Thermal Degradation of Aqueous Methyldiethanolamine (MDEA) with Continuous Injection of H2S/CO2 in High Pressure Reactor

Thermal Degradation of Aqueous Methyldiethanolamine (MDEA) with Continuous Injection of H₂S/CO₂ in High Pressure Reactor

Priyabrata Pal and Fawzi Banat*

The Petroleum Institute, Abu Dhabi, United Arab Emirates

Email: fbanat@pi.ac.ae

Introduction

The removal of H₂S/CO₂ (known as acid gases) from a great variety of hydrocarbon rich natural gas is the main goal to increase the industrial and commercial utility of the hydrocarbon streams [1]. Generally, alkanolamine solvents are used to capture acid gases and there has been an increased attention for monitoring the volatile degraded products from the solvent and correlating with foaming issues to understand environmental emissions [2]. GASCO Company (Habshan, Abu Dhabi) is currently using aqueous methyldiethanolamine (MDEA, 40-50 wt%) to remove acid gases. Different researchers conducted thermal degradation studies to identify thermal degraded products from MDEA. One of the major operational problems facing the alkanolamine based absorption process is foaming. Foaming can be enhanced by introducing various contaminants like condensed liquid hydrocarbon, fine particles, surface active agents and alkanolamine degradation products [3]. Considering all these aspects, it is highly desirable to carry out thermal degradation of MDEA in presence of H₂S/CO₂ continuously and carry out foaming and corrosion studies with degraded MDEA to understand an accurate and complete knowledge of industrial process for the purification of natural gas that are technically, environmentally and economically more efficient.

Experimental

Experimental unit

Degradation studies were performed at 120°C and CO₂/H₂S partial pressure of 0.0675/2.025 bars having total pressure of 9.0 bars in presence of nitrogen for 865 hours. The gases are cooled down in the overhead condensers and the condensate is returned to the reactor. At the end of the degradation test, the samples was distilled in vacuum distillation unit and analyzed for high molecular weight degradation products.

Instrumentation

Ion chromatography (IC), proton-transfer-reaction quadrupole mass spectrometry (PTR-QMS), gas chromatography mass spectrometry (GC-MS), inductively coupled plasma optical emission spectrometry (ICP-OES) and titration equipment were used for analysis of the samples.

 

Results and Discussions

Analysis of initial MDEA solvent

The analyses of fresh and lean MDEA are presented in Table 1. Low levels of organic acids and other inorganic heat stable salts (HSS) in mg/kg were detected in fresh and lean MDEA.

Table 1 Analysis of fresh and lean MDEA

Parameter	Fresh MDEA	Lean MDEA
Sulfide as S, mg/kg	-	27
Mercaptan as S, mg/kg	-	< 0.01
CO2, wt%	-	0.0008
Total heat stable salts, wt%	0.05	1.22
Water, wt%	49.4	55.5
pH	11.51	10.54
MDEA, wt%	49.9	40.1
Unidentified, wt%	0.7	2.3
Glycolic and lactic acid	45	540
Formic acid	100	860
Acetic acid	20	1250
Propionic acid	< 10	430
n-Butyric acid	< 10	160
Chloride	-	65
Phosphate	-	10
Sulfate	-	110
Thiosulfate	-	25
Suspended particles, ppmw	-	2

Kinetic studies of thermal degradation

The reaction rate was measured at 120°C and their reaction rate constants (kA ) were determined from first order kinetic equation as:                             lnCC0=kt                                                                 (1)

where, k  is first order rate constant. Table 2 summarizes the rate constant data obtained for MDEA degradation as well as formation of different degraded products observed in PTR-QMS with coefficient of regression values. Similarly, first order kinetics was also observed for MDEA degradation having rate constant to be 2.546 x 10^-5 hr^-1 [4]. It was observed that for formation of degraded compounds rate constant values are higher for fresh MDEA compared to lean solution.

Table 2 Rate constant of thermally degraded fresh and lean MDEA (in brackets)

Parameter	Methyl-diethanolamine		Methyl-ethanolamine		Dimethyl-acetamide	Dimethyl-aminoethanol	Diethanol-amine	Triethanol-amine
k x 104 (hr-1)	1.12 (1.82)	23.37 (12.05)		2.34 (1.62)		19.28 (5.9)	13.25	13.24 (4.02)
Coefficient of regression	0.9143 (0.9813)	0.9812 (0.9825)		0.9989 (0.9798)		0.9968 (0.9961)	0.9838 -	0.9799 (0.9398)

Organic acids in thermally degraded MDEA

Different organic acids such as formic, acetic, propionic, n-butyric, glycolic and lactic acids were present in lean MDEA with higher amounts while traces are present in fresh MDEA. Figure1 shows the formation of formic and acetic acid over degradation period of 865 hours. The rate of formation of formic acid on fresh MDEA is higher than acetic acid while for lean MDEA the formation rate was almost equal (Figure1).

