Rapid temperature swing adsorption using microwave regeneration for carbon capture

A microwave swing adsorption (MSA) process has been used to regenerate activated carbon (AC) extrudates after adsorption of a CO2 /N2 (15/85 v/v) mixture. Heating of the sorbent bed inside the microwave oven cavity was characterised at different locations. During the desorption, the effect of hotspots is minimised by a sweeping gas flow. The regeneration using microwave heating was extremely fast (56 to 79 s), the quickest reported up to date using microwave heating, due to very high heating rates (100 to 400 °C/min). The desorption procedure was optimised by testing different powers. Cyclic operation on the adsorbent showed that it reaches a stable capacity after the third cycle, keeping 90% of the original capacity.

Introduction
Capture and storage of CO2 (CCS) is necessary to reduce CO2 emissions to minimise their effect on climate change [1]. Current technologies for CCS include amine scrubbing, which suffers from several drawbacks, such as high energy intensity and corrosion of equipment. Adsorption is a promising alternative which could improve the energetics of CCS [2]. Microwave swing adsorption (MSA) is a technology at an early stage of development but which has great potential, as the heating in the regeneration step does not depend on conduction and heat losses are minimised by selectively heating the targeted components of the system – in this case, the adsorbent or the adsorbate [3]. However, several unique features of microwave heating have to be further studied to make MSA an industrial reality, such as inhomogeneous heating, i.e. the creation of hot spots.
The present study focuses on the influence of different microwave heating parameters (microwave power and irradiation time) as well as microwave operation modes (continuous vs stepwise) on CO2 desorption. A binary gas mixture of CO2 and N2 (15:85 v/v), resembling flue gas composition, was used as the feed gas, using AC extrudates to capture CO2.

Experimental
A multimode microwave oven with a cuboid cavity was used for microwave heating and desorption experiments. Cyclic sorption experiments were carried out on a packed bed of the exttrudates at 25 °C using a mixture of CO2 and N2 (15:85 v/v), at a total flow rate of 100 Nml/min. Once saturation is reached inside the column, desorption was carried out by microwave heating at a specific power during a given time under a flow of 100 Nml/min of N2.
Results
The AC material presents selectivity towards CO2 in competitive adsorption conditions. The heating in the cavity is inhomogeneous and hot and cold spots are present, the influence of which can be minimised during the desorption step by flowing a sweeping gas. Among the heating procedures tested, the one that led to a more efficient desorption was 250 W for 24 s, which achieved complete desorption in just 56 s. This AC is suitable for rapid MSA cycling, as was shown by performing 11 consecutive experiments, with only a decrease in the adsorption capacity of less than 5% in the first 3 cycles, remaining constant after that (Fig. 1). The material’s textural properties before and after the cycles were assessed, showing that microwave radiation does not damage it in any way.

Conclusions
The present study shows that the MSA process using AC is apt for CO2 capture and presents an unprecedently fast regeneration. Higher powers favour a more time-efficient regeneration. In cyclic experiments, it is evident that microwave irradiation can fully regenerate the adsorption bed. It is also demonstrated that adsorbent regeneration using microwave heating has some particularities (like hot spots, cold spots and selective heating) that, if managed correctly, could drive adsorption-desorption processes much more rapidly, making MSA an interesting option for using renewable energy in electrified CO2 capture.

References
[1] C. Song, Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing, Catalysis Today, 115 (2006) 2–32.
[2] K.T. Chue, J.N. Kim, Y.J. Yoo, S.H. Cho, R.T. Yang, Comparison of Activated Carbon and Zeolite 13X for CO2 Recovery from Flue Gas by Pressure Swing Adsorption, Industrial and Engineering Chemistry Research, 34 (2002) 591–598
[3] R. Cherbański, E. Molga, Intensification of desorption processes by use of microwaves–An overview of possible applications and industrial perspectives, Chemical Engineering and Processing: Process Intensification, 48 (2009) 48–58.