Exhaust air treatment
Volatile organic compounds (VOC) can be separated from off gas streams using one of the following processes: biological cleaning, chemical transformation (oxidation and other reactions), adsorption (using active carbon, zeolite or silica gel), absorption (scrubbers) and condensation (with mechanical cooling devices or by cryogenic cooling). The method applied depends mainly on the typ of VOC and on waste gas parameters like volume flow, concentration, pressure and temperature or other technological demands (e.g. solvent recovery or removal). Important are also the recommended clean gas parameters and local legislation (safety and environmental).
Each type of technology has its advantages but is also linked to individual disadvantages and restrictions. To find the best technology is difficult not only because of the wide range of available waste gas cleaning processes but sometimes also because of the complexity of the individual emission situation. The challenge arises in finding a method which is not only technologically and economically efficient but also meets all saftety-, environmental- and legislative requirements.
Important criteria for the choice of the right purification process are the waste gas volume flow and its load. The diagram above qualitatively compares the application area of the different processes.
Oxidation is a common method for processing large volumes of permanent waste gas flows with medium load where all organic compounds present should be mineralised to CO2 and water. This is possible by direct incineration (TNV-thermal incineration at 700°C to 800°C) or catalytic oxidation (KNV-catalytic oxidation at 300°C to 400°C). Both typs of oxidator can be equipped with recuperative or regenerative heat recovery systems to increase efficiency.
Flow rates in thousands or tens of thousands of cubic meters per hour and concentrations up to 25% LEL (lower explosion limit) of flammable VOCs are usual for this application; and oxidation, with quite low investment and medium running costs, is the best solution.
Problems can arise with the residual oxidation compounds when halogenated VOCs are present in the waste gas. A further disadvantage is that recovery of any valuable substances present in the waste gas is not possible. Current official regulations and legislation make it sometimes difficult to gain approval for this type of plant.
Capture of selected compounds using an adsorption medium is a waste gas cleaning method which also makes solvent recovery possible. Adsorption can be carried out using activated carbon, ceramic materials or organic polymers. The processes’ critical parameters are the molecular weight and chemical structure of the VOC (i.e. its affinity to the adsorption material being used), waste gas temperature (the lower the better); and its relative humidity. The method can be ideally applied where a single solvent at low to medium concentration has to be removed from a high volume flow of carrier gas.
The main problems of this technology are related to the following: removal of the adsorbed compounds from the adsorption material (desorption), overheating through adsorption heat at high pollutant levels (“hot-spots” and fire risk); and differences in the affinity of the VOC to the adsorption medium.
Desorption is usually carried out using steam, hot air and sometimes nitrogen and results in production of a certain amount of contaminated water requiring wastewater treatment, which in turn leads to additional investment and running costs.
Adsorption plants are usually supplied as stand alone, fully operational devices (with and without solvent recovery). There are also processes where the pollutant is only concentrated and then further treated using another process (for example, condensation). Occasionally smaller adsorbers function as further cleaning steps or police filters in other processes. A further very simple application is the use of containers of activated carbon which can then be disposed of (incinerated).
With this method VOCs are captured by a liquid medium, usually in scrubbers or columns. The waste gas gets into direct contact with the absorption medium (water or organic solvent) and VOCs are removed from the gas and dissolved in the liquid. The resulting saturated liquid then has to be regenerated or treated (e.g. biological waste water treatment). Although small laboratory washers are quite common absorption is seldom used in industrial plants.
Condensation / solvent recovery
Condensation units mainly consist of a condenser (shell and tube heat exchanger) and a cooling source, e.g. brine or a mechanical cooling machine. Condensed solvents leave the condenser with a high quality because there is no dilution or pollution of condensate during the process, therefore reuse of the solvents in production is often possible. However, significant recovery rates are only achievable when vapour concentrations are very high or condensation temperatures very low. Conventional condensation units can only be used for vapour recovery, not for waste gas cleaning. The gas load (vapour concentration) is reduced, concentrations low enough to meet TA-Luft (German Technical Guideline for Clean Air) regulations are not achieved.
Cooling with liquid nitrogen (boiling point -196°C) lowers the clean gas temperature to such an extent that the waste gas concentration can be reduced to the mg/m³ range. The condensers required for this must be designed in such a way that formation of ice and fog does not impair the desired cleaning effect.