Identifying Heat Stable Salts in Methyldiethanolamine MDEA Solutions

In most cases natural gas is processed to remove the presence of acidic gases, such as carbon dioxide (CO2), hydrogen sulfide (H2S), and other contaminants. This practice ensures products meet their specifications. This article demonstrates how different heat stable salts (HSS) can be separated under different isocratic conditions. It also explores the usage of an organic modifier in the eluent to allow HSS separation.

The presence of contaminants can lead to corrosion in amine treatment plants. Alkylanolamines or alkanolamine like methyldiethanolamine (MDEA) are used for removing acidic gases from the natural gas. The acid gases are absorbed by the amine solutions, which asa result are removed from the natural gas.

The acid gases can then finally be removed from the amine solutions. The amine solutions also absorb other types of salt-forming contaminants. Certain contaminants which are absorbed do not degrade upon heating and as a result they are known as heat stable salts.

The residual HSS can cause corrosion to amine treatment plants and tend to decrease the amine solutions’ absorption capacity resulting in an increase in solvent viscosity. Also before disposing the waste it must be properly profiled.

It is important to track HSS in amine solutions so appropriate action can be taken to control them. Formate, acetate, sulfate, sulfite, oxalate, thiosulfate and thiocyanate are some of the standard HSS. Anion exchange chromatography can be used to track the presence of HSS in amine solutions.

Reagents and Instruments

Following reagents were used for the analysis:

  • Oxalic acid, CAS 144-62-7
  • Sodium sulfite, CAS 7757-83-7
  • Sodium thiocyanate, CAS 540-72-7
  • Sodium thiosulfate, CAS 7772-98-7
  • Sodium carbonate, CAS 497-19-8
  • Sodium bicarbonate, CAS 144-55-8
  • Isopropanol, CAS 67-63-0
  • Sulfuric acid, CAS 7664-93-9
  • 1000ppm chloride (Metrohm USA ERA-IC1002)
  • 1000ppm formate (Metrohm USA ERA-IC1011)
  • 1000ppm sulfate (Metrohm USA ERA-IC1007)
  • Acetone, HPLC grade, CAS 67-64-1
  • Ultrapure water, resistivity >18 MΩ-cm (25 °C)
  • Methyldiethanolamine, CAS 105-59-9
  • Acetonitrile, HPLC grade, CAS 75-05-8

Following Instruments were used for the experiment:

Samples and Solutions

The samples used for the analysis were a 30% MDEA solution with a synthetic HSS mixture and 10% MDEA solution with synthetic HSS mixture. The solutions used were MSM 500mM H2SO4 + stream, and eluents such as 2mM Na2CO3 + 2.0mM NaHCO3 + 20% acetonitrile; 2mM Na2CO3 + 2.0mM NaHCO3 + 15% acetone; and 2mM Na2CO3 + 2.0mM NaHCO3 + 20% acetone. Salts and certified stock solutions were used for gravimetric preparation of standards. Table 1 shows standards in ultrapure water in parts per million.

Table 1. In ultrapure water (ppm)

S1 S2 S3 S4 S5
Formate 9.943 12.46 19.60 49.36 101.3
Chloride 0.0982 0.1231 0.194 0.487 1.000
Sulfite 1.990 2.495 3.925 9.882 20.28
Thiocyanate 5.008 6.277 9.874 24.86 51.02
Sulfate 0.9459 1.185 1.865 4.696 9.637
Oxalate 2.106 2.639 4.153 10.46 21.46
Thiosulfate 14.82 18.57 29.22 73.57 151.0

Sample Preparation

A level 2 standard was made in 30% v/v MDEA solution and 10% v/v MDEA solution for sample preparation. IC parameters are as follows:

  • Column temperature — 40°C
  • Eluent flow — 0.7mL/min
  • MSM Regenerent — 500mmol/L sulfuric acid
  • Sample loop — 10pL
  • MSM rinsing — Stream
  • Degasser— On
  • MCS — On
  • Recording time — 65min
  • MSM—Automatic stepping

Peak area is used for calculations for all the analytes through automatic integration with the MagIC Net™ 3.0 software.

Instrumentation Setup

Figure 1 shows the instrumentation setup, and Figure 2 shows an overlay of calibration standards. Figures 3, 4, 5, 6, 7, 8 and 9 show the formate calibration curve, chloride calibration curve, sulfite calibration curve, thiocyanate calibration curve, sullfate calibration curve, oxalate calibration curve, and thiosulfate calibration curve respectively.

Instrumentation setup

Figure 1. Instrumentation setup

Overlay of calibration standards

Figure 2. Overlay of calibration standards

Formate calibration curve

Figure 3. Formate calibration curve

Chloride calibration curve

Figure 4. Chloride calibration curve

Sulfite calibration curve

Figure 5. Sulfite calibration curve

Thiocyanate calibration

Figure 6. Thiocyanate calibration

Sullfate calibration curve

Figure 7. Sullfate calibration curve

Oxalate calibration curve

Figure 8. Oxalate calibration curve

Thiosulfate calibration curve

Figure 9. Thiosulfate calibration curve

The level 2 heat stable salt standard was used to spike 30% and 10% MDEA solutions and prepare the samples. Figure 10 shows a HSS overlay. It was found that MDEA reduced the sulfite peak sensitivity as predicted. HSS recoveries in the MDEA matrices are summarized in Table 2.

An overlay of HSS in water (black trace), in 10% MDEA (red trace) and in 30% MDEA (green trace)

Figure 10. An overlay of HSS in water (black trace), in 10% MDEA (red trace) and in 30% MDEA (green trace).

Table 2. Recoveries of the HSS in the MDEA matrices

Analyte Spike Amount (ppm) Spike Conc (ppm) Recovery (%) Spike Amount (ppm) Spike Conc (ppm) Recovery (%)
Formate 12.57 11.57 92.01 12.63 11.52 91.19
Sulfite 2.517 1.893 75.21 2.528 1.353 53.52
Thiocyanate 6.333 6.391 100.9 6.360 6.437 101.2
Sulfate 1.196 1.206 100.8 1.201 1.164 96.92
Oxalate 2.663 2.502 93.95 2.675 2.507 93.72
Thiosulfate 18.74 18.87 100.7 6.360 6.437 101.2

Acetate and formate are present in the MDEA which was used in the analyses. Next, 30 % and10 % MDEA unspiked solutions were ran to determine an exact recovery for chloride and formate. These were employed as matrix blanks.

To remove the formate contributed by the MDEA, blank subtraction was utilized. Recoveries of sulfite were reduced considerably because of the interaction between sulfite and MDEA.

Organic Modifier

An eluent can be added with organic modifiers, which not only enhances the separation but also improves the elution speed of thiocyanate. Figure 11 shows an offset of chromatograms, displaying 1ppm of anions and HSS in 30% MDEA.

Offset of chromatograms showing 1ppm of HSS and anions in 30% MDEA.

Figure 11. Offset of chromatograms showing 1ppm of HSS and anions in 30% MDEA.

Here, the black, red and green traces show 15% acetone, 20% acetone, and 20% acetonitrile, respectively, in the eluent. When using acetonitrile, the elution of thiocyanate occurs before phosphate elution, and theh elution of sulfite occurs after phosphate elution.

When using acetone, elution of thiocyanate occurs after the elution of sulfite. When a large amount of acetone is used, the thiocyanate shifts inbetween phosphate and sulfite.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.

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