Principle diagram
Principle and installation description
Pertraction involves the extraction of organic compounds (volatile and non-volatile) from liquids (incl. water) with the aid of membranes. The used membranes have no selectivity. A hydrophobic micro-filtration membrane helps to realise a high contact area between the organic extraction product and the to-be-purified liquid. Further, it also prevents the two phases from mixing. This means the liquid and extraction product do not need to be separated, which saves time and money. The membrane also enables the flows of both phases to be regulated flexibly and independently, whereby it is easy to optimise the process. It then becomes possible to also bring small quantities of extractant in contact with large quantities of to-be-treated liquids. This keeps installations compact.
A pertraction installation consists of one or multiple membrane modules in a series configuration (membranes are normally in a hollow fibre configuration in order to realise maximum membrane surface per volume). The extraction liquid thus flows down one side of the membrane (inner side of hollow fibre). And the wastewater is passed along the other side of the membrane (outer side of hollow fibre). The pores of the membrane are then filled with the organic extraction product. The polluted substances diffuse from the wastewater, through the membrane and to the extractant. The extractant can be regenerated using (amongst other things) a vacuum film vaporiser. It is possible to reuse the extractant.
Selectivity can be influenced by the choice of extraction product. In a number of cases, the distribution coefficients of the various to-be-removed substances do not or barely differ, whereby selective separation becomes difficult or expensive because multiple steps need to be implemented. However, in order to remove organic substances from water, for example, selective separation is generally not needed.
When selecting the extraction product, in addition to standard extraction criteria (affinity to to-be-removed components, chemical stability and toxicity), one must also take into account the membrane system i.e. the membrane’s chemical resistance and viscosity. Because of the low quantity of required extraction products, more expensive extractants can also be considered. Further, a density difference between the to-be-treated flow and the extractant is not required, which is desirable for conventional extraction due to the separation required afterwards.
The process has been demonstrated at pilot scale at various industrial locations. A full-scale installation of 15 m3/h has been operational at an industrial site in the Netherlands since 1998. Pertraction modules (incl. membranes) have, in the meantime, become commercially available.
Specific advantages and disadvantages
The main advantages of pertraction over normal extraction[1], are the use of a much lower quantity of extraction product and the ability to work without implementing an often time-consuming phase separation between to-be-treated liquid and extractant. Installations are also very compact and use little energy.
Pertraction works for a broad spectrum of organic compounds and also works in low water volumes and if high yields are needed when alternatives like stripping, active carbon treatment or biological purification are no longer appealing.
The membrane could become polluted if membrane-polluting components are present.
Application
Pertraction can be cost-effectively implemented for the removal of organic components from industrial process water, wastewater or polluted groundwater. Here are a few examples of sectors where pertraction is used: Process and wastewater flows from the chemical and petrochemical industries, metal industry, tank cleaning companies, garages, chemical laundries and wood conservation companies, as well as the pharmaceutical industry, small-scale wastewater purification plants, refineries and paint, gloss, varnish and printing-ink production.
The technique can be used for the removal of a broad spectrum of components (see ‘effectiveness’ paragraph). It is possible to effectively remove pollutants that are difficult to biodegrade, as well as pollutants that are easy to biodegrade.
It is also possible to remove unwanted metal elements from (amongst other things) passivation baths in the galvanisation industry, using a specific form of pertraction – namely, emulsion pertraction. The only difference with normal pertraction is that the watery extraction product is dispersed in an organic liquid.
Boundary conditions
For hydrophilic pollutants, like phenol and ethanol, it is sometimes difficult to find a suitable extraction product.
The ratio of pollution concentration in extraction product and water phase must be at least 100.
Effectiveness
Pertraction can be used to remove a broad spectrum of components, such as persistent organic substances (pesticides) and organic compounds (MAH’s and PAH’s, biodegradable compounds as well as non-biodegradable compounds).)
The yield for removing hydrocarbons is generally high, also at low concentrations (ppb level). Values of more than 99.5% can be realised.
This process is particularly efficient at low pollution concentrations.
Support substances
As in all extraction processes, this process uses an extraction product. During pertraction, the extractant quantity can be kept low. The extractant can also be easily regenerated and reused.
Environmental aspects
The low quantity of extractant can be regenerated and reused. Energy use is low.
Costs
The cost of pertraction is greatly determined by the quantity and composition of the to-be-treated flow and the to-be-realised end concentrations. The extraction quantity plays a major role during process optimisation. A larger quantity of extraction product means greater driving force and thus a smaller membrane surface. A smaller extractant quantity results in a smaller regeneration unit.
To provide an example, costs have been estimated for the treatment of wastewater with trichloroethylene pollution. The waste flow has a volume of 10 m3/h and a concentration of organic substances of 10 mg/l. To remove pollutants up to 10 ?g/l, the cost amounts to ca 0.5 € per m3 of treated water (price 2008). This makes pertraction cheaper than air stripping (also see technical file W44) followed by active carbon filtration (also see technical file W48), or only active carbon filtration with 2 filters in series configuration. This also applies to volumes of 1.5 and 100 m³/h.
The situation is clearly different for biodegradable substances like toluene. In this case, pertraction is only more favourable than biological purification or active carbon filtration for low water volumes and high removal percentages.
Comments
There are currently few references available for this technique. The stability of membranes cannot be sufficiently guaranteed at this moment in time.
Complexity
Pertraction is a relatively simple process. The process parameters for process optimisation are the type and quantity of extractant.
Level of automation
Far-reaching automation is possible.
References
- Kiani A. et al., Solvent extraction with immobilized interfaces in a microporous hydrophobic membrane Journal of Membrane Science, 20, 125-145, 1984
- Klaassen R. et al., Removal of hydrocarbons from waste water by pertraction Proceedings of the 7th International Symposium on Synthetic Membranes in Science and Industry, Tübingen, Germany, 316-319, August 29-September 1, 1994
- Klaassen R. et al., Membrane Contactor Applications, Proceedings of the 11th Aachener Membran Kolloquium, Aachen, March 2007, 19-29, 2007
- SPA Industrial wastewater, Info sheet no. 04
- TNAV, supplier survey, 2008
- VITO-SCT, review of technical files WASS, 2008
- website TNO, Netherlands: www.tno.nl
[1] Method for separating a mix of substances based on a difference in solubility.
Version February 2010