VOC treatment is an evironmental problem in many industrial sectors.
Volatile organic compounds (VOCs) are all those organic compounds that exist in a gas or very volatile liquid state at ordinary room temperature. Formally VOCS are all those organic compounds that have a vapour pressure equal to or higher than 0.01 kPa or an equivalent volatility in the particular conditions of use at 20ºC. VOCs usually have less then twelve carbon atoms in their chain and contain other elements such as oxygen, fluoride, chlorine, bromine, sulphur or nitrogen.
There are more than one thousand different VOCs, but the most abundant in the air are methane, toluene, n butane, i-pentane, ethane, benzene, n-pentane, propane and ethylene. These compounds are generated in all those industrial processes in which organic solvents (such as acetaldehyde, benzene, aniline, carbon tetrachloride, 1,1,1-trichloroethane, acetone, ethanol, etc.) are used. There are many activities that could produce VOC emissions. They generally belong to the follow industrial sectors:
- Iron and steel industry.
- Plastic industry.
- Food industry.
- Timber industry.
- Paint, varnish and lacquer industry.
- Livestock industry.
- Pharmaceutical industry.
- Cosmetics industry
Regarding their danger for human health and harmful effects on the environment, VOCs are classified in 3 groups:
- Compounds that are extremely dangerous for our health: benzene, vinyl chloride and 1,2 dichloroethane.
- Class A compounds: those which could cause significant damage to the environment such as acetaldehyde, aniline, trichloroethylene, etc.
- Class B compounds: have less impact on the environment. Acetone and ethanol belong to this group, among others.
There are some VOCs that destroy the stratospheric ozone layer, such as carbon tetrachloride. In addition, all VOCs, in combination with nitrogen oxides and sunlight, are ozone precursors at ground level (tropospheric ozone), which is very bad for health as it causes severe respiratory damage. This effect is known as photochemical smog and it is displayed as a brown-grey coloured fog in large cities that are usually sunny and have VOC and nitrogen oxide emissions.
For these reasons, current European legislation sets out ever more restrictive limits on the emission of these compounds. Therefore, in industrial activities that are susceptible to generating VOC, emissions must be controlled and, when necessary, treated efficiently. There are several viable techniques for VOC treatment, among which the most common are:
Gas phase advanced oxidation (GPAO): this technique consists of 4 stages. In the first stage, the air to be treated is subjected to an absorption process in water and ozone. The soluble gases that dissolve in the water are oxidised by the ozone to CO2. In stage 2, ozone is added to the gases resulting from stage 1 and the mixture is irradiated with high-intensity ultraviolet light. The ozone is transformed into OH radicals, which are extremely reactive with the VOCs. The oxidation produces a particulate aerosol, which are separated in stage 3 with an electrostatic precipitator. The resulting air, which is free of VOCs and of odours, may be released into the atmosphere. Finally, in stage 4 the remaining ozone is transformed into oxygen with a catalyst.
It is a robust technique for a great variety of VOCs, which is ideal for low flows, with a low operating cost and high energy efficiency.
Regenerative thermal oxidation (RTO): this process is carried out inside towers filled with ceramic material in which the pollutants are oxidised at 750ºC. The system has a thermal efficiency greater than 95%, such that the consumption of gas to maintain the temperature is low.
It is a very versatile technique as regards the flow to be treated (1,000-100,000 Nm3/h), which is ideal for medium-high concentrations of VOCs and optimal for a great variety of VOCs.
Regenerative catalytic oxidation (RCO): this process is similar to RTO but the presence of a catalyst in the combustion chamber makes it possible to operate at lower temperatures, in the range of 300-350ºC. The system has a thermal efficiency greater than 98% and does not consume gas when the autothermal point is reached.
It is an ideal technique for low or medium airflows (1,000-30,000 Nm3/h) for medium or low VOC concentration, which has a low operating cost.
Zeolite rotor concentrator + RTO: this technique is based on the operation of a wheel with a porous material (Zeolite) in which the VOC accumulate through an adsorption process to obtain a higher concentration. The VOCs are then treated in a regenerative thermal oxidation (RTO) unit.
It is an ideal technique for treating large air flows that contain low concentrations of VOC.
Hence, given the danger for people and the environment, the emissions that may contain VOCs must be controlled and, if necessary, treated. To do so, the technique that best fits the particular conditions in each case should be implemented. This will depend on parameters such as the flow under treatment, concentration of VOCs, conditions of the operation, etc.