AL
mixture; the flame is then ignited and propa- gates through a combustor until all the fuel is consumed. This cycle is repeated continuously at a rate of 3–4 Hertz. The differences in specific emission lifetimes combined with the kinetics of the flame provide information on time and wavelength that can be used to improve the PFPD’s selectivity and decrease the observed noise to enhance selectivity and sensitivity. The propagating flame uses low combustible gas flow rates, increasing the relative analyte con- centration. Constant pulsing of the flame also produces a “self-cleaning” effect that reduces system maintenance, decreases downtime and eliminates coking problems commonly en- countered with other sulfur selective detectors.
In comparison to standard FPDs, the PFPD of- fers unambiguous sulfur recognition as a result of improved selectivity against hydrocarbon matrices, lower detection limits and equimolar sulfur response. Since it gives an equimolar re- sponse for sulfur compounds, any unknown or uncalibrated peak in the chromatogram can be quantitated using the S response of a calibrated compound. This permits total sulfur content in a sample to be calculated from the total of individually quantified components.
Standards are generated using permeation tubes held at constant temperature in a per- meation tube oven. These tubes contain a pure compound sealed inside a wafer or tube device that diffuses across the permeation membrane at a defined rate and uncertainty range at a given temperature. The permeation tube oven is held at a constant temperature to produce a constant diffusion rate. Standard concentra- tion is varied by changing the gas-diluent flow through the permeation tube oven.
Experimental Instrument operating conditions are shown in
Table 1. The PFPD was tuned for optimum sulfur response, configured for sulfur and hydrocarbon detection, and was run in the linearized mode (square root on). Simultaneous, mutually selec- tive sulfur and hydrocarbon chromatograms are produced from a single detector using the dual gate capability of the 5383 PFPD. A FlexStream gas standards generator (KIN-TEK, La Marque, Texas) was used to calibrate the instrument. Standards were generated using hydrogen sulfide (H2
S), carbonyl sulfide (COS) and methyl AMERICAN LABORATORY 21 MARCH 2016
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mercaptan (MeSH) permeation devices and a permeation tube oven held constant at 35 °C. Concentrations were varied by changing the ni- trogen flow through the permeation oven from 200 mL/min to 3600 mL/min. Eight calibration points were generated. For H2
S the calibration
range was 0.104 to 1.882 ppmv, for COS 0.116 to 2.081 ppmv and for MeSH 0.040 to 0.723 ppmv. GC software was used to generate calibration curves for the compounds listed. Two natural gas samples were analyzed (Figures 2 and 3).
Results and discussion The S-PRO-Select GC with 5383 PFPD and Select Low Sulfur column provide a fast, reliable meth- od for the analysis of sulfur in natural gas and other gaseous fuels. All key sulfur compounds were well separated from the methane hydro- carbon matrix (Figures 2 and 3). Two samples of H2
S and COS were also analyzed. One sample
was in a 50/50 ethylene/ethane mix and the other was in a 50/50 propylene/propane mix.
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