uop HYDROGEN SULFIDE, MERCAPTAN SULFUR, AND CA RBONYL SULFIDE IN HYDROCARBON HYDROCARBON GASES BY POTENTIOMETRIC TITRATION UOP Method 212-05 SCOPE This method is for determining hydrogen sulfide (H 2S), mercaptan sulfur (RSH) and carbonyl sulfide (COS) in gaseous hydrocarbons and in typical liquefied petroleum gas (LPG) consisting of C 3 and/or C4 hydrocarbons. Also covered is the determination determination of mercaptan sulfur in LPG LPG which may contain a wide range of hydrocarbon types ranging from ethane to such gasoline boiling range hydrocarbons as pentane pentane and hexane. Each sulfur type can be determined determined from less than 1 to several thousand mass-ppm sulfur (less than 0.1 to several thousand grains per 100 cu ft).
REFERENCES ASTM Method D 1070, “Relative Density of Gaseous Fuels,” www.astm.org ASTM Method D 6667, “Total Volatile Sulfur in Gaseous Hydrocarbons and Liquefied Petroleum Gases by Ultraviolet Fluorescence,” www.astm.org Bruss, D.B., Wyld, G.E.A., and Peters, E.D., Anal. Chem., 29, 807 (1957) Handbook of Chemistry Chemistry and Physics; www.crcpress.com
Lykken, L., and Tuemmler, F.D., Ind. Eng. Chem., Anal. Ed., 14, 67 (1942) Tamele, M.W., Ryland, L.B., and Irvine, V.C., Ind. Eng. Chem., Anal. Ed., 13, 618 (1941) UOP Method 163, “Hydrogen Sulfide and Mercaptan Sulfur in Liquid Hydrocarbons,” www.astm.org UOP Method 516, “Sampling and Handling Gasolines, Distillate Fuels, and of C 3-C4 Fractions,” www.astm.org UOP Method 948, “Relative Density of Gas Mixtures by Calculation from Composition,” www.astm.org UOP Method 999, “Precision Statements in UOP Methods,” www.astm.org IT IS THE USER'S RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TO DETERMINE THE APPLICABIL ITY OF REGULATORY LIMITATIONS PRIOR TO USE. USE. EFFECTIVE HEALTH AND SAFETY PRACTICES ARE TO BE FOLL OWED WHEN UTILIZING THIS PROCEDURE. PROCEDURE. FAIL URE TO UTILIZE UTILIZE THIS PROCEDURE IN THE MANNER PRESCRIBED PRESCRIBED HEREIN CAN BE HAZARDOUS. MATERIAL SAFETY DATA SHEETS (MSDS) OR EXPERIMENTAL MATERIAL SAFETY DATA SHEETS (EMSDS) FOR ALL OF THE MATERIALS USED IN THIS PROCEDURE SHOULD BE REVIEWED FOR SELECTION OF THE APPROPRIATE PERSONAL PROTECTION EQUIPMENT (PPE). © COPYRIGHT 1959, 1968, 1972, 1977, 2003, 2004, 2005 UOP LLC. All rights reserved. Nonconfidential UOP Methods are available from ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United United States. The UOP Methods may be obtained through through the ASTM website, www.astm.org, www.astm.org, or by contacting Customer Service at
[email protected],
[email protected], 610.832.9555 FAX, or 610.832.9585 PHONE.
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OUTLINE OF METHOD The sample, taken either from a sample cylinder or directly from a refinery stream according to the procedures described in UOP Method 516, is scrubbed first through a potassium hydroxide solution and then through a monoethanolamine solution. solution. The potassium hydroxide solution solution contains chelating agents which inhibits the oxidation oxidation of sulfur compounds by chelating heavy metals. A potentiometric titration of the absorbed hydrogen sulfide and mercaptan sulfur in the potassium hydroxide solution follows, using either an aqueous or an alcoholic silver nitrate titrant. The monoethanolamine monoethanolamine solution, which contains the absorbed carbonyl sulfide is titrated potentiometrically with alcoholic silver nitrate in an acidic titration solvent. The electrode system for both titrations is a silver/silver sulfide electrode with a glass reference electrode. The concentration of each analyzed analyzed component is calculated from the titration titration curve. Either an automatic automatic (preferred) or a manually-operated manually-operated titrator may be used.
APPARATUS APPA RATUS References to catalog numbers and suppliers are included as a convenience to the method user. Other suppliers may be used if equivalent performance can be obtained. In addition to the apparatus listed below, all of the sampling-related apparatus specified in UOP Method 516 for C 3-C4 fractions is also required for this method. Balance, capable of weighing 5 kg to the nearest 0.5 g Balance, readability 0.1-mg Barometer , Fisher Scientific, Cat. No. 02-406
Instruments, Cat. No. 020212209, two or more required required Beakers, electrolytic, 250-mL, Brinkmann Instruments, 4000-mL, Fisher Scientific, Cat. No. 02-583G. Drill a hole in the bottom bottom to Beaker, stainless steel, 4000-mL, fit the selected neoprene stopper. Supply, Cat. No. No. MMM-96 Cleaning pad , synthetic, mildly abrasive, Scotch-Brite ™, Runco Office Supply, Brinkmann Instruments, Cat. No. 020948507. 020948507. Electrode, combination silver/glass titrode, Brinkmann electrode should be dedicated to sulfur analysis.
