<<

Gas Chromatographic Analysis for Hydrogen , Organic , Mercaptans, and Carbon Dioxide in Hydrocarbon Matrices Using an Electrolytic Conductivity Detector*

R.G. Schiller** and R.B. Bronsky, American Natural Service Company, 5943 Tireman, Detroit, Michigan 48204 Downloaded from https://academic.oup.com/chromsci/article/15/12/541/349030 by guest on 02 October 2021

Abstract Experimental Hydrogen sulfide, mercaptans, and organic sulfides can be A Tracor 550 Chromatograph was used in the experimenta- determined in hydrocarbon matrices using GC and an electro- tion. The chromatograph was equipped with a Flame Photo- lytic conductivity detector. Sensitivity is in the ppb level and interference from the hydrocarbon is minimal. The response metric Detector and a Tracor 310 Hall Electrolytic Conductiv- is linear simplifying calculations when compared to the FPD. ity Detector. A CSI 38 Digital Integrator was used for quanti- Carbon dioxide can also be determined with the same proce- fication. The gaseous sample was injected through a 10 port dure and its detection limit is approximately 0.1 percent. stainless steel valve equipped with a 10 cc sample loop. The columns used were: a 3ft x '/iin stainless steel column packed Introduction with acetone-washed Porapak QS 80/100 mesh (5) carrier gas The Flame Photometric Detector (FPD) has been used in flow 30 cmVmin (this column was used for programmed anal- the past for the determination of gaseous compounds in yses from 60°C - 160°C), a 36ft x l/8in Teflon column filled hydrocarbon matrices (1). This technique is satisfactory for with 40/60 mesh Teflon powder and flow coated with a poly- natural gas containing (

Reproduction (photocopying) of editorial content of this journal is prohibited without publisher's permission. JOURNAL OF CHROMATOGRAPHIC SCIENCE»VOL. 15 DECEMBER 1977*541 REACTION GAS ION EXCHANGE BED Downloaded from https://academic.oup.com/chromsci/article/15/12/541/349030 by guest on 02 October 2021

SOLVENT RESERVOIR

RECORDER

Figure 1. Diagram of the Hall Electrolytic Conductivity Detector.

of conversion of carbonyl sulfide or dimethyl disulfide. It was The Oxidative Mode thought that the column was retaining these two compounds In this mode, the furnace temperature, oxidant flow (air or but when the column was switched to the FPD, excellent res- oxygen), and furnace contact time are used as a means of ponse for these two compounds was achieved. is in- selectively converting as much of the sulfur compounds to cluded in Figure 2 showing the conversion of 100% methane sulfur oxides with as little conversion of the hydrocarbons to to carbon dioxide at various temperatures. carbon dioxide as possible. This mode therefore requires the Hydrocarbons other than methane (C2 - C6) including un- manipulation of many variables to achieve maximum sensitiv- saturates and aromatics were tested and gave a response ity and selectivity. similar to methane. Selectivity between mercaptan and sulfide Hall (4) recommended the use of a non-aqueous solvent to can be achieved at the lower furnace temperature (600° C). promote selectivity between sulfur dioxide and carbon dioxide Sulfides should be determined, for best sensitivity, between and the use of a 1 mm i.d. quartz tube packed with quartz 8OO-9OO°C. wool to increase contact time of the chromatograph effluent in The quartz tube (6 cm long with 1 mm i.d.) is extremely dif- the furnace (7). Since hydrocarbons are more stable than the ficult to pack with quartz wool. Tests were run to determine sulfur compounds, a low furnace temperature and minimal whether this packing is necessary. Two 1 mm quartz tubes amounts of oxidant would seem to be necessary to improve were used; one with and the other without the quartz wool. All selectivity. With this in mind, the instrument conditions were other parameters were held constant. adjusted using methanol as a solvent, and air as the oxidant at Three (3) ppm methyl mercaptan was used as the sulfur a flow of 4-5 cc/minute. The furnace was equipped with a 1 compound in a nitrogen matrix. The packed tube, in this in- mm i.d. quartz tube with 2 mm in length of quartz wool stance, improved the sensitivity but increased the tailing. The packing. The furnace temperature was varied to check the effect of simultaneous eluting hydrocarbon and sulfur com- conversion of specific sulfur compounds to the sulfur oxides. pounds was checked. A short 6 in Porapak QS column and A three ppm standard of each compound: hydrogen sulfide, a 130°C oven temperature was used. A 3 ppm methyl mercap- ethyl mercaptan, dimethyl sulfide, carbonyl sulfide, and di- tan standard was made up in a methane matrix and also in a methyl disulfide in nitrogen was introduced into the chro- nitrogen matrix. The unpacked tube had a 20% reduction in matograph through the 10 ml gas sample loop. Figure 2 shows sensitivity when the methane eluted with the methyl mercap- the conversion relationships. There was no indication tan. The packed tube showed little change in sensitivity when

