What is Hydrogen Sulfide?
Hydrogen sulfide is a chemical compound with the chemical formula H2S. This means it is made of two hydrogen atoms, and one sulfide. It is a colorless gas with an obvious identifying characteristic; it has a distinct rotten egg odor. Slightly denser than air, it can be incredibly explosive. When burning with oxygen, hydrogen sulfide burns blue, to form sulfur dioxide, and water. It is somewhat soluble in water, and acts as a weak acid.
- Dihydrogen monosulfide
- Dihydrogen sulfide
- Sewer gas
- Sulfurated hydrogen
- Sulfureted hydrogen
- Sulfuretted hydrogen
- Sulfur hydride
- Hydrosulfuric acid
- Hydrothionic acid
- Thiohydroxic acid
- Sulfhydric acid
Where is Hydrogen Sulfide found?
Like many other odorous organic compounds, hydrogen sulfides are produced by decaying organic matter, such as plants or animals. It is also released by human and animal waste, and is released by sewage, and manure. Another common, and somewhat infamous source for this odorous gas is nature sulfur springs, as a natural gas.
Hydrogen sulfide is also a common byproduct in many industrial production and refining processes, such as petroleum, pulp and paper, textiles, and food packaging.
Why is hydrogen sulfide a concern?
Hydrogen sulfide is an incredibly dangerous volatile organic compound, or VOC. It is very corrosive, and can destroy metals, including stainless steel, when not correctly abated. For most abatement processes, removing H2S is the very first step. This step must be completed, in order to avoid the destruction of other equipment, including abatement technologies. In addition to being highly flammable and explosive, it is very dangerous to human health. Prolonged exposure, even at very low concentrations, can cause serious nausea, headaches, and eye irritation. Loss of appetite and fatigue are also common effects of low concentration exposure, as well as digestive upset, and irritation. The more serious effects, which occur with higher concentrations, include loss of consciousness, permanent damage to the eyes, respiratory issues, infection, and pulmonary edema.
At high enough concentrations, such as 500-700 ppm, exposure can lead to near instant death. There have been several cases where hydrogen sulfide was not being handled properly in plants, and plant workers have died.
How do we treat hydrogen sulfide?
A SO2 acid gas water quench and scrubber package is designed to remove the thermal oxidation formed SO2. Up to 99% of SO2 can be removed.
An adiabatic quench section cools the oxidizer exhaust to below 180°F. Recirculation pump(s) provides water into the quench through spray headers, and the water that is not evaporated flows to the recycle sump. Approximately 50% of the acid gas is scrubbed in the quench section.
In a wet scrubber, water or scrubbing liquid, is the media which removes pollutants from the dirty incoming streams. The contaminated air enters the bottom of a countercurrent vertical packed tower scrubber, and water and a basic solution are sprayed in the top of the tower. The acid gases are absorbed by the solution as the air passes up the column. The air passes through a mist eliminator section, to remove entrained water, before exiting the scrubber column. When the water is re-circulated, the addition of fresh water is necessary to purge contaminants that accumulate, and to replace evaporation losses. Fresh water may be added to the recycle reservoir, either continuously, or on a periodic basis.
Sodium hydroxide is added to the recirculating water to neutralize the adsorbed SO2. The sodium hydroxide addition rate is controlled by a pH analyzer, and fed by a metering pump.
The reaction of SO2 with caustic produces a mixed solution of sodium sulfite (Na2SO3) and sodium bisulfite (NaHSO3) as shown by the following reactions:
SO2 + NaOH = NaHSO3
NaHSO3 + NaOH = Na2SO3 + H2O
The relative amounts of the two byproducts formed in the scrubber will vary depending on the operating pH set point.
By installing a two-stage scrubber in series, the pH set point in Stage 1 is minimized (pH ? 7) so that the byproduct formed will be mainly the 1:1 salt of NaOH with SO2 (sodium bisulfite) rather than the 2:1 salt (sodium sulfite). This will minimize the consumption of NaOH. This is achieved by using the 1st scrubber tower as an established SO2 removal workhorse. Regardless of the concentration of the SO2, the 1st tower shall be set up to operate under continuous conditions to remove a majority of the SO2 being converted in the upstream RTO. The 2nd tower that includes the same instrumentation as the 1st tower shall act as a polisher to the 1st tower and allow the equipment to be fine-tuned to achieve more consistent effluent waste water.
There are various options for wastewater treatment. The best choice will vary depending on the site.
One option is doing what most SO2 scrubbing installations do. They treat their smelly wastewater by sending it to large aeration basins where the blowdown is cooled and diluted enough to suppress its odor, and air is bubbled through the liquid long enough to convert nearly all the NaHSO3 and Na2SO3 to sodium sulfate.
NaHSO3 + Na2SO3 + NaOH + O2 = 2 Na2SO4 + H2O
Only then will the wastewater will be safe to discharge.
Additional articles in the GCES series ‘Abating Hazardous Air Pollutants’ include:
Part 2: Chlorine Abatement
Part 6: SOx, the compounds of sulfur and oxygen molecules including Sulfur Monoxide, Sulfur Dioxide and Sulfur Trioxide
Part 11: Sulfuric Acid – H2SO4
Part 12: Ethylene Oxide – EtO