1、,Y GPA STD*22b5 68 W 3824699 00002b8 O W GPA Standard 226548 GPA Standard for Determination of Hydrogen Sulfide and Mercaptan Sulfur In Natural Gas (Cadmium Su Ifate-lodometric Titration Method) Adopted as a Tentative Standard, 1965 Revised and Adopted as a Standard, 1968 Reprinted 1974 - 7980 Gas P
2、rocessors Association 181 2 First Place Tulsa, Oklahoma 741 03 COPYRIGHT Gas Processors Association (GPA)Licensed by Information Handling ServicesGPA STD*2265 68 m 3824699 O000269 2 m Non-Mem bers: Single copies . .$ 2.00 1 O or more copies. each 1.50 Members: Single copies 1 . 00 1 O or more copies
3、. each . 75 i i COPYRIGHT Gas Processors Association (GPA)Licensed by Information Handling ServicesGPA STD*2265 68 3824699 0000270 9 GPA. Method for Determination of Hydrogen Sulfide and Mercaptan Sulfur In Natural Gas (Cadmium Sulfate-lodometric Titration Met hod) SCOPE 1. This method is applicable
4、 to the determination of hydrogen sulfide and mercaptan sulfur in natural gas. It is applicable over the concentration range of O to 5 grains hydrogen sulfide and O to 1 grain mercaptan sulfur per 100 CU. ft. of natural gas. OUTLINE OF METHOD 2. A measured volume of natural gas is bubbled through ne
5、utral cadmium sulfate solution to re- move hydrogen sulfide, and then through basic cadmium sulfate to scrub out the mercaptans. The amounts of hydrogen sulfide and mercaptan sulfur in the absorbers are then determined iodometricall y. APPARATUS 3. (a) Gas Washing Bottle - Side inlet, coarse porosit
6、y fritted disc (40-60 microns maxi- mum pore size), and 24/40 outer joint at the outlet. The diameter of the fritted disc is 60 mm, and the approximate O.D. x height of the bottle is 70 x 280 mm (Note 1). Spray Traps - 24/40 inner joint, for use on the outlet side of the gas washing bottle (Note 2).
7、 Test Meter - Dial divisions graduated to 0.01 CU. ft. and accurate to 0.5 per cent when measuring gas volumes from 2 to 10 CU. ft. The meter should be calibrated before use and should be a type that is usable in the field. .(d) Burette - 10-ml, graduated in 0.02 ml, with side fill. (e) Thermometer
8、- Graduated in 1 OF divisions. NOTE i. A riritable bottle is available commercially tinder Corning Catalog No. 3 17jO. NOTE 2. A siritable spray trap is available commercially wider Corning Catalog No. 37721 (specify spray trap only). (f) Aspirator Bulb - Pressure-vacuum, with a short piece of gum r
9、ubber tubing attached to each end. (g) Barometer. (h) Stop Watch. (i) Tubing and Connections - Glass, aluminum, or stainless steel tubing is satisfactory. Con- nections between tubing and apparatus should be made with gum rubber tubing, with a minimum amount of rubber exposed to the gas sample. (j )
10、 Pipettes - Volumetric, Class A, accurate to within tolerances specified by U. S. Bureau of Standards and calibrated to deliver a specified volume. (k) Graduated Cylinders - 200 cc, 50 cc, and 10 cc standard graduated cylinders. (1) Leveling Bottle - For filling burette with sodium thiosulfate solut
11、ions. REAGENTS AND MATERIALS 4. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the Ameri- can Chemical Society, where such specifications are available. References to water indicate dis- tilled or deionized water.
12、 (a) Cadmium Sulfate Solution A. 10 grams of cadmium sulfate hydrate ( 3CdS048H,0) per liter of solution. (b) Cadmium Sulfate Solution B. 140 grams of cadmium sulfate hydrate ( 3CdS048H,0) per liter of solution. (c) Sodium Hydroxide Solution, 0.1 M. (d) Standard Iodine Solutio?, 9.1 N. Weigh 13.0 g
13、of resublimed iodine into a 250-ml beaker. Add 22 g of potassium iodide and 100 ml of of water. Stir until sojution is complete, dilute to 1 liter, mix thoroughly, and store in an amber, glass-stoppered bottle. 3 COPYRIGHT Gas Processors Association (GPA)Licensed by Information Handling ServicesGPA
14、STD*22b5 68 m 3824b 00002L O m Standardize this solution vs. standard 0.1 N sodium thiosulfate not more than one day from the time it is used. (e) Standard Iodine Solutions, 0.05 and 0.01 N. Prepare by exact dilution of the standard 0.1 N iodine solution. (f) Standard Sodium Thiosulfate Solution, 0.
