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Removing VOC emissions of the chemical industry

Exhaust air cleaning with regenerative thermal oxidation
Removing VOC emissions of the chemical industry

When it comes to pollution caused by the production in chemical plants, the most efficient solution in the environmental technology market is called regenerative thermal oxidation (RTO). Some of the main pollutants emitted into the atmosphere by the chemical industry are volatile organic compounds (VOCs). For a chemical company in Spain, Tecam installed an RTO system that removes up to 99.9 % of the VOCs emitted.

Regenerative thermal oxidisers (RTO) are designed to treat air volumes from 2000 to 200 000 Nm3/h as well as medium to high solvent concentrations. They count with many technology advantages such as being adaptable for small, medium and large air flows and a wide range of VOC emissions to be treated. They also have a high thermal efficiency and a low operating and maintenance cost. In addition, they do not generate any waste and they can generate heat recovery for external processes. Depending on the project’s technical needs, RTOs can be accompanied by other equipment such as quenches or scrubbers. A scrubber is responsible for eliminating acid gases, usually generated by the oxidation of VOCs with halogenated compounds, whereas a quench reduces the temperature at the RTO outlet. Tecam designs, assembles and installs equipment for VOC emissions treatment through regenerative thermal oxidisers.

VOC reduction of up to 99,9 %

But how is it possible to achieve a VOC emissions reduction of up to 99,9 % by RTO? The Tecam regenerative thermal oxidisers consist of towers housing a ceramic bed and a common combustion chamber where solvent oxidation takes place. The towers’ interior insulation and the combustion chamber, along with the use of more evolved ceramic materials, result in low equipment gas consumption. A conical closing valve system achieves very high purification efficiency, avoiding VOC emissions going out untreated. That valve system also allows the equipment to be closed during overnight or weekend shut-downs, hence allowing the customer to keep a high temperature inside the RTO equipment, and thus a faster start-up, with lower equipment gas consumption for VOC emissions treatment.

The polluted process air is sucked in by the main fan usually located upstream of the oxidiser. For processes with high concentrations of suspended particles, the fan usually is located downstream of the oxidiser. The fan forces the process air through the first ceramic tower, and the process air is heated by the ceramic. When the air has passed the ceramic bed, it reaches the combustion chamber, where the oxidation takes place. The oxidation temperature depends on the compounds to be treated. For emissions without halogenated compounds, an oxidation temperature between 800 and 900 °C is chosen. If there are halogenated compounds within the polluted air, the temperature should be at least 1100 °C to ensure complete oxidation.

Parallel to the process air inlet through the first chamber, the already oxidised air passes through the second ceramic tower to exchange the heat from the air to the tower. Thus, the gas gets cooled and the ceramic bed gets heated. After passing through the second tower, the clean air is sent to the stack. The third ceramic tower is used to recirculate the purge, since in the sequence of valves it must be ensured that all the air is oxidised. This sequence is repeated between 45 and 90 s, to ensure that each tower works the same way. The system of three ceramic towers is the most used nowadays, although there are also RTOs with two towers and even with five towers. Due to the purge circulation system in the three-tower RTO, it is ensured that all gases pass through the combustion chamber. Thanks to the special design of the combustion chamber in Tecam‘s RTO plants and the valves, which have an air tightness of 99,9 %, a high degree of purification is achieved. The ceramic towers allow the recovering of the thermal energy of the combustion chamber.

Successful project in Spain

A world‘s leading chemical company contacted Tecam in 2018 in order to eliminate the VOC emissions from one of their production sites in Spain. The specific challenge of this project was treating emissions with a high load of halogenated compounds with high corrosion risk. The presence of acidic inorganic compounds and the explosive internal process environment in the chemical reactor required high quality inspection processes and a rigorous project audit.

One of the most important challenges was to guarantee the emission limits for high-risk compounds, which according to the regional authorities had to be below
2 mg/Nm3. Some of the compounds of the total mix of VOCs were considered as of high environmental risk and had to comply with that limit. The finally installed exhaust air cleaning plant emits between 3 and
6 mg/Nm3 of total VOC. The value of 2 mg/Nm3 for each risk compound therefore is guaranteed.

The concentration of 4180 mg/Nm3 VOCs in the customer‘s process was very high. After analysing the customer’s technical requirements, Tecam proposed a custom-made solution consisting of a regenerative thermal oxidiser for 5000 Nm3/h with a quench and a scrubber system to eliminate the acids generated during the thermal treatment of the halogenated compounds. Due to the high VOC concentration, a dilution of 5000 Nm3/h of fresh air was carried out, which allowed to work with total safety and at the same time maintain the autothermal conditions. The equipment was designed for high temperature (1200 ºC) to fulfill the requirements of the administration regarding the halogenated compounds. After installation, the custom-made RTO technology solution managed to eliminate 99.9 % of the VOC emissions.

RTO is not only suitable for the chemical sector. This exhaust air cleaning solution is as well applied in the pharmaceutical, petrochemical and oil & gas industry, and many others.

Tecam Group, Barcelona Spain

Author: Bernat Sala



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