Using a Pulsed-Flame Photometric Detector to Analyze Impurities in Propane/Propylene

Conversions of high-grade ethylene (C2) and propane/propylene (C3) feedstocks into intermediates such as 1-butene and end products (polypropylene and polyethylene) are some of the key processes in the petrochemical sector. These processes are the building blocks for a wide range of products, including plastics, and represent a large industry with 55 million metric tons of polypropylene produced in 20131.

Unfortunately, C2 and C3 feedstocks often contain trace levels of sulfur species COS and H2S, which corrode equipment and pipes, inhibit or damage catalyst beds, and reduce product yield and purity2.

The need for a reliable and fast analysis method for H2S and COS in both C2 and C3 feedstocks is obvious, but due to the poor separation of the impurities from the matrix when coupled with the quenching of the PFPD detector signal by propane/propylene makes sulfur a difficult application in C2 and C3.

This article outlines a fast, robust, and reliable technique for the analysis of sulfur contaminants in C2 and C3 feedstocks. The technique uses separation by gas chromatography, an automated gas loop injection system, and pulsed-flame photometric detection that has the ability to detect sulfur at better than 0.1 ppmv.

Experiment

The OI Analytical S-PRO System (Figure 1) fitted with the 5383 PFPD was used in this analysis. It features a custom-configured gas chromatograph with two electronically controlled air-actuated valves for sulfur analysis in gas phase samples.

The Agilent Low Sulfur Select column was used, as it can easily handle the separation of H2S and COS from C2 and C3 matrices, respectively. Table 1 shows the operating conditions of the instrument.

OI Analytical S-Pro Select GC System with 5383 PFPD

Figure 1. OI Analytical S-Pro Select GC System with 5383 PFPD

Table 1. Instrument configuration and operating conditions

S-PRO Select GC System
Permeation Oven 35°C
Nitrogen dilution gas
Dilution gas flow rate 200 – 4500 mL/min
Permeation Devices H2S wafer device; permeation rate = 809 ng/min at 40°C
COS wafer device; permeation rate = 1586 ng/min at 40°C
MeSH wafer device; permeation rate = 441 ng/min at 40°C
Automated Injection System 4-port selection valve
6-port GSV with 1-mL Sulfinert®-coated sample loop
Automated, air-actuated valves
All lines Sulfinert coated
Valve oven temperature 110 °C
Volatiles Interface 200°C
Split mode
Split ratio 10:1
Sulfinert coated
GC Column Agilent J&W Select Low Sulfur Column
60-m x 0.32-mm ID
Helium carrier gas, 1.2 mL/min
Oven Program (Agilent 7890A) 40°C for 10 min
30°C/min to 180°C
Hold for 0.5 min*
Total run time 15.2 min
Sulfur Detection Pulsed Flame Photometric Detector (PFPD)
2-mm combustor, BG-12 filter, R1924 PMT
Detector base temperature 250°C
H2/air ratio tuned for optimum sulfur emission
6-24 msec sulfur gate (linear mode; square root on)
1-2 msec hydrocarbon gate

*Oven hold time may be extended to include later eluting compounds.

The PFPD was tuned for optimum sulfur response and configured for detection of sulfur and hydrocarbons, with sulfur run in the linearized mode, i.e., with the square root function on.

Using the dual time gate capability of the PFPD, mutually selective hydrocarbon and sulfur chromatograms are produced simultaneously from a single detector.

The instrument was calibrated for MeSH, COS, and H2S using certified wafer-type permeation devices and a permeation oven maintained at a steady temperature of 35°C. Changing the nitrogen flow rate through the permeation oven helped vary the concentrations of the compounds.

The calibration range for MeSH was 0.032 to 0.719 ppmv, for H2S it was 0.083 to 1.881 ppmv, and for COS 0.093 to 2.08 3ppmv. Gas samples were introduced via the sample inlet of the S-PRO sample pathway. Examples of standard chromatograms are shown in Figures 2 and 3.

High standard

Figure 2. High standard

Low standard

Figure 3. Low standard

Results and Discussion

Calibration

A ten-point calibration was analyzed and the Agilent GC Chemstation Open Lab data system was employed to create calibration curves. Linearity was determined for the three compounds with a correlation coefficient of 0.999 (Figure 4).

Calibrations

Figure 4. Calibrations

System Stability

A repeatability study was carried out over a period of two weeks with 110 replicate injections of standards. Relatively low %RSDs indicate the stability of the GC system and PFPD (Table 2).

Table 2. Repeatability study for sulfur compounds

Compound Name Concentration ppmv in Nitrogen %RSD
H2S 0.188 10.96
COS 0.209 4.31
MeSH 0.072 8.81

Samples

Manufactured samples comprising of different sulfur compounds in methane, ethylene/ethane, and propylene/propane matrices were studied.

It was easy to separate sulfur compounds from the methane and propylene/propane matrices. However, it is difficult to separate H2S from the C2 matrix as H2S elutes between ethane and ethylene.

When some samples are analyzed, there is a balance between reporting the desired levels and obtaining the results without matrix interference. It is important to monitor co-elutions as split ratios and matrices are changed (Figures 5-10).

Sulfur in Ethylene/Ethane (10 split)

Figure 5. Sulfur in Ethylene/Ethane (10 split)

Sulfur in Ethylene/Ethane (10 split)

Figure 6. Sulfur in Ethylene/Ethane (10 split)

Sulfur in Propylene/Propane (10 split)

Figure 7. Sulfur in Propylene/Propane (10 split)

Sulfur in Propylene/Propane (20 split)

Figure 8. Sulfur in Propylene/Propane (20 split)

Sulfur in Natural Gas (20 split)

Figure 9. Sulfur in Natural Gas (20 split)

Sulfur in Methane (100 split)

Figure 10. Sulfur in Methane (100 split)

Conclusion

Equipped with 5383 PFPD and Agilent Select Low Sulfur column, the OI Analytical S-Pro GC System provides a fast, robust, and reliable method to study sulfur compounds, including COS and H2S in various matrices.

The system can be used for analyzing lighter sulfur compounds in an array of matrices without any need for significant changes to the chromatographic conditions. As a result, the matrix concentration can be balanced with the preferred sulfur sensitivity and GC split ratios to optimize performance for different analyses.

References

1. Market Study: Polypropylene (3rd Edition), Ceresana December, 2014.

2. ASTM Standard D6228-10 Standard Test Method for Determination of Sulfur Compounds in Natural Gas and Gaseous Fuels by Gas Chromatography and Flame Photometric Detection, ASTM International, West Conshohoken, PA, www.astm.org.

Acknowledgements

Analytical column provided by Agilent. Natural gas samples were provided by DCG Partnership in Pearland, Texas. The permeation diluter and permeation devices were provided by KIN-TEK in La Marque, Texas.

Sulfinert® is a registered trademark of Restek Corporation

This information has been sourced, reviewed and adapted from materials provided by OI Analytical.

For more information on this source, please visit OI Analytical.

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