Foaming of aqueous piperazine and monoethanolamine for CO2 capture

International Journal of Greenhouse Gas Control 5 (2011) 381–386

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Foaming of aqueous piperazine and monoethanolamine for CO2 capture Xi Chen, Stephanie A. Freeman, Gary T. Rochelle Department of Chemical Engineering, The University of Texas at Austin, 1 University Station C0400, Austin, TX 78712-0231, United States

a r t i c l e

i n f o

Article history: Received 28 January 2010 Received in revised form 26 August 2010 Accepted 18 September 2010 Available online 23 October 2010 Keywords: Foaminess Foam stability Piperazine Monoethanolamine Formaldehyde

a b s t r a c t The cause of foaming in aqueous amines used for CO2 absorption was investigated in this study. The effect on foaming of amine concentration and various additives, including electrolytes, liquid hydrocarbon, and degradation products, was measured by a standard method. Both aqueous piperazine (PZ) with 0.3 mole CO2 /mole alkalinity (˛) and 7 m monoethanolamine (MEA, ˛ = 0.4)) were studied. Formaldehyde at 270 mM substantially increases foaming in PZ. PZ foamed after 163 h of oxidative degradation, but this effect was greatly mitigated with an oxidation inhibitor. Silicone antifoam of 1 ppm reduced the foaminess by 20 times. The tendency of 8 m PZ to foam was increased by 40% with the addition of iron (II) up to a concentration of 1.5 mM, but dissolved iron had no significant effect on 7 m MEA. The tendency to foam and foam stability of 8 m PZ solutions was only slightly affected by 1 mM iron (III), 0.1% heptane in water, 5 mM of copper sulfate, or 100 mM of an oxidation inhibitor. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Foaming is a problem that is widely encountered in gas treating plants and normally leads to serious consequences such as loss of absorption capacity, reduced mass transfer area and efficiency, and carryover of amine solution to the downstream plant. Foaming can be induced by various chemical contaminants including condensed liquid hydrocarbon, fine particulates like iron sulfide, additives containing surface active chemicals, and amine degradation products (Abdi and Meisen, 2000; Al-Dhafeeri, 2007; Pauley, 1991; Pauley et al., 1989a, 1989b; Spooner et al., 2006; Stewart and Lanning, 1994; von Phul, 2001). Relatively few studies involving systematic and quantitative investigation of foaming in amine solutions have been published. Pauley studied the effect of hydrocarbon and organic acids on the foaming tendency of monoethanolamine (MEA), methyldiethanolamine (MDEA), diethanolamine (DEA), and formulated MDEA (Pauley et al., 1989a,1989b). All the contaminants investigated were found to increase the foaming tendency and foam stability of amine solutions to various extents. McCarthy and Trebble studied the foaming tendency of DEA solutions in the presence of various contaminants such as carboxylic acids (McCarthy and Trebble, 1996). They found that only those carboxylic acids with more than five carbons substantially enhanced the foaminess compared to a clean DEA solution. Thitakamol and Veawab systematically investigated the effects of process parameters on foaming behavior of MEA, MDEA, and 2-Amino-2-Methyl-Propanol (AMP) and their mixtures (Thitakamol and Veawab, 2008). Ranges of solution volume and gas flow rates were identified and used for measuring the foaminess coefficient. They found that most clean amine solutions did not foam, but the addition of degradation prod1750-5836/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2010.09.006

ucts or corrosion inhibitors increased the foaming tendency by up to 23%. The solution volume and gas flow rate used in our study is based on their recommendations. Concentrated (8 m) PZ has been identified as a promising solvent for CO2 capture from coal-fired flue gas (Freeman et al., 2010a,2010b). It has high absorption capacity and a fast rate of reaction with CO2 (Bishnoi and Rochelle, 2000; Dugas and Rochelle, 2009). Foaming was observed in earlier pilot plant experiments with K2 CO3 /PZ (Chen, 2007). Foaming has also been observed in recent bench-scale measurements of oxidative degradation in PZ systems (Freeman et al., 2010a). This study focused on finding the main causes for PZ foaming. The results obtained will be used for further study of the foaming effect on the CO2 capture process, and developing efficient means for foaming control. 2. Experimental methods 2.1. Experimental setup Foaming tests were performed using a standard test method for foaming of lubricating oils (ASTM D892) as modified by Thitakamol (Thitakamol and Veawab, 2008). As shown in Fig. 1, the experimental setup included a 1000 ml graduated cylinder, a water bath equipped with an immersion digital temperature controller, a gas diffusing stone (1 in. diam., porous fused crystalline alumina, average pore size = 60 ␮m, Fisher Scientific) and a gas flow rotameter. Nitrogen instead of air was used to bubble solutions in order to prevent oxidative degradation and minimize variation of CO2 loading of tested solutions during the course of experiments.

