VOC concentrator

VOC concentrators are specialised systems designed to reduce both the capital investment and operating costs associated with treating large airflows containing low concentrations of volatile organic compounds (VOCs).

Rather than treating a large volume of contaminated air directly, a VOC concentrator captures the VOCs from the process air and transfers them to a much smaller air stream. This significantly reduces the volume of air that requires downstream treatment, resulting in smaller equipment, lower energy consumption, and reduced operating costs.

As such, VOC concentration is often used as an intermediate step in an air treatment system, particularly when direct treatment of the full airflow would be technically or economically challenging.

How does a VOC concentrator work?

The most common type of VOC concentrator uses a zeolite wheel. Zeolite is a highly porous material with an extremely large internal surface area. This porous structure provides countless adsorption sites where VOC molecules can attach themselves as contaminated air passes through the wheel.

During operation, the process air is directed through the zeolite matrix. The VOCs are adsorbed onto the zeolite surface, while the purified air is discharged from the system.

Over time, the adsorption capacity of the zeolite becomes exhausted as the available adsorption sites are occupied. To restore the adsorption capacity, the zeolite must be regenerated by removing the accumulated VOCs. This is achieved by passing a hot stream of air or nitrogen through the zeolite. The elevated temperature causes the VOCs to desorb from the surface, leaving the zeolite material intact and ready for reuse.

Industrial VOC concentrators perform this adsorption and regeneration process continuously. To achieve this, the zeolite is arranged in a slowly rotating wheel. The largest section of the wheel is exposed to the process air and continuously adsorbs VOCs, while a smaller section is simultaneously regenerated using heated air.

As the wheel rotates, freshly regenerated zeolite continuously enters the adsorption zone, enabling uninterrupted operation and stable performance.

Where are VOC concentrators used?

Depending on the application, VOC concentrators can achieve concentration factors of up to 25:1. Even higher concentration ratios are possible by combining multiple concentration stages. They are particularly attractive in applications where:

VOC concentrations are relatively low.

VOC concentrators are most effective when treating dilute emissions, typically below 2 g/Nm³.

The process air temperature is moderate.

As a rule of thumb, inlet temperatures should generally remain below approximately 50°C. Higher temperatures may reduce adsorption efficiency and require upstream cooling.

Airflow rates are high.

The economic benefits are greatest when a large airflow can be reduced to a much smaller volume prior to final treatment.

The VOCs have suitable adsorption characteristics.

Compounds such as toluene, xylene, ethylbenzene, ketones, esters, and many solvent vapours adsorb well onto zeolite. Heavy compounds that form sticky deposits, tars, or aerosols are generally less suitable.

Example application

A production facility generates a ventilation airflow of 100,000 Nm³/h at 20°C, containing approximately 0.2 g/Nm³ of toluene and xylene. This air stream could be treated directly using a regenerative thermal oxidizer (RTO). However, because of the high airflow and relatively low VOC concentration, the required RTO would be very large. In addition, supplementary natural gas would be required to maintain the oxidation temperature, resulting in considerable operating costs.

By combining a VOC concentrator with an RTO, both the investment cost and the operating cost of the overall installation can be reduced substantially, while maintaining high VOC removal efficiency. A more economical solution may consist of the following treatment train:

Dust filtration

Removal of particulate matter and contaminants that could foul or block the zeolite surface.

VOC concentrator

Concentration of the VOCs from the original airflow of 100,000 Nm³/h into a regeneration stream of approximately 5,000 Nm³/h, increasing the VOC concentration from 0.2 g/Nm³ to approximately 4 g/Nm³.

Regenerative Thermal Oxidizer (RTO)

Treatment of the concentrated VOC stream. Because the airflow is significantly smaller and the VOC concentration is much higher, the RTO can be considerably smaller and may operate autothermally, eliminating or greatly reducing natural gas consumption.

FAQ: questions about VOC concentrators

Can a VOC concentrator handle fluctuating VOC concentrations?

Yes.

VOC concentrators can generally handle moderate fluctuations in VOC concentration and airflow. However, extremely high concentrations may require larger equipment or alternative treatment approaches.

Does the process air need pre-treatment?

In many applications, yes. Dust, aerosols, oil mist, and other contaminants can clog or foul the zeolite surface and reduce performance. Filtration is therefore commonly installed upstream of the concentrator.

Can a VOC concentrator reduce energy consumption?

Yes.

By reducing the airflow that requires treatment, downstream equipment such as regenerative thermal oxidizers can be significantly smaller and may consume substantially less fuel.

Which VOCs are suitable for treatment with a VOC concentrator?

VOC concentrators are particularly effective for solvent vapours such as toluene, xylene, ethylbenzene, ketones, esters, and alcohols. Compounds that form sticky deposits, aerosols, or tars are generally less suitable, as they may foul the zeolite surface. Compounds that are too volatile, with a boiling point generally less than 40°C, are not inclined to stick to the zeolite and are hence not suited to this process.

Can a VOC concentrator be used as a standalone treatment system?

No.

A VOC concentrator does not destroy or permanently remove VOCs from the process air. It merely transfers them from a large airflow into a smaller, more concentrated air stream. A downstream treatment technology, such as an RTO, catalytic oxidizer, or solvent recovery unit, is therefore usually required.

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