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. CAS 141-43-5 2-Aminoethanol

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.

Following reagents were used for the analysis:

Following Instruments were used for the experiment:

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)

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:

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

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.

Figure 2. Overlay of calibration standards

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.

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

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.

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.

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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Nice Article, it should be useful for many researchers & technicians

Dear Sirs, I faced a problem in this analysis where the Sulfite and Sulfate peaks overlap when the concentration is high enough while in calibration the peaks are perfectly separated. Please advise. Thank You in advance.

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