542OECEMBER1977 JOURNAL OF CHROMATOGRAPHIC SCIENCE«VOL. 15 Downloaded from https://academic.oup.com/chromsci/article/15/12/541/349030 by guest on 02 October 2021

DEGREES C/100

Figure 2. Conversion and response relationship at various furnace temperatures A: Three (3) ppm ethyl mercaptan, sensitivity (3x1) B: Three (3) ppm hydrogen sulfide, sensitivity (3 x 4) C: Three (3) ppm dimethyl sulfide, sensitivity (3x1) D: 100% methane, sensitivity (3x1)

the methane eluted with the methyl mercaptan (Figure 3). It showing the approximate detection limit with the present con- appears that the packing is very important in reducing hydro- figuration. Detection limits for the lower boiling sulfur com- carbon interference. pounds (e.g., hydrogen sulfide, methyl mercaptan) are better Methanol proved to be the best solvent for the Hall Electro- than with high boiling sulfur compounds because of less peak lytic Conductivity Detector. Other solvents such as ethanol spreading. The determination of carbon dioxide at mole per- and propanol were tried, but the tailing was excessive with cent levels is possible using this detector. Figure 5 shows that these solvents. Up to 8% v/v water was added to the methanol carbon dioxide is not linear and standards closely matching to increase sensitivity but the signal-to-noise ratio was best samples would be necessary. This technique compares with with pure methanol. A satisfactory oxidant flow was approxi- that of a thermal conductivity detector. mately 5 cc/min. Faster flow increased the tailing while slower Figure 6 is a chromatogram of a typical odorized natural flow showed little change. gas. The gas was odorized with a dimethyl sulfide and tertiary The following optimal conditions are found: Solvent; meth- butyl mercaptan blended odorant. The other mercaptans and anol, oxidant; air at 5 cc/min, quartz tube; 6 cm long, 1 mm hydrogen sulfide detected were present before odorization. i.d. packed with 2 mm of quartz wool, furnace temperature; Figure 7 is a chromatogram of a high pressure light conden- variable depending on the sample. Using these conditions and sate liquid sample. The sample was vaporized and introduced a furnace temperature of 750°C the linearity of response was into the gas sample loop. If one wishes to distinguish between investigated. Hydrogen sulfide and methyl mercaptan were simultaneous eluting mercaptan and sulfides one can pass the checked and both were linear up to 30 ppm. Dimethyl sulfide sample through caustic before injection. The caustic removes standards at 30 ppm were linear up to 800° C furnace tempera- the carbon dioxide, hydrogen sulfide and the mercaptans. ture, but beyond 800°C the linearity was lost. Table I is the weight percent hydrocarbon analysis of the Figure 4 is a chromatogram of ethyl mercaptan at 500 ppb condensate.

JOURNAL OF CHROMATOGRAPHIC SCIENCE«VOL. 15 DECEMBER 1977»543 3 I \

t 2

5 5

4-

.1 Downloaded from https://academic.oup.com/chromsci/article/15/12/541/349030 by guest on 02 October 2021 3- y- :j .: 1 ' 8 !— 'I

•-•T • 1 2 MOLE % CARBON DIOXIDE

• : Figure 5. Linearity of carbon dioxide at mole % levels. ll Column: 3ft x V* in stainless steel Porapak QS \ 3 •••%« Carrier gas flow 30 cm /min Column oven 40° isothermal Figure 3. Composite chromatograms of 3 ppm methyl mer- Furnace temperature 750°C, sensitivity 1x3 captan showing the effects of quartz wool packing. 1. Furnace temperature 700°C, quartz wool, nitrogen matrix 2. Furnace temperature 700°C, quartz wool, methane matrix 3. Furnace temperature 800°C, quartz wool, nitrogen matrix 4. Furnace temperature 800°C, quartz wool, methane matrix 5. Furnace temperature 800°C, no quartz wool, nitrogen matrix 6. Furnace temperature 800°C, no quartz wool, methane matrix 7. Furnace temperature 700°C, no quartz wool, nitrogen matrix 8. Furnace temperature 700°C, no quartz wool, methane matrix

