Materials and methods
4.1. Oxidative degradation setup 1
The setup used for most of the oxidative degradation experiments performed in the scope of his work, was an open-batch setup and based on the work of Vevelstad (2013) and Fytianos (2016). First, one setup of three parallel reactors was built and after that was successfully in use another, identical setup was made. This allowed for six parallel reactions to run at the same time.
Figure 4.1: Schematic of oxidative degradation setup 1, of which there were two.
The experimental conditions were meant to simulate absorber conditions, so the temperature was kept constant at 60 °C throughout every experiment. Temperature control was ensured by making the reactors of double-jacketed glass, meaning that there is a large contact area between the heating liquid, which was water provided by a circulating heating bath. The temperature of the Graham condensers was kept constant by using a circulating cooling bath with water kept at 5 °C. Each reactor has a total volume of ~275 mL and a liquid volume of ~200 mL was used during the experiments. The gas mixture was provided though five Alicat mass flow controllers (MFC), two regulating the pressure of CO2 and O2, and three MFCs connected after the two gases had been mixed, providing an equal gas flow to each reactor. A bleed valve coupled in parallel with the three MFCs going into the reactors, ensured release of excess gas mixture. The gas is passed through an empty gas wash bottle and
Bleed MFC MFC
MFC
Condenser Condenser
Condenser
Reactor 1 Reactor 2 Reactor 3
Vent Vent
Vent
O2 CO2
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through a Pyrex® gas dispersion tube (GDT) of porosity grade 1 into the liquid, to increase mass transfer from the gas to the liquid phase. The GDTs were either new, or thoroughly cleaned with sulphuric acid (H2SO4), and deionized water between experiments. Magnetic stirring was maintained constant throughout every experiment at a rate of ~200 rpm. Each reactor has three openings, one at the top, for the condenser, one on the side, where a thermometer adapter ensured a tight fit around the GDT going into the reactor and a third opening covered with a screw cap with septum, for sampling.
Figure 4.2: Photo of oxidative degradation setup 1. Photo: Per Henning, NTNU.
Each experiment started with adding ~200 mL of an accurately weighed amine solution, which was typically pre-loaded with CO2 and contained 0.5 mM of iron sulphate heptahydrate (FeSO4∙7H2O), into pre-heated reactors with the cooling system on. The GDT was already secured and connected and as soon as the sample was added to the reactor, the septum-containing cap was put in place. Magnetic stirring and gas flow were commenced as soon as all reactors were filled. Sampling was performed regularly through the septum, using a syringe with a needle. Each sampling removed 2-3 mL of the liquid and every sample was accurately weighed, to ensure knowledge of the mass balance throughout the experiment. The typical duration of an experiment was 21 days, with sampling two times per week.
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The following subchapters contain results from two different method validation experiments, that are not included in the published papers.
4.1.1 Catalysis of oxidative MEA degradation
One experiment testing the catalytic effect of copper was performed in three parallel reactors in Setup 1. The experimental procedure given in section 4.1 was followed, with the exception of 0.5 mM CuSO4 addition instead of FeSO4∙7H2O. the experiment was performed in 30 wt% MEA (aq.) with a loading kept at approximately 0.4 molCO2 molMEA-1. The gas mixture sparged into each reactor consisted of 98% O2
and 2% CO2, and the gas flow rate was kept at 60 mL min-1.
Figure 4.3: Relative alkalinity remaining in the 30 wt% MEA (aq.) solution throughout three weeks under oxidising conditions when added Fe2+ or Cu2+.
As can be seen in Figure 4.3, the rate of copper-catalysed oxidative degradation of 30 wt% MEA (aq.) is comparable to that of iron-catalysed degradation. The data for MEA 30 wt% with Fe2+ is from the publication printed in chapter 7, the data for the copper-catalysed experiments is found in Table 4.1. The alkalinity show in the figure was measured by titration, then corrected to CO2-free concentration, and under the assumption that water loss is linear, corrected for loss of mass (water) throughout the experiment.
Table 4.1: Measured concentrations of alkalinity (section 4.4.1) and CO2 (section 4.6.1) for oxidative degradation of 30 wt% MEA (aq.) with 0.5 mM CuSO4 in three parallel reactors (a, b, and c).
