1、COMBINED NITROGEN IN LPG BY CHEMILUMINESCENCE UOP Method 936-95 SCOPE This method is for determining combined nitrogen in liquefied petroleum gas (LPG). The method determines all nitrogen compounds that are extractable by aqueous acid, including ammonia, amines, nitriles and amides. The method is pa
2、rticularly useful for LPG samples containing nitrogen species, such as ammonia and amines, which can readily adsorb on stainless steel surfaces in contact with the LPG. The procedure yields quantitative results in the range of approximately 0.1 mass-ppm to approximately 3 mass-%. OUTLINE OF METHOD N
3、itrogen compounds are quantitatively extracted from an LPG sample using a solution of 2-N acetic acid in water. A measured mass of LPG is collected in a cleaned, stainless steel, sample cylinder. A known amount of acid solution is added to a second cylinder, the two vessels are connected and the aci
4、d solution is transferred under helium pressure into the LPG sample cylinder. The nitrogen components are extracted into the acid solution by mixing the liquids for several minutes. The LPG and acid extract are allowed to separate. The acid extract is withdrawn and analyzed for total nitrogen by che
5、miluminescence, utilizing ASTM Method D 4629 with the optional boat inlet system. APPARATUS References to catalog numbers and suppliers are included as a convenience to the method user. Other suppliers may be used. Adapter, male, 2 required, for charging acetic acid cylinder, available from Dearborn
6、 Valve and Fitting, Swagelok, Cat. No. SS-4-TA-1-4 (see also Connector) Balance, readability 0.1-mg Balance, readability 0.1-g, capacity 4000-g Beaker, polypropylene, 100-mL, Fisher Scientific, Cat. No. 02-591-17B IT IS THE USERS RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TO
7、 DETERMINE THE APPLICABILITY OF REGULATORY LIMITATIONS PRIOR TO USE. EFFECTIVE HEALTH AND SAFETY PRACTICES ARE TO BE FOLLOWED WHEN UTILIZING THIS PROCEDURE. FAILURE TO UTILIZE THIS PROCEDURE IN THE MANNER PRESCRIBED HEREIN CAN BE HAZARDOUS. MATERIAL SAFETY DATA SHEETS (MSDS) OR EXPERIMENTAL MATERIAL
8、 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 1995 UOP LLC ALL RIGHTS RESERVED UOP Methods are available through ASTM International, 100 Barr Harbor Drive, PO Box C700, We
9、st Conshohocken PA 19428-2959, United States. The Methods may be obtained through the ASTM website, www.astm.org, or by contacting Customer Service at serviceastm.org, 610.832.9555 FAX, or 610.832.9585 PHONE. 2 of 16 936-95 Calculator, capable of least squares linear regression calculation, required
10、 only if the nitrogen analyzer does not permit direct calibration and direct reading of the nitrogen in the extract Clamps, for ring stand and sample cylinders, several required, with holders and cast iron ring, Fisher Scientific, Cat. Nos. 05-769-8, 05-754 and 14-050C, respectively Connector, stain
11、less steel, used to connect the 300-mL and the 1000-mL cylinders vertically together and consisting of a male connector, Swagelok, Cat. No. SS-400-1-4 joined together with one male adapter, Swagelok, Cat. No. SS-4-TA-1-4. All parts are available from Dearborn Valve and Fitting. Cylinders, for sample
12、 collection and analysis, type 304 stainless steel, high pressure, 12,400 kPa gauge (1800 psi) maximum working pressure, 1000- and 300-mL capacity, Whitey, Cat. Nos. SS-304L-HDF4-1000 and -300, respectively. Equip the cylinders with two stainless steel valves each, Whitey, Cat. No. SS-16DKM4-F4, and
13、 two pipe-plugs each, Cajon, quarter-inch P-male-NPT, Swagelok, Cat. No. SS-4-P. All parts are available from Dearborn Valve and Fitting. Cylinders, graduated, borosilicate glass, 50-, 100- and 250-mL, Fisher Scientific, Cat. Nos. 08-556C, D and E, respectively Extractor (optional), special construc
