CO 2 Absorption Solvent Degradation Compound Identification Using Liquid Chromatography-Mass Spectrometry Quadrapole-Time of Flight (LCMSQTOF)

The degradation of the alkanolamine solvent used in the removal of acid gases from natural gas streams due to exposure to contaminants, thermal degradation and presence of oxygen or oxygen containing compounds will change the solvent properties, such as heat transfer coefficient, diffusion coefficient, and mass transfer coefficient of the solvent. Therefore, characterization and quantification of amine degradation product becomes one of the important analyses to determine alkanolamine solvent’s health. In order to identify degradation products of alkanolamine solvent, analytical strategies by using mass spectrometry (MS) as detector have been studied extensively. In this work, due to the low concentration of the amine degradation product, a method was developed for identification of alkanolamine degradation products using LCMS-QTOF technique. A strategy for identification of trace degradation products has been identified. Six (6) alkanolamine degradation products had been identified by using LCMS-QTOF targeted analysis in the blended alkanolamine solvent used in natural gas processing plant. Another fifteen (15) molecular formulas having similarity in chemical structure to alkanolamine degradation products were identified using untargeted analysis strategy, as possible compounds related to degradation products. Using LCMS-QTOF via targeted and untargeted analysis strategy, without tedious column separation and reference standard, enables laboratory to provide a quick and indicative information for alkanolamine solvent’s organic degradation compounds identification in CO 2 adsorption, within reasonable analysis time.


Introduction
Removal of acid gases from natural gas streams using blended alkanolamine solvent has been widely used since decades ago [1]. In removing acid gases, many technology options are available but by far the most popular is the absorption by alkanolamine solvents. Several alkanolamine solvents have been proposed for acid gases removal. Among the common alkanolamines used are monoethanolamine (MEA), diethanolamine (DEA), di-isopropanolamine (DIPA) and methyldiethanolamine (MDEA) [2]. In a conventional acid gases removal plant, both absorption and desorption of acid gas are involved. The acid gas is absorbed by the alkanolamine solvent in the absorber. In the desorber, the acid gas is released by increasing the temperature of the column to break the chemical bonding of the akanolamine with the acid gases adsorbed [3].
Amine solvent can degrade due to exposure to contaminants, such as SO x , NO x , halogen compound, hydrocarbons, and other contaminants [4], which may be introduced from equipment components and maintenance activities. In addition, thermal degradation [4] [5] can happen during amine regeneration which is normally carried out around its boiling point. Presence of oxygen or oxygen containing compounds will also cause oxidative degradation [4] [5] especially under high temperature condition. These degradation processes can occur simultaneously and produce various degradation products that will eventually affect solvent properties, such as viscosity and surface tension. The change in solvent physical properties can potentially affect heat transfer coefficient, diffusion coefficient, and mass transfer coefficient in amine solvent. This may introduce operational problems, such as reduced solvent capacity, increased energy consumption, corrosion, fouling, and foaming. Foaming of amine is a common problem in natural gas processing plant which increases down time and reduces throughput [6]. It often occurs due to presence of amine degradation product, such as heat stable salts (HSS), though presence of corrosion inhibitors, hydrocarbon, and iron sulphide particles originating from corrosion [7] [8] are also the usual suspects. Therefore, characterization and quantification of amine degradation product becomes one of the important analyses to determine amine solvent's health for foaming prevention.
The type of alkanolamine degradation product and its relevant degradation reaction were mentioned and discussed in many literatures related to CO 2 adsorption. For example, degradation products from MDEA and Piperazine were compiled and tabulated in Table 1.
In order to identify degradation products of alkanolamine solvent, analytical strategies by using mass spectrometry (MS) as detector were mentioned in few literatures e.g. LC-MS, GC [11] trace concentration detection, LCMS-QTOF was commonly used due to its high sensitivity feature. In CO 2 absorption studies, many authors discussed the degradation products of alkanolamine, degradation path and solution to resolve issues caused by degradation products. But identification strategy on MS acquired data was seldom discussed in detail. Instead, a comprehensive identification strategy in characterization of trace degradation products using MS or MSMS had been extensively discussed in pharmaceutical or drug impurities/degradation products study [17] [19] [20] [21] [22], probably due to its stringent requirement for pharmaceutical product. Further, there is no standard strategy to derive unequivocal identification of trace degradation products qualitatively and it had been done in many different approaches. It is crucial to assure quality of the result finding to be reliable. In this work due to the low concentration of the alkanolamine degradation product, a method was developed for identification of alkanolamine degradation compounds using LCMS-QTOF technique. A strategy for identification of trace degradation products will be discussed.

Alkanolamine Samples
Three types of alkanolamine solutions were used in this study. These include freshly prepared using purchased chemicals (Sample A1 and Sample A2), alkanolamine solutions taken from a natural gas processing plant used for 3 years Sample A1 was prepared fresh with 30% methyl diethanolamine (MDEA) and Sample A2 was prepared with 7% piperazine, both diluted in ultrapure water.
Both Sample A1 and A2 were used as baseline, for the identification of amine degradation products.

LCMS-QTOF Equipment
The  Table 2. Gas temperature was optimized at 125˚C. At temperature above 125˚C, alkanolamine compounds was undetectable which possibly caused by amine degradation at ion source.