The
Fisher Scientific, Cat. Nos. 10-210-5E, 10-210-5E, -5F, Flasks, volumetric, Class A, 250-, 500-, and 1000-mL, Fisher and -5G, respectively Flask, volumetric, Class A, amber, for light sensitive materials, 100-mL, Fisher Scientific, Cat. No. 10-229C Funnel, separatory, 250-mL, Fisher Scientific, Cat. No. 10-437-5C Gas washing bottle, 125-mL. The type fitted with sintered glass disk of coarse porosity is suitable and can be obtained commercially, Reliance Glass, Cat. No. LG-3761-100, or, Fisher Scientific, Cat. No. 03-040A. Because light is the most serious serious factor affecting recovery of carbonyl sulfide, bottle with black tape. In order to view the bubbling, bubbling, a vertical completely mask the gas washing bottle slit, 2-3 mm wide, may be cut in the tape, which can taped over except when needed. Gas washing bottle, 250-mL, Reliance Glass, Cat. No. LG-3690-110 Gas washing bottles, 250-mL. The type fitted with a sintered glass gas-distribution plate of coarse porosity is suitable and this type can be obtained commercially, Reliance Glass, Cat. No. LG212-05
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3762-102, or Fisher Scientific, Cat. No. 03-040B, two required. The maximum gas flow rate for this type of gas washing bottle is about 30 L/hr (1 cu ft/hr). If no COS scrubber is connected downstream of the caustic scrubbers, an alternative gas washing bottle, suitable for a higher flow rate, may be used. This type is fitted with a perforated disk gas distributor (see Figure 1), and is available on special order from Reliance Glass. The rate with this distributor can be as high as 90 L/hr (3 cu ft/hr), provided that no COS scrubber is connected downstream from the caustic scrubber . The scrubbing rate for the COS scrubber must not exceed 30 L/hr .
Gauge, pressure, stainless steel, 0 to 2800 kPa gauge (0 to 400 psig) range, Matheson Tri-Gas, Cat. No. 63-2242, with adapter to fit the specific sample cylinders used Graduated cylinders, 5-, 100-, and 1000-mL, Fisher Scientific, Cat. No. 08-550A, -550E, and 550H, respectively Laboratory expansion valve, Swagelok Type SS-3NRS4 (regulating stem 316 stainless steel valve and fittings), Swagelok. Wrap the valve with electrical heating tape and provide a variable transformer to control the temperature of the tape. (See Figure 2 for laboratory expansion valve apparatus arrangement.) To ensure that the valve provides a smooth, regular flow without any blockages, it must be cleaned regularly, by rinsing with water,. Additional maintenance requires either a rebuild kit, returning to the manufacturer for repair, or replacement.
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Pipets, Class A, 1-, 2-, 3-, 5-, 10-, 15-, 25-, and 50-mL, Fisher S cientific, Cat. Nos. 13-650-2B, -2C, -2D, -2F, -2L, -2M, -2P, and -2S, respectively Pipet , Mohr, 5-mL, Fisher Scientific, Cat. No. 13-665K Pipet filler , Fisher Scientific, Cat. No. 03-692-35 Refinery expansion valve, Swagelok Type SS-3NRS4 (regulating stem 316 stainless steel valve and fittings), Swagelok. Since the use of electrically-heated expansion valves is not permitted in refineries, use a hot water heating system for vaporizing an on-stream sample. (See Figure 3 for refinery expansion valve apparatus arrangement.) To ensure that the valve provides a smooth, regular flow without any blockages, it must be cleaned regularly, by rinsing with water. Additional maintenance requires either a rebuild kit, returning to the manufacturer for repair, or replacement. Regulator , nitrogen, two-stage, high-purity, delivery pressure range 15-200 kPa (2-30 psi), Matheson Tri-Gas, Model 3121-580 Stoppers, neoprene, sizes to fit specific apparatus. Fisher Scientific, Cat. No. 14-141*. Bore holes as necessary using Boring Machine, Fisher Scientific, Cat. No. 07-855. *Select sizes to fit containers. Stopwatch, Fisher Scientific, Cat. No. 14-648-3 Tape, heating, electrical, Fisher Scientific, Cat. No. 11-463-54A Tape, polyethylene, black, Fisher Scientific, Cat. No. 11-866-2 Thermometer or thermocouple, capable of reading heated expansion valve temperature of 38-43ºC (100-110ºF) 212-05
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Titrator, potentiometric, recording, ±2000-mV range, 1-mV resolution, capable of reducing the titration rate to a minimum in the vicinity of the endpoint, with dispenser having a volume readout of 0.00 - 99.99 mL, 0.0001 of the buret volume resolution, Metrohm Model 836 Titrando system with optional sample changer, and 20-mL buret, Brinkmann Instruments Transformer, variable, Fisher Scientific, Cat. No. 11-472-76 (for 120V) Tubing, rubber, 3/16” ID, Fisher Scientific, Cat. No. 14-167C Tubing, stainless steel, ¼” OD, Alltech Associates, Cat. No. 30306ST Wash bottle, for 2-propanol, Fisher Scientific, Cat. No. 03-409-20D Wet test gas meter or calibrated gas receiver, 225 L/hr capacity, Meters & Controls, American Meter, Cat. No. AL 17-1. An equivalent meter, calibrated in cu ft, is available as Cat. No. AL 17, see APPENDIX .