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X. Chen et al. / International Journal of Greenhouse Gas Control 5 (2011) 381–386

30

Foaminess for PZ Foaminess for MEA

25

60

Break Time for PZ Break Time for MEA

20

-3

2

70

Fig. 1. Schematic diagram for foaming experimental setup.

2.2. Materials PZ (99%, Alfa Aesar) and MEA (+99%, Acros) were used without further purification. Amine solutions were prepared by dissolving amines in deionized water followed by sparging the solutions with CO2 (99.99%) to achieve the desired loading. The typical solution compositions used in this study were 8 m PZ with ˛ = 0.3 (moles CO2 /mole alkalinity) and 7 m MEA with ˛ = 0.4. Ferrous (II) sulfate (99%, Reagent A.C.S, Spectrum), ferric (III) chloride (Certified A.C.S, Fisher Chemical), cupric (II) sulfate (Analytical Reagent, Mallinckrodt), formaldehyde (37 wt% water solution, Certified A.C.S, Fisher Chemical), and formic acid (88 wt% water solution, Certified A.C.S, Fisher Chemical) were used without further purification. The antifoam was Q2-3183A obtained from Dow Corning, with silicone as the main component. 2.3. Experimental procedures A 1000 ml graduated cylinder containing 400 ml test solution was placed in the water bath that had been heated to 40 ◦ C. The diffuser was immersed into the solution and the system was allowed approximately 20 min to reach thermal equilibrium. The initial solution volume was recorded. Then nitrogen was introduced to the graduated cylinder at a fixed flow rate of 2 × 10−3 m/s (with respect to the cross section area of the graduated cylinder). A stopwatch was used to track duration of bubbling time. Since the interface between liquid and foam was hard to identify for most test solutions, the total volume of contents in the cylinder (liquid and foam), instead of the volume of foam only, was recorded every minute. Each foaming test was run for 25 min. The total volume seemed to be relatively constant 5 or 6 min after experiments were started, therefore the data recorded during the last 15 min was averaged and reported as the steady-state value. Prior to testing each specific additive, neat amine solution (without any additive) was run as a base line. Since the results for neat solutions were not exactly the same each time, normalized foaminess was reported to compare different additives.

50 15 40 10

30

5

20 10

Break time (s)

Foaminess (10 m *s)

80

1

2

3

4

5

Amine (m)

6

7

8

9

0

Fig. 2. Effect of amine concentration on foaminess and break time at 40 ◦ C.

(Bikerman, 1973; Thitakamol and Veawab, 2008). The normalized foaminess (F*) was obtained by dividing F by the foaminess of the neat amine solution (F0 ): F∗ =

F F0

The break time of foam (t, second) was defined as the period of time for foam to break completely after gas flow was discontinued. Break time was used to estimate foam stability. 3. Results and discussion 3.1. Amine concentration Foaminess increased as PZ was varied from 2 m to 8 m at 40 ◦ C with ˛ = 0.3 (Fig. 2). This is believed to be mainly due to increased viscosity. As the viscosity of the bulk solution is increased, the drainage of liquid in foam films and the subsequent coalescence is retarded (Ivanov and Dimitrov, 1988), which allows foam to propagate to a greater extent. Increase in ionic species with amine concentration might also contribute to the stabilization of foam through electrostatic repulsive forces (Exerowa et al., 1997). Attributed to the same reasons, the foam break time increased from 5 to 29 s with 2 to 8 m PZ, reflecting enhanced foam stability. Freeman reported that viscosity of 7 m MEA at 0.4 loading is about 1/4 of that of 8 m PZ at 0.3 loading (Freeman et al., 2010a, 2010b). Consistently, the foaminess for 7 m MEA solution, about 20 × 10−3 m2 s is found to be much lower than that for 8 m PZ solution, around 80 × 10−3 m2 s (Fig. 2).

