Article pubs.acs.org/IECR
Identification of Methyldiethanolamine Degradation Products and Their Influence on Foaming Properties during the Desulfurization Process for High-Sulfurous Natural Gas Yucheng Liu,*,† Danni Wu,† Mingyan Chen,† Bo Zhang,† Ju Chen,† and Yuanzhi Liu‡ †
School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China Puguang Branch of Zhongyuan Oilfield Company, Dazhou 636156, China
‡
ABSTRACT: The effects of complex degradation products on foaming behavior and physical properties of a 50 wt % methyldiethanolamine (MDEA) solution were investigated systematically. The degradation products in lean amine solution were detected and quantified by gas chromatography−mass spectrometry (GC−MS). The degradation products were identified as N,N-dimethylethanolamine (DMEA), ethylene glycol (EG), 1-(2-hydroxyethyl)-4-methylpiperazine (HMP), diethanolamine (DEA), triethanolamine (TEA), and bicine (N,N-bis(2-hydroxyethyl)glycine), respectively. Considering the standard ASTM D892 for foam performances testing of lubricating oils, the effects of different degradation products on the foaming behavior of the MDEA solution were discussed. Results showed that the degradation compounds exhibited significant impact on foam stability promotion, even with small amounts, especially HMP and bicine. With the addition of 0.1 wt % HMP, the foam height increased from 130 mm to 325 mm; meanwhile, break time increased from 8.96 to 31.72 s. A maximum in foam height and break time was observed as the bicine was increased to 0.2 wt %. Some physical properties, particularly the surface tension, viscosity, density, and pH value of solutions had an important influence on bubble behavior.
1. INTRODUCTION The natural gas produced from Puguang (Dazhou, China) gas fields is a special high-sulfurous gas. Aqueous MDEA solution, which has the advantages of negligible corrosion, stabilization, relatively low reaction heat, and highly selective absorption of H2S from a gas stream mixture with CO2,1,2 is widely used as a desulfurization agent in the purification plant. However, during the operation process, the foaming of the amine liquid caused by metamorphic pollution is serious. The according problems appear, such as absorbent loss, pipeline corrosion, decreased hydrocarbon yield, and fluctuating operation parameters.3−5 There are three main degradation mechanisms of the amine: oxidative degradation, degradation reacted with CO2 and thermal degradation.6−12 Chakma et al.6 first discovered MDEA degradation by CO2 and studied the degradation mechanism and kinetics. Subsequently, many further studies were published on this topic. The irreversibility of the degradation reaction between MDEA and CO2 resulted in the generation of more than 10 organic products (e.g., EG, DEA, TEA, HMP, and DMEA).6−8 Rooney et al.9 carried out the oxidation experiments on 50 wt % MDEA solution at 82 °C. They reported the generation of DEA, even at that relatively low temperature. Another significant product of the oxidation deterioration of MDEA was bicine. It was reported that the desulfurization of a gas plant exposed to oxygen resulted in the generation of a high amount of bicine. The authors demonstrated that oxidative degradation gave rise to the formation of bicine.10,11 Bedell and Worley12 studied several MDEA system stability tests. The results indicated that the thermal degradation of MDEA at 182 °C within 4 days virtually did not occur. They thought that N-(2-hydroxyethyl)2-methylglycine (HEMgly) was the initial oxidation product of MDEA, and then it disproportionated to bicine and DMEA. © 2015 American Chemical Society
Organic contamination resulting from degradation is one of the most important factors causing foaming.13,14 Therefore, research into the effects of degradation compounds on the foaming properties of amine liquids is necessary. Extensive research has been devoted to the impact of solid particles (e.g., activated carbon, FeS, and Fe(OH)3), methanol, inorganic salts, corrosion inhibitors, and liquid hydrocarbons on the foam of desulfurizer solution.4,13,15−17 Unfortunately, no previous data have been published on the effects of various degradation products on the foaming properties of amine liquids. Much work has been done on defoaming methods including physical and chemical ones. Generally, however, the most efficient foam control is realized by