REAGENTS AND MATERIALS References to catalog numbers and suppliers are included as a convenience to the method user. Other suppliers may be used. References to “water” mean double deionized or distilled water purged with nitrogen for 10 to 15 minutes before use to remove dissolved oxygen. Unqualified references to solutions mean aqueous solutions. In addition to the reagents and materials listed below, the reagents and materials specified in UOP Method 516 for C 3-C4 fractions are also required for this method. Acetic acid , glacial, Certified ACS Plus, Fisher Scientific, Cat. No. A38-212
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Acid titration solvent. Dissolve 2.7 g of sodium acetate trihydrate in 20 mL of water and 975 mL of anhydrous ethyl alcohol. Add 4.6 mL of glacial acetic acid to the solution. Purge the solution with nitrogen for 10 to 15 minutes prior to use to remove dissolved oxygen. Ammonium hydroxide, concentrated, Certified ACS Plus, Fisher Scientific, Cat. No. A669-212 Cadmium chloride, Certified ACS, Fisher Scientific, Cat. No. C10-500 Cadmium chloride solution. Dissolve 100 g of cadmium chloride in 500 mL of water. Add 10 mL of concentrated hydrochloric acid and make up to a final volume of 1 L with water. Purge with nitrogen for 10 to 15 minutes before using. Chelating solution, a solution of chelating agents found effective in reducing the deterioration of sulfides and mercaptides in caustic solutions. The recommended usage is 2 mL/100 mL of caustic solution. This chelating solution is made up from the constituent chemicals:
Diethylenetriamine pentaacetic acid, sodium salt (Na5DTPA), Acros Organics, Fisher Scientific, Cat. No. 40729-0010 (40% aqueous solution) Diethylenetriamine pentaacetic acid, sodium salt , 20 mass-% solution. Dilute diethylenetriamine pentaacetic acid, sodium salt, 40% aqueous solution, with water to make a 20 mass% solution. N-(2-hydroxyethyl) ethylenediaminetriacetic acid, sodium salt , (Na3HODTA), Aldrich, Cat. No. 16,153-5, or Acros Organics, Fisher Scientific, Cat. No. 34870-0010 N-(2-hydroxyethyl) ethylenediaminetriacetic acid, sodium salt, 20 mass-% solution. Dissolve N-(2-hydroxyethyl) ethylenediaminetriacetic acid, sodium salt, in water to make a 20 mass-% solution. Ethylenediaminetetraacetic acid, sodium salt (Na 4EDTA), Aldrich Chemical, Cat. No. E2,6290, or, Acros Organics, Fisher Scientific, Cat. No. 14786-5000 N,N-bis(2-hydroxyethyl)glycine (Bicine), Aldrich Chemical, Cat. No. 16,379-1, or, Acros Organics, Fisher Scientific, Cat. No. 17265-1000
Mix 1:1 by weight ethylenediaminetetraacetic acid, sodium salt, and N,N-bis(2-hydroxyethyl) glycine. Dissolve in water to make a 20 mass-% solution. Mix equal volumes of each of the three 20 mass-% solutions to make the final chelating solutiuon. Detergent , LiquiNox, Fisher Scientific, Cat. No. 04-322-15B Ethyl alcohol, anhydrous, spectro grade, Acros Organics, Cat. No. 22409-5000 Hydrochloric acid , concentrated, Certified ACS Plus, Fisher Scientific, Cat. No. A144S-500 Lead acetate test paper , Fisher Scientific, Cat. No. 14-862 Leak test solution, soap solution, Snoop, Swagelok, Cat. No. MS-SNOOP-8OZ Monoethanolamine (ethanolamine), 99%, Acros Organics, Cat. No. 14958-0025 Monoethanolamine, 5 vol-% solution in ethyl alcohol. Prepare by pipetting 5 mL of monoethanolamine into an amber 100-mL volumetric flask. Fill to the mark with ethyl alcohol. Store in a dark place. Purge with nitrogen for 10 to 15 minutes before using. Discard after one day. Nitrogen, high purity, 99.99% 212-05
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Potassium hydroxide, 50 w/v-% solution, Fisher Scientific, Cat. No. LC19260-2 Potassium hydroxide, 40 w/v-% solution. Prepare by diluting four parts of 50 w/v-% solution with one part of water. Purge with nitrogen for 10 to 15 minutes before using. Use only for samples high in hydrogen sulfide. Potassium hydroxide, 10 w/v-% solution, Fisher Scientific, Cat. No. LC19200-2 nitrogen for 10 to 15 minutes before using.
Purge with
2-Propanol, Certified ACS grade, Fisher Scientific, Cat. No. A416-4. Purge with nitrogen for 10 to 15 minutes before using. Silver nitrate, 0.1- M solution, standardized at 25ºC against NIST potassium chloride, lot analysis supplied, Fisher Scientific, Cat. No. SS72-4 Silver nitrate, 0.01- M standard aqueous solution. Prepare by pipetting 50 mL of the 0.1- M solution into a 500-mL volumetric flask. Dilute to the mark with water, cap and invert several times to mix thoroughly. The molarity of this solution is one-tenth the molarity of the purchased, nominally 0.1- M solution. Silver nitrate, 0.01- M standard alcoholic solution. Prepare by pipetting 50 mL of the 0.1- M solution into a 500-mL volumetric flask. Dilute to the mark with 2-propanol, cap and invert several times to mix thoroughly. The molarity of this solution is one-tenth the molarity of the purchased, nominally 0.1- M solution. Sodium acetate trihydrate, Certified ACS grade, Fisher Scientific, Cat. No. S209-500 Sodium carbonate, 5 w/v-% solution, Fisher Scientific, Cat. No. LC22970-2. Purge with nitrogen before using. Sodium hydroxide, 20 w/v-% solution, Fisher Scientific, Cat. No. LC24090-2 Sodium sulfide, 3 w/v-% solution, Fisher Scientific, Cat. No. LC24920-2 Wipers, Kimwipes, Ex-L, Fisher Scientific, Cat. No. 06-666A
SAMPLING Refer to UOP Method 516, Sampling and Handling of Gasolines, Distillate Fuels, and C 3-C4 Fractions, for the required sample cylinders (Silcosteel recommended), their preparation and cleaning, and sampling procedures for C 3-C4 fractions.
SCRUBBING OF GASES If analysis of COS is not required, the COS scrubber may be eliminated, and, if desired, scrubbers with perforated disk distributors may be used at the higher flow rate. In CALCULATIONS , it is assumed that the gas is saturated with water when metered. For greatest accuracy, it is advisable to provide a water saturator between the last scrubber and the meter.
Scrubbing in t he Laboratory CAUTION : This method involves venting a pressurized cylinder into vessels containing a caustic solution. The pressure in the LPG cylinder should be checked with a pressure gauge prior to scrubbing to ensure that the cylinder is under LPG vapor pressure only. Additional pressure may
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deliver too high a flow rate into the caustic scrubbing solution and cause splashing. Also see NOTES AND PRECAUTIONS . •
Special care should be taken when handling the LPG during the scrubbing procedure to avoid caustic splashing. Use a face shield during the scrubbing procedure.
