Synonyms, abbreviations and/or process names
- Heat exchanger
- Odour control condensation (OCC)
Removed components
Condensable substances:
- Water vapour
- Odour
- Fats
- Solvents
Diagram
Process description
The gas stream is cooled with a cooling medium (cold wall in a heat exchanger or a liquid). The reduction in temperature lowers the vapour pressure of pollutants in the gas stream. If the vapour pressure drops below the partial pressure of the pollutant, the substance will condense into mist or droplets. The mist or droplets must later be separated using a mist or droplet separator.
During the condensation of water, pollutants that are soluble in water (acids, alcohols, ammonia…) will partly dissolve in the condensate. This may cause strong odour reductions to be realised. For odours, a condenser is normally followed by another treatment technology.
The condenser can be implemented as a direct or indirect condenser. These options are further discussed under ‘variants’.
The cooling water used to cool the gas stream must in-turn be cooled with surrounding air. This can take place in open or closed cooling towers (see ‘variants’).
Variants
Direct and indirect condenser
Direct and indirect condensers are the main condenser variants.
In the direct condenser, there is direct contact between gases and the cooling medium. This provides very good heat transfer. This condenser is normally set up as a spraying chamber. It is primarily suited to gas streams that pollute classic heat exchangers, for example, if sticky or dust-laden streams are concerned. Along with the to-be-condensed product, soluble substances and part of the dust will also be collected.
When water is used as a cooling medium, this system is actually a hybrid system in which the water of a scrubber is cooled. The cooling of water can be combined with almost all types of scrubbers. The material choice for heat exchangers is important, in order to avoid corrosion.
In indirect condensers, the to-be-treated air is passed through a liquid-gas heat exchanger. The cooling medium is thus separated from the to-be-treated gas stream. This has two main advantages:
- No pollution of the cooling medium
- During the condensation of solvents, a separation stage is not needed to separate the solvent from the cooling medium.
Sticky substances may cause pollution of the heat exchanger, whereby the yield becomes lower and a risk of blockage exists. In order to reduce pollution, water can be sprayed over the surface of the heat exchanger, so that the surface is washed clean and the likelihood of substances sticking to it is reduced. If there is excessive pollution, then one needs to change to a direct condenser.
Type of cooling medium
In addition to the choice between direct and indirect condensers, one must also choose the type of cooling medium. The cooling medium determines the lowest operating temperature. The residual solvent concentration is determined by the temperature and decreases (logarithmically) when temperature decreases. For specific solvent compositions, a specific residual concentration can be realised by selecting the right condensation temperature. The following temperature levels can be realised:
- Cooling water: down to 25 °C
- Cooling water with mechanical cooling: down to 2 °C
- Brine: -14 °C
- Ammonia condensation circuit: -40 °C for one stage and -60 °C for two stages
- Cryogenic condensation: down to -120 °C, in practice mostly between -40 and -80 °C
In order to abate odour, one normally opts for cooling with cooling water, without extra mechanical cooling. The condensation temperature of waste gases must exceed 40 °C. Cooling until freezing point is also possible but energy use and cost aspects for the installation increase drastically. A reduction of the condensation temperature from 25 °C to 5 °C has relatively little effect because the majority of the water vapour has already condensed at 25 °C.
Condensation at low temperatures (-20 to -120 °C) is used to reduce and recuperate solvents because solvents normally have a high vapour pressure.
Open and closed cooling towers
In open cooling towers there is direct contact between the air and the cooling water. These systems can be recognised via the condensation plume which rises from the towers. These systems are also able to cool the water to wet bulb temperature.
Closed cooling towers or air coolers include a heat-exchanging surface through which air is blown using a ventilator. There is no direct contact between the cooling water and the outside air. In these systems the air can be cooled up to 5 – 10 °C higher than the outside temperature. These systems can be improved by spraying water on the surface, so that extra cooling is realised via vaporisation.
If cooling water comes into contact with to-be-treated flue gases, it is better to opt for a closed system because otherwise one runs the risk of stripping the separated substances back into the environment, whereby one encounters a secondary source. This is primarily a problem in the presence of odour components.
Efficiency
The efficiency is determined by:
- Pollutant type (vapour pressure, water solubility)
- Concentration of pollutants
- Water content
- Condensation temperature
- Quantity of suppletion water for direct condensation
It must be examined on a case-by-case basis whether cooling helps to realise sufficient condensation of pollutants.
In the case of odours, removal is partly realised by condensing substances and partly by absorbing odour molecules in condensed droplets. Yields of 50 – 90 % can be realised in odour-removal of highly vapour-laden air. Odour reduction yield is determined by the humidity of incoming air, pollutant load and the type of pollutant found in waste gases.
Boundary conditions
- Dew point temperature above 40 °C.
- Condensable organic substances
- In indirect condensation, one must ensure a low dust and fat content in order to prevent the condenser from blocking. By making specific adjustments to the condenser, like placing a water sprayer earlier in the process, fouling of the condenser can be reduced by the water's washing effect.
Auxiliary materials
For cooling water
- Corrosion inhibitors
- Biocides to avoid growth of bio-organisms
Coolant circuit
Ammonia, organic substances or freons for the cooling circuit.
Environmental aspects
Condensate which must be sent for water treatment or disposed of.
Energy use
Determined by the installation type and the flue gas type:
- For cooling water: A recirculation pump and a ventilator for the cooling air
- For mechanical cooling: A recirculation pump, a compressor for the gas and a ventilator for the cooling air
- Cooling capacity is primarily determined by the gas and condensation temperature, the humidity of the flue gas and the flow rate.
Cost aspects
- Investment
- 5 000 EUR for 1 000 Nm³/h (excluding cooling-water provision): pump; piping, cooling towers) [1]
- Operating costs
- Personnel costs: ca. 2 hours per week (excl. cooling system) [1]
Advantages and disadvantages
Advantages
- Good for separating larger pollution content
Disadvantages
- Cooling water is required, so an air cooler or cooling towers must be placed.
- Varying yields for various gas compositions and humidity levels
Applications
Condensation is primarily used for humid odour-laden waste gases or for very high solvent concentrations (> 50 g/Nm³). In the case of solvents, one would normally switch to cryo-condensation.
It is implemented for:
- waste gases in the foodstuffs sector
- waste gases from indirect drying installations: These waste gases consist of water vapour containing a low level of non-condensable gases, which means large reductions in the to-be-treated gas volume can be realised.
Condensation is also normally used as a pre-treatment to collect most of the encountered pollutant. Another technique is then later employed to meet emission standards or to solve odour problems. These later techniques are then less loaded, whereby costs are normally lower.
References
- EPA technical bulletin: Refrigerated condensers for control of organic air emissions December 2001
- BREF: "Common waste water and waste gas treatment /management systems in the chemical sector" EIPPC, February 2002
- Factsheets on Air-emission reduction techniques, www.infomil.nl, Infomil
- J.C.Mycock, et Al.:" Air pollution control engineering and technology" Lewis publishers, 1995
- Solvent capture for recovery and re-use from solvent laden gas streams, Environmental Technology Best Practice programme, guide GG 12
- J. Van Deynze, P. Vercaemst, P. Van den Steen and R. Dijkmans., Best Available Techniques for paint, varnish and printing ink production, 1998