UOP 678-2004 DISSOLVED MOLECULAR OXYGEN IN LIQUID HYDROCARBONS BY ELECTROCHEMICAL DETECTION.pdf

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1、 uop IT IS THE USERS RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TO 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 PRES

2、CRIBED 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 1970, 1988, 2004 UOP LLC. All righ

3、ts reserved. Nonconfidential UOP Methods are available from ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. The UOP Methods may be obtained through the ASTM website, www.astm.org, or by contacting Customer Service at serviceastm.org, 610.832.9

4、555 FAX, or 610.832.9585 PHONE. DISSOLVED MOLECULAR OXYGEN IN LIQUID HYDROCARBONS BY ELECTROCHEMICAL DETECTION UOP Method 678-04 SCOPE This method is for estimating the dissolved oxygen in liquid hydrocarbons by electrochemical detection. The preferred application is online analysis, although the an

5、alysis may also be performed in the laboratory. The lower detection limit is 0.1 mass-ppm while the upper limit is dependent on the solubility of oxygen in the specific sample. Carbon dioxide, ammonia, hydrogen sulfide, and mercaptans at concentrations above 200, 300, 15 and 15-ppm (see Note 1) resp

6、ectively, will interfere with the oxygen determination. Materials that contaminate the membrane or volatile components that are either oxidized, reduced, or react with the sensor electrodes will also interfere. The sampling system associated with a process unit is to be specified depending on the sp

7、ecific process unit. REFERENCES ASTM Method D 2779, “Estimation of Solubility of Gases in Petroleum Liquids,” www.astm.org ASTM Method D 3764, “Validation of Process Stream Analyzer Systems,” www.astm.org ASTM Method D 4052, “Density and Relative Density of Liquids by Digital Density Meter,” www.ast

8、m.org ASTM Method D 6621, “Performance Testing of Process Analyzers for Aromatic Hydrocarbon Materials,” www.astm.org IUPAC Solubility Data Series, Vol. 7, Oxygen and Ozone, Ed. R. Battino, Pergamon Press, NY (1981) UOP Method 888, “Precision Statements in UOP Methods,” www.astm.org OUTLINE OF METHO

9、D The method is performed either on-line or in the laboratory. The on-line analysis is performed by installing the oxygen sensor in a process stream. After the probe has equilibrated, the stream can be continuously monitored. Copyright by ASTM Intl (all rights reserved);Reproduction authorized per L

10、icense Agreement with Monique Tyree (IHS); Mon Jan 17 14:46:54 EST 20052 of 8 678-04 The laboratory analysis is performed by obtaining a representative sample and equilibrating it to the temperature at which the analysis is desired, normally the temperature of the process stream. The probe is placed

11、 into the stirred sample, allowed to equilibrate and the fugacity of oxygen (equal to the partial pressure of oxygen at the expected analytical conditions) is obtained. An estimate of the dissolved oxygen content is then calculated. An alternative laboratory analysis, using samples obtained in sampl

12、e cylinders, is described in the APPENDIX. DEFINITION Fugacity, a measure of the escaping tendency of a component in solution. Fugacity is defined in Equation 1: = 0+ RT ln f (1) where: = chemical potential of a pure gas at pressure, P 0= chemical potential of a pure gas at standard state conditions

13、 R = gas constant T = temperature, K f = fugacity At normal operating conditions, where the oxygen pressure, P, is much less than the oxygen critical pressure, 5,036 kPa (49.7 atm), the fugacity of oxygen is approximately equal to the oxygen partial pressure. APPARATUS References to catalog numbers

14、and suppliers are included as a convenience to the method user. Other suppliers may be used. For online analysis: Dissolved Oxygen Analyzer with Probe, Orbisphere Model 3600 or 3650, with sensor Model 311xx, Hach Ultra Analytics. Consult manufacturer for models and options for the specific applicati

15、on. Sampling port, appropriate to the installation For laboratory analysis: Bath, water, with cover and temperature display, general purpose, operating temperature range of ambient to 60C with a uniformity of 0.05C, Fisher Scientific, Cat. No. 15-462-5 Bottle, Teflon FEP, 500-mL, wide mouth, round,

16、with Tefzel ETFE screw cap, Fisher Scientific, Cat. No. 02-923-30C Dissolved Oxygen Analyzer with Probe, Orbisphere Model 3650, with sensor Model 311xx, Hach Ultra Analytics. Consult manufacturer for models and options for the specific application. Stirring apparatus, magnetic, variable speed, subme

17、rsible, Fisher Scientific, Cat. No. 11-497-5 Stirring bar, magnetic, Teflon, Fisher Scientific, Cat. No. 14-511-64 Copyright by ASTM Intl (all rights reserved);Reproduction authorized per License Agreement with Monique Tyree (IHS); Mon Jan 17 14:46:54 EST 20053 of 8 678-04 REAGENTS AND MATERIALS Ref