All scrubbing operations should include the use of the chelating solution, 2 mL for each 100 mL of potassium hydroxide, in order to reduce the deterioration of sulfides and mercaptides in caustic solutions. Scrubbing in a laboratory is carried out by mounting the sample cylinder and adapter (consisting of connecting lines, expansion valve, and stainless steel tubing) vertically over the scrubber and connecting it as shown in Figure 2. 1. Equip the first scrubber with a 2-hole neoprene stopper. One hole is to accept the stainless steel tubing on the sample cylinder and the other is for the stainless steel tubing vent into the next scrubber in the train. 2. Place the appropriate solutions in the gas washing bottles of the scrubbing train. See following sections for specific solutions for each application. 3. Connect the last scrubber to a wet test meter to measure the volume and rate of scrubbing. Vent the flow out of the wet test meter into a hood or other safe area. •
Use a minimum length of rubber tubing for all connections, consistent with being able to connect the inlet and the outlet of a scrubber with one piece of rubber tubing to maintain a closed system.
4. Purge the connected scrubbers with nitrogen for 10 min. 5. Mount the adapter vertically above the first scrubber. 6. Place the LPG sample cylinder in a vertical position in a hood or well-vented area. If the cylinder has an outage tube, the outage tube must be at the top. Attach the gauge to the valve at the top of the cylinder and check the pressure in the cylinder. See NOTE 1. Briefly open the bottom valve (A) to check that no water or sediment is present in the LPG. If water or sediment is determined to be present, discontinue the analysis and obtain a clean sample. •
LPG samples are usually contained in a cylinder having valves on both ends or, in some cases, a cylinder where one of the valves is connected to an eductor tube. If the sample cylinder contains an eductor tube, invert the cylinder (both valves on the bottom) and briefly open the valve not connected to the eductor tube to check that no water or sediment is present.
7. Weigh the sample cylinder to the nearest 0.5 g. 8. Position the cylinder directly over the adapter and connect it to the adapter. Pass the stainless steel tubing through the remaining hole in the neoprene stopper to a depth of 1 cm from the bottom of the scrubber. 9. Turn on the power to the heating tape and adjust the transformer so that the temperature of the gas expansion valve is 38-43ºC. 10. Record the initial wet test meter readings (L) as well as the wet test meter temperature (°C) and the atmospheric pressure (mbar). 11. Ensure that the gas expansion valve is fully closed.
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12. Open the lower valve of the sample cylinder all the way. Check for leaks at the fittings using the leak test solution. 13. Slowly open the gas expansion valve and start the flow of sample through the scrubber and wet test meter. Adjust the flow of gas so as not to exceed 30 L/hr (1.5 L in 3 minutes). (Excess flow can cause the caustic scrubbing solution to splash). If the gas washing bottles are of the type fitted with a perforated disk gas distributor, and no COS scrubber is connected downstream from the caustic scrubber, the rate can be as high as 90 L/hr. 14. After the appropriate volume of sample has been scrubbed, turn off the heat and close the lower valve of the sample cylinder. 15. After gas bubbles cease to come out of the tip of the adapter, disconnect the adapter from the cylinder. Raise the adapter just high enough so that its tip is out of the scrubber solution and wash the inside of the adapter with about 5 ml of water, collecting the washings in the scrubber solution. 16. Remove the adapter from the scrubber and disconnect the vent line to the next scrubber. Cap the first scrubber with a neoprene stopper. Disconnect the remaining scrubbers and quickly connect the inlet of each scrubber to its own outlet with the rubber tubing used to connect the scrubbers, thus minimizing exposure to air. 17. Record the volume of the gas metered. Reweigh the sample cylinder to the nearest 0.5 g to determine the weight of sample scrubbed (see NOTES AND PRECAUTIONS ).
Scrubbing in the Refinery Scrubbing in the refinery follows the same procedures as scrubbing in the laboratory except for the following changes. When the scrubbing of a gas or LPG sample is to be carried out in a refinery, an electrically-heated expansion valve cannot be used because of the danger of explosion. Therefore, an expansion valve heated by warm water is employed. Figure 3 shows the vaporization apparatus used in on-stream scrubbing at the refinery. Follow all applicable refinery safety practices, modifying the apparatus and procedure if necessary. The heat necessary to warm the water is supplied by refinery steam lines. A cold-water jacket, located upstream of the expansion valve, is necessary when using warm water to heat the expansion valve in order to avoid conductive heating of the line upstream from the valve. The cold water for the cooling jacket is supplied by the refinery water lines. Determine the weight of the sample from the volume of gas scrubbed as measured by the wet test meter. Vent the meter in a safe area. The following sample types (A through D) are typical of those found in various refinery streams. Procedures can be modified for other sample types as necessary.
Samples Containing Relatively High Hydrogen Sulfide and Carbonyl Sulfide Concentrati ons, and Low Mercaptan Concent ration (Sample Type A) For such samples as gases from catalytic cracking that contain in excess of 1000 ppm of hydrogen sulfide and some carbonyl sulfide, more concentrated solution of potassium hydroxide is required. The low (less than 100 ppm) mercaptan concentration is typically not measured. If it is necessary to measure the low mercaptan concentration, repeat the analysis using the procedure in Sample Type C. 1. Place 50 mL of 40 w/v-% potassium hydroxide solution and 1 mL of the chelating solution into each of two 250-mL scrubbers and mix by swirling. 212-05
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The chelating solution is required to prevent loss of sulfide.
2. Place 20 mL of 5 vol-% alcoholic monoethanolamine solution into the third scrubber (125-mL), which has been masked in black tape to exclude light. 3. Scrub about 60 L (2 cu ft) of gas. 4. Titrate the monoethanolamine scrubber solution as soon as possible after a sample has been scrubbed (see PROCEDURE ). Combine the two potassium hydroxide scrubber solutions and titrate the same day (see PROCEDURE ).
Samples Containi ng Relativ ely Low Hydro gen Sulfide, Mercaptan, and Carbon yl Sulfide Concentr ations (Sample Type B) For purposes of this method, low concentrations of sulfur types are generally considered to be less than 100 ppm. 1. Place 100 mL of 10 w/v-% potassium hydroxide solution and 2 mL of the chelating solution into each of two 250-mL scrubbers and mix by swirling. 2. Place 50 mL of 5 vol-% alcoholic monoethanolamine solution into the third scrubber (125-mL), which has been masked in black tape to exclude light. 3. Scrub about 300 L (10 cu ft) of gas. 4. Titrate the monoethanolamine scrubber solution as soon as possible after a sample has been scrubbed (see PROCEDURE ). Combine the two potassium hydroxide scrubber solutions and titrate the combined solution the same day (see PROCEDURE ).