adding defoamers whose main role is to prevent formation of excessive foam. Various defoamers are available and widely used during the natural gas sweetening process, such as polyether defoamer, organic silicon defoamer, and polyether modified polysiloxane defoamer.18,19 In addition, a series control measures can reduce foaming, which include the timely supply of fresh amine solution and the control of oxygen.10,13 One basic objective of the present work was to use GC−MS to identify the principal degradation compounds in a lean amine of MDEA aqueous solution, and then added these compounds to fresh 50 wt % MDEA aqueous solution for foaming experiments. The experiments aimed to quantify the effects of each compound on the foaming behavior. The knowledge acquired will subsequently be used to provide a Received: Revised: Accepted: Published: 5836
November 9, 2014 May 8, 2015 May 12, 2015 May 12, 2015 DOI: 10.1021/ie504432d Ind. Eng. Chem. Res. 2015, 54, 5836−5841
Industrial & Engineering Chemistry Research
Article
altitude difference of the solution between the initial and the final conditions, and it represented the foaming ability of the solution. The foaming break time (t/s) was the period of time from the cessation of ventilation to the completed rupture of the bubble, and it was used to indicate foam stability. The basic outline of the experimental device was presented in Figure 2. The device constituted a flow meter (0−1.5 L/min), a
basis for developing corresponding countermeasures for foaming control.
2. EXPERIMENTAL METHODS 2.1. Sample Preparation. The chromatographic column that was used was nonpolar, so it induced hydrolysis of the stationary phase, despite the presence of a small amount of water. Therefore, water needed to be removed from the sample prior to injection, and the lean amine used in the Puguang purification plant had to be enriched simultaneously. Vacuum distillation was chosen and the temperature was controlled between 85 and 90 °C. The distillation residue collected from the bottom of the flask was reserved for GC−MS. The difference in appearance between the lean amine and the vacuum distillation residue is shown in Figure 1.
Figure 2. Schematic diagram of the experimental device.
Figure 1. Comparison of the appearance of the lean amine (left) with the vacuum distillation residue (right).
2.2. GC−MS Test Conditions. For the quick and efficient determination of degradation products, an Agilent 7890A gas chromatograph connected to an Agilent 5975C mass spectrometer was used. The chromatographic column used for the separation of the compounds was an HP-5MS elastic quartz capillary vessel column (60 m × 0.25 mm i.d., 0.25 μm film thickness). The specific test conditions are shown in Table 1.
foaming tube with graduations and a diffuser, and a superheated water bath equipped with a digital temperature controller. The blister tube used in the experimental process was generally a casing pipe with an inner diameter of 30 mm and a height of 500 mm. As an inert gas, industrial-grade nitrogen was utilized instead of air to prevent further oxidation degradation. In addition, a stopwatch was used to track the bubbling time. 2.4. Experimental Procedures. The test solution was placed into a graduated cylinder on the scale at 100 mm and heated to 40 ± 0.5 °C for at least 20 min by a circulating water bath to reach thermal equilibrium. A diffuser for dispersing gas into the heated test fluid was placed into the MDEA solution for approximately 5 min until equilibrium was reached. A constant flow rate of 0.25 L/min of nitrogen passed through the flow meter into the solution for 25 min ± 5 s and was then released to the atmosphere from the top of the blister tube. For most of the test solutions, there was no clear boundary between liquid and foam. Therefore, the total height of the system (liquid and foam) was recorded every minute. It was found that the total height increased to the maximum at the beginning, and then gradually decreased until reaching a steady state. In most experiments, the total height changed with time before stabilizing approximately 10 min after the beginning of the experiment. Consequently, the average value of the data recorded during the last 15 min was used as the foam height of the test solution. 50 wt % MDEA solution (without any additive) was used as a benchmark for the full-dose trials.
