(522a) 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

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)

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).

\s

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.

\s

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

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

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.

 
\s

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

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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]
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Gas Cont., 5(2011) 401-404.

[6]
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