Optimisation of ultrasonic intensification for CO₂ stripping

One of the greatest challenges for large-scale implementation of CO₂ capture is the associated cost. The investment cost is related to the equipment size, while the operational cost is mainly due to the steam needed for regeneration of the solvent.

The main objective of this project is to improve stripping of CO₂ from the absorbent before recycling by the assistance of ultrasound, confirm and optimise the technology by an experimental set-up emulating an actual reboiler with solution of 30 wt% monoethanolamine (MEA), and thereby provide key data for its engineering.

The work has been conducted over 2 years from April 2014 to April 2016. Based on a two stage experimental set-up on a kettle reboiler scale model with ultrasound, effects of ultrasound on CO₂ stripping enhancement was quantified.

We achieved over 300 % improvement compared to the bubbling by heat of steam pipe in the reboiler, indicating that the mass transfer can be intensified significantly. The specific energy saving was over 50 % when focussing on CO₂ stripping in the reboiler. The normalized specific energy saving reached 13 % when the extra vapour by US that enters a desorption column for further CO₂ stripping was taken into account. This normalized specific energy was found to be 3.6 MJ/kgCO₂, as compared to a typical MEA-based industrial case at 4.2 MJ/kgCO₂.

Estimates for cost, energy and secondary effects on the absorption-desorption plant indicate cost savings in the order of 5.5% based on CO₂ capture in a NGCC plant. Further optimisation is possible. No significant HSE issues have been identified and a risk analysis did not reveal any showstoppers. The technology is solvent neutral in the sense that it may be used with any liquid-based absorbent and absorption plant technology. Ultrasound can be used also with other classes of absorbents than amines.

Based on the experiments and observations, two possible methods of ultrasound application by piggy-backing to an existing process in CO₂ stripping are suggested for industry.