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Figure1. Organic acids as HSS on thermal degradation of fresh and lean MDEA

Degradation products

It was observed that MDEA dissociated to diethanolamine (DEA) and finally monoethanolamine by delakylation/demethylation [5]. DEA was produced initially from MDEA fragmentation, in which the protonated MDEA was simply demethylated using the charge-remote fragmentation mechanism [6]. The second fragmentation involved the activation of the nitrogen atom with a hydrogen atom. This attack at the nitrogen atom causes one of the hydroxyethyl groups to leave the protonated MDEA to produce [C₃H₉ON+H]⁺ (methylethanolamine). It was observed that after degradation of fresh and lean MDEA formation of DEA and methylethanolamine were relatively in same range with increasing degradation time as shown in Figure2. In this thermal degradation N,N-dimethylacetamide (DMA) was also identified. The molar concentrations of DMA increased with time and were greater for fresh MDEA as compared to lean MDEA. This may due to instability of the acetamide group which may further hydrolyze to produce organic acids.

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Figure2. Change of concentration with time for degraded products

Distillation and residue analyses

To determine the level of high-boiling degradation products, the solvent collected at the end of the degradation experiment was distilled using vacuum distillation. It was observed that residue content was almost twice for lean MDEA as compared to fresh MDEA (Table 3). The higher residue content indicated high molecular weight degradation products with higher concentration in lean MDEA.

Table 3 Distillation and residue analysis of fresh and lean MDEA

Distillation yields	Fresh MDEA	Lean MDEA
	wt%	wt%
Water fraction	45.80	53.3
MDEA fraction	47.90	41.0
Residue	1.44	2.74
Loss	4.82	3.03
Unidentified	> 90%	> 90%

Corrosion parameters

The corrosion potential for fresh and lean MDEA was measured using iron solubility test (IST; the ability of a solvent to dissolve iron in the solution) and complexing power (CP; ability of the solvent to keep metals in solution). The analysis of MDEA before and after degradation experiments are shown in Table 4. It was observed that lean MDEA has relatively higher IST value and can be well explained with higher acid content. However, the increase in corrosion potential with aging time is comparable with fresh MDEA. The low value of CP of the lean MDEA was not likely to have any corrosion problems in gas sweetening unit.

Table 4 Corrosion performance of fresh and lean MDEA

Parameter	Fresh MDEA		Lean MDEA
Aging time, hr	0	865	0	865
Iron solubility, ppm	5	119	225	292
Complexing power	low	low	low	low

Foaming tendency of solvents

The MDEA collected at the end of the degradation experiments were tested for foaming using N₂ [3]. The foam height and foam break time indicates relatively higher foaming tendency for degraded lean MDEA (Figure3). This could be due to higher heat stable salts present. The higher foaming tendency could also be related to potential hydrocarbons, emulsifiers etc. present in the solvent.

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Figure3. Effect of nitrogen flow rate on foam height and foam break time

Conclusions

It was observed that thermal degradation of MDEA obeyed first order kinetics at 120°C. The major degradation products from both the fresh and lean MDEA were found to be similar. However, the concentrations of degradation products from lean MDEA were significantly higher. The higher acid content indicated oxidative degradation and possible oxygen ingress or presence of trace sulfur in the feed gas. The suspended particles of 2 ppmw indicated good solvent filtration efficiency. High IST value and lower CP of lean MDEA indicated lower corrosion potential of the solvent. However, the loss of MDEA due to degradation was significantly higher in the lean MDEA after degradation experiments. The lean MDEA sample showed relatively higher foaming tendency as seen from higher foam height and foam break time in the foaming tests.

Acknowledgement

The authors are grateful to The Petroleum Institute Gas Processing and Materials Science Research Center (Abu Dhabi) for funding the project (GRC 006), Shell Technology Centre Amsterdam (The Netherlands) and GASCO Company (Habshan, Abu Dhabi).

References

[1] Kohl, A.L. and Nielsen, R.B. Gas purification, 5th ed.; Gulf Publishing Company: Houston, TX, 1997.

[2] Mazari, S.A., Ali, B.S., Jan, B.M., Saeed, I.M., Nizamuddin, S. An overview of solvent management and emissions of amine-based CO₂ capture technology, Int. J. Green. Gas Cont., 34(2015) 129-140.

[3] Alhseinat, E., Pal, P., Ganesan, A., Banat, F. Effect of MDEA degradation products on foaming behavior and physical properties of aqueous MDEA solutions, Int. J. Green. Gas Cont., 37(2015) 280-286.

[4] Pal, P., AbuKashabeh, A., Al-Asheh, S., Banat F. Accumulation of heat stable salts and degraded products during thermal degradation of aqueous methyldiethanolamine (MDEA) using microwave digester and high pressure reactor, J. Nat. Gas Sci. Eng., 21(2014) 1043-1047.

[5] Bedell, S.A., Worley, C., Darst, K. and Simmons, K., Thermal and oxidative disproportionation in amine degradation – O₂ stoichiomery and mechanistic implications, Int. J. Green. Gas Cont., 5(2011) 401-404.

[6] Gross, M.L. Charge-remote fragmentation: an account of research on mechanisms and applications, Inter. J. Mass Spectrom., 200(2000) 611–624.