Effects of operating parameters

8.2.2.1 Effect of temperature and O2pressure

The concentration profiles for AMP and MEA decreased linearly with degradation time when plotted in a semi-logarithmic scale. This suggests an overall first order reaction with respect to the amines. Equation 4.2 provides a relationship for the concentration of AMP (or MEA) with time and allows the determination of an apparent first order rate constant. Figure 8.1 shows an example of ln(c/c0) against reaction time in a mixture of 3mol/kg AMP and 2 mol/kg MEA that was degraded at 120°C and 250 kPa O2. The slopes of the plots represent the apparent first order reaction rate constants for AMP and MEA, respectively.

0 400000 800000 1200000

-0,8 -0,4 0,0 0,4 0,8 1,2

ln(c/c0)

Reaction time (Sec) MEA

AMP

Figure 8.1 Plot of ln (c/c0) against reaction time ( 3mol/kg AMP + 2 mol/kg MEA degraded at 120°C and 250 kPa O2).

All the calculated first order reaction constants for AMP and MEA under other reaction conditions are presented in Table 8.2. Although MEA and AMP oxidation could be oxygen mass transfer limited at the experimental conditions, it does not affect comparison of the

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relative degradation rates of AMP and MEA in the blends. The results show that the rate of MEA degradation was higher than that of AMP in all the different blend compositions at all experimental conditions. As can be seen in Table 8.2, both of the overall rates of AMP degradation and MEA degradation increased with rising temperature as expected. It should be noted that the duration of each experiment was only 1 week, the degradation of AMP was low, and thus the measured reaction rate constants could have a large error.

Generation of carboxylate ions was measured in all samples. The comparison of formate and oxalate concentration for the case of 3mol/kg AMP-2mol/kg MEA at 120°C and 140°C is shown in Figure 8.2. Generation of both formate and oxalate was enhanced at higher temperature, as expected with increased oxidation rates of AMP and MEA. This result indicates that the earlier analyzed enhanced degradation rates of AMP and MEA at higher temperatures are potentially true which is also reflected in increased concentration of degradation products.

0 20 40 60 80 100 120 140 160 180

0 5 10 15 20 25 30 35

Concentration (mmol/kg)

Reaction time (hr) Formate

Oxalate

Figure 8.2Effect of temperature on generation of formate (A < # B %(/-"

points) or 140°C (solid points). The experiments were conducted with 3 mol/kg AMP + 2 mol/kg MEA under 250 kPa O2.

Based on previous assumption that AMP and MEA may be oxidized in parallel in the AMP/MEA blends, and the fact that acetone, 2, 4-lutidine and DMOZD are universally found during AMP oxidative degradation, we therefore take these three main degradation products

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of AMP as its degradation indicators. Figure 8.3 shows the amount of formation of acetone, 2,4-lutidine and DMOZD after 168 hours reaction time using a mixture of 3 mol/kg AMP and 2 mol/kg MEA under 250 kPa O2. As can be seen in Figure 8.3, the amount of acetone, 2, 4-lutidine and DMOZD increased with rising temperature. As proposed in Chapter 6, it is believed that acetone is an intermediate of AMP oxidation which decomposes to acetic acid and formaldehyde in the presence of OH radicals. Therefore, the end result of the formation rate of acetone increased less than those of 2, 4-lutidine and DMOZD.

90 100 110 120 130 140 150

0,00 0,04 0,08 0,12 0,16 0,20

Concentration (mol/kg)

Temperature (⁰C) Acetone

2,4-Lutidine DMOZD)

Figure 8.3 Comparison of the amount of formation of acetone, 2, 4-lutidine and DMOZD with 3mol/kg AMP + 2 mol/kg MEA from 100 to 140°C under 250 kPa O2after 168 hours.

As discussed in Chapter 5, the oxidation of concentrated AMP could be O2 mass transfer limited under accelerated conditions. Since AMP degradation by oxygen in the reaction vessel is a two-phase system. O2should be dissolved into the aqueous solution first.

The dissolved O2reacts with amine followed by degradation. For the same reason, the overall degradation rates of AMP and MEA were also affected by O2partial pressure as they were oxidized individually. The degradation rates were expected to increase with increasing O2

partial pressure due to the increased dissolved O2in the aqueous solutions.