2.4. Data analysis 3.2. Oxidation products By subtracting the original liquid volume from the total volume in the cylinder, the total gas volume contained in the foam was obtained. The foaminess (F, m2 s) defined in this study was: F=

Vg Vt − V0 = G G

where Vg is the total steady volume (m3 ) of gas trapped in the liquid, V0 is the original liquid volume (m3 ), Vt is the total steady volume (m3 ) of content in the cylinder during foaming, and G is the superficial velocity of gas (m/s). Note that the foaminess defined in this study is different from the foaminess coefficient reported by other literatures, which is the ratio of total foam volume to gas flow rate

Freeman showed that formate is one of the primary oxidation products of PZ (Freeman et al., 2010a, 2010b). The mass balance between the loss of PZ and the increase of oxidation products was not achieved and formaldehyde is believed to be one of the important intermediate products of oxidation that has not been accounted for. Formic acid and formaldehyde were added to CO2 loaded PZ solutions to study their effect on foaminess. Foaminess increased only slightly from 81 × 10−3 to 85 × 10−3 m2 s with the addition of 0.5 M formic acid to 8 m PZ. As shown in Fig. 3, the foaminess increased significantly with formaldehyde. With 270 mM formaldehyde, the volume of the

X. Chen et al. / International Journal of Greenhouse Gas Control 5 (2011) 381–386

383

350

125

350

300

120

300

250

115

200

160

150

120 100

80

2

200

Break time (s)

-3

2

t

110

F

105

200

t 150

100 100

95

50

40

250

-3

F

240

50

90 0

0

50

100

150

200

[HCHO] (mM)

250

0 300

Fig. 3. Foaminess and break time as a function of formaldehyde concentration for 8 m PZ solution with ˛ = 0.3 at 40 ◦ C. The value of F reported for [HCHO] = 270 mM is an estimation and less than the actual value.

foaming solution exceeded the limit of the graduated cylinder (1000 ml). Therefore the foaminess could only be estimated to be greater than 319 × 10−3 m2 s, more than 3 times greater than the original neat solution (indicated by the arrows in Fig. 3). In addition, the foam layer that formed had a break time greater than 300 s. The viscosity of PZ solutions with 270 mM formaldehyde was the same as that of neat solutions. Therefore viscosity does not play a role in increasing foaminess. Sandler reported a condensation reaction between formaldehyde and PZ (Sandler and Delgado, 1969). The PZ solution was observed to turn slightly turbid as HCHO was added under stirring. The products, which may be oligimers or polymers, may be surface active and appear to enhance foam stability and increase foaminess. With 7 m MEA solution, the addition of 480 mM formaldehyde also caused a significant increase in foaming tendency. A PZ solution with 5 mM Cu2+ that was oxidized at 55 ◦ C for over 4 weeks was found to foam like 8 m PZ with 270 mM formaldehyde. Unfortunately, analytical methods have not been developed to determine the formaldehyde in the oxidized solution.

85

Break time (s)

Foaminess (10 m *s)

280

Foaminess (10 m *s)

320

0

0.2

0.4

0.6

2+

0.8

1

1.2

1.4

0 1.6

[Fe ](mM) Fig. 4. Foaminess and break time as a function of FeSO4 concentration for 8 m PZ solution with ˛ = 0.3 at 40 ◦ C.

