Quantification of Sulfur Content in LiquefiedPetroleum Gas (LPG) using the ThermoScientific iPRO 5000 Series Total Nitrogen/Total Sulfur (TN/TS) AnalyzerAngela Seipel, Kristian Hoffman, Application Specialists, Thermo Fisher Scientific, Cambridge, UK

IntroductionLiquefied Petroleum Gases (LPG) are a group ofhydrocarbon gases typically comprising three or fourcarbon atoms. The primary constituents of LPG arepropane (C3H8), propylene (C3H6), butane (C4H12) andbutylenes (C4H8). There is also increasing interest in theuse of dimethly ether (C2H6O) as a potential substitute forpropane, despite the drawbacks associated with itsreactivity towards valve-seals and tubing.

Being colourless and odourless necessitates theaddition of odorants to LPGs to aid detection. Theseodorants are normally the sulfur containing compoundsmercaptans, which are added at an approximateconcentration of ~10 ppm. Measurement of the totalsulfur (TS) content of natural gas streams is thereforeimportant both in avoiding sulfur related pipelinecorrosion and for determining the precise quantity ofodorant that must be added to LPGs.

LPG and other light hydrocarbons are also finding useas feedstocks for a variety of fuel cell technologies. Fuelcells and reformer catalysts are generally sulfur intolerantand the need for low level sulfur measurements in the fuelcell industry is a growing trend. Furthermore, LPG isincreasingly used as an automotive fuel and must thereforecomply with legislation concerning the sulfur content ofautomotive fuels.

The sulfur analysis of LPGs by ultraviolet (UV)fluorescence is governed by ASTM D6667 [1] and isapplicable to samples containing 1.0 – 100 mg/kg ofsulfur. This application note describes a direct injectiontechnique, which is fully compliant with ASTM D6667,for measuring low levels of sulfur in LPG using theThermo Scientific iPRO 5000 S Total Sulfur analyzer.

ExperimentalLPG was introduced into the combustion analyzer using aThermo Scientific EGM-II gas and LPG sampleintroduction accessory. The EGM-II contained a ten-portrotary valve fitted with two sample loops (one for gassamples and the other for LPG samples). The sampleloops allowed for the delivery of known quantities of gasor LPG into the combustion analyzer, with the gas loopdelivering 10 ml per loop and the LPG loop delivering100 µl per loop. The EGM-II was used to introduceseveral loops of a thiophene doped butane LPG into theiPRO 5000™ S analyzer. Air Products UK supplied thethiophene doped LPG, which contained 10.15 ppm ofthiophene in butane. The total sulfur content, as afunction of the number of loops, was then measured intriplicate and used to calibrate the instrument prior todetermining the sulfur content of an LPG sample ofunknown sulfur content.

The measurement process was controlled using theThermo Scientific NSX Visual Software. The software wassupplied with pre-loaded ASTM-compliant methods andthe analysis was performed in accordance with methodASTM D6667, which resulted in the system parametersshown in Table 1.

The LPG was vaporized and mixed with argon carrier gas in the EGM-II and injected directly into theiPRO 5000 S analyzer, the Total Sulfur configuration ofthe iPRO 5000 Series TN/TS analzyer, via the jetPRO™

direct spray injector. Complete oxidation of the sampleoccurred in the oxygen-rich environment of the hightemperature furnace. Combustion products were driedusing a permeable membrane drying tube containedwithin the instrument and the total amount of sulfur wasthen determined by SO2 UV-fluorescence. Signalprocessing and analysis was handled automatically by theNSX Visual Software.

Parameter System value

Argon flow (EGM-II) 250 ml/min

Argon flow (furnace) 25 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 975 °C

Temperature furnace II 1075 °C

Loop volume 100 µl

Table 1: System parameters used on the iPRO 5000 S analyzer in thedetermination of sulfur by ASTM D6667.

Key Words

• iPRO 5000™

Series

• ASTM D6667

• jetPRO™ direct spray injector

• Liquefied Petroleum Gas (LPG)

• Sulfur

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ResultsPresented in Table 2 are the results of injecting knownquantities of sulfur-containing butane LPG into the iPRO5000 S analyzer. The data was used to generate a linear

Figure 1: First order calibration for the iPRO 5000 S analyzer and EGM-IIsystem. Each loop delivered 3.8 mg/kg of sulfur into the analyzer. The signalresponse of the sulfur detector exhibited excellent linearity with R2 = 0.998.

calibration graph for the instrument, which is shown inFigure 1. Representative peak shapes for a typical analysisare shown in Figure 2.

Figure 2: Peak shapes arising from the injection of three repetitions of fourloops of thiophene-bearing butane LPG. The thiophene content was 10.15mg/kg, corresponding to a sulfur content of 3.8 mg/kg. The high degree ofreproducibility was typical of the measurements and is consistent with thelow RSDs presented in Table 2 and Table 3.

Finally, a sample of LPG with unknown concentrationwas analyzed for sulfur content using the calibration datapresented in Figure 1. The results of the analysis arepresented in Table 3 and reveal the sulfur content to be23.36 mg/kg.

ConclusionA Thermo Scientific iPRO 5000 S analyzer, combined withan EGM-II gas and LPG sample introduction accessorywas used to determine low levels of sulfur in LPG bydirect injection. The procedure was fully compliant withASTM D6667 and was shown to result in excellent levelsof accuracy and precision.

References[1] ASTM D6667 Standard Test Method for Determination of Total VolatileSulfur in Gaseous Hydrocarbons and Liquefied Petroleum Gases byUltraviolet Fluorescence

Number Injected Concentration Replicate 1 Replicate 2 Replicate 2 Mean of loops volume (µl) S (mg/kg) concentration concentration concentration concentration SD RSD (%)

mg S/kg mg S/kg mg S/kg mg S/kg mg S/kg

1 100 3.8 3.48 3.55 3.49 3.50 0.03 0.93

2 200 7.6 7.43 7.42 7.38 7.41 0.02 0.33

4 400 15.2 15.77 15.03 16.18 15.66 0.51 3.26

8 800 30.4 31.84 30.95 30.55 31.11 0.65 2.09

10 1000 38.0 36.73 37.66 37.55 37.31 0.50 1.35

Table 2: Sulfur concentrations resulting from UV fluorescence-based detection of SO2 following complete combustion of known quantities of sulfur-containingbutane LPG. LPG was delivered into the combustion analyzer using a Thermo Scientific EGM-II, fitted with a 100 µl LPG sampling loop.

Number Injected Concentration Replicate 1 Replicate 2 Replicate 3 Mean of loops volume (µl) S (mg/kg) concentration concentration concentration concentration SD RSD (%)

mg S/kg mg S/kg mg S/kg mg S/kg mg S/kg

6 600 22.8 23.19 23.32 23.57 23.36 0.19 0.80

Table 3: Sulfur concentration corresponding to the injection of 6 100 µl loops of butane LPG of unknown concentration.