15、1 N. Dissolve 25 g of sodium thiosulfate (Na,S,0,-5H20) in water and add 0.01 g of sodium carbonate to stabilize the solution. Dilute to 1 liter and mix thoroughly. Standardize by any accepted procedure by which the normality may be determined with an accuracy of +- 0.0002. (g) Standard Sodium Thios
16、ulfate Solutions, 0.05 and 0.01 N. Prepare by exact dilution of the 0.1 N standard sodium thiosulfate solution. (h) Starch Solution. A freshly prepared solution of any soluble starch suitable as an iodine indicator may be used. One suitable method of preparation is as follows: make a suspension of 2
17、.5 g of arrow- root starch in a small amount of water. Add the suspension to 1 liter of boiling water and boil for 5 to 10 minutes, allow to cool, and decant the clear portion into glass-stoppered bottles. (i) Hydrochloric Acid, concentrated, sp. gr. ap- proximately 1.19. PROCEDURE 5. (a) Sampling.
18、The gas to be analyzed must be sampled at the source. It is not permissible to transport the gas to the laboratory in a cylin- der of any type, The sampling point should be located directly on the main flow line and the connections preferably a center line tap. Since it is usually unavoidable that t
19、he tap line be of iron, thorough purging of the tap line before sampling is necessary to remove any “dead” gas having a low hydrogen sul- fide content due to reaction with the iron tap line. The connection from the sampling point to the absorption bottle should be as short as possible. Clean all gla
20、ssware with chromic acid solution and rinse with distilled water prior to test. Assemble the absorption train as shown in Figure 1. Add 150 ml of cadmium sulfate solution A or B (see Table I) to each of the three absorbers. Replace the spray traps on the two hydrogen sulfide absorbers. Add 50 ml of
21、0.1 M sodium hydroxide to the third absorber and replace the spray trap. Purge sample line to absorbers. Record the initial test meter reading. Start the flow of gas through the system by opening the con- trol valve on the sampling line. Set the rate not to exceed 6 CU. ft. per hour. Record the temp
22、erature of the test gas several times during the sampling period. The amount of sample required and the concentration of cadmium sulfate used de- pends upon the hydrogen sulfide content of the gas. A close enough approximation of the hydrogen sulfide content for use of this table may be quickly obta
23、ined by means of a hydrogen sulfide detector (Note 3). Other- wise a preliminary run to arrive at the approximate concentration will be necessary. The following volumes and concentrations are recommended: TABLE I Expected HsS Expected Mercaptan Gas Concentration Concentration Sulfur Concentration, V
24、olume Cadmium Grains/100 SCF Grains/100 SCF SCF Suiatc 1 to 2 2 to 3 3 to 4 4 to 5 (b O to 0.25 O to 0.25 14 10 g/liter 0.25 to 0.50 0.25 to 0.50 10 10 g/liter 0.50 to 0.75 0.50 to 0.75 8 10 g/liter 0.75 to 1.0 0.75 to 1.0 6 10 g/liter - 18 140 g/liter - 15 140 g/liter - 12 140 g/liter - 10 140 glli
25、ter After sufficient sample has been passed, stop the flow of gas through the system by shutting the control valve. Disconnect the apparatus. Rinse out spray traps with water and add the rinse water to the absorber. Place rubber policemen over inlet and outlet tubes of the absorber bottles. Record t
26、he final meter reading, the barometric pressure, and the average temperature of the gas that has passed through the meter. Transport the absorption bottles containing the samples to the laboratory, unless it is convenient to titrate in the field. (CAUTION: The titra- tion should never be made in the
27、 sunlight, or at a temperature in excess of 80F.) Titration. Use the following information as a guide in selecting the proper concentration of iodine and sodium thiosulfate solutions. TABLE II Concentration of HnS or Mercaptan Sulfur Concentration, Reagents Grains/100 SCF Recommended O to 0.5 0.01 N
28、 0.5 to 2 0.05 N 2 to 5 0.1 N 4 PROBLEM HARD COPY COPYRIGHT Gas Processors Association (GPA)Licensed by Information Handling ServicesThe Hydrogen Sulfide Absorbers No. 1 and No. 2. Remove outlet tubes, add 20 ml of iodine solution by pipette (see Table II for recommended concentra- tion) and replace
29、 the tube and cap. Mix the contents well by rocking and shak- ing the bottle several times, and by forcing the solution through the disc and up into the inlet bubble trap, using the aspirator bulb attached to the outlet tube. Again remove the outlet tube and cap and add 50 ml of the concentrated hyd
30、rochloric acid. Quickly replace the cap and mix contents by gently rocking the scrubber. Let sit for 15 minutes. The complete removal of cadmium sul- fide from the pores of the fritted glass disc is expedited by causing the solution of the acid and iodine to pass repeatedly through the plate, using
31、the aspirator bulb. Make certain that an excess of iodine is present throughout the mixing procedure, as is evidenced by the yellow iodine color. Titrate the excess iodine with sodium thiosulfate of matching normality, adding the starch solution as an indicator just before the equivalent point is re
32、ached. The titrations must be conducted in the absorption vessel, with the contents being agitated by alternately forcing air through the disc from the inlet side and forcing the solution down into the lower chamber and up into the inlet bubble trap. Record burette read- ings to the nearest .O1 ml.
