through inlet 10b of the ejector 10. The sulfur containing scrubbed liquid along with air is ejected via the outlet 10c of the ejector 10 into the inlet 11 of the regeneration chamber 9. In the regeneration chamber 9, the sulfur containing liquid having co-ordination complex of iron in reduced Fe2+ state reacts with oxygen present in the air. The oxygen converts the co-ordination complex of iron in reduced Fe2+ state to oxidized Fe3+ state, thus regenerating the scrubbing liquid. The sulfur containing regenerated scrubbing liquid is collected in the regeneration tank 15. Through the line 17 and the line 18, sulfur containing regenerated liquid is passed to the first filtration unit 19 and the second filtration unit 20, respectively. After filtration, the sulfur free scrubbing liquid is collected in the filter tank 21. From the filter tank 21 the sulfur free scrubbing liquid is supplied to the scrubbing chamber 1 via the line 23. The sulfur free scrubbing liquid having co-ordination complex of iron in oxidized Fe3+ state is used again for co-injection with the gaseous mixture, thus forming a closed loop system with zero wastage of the scrubbing liquid.
Example 2:
The gaseous mixture is injected into the scrubbing liquid filled scrubbing chamber. The height of the scrubbing chamber is about 2m. The H2S concentration in the gaseous mixture is the range of 5% to 20%. The injection of gaseous mixture is achieved by sparging the gaseous mixture at a flow rate of 4.5 m3/hr to 6.0 m3/hr. The sparger has a nozzle area of 300 mm2. After scrubbing the concentration of H2S in the outlet gas ranged from 0 ppm to 20 ppm depending upon the H2S concentration in the gaseous mixture. After scrubbing the sulfur containing scrubbed liquid having co-ordination complex of iron in reduced Fe2+ state is passed to the next step of regeneration.
For regeneration the sulfur containing scrubbed liquid having co-ordination complex of iron in reduced Fe2+ state is reacted with oxygen present in the air. Initially, the sulfur containing scrubbed solution is filled in the regeneration chamber. The height of the regeneration chamber is about 3 meter. The air is sparged into the sulfur containing scrubbed liquid at a flow rate of about 7.0 m3/hr to 15.0 m3/hr. The sparger has a nozzle area of about 75 mm2. In another example, the nozzle area of the sparger is about 300 mm2. The time taken to regenerate 700 litres of the sulfur containing scrubbed liquid is observed to be in the range of 50 minutes to 135 minutes, depending upon the flow rate of air. With higher nozzle area of the sparger (300 mm2) the time taken to regenerate 700 litres of sulfur containing scrubbed liquid is observed to be 50 minutes. The invention thus provides a method for removing hydrogen sulfide from a gaseous mixture which uses liquid column for scrubbing of hydrogen sulfide. Thus problem of choking of the scrubbing chamber due to precipitated sulfur is not encountered. Also, the invention provides a method that regenerates the scrubbing liquid fully and reuses it for further scrubbing without any wastage.
INDUSTRIAL APPLICABILITY:
The invention provides a method and a system for removal of hydrogen sulfide from a gaseous mixture. The method found application for removal hydrogen sulfide from biogas in industries like distilleries, breweries, landfills, food processing industries, diary industries, starch industries and other industries which generate hydrogen sulfide containing gas. The removal of hydrogen sulfide is essential both from environmental reasons as well as economical use of the gas generated.
The foregoing description of the invention has been included merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.