Synonyms, abbreviations and/or process names
Air separation with membranes
- vinyl chloride
N2 removal from air with simultaneous production of O2-enriched air
H2 from refinery gas
- compressed air
Air laden with solvents is compressed and passed via a membrane. The membrane stops the air and allows solvents to pass through. The to-be-cleaned air is subject to overpressure (0.1 - 1 Mpa). The other side of the membrane is kept under underpressure (approximately 0.2 – 150 kPa) via a vacuum pump. Due to the difference in partial pressure, the solvents migrate through the membrane.
As a result, one gets a solvent-poor and a solvent-rich gas stream. The solvent-poor gas stream must be further treated via another purification technology like adsorption, photo oxidation, afterburning, biological purification…
The solvents are recuperated from the solvent-rich stream by performing condensation. This is possible, as the figure shows, after mixing with the original stream. Recuperation can also take place before mixing with the entry stream. There are a few viable variants.
For high solvent concentrations, prior condensation will take place in the heat exchanger after the compression phase, where the solvents will be separated. This is useful for reducing the load on the membrane.
The installation is dimensioned on the basis of flow rate, solvent concentration, solvent type, membrane type (surface load), required recuperation grade, required solvent concentration…
Because solvent concentration varies in the concentration build-up of solvents, from the lowest explosion limit to the highest explosion limit, there is a risk of explosion in the membrane module. This must be considered during the design phase.
Membrane installations are also available for the production of nitrogen gas, nitrogen-enriched air (82% instead of 78% for NOx reduction in engines) and oxygen-enriched air (31% instead of 21% for clinical purposes or combustion purposes).
The membrane configuration depends on the supplier: This varies from smooth membranes to hollow fibre membranes. There is also a choice between organic and inorganic membranes. Inorganic membranes are more resistant against temperature and fouling. These membranes are primarily used in gas purification of nuclear fuels and for the separation of mixes of hydrogen/nitrogen, hydrogen/methane, helium/methane, oxygen/nitrogen, CO2/CH4, hydrocarbons from air…. Inorganic membranes are more resistant against extreme gas conditions (e.g. temperature-related).
Dichloromethane : Inlet concentration: 10 – 30 %
End concentration: 150 mg/Nm³
1,2 Dichloromethane : Inlet concentration: 80 g/Nm³
End concentration: 320 mg/Nm³
Butane : Inlet concentration: 300 ppm
End concentration: 70 ppm
Ambient temperature (determined by membrane)
Pressure: 3.5 bara  (bara = absolute pressure)
5 barg in process  (barg = overpressure compared to ambient pressure or = bara - 1)
Up to 100 bara in inorganic membranes 
Solvent concentration minimum a few g/Nm³
Dust: Very low concentration needed to protect the membrane
Relatively low gas volumes: Capacities from 2 100 – 3 000 Nm³/h have been recorded 
Solvent concentrations up to 90% can be realised 
Solvent concentrations: Typically between1 -10% 
The membrane will need to be replaced periodically. The life-span guarantee for membranes is 5 years .
The technique is used for recuperation, which means the concentrated stream is reused.
Residual emissions normally need to be further treated. In particular cases, the solvent concentration can be immediately reduced to beneath the limit value by using membrane separation.
Energy use consists of ventilator costs and costs for the vacuum pump. The pressure drop over the membrane is 0.1 -1 MPa. The total energy use amounts to between 250 kWh/1 000 Nm³/h  and 300 kWh per 1 000 Nm³/h  (incl. ventilator)
- Operating costs
Personnel: 4 days per year
Energy costs: 250 kWh per 1 000 Nm³/h  (incl. ventilator)
300 kWh per 1 000 Nm³/h  (incl. ventilator)
Cost-saving possible via recuperated solvents
Total operating costs typically less than 50 EUR per 1 000 Nm³
Technical life-span of membrane;
Required end concentration;
Type of components
If a product with a high unit price is recuperated, money-return periods of 4 months to 1 year have been stated. This is only possible with favourable process conditions. In less favourable cases the technique will not pay for itself, but will be classed as a saving compared to other air purification techniques.
Production of nitrogen gas, prices 1995 
Capacity: 3 ton/day
Purity: 95 % N2
Investment costs: 90 000 USD
Membrane replacement: 16 USD/day
Energy: 35 USD/day
Amortisation: 46 USD/day
Other: 9 USD/day
Total cost per day: 106 USD/day
Costs for nitrogen: 35 USD/ton
Production of oxygen-enriched air, prices 1995 
Capacity: 10 ton/day oxygen/day
Purity: 35 % O2
Investment costs: 288 000 USD
Membrane replacement: 38 USD/day
Energy: 86 USD/day
Amortisation: 138 USD/day
Other: 18 USD/day
Total cost per day: 280 USD/day
Costs for nitrogen: 28 USD/ton
3-phase system to recuperate freon CFK-113 
Capacity: 850 Nm³/h
Ingoing concentration: 3% in air
End concentration: 0.075 %
End product: Clear liquid from CFK-113
Investment costs: 650 000 USD
Energy costs: 63 500 USD/year
Operating cost incl. amortisation: 44 USD per 1 000 Nm³ feed
Cost per kg product: 0.46 USD/kg
Advantages and disadvantages
Re-use of raw materials
Easy to use
No residue produced
Membrane filtration is only a concentration technique and must be followed-up by a second phase (e.g.
condensation or cryocondensation)
Risk of explosion
Nitrogen gas production (97 – 98% purity) and production of air enriched with oxygen.
Recuperation of benzene and solvent vapours from storage tanks (occurring during filling).
Recuperation of 1.2-dichloroethane in the production of monovinylchloride.
Recuperation of monovinylchloride in PVC production.
Recuperation of dichloromethane from concentrated flue gases from an industry that uses this substance as a
Recuperation of olefin monomers from flue gases from degassing polyolefins in polymer production.
Hydrogen recuperation from refinery gas.
Recuperation of organic substances from refinery flue gases.
Continuous removal of the product formed in a chemical reactor when maintaining ideal reaction conditions and
encouraging effective reactions.
Helium recuperation from hydrocarbons or nitrogen gas.
Hydrogen separation from synthesis gas for correcting the CO/H2 ratio, from nitrogen gas in ammonia production and
from hydrocarbons for hydrogen recuperation.
CO2 separation from natural gas and biogas, to increase gas value.
Drying of air and other gases.
- Supplier information
- Factsheets on Air-emission reduction techniques, www.infomil.nl, Infomil
- BREF: "Common waste water and waste gas treatment /management systems in the chemical sector" EIPPC, February 2002
- Environmental Technology Monographs Handbook, Handbook on Gaseous Waste, Envi Tech Consult, The Netherlands, 1993
- R.D. Noble , S.A. Stern: Membrane Separation Technology: principles and applications, Elsevier Science 1995
- Solvent capture for recovery and re-use from solvent laden gas streams, Environmental Technology Best Practice programme, guide GG 12