1. BY: SUJAN KUMAR DALBEHERA
Hydrogen Sulphide is a major toxic gas coming out from a crude oil refinery”. Its capture & utilization is a major challenge to refiners & environmentalist. Can it be utilized in a better way to save environment.
BY: SUJAN KUMAR DALBEHERA

2. INTRODUCTION:
Hydrogen sulphide gas is found in petroleum and natural gas. Oil or natural gas is considered sour if it has a high percentage of hydrogen sulphide. The gas has a characteristic rotten egg odour at low concentrations.
Natural gas can contain up to 28 percent hydrogen sulphide gas, consequently,  it may be an air pollutant near petroleum refineries and in oil and gas extraction areas.
The principal source of anthropogenic hydrogen sulphide is as a by-product in the purification of natural gas and refinement of crude oil.   It is a highly toxic, flammable gas with an auto-ignition temperature of 292celsius
Atmospheric releases of hydrogen sulphide represent the most significant public health concern for the geothermal energy industry.

3. Need for Hydrogen Sulphide Removal:
Hydrogen sulphide concentration must be less than 6 mg/m3 (43 ppm) to prevent the effect of
corrosion in process equipment
Most industrial personnel gas detectors are set to 20 ppm of H2S in air as a limit for the escape mode from an area.
The health effects of hydrogen sulfide depend on several factors such as how much hydrogen sulfide you are exposed to and the length of that exposure.
Exposure to low concentrations of hydrogen sulfide may cause irritation to the eyes, nose, or throat. It may also cause difficulty in breathing for some asthmatics.

4. How is hydrogen sulphide used?
Hydrogen sulfide is used primarily in the production of sulphur and sulphuric acid.
It can also be used to make other chemicals such as sodium sulphide and sodium hydrosulphide, which are used to make a variety of products including dyes, pesticides, and pharmaceuticals.
Hydrogen sulphide is utilized in the purification of nickel and manganese as well as hydrochloric and sulfuric acids.
It is used in metallurgy, the nuclear industry, and in laboratory experiments. It is also an agricultural disinfectant.

5. Absorption of acid Gases
The absorption process is used to remove a component (solute) from a gas stream by contacting the gas with a liquid solution (solvent).
The choice of solvents depends on its physical properties, such as high solubility, and ease of
recovery.
Absorption requires the mass transfer of a substance from the gas stream to the liquid
stream. If the concentration of the vapour and liquid are in equilibrium, no driving force exist, so no mass transfer will occur.

6. Chemical Solvents:
Amine Process:- Amine gas treating, also known as gas sweetening and acid gas (AG)
removal, refers to a group of processes that use aqueous solutions of variousamines to remove H2S and CO2 from gases.
Sweetening processes involve the removal of H2S and mercaptans from refinery streams.
The most commonly used amines in gas treating are primary monoethanolamine (MEA), secondary diethanolamine (DEA) and tertiary methyldiethanolamine (MDEA).
DEA is much less corrosive to carbon steel than MEA, and DEA is less volatile than MEA. MDEA is much less reactive than either DEA or MEA
A typical amine gas treating process includes an absorber and a regenerator

7. In the absorber, the gas stream, which contains H2S and CO2, enters from the bottom. The lean amine solvent comes from the absorber top.
The resultant (rich) amine is then routed into the regenerator to produce a regenerated or (lean) amine that is recycled for reuse in the absorber.
This H2S-rich stripped gas stream is then usually sent to a Claus process to convert it into elemental sulphur.

8. Carbonate Process:-
In this process, hot potassium carbonate (K2CO3) is used to remove both CO2 and H2S. It can also remove COS.
It can be observed that high CO2 partial pressure, in the range of 2–6 bar and temperature between 110–116 degree Celsius are required to keep KHCO3 KHS in solution.
Therefore, this process cannot be used for streams that contain H2S only because KHS is very hard to regenerate unless a considerable amount of KHCO3 is present.

9. Physical Solvents:
Physical solvents allow the absorption of acid compounds without any chemical reaction.
The difference in H2S and CO2 physical solubility gives the solvents their selectivity.
Organic solvents are used in these processes to absorb H2S more than CO2 at high pressures and low temperatures.
Regeneration is carried out by releasing the pressure.

10. Selexol Process: Selexol is a physical solvent, unlike amine-based acid gas removal solvents
that rely on a chemical reaction with the acid gases. It is dimethyl ether of polyethylene glycol.
Selexol usually requires less energy than the amine-based processes. It has high selectivity for H2S over CO2 that equals to 9–10.
Morphysorb Process:- The morphysorb process uses a mixture of N-formyl morpholine and Nacetyl
morpholine, for the removal of acid gases
The main advantages of this process are:
Higher solvent loading and hence a lower circulation rate.
Lower absorption of hydrocarbons.
Low corrosion and low environmental hazard.

11. Membrane Absorption:-
Selective permeation for gases occurs depending on the solubility at the surface contact between the gas and the membrane.
The acid gas basically diffuses through the membrane if high pressure is maintained to ensure a high permeation rate.
A membrane such as the Spiral Wound, has a high selectivity for H2S and CO2 over methane and other gases. For example, it has a permeation rate of 10 and 6 for H2S and CO2,respectively, while for methane, it is only 0.2.

