Catalytic methods to treat hazardous air pollutants are experiencing a comeback. In the past 15-20 years they have often been replaced by other thermal oxidation processes on account of the catalysts’ susceptibility to poisoning and/or masking. Catalytic oxidation of VOCs owe their comeback to two persuasive benefits: low energy needs and notably lower NOx emissions.

The air pollution control specialists Environmental and Energy Systems (EES), a business unit of the Dürr machinery and plant manufacturing group, prefers to apply catalytic methods when treating air pollutants in the chemical processing industry. Classical areas of application are abatement systems used for PTA production facilities, for calcination processes (catalyst manufacturing) and for manufacturing of maleic anhydride, olefines, natural rubber and acrylic.

For more cost-efficiency, the treatment of smaller exhaust air streams and lower VOC loads is becoming the predominant trend in chemical processing, petrochemical, and pharmaceutical manufacturing industries. Since solvents remain a cost factor, the quantity of solvents discharged with the exhaust air stream should be as low as possible. Advanced production processes consider this issue more and more in their calculations.

Some important standards for selecting the appropriate procedure are process parameters such as flow rate, temperature, particulates in the contaminated air and the species of VOCs. To choose the right catalyst, it is important to know what type of VOCs (aromatic, oxygenated, halogenated hydrocarbons or alcanes) have to be removed. An effective catalyst for short-chain alcane methane is currently not available. The aim is, however, that future catalyst types will be capable of removing this substance.

In order to offer the most suitable catalyst, Dürr is partnering with catalyst manufacturers like Haldor Topsoe to make beneficial use of their wide-ranging portfolio.

Catalyst poisoning can de-activate the catalyst within a short operating time. The catalyst material may have to be replaced in order to keep emission levels within specified limits. The active substances of the catalysts and the catalyst poisons have not significantly changed in the past decades. Substances such as silicon, phosphorus, sulfur, halogens, and metals – in particular germanium, arsenic, selenium and tellurium – are the materials that are most often cited to cause poisoning. These substances can be tolerated in low concentrations, but certain limits must not be exceeded.

The last important criterion for selecting the best abatement method is the VOC concentration itself. Catalytic methods differ from comparable thermal procedures methods. Catalytic methods can be auto thermal at lower concentrations than thermal methods. This means that no additional energy needs to be supplied to sufficiently pre-heat the contaminated air to the necessary temperature when it flows through the catalyst. When VOC concentrations are too high, the air has to be diluted before it passes through the abatement system. However, certain limits are set. If the treated air stream is diluted too much, the air flow rate increases disproportionately, which results in higher costs for the individual equipment units and the catalyst.

The benefits of catalytic abatement methods result ultimately from the operating costs saved in primary energies. VOCs can be removed without any additional energy when VOC concentrations are low. As a result, fuel consumption, CO2, and NOx decrease in comparison to other thermal methods.