Increasing the Efficiency and Sustainability of Vapour Phase Activated Carbon
The performance of activated carbon is of course variable depending on the contaminant. Some organic compounds adsorb to vapour phase activated carbon very efficiently indeed, sometimes in excess of 40% weight for weight, others conversely adsorb very poorly. For vapour phase activated carbon the efficiency of adsorption is also greatly influenced by humidity and temperature. The efficacy and indeed viability of the treatment approach therefore depends not just on the contaminant and it’s concentration but also on the condition of the vapour in which it is carried. It also depends on the mass expected.
Activated carbon in vessels situated outside in ambient weather conditions will perform very badly in cold weather as the vapour cools. Condensation will cause accumulation of water in the vessels and water vapour will preferentially adsorb to the carbon at the cost of the contaminant. A worst case scenario could be an air stripper or multi-phase extraction system delivering wet warm air to a cold carbon vessel.
Likewise, consider the activated carbon isotherm for a given contaminant at different temperatures. The higher the temperature, the less efficient the contaminant uptake. A soil vapour extraction system, and in particular a multi-phase extraction system will inherently heat up the extracted soil gas through the system. The harder the blower or pump is working the hotter the discharged soil gas will be. The temperature of soil gas discharged from an SVE system can quite typically be in the region of 40oC. A hard working MPE system can heat the extracted soil gases to 80 oC. Depending on the contaminant this can very significantly reduce the activated carbon performance.
Vapour Conditioning System Developed by Cornelsen
Cornelsen has developed a vapour conditioning system wherein extracted soil gases discharged from the SVE or MPE system (or air stripper) are cooled, moisture is condensed out and then the gases are reheated to ensure that the relative humidity is below 70%, thus preventing water vapour using up the activated carbon pore spaces.
A simple cost benefit analysis considering the additional process plant vs the consumption of activated carbon will quickly inform the project team of the relative value of vapour conditioning.
In 2013/2014 Cornelsen undertook two projects where the benefit of deploying vapour conditioning prior to the activated carbon could be measured in terms of several hundred thousand pounds. Both projects involved dichloromethane which adsorbs relatively poorly to vapour phase activated carbon. As one of those projects entail steam injection, two vapour conditioning systems were required. One system was installed at the inlet of the SVE system to cool the gases to a safe temperature within the operational limit of the blower (Note normally this will be 40oC), then a second system at the discharge given the inevitable heat increase which occurs by energy transfer through the blower.
Contact Cornelsen for a Cost Benefit Analysis
The cost and benefits are easy to assess and the solution quite simple to deliver. The choice between vapour conditioning vs increased activated carbon consumption can be as simple as straight maths. The added sustainability associated with the use of less activated carbon and all the transport and works associated with changing the carbon are similarly persuasive. Conversely, where the contaminant mass is low the poor performance may have little impact on the overall volume of activated carbon required and therefore the costs may exceed the benefit. The same may be true for organic contaminants which inherently adsorb more efficiently.
In summary, we recommend a simple cost benefit analysis based on isotherms obtained from activated carbon suppliers at different temperatures. Cornelsen will be pleased to assist in the cost benefit analysis and in the provision of the cooling/warming and heat exchange equipment. Use the Contact Form to send us a message today.