Table 1. GC−MS Test Conditions chromatographic conditions injection port temperature detector port temperature carrier gas carrier flow line speed column temperature split ratio injected sample size
280 °C 280 °C 99.999% helium 1.5 mL/min 45 cm/s initial temperature at 50 °C for 5 min, then increased at a rate of 10 °C/min to 280 °C with a hold time of 3 min 20:1 0.2 μL mass spectrometry conditions
ionization source ionization voltage filament current transfer line and ion source temperature acquisition mode solvent delay
electron impact (EI) 70 eV 100 μA 280 °C full-scan 2 min
3. RESULTS AND DISCUSSION 3.1. Degradation Products’ Distribution and Relative Quantity. To identify and quantify the vacuum distillation residues, GC−MS was used. Figure 3 presented the total ion chromatogram of the lean amine after pretreatment, and the major peaks corresponding of the main MDEA degradation products were summarized in Table 2. The main purpose of the pretreatment is to concentrate the target substances. Organic sample preparation methods that can be used include liquid−liquid extraction (LLE), distillation,
2.3. Experimental Setup. Different quantities of the detected compounds were added to fresh 50 wt % MDEA solution. The foaming experiments were then performed according to the standard ASTM D892 test method for the foaming characteristics of lubricating oil, with the exception that the solution was heated with circulating water rather than an ordinary water bath.20 The foam height (Δh/mm) was the 5837
DOI: 10.1021/ie504432d Ind. Eng. Chem. Res. 2015, 54, 5836−5841
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Figure 3. Total ion chromatogram (TIC) of the vacuum distillation residue.
Table 2. Chemical Composition (in Percent) of the Vacuum Distillation Residue
a
Retention time. bBoiling point. cMolar mass.
blowing, adsorption, solid phase extraction (SPE), and
on the grounds that they were all strong polar substances. The pretreatment experiment was implemented by vacuum distillation because the boiling points of those products were significantly greater than that of water. From the perspective of
supercritical fluid extraction (SCFE). Enrichment of the organics enumerated above from the 21,22
water by liquid−liquid extraction had been less than satisfactory 5838
DOI: 10.1021/ie504432d Ind. Eng. Chem. Res. 2015, 54, 5836−5841
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Figure 4. Effects of the addition of different degradation products on foam height and break time of 50 wt % MDEA solution: (△) foam height; (□) break time.
very small quantity of HMP changed the foaming tendency of 50 wt % MDEA solution significantly. With the addition of 0.1 wt % of HMP, the foam height increased from 130 mm (50 wt % MDEA solution) to 325 mm. Break time also rose from 8.96 to 31.72 s. To achieve the same effect, the concentration of DEA and TEA needed to be more than 10 wt %. As seen from Figure 4, the effect of bicine on the 50 wt % MDEA solution was also studied. A maximum in foam height and break time was observed as the bicine was increased to 0.2 wt %. Then the foam height and break time decreased with further addition of bicine slightly. The effects of different degradation products addition on 50 wt % MDEA solution surface tension, viscosity, density, and pH value are shown in Figures 5 and 6. The results in Figures 5 and 6 indicated that the addition of these organic components reduced the surface tension and improved the viscosity and density of the 50 wt % MDEA aqueous solution. The surface tension primarily had a fundamental impact on the foamability. Normally, the lower was the surface tension, the better was the foaming ability.17 Therefore, the foam could easily form or the foam capacity was strengthened. Degradation products could easily adsorb on the surface of the bubbles, increasing their outer membrane strength and thereby enhancing the foam stability. Notably, despite their high viscosity, the 50 wt % MDEA solutions with the concentration of DEA and TEA more than 10 wt % achieved the same effect, just like other degradation products. This may be explained as the high bulk viscosity which could stop the bubbles from rising. As depicted in Figure 6, degradation products of different concentrations played a minor role in pH value, except bicine. From observation, bicine had significantly changed the pH
