Determining the Efficiency of Catalysts through Rapid Analysis of Sulfur Dioxide

Nearly 180 million tons of sulfuric acid (H₂SO₄) are used for various applications, for example as formulation of insecticides, fertilizers, and detergents, around the world every year. It is one of the most manufactured chemicals, with billions of pounds being produced and sold in the United States alone. For producing H₂SO₄, sulfur dioxide (SO₂) has to be oxidized, which produces sulfur trioxide (SO₃). The SO₃ interacts with water to form H₂SO₄. Most of the H₂SO₄ is manufactured using this method.

The oxidation of SO₂ is made possible by catalysts. The SO₂ analysis performed at the inlet and outlet of the catalytic bed establishes the performance and conversion efficiency of the catalyst. A percent level of SO₂ is supplied at the catalytic bed inlet. Once the catalytic conversion of SO₂ to sulfuric acid is completed, the concentration of the SO₂ is usually about 100 ppm when it exits the bed outlet.

The Micro GC Fusion

In most cases, obtaining results quickly and reliably on-site is preferable as any delay during transportation of the sample to an analysis lab could compromise the integrity of the sample. A vast concentration range of SO₂ can be accurately analyzed using gas chromatography (GC) .

The Micro GC Fusion is a compact, portable GC instrument. It is designed for analyzing SO₂ across a wide linear range. It has a microelectromechanical systems (MEMS) based thermal conductivity detector (TCD), which makes it capable of measuring a broad concentration range of compounds within a minute at great accuracy.

Experiment

A 12 m Rt-Q-Bond column and a variable volume injector make up the Micro GC Fusion. The Rt-Q-Bond column was added to the design as it offers superior separation of the SO₂ peak from the adjacent water peak. The variable volume injector enables numerous types of samples to be analyzed.

Two of Air Liquide’s calibration gas standards were used for this experiment. The first standard was made up of 12% SO₂ in air to imitate the initial concentration of SO₂ at the catalytic bed inlet. The second standard was made up of 100 ppm of SO₂ in air to imitate the SO₂ concentration exiting the bed. The technique was formulated to elute SO₂ rapidly, as well as to maintain the separation of the water peak.

A 100 mL gas tight syringe was used to deliver the 12% SO₂ and 100 ppm SO₂ calibration gases. The next step was diluting the 12% SO₂ calibration gas to 6% and 0.96% using the syringe. The diluted gas was then fed into the instrument (Table 1), and analysis was carried out of multiple injections of each concentration. The average area counts and concentrations of each of the four calibration standards were plotted to create a calibration curve.

Table 1. Air Liquide SO₂ calibration gas standard concentration information

The 12% SO₂ calibration gas standard was used to perform 10 consecutive runs for estimating the relative standard deviation (% RSD) for retention time and peak area. A 1/16 inch Restek SilcoNert^® piece of tubing was used for directly connecting this calibration gas to the sample inlet.

Results

Within a minute, SO₂ is separated from the solvent peak, air (Figure 1). Figure 2 illustrates chromatograms corresponding to the four calibration standard concentrations.

Figure 1. Chromatogram of 12% SO₂ in air

Figure 2. Chromatogram overlay of four concentrations of SO₂

Figure 3 depicts the calibration curve, which displays superior linearity of SO₂ concentrations, ranging from 100 ppm to 12%.

Figure 3. Calibration curve for SO₂

Exceptional repeatability can be observed from the % RSD calculations for both area and retention time. The % RSD values for area count and retention time over 10 runs are 0.43% and 0.14%, respectively (Table 2).

Table 2. Repeatability data for the 12% SO₂ calibration gas standard - ten runs

Conclusion

The results have demonstrated the analysis of 100 ppm to 12% SO₂ at the catalytic bed’s inlet and outlet, with excellent linearity and precision, using the 12 m Rt-Q-Bond column.

On-site analyses can be directly performed within 60 seconds to observe the performance and conversion efficiency of the catalyst to improve the production efficiency of H₂SO₄.

This information has been sourced, reviewed and adapted from materials provided by INFICON Inc..

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