Activated carbon adsorption for the removal of industrial odours
Odour perception is subjective, complicating control efforts due to differing expectations of effectiveness. Odour streams often contain diverse molecules requiring varied removal methods. Despite this complexity, activated carbon filtration is highly effective at removing compounds like hydrogen sulfide, ammonia, terpenes, and thiols, making it ideal for industries such as wastewater treatment, food processing, and asphalt production.
Overview of activated carbon technology
Activated carbon adsorption basics
Activated carbon captures odour molecules (adsorbates) on its surface through two main methods:
- Physisorption: Molecules attach weakly to the carbon surface due to intermolecular forces. This bond is reversible.
- Chemisorption: Molecules form stronger, often permanent bonds with the carbon surface. This method is useful for trapping compounds like ammonia and hydrogen sulphide.
Regenerating Activated Carbon :
The adsorption capacity of saturated activated carbon can be restored by desorption, also called regeneration or reactivation. This involves a high-temperature pyrolysis process (700–1000°C) with steam and no oxygen, which removes adsorbates. This process restores the characteristics of the activated carbon, which can then be reused, reducing waste. Exhaust aftertreatment systems, such as scrubbers or afterburners, are used to destroy the desorbed molecules.
Coal-based coal vs. renewable shell-based coal
Activated carbons can come from coal or renewable sources like nutshells.
- Coal-based activated carbons, derived from fossil sources, tend to have varied pore sizes, suitable for capturing larger molecules.
- Renewable shell-based carbons, usually have a larger volume of micropores. This makes shell-based activated carbons more suitable for capturing smaller VOC molecules.
DESOTEC's research has shown that shell-based activated carbons can achieve higher loading capacities than mineral-based carbons for certain molecules. In addition, shell-based carbons release adsorbates more gradually, which reduces the risk of exceeding emission limits during e.g. batch operations.
Renewable precursors are gaining interest for reducing the environmental impact of activated carbon production. Life cycle assessments (LCA) show that using waste biomass, like nutshells, instead of coal is more climate-friendly, with even greater benefits when coal is reactivated rather than replaced. DESOTEC activated carbon range include more and more renewable grades, gradually replacing coal-based grades in numerous applications when suitable.
Key factors in filtration solution design
There are a series of operational parameters to take into account when designing an optimal odour filtration solution with activated carbon filters.
Flow rate
The flow rate of the polluted stream through the filter bed is crucial to optimise the filtration performance. In industrial applications, the Mass Transfer Zone (MTZ) - where the gradient between influent and effluent occurs - widens due to diffusion, competing adsorption and varying adsorption concentrations.
A higher flow rate lengthens the MTZ and, if it exceeds the filter length, the desired effluent concentration may not be reached. Thus, a longer carbon bed with a constant cross section improves the contact time and reduces the risk of non-compliance with odour emission limits.
Temperature
Higher temperatures tend to reduce the adsorption capacity, especially in physisorption processes, which are exothermic. Operating temperatures should not exceed 60 °C to avoid safety risks.
Relative Humidity (RH)
Adsorption capacity usually decreases with increasing RH. High RH can cause water vapour to condense in the pores of the activated carbon, which could reduce filtration performance. To control high humidity, the influent can be cooled to condense the water before the flow reaches the filter, and in addition, filters should be designed for efficient water drainage and minimal moisture contact with the carbon.
Contaminant Composition:
Understanding and analysing the composition of the contaminated stream is essential to maximise removal efficiency. While virgin activated carbon can physically adsorb organic odours, chemisorption is necessary for inorganic molecules. Secondary contaminants, such as dust should be filtered out before reaching the carbon to avoid clogging.
Pressure drop
The pressure drop in the coal bed increases with the flow rate and can be aggravated by dust accumulation, which shortens the life of the coal. Excessive pressure drop requires more powerful fans, which increases energy consumption. To mitigate this, larger cross sections or parallel filter setups can be used.
Industrial design considerations
Filter design must account for variations in air and water stream composition due to factors like process changes, contaminant types, and concentration levels. Proper filter sizing is essential for both performance and cost efficiency, though it can be challenging due to uncertainties in gas composition and flow rates, especially in high humidity. Pilot installations aid in estimating filter lifespan and carbon consumption, while mobile filtration units offer flexibility and reduce financial risks compared to fixed systems.
Mobile filters, generally safer and more cost-effective than stationary ones, lower manual labour and handling risks associated with spent carbon. They are particularly useful in high-flow situations, enabling faster, safer replacements with minimal downtime. Filters can be arranged in series or parallel based on needs: parallel setups suit higher flow rates and lower pressure drops, while series configurations optimize carbon usage, ensuring emission compliance by allowing the second filter to activate once the first is saturated.
Case Study: Odour Treatment for a Chemical Producer's Wastewater Plant
This case study illustrates DESOTEC’s capacity to deliver fast, effective, and sustainable filtration solutions tailored to client needs in the specialty chemicals sector.
Challenge
A specialty chemicals company, a DESOTEC client, faced odour issues at their wastewater treatment plant due to high levels of odorous compounds like alcohols (757 µg/m³) and organic sulphur compounds (1,846 µg/m³). These volatile compounds, including dimethyldisulphide and 2-ethyl-1-hexanol, generated complaints from employees and neighbours, especially in warmer months. The company sought an effective, quick solution to address these odours.
Solution
DESOTEC implemented its mobile AIRCON 3000 filtration system with B-PURE 10 NB renewable activated carbon. The system captured odorous emissions from the buffer tank and vented clean air through a chimney, installed within 30 minutes using Plug & Play technology, without interrupting operations.
Results
The filtration system achieved a 99.5% reduction in odour compounds. Operational for three years, it has effectively prevented further complaints. The mobile filter requires annual replacement to ensure optimal performance, and DESOTEC manages the safe and sustainable disposal of spent filters, reactivating the carbon to create a circular, environmentally friendly process.
Takeways
- Effective and Rapid Odour Mitigation
DESOTEC’s mobile filtration cut odorous VOCs by 99%, quickly resolving complaints and ensuring compliance during seasonal challenges.
- Minimal Operational Disruption
The Plug & Play design enabled fast setup and filter replacements without downtime, ensuring seamless business operations.
- Sustainable and Circular Waste Management
DESOTEC’s off-site filter reactivation minimizes environmental impact, simplifies waste handling, and aligns with customer values.
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Odor control
Industries are at the heart of communities. By keeping the air smelling fresh, your company fosters good relations with the local community. -
Our Filtration Solutions
We offer a wide range of filter models and carbon grades to suit your industrial water or air purification requirements. -
Waste and Wastewater Management
Sustainable air and water purification for waste management and recycling to protect our environment and control odors.
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