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Making incineration more effective

Oily wastewater can often contain inorganic components, such as calcium, sodium, and heavy metals in the form of salts, hydroxides, oxides, and sulphates. The patented process developed by Rinheat and Ekokem makes handling the resulting treatment residue cost-effective and increases throughput capacity as well.

Incineration is often used to dispose of the residue resulting from the treatment of oily wastewater, such as the bilge water discharged by ships in harbour. The material is sprayed into a combustion chamber or kiln, where the organics present are destroyed in the high temperature and water escapes with the flue gas, while non-organic material is removed using post-kiln filters.

Although effective, this approach is energy-intensive. It is often impractical to use more efficient, indirect multi-stage evaporation, however, due to heavy scaling. If most of the water could be removed prior to incineration using another technique, however, considerable energy savings could be made.

Rinheat, a specialist in heat recovery systems for mechanical pulping, and Ekokem, Finland’s leading hazardous waste management company, have jointly developed a process to do this by using the spent oil as a heat transfer medium. Watery oil is injected into hot oil, which is subsequently flashed to boil-off water. The evaporated steam is condensed, decanted, and the steam stripped to produce condensate that is clean enough to be sent for normal municipal treatment.

As waste lots typically come from numerous sources and volumes vary, they tend to form separate layers. The process uses two feed tanks for equalization purposes and vigorous mixing for homogeneity.

Stripping out the problem

Known as Non-Fouling Evaporation, the process was first tested at Ekokem’s site in Pori, and the plant there has been permanently modified to provide approx. 2 t/h of capacity, and is mainly used to dry spent motor oil. A new, slightly larger, purpose-built plant followed at Ekokem’s main site in Riihimäki.

Contaminated water is injected into heated circulating oil, and the resulting mixture is flashed in a vacuum vessel. Practically all the water is evaporated, as well as any volatile organics if they are present. Following condensing and decanting, the feed goes to a packed bed stripper equipped with a steam heater.

The light hydrocarbons in the stripper overhead are normally sent to an incinerator, while the clean flash condensate is sent for municipal treatment. Non-condensable gases are removed and sent to a high-temperature kiln.

No scaling problems have been encountered on heat exchanger surfaces, and suspended solids contents of up to 25% by weight have been tested without any problems, thanks to a continuous bleed to the incinerator.

The oil lost with this bleed is generally replenished by oil in the feed, but where oil content is too low a centrifuge can be used to reduce the need for replacement oil. The quality of the post-stripping condensate depends on the feed, and should quality issues arise an activated carbon filter or other type of adsorptive unit can easily be added.

> Arvi Artamo
(Published in HighTech Finland 2009)