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Odours arising from Chemical and Petrochemical Plants

The process of refining crude oils generates a sizable volume of wastewater. Within crude oil there are significant quantities of Sulphides and Ammonia. These levels are very high and will frequently generate odour at ground level well past the boundary of the plant. It is essential as part of any odour management process to maintain a significant buffer zone to any inhabited areas nearby the plant.

In addition, organic residues present in the wastewater streams are themselves odorous by nature. These compounds have also potentially a long-term impact on public health. Benzene, frequently found in air emissions from oil refineries is classified as a cacogenic compound. This makes the requirement for containment and treatment critical from the view of occupational and community safety.

The normal practice is to collect and vent the air to the flare stack, however frequently the volume of air to be treated exceeds the capacity of the flaring system and alternative treatment technologies need to be considered.

Management of Toxic Vapours and Odorous gas emissions from Petrochemical Plants

The gas emission composition from the front-end separation technology and from the sludge handling area are similar and dealt with in the same manner.

The concentration of Hydrogen Sulphide in the collected gas stream will tend to exceed 500 ppm which on balance does not favour either Biological odour control systems or Activated Carbon. Generally, the preferred method of treatment is to adopt a two-stage process utilising Caustic Venturi Scrubber followed by Catalytic Caustic / Hypochlorite packed column scrubbing. Nickel Oxide catalyst is employed in this scrubbing system as it is the only satisfactory wet chemical scrubbing process able to treat organic compounds in the gas stream.

The cover design of the process equipment is critical in this application as fugitive emissions are potentially toxic. It is normal practice to have very high extraction rates to ensure a high under cover vacuum is maintained, preventing emission release.

When considering covering processes such as Dissolved Air Flotation plants, always take into consideration the problem of foaming and aerosols. There needs to be a disengagement section and a solids collection filter in the foul air stream.

The alternatives in treatment technology for managing odours from Petrochemical sources are:

• Activated Carbon for small areas emissions sources of very low contaminant concentration.
• Activated Carbon can also be used for high concentrations where steam and hot air are used to regenerate the carbon and recover the volatile compounds.
• BioTrickling Filters and Activated Carbon are suitable for streams containing Hydrogen Sulphide levels up to 200 ppm. If there are appreciable levels of Volatile Organic Carbon in the gas stream then a two stage BioTrickling Filter needs to be considered. Note that Dimethyl Sulphide and Dimethyl Disulphide are very difficult to treat Biologically. Activated Carbon must always be used for final polishing of the gas stream.
• Biofilters are only suitable when Hydrogen Sulphide levels do not exceed 35 ppm. The units tend to be very large so land area must be sufficient to cater for the system requirements.
• If the wastewater treatment plant generates sludge, provided that the Hydrogen Sulphide levels are low enough, consider Biofilters for odour treatment.
• If Hydrogen Sulphide levels are exceedingly high, consider a venturi caustic scrubber in combination with a caustic / hypochlorite catalytic packed column scrubber.

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