14、tion, see APPENDIX Flasks, volumetric, 100-, 1000- and 2000-mL, with caps, Fisher Scientific, Cat. Nos. 10-210-5C, -5G and -5H, respectively Gauge, pressure, 0-20, 700 kPa gauge (0-3000 psig), 2-inch diameter, 0.25-inch MNPT, used for testing the pressure of an unknown LPG sample gas cylinder, Mathe
15、son Gas Products, Model 63-2233 Jar-Mill (optional), 20-300 rpm, Model 755 RMV, Fisher Scientific, Cat. No. 08-381-1. Required only if optional extractor is used. Nitrogen analyzer, total nitrogen by chemiluminescence, with boat control system for aqueous sample, Mitsubishi Instruments, TN-10, avail
16、able from Cosa Scientific Oven, drying, capable of maintaining 105C Pipet, Mohr, 1-, 5- and 25-mL, Fisher Scientific, Cat. Nos. 13-665-F, -K and -N, respectively Quartz beads, -10 to + 60 mesh, Pyromatics, Cat. No. 7323 Regulators, helium, two-stage, Matheson Gas Products, Models 3104C-580 (used for
17、 the nitrogen analyzer) and 3030-580 (used for acetic acid transfer between cylinders) Regulator, oxygen, two-stage, high purity, Matheson Gas Products, Model 3813-CGA-540, cleaned for oxygen service by the supplier, used with Nitrogen Analyzer Ring stand, with rectangular base, 170- x 280-mm base w
18、ith 910-mm rod, Fisher Scientific, Cat. No. 14-670D 3 of 16 936-95 Sample vials, for extracted sample, Kimble Scientific Products, Cat. No. 60957-1, with Quorpack black phenolic caps and polyseal liners, Cat. No. 02-883-5G, available from Fisher Scientific Syringe, 50-L, Hamilton, Model 705N, Fisher
19、 Scientific Products, Cat. No. 14-824-5 Tubing, polyethylene, “POLYFLOW”, for charging acetic acid cylinder, 6.4-mm (0.25-inch) ID, Lesman Instruments, Cat. No. 44-P-N Tubing, Tygon, R3603, for charging acetic acid cylinder 6.4-mm (0.25-inch) ID, Fisher Scientific, Cat. No. 14-169-3C Tubing, vacuum,
20、 1/4-inch ID (6.4-mm), Fisher Scientific, Cat. No. 14-173C Vacuum pump, aspirator, Nalgene, Fisher Scientific, Cat. No. 09-960-2 Valve, check, 20,700 kPa gauge (3000 psig), Matheson Gas Products, Model 400V REAGENTS AND MATERIALS All reagents shall conform to the specifications established by the Co
21、mmittee on Analytical Reagents of the American Chemical Society, when such specifications are available, unless otherwise specified. References to water mean deionized or distilled water. References to catalog numbers and suppliers are included as a convenience to the method user. Other suppliers ma
22、y be used. Acetic acid, approximately 99.7%, Fisher Certified, Fisher Scientific, Cat. No. A38-212 Acetic acid, approximately 2-N. Prepare two liters by slowly adding 232 mL of concentrated acetic acid from a graduated cylinder to approximately 1500 mL of water in a 2000-mL volumetric flask. After t
23、he heat of dilution dissipates, dilute to volume with water and mix thoroughly by capping the flask and inverting at least 10 times. Acetone, Fisher Certified, Fisher Scientific, Cat. No. A18-4 Helium, Ultra High Purity, 99.999%, Matheson Gas Products Isooctane, Certified, Fisher Scientific, Cat. No
24、. 0299-4 Isooctane-acetone, 9 to 1, Measure 900 mL of isooctane into a 1000-mL volumetric flask and fill with acetone to the mark. Cap and mix thoroughly by inverting at least ten times. Methanol, Fisher Certified, ACS, Fisher Scientific, Cat. No. A412-4 Methanol-acetone, 1 to 1. Measure 500 mL of a
25、cetone with a graduated cylinder into a 1000-mL volumetric flask and fill to the mark with methanol. Cap the flask and mix by inverting at least ten times. 4 of 16 936-95 Oxygen, 99.98%, Ultra-High Purity, Matheson Gas Products pH paper, CF strips, 0 to 14 pH units, Fisher Scientific, Cat. No. 09-87
