Design, Fabrication and Supply of Equipment
For the treatment of Hydrogen Sulphide
The removal of hydrogen sulphide is a basic requirement when working with the output from anaerobic digesters located at, for example, pigs farms or power generation plant using landfill or biogas.
Where the objective is to destroy the transporting gas as well as the hydrogen sulphide, Organics is able to offer thermal oxidisers that can reduce 20,000 ppm of hydrogen sulphide to less than 0.5 ppm.
The Case for h2s removal
The Anaerobic digestion of organic matter produces methane and carbon dioxide. It can also lead to the production of other trace gases, such as hydrogen sulphide. Hydrogen sulphide can be a health and safety hazard. The gas has an unpleasant odour and can cause the rapid corrosion of carbon steels.
Hydrogen sulphide can be removed from gas streams by means of chemical oxidative scrubbing. Such scrubbers employ water-based reagents containing oxidants such as sodium hydroxide, sodium hypochlorite, potassium hydroxide and ethanolamines. Each chemical oxidant has its advantages and disadvantages for specific applications.
Key Features of Hydrogen Sulphide Scrubbers
Removes noxious odours
Prevent corrosion in plant and equipment
Prevent emissions of sulphur dioxide
Remove local health hazards from the atmosphere
Meets all environmental standards for hydrogen sulphide control
The specification employed in the design is based on the maximum gas flow rate, the optimum operating temperature, the maximum inlet H2S content, minimum outlet H2S content, minimum operating pressure and allowable pressure drop. One of the advantages of this process is that it does not require regular chemical additions. It is also generally easy to operate.
The BIO-SCRUBBER process
The process is comprised of a random packed-bed, regular-sized packing material contained in a sealed vessel. The packing material is used as a support medium for the growth of hydrogen sulphide consuming bacteria. These bacteria become active at a low pH.
After initial seeding, the system requires nutrient feeding, which would normally entail carbon from the digester wastewater effluent line and regular removal of excess bacteria to ensure the pH does not go too low.
Spent liquids are safe and can be disposed of to a standard wastewater drain. Gas flows through the packed-bed and hydrogen sulphide biochemically reacts with the bacteria. Air is also introduced to the bio-reactor to maintain the bacteria in an optimum condition. The ideal operating temperature for the process is 35 to 40º Celsius.
In cold climate zones, the temperature can be maintained by connection to the cooling system of an engine or a take-off point from a boiler, which are the normal applications requiring protection from hydrogen sulphide. In a warmer climate, the thermal load for maintaining this temperature is quite small and can normally be ignored.
A minimum ambient temperature of 20oC is adequate for the maintenance of system performance. Further analysis of this aspect will be required for Brazil. In Thailand, insulation is not required.
STANDARD SCOPE OF SUPPLY
Organics has supplied hydrogen sulphide control equipment to many projects around the world. These have ranged from landfill sites with a high H2S concentration to anaerobic digestion plant dedicated to the treatment of organic waste material both from agriculture and waste food for human consumption. The standard scope of supply for a component delivery will be comprised of the following main components:
- Main bio-scrubber vessel. There are a number of options for the selection of a suitable containment system, ranging from glass-reinforced plastic to glass or rubber-lined steel tanks and suitably treated concrete vessels. The selection of which containment system to use is based upon plant-capacity, location of use, location of manufacture and overall general compatibility with any existing facilities and systems.
- Packing media. This is selected to maximise surface area for bacterial growth whilst minimising the possibility of blockages forming during operation.
- RIC® SYSTEM
The Rapid In-situ Cleaning system is an innovative system that facilitates cleaning without the need to remove the packing. The system reduces the time necessary to clean the unit and so reduces downtime and optimises operational hours.
- Gas and liquid distribution systems. Whilst a bio-scrubber is not a counterflow chemical scrubber or stripper, it does need to be designed to ensure proper distribution of flows within the vessel.
- Ancillary components. These include circulation pumps, air blowers, valves, filters, instrumentation and controls. In colder climates, heating systems are required to maintain process temperatures.
Controlling the release of H2S
Inhalation represents by far the most significant route of entry for hydrogen sulphide. Exposure to levels of hydrogen sulphide above 500 ppm even for short periods will generally result in rapid or immediate respiratory failure or collapse. Death will result unless the victim is rescued immediately and given pulmonary resuscitation. Exposure to levels below 500 ppm may result in serious consequences including unconsciousness and death.
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