Samples Containi ng Carbonyl Sulf ide, Relatively High Concentrations of Hydrogen Sulfide, and Low Concentrations of Mercaptan Sulfur (Sampl e Type C) For purposes of this method, high concentrations of sulfur types are generally considered to be greater than 1000 ppm, and low concentrations of sulfur types are generally considered to be less than 100 ppm. Mercaptan analysis: In some cases, samples will have a high ratio of hydrogen sulfide to mercaptan sulfur. Where this condition exists, the accuracy of the mercaptan sulfur determination may be affected adversely if additional scrubbers are not placed in the sampling train before the potassium hydroxide scrubbers. 1. Charge the first 250-mL scrubber downstream from the expansion valve with 100 mL of 5 w/v% sodium carbonate solution. Charge the second 250-mL scrubber with 100 mL of 5 w/v-% sodium carbonate solution. 2. Follow these 2 scrubbers with an empty scrubber which serves as a knockout trap, then follow with two 250-mL gas scrubbers each containing 50 mL of 40 w/v-% potassium hydroxide solution and 1 mL of chelating solution. 3. Scrub approximately 300 L (10 cu ft) of the gas.
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4. Combine the two potassium hydroxide scrubber solutions and titrate the combined solution the same day as described in the PROCEDURE . A separate analysis must be carried out to determine the respective concentrations of hydrogen sulfide and carbonyl sulfide in samples of this type. The sampling procedure is identical to that described previously for gases which are relatively high in hydrogen sulfide (Sample type A).
Samples Containing Mercaptan Sulfur Only and Appreciable Concentratio ns o f Pentane and Higher B oili ng Materials (Sample Type D) For purposes of this section, “appreciable concentrations” of pentane and higher boiling materials are indicated when, after the scrubbing is complete, there is at least a 5-10 mL hydrocarbon layer present in the scrubber. 1. Place 100 mL of 10 w/v-% potassium hydroxide solution and 2 mL of the chelating solution in each of two 250-mL scrubbers and mix by swirling. 2. Scrub about 60 L (2 cu ft) of gas. 3. If there is a hydrocarbon layer present in the scrubber after sampling, remove the hydrocarbon layer by using a nitrogen-purged separatory funnel. 4. Return the aqueous portion to the scrubber and replace the bubbler to exclude air. 5. Analyze the hydrocarbon phase for mercaptan sulfur using UOP Method 163. Use the entire weight of sample scrubbed as the sample weight in the calculation of mercaptan sulfur in accordance with UOP Method 163. 6. Titrate the aqueous phase as described in the PROCEDURE . 7. Sum the results of the titration and U163 to obtain the total mercaptan content.
PROCEDURE The analyst is expected to be familiar with general laboratory practices, the technique of titration, and with the equipment being used. Caustic solutions containing sulfur compounds are quite susceptible to oxidation in air. The solutions should, therefore, be protected by blanketing with nitrogen or other inert gas. Titrate solutions immediately after they are ready.
Silver/Glass Electrode Preparation and Reconditioning Proper electrode preparation is essential to obtain reproducible and noise-free titration curves having good endpoints. When in use, the electrode should be reconditioned on a weekly basis. An electrode should be dedicated to sulfur analysis. If it is used for any other titrations, it must be reconditioned before use. Prepare and recondition the silver-silver sulfide electrode as follows: 1. Clean the silver surface with a cleaning pad. Rinse with water and dry. 2. Immerse the electrodes in a solution containing 100 mL of 2-propanol, 1 mL of ammonium hydroxide, and 3 mL of 3 w/v-% sodium sulfide solution.
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3. While stirring at a moderate speed, slowly add approximately 10 mL of 0.01- M silver nitrate solution over a period of 10 minutes. A film of silver sulfide will be deposited on the silver electrode. 4. Wipe the excess silver sulfide from the electrode with a wiper. •
The electrode should be cleaned after each titration by rinsing with water.
Hydrogen Sulfide and Mercaptans 1. Prepare the titrator for operation according to the instrument manufacturers instructions, with the specified electrode in place and 0.01- M silver nitrate as the titrant. •
Alcoholic silver nitrate may also be used for the sulfide and mercaptide titration to avoid changing titrant for the carbonyl sulfide titration.
2. Check the contents of the caustic scrubbers qualitatively using lead acetate paper. If there is no perceptible coloration of the paper, transfer the contents of the scrubbers into a 250-ml electrolytic beaker. Proceed with Step 3. If the lead acetate paper shows any coloration, quantitatively transfer the contents of the caustic scrubbers into a 250-ml volumetric flask. Proceed with Step 4. 3. Wash the scrubbers (bottles and bubblers, not the rubber hose) with water and add the washings to the same beaker. Add approximately 1 mL of concentrated ammonium hydroxide. Proceed with Step 6. 4. Wash the scrubbers (bottles and bubblers, not the rubber hose) with water and add the washings to the volumetric flask. Dilute to volume with water and mix by swirling. Again, check the diluted solution with lead acetate paper and use Table 1 as a guide in selecting the proper aliquot size. •
Caustic solutions containing high levels of mercaptide and low levels of sulfide are titrated separately for the two components. A sample is titrated as above to the mercaptide endpoint, and then a larger sample is titrated only to the sulfide endpoint. The mercaptide concentration is determined by difference.
•
If the ratio of sulfide sulfur to mercaptide sulfur is greater than 10:1, then the mercaptide result is considered to be qualitative.
