Using Isocratic Ion Chromatography to Quickly Determine Heat Stable Salts

Acid gases, such as hydrogen sulfide and carbon dioxide are often present in raw or sour natural gases. Absorption and removal of acidic gases can be acheieved using diglycolamine (DGA) and methyl-diethanolamine (MDEA), which are typically used as gas-treating agents. Contaminants are extracted using the amine solutions which cause the formation of salts of sulfur species and organic acids, e.g. thiocyanates, thiosulfates, formic acid, formate, oxalic acid, oxalates, sulfites and sulfates.

These anions, known as heat stable salts (HSS), can be determined using a simple isocratic seperation. This article demonstrates the isocratic separation of different HSS, and explores the application of organic modifiers in the eluent to enable more rapid separation of HSS.

Anion Exchange Chromatography

Acidic gases from either raw gas or from refinery gas streams are allowed to pass via the amine solution and are neutralized to form amine salts. Gas sweetening with an amine treatment process is a well-established technique for the removal of acidic gases. Depending on process requirements different amine combinations like MDEA or alkanolamines can be used.

Other contaminants, which are not acidic gases, can also be absorbed and form salts. Heating and reusing the amine solutions eventually removes the acidic gas. Some of the contaminants that are absorbed, such as HSS, do not degrade even following heating.

HSS is produced by a neutralization reaction between an organic/inorganic acid and the alkaline amine solvent. HSS accumulation reduces the absorption capacity of CO2 and it has also been shown to increase corrosion in the system. In order to ensure a safe and efficient amine treatment facility the continuous monitoring of amine regeneration and HSS formation are important.

Anion exchange chromatography can be used for tracking HSS in amine solutions. Gradient IC techniques are traditionally applied to elute thiosulfate and thiocyanate, which are strongly bound polarizable ions. However the gradient IC method is complex and takes up a significant amount of time.

Instrumentation and Separation of Heat Stable Salts

The instruments used for the analysis were the 858 Professional Sample Processor, the 930 Compact Flex IC w/sequential suppression, and a conductivity detector (Figure 1).

Compact Flex IC w/sequential suppression, 858 Professional Sample Processor, and conductivity detector

Figure 1. Compact Flex IC w/sequential suppression, 858 Professional Sample Processor, and conductivity detector

For sample preparation a HSS standard was used to spike 30% and 10% MDEA solutions. An HSS overlay in 30% MDEA (green), in 10% MDEA (red) and in water (black) is illustrated in Figure 2. As predicted, the MDEA reduced the sulfite peak sensitivity.

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

Figure 2. An overlay of HSS in water (black trace), in 10% MDEA (red trace) and in 30% MDEA (green trace).

Method parameters and instrument settings have been optimized with respect to the eluent, separation column, flow rate and column oven temperature.

  • Column: Metrosep A Supp 5-250/4.0
  • Column temperature: 40°C
  • Eluent: 2mmol/L sodium hydrogen carbonate; 2mmol/L sodium carbonate; 20% acetone
  • Flow: 0.7mL/min
  • Loop: 10µL
  • Suppressor: Sequential suppression (MSM, MCS)
  • MSM Regen.: 500mmol/L sulfuric acid
  • Degassing: ON
  • Detector: Conductivity

Standards and Calibration

Certified stock solutions or salts were used for gravimetric preparation of standards. A linear regression from the standards was used to prepare calibration curves. Care was taken to ensure that relative standard deviations (RSDs) were below 5% for all of the calibration curves for all of the analytes, as shown in Table 1.

Table 1. Standards and calibration of all analytes

Analyte Std 1
Coric
(mg/L)
Std 2
Coric
(mg/L)
Std 3
Coric
(mg/L)
Std 4
Coric
(mg/L)
Std 5
Coric
(mg/L)
Corr.
Coeffic.
Formate 9.943 12.46 19.60 49.36 101.3 0.9989
Sulfite 1.990 2.495 3.925 9.882 20.28 0.9999
Thiocyanate 5.008 6.277 9.874 24.86 51.02 1.0000
Sulfate 0.946 1.185 1.865 4.696 9.637 0.9998
Oxalate 2.106 2.639 4.153 10.46 21.46 1.0000
Thiosulfate 14.82 18.57 29.22 73.57 151.0 1.0000

HSS Recovery

HSS recoveries in the MDEA matrices are illustrated in Table 2.

Table 2. Recovery of HSS

Analyte Spike Amount (mg/L) Spike Conc (mg/L) Recovery (%) Spike Amount (mg/L) Spike Conc (mg/L) 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

In these analyses the MDEA included both formate and acetate. In order to determine an exact recovery for formate, non-spiked solutions containing 30% and 10% MDEA were examined and employed as matrix blanks. The formate contributed by MDEA was removed through blank subtraction. The interaction between MDEA and sulfite reduces the recovery of the latter considerably.

Organic Modifiers Influence

Organic modifiers not only enhance separation but also accelerate thiocyanate elution. As a result they can be utilized in the eluent. Figure 3 shows an offset of chromatograms displaying 1ppm of anions and HSS in 30% MDEA. The black, red and green traces have 15% acetone, 20% acetone, and 20% acetonitrile respectively in the eluent.

Chromatograms showing 1ppm of HSS and anions in 30% MDEA

Figure 3. Chromatograms showing 1ppm of HSS and anions in 30% MDEA.

While using acetonitrile, the elution of sulfite and thiocyanate occurs after and before phosphate (respectively). Whilst using acetone, the elution of thiocyanate occurs after sulfite because higher amounts of acetone shift the thiocyanate inbetween the phosphate and sulfite.

Conclusion

Optimized isocratic conditions are used for separating HSS, without the use of time-consuming and complicated gradients. Recovery results of HSS show the lack of matrix effect and demonstrate that the new yet simple isocratic technique is ideal for industrial applications.

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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