https://doi.org/10.1016/j.ijggc.2018.10.008
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2.2. Microwave generator setup
As an exhaustive description of the microwave regeneration setup can be found elsewhere (McGurk et al., 2017), only a succinct one will be given here. Microwave regeneration of the CO2-loaded aqueous MEA solutions were all performed with a setup, shown in Fig. 1, composed of a microwave magnetron operating at 2.45 GHz and controlled by an Alter SM445 power supply which maximum output power is 1.2 kW. A single mode waveguide directed the microwave energy towards respectively: (i) a dual-directional coupler (GAE Inc., GA310x, calibrated to take into account the presence of the quartz reactor) which measure the forward and reflected microwave power flow, (ii) a resonant cavity where the liquid sample is located, and (iii) a sliding short circuit (Sairem) to reflect the microwave wave at normal incidence. Non-absorbed power was directed and absorbed by a water dummy-load.
Fig. 1. Experimental setup used for CO2 absorption and regeneration of the solution by microwave. (1) Magnetron, (2) waveguide, (3) circulator with water dummy load, (4) dual-directional couplers, (5) sample cavity, (6) sliding short circuit, (7) quartz reactor, (8) gas cylinders, (9) mass flow controllers, (10) fiber optic temperature sensor and signal conditioner, (11) water condenser, (12) CO2 sensor, (13) mass flow meter, (14) gas exit.
In the sample cavity, 5 g of solution was contained in a cylindrical quartz reactor which had an outside diameter of 17 mm and a thickness of 1.5 mm. The temperature of the solution was measured at the center of the liquid bulk every second with a fiber optic sensor probe (Opsens OTG-MPK8) suited for use with amine solutions and microwave irradiation. Gases were fed via two calibrated mass flow controllers (Brooks Instruments GF-Series, 0–400 ml/min N2, 0–100 ml/min CO2) and bubbled at the bottom of the solution through a small quartz tube. The outlet gas stream of the reactor passed successively through a cold trap, a non-dispersive infrared CO2 sensor (COZIR-W-100, CO2 calibration range: 0–70% v/v) and finally a flow meter (Brooks Instruments SLA5860, 0–500 ml/min) before exiting through an exhaust line. All experiments were performed at atmospheric pressure and to avoid unexpected CO2 absorption by the solution, all gas lines were purged with N2 before and between experimental runs.
2.3. Absorption procedure
In order to obtain a CO2-loaded aqueous MEA solution and perform the regeneration, CO2 absorption was first accomplished by feeding the quartz vial with a binary gas mixture of 20% CO2 and 80% N2 which was bubbled through the 5 g MEA solution at a total flow rate of 100 ml/min and at ambient temperature. The outlet CO2 flow rate was calculated by multiplying the CO2 sensor and flow meter readings. A blank absorption run was performed with an empty reactor to allow the absorbed quantity of CO2 to be calculated by integrating the difference between the breakthrough blank and sample CO2 outlet flow rate curves. The CO2 loading was calculated as the ratio of the absorbed quantity of CO2 (mol) to the amount of amine (mol) in solution.
2.4. Regeneration procedure
Once a loaded MEA solution was obtained, the microwave regeneration step could be performed. A N2 flow was first set at 100 ml/min to purge the gas lines from CO2 and then acted as a sweeping gas. The microwave source was turned on at an initial power of 100 W (unless stated otherwise) to heat the solution from ambient to the desired regeneration temperature and then the power was manually reduced and controlled to maintain a constant temperature. On the setup, the dual-directional couplers measured the forward and reflected microwave power flows and the difference between them represented the amount of energy absorbed by the solution. Temperature selection in this work (70–90 °C) was based on the results of McGurk (McGurk et al., 2017) who showed that microwave regeneration was effective and quite fast at these temperatures. Higher temperatures were not selected to keep solvent evaporation rate as low as possible and to avoid significant solvent loss (which could be quite detrimental for a small sample as solution concentration can change over time). Furthermore, an optimized process operating at a low temperature will allow reducing heat losses as well as solvent degradation and corrosion problems during an industrial application. At the end of the regeneration time, the magnetron was turned off and the solution was allowed to cool down. The amount of stripped CO2 was determined by direct integration of the outlet CO2 flow rate which was calculated by multiplying the CO2 sensor and flow meter readings.
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