Purification of Volatile Organic Compounds (VOCs)
Volatile organic compounds (VOCs) are products that can be harmful to health and cause significant damage to natural resources. To minimize these adverse effects the Royal Decree 117/2003 was published on the limitation of emissions of volatile organic compounds due to the use of solvents in certain activities, active October 31, 2007 to all affected industries. The Royal Decree established a limit for solvent consumption and emissions of VOCs in the gases emitted from chimneys and diffuse emissions in every affected activity.
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.
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%.
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.
Recuperative Thermal Oxidation 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%.
With catalytic oxidation, the main difference is that combustion is achieved at lower temperatures (200-400°C) due to the presence of a catalyst in the combustion chamber. 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.
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.
In the case of very high airflow rates (> 10,000 Nm3 / h) with a very low concentration of VOCs (<1g/Nm3), the fuel consumed by these technologies is quite high. In order to reduce consumption, 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 to be purified.
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.
The most common technology in this group is activated carbon adsorption. 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 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.
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.)..