- VOC definition
- Activities associated with VOC emissions
- Danger for human health and environment
- Technologies for VOC treatment
- Destructive technologies
- Non-destructive technologies
- Hybrid technologies
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.
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
Danger for human health and harmful effects on the environment
- 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.
In order to select the best technology for purification of volatile organic compounds (VOCs) one must consider the volume, concentration of VOCs, air temperature and humidity, the solvents present, permitted emissions limits and the possible presence of dust and other contaminants. On their behalf, the company must assess available resources, the temporal distribution of contaminating emissions as well as the possibility of recovering solvents and thermal energy.
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.
Technologies VOC treatment
Treatment technologies can be divided in two groups: the destructive and non-destructive. Destructive treatments are those in which the VOCs are transformed into other substances through an adequate procedure, while non-destructive treatment consists of the physical or chemical separation of VOCs from the air to be treated.
- Regenerative thermal oxidation (RTO): Like all other oxidative techniques, oxidizes VOCs in a combustion chamber with a burner. VOCs are transformed into CO2 and H2O. RTO is characterized by the presence of towers (normally 2 or 3) filled with ceramic material that holds and transfers the heat of combustion air treated during successive process cycles. With these towers, it is possible to achieve thermal recuperation efficiency above 95%, such that the consumption of gas to maintain the temperature is low.
RTO therefore is a technology with reduced fuel consumption. Moreover, if the concentration of solvents is greater than 1.5 – 2 g/Nm3, RTO becomes an auto thermal process with practically zero consumption. The operating temperature is between 750 and 1,250 ºC. At this temperature all organic substances can be oxidized.
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.
- Recuperative Thermal Oxidation: It is a simple technology with a low investment cost but higher management costs. It consists of a combustion chamber with a burner and a heat exchanger that heats incoming air and cools purified air.
Using this technique it is possible to achieve thermal recuperation efficiency of about 65%.
This technology requires lower investment costs than the regenerative one but has higher management costs due to higher fuel consumption. Recuperative thermal oxidation is a technology that makes it possible to eliminate pollutes that are in a gas when subjecting the latter to a sufficiently high temperature. For the process to be effective and for the pollutants to be able to fully oxidised, it is necessary to maintain a minimum temperature (between 700ºC and 1200ºC) for a minimum time (0.6-2 seconds).
The pollutants that are normally eliminated with this technology may be organic (VOCs, odours, etc.) or inorganic (CO, H2S, HCN, etc.). It is a technology that is used when then gas flow is below 50,000 Nm3/h and the concentration of pollutants is 5-20 g/Nm3.
- 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, due to the presence of a catalyst in the combustion chamber. The system has a thermal efficiency greater than 98% and does not consume gas when the autothermal point is reached.
This equipment is compact, requires less space and works at lower temperatures, consuming less fuel than recuperative thermal oxidation. To apply this technology, all solvents must be well studied, as there may be some products that poison the catalyst and warrant its replacement.
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.
- 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.
- Biofiltración: For some specific cases with low concentrations and uniform in time of biodegradable solvents and soluble in water, there is the possibility of using a biofilter in which microorganisms are responsible for degrading the organic matter. Biofiltration, although characterized by having low operating costs, presents some drawbacks due to the microorganisms need for stable conditions of humidity, temperature and food supply. In the case that these conditions change suddenly, hazards to the substrate are possible.
For all oxidative techniques, it must be kept in mind that in the presence of chlorinated compounds and other halogenated compounds; they become HCl type products that cannot be emitted to the atmosphere. In the presence of halogenated compounds, a scrubber is necessary to treat the acidic emissions generated.
- Activated Carbon Adsorption: It is the most common technology in this group
With this technology, the air to be treated is passed through a bed of activated carbon that retains the VOCs. The activated carbon becomes loaded with VOCs and reaches saturation losing its adsorbent capacity.
At this point we can dispose of this coal, managing it as waste and replacing it with a new carbon or regenerate the carbon with steam or an inert gas (nitrogen), which allows the recovery and reuse of solvents in the production process.
