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ADSORPTION PLANTS
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In case of low concentration of Volatile Organic Compounds (VOC) in the exhaust air, adsorption plants represent the best combination between reduced running costs and abatement efficiency.
Adsorption process is operated by mean of zeolite or activated carbon, depending on the kind of pollutants, concentration and desired performances.
Most of adsorption plants are designed in connection to thermal oxidizers to allow continuous desorption and combustion of the VOCs.
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Zeolite Adsorption Rotor (ADR) |
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The adsorption rotor is an integrated system for air pollution control. By means of the adsorbing properties of the zeolite, this system removes VOCs from exhaust air capturing them inside the honeycomb structure.
The exhaust air leaving the rotor concentrator has a low concentration of VOCs and is conveyed directly to the clean air stack. Part of this air is then heated up and used for purging the rotor itself from the adsorbed VOC. The purging air, with high VOCs concentration, is then conveyed to an oxidizing system (RTO).
The ADR is a dynamic system: the adsorbing zeolite is inserted in a wheel, continuously and slowly rotating. The wheel has three sections: adsorption, cooling, desorption. Each section is separated from one to the other and perfectly tight. The exhaust air passes mainly through the adsorbing section where the VOCs are adsorbed by the zeolite. Part of the exhaust air ( 5% to 10% depending on the process conditions) is conveyed through the cooling section. This same air is then heated up and conveyed back to the desorption section for purging the rotor from VOCs.
Thus the rotor concentrator continuously passes throughout the three section, with continuous cycles of adsorbing and purging.
The main advantages of this system is the reduction of energy consumptions:
- the exhaust air flow to be oxidized is reduced (gas consumption reduction)
- the VOCs concentration is increased leading to almost autothermal operation of the oxidizing system (gas consumption reduction)
- Most of the exhaust air passes only through the rotor concentrator with very low pressure drop compared to an oxidizer (Electrical energy consumption reduction)
- The overall dimensions of the complete plant (ADR+RTO) are lower. (Installation costs reduction) |
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Adsorbers with activated carbons |
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The adsorption plant capitalizes the remarkable adsorption properties of the activated carbons, materials with vegetarian or mineral origin with an extremely high internal porosity. The numerous microscopic thin long pores, of the activated carbon, are developed in depth and can reach a surface ranging between 500 and 2.000 square meters per gram of material.
Their adsorptive capacities are very suitable for the treatment of organic compounds with a molecular weight of 50 up to approximately 200. Normally, compounds with a lower molecular weight are not absorbed sufficiently, due to their smal dimension; on the contrary, organic compounds with a high molecular weight get adsorbed so strongly, that during the phase of regenerating it is difficult to remove them.
The level of adsorption is determined by several parameters, such as temperature, pressure, type and concentration of the pollutants, molecular weight, grade of humidity and presence of particulates in the emission flow.
Other elements influencing the adsorption, are also the quantity of activated carbons used, the size and shape of the pores, eventual chemical reaction, the dwell time between activated carbons and pollutants to be adsorbed. |
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