https://doi.org/10.1016/j.petlm.2016.11.002
“We employ a laboratory-scale bubble column reactor with 30 wt% aqueous MEA solution within a single mode microwave applicator at 2.45 GHz. The basis of the microwave regeneration experiment is outlined in Fig. 1 and consists of a WR340 rectangular waveguide (86.36 mm × 43.18 mm) connected to a 2450 MHz magnetron (GAE Inc., GA4001, maximum power 1.2 kW, peak voltage 4.5 kV) controlled by an Alter SM445 switching power supply. The waveguide incorporates a three-port circulator (GAE Inc., GA1105) with a cross dummy load, a dual-directional coupler (GAE Inc., GA310x, coupling factor 60 dB, directivity 23 dB) with GA3301 coupler power interfaces, a three-stub tuner (GAE Inc., GA1005), universal waveguide applicator (GAE Inc., GA600x), and a sliding short circuit (Sairem). The microwaves are generated by the magnetron with an adjustable input power setting and are then directed through the circulator and down the longitudinal axis of the waveguide.
Fig. 1. The configuration of the microwave regeneration experiment. (1) Magnetron, (2) circulator, (3) dual-directional coupler and interfaces, (4) three-stub tuner, (5) universal waveguide applicator, (6) sliding short circuit, (7) water dummy load, (8) quartz bubbler, (9) gas inlet to bubbler, (10) fibre-optic temperature sensor and signal conditioner, (11) CO2 mass flow controller, (12) N2 mass flow controller, (13) zeolite/silica drying column for CO2 gas line, (14) zeolite/silica drying column for N2 gas line, (15) CO2 gas cylinder and regulator, (16) N2 gas cylinder and regulator, (17) gas outlet from bubbler, (18) water condenser, (19) humidity meter, (20) CO2 sensor, (21) mass flow meter, (22) gas exit. Numbers 2 to 6 make up the waveguide. Numbers 1, 3, 10, 11, 12, 19, 20 and 21 are connected to an experimental computer through a National Instruments DAQ interface and LabView routine for data acquisition and control. A representation of the propagating electric field magnitude along the waveguide is also shown. This figure was adapted with permission from Gerling Applied Electronics, Inc.
The WR340 waveguide propagates the microwave electric field in the dominant TE10 mode (guide wavelength = 17.35 cm), as portrayed in Fig. 1, meaning that the electric field is uniform and transverse to the direction of propagation. After passing through the dual-directional coupler and three-stub tuner, the input wave travels through the waveguide applicator and interacts with the loaded sample. The wave then undergoes reflection at normal incidence from the short circuit and is directed back through the sample region along the reverse direction of the input wave, where finally the power is dumped into the dummy (water) load. A superposition of the forward and reflected waves creates a standing wave within the sample interaction region. The three-stub tuner and sliding short circuit are manually adjusted before each set of experiments to minimize impedance mismatch of the sample load and shift the peak of the electric field intensity to the sample. This ensures maximum power transfer efficiency during the microwave heating. The dual-directional coupler monitors the forward and reverse power flow across the waveguide.
A solution of 30 wt% MEA (Sigma Aldrich, >99% purity) in distilled water was contained in a cylindrical quartz bubbler reactor (17 mm × 135 mm) and placed in the waveguide applicator. Quartz was chosen due to its transparency in the microwave region (ε” < 0.001) [28] and the bubbler was sealed using PTFE collars. The temperature of the liquid was monitored using a fibre-optic temperature sensor (Opsens OTG-MPK8) and signal conditioner, suited for use with amine solutions and microwaves. Research grade gases (Linde Group, >99.99% purity) were fed into the quartz bubbler via two mass flow controllers (Brooks Instruments GF-Series, 0–400 mL/min N2, 0–100 mL/min CO2). The outlet gas line from the bubbler was cooled and connected to a cold trap to minimize evaporation losses. The outlet gas stream from the bubbler passed successively through a humidity meter and non-dispersive infrared (NDIR) CO2 sensor (COZIR-W-100) followed by a mass flow meter (Brooks Instruments SLA5860, 0–500 mL/min) before exiting through an exhaust line. All experiments were performed at a total pressure of 1 bar. The gas lines were purged with N2 before and between experimental runs.
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