remained on the top of the solutions, stable for at least 300 s after the gas flow was stopped. The effect of Fe2+ on MEA solution was also studied, as shown in Fig. 5. A maximum in foaminess was observed as the [Fe2+ ] was increased from 0 to 1 mM, but overall the foaming tendency of MEA solutions was not significantly changed by addition of Fe2+ based on the observation in this work. 3.4. Ferric ion Dissolved Fe2+ can be easily oxidized to ferric ion, Fe3+ . Ferric chloride was added to neat amine solution to a concentration from 0.01–1 mM. The change in foaminess of PZ solutions due to the addition of Fe3+ is shown in Fig. 6. Foaminess increased slightly with Fe3+ concentration first, but peaked at 0.2 mM, then dropped and leveled off at higher concentrations. The ferric ion has a better solubility in amine solutions and may not be able to form fine particles to stabilized foam as ferrous ion did. 25

12

3.3. Ferrous ion 11

F

24

-3

2

Foaminess (10 m *s)

t

10 9

23

8 22

7

Break time (s)

Steel materials are used for most gas treating facilities, making it necessary to study the effect of dissolved ferrous or ferric ions on foaming. A solution of 0.1 M FeSO4 with 0.05 M H2 SO4 was added to 8 m PZ solution under strong stirring at a rate of 1 drop/s. The amount of Fe2+ in the amine solution was varied from 0 to 1.5 mM to cover the possible range of Fe2+ content in a real gas treating system. As shown in Fig. 4, the foaminess of the solution was increased by about 40% as Fe2+ was increased to 0.5 mM. Then the foaminess decreased slightly with further addition of Fe2+ . It was found that the amine solution turned from light yellow to dark orange as [Fe2+ ] was gradually increased. Moreover, a layer of orange precipitation was visible on the bottom of the solution container after stirring was stopped. Du et al. (2003) and Gonzenbach et al. (2006) suggested that if particles have the correct range of surface energy, they could be favorably absorbed to the interface and form closely packed layer, thus preventing or reducing disproportionation and coalescence of bubbles. It was thus inferred that the fine particles composed of ferrous oxides or ferrous hydroxide in the amine solution might contribute to the increase of foaming tendency. When Fe2+ was greater than 0.5 mM, a foam layer of 3–4 mm in thickness

6 21 5 20

0

0.2

0.4

2+

0.6

0.8

1

4 1.2

[Fe ] (mM) Fig. 5. Foaminess and break time as a function of FeSO4 concentration for 7 m MEA solution with ˛ = 0.4 at 40 ◦ C.

X. Chen et al. / International Journal of Greenhouse Gas Control 5 (2011) 381–386

42

86

40

84

38

82

36

80

34

78

32

-3

2

88

Table 1 Effect of different chemical additives on normalized foaminess and break time for 8 m PZ solution with ˛ = 0.3 at 40 ◦ C.

Break time (s)

Foaminess (10 m *s)

384

F* = F/F0

t (s)

Cu2+ (5 mM) Cu2+ (5 mM) + Inhibitor “A” (100 mM) Cu2+ (5 mM) + Inhibitor “A” (100 mM) + Fe2+ (0.1 mM) V5+ (10 mM) + Fe2+ (0.1 mM)

0.98 0.85 0.90 0.77

30 33 35 28

Table 2 Effect of inhibitor A on foaming tendency of degraded amine solution (8 m PZ, ␣ = 0.3, oxidized at 55 ◦ C). Additvies to 8 m PZ

76

72

30

F t

74

0

0.2

0.4

0.6

0.8

1 mM Fe2+ 1 mM Fe2+ + 100 mM A

28

1

1.2

26

3+

[Fe ] (mM) Fig. 6. Normalized foaminess and break time as a function of FeCl3 concentration for 8 m PZ solution with ˛ = 0.3 at 40 ◦ C.

3.5. Liquid hydrocarbon Previous studies have suggested that hydrocarbon be an important cause of foaming observed in some plants (Abdi, 2001; Al-Dhafeeri, 2007; Pauley et al., 1989a, 1989b). Heptane was used in this work to study the effect of hydrocarbon on foaming. The solubility of heptane in pure water at 40 ◦ C is about 4.6 × 10−7 moles heptanes per mole water, which is calculated from the semiempirical equation suggested by Marche and co-workers (Marche et al., 2003). It is difficult to add a small quantity of heptane below the solubility limit, so the starting molar ratio of heptane to water was 8.7 × 10−6 and gradually increased to 9 × 10−3 . As shown in Fig. 7, a very small quantity of heptane did not change the foaming tendency of PZ solution. As nheptane /nH2 O was increased to 9 × 10−3 , both foaming tendency and foam stability decreased and heptane started to act as a defoamer. At this concentration it could

