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
| Component |
Calibration Gas 1 |
Calibration Gas 2 |
Dilution 1 (50 mL 12% SO2, 50 mL air) |
Dilution 2 (8 mL 12% SO2, 92 mL air) |
| SO2 |
12% |
100 ppm |
6% |
0.96% |
| Air |
Balance |
Balance |
Balance |
Balance |
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.
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Figure 1. Chromatogram of 12% SO2 in air
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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%.
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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
| Compound |
Retention Time (s) |
RT %RSD |
Area %RSD |
| 12% SO2 |
51.3 |
0.14 |
0.43 |
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.
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This information has been sourced, reviewed and adapted from materials provided by INFICON Inc..
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