Coming across the feed water analysis, the hydrogen sulfide level of 40 ppm is considered high. Hydrogen sulfide, H2S, is a common taste and odor compound found in underground water source, characterized by a rotten-eggs odor and metallic taste. Hydrogen sulfide gas occurs naturally in crude petroleum, natural gas, volcanic gases and hot springs. It also can result from bacterial breakdown of organic matter and be produced by human and animal wastes.
- Hydrogen sulfide (H2S) is a colorless, flammable gas
- H2S has a pungent odour reminiscent of rotten eggs. The odour is detectable at very low concentrations.
- But! Odour perception is unreliable as a warning of high exposures; olfactory fatigue may develop at concentrations of 100 ppm and above.
- Exposures for a few hours to concentrations of 500 ppm and above are likely to be fatal. Exposure for a few minutes to concentrations of 1000 ppm and above is likely to cause rapid unconsciousness and death.
Although hydrogen sulfide is not considered as a harmless gas with respect to the membranes, however, it could create a major problem if this gas oxidized inside the membranes. Hydrogen sulfide exists as dissolved gas in the feed water. When this gas oxidize it forms elemental sulfur that will precipitate and foul the membranes which will end up in blocking them and results in interruption of the RO process. Hence, it is the design engineers decision to either treat the hydrogen sulfide separately as a pre-treatment prior to the RO, where, the risk of oxidation ad fouling will be eliminate, or the hydrogen gas will be allowed to pass from the source of the feed water to the membranes in a closed loop without letting the water get in touch with the air or any oxidizing agent. This step will keep the hydrogen sulfide in gaseous form where it will pass the membranes and leave with the product water unaffected where it will be removed as a post treatment step after the RO process.
A common treatment for hydrogen sulfide in water is aeration. Bringing air into intimate contact with water for the purpose of exchanging certain components between the two phases is called aeration (Degasification). Oxygenation is one of the purposes of aeration. Others are removal of volatile organic substances, hydrogen sulfide, ammonia, and volatile organic compounds. The undesired hydrogen sulfide is removed as a volatile gas by venting it into a waste pipe or to the outdoors. These reactions are reversible, and aeration may also causes dehydration resulting in releasing the gas from water. Henry’s law is applicable to this type of equilibrium for consideration.
Aeration requires acidic conditions (pH 4.0-6.0), and usually large space requirements.
In addition, the vented gas from the Degasifier should be introduced for further treatment using scrubbing technique and injection of caustic soda in order to prevent its release to the atmosphere.
Hydrogen sulfide will be stripped off from the feed water coming from the feed water tanks using two degasifiers in parallel. The water contains approximately 40 ppm of hydrogen sulfide gas (i.e. HS– & dissolved H2S).
Unit dimensions for 99% removal of H2S
Table Stripper dimensions
|Tower Diameter:||1700 mm|
|Packing Height:||4000 mm|
|Clean Packing Pressure Drop:||12 mm.WC|
|Air/Water Ratio:||30:1 (m3/m3)|
|Air Flow Rate:||5000 m3/h|
|Total Sulfides Removal:||99%|
|Off-gas H2S Concentration:||950 ppmv|
The scrubbing process is a unit operation in which one or more components of a gas stream are selectively absorbed into an absorbent. The term “scrubbing” is used interchangeably with “absorption” when describing this process. In wet scrubbing, water is the most common choice of absorbent liquor. In special cases, another relatively nonvolatile liquid may be used as the absorbent.