42 100

F t

40

39 80

Break time (s)

-3

2

90

38 70 37

60

10

-5

0.0001

0.001

0.01

F (10−3 m2 s) 70-h degradation

163-h degradation

85 92

300 68

be observed that heptane droplets were dispersed in the solution. Wasan et al. (1994) suggested that oil droplets may enter liquid thin film, spread on the gas–aqueous liquid interface and act as a foam breaker. 3.6. Corrosion inhibitor and oxidation inhibitor Copper (II) and vanadium (V) are common chemicals added as corrosion inhibitors to amine solutions. The proprietary oxidation inhibitor, “A”, may be used used to curb amine oxidation. The effect of these additives on the foaming tendency of PZ is shown in Table 1. The normalized foaminess, F*, was found to be less than 1 with additions of corrosion or oxidation inhibitor, which means these inhibitors themselves do not contribute to foaming. Although the addition of inhibitor A itself did not change the foaming tendency of the amine solution, it can retard the oxidation degradation process. If oxidation products are the main contributors to foaming, amine solution degraded in the presence of A should have a smaller foaming tendency than that of amine solution degraded without A. This is confirmed by foaming results of oxidatively degraded solutions, as shown in Table 2. The amine solutions were degraded by the method of Sexton (2008) at 55 ◦ C under violent agitation in an environment of 98% O2 and 2% CO2 for a period of 70 or 163 h prior to the foaming test. The addition of 100 mM Inhibitor A in PZ solutions was found to decrease foaminess from > 300 to 68 after 163 h of oxidation. 3.7. Antifoam

41

Foaminess (10 m *s)

Additives to 8 m PZ, ˛ = 0.3

36

n_Heptane / n_H O 2

Fig. 7. Foaminess and break time as a function of molar ratio of heptane to water for 8 m PZ solution with ˛ = 0.3 at 40 ◦ C.

The effectiveness of antifoam (Dow Corning, Q2-3183A) in eliminating foaming of PZ solutions was tested. The antifoam was added to the amine solution prior to the start of foaming test. As shown in Table 3, as low as 1 ppm antifoam was sufficient to reduce the foaminess by 15–20 times as well as greatly destabilize the foam. One of the common theories about antifoaming mechanism suggests that antifoam oil can spread across interface, replace the original stabilizing agents and lead to film rupture Table 3 Effect of antifoam on normalized foaminess with 8 m PZ at ˛ = 0.3 containing Fe2+ or formaldehyde at 40 ◦ C. Fe2+ (mM)

HCHO (mM)

Antifoam (ppm)

F* = F/F0

T (s)

0 1.5 1.5 0 0 0

0 0 0 270 270 270

0 0 1 0 1 2

1.00 1.32 0.09 3.33 0.18 0.12

34 >300 <2 N/A 20 <8

X. Chen et al. / International Journal of Greenhouse Gas Control 5 (2011) 381–386

385

Table 4 Summary of foaminess and break time measurements for PZ with ˛ = 0.3 and MEA with ˛ = 0.4 with different additives at 40 ◦ C. Amine (m)

Additives (mM)

Foaminess (10−3 m2 s)

Break time (s)

Normalized foaminess F*

PZ/2 PZ/4 PZ/6 PZ/8

None None None None

16.7 19.5 34.9 78.8

5 7 12 29

0.21 0.25 0.44 1.00

PZ/8 PZ/8 PZ/8 PZ/8 PZ/8 PZ/8

Fe3+ /0.01 Fe3+ /0.1 Fe3+ /0.2 Fe3+ /0.3 Fe3+ /0.5 Fe3+ /1

73.1 76.5 87.0 81.1 79.1 78.7

27 28 31 35 35 40

1.00 1.05 1.19 1.11 1.08 1.08

PZ/8 PZ/8 PZ/8 PZ/8 PZ/8 PZ/8 PZ/8

None Fe2+ /0.1 Fe2+ /0.2 Fe2+ /0.3 Fe2+ /0.5 Fe2+ /1.0 Fe2+ /1.5 Fe2+ /1.5 Antifoam/1 ppm