18、erences to catalog numbers and suppliers are included as a convenience to the method user. Other suppliers may be used. Recharge kit, Orbisphere, Cat. No. 32706, Hach Ultra Analytics Tape, Teflon, thread seal, Fisher Scientific, Cat. No. 14-831-300B PROCEDURE Oxygen Sensor The sensor electrodes are

19、separated from the liquid sample by a membrane, therefore, the sensor actually determines the fugacity of oxygen in the sample. The fugacity is proportional to the dissolved oxygen content. Henrys Law, which relates fugacity to the dissolved gas content, can be expressed as the fugacity of oxygen in

20、 the sample equaling the concentration of dissolved oxygen in the sample times the solubility, or Henrys Law Constant. The gas solubility is temperature, pressure and liquid composition dependent and can be estimated (see Note 2) by ASTM D 2779 and from the sample temperature and relative density, A

21、STM D 4052. On-Line Analysis The dissolved oxygen probe is installed in the process stream according to process specifications. The preferred location of the probe is at a position where the temperature of the process liquid is reasonably constant and well below the temperature and operating limitat

22、ions of the probe. Auxiliary temperature readouts are not required because a temperature measurement sensor is built into the probe. Provisions must be made to allow removal of the probe from the process stream for servicing without requiring the process stream to be shut down. The probe must be ser

23、viced between two and eight times per year, depending on the temperature and the oxygen level being measured. The maintenance procedure for the probe consists of cleaning the electrodes, replacing both the electrolyte and the membrane and recalibration. Consult the instrument manual for information

24、required to operate and maintain the analytical system. Approximately 30 minutes are required for the probe to re-equilibrate when returned to service. The analyzer will determine the fugacity of oxygen (see Note 3), normally in kPa. An estimate of the dissolved oxygen concentration is then calculat

25、ed from the oxygen fugacity, the process stream temperature and the relative density (see Note 4 and CALCULATIONS). Laboratory Analysis 1. Verify the instrument calibration, and if necessary perform the maintenance procedure on the oxygen probe as outlined in the manufacturers service manual. This p

26、rocedure includes cleaning the electrodes, replacement of the electrolyte and the membrane, instrument warm-up, and analyzer recalibration. 2. Wrap a section of the probe, 10 to 15 mm in width, approximately 100 mm from the sensor head, with enough Teflon tape to provide the probe with a snug fit wh

27、en placed in the neck of the Teflon sampling bottle. 3. Take care when sampling to maintain sample integrity. For best results, the sample must be analyzed as quickly as possible. To accomplish this, prepare a water bath at the required temperature before the sample is taken. For process stream samp

28、les the temperature chosen Copyright by ASTM Intl (all rights reserved);Reproduction authorized per License Agreement with Monique Tyree (IHS); Mon Jan 17 14:46:54 EST 20054 of 8 678-04 will typically be the same as the stream. For samples other than process stream samples, use ambient temperature u

29、nless otherwise specified. 4. Fill a 500-mL Teflon sampling bottle to overflowing with the sample and empty it. Repeat this rinsing procedure 2 more times. Fill the sampling bottle to overflowing a fourth time and cap it immediately (see Note 5). Wipe the outside of the bottle dry. A sample can be o

30、btained from either a flowing process stream or from a container 5. Place the sample bottle in the water bath and allow it to reach thermal equilibrium. 6. Remove the sample bottle from the water bath and place in a beaker or tray to contain sample overflow. Perform steps 6 through 8 in immediate su

31、ccession to maintain the temperature of the sample. 7. Remove the bottle cap, place the stirring bar in the bottle and lower the probe into the sample. Seat the Teflon wrapped probe snugly in the neck of the bottle. Wipe the outside of the bottle dry. 8. Place the bottle on the magnetic stirrer in t

32、he water bath and adjust the stirrer to a rapid stirring rate. 9. Record the analyzer reading as soon as the probe and sample have equilibrated. Typically, equilibration time will not exceed 25 minutes. The analyzer will determine the fugacity of the sample. An estimate of the dissolved oxygen conce

33、ntration is calculated from the oxygen fugacity using the sample temperature and relative density (see Note 4 and CALCULATIONS). CALCULATIONS Estimate of Henrys Law Constant Henrys Law Constant, or oxygen solubility, can be estimated from the sample temperature, relative density and ASTM D 2779 (see

34、 Note 2). Alternatively, a more precise estimation can be obtained for some sample compositions through the use of Data Tables (2). ASTM D 2779 is used to estimate the Ostwald coefficient, which is used to calculate the Henrys Law Constant. Henrys Law Constant is estimated from the calculation shown