12. Claus process:
The Sulfur Recovery Unit (SRU) is based on the modified Claus Process, which is a two-step reaction scheme overall.
A portion of the total H2S is burned in the Reaction Furnace to form SO2. Then, the H2S and SO2 react, at an optimal 2:1 ratio, to form elemental Sulfur (Sx) across the Claus Reactors.
After each catalytic stage, liquid sulfur is recovered in the Claus Condensers.
The remaining unreacted H2S and SO2 then proceed to the next stage, where the equilibrium-limited Claus reaction continues in the presence of Claus catalyst.

13. Process Description:
The Claus reaction to convert H2S into elemental sulfur requires the presence of one mole of SO2 for each two moles of H2S: (1) 2H2S + SO2 → 3S + 2H2O
To provide that ratio of components, the first step in the Claus process is the combustion of one-third of the H2S in the feed gas: (2) H2S O2 → SO2 + H2O
Combining equations (1) and (2), the overall process reaction is: (3) 2H2S + O2 → 2S + 2H2O
Demerits of claus process:- It is that the energy stored in hydrogen sulfide is partially wasted by the formation of hydrogen oxide. In fact, the energy stored in H2S could be utilized for the generation of hydrogen, a potential energy source in future, or other chemical products like H2O2
additional tail gas treatment and inflexibility to adjust to changes

14. Various methods that could possibly make better use of hydrogen sulfide have been studied in recent years
Thermal decomposition: 
High temperature above 1000 K for significant conversion of H2S is often required. Besides, high pressure and proper catalyst like molybdenum sulfide and other metal sulfide are commonly suggested, too. Interestingly, solar furnace was also suggested as the thermal source from the energy source point of view.
Electrochemical method : 
Electrochemical method like direct electrolysis is often carried out in basic solutions where H2S is absorbed. Anode poisoning by sulfur is a big challenge. In addition, chemical redox couples such as I3−/I− and Fe3+/Fe2+ are also introduced for indirect electrolysis of H2S. The main problem of electrochemical method is the high electricity costs today.

15. plasmachemical method:
Plasma generated from microwave, ozonizer, and glow discharge was also reported to be an active species to induce the decomposition of H2S into H2 and S. In comparison, the plasma method is relatively clean and effective. However, similar to electrochemical method, the big obstacle of the plasmachemical method is the use of electricity.
photocatalytic method:
In contrast to others like thermal and electrochemical methods, the photodecomposition of H2S is much less mature. Nevertheless, it is a very attracting method.

16. Photocatalytic Recovery:
The system consists of three major units:
A - the scrubber and filtration apparatus;
B - an H2S stripper or other outgassing unit;
C - the photoreactor.

17.  Hydrogen sulfide, H2S, coming from a hydrodesulfurization plant, a sour natural gas well, or other source, is bubbled into a scrubber where it encounters disulfide solution (10) coming from the photoreactor
An outlet (20) with a control valve (25) is in place to maintain an overpressure of H2S in the headspace and to function as a pressure relief valve.
The H2S lowers the pH of the scrubber vessel solution (30), to about , whereupon the disulfide anion decomposes, freeing elemental sulfur, S°. The sulfur is collected by filtration (40) of the suspension through a ceramic frit or other fine, porous media.
The filtrate continues onto the stripper unit (B), where the excess H2S is removed from the sulfide solution. The H2S is returned (50) to the foreline of the scrubber unit, while the sulfide solution (60) is fed to the photoreactor (C).

18. The sulfide solution inside the photoreactor (70), consisting mainly of bisulfide ion, HS-, is oxidatively converted to elemental sulfur and then complexed with excess sulfide ion to make disulfide ion, while water is reduced to make H2, under the action of light source (80). 
The H2 gas percolates out the top of the photoreactor, while the disulfide solution is returned to the scrubber.

19. Hydrogen Sulphide Scrubbers:
The process uses the counter current gas liquid contacting with the gas being taken from the bottom of the packed scrubber column and the scrubbed liquid is pumped form the top in a two stage scrubbing operation.
The gas coming out of the scrubber column, which is free of hydrogen sulphide, is then scrubbed with water for cleaning any minute quantities of chemical carried over. The clean gas thus obtained is fit for the end application.
The scrubbed solution containing sulfur is then passed through filter press for sulphur removal. The clear filtrate is then regenerated in a countercurrent with air in a packed re-generation column.

20. The System and its major elements :
Scrubber
b. Pre scrubber
c. Regeneration tower
d. Filter press
e. Wash tower
f. Biogas blower
g. Air blower
h. Pumps

21. Advantages of this process:
H2S is converted gas flow rates and inlet hydrogen sulfide concentrato solid Sulfur, which has commercial value.
Tailor made for individual gas flow rates and inlet hydrogen sulfide concentration.
Low H2S content in the cleaned gas (< 100 ppm v/v basis) irrespective of the inlet hydrogen Sulphide concentration.
The system is capable of handling fluctuations in H2S concentration and biogas flow rate.
This process is highly selective for H2S with little H2S being removed.
The chemical used is regenerated implying low running costs compared to other systems.
The system operates ambient temperature and pressure

22. Sulfur Quality and its utility:
Utility and Chemical requirement:
Process control and operator requirements:
Cost of electricity produced:
Maintenance requirements: