Identification of Amines Found in Refineries via Ion Chromatography Using a High Capacity Cation Exchange Column

The presence of gases like hydrogen sulphide and carbon dioxide renders petroleum and natural gas products “sour”. These acidic gases cause corrosion; hence, their presence in the final gas products is unacceptable. In natural gas plants and refineries the removal of acid gases was done using alkanolamines. Amines, being weak bases, react with these acid gases to form salts. This salt forming reaction between amines and acids is called gas sweetening.

The amine content in gas sweetening solutions needs to be monitored to optimize the amine content and to  profile waste, which minimizes the plant maintenance effort. The type of amine used for gas sweetening varies for each plant. The two main factors that affect the choice of amines are the acid being treated in the gas and the extent to which the acid gas dissolves in the amine.

This article looks into the use of cation exchange chromatography with nonsuppressed conductivity detection for separating several types of amine that can be used in gas sweetening solutions. The amines that are analyzed are: diethanolamine (DEA), 3-methoxypropylamine (MOPA), monoethanolamine (MEA), triethanolamine (TEA), methyldiethanolamine (MDEA), diglycolamine (DGA) and cyclohexylamine. Additionally the separation of amines in the presence of standard cations is also explorede.

Reagents

The following reagents were used for this experiment:

  • Oxalic Acid, CAS 144-62-7
  • Nitric Acid, CAS 7697-37-2
  • Dipicolinic Acid, CAS 499-83-2
  • Ultrapure water, resistivity >18 MO•cm (25 °C)
  • Acetone, HPLC grade, CAS 67-64-1
  • Monoethanolamine, CAS 141-43-5
  • Diethanolamine, CAS 111-42-2
  • Triethanolamine, CAS 102-71-6
  • 3-methoxypropylamine, CAS 5332-73-0
  • Cyclohexylamine, CAS 108-91-8
  • Diglycolamine, CAS 929-06-6
  • Methyldiethanolamine, CAS 105-59-9
  • Custom Cation Mix 2 (REAIC1230, MUSA)

Instruments

The instrumental setup is given in the table below. Figure 1 shows the experimental setup.

Instrumentation Setup

Figure 1. Instrumentation Setup

940 Professional IC Vario ONE 2.940.1100
IC conductivity detector 2.850.9010
858 Professional Sample Processor – Pump 2.858.0020
MagIC Net TM 3.0 6.6059.302
Metrosep C 6 250/4.0 6.1051.430
10 µL loop 6.1825.230

Solutions

The solutions used were Eluent - 1mM HNO3 + 1.5mM Oxalic acid + 0.75mM Dipicolinic acid + 1% acetone.

Samples and Standards

The sample used was ultrapure water which was spiked with amines and cations. The standards were prepared from certified stock solutions or from bulk chemicals.

The table below gives the composition of ultrapure water.

S1 S2 S3 S4 S5 S6
Li 0.9693 2.441 4.807 9.646 24.68 49.34
Na 0.9693 2.441 4.807 9.646 24.68 49.34
NH4 0.9693 2.441 4.807 9.646 24.68 49.34
MEA 0.9840 2.478 4.880 9.792 25.05 50.08
DEA 0.9818 2.472 4.869 9.770 25.00 49.97
K 0.9693 2.441 4.807 9.646 24.68 49.34
DGA 0.9839 2.477 4.879 9.791 25.05 50.08
TEA 0.9792 2.466 4.856 9.744 24.93 49.84
MDEA 0.9799 2.467 4.860 9.752 24.95 49.88
MOPA 0.9835 2.476 4.877 9.787 25.04 50.06
Mg 0.9693 2.441 4.807 9.646 24.68 49.34
Ca 0.9693 2.441 4.807 9.646 24.68 49.34
Cyclo 0.9772 2.461 4.846 9.752 24.88 49.74

Sample Preparation

Samples were injected into the IC directly. The table below shows the IC parameters.

Eluent Flow 0.9 mL/min
Column temperature 40°C
Sample loop 10 µL
Degasser On
Recording Time 60 minutes

Calculation

Calculations were done using the peak area of all the analytes via automatic integration with the MagIC Net 3.0 software.

Calibration

The tables and graphs that follow depict the calibration standards for various cations.

Overlay of calibration standards

Figure 2. Overlay of calibration standards

Lithium

Figure 3. Lithium

Sodium

Figure 4. Sodium

Ammonium

Figure 5. Ammonium

MEA

Figure 6. MEA

DEA

Figure 7. DEA

Potassium

Figure 8. Potassium

DGA

Figure 9. DGA

TEA

Figure 10. TEA

MDEA

Figure 11. MDEA

MOPA

Figure 12. MOPA

Magnesium

Figure 13. Magnesium

Calcium

Figure 14. Calcium

Cyclohexylamine

Figure 15. Cyclohexylamine

Among the conditions used, acetone showed a profound effect on separation. The stacked chromatograms depicted in Figure 16 represent 1.25% acetone in eluent (top/red) and 1% acetone in eluent (bottom/black). When the acetone concentration is greater, cyclohexylamine will elute in less than 40min. However co-elution of magnesium, calcium, DEA and MEA is also observed.

Stacked chromatograms

Figure 16. Stacked chromatograms

The retention of amines is influenced using other organic modifiers. The comparison of the eluent that is prepared with 1% acetone (black) and the eluent prepared with 1% acetonitrile (red) is shown Figure 17. Co-elution of morpholine and MOPA is observed when acetone is used as an organic modifier. However morpholine and MOPA are separated when acetonitrile is used. An overlap of morpholine over MDEA is observed.

Comparison of eluent

Figure 17. Comparison of eluent

Conclusion

The work presented in this article has shown the possibility of separating amines based on the provided chemistry. Additionally, the effect of an organic modifier on separation has also been demonstrated. The calibration curves presented in this article are based on quadratic regressions. For smaller calibration concentration ranges linear regression may be used.

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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