Day Alkalinity (w/ CO2)
CO2
concentration Loading Alkalinity (w/o CO2)
Alkalinity (corrected) [mol kg−1] [g kg−1] [molCO2 molMEA−1] [mol kg−1] [mol kg−1]
0 4.511 85.81 0.43 4.898 4.898
3a 4.340 83.31 0.44 4.701 4.659
3b 4.349 - - 4.715 4.674
3c 4.345 - - 4.710 4.653
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Day Alkalinity (w/ CO2)
CO2
concentration Loading Alkalinity (w/o CO2)
Alkalinity (corrected) [mol kg−1] [g kg−1] [molCO2 molMEA−1] [mol kg−1] [mol kg−1]
7a 4.127 81.17 0.45 4.462 4.369
7b 4.164 82.47 0.45 4.507 4.416
7c 4.152 82.21 0.45 4.493 4.367
10a 3.765 71.42 0.43 4.034 3.913
10b 4.043 - - 4.355 4.228
10c 4.016 - - 4.320 4.147
14a 3.337 60.86 0.41 3.540 3.391
14b 3.831 71.86 0.43 4.107 3.940
14c 3.753 69.06 0.42 4.012 3.787
17a 3.042 53.52 0.40 3.205 3.041
17b 3.663 - - 3.908 3.715
17c 3.609 - - 3.842 3.581
21a 2.619 45.09 0.39 2.737 2.565
21b 3.443 61.90 0.41 3.656 3.434
21c 3.376 59.98 0.40 3.578 3.278
4.1.2 Oxidative degradation with 1% vs 98% O2
To test whether degradation takes place also with very low pO2, an experiment was run replacing pure O2, with a mixture of 99% N2 and 1% O2 in 30 wt% MEA (aq.).
The instructions in section 4.1 were otherwise followed, resulting in a 60 mL min-1 flow rate of approximately 1% O2, 2% CO2 and 97% N2 into each reactor. The solution contained 0.5 mM FeSO4∙7H2O to catalyse the reaction.
Figure 4.4: Oxidative stability of 30 wt% MEA (aq.) with high and low pO2.
In Figure 4.4 the loss of alkalinity for MEA sparged with 1% O2 is compared to that of 98% O2 (the same data as in Figure 4.3). In the figure, alkalinity of the CO2-free
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solution is given, corrected for water loss throughout the experiment. It can be seen that the loss of amine is negligible in this case, that more oxygen is required to induce a significant loss of alkalinity in the three weeks the experiments take. The measured concentrations of amine and CO2 are given in Table 4.2.
Table 4.2: Measured concentrations of alkalinity (section 4.4.1) and CO2 (section 4.6.1) for oxidative degradation of 30 wt% MEA (aq.) with 1% O2 in three parallel reactors (a, b, and c).
Day Alkalinity (w/ CO2)
CO2
concentration Loading Alkalinity (w/o CO2)
Alkalinity (corrected) [mol kg−1] [g kg−1] [molCO2 molMEA−1] [mol kg−1] [mol kg−1]
0 4.514 81.33 0.41 4.881 4.881
3a 4.548 83.37 0.42 4.928 4.883
3b 4.541 - - 4.929 4.887
3c 4.511 - - 4.919 4.882
7a 4.558 84.10 0.42 4.942 4.838
7b 4.597 89.34 0.44 5.008 4.910
7c 4.577 99.78 0.50 5.034 4.946
10a 4.620 84.82 0.42 5.012 4.862
10b 4.595 84.29 0.42 4.983 4.843
10c 4.615 84.32 0.42 5.004 4.879
14a 4.667 89.75 0.44 5.086 4.872
14b 4.646 89.77 0.44 5.063 4.864
14c 4.641 90.22 0.44 5.060 4.883
17a 4.672 90.97 0.44 5.097 4.837
17b 4.695 90.14 0.44 5.118 4.875
17c 4.650 91.30 0.45 5.075 4.859
21a 4.690 92.69 0.45 5.125 4.802
21b 4.669 92.93 0.45 5.103 4.803
21c 4.700 91.74 0.44 5.131 4.861
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