85.8 92.2 98.8 106.3 122.1 116.83 113.33

30 35 39 48 >300 >300 >300

1.00 1.07 1.15 1.24 1.42 1.36 1.32

7.5

<2

0.09

21.0 20.5 20.5 21.0 23.5 24.47 24.23 20.6

5 6 7 8 10 11 10 8

1.00 0.98 0.98 1.00 1.12 1.17 1.15 0.98

>303.5

>300

N/A

PZ/8 MEA/7 MEA/7 MEA/7 MEA/7 MEA/7 MEA/7 MEA/7 MEA/7 PZ/8 degraded oxidatively

None Fe2+ /0.001 Fe2+ /0.01 Fe2+ /0.1 Fe2+ /0.2 Fe2+ /0.3 Fe2+ /0.5 Fe2+ /1.0 Cu2+ Inhibitor A

PZ/8 PZ/8

None Cu2+ /5.0

88.3 86.3

31 30

1.00 0.98

PZ/8

Cu2+ /5.0 Inhibitor A/100

75.0

33

0.85

PZ/8

CuSO4 /5.0 Inhibitor A/100 FeSO4 /0.1

79.8

35

0.90

PZ/8

V5+ /10.0 Fe2+ /0.1

67.6

28

0.77

PZ/8 PZ/8

None Formic acid/500

80.5 85.1

28 30

1.00 1.06

PZ/8 PZ/8 PZ/8 PZ/8 PZ/8

None Formaldehyde/10 Formaldehyde/30 Formaldehyde/90 Formaldehyde/270 Formaldehyde/270 Antifoam/1 ppm

95.8 102.8 107.4 133.6 >319

34 34 35 50 >300

1.00 1.07 1.12 1.39 3.33

17.4

20

0.18

PZ/8 PZ/8

Formaldehyde/270 Antifoam/2 ppm

12.0

15

0.12

PZ/8 PZ/8 PZ/8 PZ/8 PZ/8

None Heptane/40 ppm Heptane/430 ppm Heptane/430 ppm Heptane/40,850 ppm

95.0 102.4 101.1 102.5 65.2

34 34 35 50 N/A

1.00 1.08 1.06 1.08 0.69

(Pugh, 1996). Strong correlation between antifoam efficiency and antifoam spreading has also been reported (Jha et al., 2000). All the results for foaminess and foam stability are tabulated in Table 4. 4. Conclusions Formaldehyde at 270 and 500 mM, respectively, was found to dramatically increase the foaminess of 8 m PZ and 7 m MEA. Inhibitor A (100 mM) reduced foaminess when 8 m PZ was exposed to 98% O2 and 2% CO2 for 163 h. Foaming was effectively inhibited

by the addition of 1 ppm silicone-based antifoam (Dow Corning Q23183A). A higher concentration of piperazine has a higher foaming tendency, probably resulting from increased viscosity. The presence of 1 mM Fe2+ in solution increased the foaming tendency of PZ solution by up to 40%, but it does not significantly affect foaming of MEA solution. Fe3+ up to 1 mM only slightly changes foaminess of PZ solution. Addition of corrosion inhibitor Cu2+ or V5+ and oxidation inhibitor A did not increase foaming. Formic acid at a concentration of 0.5 M had no effect on the foaming tendency of PZ solutions. Heptane has a negligible effect on PZ solutions as, but it can destabilize foam as nhep /nH2 O is increased above 9 × 10−3 . Although some