- Cryogenic condensation: It is a process that is based on the freezing of air to be treated at extremely low temperatures using liquid nitrogen or another cryogenic fluid. The contaminated air is progressively cooled in condensers, below its dew point, resulting in condensation of VOCs and their separation from the gas phase.
This technology is not only useful for the purification of VOC emissions, but also allows for the condensation and recovery of expensive raw materials and contaminants that are usually present in emissions of processes where organic solvents are involved.
Cryocondensation is a clean and non-destructive method because it recovers vapor emissions in liquid form that would otherwise be released into the atmosphere. This is accomplished by carrying out a controlled cooling of the process vapours of a determined substance to reach its dew point in the moment when it starts its condensation.
By use a of condensation column, the contaminated air current is crossed with VOCs, circulating liquid nitrogen in a counter current flow which cools the air with a volatile substance below the condensing temperature (reaching up to -200ºC). This produces the freezing of the moisture in the air and obtains a liquid product that can be reused in the process. The nitrogen used can be reutilized by a small compression station for use as gas in manufacturing or it can be discharged into the atmospheres if there is no use for it.
The range of available equipment covers a wide spectrum of recoverable solvents such as: toluene, acetone, methanol, chlorinated derivatives, hydrocarbons, etc.
Cryocondensation can treat different currents, flows, and pressures. Its systems can even be custom designed for each case. As mentioned before, there exists the possibility to refuse the condensed solvents as well as the nitrogen generated.
Thanks to its properties, liquid nitrogen is used as a refrigerating agent that allows for the condensation of all substances considered VOCs in a range between -30 and -120 ° C.
The condensing temperature is determined by the components to be treated and by the ppm that we wish to achieve in the emission current.
- Physical/chemical absorption: Physical/chemical absorption consists of the retention of pollutants in an aqueous solution flowing in a countercurrent inside washing towers. A reagent may be added to the aqueous treatment solution that will react with the pollutant, favoring its elimination. The washing towers must be accompanied by a system for treating the water that has absorbed the contaminants. In the case of VOCs, this technology is applicable in cases in which the products are soluble in water (acetone, alcohols, etc.).
- 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.
The first step is a Rotor Concentrator, which is a ‘wheel’ filled with zeolites that adsorb the VOCs in the incoming air. The air is purified upon exit. A small portion of purified air (between one tenth and one fifteenth) is heated to 200 °C and passed upstream to desorb the VOCs retained in the zeolites. In this way, we obtain an airflow 10-15 times lower than the initial with a concentration 10-15 times higher than the initial. This air is then sent to the oxidation unit (RTO) to be purified.
This is a wastewater treatment process that combines thermal separation of soluble substances in water with the purification of volatile organic compound (VOC).
The appropriate waste to be treated by evapo-oxidation (evaporation and oxidation processes) are waters of organic nature (not organ halogen), with or without the presence of salts and other inorganic compounds (by-products of nitrogen, sulfur), with low net calorific values (NCV), those that are non-flammable nor solvents and that contain significant COD values.
In the first stage, the effluent undergoes an evaporation process generating water vapor that carries the volatile compounds that already have a lower boiling point than water. Additionally, they also carry all the substances that form azeotropic mixtures.
After this first stage, the water vapor that is obtained is sent together with the volatile substances to an oxidation chamber where the water vapor is burned, thereby preventing its emission into the atmosphere or any other contaminating activity.
Thus, thermal oxidation of the water vapor can completely destroy volatiles within the effluent.
Another option is to use these volatile compounds (if and when they are in elevated presence) to carry out an auto thermal process since the heat generated during their combustion is sufficient enough to not require external heat. Thus, one can obtain the energy needed to power the process itself.
Furthermore, the first process of evaporation that the effluent is subjected to before the phase of vapor oxidation, results in concentrated organic waste that is found in the effluent. This waste can be sent to waste management or undergo a secondary phase of concentration for recovery and enhancement.
It should be noted that it is also possible to utilize the evapo-oxidation procedure with low calorific vapours as well as for the elimination of odorants.
Although evapo-oxidation is a procedure that offers very good results, it is not the only technology to treat effluents that contain VOCs. A variation of this process is stripping columns with vapor or hot air in a counter current in order to subsequently utilize OTR systems for the thermal oxidation of the volatiles.