33、The Mercaptan Scrubber No. 3. Follow the same procedure described for ab- sorbers No. 1 and No. 2, except add 50 ml of HC1 without agitation, immediately before adding the iodine solution. Shake well and let stand for 15 minutes. Com- plete the titration as described for the hydrogen sulfide absorbe
34、rs. Reagent Blanks (a) Hydrogen Sulfide. Make a blank determination at least once a day and also when new solutions are prepared. Put 150 ml of cadmium sulfate solution from the same batch used for absorption of hydrogen sul- fide in a clean absorption bottle and add 10 ml of iodine. Acidify with 50
35、 ml of concentrated hydrochloric acid and mix thoroughly. Back titrate with sodium thiosulfate using the aspirator bulb for mixing and the starch solution as an indicator as specified in Paragraph 1. Calculate the amount of thiosulfate that would be required to use up the 10 ml of iodine by the rela
36、tion V2 = N,V,/N2 where N, = normality of iodine, VI = volume of iodine, N2 = normality of thiosulfate, and V2 = volume of thiosulfate. If the amount of thio- sulfate used in the blank determina- tion is greater than this value the difference in the two values should be subtracted as “ml blank” in t
37、he calculations, paragraph 6. If the amount of thiosulfate used in the blank determination is less than this value, the difference in the two values should be added as “ml blank” in the calculations. Mercaptan Sulfur. Determine a sec- ond blank using the same solutions as for hydrogen sulfide but wi
38、th the addition of 50 ml 0.1 M sodium hydroxide. CALCULATIONS 6. Calculate the concentrations of hydrogen sulfide and mercaptan sulfur as follows: = Ist absorber + 2nd absorber Grains mercaptan sulfur/100 CU. ft. = 1 (I2 N,) - (T-N,) (F) (B) (.495x100) v (B-P,) Where: Il = ml iodine added to first a
39、bsorber or second absorber I, = ml iodine added to third absorber NI = normality of iodine T, = ml thiosulfate Irr ml blank, first absorber T, = ml thiosulfate * ml blank, third absorber N, = normality of thiosulfate or second absorber 460 + t 520 F = temperature correction factor = t = temperature
40、of gas at meter, Fahrenheit V = uncorrected volume of sample, CU. ft. B = barometric pressure, inches mercury P, = vapor pressure of water at temperature t, .263 = gr hydrogen sulfideSCF .495 = gr mercaptan sulfur ;SCF inches mercury 5 COPYRIGHT Gas Processors Association (GPA)Licensed by Informatio
41、n Handling ServicesGPA STDx2265 68 m 3824699 0000273 4 m PRECISION 7. The following data should be used for judging the acceptability of results (95 per cent probability) : (u) Repeatability-Duplicate results by the same operator should not be considered suspect unless they differ by more than the f
42、ollowing amounts for the indicated concentration ranges: Concentration, grains/100 CU. ft. Repeatability 0.1 . 0.03 0.5 . 0.06 1.0 . 0.11 3.0 . 0.26 5.0 0.36 GAS WASHING BOTTLES I50 ML 150ML Cd SO4 CdS04 (b) Reproducibilitg-The result submitted by each of two laboratories should not be considered su
43、spect unless the two results differ by more than the following amounts for the indicated concentration ranges: Concentration, grains/100 CU. ft. Reproducibility 0.1 0.04 0.5 0.09 1.0 0.15 3.0 0.36 5.0 0.50 METER COARSE FRITTED /DISK I50 ML CdSO4 PLUS 50 ML 0.1 M NaOH Figure 1 Diagram of Apparatus 6 COPYRIGHT Gas Processors Association (GPA)Licensed by Information Handling Services