the detection results of GC−MS, the method was efficient and reproducible. 3.2. Effects of Degradation Products on Foam Formation. The purpose was to explore the effects of different degradation products on the foam formation in the blend of MDEA−water. The addition of different degradation products may determine, to some extent, the changes of the physical properties of 50 wt % MDEA solution. To further study the bubble behavior in solution, the physical characterizations (including surface tension, viscosity, density, and pH value) of the tested solutions were detected, as well. Figure 4 represented the effects of these degradation products at the various concentrations on the foam formation of the 50 wt % MDEA solution. As presented in Figure 4, first, the fresh 50 wt % MDEA solution did not generally form stable foam, and its foam system did not have very good foaming properties. The solutions containing degradation products provided greater foam height and break time than those without degradation products. It was observed that the foam height results were roughly in line with the trend seen in break time. It showed an upward trend with the increase of concentration of the different compounds (except bicine). Figure 4 also showed that the upward trend was rather more slow at relatively low concentration. However, the greater the concentration was, the more obvious was the foam behavior. Therefore, these degradation products played an important role in facilitating the foaming behavior and foam stability of the 50 wt % MDEA aqueous solution. The presence of a small amount substances had a great influence on the foam properties, especially HMP and bicine. HMP seemed to have more influence on foaming than others. As shown in Figure 4, a 5839
DOI: 10.1021/ie504432d Ind. Eng. Chem. Res. 2015, 54, 5836−5841
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Figure 5. Effects of the addition of different degradation products on surface tension and viscosity of 50 wt % MDEA solution: (△) surface tension; (□) viscosity.
Figure 6. Effects of the addition of different degradation products on solution density and pH value of 50 wt % MDEA: (△) density; (□) pH value.
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DOI: 10.1021/ie504432d Ind. Eng. Chem. Res. 2015, 54, 5836−5841
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value of the 50 wt % MDEA solution. The bubble behavior of the solution with added bicine could be ascribed to the fact that bicine could neutralize alkaline substances and thus obviously reduce the pH value of the solution. The electrostatic repulsion between bilayers was impaired by this neutralization reaction, which reduced the surface tension of the solution, and enhanced foam formation.23,24 As the reaction ultimately reached equilibrium, the foam performance tended to be stabilized.
4. CONCLUSIONS A reliable and accurate GC−MS method was developed and implemented to identify and quantify the lean amine in natural gas treatments plants in the Puguang gas fields that was used for sour gas removal. DMEA, EG, HMP, DEA, TEA, and bicine were detected. Vacuum distillation was used to pretreat the organic impurities in the MDEA solution. The effects of the major degradation products were studied by single factor tests, and the testing results indicated that all of the compounds promoted foam stability. Even with only a small amount of substances added, there was still a significant impact on the foam properties, especially HMP and bicine. HMP seemed to have more influence on foaming than others. With the addition of 0.1 wt % of HMP, the foam height increased from 130 mm (50 wt % MDEA solution) to 325 mm. Break time also rose from 8.96 to 31.72 s. To achieve the same effect, the concentration of DEA and TEA needed to be more than 10 wt %. A maximum in foam height and break time was observed as the bicine was increased to 0.2 wt %. Then the foam height and break time slightly decreased with further addition of bicine. The foam properties of the MDEA solution were influenced by some physical properties, particularly the surface tension, viscosity, density, and pH value. These data can provide the basis for the future development of foaming control.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +8602883037326. Fax: +8602883037305. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
Financial support received from the National Science and Technology Major Projects (No. 2011ZX05017) and SWPU Pollution Control of Oil & Gas Fields Science & Technology Innovation Youth Team (No. 2013XJZT003) is gratefully acknowledged.
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DOI: 10.1021/ie504432d Ind. Eng. Chem. Res. 2015, 54, 5836−5841