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Most of the oxidation experiments in this project were performed with an oxygen partial pressure of 250 kPa. In order to investigate the effect of O2 partial pressure on the degradation of AMP in the AMP/MEA blends, two of the eight experiments were conducted at different O2pressures. For a 2 mol/kg AMP and 3 mol/kg MEA blend at 120°C, the first order reaction constant for AMP increased by 49% when O2pressure was increased from 250 kPa to 350 kPa, and the first order reaction constant for MEA increased by 61%. This result demonstrates that O2partial pressure has a significant effect on both oxidation rates of AMP and MEA in the blends. The type of identified products is exactly same at the two different O2partially pressures. However, oxidation of concentrated AMP and MEA could be O2mass transfer limited under accelerated conditions, and thus higher O2pressure could cause higher degradation rates of AMP and MEA. The amount of formation of the major three products of AMP at two different O2pressures is shown in Figure 8.4.

200 250 300 350

0,00 0,04 0,08 0,12 0,16

Concentration (mol/kg)

Oxygen pressure (kPa) Acetone

2,4-Lutidine DMOZD

Figure 8.4 Comparison of the amount of formation of acetone, 2, 4-lutidine and DMOZD with 2mol/kg AMP + 3 mol/kg MEA at 120°C under 250 kPa and 350 kPa O2after 168 hours.

8.2.2.2 Effect of blend composition

Another factor explored in this work is the effect of blend composition on the oxidation of AMP in the blends. As can be seen in Table 8.2, the degradation rate of AMP decreased when the initial concentration of AMP was fixed at 4 mol/kg while the initial concentration of MEA was increased. Figure 8.5 shows the amount of formation of acetone, 2, 4-lutidine

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and DMOZD after 168 hours degradation of the AMP/MEA blends. The data clearly show that the amount of acetone, 2, 4-lutidine and DMOZD decreased with raised MEA concentration. The degradation rate of AMP and the formation rates of the indicators decreased indicating MEA protects AMP from oxidation in the mixture when initial MEA concentration is increased. As mentioned above, AMP and MEA could be degraded separately in the blends, but MEA degrades faster than AMP. On the other hand, the oxidation of concentrated MEA and AMP could be O2 mass transfer limited under the experimental conditions. In this case, MEA competed with AMP for O2 and thus be the reason for a decreased degradation rate of AMP.

4/1 4/2 4/4

Figure 8.5 Comparison of the amount of formation of acetone, 2, 4-lutidine and DMOZD with AMP/MEA blends at 120°C and 250 kPa O2after 168 hours.

8.3 Conclusions

An aqueous AMP/MEA blend (3mol/kg AMP +2mol/kg MEA) with a CO2 loading of 0.3 mol CO2/mol total amine was degraded at 135°C. The rate constants for the reaction of CO2with AMP and MEA were 1.9 × 10^-8s^-1and 1.4 × 10^-7s^-1, respectively. The rate of the reaction of MEA with CO2is approximate one order of magnitude higher than that of AMP.

Since both MEA and AMP can attack oxazolidinones derived from MEA and AMP, cross products, AMPAE, EDAMP and HTBI were formed during thermal degradation in the

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presence of CO2. However formation of cross products would not be confirmed yet by use of authentic standards.

For AMP/MEA blends, the major oxidative degradation products include ammonia, acetone, 2, 4-lutidine, 4, 4-dimethyl-2-oxazolidinone, hydroxyethyl)formamide, N-(2-hydroxyethyl)acetamide, N-(2-hydroxethyl)imidazole, and formate. As compared with oxidative degradation of single MEA and AMP, no cross product was found in the degraded mixtures. This result indicates that AMP and MEA are degraded in parallel in the blends.

The pseudo first order reaction rate constants for AMP and MEA in AMP/MEA blends were measured. Both the overall degradation rates of MEA and AMP increased with raising temperature and oxygen partial pressure, respectively. Acetone, 2, 4-lutidine and DMOZD were three typical degradation products of AMP. It was found that acetone formation rate increased less than those of 2, 4-lutidine and DMOZD. These results indicate that acetone was an intermediate as proposed in earlier the AMP degradation pathway. MEA degraded faster than AMP in AMP/MEA blends under different experimental conditions. When increased initial MEA concentration, the amount of AMP loss decreased indicating that MEA protects AMP from oxidation. The inhibition effect of MEA on AMP degradation could be due to the fact that MEA degraded faster than AMP in the blends, however, this “protecting”

effect of MEA is not expected to operate in a situation with high O2 partial pressure.

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