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additives tested in this study did not affect foaming tendency by themselves, the possibility that they could act as foaming promoters when other contaminants are present in the solutions is not excluded. Acknowledgement This research was supported by the Luminant Carbon Management Program at the University of Texas at Austin. References Abdi, M.A., Meisen, A., 2000. Amine degradation: problems, review of research achievements, recovery techniques. In: Proceedings of the 2nd International Oil, Gas and Petrochemical Conference, Tehran, Iran. Abdi, M.A., 2001. Improve contaminant control in amine systems. Hydrocarbon Processing 80 (10), 102. Al-Dhafeeri, M.A., 2007. Identifying sources key to detailed troubleshooting of amine foaming. Oil and Gas Journal 105 (32), 56. Bikerman, J.J., 1973. Applied Physics and Engineering, No. 10: Foams [Physicochemical Aspects]. Bishnoi, S., Rochelle, G.T., 2000. Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility. Chemical Engineering Science 55 (22), 5531–5543. Chen, E., 2007. Carbon dioxide absorption into piperazine promoted potassium carbonate using structured packing. PhD. Dissertation. The University of Texas at Austin, Austin, TX. Du, Z., et al., 2003. Outstanding stability of particle-stabilized bubbles. Langmuir 19 (8), 3106–3108. Dugas, R., Rochelle, G., 2009. Absorption and desorption rates of carbon dioxide with monoethanolamine and piperazine. Energy Procedia 1 (1), 1163–1169. Exerowa, D., et al., 1997. Foam and Foam Films: Theory, Experiment, Application. Freeman, S.A., et al., 2010a. Degradation of aqueous piperazine in carbon dioxide capture. International Journal of Greenhouse Gas Control 4 (5), 756–761.

Freeman, S.A., et al., 2010b. Carbon dioxide capture with concentrated, aqueous piperazine. International Journal of Greenhouse Gas Control 4 (2), 119–124. Gonzenbach, U.T., et al., 2006. Ultrastable particle-stabilized foams. Angewandte Chemie International Edition 45 (21), 3526–3530. Ivanov, I., Dimitrov, D., 1988. Thin film drainage. Surfactant Science Series 29, 379–396 (Thin Liq. Films). Jha, B.K., et al., 2000. Silicone antifoam performance: correlation with spreading and surfactant monolayer packing. Langmuir 16 (26), 9947–9954. Marche, C., et al., 2003. Solubilities of n-Alkanes (C6 to C8 ) in Water from 30 ◦ C to 180 ◦ C. Journal of Chemical and Engineering Data 48 (4), 967–971. McCarthy, J., Trebble, M.A., 1996. An experimental investigation into the foaming tendency of diethanolamine gas sweetening solutions. Chemical Engineering Communications 144, 159–171 (Print). Pauley, C.R., 1991. Face the facts about amine foaming. Chemical Engineering Progress 87 (7), 33–38. Pauley, C.R., et al., 1989a. Analysis of foaming mechanisms in amine plants. In: Proceedings – Laurance Reid Gas Conditioning Conference, pp. 219–247. Pauley, C.R., et al., 1989b. Ways to control amine unit foaming offered. Oil and Gas Journal 87 (50), 67–75. Pugh, R.J., 1996. Foaming, foam films, antifoaming and defoaming. Advances in Colloid and Interface Science 64, 67–142. Sandler, S.R., Delgado, M.L., 1969. Reinvestigation of the reaction of piperazine with aldehydes. Journal of Polymer Science: Part A-1 7, 1373–1378. Sexton, A., 2008. Amine oxidation in CO2 capture processes. Ph.D. Dissertation. The University of Texas at Austin, Austin, TX. Spooner, B., et al., 2006. Iron sulphides-friend or foe? In: Proceedings of the Laurance Reid Gas Conditioning Conference, p. 109. Stewart, E.J., Lanning, R.A., 1994. Reduce amine plant solvent losses. Hydrocarbon Processing 73 (5), 67–81. Thitakamol, B., Veawab, A., 2008. Foaming behavior in CO2 absorption process using aqueous solutions of single and blended alkanolamines. Industrial and Engineering Chemistry Research 47 (1), 216–225. von Phul, S.A., 2001. Sweetening process foaming and abatement. In: Proceedings of the Laurance Reid Gas Conditioning Conference, pp. 251–280. Wasan, D.T., et al., 1994. Mechanisms of aqueous foam stability and antifoaming action with and without oil: a thin-film approach. Advances in Chemistry Series 242, 47–114 (Foams: Fundamentals and Applications in the Petroleum Industry).