Table 1 Selection of Aliquot Size Color o f lead acetate paper after testing the diluted scrubber solution dark brown brow n light brown
Al iq uo t t o b e tak en fo r titration, mL 1 to 5 5 to 20 20 to 50
5. Pipet an aliquot into an electrolytic beaker. Add approximately 1 mL of concentrated ammonium hydroxide. Dilute to about 50 mL with water. 6. Titrate with 0.01- M silver nitrate, automatically or manually (see Manual Titration) depending on the equipment available. Figure 4 shows three typical titration curves for samples containing mercaptan only, hydrogen sulfide only, and both mercaptan and hydrogen sulfide in the same sample. Samples containing only mercaptan will show an inflection in their titration curves at about 0.0 to +0.3V (Curve A). Samples 212-05
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containing only hydrogen sulfide will show an inflection in their titration curves at about -0.4 to +0.2V (Curve B). When both hydrogen sulfide and mercaptan are present, 2 inflections (Curve C) will be seen. The first inflection, at about -0.4 to -0.2V. indicates hydrogen sulfide and the second, at about 0.0 to +0.3V, indicates mercaptan.
Mercaptans Only 1.
Prepare the titrator in the same manner as for the sulfide and mercaptan titration.
2.
Transfer the caustic scrubber solution from each of the two scrubbers to two separate 250-mL electrolytic beakers. Wash each scrubber (bottles and bubblers, not the rubber hose) with 50 mL of water and add the water washings to each respective beaker. Add approximately 1 mL of concentrated ammonium hydroxide to each of the two beakers.
3.
Using 0.01- M silver nitrate as the titrant, titrate automatically or manually (see Manual Titration) depending on the equipment available.
Figure 4 shows a typical titration curve for samples containing only mercaptan sulfur (Curve A).
Carbonyl Sulfide 1.
Prepare the titrator in the same manner as for the sulfide and mercaptide titration.
2.
Transfer the monoethanolamine scrubber solution to an electrolytic beaker. Wash the scrubber (bottle and bubbler, not the rubber hose) with two 50-mL portions of the acid titration solvent and add the washings to the electrolytic beaker. 212-05
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3.
Using 0.01- M alcoholic silver nitrate, titrate automatically or manually (see Manual Titration) depending on the equipment available.
Figure 5 shows a typical titration curve for samples containing carbonyl sulfide. The inflection point on a carbonyl sulfide titration curve occurs at about -0.1 to +0.1V.
Manual Titration Automatic titration using the listed equipment is recommended. However, if only manual titration equipment is available, manual titration can be performed. Add the silver nitrate solution in increments of 0.2 mL and record the emf after each addition until the endpoint is approached. Then, reduce the increments to 0.1 mL. The endpoint is found where ∆emf/∆mL is a maximum, i.e., where the change in emf per increment of titrant added is the greatest. If both sulfide and mercaptide are present, 2 inflections will be observed. Make a plot of the titration, i.e., emf vs. volume of silver nitrate. Estimate the midpoint of the inflections by inspecting the titration curve, or plot ∆emf/mL and take the maximum as the endpoint. Record the volumes of silver nitrate used for the titration of the sulfide and mercaptide respectively. (See Figure 4 for typical titration curves.) Use the same procedure when the carbonyl sulfide is titrated manually. (See Figure 5 for a typical titration curve.)
CALCULATIONS The calculations below use metric units. When calculating values using metered gas volumes of sample, and the gas meter is calibrated in cubic feet, use the calculations in the APPENDIX . The APPENDIX also includes calculations for reporting results in grains of sulfur per 100 cubic feet. 212-05
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Hydrogen Sulfide To calculate the concentration of hydrogen sulfide as sulfur, in mass-ppm, from the metered gas volume of sample used, use Equation 1.
Hydrogen sulfide as sulfur, mass-ppm =
16030 AMY VCDZ
(1)
where:
A = volume of standard silver nitrate solution used for titrating the sulfide, mL C = factor for converting the metered gas volume to 15ºC and 1013 mbar, calculated from Equation 2: Correction factor =
288(P PV ) 1013 (273
t)
(2)
where:
P = barometric pressure, mbar PV = vapor pressure of water at temperature t , mbar. At normal room temperature, 22°C, the vapor pressure of water is about 27 mbar. For a more precise number, or for a different temperature, consult a technical handbook such as the Handbook of Chemistry and Physics. t = meter temperature, ºC 273 = absolute zero, °C, absolute value 288 = 273, previously defined + 15, standard temperature, °C 1013 = standard pressure, mbar D = gas density corrected to 15ºC and 1013 mbar, g/L, (obtained by ASTM Method D 1070, Relative Density of Gaseous Fuels, or UOP Method 948, Relative Density of Gas Mixtures by Calculation from Composition) M = molarity of the standard silver nitrate solution used for titration, moles/L V = metered gas volume before correction to 15ºC and 1013 mbar, L Y = total volume to which the potassium hydroxide scrubber solution is diluted before aliquoting, mL. (If the entire scrubber contents are titrated, delete the terms Y and Z from the equation since they become identical and cancel out.) Z = volume of the aliquot taken for titration, mL 16030 = factor for converting to mass-ppm sulfur, equal to the product of: (32.06)(10 6 ) (2)(10 3 ) where:
32.06 = 106 = 2 = 103 =
molecular weight of sulfur, g/mole conversion of g to µg conversion of moles of silver nitrate to moles of sulfur from H 2S conversion of L to mL
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To calculate the concentration of hydrogen sulfide, in mass-ppm, from the measured cylinder weight difference, use Equation 3.
Hydrogen sulfide as sulfur, mass-ppm =
16030 AMY GZ
(3)
where:
G = weight of sample taken by cylinder difference, g A, M, Y, Z = previously defined 16030 = previously defined Mercaptan Sulfur To calculate the concentration of mercaptan sulfur, in mass-ppm, from the metered gas volume of sample used, use Equation 4.
Mercaptan sulfur as sulfur, mass-ppm =
32060BMY VCDZ
( 4)
where:
B = volume of standard alcoholic silver nitrate solution used for titrating the mercaptide, mL C, D, M, V, Y, Z = previously defined 32060 = factor for converting to mass-ppm sulfur, equal to the product of: (32.06)(10 6 ) (10 3 ) where:
32.06 = molecular weight of sulfur, g/mole 106 = conversion of g to µg 103 = conversion of L to mL To calculate the concentration of mercaptan sulfur, in mass-ppm, from the measured cylinder weight difference, use Equation 5.
