Desorption in activated carbon filters: an underestimated risk
Highlights
- Desorption is the reverse of adsorption: compounds adsorbed by activated carbon can be released again when conditions change.
- Temperature is often the critical factor: a temperature rise around and inside the filter promotes desorption, which can lead to a breakthrough and therefore a drop in adsorption efficiency.
- Insulation and cooling are essential: they keep the temperature inside the filter stable, prevent hotspots and thus limit the risk of desorption.
Activated carbon is a highly reliable adsorbent for removing contaminants from air and biogas. But adsorption is not a one-way process. Under certain conditions, including high ambient temperatures, the reverse can occur: desorption.
Alongside a well-considered choice of carbon type and filter solution, monitoring the process conditions is also essential. Thanks to their thermal insulation and integrated active cooling, Cargen’s AirPure L filters can help prevent the adverse effects of higher ambient temperatures on your process.
What is desorption and why does it occur?
Desorption is the process in which previously adsorbed molecules are released from the pore structure of activated carbon and return to the gas or liquid phase. It is an equilibrium process that goes hand in hand with adsorption, since adsorption is reversible. When the process conditions change (such as temperature, pressure, concentration or pH), this equilibrium can shift, allowing molecules to desorb and thus be released again.
Effect of Temperature
Temperature is one of the most decisive factors for desorption. Because adsorption is an exothermic process, a rising temperature inside the filter shifts the equilibrium towards desorption. This results in a declining adsorption capacity. The kinetic energy of the components to be captured also increases at higher temperatures, making them more mobile and allowing them to escape from the pores more easily.
Desorption in biogas purification
When upgrading biogas to biomethane, both H2S and VOCs must be captured. H2S is captured primarily through a (more) stable chemical bond (chemisorption). VOCs, however, are mainly adsorbed via reversible physisorption and will be more susceptible to desorption as the temperature rises. Released VOCs can result in emission peaks, lowering the purity of the biogas. Desorbed VOCs will then be adsorbed or condensed on the downstream membrane, resulting in membrane fouling.
Why is desorption harmful?
Desorption can have significant negative consequences for the operation of an activated carbon filter:
- Quality issues: in air treatment, this can lead to exceedances of emission limits or odour problems. In biogas purification specifically, VOC breakthrough can foul the membranes, hampering biomethane injection.
- Breakthrough: sudden release of adsorbed pollutants, which can lead to unwanted emission peaks.
- Reduced process reliability: unpredictable behaviour complicates sizing and monitoring. A misinterpretation can result in a premature filter change (because the carbon is wrongly believed to be saturated), leading to unnecessary costs.
Solution
All of Cargen’s AirPure L filters are thermally insulated for maximum stability and equipped with a cooling coil to actively regulate the temperature inside the filter. This guarantees not only a high initial adsorption capacity, but also reliable operation as ambient temperatures rise.
Conclusion
Temperature fluctuations in a filter bed can lead to faster breakthrough, reduced adsorption capacity and uncontrolled emissions. By actively controlling the process conditions with insulated mobile filters fitted with a cooling coil, desorption can be prevented. Choosing Cargen’s AirPure L filter gives the customer the assurance of an efficient and smoothly running adsorption process.
