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Application Note: 42166

Ultra Low Levels of Sulfur and Nitrogen in Aromatic Hydrocarbons and Oxygenates using the Thermo Scientific iPRO 5000 Series Analyzer Kristian J. Hoffman, Angela Seipel, Application Specialists, Thermo Fisher Scientific, Cambridge, UK

(TS) present in aromatic hydrocarbons and oxygenates. Sulfur analysis in aromatics was conducted using ultraviolet fluorescence in accordance with ASTM D7183 [1], which is applicable to aromatic hydrocarbons with sulfur content in the range 0.5 – 100 mg/kg. Nitrogen analysis in aromatics was conducted using chemiluminescence based detection in accordance with ASTM D7184 [2] which is applicable to aromatic hydrocarbons with nitrogen content in the range 0.03 – 1 mg/kg. ASTM D4629 [3] is suitable for the determination of total nitrogen in liquid hydrocarbons with nitrogen content in the range 0.3 – 100 mg/kg. These hydrocarbons may include bio-fuels. A Thermo Scientific iPRO 5000 Series Total Nitrogen and Total Sulfur combustion analyzer fitted with a Thermo Scientific jetPRO direct spray injector was used to perform the analysis described below.

Key Words • iPRO 5000™ NS • Aromatic Hydrocarbons • ASTM D7183 • ASTM D7184 • ASTM D4629 • Ethanol • jetPRO™ direct spray injector • Nitrogen • Sulfur

Introduction Even very low levels of sulfur and nitrogen are detrimental to the effectiveness of catalysts used in refining processes. Refineries therefore require the ability to monitor nitrogen and sulfur content of feedstocks in the sub-mg/kg region. Current catalytic exhaust systems are highly efficient and can dramatically reduce emissions of hydrocarbons, nitrogen oxides and carbon monoxide. However, their efficiency is strongly degraded by the presence of sulfur. The introduction of modern fuel-injection methods, such as Gasoline Direct Injection (GDI), to internal combustion engines has the potential to significantly reduce fuel consumption. Unfortunately, a characteristic feature of GDI engines is the increased emission of nitrogen oxides, which must then be catalytically reduced to nitrogen. The reduction process requires optimal performance of the catalyst and the presence of even very low levels of sulfur results in inefficiency in the process, which in turn drives up fuel consumption. Government directives increasingly call for the use of fuel derived from renewable sources. These sources can include oxygenates such as ethanol, which can be derived from sugar and corn and is naturally very low in sulfur and nitrogen. The blending of low sulfur ethanol into gasoline can be used to reduce the overall sulfur and nitrogen content of gasoline. This type of activity further drives the requirement to measure sulfur and nitrogen at trace levels. This application note describes a direct injection, oxidative combustion technique for the measurement of ultra-low levels of total nitrogen (TN) and total sulfur

Experimental A Thermo Scientific AS3000 autosampler was used in conjunction with a Thermo Scientific iPRO 5000 NS analyzer to directly inject samples into the furnace. Data acquisition and peak analysis was fully automated by the Thermo Scientific NSX Visual Software. System parameters were set by the NSX Visual Software according to the selected pre-loaded methods: ASTM D7183 and ASTM D7184 for the analysis of aromatics. This automatically resulted in the fully ASTM compliant system parameters presented in Table 1. Parameter

System value

Argon flow

50 ml/min

Oxygen primary flow

320 ml/min

Oxygen secondary flow

50 ml/min

Oxygen makeup flow

400 ml/min

Oxygen ozonator flow

50 ml/min

Temperature furnace I

980 °C

Temperature furnace II

1000 °C

Injection speed

2.0 µl/s

Sample volume

140 µl

Table 1: System parameters for the iPRO 5000 NS analyzer.


Calibration of the iPRO 5000 NS analyzer was achieved using standards containing equal molar quantities of sulfur and nitrogen – derived from pyridine and thiophene – dissolved in toluene. Several standards covering the range 0 to approximately 750 µg/kg were prepared by serial dilution of stock standards and were then used to provide calibration of the nitrogen and sulfur detectors in the sub ppm regime. Samples of toluene, benzene and ethanol were then analyzed for their total nitrogen and total sulfur concentration. These aromatic and oxygenate samples were provided by a large multinational chemical company. All samples were injected directly into the iPRO 5000 NS analzyer using the AS3000 autosampler and all measurements were made in triplicate.

Representative peak shapes for TN and TS analysis of aromatic hydrocarbons in the 0 – 1 mg/kg range using direct injection are displayed in Figure 3 and Figure 4, respectively.

Results Presented in Figure 1 and Figure 2 are the nitrogen and sulfur calibration lines determined on the iPRO 5000 NS analyzer. It is clear that both show excellent linearity, with R2 = 0.998 and R2 = 0.999 for the TN and TS channels, respectively.

Figure 1: Linear calibration line for the iPRO 5000 NS analzyer nitrogen detector in the 0 – 750 µg/kg range.

Figure 2: Linear calibration line for the iPRO 5000 NS analyzer sulfur detector in the 0 – 750 µg/kg range.

Figure 3: Representative peak shapes for 3 replicates of a 160 ppb nitrogen standard using an iPRO 5000 NS analyzer. Post analysis peak smoothing can be applied to the data using the NSX Visual software.