26、6-17 Pyridine, 99.9%, Aldrich Chemical, Cat. No. 27,040-7 Sulfuric acid, reagent grade, ACS, 95.5%, Fisher Scientific, Cat. No. A300-212 Sulfuric acid, 10%. Prepare one liter by slowly adding 100 mL of concentrated sulfuric acid from a graduated cylinder to approximately 800 mL of water in a 1000-mL
27、 volumetric flask while cooling in ice water. After the heat of dilution dissipates, dilute to volume with water. Mix thoroughly by capping the flask and inverting at least 10 times. Teflon-tape, PTFE, roll, 12.5-mm (0.5-inch), Fisher Scientific, Cat. No. 14-831-300A PROCEDURE Cylinder Preparation 1
28、. Clean the interior surfaces of the 1000-mL LPG sample cylinder and valves as follows: Rinse the cylinder with successive 100-mL portions of the isooctane-acetone mixture until the rinse is colorless and follow with 100-mL rinses of the methanol-acetone mixture until the rinse is colorless. Rinse w
29、ith one 100-mL portion of the 10% sulfuric acid solution. Rinse with water until the rinse is neutral to the pH paper. Appropriately discard the washes. 2. Remove the water by rinsing the 1000-mL cylinder and valves with two successive 100-mL portions of the methanol-acetone and discard the rinses.
30、Rinsing with the solvents, in the order given, removes adsorbed nitrogen compounds. Any convenient procedure that assures contact of the wetted valve and cylinder surfaces with solvents may be used. One workable procedure is outlined in Note 1. 3. Flush the cylinder with helium to remove the remaini
31、ng solvents and then dry in the drying oven for at least one hour at 105C. Nitrogen may be substituted for the more expensive helium in this step. 4. Allow the cylinder to cool to room temperature. Tare the cylinder and record the tare mass to the nearest 0.1 g. The weight of the cylinder should not
32、 include any detachable parts, such as end-caps, tags, etc. to assure that an accurate mass is recorded. Sample Collection 1. Collect sufficient LPG in the 1000-mL sample cylinder to fill it approximately half full with liquid, according to UOP Method 516. In no case should a cylinder be filled comp
33、letely with liquid because the hydrostatic pressure in the cylinder will exceed the rated pressure of the cylinder, if the liquid expands after filling. The mass of the LPG will be approximately 250 g, but should never exceed 300 g when the cylinder is half full. 5 of 16 936-95 The LPG sample size s
34、hould be adjusted at the source, based on the nitrogen concentration. A sample size of 500 mL was selected to analyze samples containing 300 mass-ppm N or less. If the nitrogen content is greater than 300 mass-ppm, the LPG sample size must be adjusted as shown in Table 1. Higher nitrogen concentrati
35、ons also require the use of more extractant, as described later in the method. The ratio of extractant to LPG must never be less than the values shown in Table 1. If larger volumes of extractant are injected accidentally, the analysis is still accurate. Table 1 Quantities of LPG and Extractant to Us
36、e for Various Nitrogen Concentrations Mass LPG, g N, mass-ppm Extractant, g 250 0.1 to 300 50 g 50 300 to 3,000 100 g 25 3,000 to 30,000 250 g 2. Measure and record the pressure in the half-filled 1000-mL cylinder using the pressure gauge (0-20,700 kPa) and indicate the pressure on a tag to be attac
37、hed to the cylinder. The pressure of the sample cylinder should not exceed 500 psi. If the pressure is greater than 500 psi, the sample is likely not pure LPG and should not be analyzed. Consult with the plant engineer or originator of the sample. 3. Reweigh the cylinder and record the LPG sample ma
38、ss, to the nearest 0.1 g, on a tag to be attached to the cylinder as the final cylinder mass minus the tare mass from Cylinder Preparation, Step 4. Extraction 1. Clean the 300-mL cylinder in the same manner as directed for the 1000-mL cylinder under Cylinder Preparation, Steps 1 through 4. 2. Mount
