Reduction of nitrosamines in CO₂ capture by UV-light

The project is related to possible environmental problems with emissions to air from CO₂ capture plants. In amine- and amino acid-based CO₂ capture plants, nitrosamines and nitramines may be formed as a result of degradation processes in the presence of oxygen, NO and NO2. It is important to be able to keep nitrosamine levels in the process at a low level, and especially to avoid the release of these substances into the atmosphere. One method that has been proposed for reducing the nitrosamine content in aqueous solutions is irradiation with UV light. This type of nitrosamine removal is known from the treatment of drinking water.

The project aims to investigate whether UV radiation can be an effective method for controlling the nitrosamine content in absorption processes for CO₂ capture based on amines. Treatment of both the process solution and the water washing part of the process has been investigated and the effect of UV light measured.

In 2011, experiments were run both on a small scale in a built-up circulation loop and in a pilot plant under more real conditions with CO₂ absorption and regeneration of absorbent. The two nitrosamines examined are NDMA and NDELA. Experiments were run with solutions that varied from water-clear solutions with small amounts of amines, washing water solutions from pilot testing and diluted and undiluted solutions of used MEA and DEA solvents. In parallel with the degradation tests, surveys of penetration depth for UV light were run in the various solutions to increase the understanding of the processes that take place. It was found that the degradation process depends on the availability of both UV light and nitrosamine so that in clean water, with great penetration depth and where UV light is not absorbed by anything other than the nitrosamines, the kinetics become first order in terms of nitrosamine concentration. In colored solutions, typically for degraded amine solutions, other components will absorb UV light and convert it to heat before hitting nitrosamines.

These are solutions with low penetration depth. In this case, the rate of degradation is to a small extent dependent on the nitrosamine concentration and a zero-order kinetics is seen. Based on this, and confirmed by pilot experiments in both process solution and wash water, it was concluded that degradation with UV light will probably not be an appropriate method for removing nitrosamines from the process solution, but that it may work well in the water wash part of an amine-based process. It was further found that NDELA was degraded about twice as fast as NDMA. Experiments were also run to investigate whether exposure to UV light increased the rate of degradation products. It was found that this did not happen under conditions where oxygen (air) was not blown in.

Publications:

Knuutila H. ,Asif N. and Svendsen H.F., Destruction of nitrosoamines with UV-light, Energy Procedia, Volume 37, 2013, Pages 743–750.

Knuutila, H., Asif, N., Vevelstad, S.J., Svendsen, H.F. Formation and destruction of NDELA in 30wt% MEA (monoethanolamine) and 50wt% DEA (diethanolamine) solutions. Oil & Gas Science and Technology-Revue. Published online 24 October 2013.