Figure 4: Representative peak shapes for 3 replicates of a 160 ppb sulfur standard using an iPRO 5000 NS analyzer. Post analysis peak smoothing can be applied to the data using the NSX Visual software.


The peak areas and standard deviations for each of the calibration standards used to generate Figure 1 and Figure 2 are presented in Table 2 and Table 3, respectively. Concentration of TN standard (µg/kg)

0

Replicate 1 concentration µg N/kg

Replicate 2 concentration µg N/kg

Replicate 3 concentration µg N/kg

Mean concentration µg N/kg

SD µg N/kg

RSD (%)

-6.06

-8.32

-9.16

-7.85

-0.09

1.22

11.49

2.33

8.87

6.22

5.80

0.13

2.26

22.99

24.38

15.43

23.71

21.17

0.66

3.10

45.98

59.98

49.45

36.08

48.49

3.09

6.37

91.95

121.97

87.59

121.37

110.31

8.70

7.88

183.91

198.38

179.16

191.60

189.71

5.62

2.96

367.82

372.66

347.40

358.21

359.43

9.13

2.54

735.63

749.57

732.91

735.89

739.45

7.48

1.01

Table 2: TN signal concentrations obtained during calibration of an iPRO 5000 NS analyzer. Three replicates of 140 µl were performed for each of the calibration standards in the approximate range 0 – 750 µg/kg. The data was taken in weight/volume units and then converted to weight/weight units in software.

Concentration of TS standard (µg/kg)

Replicate 1 concentration µg S/kg

Replicate 2 concentration µg S/kg

Replicate 3 concentration µg S/kg

Mean concentration µg S/kg

SD µg S/kg

RSD (%)

0

2.21

0.31

3.74

2.08

0.36

16.87

11.49

10.93

11.37

8.62

10.31

0.82

8.01

22.99

26.07

19.82

18.61

21.49

2.91

13.54

45.98

42.45

49.39

46.51

46.11

2.97

6.43

91.95

92.4

93.74

91.08

92.4

1.22

1.32

183.91

180.78

183.98

193.31

186.02

6.29

3.38

367.82

359.07

346.08

359.63

355.17

7.14

2.01

735.63

741.17

764.3

718.45

741.31

22.76

3.07

Table 3: TS signal concentrations obtained during calibration of an iPRO 5000 Series NS analyzer. Three replicates of 140 µl were performed for each of the calibration standards in the approximate range 0 – 750 µg/kg. The data was taken in weight/volume units and then converted to weight/weight units in software.

Finally, the iPRO 5000 NS analzyer was used to measure the nitrogen and sulfur content of a benzene, toluene and an ethanol sample. The nitrogen data obtained in this study is presented in Table 4 and the sulfur data is presented in Table 5.

Sample Type

Toluene

Replicate 1 concentration µg N/kg

Replicate 2 concentration µg N/kg

Replicate 3 concentration µg N/kg

Mean concentration µg N/kg

SD µg N/kg

RSD (%)

79.15

61.40

55.37

65.30

3.94

6.04

Benzene

89.49

93.78

110.84

98.04

4.67

4.76

Ethanol

1677.35

1835.84

1548.33

1687.17

62.93

3.73

Table 4: TN signal concentrations for aromatic and oxygenate samples. Three replicates of 140 µl were performed for each analysis.

Sample Type

Replicate 1 concentration µg S/kg

Replicate 2 concentration µg S/kg

Replicate 3 concentration µg S/kg

Mean concentration µg S/kg

SD µg S/kg

RSD (%)

Toluene

42.82

45.42

45.63

44.62

1.37

3.07

Benzene

33.38

33.92

38.22

35.17

2.24

6.38

Ethanol

717.61

773.52

751.20

747.44

27.88

3.73

Table 5: TS signal concentrations for aromatic and oxygenate samples. Three replicates of 140 µl were performed for each analysis.


Conclusion

In addition to these

A Thermo Scientific iPRO 5000 Series Total Nitrogen and Total Sulfur combustion analyzer was used to determine the nitrogen and sulfur content of aromatics in the 0 – 750 µg/kg regime. The instrument was found to exhibit excellent detection capabilities for both nitrogen and sulfur and was fully compliant with ASTM D7183 and D7184. An oxygenate, in the form of ethanol, was also analyzed for total nitrogen and total sulfur content. Efficient sample introduction via the Thermo Scientific jetPRO direct spray injector ensured that the analysis time was approximately 3 minutes per replicate for all the samples measured.

offices, Thermo Fisher

References [1] ASTM D7183 Standard Test Method for Determination of Total Sulfur in Aromatic Hydrocarbons and related Chemicals by Ultraviolet Fluorescence. [2] ASTM D7184 Standard Test Method for Ultra Low Nitrogen in Aromatic Hydrocarbons by Oxidative Combustion and Reduced Pressure Chemiluminescence Detection. [3] ASTM D4629 Standard Test Method for Trace Nitrogen in Liquid Petroleum Hydrocarbons by Syringe/Inlet Oxidative Combustion and Chemiluminescence Detection

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Application Note: 42166  

Ultra Low Levels of Sulfur and Nitrogen in Aromatic Hydrocarbons and Oxygenates using the Thermo Scientific iPRO 5000 Series Analyzer

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