39、the 300-mL cylinder vertically on the ring stand with the clamps and attach a vacuum hose to a male adapter at the open valve at the top of the cylinder, but keep the bottom valve closed. Apply vacuum to the cylinder at the open top valve. 3. Note the appropriate volume of 2-N acetic acid solution t
40、o be added to the 300-mL cylinder based on the nitrogen content of the LPG and Table 1 and measure the acid into an appropriately sized graduated cylinder. 4. Attach a piece of Tygon and polyethylene “POLYFLOW” tubing to the male adapter at the bottom valve, so that the polyethylene tubing is submer
41、ged at the bottom of the graduated cylinder containing the appropriate volume of acetic acid and slowly open the bottom valve to withdraw the acetic acid with vacuum into the 300-mL cylinder. Next, close first the lower, then the upper valve and disconnect the vacuum hose. Take care to control the r
42、ate of withdrawal of the acetic acid into the evacuated 300-mL cylinder so that none of the acid is aspirated out of the opened top valve. 6 of 16 936-95 5. Assemble the cylinders attached to the helium tank as shown in the Figure and pressurize the 300-mL cylinder with helium, until the pressure in
43、 the 300-mL cylinder exceeds the pressure in the 1000-mL cylinder by approximately 340 kPa gauge (50 psig). The pressure in the LPG sample cylinder was recorded in Sample Collection, Step 2. If no record is present, the pressure must be measured by attaching the cylinder to the ring stand in a verti
44、cal position, attaching a gauge to the top of the cylinder, and measuring the pressure. For this method, the pressure should not exceed 500 psi, but in no case should the pressure in the cylinder exceed the safe operating pressure of the vessel, typically 12,400 kPa (1800 psig). The two cylinders ar
45、e attached vertically to a ring stand, positioned in a hood, using the clamps. The bottom valve of the top cylinder is attached to the top valve of the lower LPG sample cylinder with the straight stainless steel connector. Any substitute connector tube should be no more than 51 mm (2 inches) in leng
46、th and the shortest possible section of straight, stainless steel tubing and fittings should be used to minimize hold up of the acetic acid. Ensure that all fittings are leak free at the pressure in the 300-mL cylinder. The cylinder assembly is shown in the Figure, without indicating the location of
47、 the ring stand and clamps. 6. Open the bottom valve of the top cylinder slowly. 7. Open the top valve of the 1000-mL LPG cylinder. The sound of the 2-N acetic acid solution being forced into the LPG sample should be audible. 8. Close the top valve of the LPG sample cylinder and then close the heliu
48、m cylinder valves. Development of heat of the sample cylinder is not expected. If, however, the cylinder should warm up due to caustic or other impurities in the sample, immediately stop the analysis and consult with the plant engineers or originators of the sample. 9. Disconnect the helium tubing c
49、onnection at the top valve of the 300-mL cylinder slowly to relieve pressure. Disconnect the 300-mL cylinder from the LPG cylinder. Remove the LPG cylinder from the ring stand and reweigh the LPG cylinder to determine the mass of the acid added (0.1 g). The weight of the cylinder should not include any detachable parts, such as end-caps, tags etc., to assure that an accurate mass is recorded. The 1000-mL LPG cylinder is plugged at both ends after weighing, using the stainless steel plugs. 10. Mix the LPG and acid mixture thoroughly by inverting and rotatin