Mercaptan sulfur as sulfur, mass-ppm =
32060BMY GZ
(5)
where:
B, G , M, Y, Z = previously defined 32060 = previously defined Carbonyl Sulfide To calculate the concentration of carbonyl sulfide, in mass-ppm, from the metered gas volume of sample used, use Equation 6.
Carbonyl sulfid e as sulf ur, mass-ppm =
16030EM VCD
(6)
where:
E = volume of standard alcoholic silver nitrate solution used for titrating the carbonyl sulfide, mL 212-05
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C, D, M, V = previously defined 16030 = previously defined To calculate the concentration of carbonyl sulfide, in mass-ppm, from the measured cylinder weight difference, use Equation 7.
Carbonyl sulfi de as sulf ur, mass-ppm =
16030EM G
(7)
where:
E, G, M = previously defined 16030 = previously defined Report each sulfur type to the nearest 1 pp m as sulfur.
NOTES AND PRECAUTIONS 1. The sample cylinder should be pressure checked with a gauge before scrubbing to ensure that it is only under LPG vapor pressure, and has not been additionally pressurized with a gas. Typically, LPG-only pressure is not greater than 1000 kPa. Additional pressure may cause caustic splashes. If the pressure is greater than 1000 kPa, check the source and history of the sample, and resample if necessary. 2. The regulating stem of the expansion valve should be cleaned and maintained regularly to ensure smooth delivery of the LPG into the caustic scrubber to avoid splashes. Cleaning is performed by rinsing with water, to ensure that it provides a smooth, regular flow without any blockages. Additional maintenance requires either a rebuild kit, returning to the manufacturer, or replacement. 3. The use of a face shield is recommended during the scrubbing process in addition to the usual safety glasses, gloves, and other appropriate personal protection equipment. 4. For refinery sampling, the sample lines should be as short as possible. Purge them with the sample prior to attaching the scrubber. 5. Use rubber tubing to connect the scrubber to the sample point and to the wet test meter because of its flexibility. Make the length as short as practical. 6. Analyze the absorber solutions as soon as possible, and protect them from air to minimize oxidation of the sulfide and mercaptide. 7. Always pass the hydrocarbon gas through the scrubber before passing it through the wet test meter. This prevents contamination of the meter with hydrogen sulfide. 8. Protect the monoethanolamine solution from exposure to light. 9. The calculations assume that, for practical purposes, the metered gas volume represents the volume of gas analyzed. The difference represents the volume of hydrogen sulfide in the sample which normally has no significant effect. For unusual samples which contain hydrogen sulfide in the percent range, the analyst may refine the calculations to take this into account. 10. Dispose of all used solutions in an environmentally safe manner.
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PRECISION AND BIAS Precision statements were determined using UOP Method 999, from precision data obtained using a Brinkmamm Metrohm Model 751GPD Titrino titrator with an automatic sample changer.
Intermediate Precision A single sample was analyzed four times by each of two analysts over a period of eight days in one laboratory. The precision data are summarized in Table 2. Two tests performed in one laboratory by different analysts on different days should not differ by more than the intermediate precision allowable difference with 95% confidence. The data in Table 2 are an estimate of intermediate precision. When the test is run routinely, a control standard and chart should be used to develop a better estimate of the long-term repeatability.
Table 2 Intermediate Precisio n, mass-ppm Sulfur Intermediate Precision LPG Sample Hydr ogen sulfid e
Mean Value 2.8
WithinLab esd 0.27
Al lo wable Difference 1.0
Mercaptan sulfur Carbon yl sul fide
5.2 0.7
0.27 0.05
1.2 0.2
Reproducibility There is insufficient data to calculate reproducibility of the test at this time.
Bias The LPG sample that was used to develop the precision statement was also analyzed for total sulfur by ASTM Method D 6667. D 6667 measured 8.6 mass-ppm total sulfur, compared to a sum of 8.7 mass-ppm sulfur for this method.
TIME FOR ANALYSIS Samples Containing Hydrogen Sulfide, Mercaptan, and Carbonyl Sulfide Sulfur The elapsed time for one analysis is four hours. The labor requirement is four hours. Additional time is required to initially assemble the apparatus. Samples with very low sulfur levels may require additional scrubbing time, up to a total of 10 hours.
Samples Containing Mercaptan Sulfur Only and Appreciable Concentrations of Pentane and Higher Boil ing Materials The elapsed time for one analysis is three hours. The labor requirement is three hours.
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SUGGESTED SUPPLIERS Acros Organics, Fisher Scientific, 711 Forbes Ave., Pittsburgh, PA 15219-4785 (412-490-8300) www.fishersci.com Aldrich Chemical Company, 1000 West Saint Paul Ave., Milwaukee, WI 53233 (414-273-3850) www.sigma-aldrich.com Alltech Associates Inc., 2051 Waukegan Rd., Deerfield, IL 60015 (847-948-8600) www.alltechweb.com Brinkmann Instruments Co., One Cantiague Rd., Westbury NY 11590 (516-334-7500) www.brinkmann.com Fisher Scientific Co., 711 Forbes Ave., Pittsburgh, PA 15219-4785 (412-490-8300) www.fishersci.com Matheson Tri-Gas, 166 Keystone Drive, Montgomeryville, PA 18936 (215-641-2700) www.mathesontrigas.com Meters & Controls, 505 W. Wrightwood Ave., Elmhurst, IL 60126 (630-279-3800) www.americanmeter.com Reliance Glass, 2220 Gateway Road, Bensenville, IL 60106 (630-766-1816) www.lab-glass.com Runco Office Supply, 1108 Lee St., Des Plaines, IL 60016 (847-297-7740) www.runcoonline.com Swagelok, 1540 N. Old Rand Road, Wauconda, IL 60084-0847 (847-526-6900) www.swagelok.com
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APPENDIX The appendix is written as a convenience for users who measure the sample quantity scrubbed from the metered gas volume of sample used, and whose equipment for SCRUBBING OF GASES is calibrated in English units (cu ft, ºF). Also included are calculations for reporting results in grains of sulfur per 100 cubic feet.
CALCULATIONS Hydrogen Sulfide To calculate the concentration of hydrogen sulfide, in mass-ppm, from the metered gas volume of sample used, use Equation A1.