35、 in Equation 2: JT31.8H = )2( where: H = Henrys Law Constant, (liters)(kPa)/mol J = estimated Ostwald coefficient, see ASTM D 2779 T = temperature, degrees Kelvin (K) 8.31 = gas constant R, converted to the appropriate units, )K)(mol()L)(kPa(, Equation 3 mLL10xatmkPa33.101x)K)(mol()atm)(mL(0568.823)

36、3( Copyright by ASTM Intl (all rights reserved);Reproduction authorized per License Agreement with Monique Tyree (IHS); Mon Jan 17 14:46:54 EST 20055 of 8 678-04 Estimate of Dissolved Oxygen Calculate an estimate of the dissolved oxygen concentration in the liquid sample using Equation 4: HPC = )4(

37、where: C = concentration of dissolved molecular oxygen, moles/liter P = fugacity (partial pressure) of oxygen as determined by probe, kPa H = Henrys Law Constant, previously defined Calculate an estimate of the dissolved oxygen concentration, mass-ppm, in the liquid sample using Equation 5: DC32000W

38、 = )5( where: C = concentration of dissolved molecular oxygen in moles/liter, previously defined D = density of sample, g/cm3W = concentration of dissolved molecular oxygen, mass-ppm 32000 = conversion factor equal to the product of 32, the molecular weight of oxygen, g/mole, times 103, the conversi

39、on factor to mass-ppm NOTES AND PRECAUTIONS 1. Special probes are available that can operate in samples containing up to 3000-ppm hydrogen sulfide. Contact the instrument manufacturer for additional information. 2. ASTM D 2779 provides for gas solubility estimates at atmospheric pressure only. The e

40、ffect of pressure on the gas solubility is assumed to be small but has not been rigorously documented. A more accurate gas solubility, if desired, must be determined empirically at specific sample pressure, temperature and composition conditions. 3. Analyzers can be factory calibrated to read direct

41、ly in dissolved oxygen concentration units. This is desirable, provided the process stream composition, temperature and pressure remain constant. 4. Often, only monitoring for relative changes and trends is needed. Therefore, the determination of the actual dissolved oxygen concentration may not be

42、necessary. 5. Major changes in sample temperature can cause the Teflon sampling bottle to deform. PRECISION Online analyzers may be validated using ASTM Methods D 3764 or D 6621. For laboratory analysis, based upon 6 replicate determinations of a kerosine sample, the estimated standard deviation (es

43、d) for dissolved oxygen at the 73 mass-ppm level was calculated to be 0.46 mass-ppm using the data reduction program in UOP Method 888. The experimental design stipulated Copyright by ASTM Intl (all rights reserved);Reproduction authorized per License Agreement with Monique Tyree (IHS); Mon Jan 17 1

44、4:46:54 EST 20056 of 8 678-04 in UOP Method 888 was not used. Duplicate results by the same operator should not differ by more than 1.7 mass-ppm (95% probability) at the stated level. TIME FOR ANALYSIS For the on-line analysis the elapsed time per run condition change is less than 0.1 hour. There is

45、 no labor requirement other than maintenance. For the laboratory analysis, the elapsed time for one sample, excluding sample collection, is 1.5 hours. The labor requirements are 0.5 hour for the first sample and 0.3 hour for each additional sample. SUGGESTED SUPPLIERS Fisher Scientific Co., 711 Forb

46、es Ave., Pittsburgh, PA 15219-4785 (412-490-8300) Hach Ultra Analytics, 481 California Ave., Grants Pass, OR 97526 (541-472-6500) Copyright by ASTM Intl (all rights reserved);Reproduction authorized per License Agreement with Monique Tyree (IHS); Mon Jan 17 14:46:54 EST 20057 of 8 678-04 APPENDIX

47、Laboratory Analysis for Samples in Cylinders APPARATUS The following items are required in addition to, or as an alternative to, those listed in the body of UOP Method 678-04. References to catalog numbers and suppliers are included as a convenience to the method user. Other suppliers may be used, u

48、nless stated otherwise. Cylinder, sampling, type 304 stainless steel, high pressure, 12,400 kPa gauge (1800 psig) maximum working pressure, one-liter capacity, Swagelok, Cat. No. 304L-HDF4-1000. Equip the cylinder with an outage tube, Swagelok, Cat. No. 304L-DTM4-F4-038, two stainless steel valves,

49、Swagelok, Cat. No. SS-14DKM4-S4, and two plugs, Swagelok, Cat. No. SS-400-P. Flow chamber, Orbisphere Model 32007.x11 or 32001.x11, Hach Ultra Analytics. Consult manufacturer for models and options for the specific application. Regulator, pressure, 0-50 psig, Omega Engineering, Cat. No. PRG101-60 Rotameter, 0.2 GPM, Omega Engineering, Cat. No. FL1651 Tubing, inch OD, stainless steel or Teflon Valves, 3-way, Swagelok, Cat. No. SS-43XS4 Valve, flow control, Swagelok, Cat. No. SS-1RS4 REAGENTS AND MATERIALS The following items