Mass Data Analysis
Sample mass data generated from QTOF detector was processed by Agilent MassHunter Qualitative Analysis Workflows 10.0. The degradation compound identification strategy was conducted using targeted and untargeted analysis.  Table 1). The degradation compounds were identified based on highest match of mass, isotope abundance and isotope spacing between the experimental and theoretical accurate mass, with minimum 80% score as basis. Example is shown in Figure 2. Untargeted analysis was conducted using compound discovery workflow to perform   Table 1, but with extended wider range about two times of typical range, to explore any larger molecular compounds probably derived from alkanolamine degradation.
Compounds with carbon number above 20 were not targeted, to eliminate complex molecular structure that unlikely to happen. In order to relate the identified possible molecular formulas with alkanolamine degradation process, the possible molecular structures of each molecular formula were found using Chemspider, and chemical structures that could have derived from alkanolamine degradation products structure or its combination were shortlisted as possible degradation products. Those identified possible degradation products from untargeted analysis were considered as possible structures, but further confirmation was not covered in this study.

Chromatography
Sample was introduced into detector via auto-sampler without column separa-

Sample A1-Freshly Prepared 30% Methyl Diethanolamine (MDEA)
In targeted analysis, two degradation products (DP) were found in sample A1 (freshly prepared MDEA), which were A1-DP1 and A1-DP2 with average score of 97.90% and 86.61%, representing degradation products of MM and TMA as shown in Table 3. The MM and TMA resulted from MDEA thermal degradation [9], most likely due to the effect of solvent storage. From the untargeted analysis result (Table 4), four compound masses were identified having molecular formula match with the CHNO elemental limit specified (C: 1 -20, H: 0 -80, N: 0 -10, O: 0 -10) and there was one potential chemical structure (Table 5) identified from Chemspider which display similar structure to morpholine and piperazine, potentially relate to alkanolamine degradation.

Sample A2-Freshly Prepared 7% Piperazine (Pz)
In targeted analysis, no alkanolamine degradation product was found in freshly prepared piperazine. From untargeted analysis (Table 6), one compound mass was identified having molecular formula match with the CHNO elemental limit specified (C: 1 -20, H: 0 -80, N: 0 -10, O: 0 -10) but no molecular structure found having similar structure to degradation product. It indicated fresh piperazine solvent did not contain any possible degradation product as it is stable during storage.

Sample B-Alkanolamine Solution Used in Natural Gas Processing Plant for 3 Years' Duration
Five (5) alkanolamine degradation products were found in sample B using targeted analysis with average mass score recorded as 82.41% -97.52% (Table 7).
The five (5) products were related to MDEA thermal degradation products [9].
In untargeted analysis result (Table 8)      ethanolamine and ethylene diamine (Table 11), which potentially relates to alkanolamine degradation. The result of targeted and untargeted analysis confirmed that sample B had been exposed to thermal degradation and formed few years, compared to the freshly prepared Sample A1 and A2.

Sample C-Alkanolamine Solution Used in Natural Gas Processing Plant for 20 Years' Duration
Six (6) alkanolamine degradation products were found in sample C using targeted analysis with average mass score recorded as 82.31% -99.64% (Table 9). Out of the six (6) products, five (5) products (C-DP1, C-DP2, C-DP3, C-DP5, C-DP6) were MDEA thermal degradation product and one (1) product (C-DP1) was related to piperazine thermal degradation [9]. In untargeted analysis result (Table 10), seventeen (17) compound masses were identified from the broad compound discovery indicating a good match with predicted molecular formula with the CHNO element specified (C: 1 -20, H: 0 -50, N: 0 -6, O: 0 -8). They had recorded average mass score between 80.66% -98.78%. Based on these seventeen (17) predicted molecular formula, thirty-eight (38) potential chemical structure were identified from Chemspider database, consisted of chemical structure similar to MDEA, piperazine and alkanolamine degradation products of morpholine, methyl amine, ethanolamine and ethylene diamine (Table 11), which potentially related to alkanolamine degradation. The result of targeted and untargeted analysis indicated sample C had been exposed to more severe thermal degradation and formed more organic degradation products in the natural gas processing plant operated for 20 years, compared to sample A1, A2 and sample B.

Identification of Peaks and Compound Correlation with Chemical Reaction
All samples except A2 contained N-methyl morpholine (MM) which was a major    Table 12. Comparison between Sample B and Sample C, in abundance % of alkanolamine products and its degradation products found via targeted analysis. It provided some qualitative indication on the reduction of MDEA and Pz alkanolamine product in sample C which had been recycled used in gas processing plant for more than 20 years compare to sample B which was 3 years. Other products mentioned in Table 1 ND 1 ND 1 ND 1 ND 1 Note 1: ND-Not Detected either the accurate mass score was below 80% or was not detected due to trace level below method detection limit.

Conclusion
Six (6) alkanolamine degradation products have been identified by using

CARBON DISTRIBUTION OF ALKANOLAMINE ORGANIC DEGRADATION PRODUCTS (SAMPLE C)
untargetted analysis targetted analysis Journal of Analytical Sciences, Methods and Instrumentation LCMS-QTOF targeted analysis in the blended alkanolamine solvent (MDEA and Pz) used in natural gas processing plant. Another fifteen (15) molecular formulas having similarity in chemical structure to alkanolamine degradation products were identified using untargeted analysis strategy, as possible compounds related to degradation product, but confirmation of its validation was not covered in this study. Using LCMS-QTOF via targeted and untargeted analysis strategy, without tedious column separation and reference standard, enables laboratory to provide a quick and indicative information for alkanolamine solvent's organic degradation compounds identification in CO 2 adsorption, within reasonable analysis time. In order to achieve higher accuracy, further extension of this LCMS-QTOF analysis using MSMS ion fragmentation would help to confirm the compound structure and investing in optimizing compound separation using analytical column will also improve the sensitivity of the method.