Hydrogen sulfide as sulfur, mass-ppm =
566 AMY VCDZ
( A 1)
where:
A = volume of standard silver nitrate solution used for titrating the sulfide, mL C = factor for converting the metered gas volume to 60ºF and 760 mm Hg. This can be conveniently ascertained from the Gas Volume Correction Chart, Figures A1 and A2. Alternatively, it can be calculated from Equation A2: Correction factor =
520 (P − PV ) 760 (460 + t)
( A 2)
where:
P = barometric pressure, mm Hg PV = vapor pressure of water at temperature t , mm Hg. At normal room temperature, 72°F, the vapor pressure of water is about 20 mm Hg. For a more precise number, or for a different temperature, consult a technical handbook. t = meter temperature, ºF 460 = absolute zero, °F, absolute value 520 = 460, previously defined + 60, standard temperature, °F 760 = standard pressure, mm Hg D = gas density corrected to 60ºF and 760 mm Hg, g/L, (obtained by ASTM Method D 1070, Relative Density of Gaseous Fuels, or UOP Method 948, Relative Density of Gas Mixtures by Calculation from Composition) M = molarity of the standard silver nitrate solution used V = metered gas volume before correction to 60ºF and 760 mm Hg, cu ft Y = total volume to which the potassium hydroxide scrubber solution is diluted before aliquoting, mL. (If the entire scrubber contents are titrated, delete the terms Y and Z from the equation since they become identical and cancel out.) Z = volume of the aliquot taken for titration, mL 566 = factor for converting to mass-ppm sulfur, equal to the product of: (32.06)(10 6 ) (2)(28.316)(10 3 ) 212-05
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where:
32.06 = 106 = 2 = 28.316 = 103 =
molecular weight of sulfur, g/mole conversion of g to µg conversion of moles of silver nitrate to moles of sulfur from H 2S conversion of cu ft to L of gas, L/cu ft conversion of L to mL
To calculate the concentration of hydrogen sulfide in grains per 100 cu ft at standard temperature and pressure (STP, 60ºF, 760 mm Hg), use Equation A3.
Hydrogen sulfide as sulfur, grains/100 cu ft at STP =
24.73 AMY 0.000418 AMY + VCZ
( A 3)
where:
A, C, M, V, Y, Z = previously defined 24.73 = factor for converting to grains of sulfur per 100 cu ft of gas at STP, equal to the product of: (32.06)(15.43)(10 2 ) (2)(10 3 ) where:
32.06 = 15.43 = 102 = 2 = 103 =
molecular weight of sulfur, g/mole conversion from grams of sulfur to grains of sulfur, grains/g conversion from per cu ft of gas to per 100 cu ft of gas conversion of moles of silver nitrate to moles of sulfur from H 2S conversion of L to mL
0.000418 = factor for correcting the metered gas volume for the volume of H 2S adsorbed by the scrubbers, equal to the product of: (23.67) (28.316)(2)(10 3 ) where:
23.67 = 28.316 = 2 = 103 =
conversion from moles to liters of gas at STP, L/mole conversion of cu ft to L of gas, L/cu ft conversion of moles of silver nitrate to moles of sulfur from H 2S conversion of L to mL
For gases containing less than 200 grains of hydrogen sulfide per 100 cu ft, the term 0.000418 AMY in the denominator, which represents the cubic feet of hydrogen sulfide in the sample, is negligible and may be eliminated from the formula. This simplifies the expression to Equation A4:
Hydrogen sulfide as sulfur, grains/100 cu ft at STP =
24.73 AMY VCZ
( A 4)
where:
A, C, M, V, Y, Z = previously defined 24.73 = factor for converting to grains of sulfur per 100 cu ft of gas at STP, previously defined 212-05
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Mercaptan Sulfur To calculate the concentration of mercaptan sulfur, in mass-ppm, from the metered gas volume of sample used, use Equation A5.
Mercaptan sulfur as sulfur, mass-ppm =
1132BMY VCDZ
( A 5)
where:
B = volume of standard alcoholic silver nitrate solution used for titrating the mercaptide, mL C, D, M, V, Y, Z = previously defined 1132 = factor for converting to mass-ppm sulfur, equal to the product of: (32.06)(10 6 ) (28.316)(10 3 ) where:
32.06 = 106 = 28.316 = 103 =
molecular weight of sulfur, g/mole conversion of g to µg conversion of cu ft to L of gas, L/cu ft conversion of L to mL
To calculate the concentration of mercaptan sulfur in grains per 100 cu ft at STP, use Equation A6.
Mercaptan sulfur as sulfur, grains/100 cu ft at STP =
49.47BMY VCZ
( A 6)
where:
B, C, M, V, Y, Z = previously defined 49.47 = factor for converting to grains of sulfur per 100 cu ft of gas at STP, equal to the product of: (32.06)(15.43)(10 2 ) (10 3 ) where:
32.06 = 15.43 = 102 = 103 =
molecular weight of sulfur, g/mole conversion from grams of sulfur to grains of sulfur, grains/g conversion from per cu ft of gas to per 100 cu ft of gas conversion of L to mL
Carbonyl Sulfide To calculate the concentration of carbonyl sulfide, in mass-ppm, from the metered gas volume of sample used, use Equation A7.
Carbonyl sulfid e as sulf ur, mass-ppm = where:
212-05
566EM VCD
( A 7)
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E = volume of standard alcoholic silver nitrate solution used for titrating the carbonyl sulfide, mL C, D, M, V = previously defined 566 = factor for converting to mass-ppm sulfur, equal to the product of: (32.06)(10 6 ) (2)(28.316)(10 3 ) where:
32.06 = molecular weight of sulfur, g/mole 106 = conversion of g to µg 2 = conversion of moles of silver nitrate to moles of sulfur from COS 28.316 = conversion of cu ft to L of gas, L/cu ft 103 = conversion of L to mL Report all sulfur types to the nearest 1 ppm as sulfur, or to the nearest 0.1 grains per 100 cu ft at STP.
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Figure A1
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Figure A2
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