Two families of VOC treatment processes extract VOCs from ambient air:
- Recovery processes
Recuperative processes aim to capture VOCs for reuse. They include adsorption with desorption, absorption, condensation and membrane separation.
- Destruction process
Destructive processes include VOC oxidation (catalytic, thermal), biological treatment, adsorption without desorption, etc .
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Adsorption
VOC molecules bind to a solid material with a large specific surface area, through the action of Van der Waals forces. A wide range of adsorbent materials are available: activated carbon, zeolite, silica gel, activated alumina and resin. Activated carbon is the most widely used. Characterized by its high specific surface area (800 to 2000 m²/g), it comes in powder, grain or fabric form. When the activated carbon is saturated, either the material is changed and treated as waste (powder), or it is regenerated by recovering VOCs through desorption. In this case, either the total pressure is reduced (vacuum desorption), orthe temperature is increased(steam, air or hot neutral gases). The VOCs are then recycled into production…
Absorption, or gas scrubbing, is based on contact between air containing VOCs and a liquid solvent. Soluble VOCs pass into the liquid. Water is used for water-soluble VOCs. For less soluble VOCs,silicone oil replaces water. These two techniques are not widely used, as water is not very effective and oil is quite costly.
Condensation
This involves transforming the gaseous VOC into a liquid, by lowering the temperature to -40°C for mechanical condensation (using a compressor and heat exchanger) and – 180°C for cryogenic condensation (using liquid nitrogen). The liquid is then separated from the air for recovery. This technique is used for polluted air with a high concentration of VOCs, the latter having a boiling point of at least 40°C. Condensation is suitable for polluted air flows of less than 1000m3/h.
Membrane separation
Air polluted with VOCs passes through a semi-permeable membrane, retaining the VOCs as it passes. The flow rate treated by membrane separation is less than 100m3/h. Separation efficiency depends on the structure of the membrane, its cut-off threshold and filtration operating conditions. The technique is costly and sensitive to variations in flow rates and concentrations.
Thermal oxidation
The aim is to burn the VOCs at at least 750°, transforming them intoCO2 and water. A VOC concentration in excess of 10g/m3 is required to balance the energy and heat input of combustion. Otherwise, the process becomes very energy-intensive. When VOCs are halogenated or sulfurated, toxic by-products may be formed, requiring additional neutralization treatment.
Catalytic oxidation
To the previous technique, a precious metal or metal oxide catalyst is added. This oxidizes VOCs at temperatures between 250° and 400°. The aim is to reduce the energy consumption associated with oxidation. However, certain elements (heavy metals, phosphorus, SO2) contribute to catalyst poisoning and deactivation.
Microbiological treatments
Microbes are used aerobically to break down VOCs intoCO2 and water. The microbial culture is either fixed or dispersed in a mobile or stationary liquid phase. A distinction is made between biofilters, biopercolators and bioscrubbers.
Biofilter
The bacteria attach themselves to an organic support (peat, wood, etc.) through which the VOC-polluted air flows. To maintain biological activity, the bacteria are occasionally sprayed with water and additional nutrients.
Biopercolator
Bacteria attach themselves to a solid support , mineral or lining. A biofilm is formed, up to several millimeters thick. Continuous watering distributes water and nutrient supplements. This creates a liquid film around the biofilm, in which oxygen and VOCs are absorbed and then transferred to the biofilm.
Biolaveur
VOCs are absorbed in a spray washing tower, then biodegraded in an activation tank containing suspended biomass. Bioscrubbers, using an oil/water emulsion, can treat compounds that are poorly soluble in water or toxic to microorganisms.
Choice of treatment
It depends on the nature of the VOCs to be treated, their minimum, maximum and average concentration in the polluted air, and its physical conditions (temperature, relative humidity, presence of dust or other pollutants, etc.).
If the process emits one or two VOCs to be treated, and the VOC(s) is (are) cost-effective to reuse, a recovery technique may be chosen; subject to an adequate VOC concentration and polluted air flow rate.
With 3 or more VOCs, it’s best to opt for a destructive technique.
It should be noted that more than 30% of manufacturers choose activated carbon-based treatment. allowing a wide range of flow rates for both destructive and recuperative applications. They also eliminate olfactory nuisances.
