The purpose of compressors is to generate compressed air. Depending on air humidity and temperature, this is accompanied by a by-product, a large quantity of condensate. Significant technical resources are engaged in order to extract this condensate from the compressed air. Aftercoolers, separators, filters and adsorption dryers serve only one purpose, the achievement of a specified compressed air quality and the removal of condensate. This condensate is the outcome of pollutants drawn in from the atmosphere, absorbed by water aerosols. All components of the aspirated air, such as hydrocarbons from the lubricating oil of the compressor, flue gas emissions (ie. sulphur dioxide) and solvent vapour as well as diverse microdusts, form an oily and greasy or highly aggressive condensate in conjunction with the precipitated moisture and the oil component, depending on the type of compressor used. Oil free condensate resembles distilled water and therefore readily absorbs mineral substances. Atmospheric air contains large quantities of acid components released by the combustion of fossil fuels. Enriched through concentration, the pH-value of the condensate can be reduced to around pH 4. With oil lubricated compressors, the concentration enrichment factor of the condensate can cause up to a 300 times higher proportion of oil than in the air.
Some compressor oils partly neutralise the pH-value of the condensate. Most condensates precipitate at a pH of 4 - 6. The problems of condensate treatment assume increasing significance through growing environmental consciousness and ever strict legislation. Oil containing compressor condensates, in particular, must be treated before being allowed to enter the drainage system. According to the Water Resources Act 1991 Part iii Ch.ii Sec.85 (4)(a), operators of oil lubricated compressors are obliged to dispose of the oil containing condensate in accordance with state of the art technology. As this is always a question of water which contains oil, in dispersed, emulsified, or soluable form, there are two possibilities:
- Collection in tanks and disposal through specialised companies or, preferably.
- Treatment by condensate trap, oil/water separator and emulsion separating equipment.
For chemically generated emulsions, emulsifying agents are used. Emulsifying agents are substances which lead to a homogeneous emulsion and prevent separation into two phases. They enable unpolarised substances such as oils, greases, hydrocarbons, solvents and petrol, etc. to be stably bound up in polarised liquids such as water. To purify such wastes or water, it is often required to de-activate the emulsifying agent, i.e. to split the emulsion. Compressor condensates are mechanically generated emulsions. By adding flocculating agents, finely emulsified oil droplets are adsorptively tied to microflocs generated by the flocculating agent.
For further enhancing flocculation, auxiliary flocculating agents are used in addition to the main agents. These permeate the forming microflocs, leading to macro flocculation and an improved sedimentation process or, alternatively, facilitate separation by causing the pollutant to float. Emulsions can also be separated by another process, ultrafiltration (see Section 11.4). This process does rely on the addition of flocculating agents. The equipment is relatively expensive but it creates filtrates of a high degree of purity (less than 1mg/L). Ultrafiltration is particularly suitable for removing the larger types of molecule. Under pressures of 2.5 - 4 bar1, smaller molecules can pass through the membrane whereas larger particles are retained. Ultrafiltration is suitable therefore for the wider field of waste water purification. This is not the place to explain separating plant in further detail as the field of application extends far beyond that of condensate treatment. The units of far greater interest in connection with condensate treatment, such as static Oil/Water Separators and emulsion separating equipment (Ultrafiltration), form the subject of subsequent sections.
