The process of purification of air containing solvents, developed with the VOC concentration technology, is based on the following principles:
- The primary current of polluted air, with a high flow rate of 20,000-80,000 Nm3/h and low concentration (0.2 - 1.2 gr/Nm3) is purified by passage through an adsorbent surface that retains VOCs.
- The surface is regenerated with a stream of warm air, generally at a temperature between 150-170°C, depending on the mixture of VOCs adsorbed and the characteristics of the adsorbent surface
- The amount of hot air regeneration is generally between 1/10-1/8 of the amount of primary air treated, so that VOCs are reemitted into the hot air regeneration flow at a concentration about 10-18 times higher than the initial step
- The hot air outgoing from the adsorbent surface is conveyed to a combustor for the complete oxidation of VOCs.
- Part of the combustion fumes, mixed with external air, are recycled as hot regeneration air.
The process is low energy consuming since the air flow sent to the combustor is very limited. In addition, VOCs at high concentration are often able to support the combustion without the use of auxiliary fuel.
We offer two versions of the solvent concentration plant.
A first version involves a static modular system in which the adsorption-desorption unit consists in a series of modules containing parallel adsorption surfaces and loaded with microporous adsorbent material, formulated in particles.
The modules – most of the time – operate as adsorbents, with the current of polluted air crossing them, getting purified and then conveyed to the chimney.
Modules are cyclically regenerated by prolonged flushing of hot and cold air flows.
The air flows coming from the desorption of the regeneration modules is sent to a combustor, preferably of recovering-catalytic type.
The management system of the plant checks the alternation of adsorption and regeneration phases in each individual module, controlling the opening and closures of internal valves thus regulating air temperatures.
Adsorbent particles' size is of the order of 1.5-2.0 mm, suitable to ensure a high specific surface exposed to the air flow with still acceptable load losses. The modular type adsorption can be expected to be loaded with activated carbon or with hydrophobic zeolite.
Use of zeolites
A solvent concentration technology involving the use of zeolites impregnated on sheets of silica-based composite materials is quite well established; sheets are assembled in a monolith, with a high active area and in the form of a cylindrical wheel, constantly exposed to the flow of polluted air and hot regenerative air.
The monolith is continuously rotating and alternatively exposes zeolites to polluted air and hot air for regeneration. In the monolith, zeolites are dispersed almost at the level of single crystals, so that the process of adsorption and desorption proceed with fast kinetics, albeit limited by the resistance of the external gaseous film that operates in a viscous regime.
For the same functional performance ratio, the amount of zeolites engages in the wheel is 1-2/10 of the equivalent amount involved in a traditional adsorbent particles surface; however, the manufacturing cost of the monolith is high and higher than the cost of an equivalent adsorption particle surface.
In the case of zeolite deactivation due to the formation of polymers in the adsorbed micropores, the monolith must be replaced.
For the manufacture of impregnated zeolite wheel monoliths, a handful of companies are available, holding the know-how of the solvent concentration process.
It is assumed that zeolites employed are of type ZSM-5 or Y at a different hydrophobic grade.