10 20 MINUTES

Figure 6. Chromatogram of odorized natural gas. Column: 3ft x VA in stainless steel Porapak QS 1O Carrier gas flow 30 cm^/min MINUTES Oven temperature programmed: 70°C-150°C at 10°C/min Furnace temperature 800°C, sensitivity 3x1 1. 96% methane Figure 4. Chromatogram of ethyl mercaptan at 500 ppb. 2. 0.25% carbon dioxide 3. 1.2 ppm hydrogen sulfide Column: 3ft x VA in stainless steel Porapak QS 3 4. 0.1 ppm methyl mercaptan Carrier gas flow 30 cm /min 5. 1.3 ppm dimethyl sulfide Gas sample size: 10 ml. 6. 0.2 ppm isopropyl mercaptan Column oven 110°C isothermal 7. 0.2 ppm normal propyl mercaptan Furnace temperature 750°C, sensitivity 1x1 8. 3.0 ppm tertiary butyl mercaptan

544»DECEMBER1977 JOURNAL OF CHROMATOGRAPHIC SCIENCE»VOL. 15 Table I. Weight % Analysis of Light Hydrocarbon Condensate

c 12.61 x 6.08 c2 c, 11.29 iC4 8.67 nC4 12.16 iCs 8.68 nCs 5.21 9.12 c6 5.86 c7 c. 4.88 c, 5.21 3.58 c10 2.27 cu 1.42 c12 Downloaded from https://academic.oup.com/chromsci/article/15/12/541/349030 by guest on 02 October 2021 1.36 c13 so 0.97 c14 MLN0TES 0.38 c,s 0.25 Figure 7. Chromatogram of a high pressure light condensate c16 Column: 3ft x 1/4 in stainless steel Porapak QS Carrier gas flow 30 cm3/min Oven temperature programmed: 80°C-1506C at 10°C/min Furnace temperature 700°C, sensitivity 3x2 1. Trace carbon dioxide Manuscript received August 12, 1977; 2. Trace hydrogen sulfide revision received October 31,1977. 3. 0.6 ppm methyl mercaptan 4. 5.0 ppm ethyl mercaptan 5. 4.5 ppm isopropyl mercaptan 6. 3.5 ppm normal propyl mercaptan 7. 3.6 ppm tertiary butyl mercaptan References 8. 6.2 ppm other butyl mercaptans 1. CD. Pearson and W.J. Hines. Determination of hydrogen sulfide, carbonyl sulfide, carbon disulfide, and sulfur dioxide in gases and hydrocarbon streams by gas chromatography/flamc photometric detection. Anal. Chem. 49:123-25(1977). Conclusions 2. S.S. Brody and J.E. Chaney. Flame photometric detector. The ap- plication of a specific detector for phosphorus and for sulfur com- The Hall Electrolytic Conductivity Detector has been pounds—sensitive to subnanogram quantities. J. Gas Chromatog. demonstrated to be a useful tool in determining a variety of 4:42-46(1966). trace sulfur compounds in a large hydrocarbon matrix. The 3. R.K. Stevens, J.D. MuUk, A.E. O'Keeffe, and K.J. Krost. Gas detector has a definite advantage over the FPD in that it is chromatography of reactive sulfur gases in air at the parts-per- billion level. Anal. Chem. 43:827-31 (1971). linear and has minimal hydrocarbon interference. The detec- 4. R.C. Hall. A highly sensitive and selective microelectrolytic con- tor has disadvantages in that the resolution of closely eluting ductivity detector for gas chromatography. J. Chromatog. Sci. 12: peaks is not as good as the FPD. Furthermore, the detector in 152-60(1974). the mode discussed has not been demonstrated to be able to 5. T.L.C. de Souza, D.C. Lane, and S.P. Bhatia. Analysis of sulfur- containing gases by gas-solid chromatography on a specially treated determine carbonyl sulfide and . The ability to anal- Porapak QS column packing. Anal. Chem. 47:543-45 (1975). yze for hydrogen sulfide and carbon dioxide with one tech- 6. P. Jones and G. Nickless. Versatile electrolytic conductivity detec- nique is quite useful since these compounds are the principal tor for gas chromatography. /. Chromatog. 73:19-28 (1972). causes of corrosion in natural gas pipelines. 7. R. Hall. Personal communication.

JOURNAL OF CHROMATOGRAPHIC SCIENCE»VOL. 15 DECEMBER 1977-545