Highlights
- Caustic scrubbing is a technology has been used for the removal of H2S from exhaust gases in different industries for many years.
- Improvements in design and control strategies, favorable prices for caustic compared with other non-regenerable H2S scavenging chemicals, and the potential for sales value for the NaHS (sodium bisulfide) product have made caustic scrubbing an economic and viable option for some refinery streams.
- The scrubbing of hydrogen sulfide (H2S) using sodium hydroxide (NaOH) solution is established technology, and has been applied in a variety of industries.
- The removal of small amounts of H2S from gas streams is typically referred to as scavenging. Caustic is often classified as an H2S scavenger, and competes with other scavengers such as solid reactive adsorbents.
- Scavengers are most often used for the removal of H2S in amounts of roughly 0.1 ton per day, or less. However, the product of caustic scrubbing of hydrogen sulfide at moderate pH – NaHS which is also referred to as NaSH – when produced at high quality can often be sold, and depending on the value of the product solution.
Mechanism
Hydrogen Sulfide Scrubber
Hydrogen sulfide gas causes a wide range of health effects. Workers are primarily exposed to hydrogen sulfide by breathing it. The effects depend on how much hydrogen sulfide you breathe and for how long. Exposure to very high concentrations can quickly lead to death.
| Worker Exposure Limits |
| NIOSH REL (10-min. ceiling): 10 ppm |
| OSHA PELs:
· General Industry Ceiling Limit: 20 ppm · General Industry Peak Limit: 50 ppm (up to 10 minutes if no other exposure during shift) · Construction 8-hour Limit: 10 ppm · Shipyard 8-hour limit: 10 ppm |
| NIOSH IDLH : 100 ppm |
| IDLH: immediately dangerous to life and health (level that interferes with the ability to escape) (NIOSH)
PEL: permissible exposure limit (enforceable) (OSHA) ppm: parts per million REL: recommended exposure limit (NIOSH) |
Chemical reaction
- In the scrubbing process, sodium hydroxide reacts with H2S dissolved in aqueous solution to form sodium bisulfide (NaHS) and sodium sulfide (Na2S).
H2S + NaOH → NaHS + H2O (1)
NaHS + NaOH → Na2S + H2O (2)
- The extent of reactions 1 and 2 are dependent upon the amount of NaOH that is available relative to the amount of H2S that is scrubbed into the solution.
- When greater amounts of NaOH are present relative to the sulfide that is present, the solution pH is higher, and continuation from reaction 1 to reaction 2 is favored.
- NaHS is a valuable product than Na2S; in order to maintain product quality, it is required to control the pH of the spent caustic solution so that NaHS is the predominant sulfide species. Na2S uses twice the amount of caustic necessary for H2S removal and it has a limited solubility in water as well.
Important Considerations for H2S Scrubber Design
There are many different possible designs for caustic scrubbers. In selecting a configuration for a caustic scrubber for H2S removal, the following are some of the factors that should be considered:
- H2S Concentration in exhaust gas
- H2S load (per day)
- Other contaminant (eg. CO2)
- By product formation
- Frequency of scrubber operation
- Cost of caustic
Features
- Can handle air flow rate from 100 to 30,000 CFM
- Removal method is Absorption / Chemisorption
- Material of Construction: PP, FRP, PP+FRP, MS+FRP, SS 304, SS 316
- High efficiency Tellerette®Tower Packing removes soluble/reactive gases, solid particulate and liquid droplets down to 7 microns
- Lower shell serves as integral sump for recycle of liquid
- Spray nozzles accessible from top of unit
- Entrainment separators prevent liquid carryover
- Special multi-bed packing designs accommodate different scrubbing solutions for removal of multiple contaminants
- Efficiency up to 99 %
Additional accessories
- Continuous pH monitoring system for scrubbing liquid
- Dosing system for adjusting pH
- PLC operated control panel
- SCADA system
Applications
- Exhaust gas scrubbing
- Municipal odor control scrubbing
- Biogas purification scrubber