1. Overview The trace oxygen content in gas is an important indicator of the quality of gas products. Except for product oxygen and intermediate products, trace oxygen in all kinds of air separation gases needs to be analyzed. Because oxygen is oxidizing, and air separation gas is mainly used as shielding gas, it is mainly for the purpose of isolating oxygen, whether it is nitrogen or argon, helium and neon in inert gas, the trace oxygen content is a major one. Quality Index. This article discusses the trace oxygen analyzer used in the air separation gas.
2. Classification, principles and characteristics Trace oxygen analysis methods are mainly colorimetric method, chemical cell method, yellow phosphorescence method, concentration cell method and gas chromatography. Among them, the colorimetric method is an earlier analysis method. It is a method prescribed by the national standard. Colorimetric analysis using copper-ammonia solution is difficult because the operation is complicated and the automatic online analysis cannot be implemented. It is rarely used nowadays. , but it is still an arbitration method. The yellow phosphorus luminescence method utilizes the oxygen and yellow phosphorus oxidative combustion for analysis. It has the characteristics of rapid analysis and continuous analysis. However, the yellow phosphorus used in this method is a dangerous chemical. The resulting product is corrosive and the detection limit is low. So it is rarely used now. This section mainly introduces the chemical battery method, the concentration battery method, and the gas chromatography method.
2.1.Chemical Battery Method The trace oxygen analyzer of the chemical battery method refers to the principle of using the redox battery to perform trace oxygen analysis. Its sensor (detector) is a chemical primary battery, mainly composed of a cathode, an anode, and an electrolyte. The above components are sealed in an inert shell. Oxygen in the gas to be measured enters the O2 near the cathode of the battery to obtain electrons. The anode is made of lead metal. The lost electrons themselves are oxidized. The electrons generated by the battery are extracted by the circuit and then compensated for amplification. The oxygen content in the measured gas can be measured. The reaction formula is as follows: O2+2H2O+4e-→40H-cathode Pb2OH-→pbo H2O2e The total anode reaction formula 2pb+O2→2pbO can be divided into primary cell method, fuel cell method, and Hertz cell method depending on the implementation, which are described below.
2.1.1. Primary cell method The primary oxygen cell micro oxygen analyzer is an early product. It is characterized by a simple structure, the detection cell is a development type, the need to add electrolyte, and can be cleaned and replaced electrodes, the use of lower cost. However, the use of lye to use, but also often add distilled water, replace the silica gel desiccant, air tightness is not easy to guarantee, maintenance is more troublesome. The nitrogen oxygen content analysis of an air separation product in our factory was originally performed using the DH-01 oxygen analyzer produced by Sichuan Analytical Instrument Factory, and also used the DH-3 type micro-oxygen analyzer from Nanjing Analytical Instrument Factory. Similar to the DH-01, the foreign product is the Elcoflux-C5 micro Oxygen Analyzer from German H&B Company. The sensitivity and stability are also good, as long as they are mastered or can be used. The electrolyte of this kind of instrument has two kinds of acidity and alkalinity, the lead generally adopts the lead electrode, the cathode adopts the inert silver or gold electrode, when the sensitivity is reduced, can deal with changing the electrode.
2.1.2 Fuel cell method This kind of instrument is an integrated, miniaturized primary battery instrument. Its sensor (detector) is a micro-fuel cell, which is mainly composed of a cathode, an anode, and electrolyte. The above components are sealed in inert. In the shell, the oxygen in the measured gas enters the cell through the (semi-permeable membrane) diffusion membrane on the cell side, but the internal electrolyte cannot seep out. The cathode is made of ruthenium metal and does not participate in the redox reaction itself. O2 near the cathode receives electrons, and the anode is made of lead metal. The lost electrons themselves are oxidized. The electrons generated by the battery are detected by the circuit, which means that the oxygen content in the measured gas is measured. Its key technology is diffuse diffusion membrane, which can quantitatively control oxygen infiltration and prevent water from seeping out. At present, there is no reliable product in China. The main disadvantage of this type of instrument is that it requires periodic replacement of the sensor, and the sensor cannot be stored for long periods of time and is sensitive to the temperature and pressure of the sample. The common instruments in China mainly include EC90 and EC910 series from British SYSTECH, 3000 series from American TELEDYNE, and GPR series from American AII. The technologies are relatively mature. When using such instruments, attention should be paid to background gas and interference impurities. Choose to get accurate results.
2.1.3 Hertz Battery The Fahrenheit battery type micro oxygen analyzer is a special case of the chemical battery method. The electrode is not consumed during use. It is similar to an electrolytic cell. The instrument provides 1.300V DC voltage to provide electron transfer energy. The gas inlet enters, the sample gas outlet discharges, the electrode, the electrolyte and the sample do not have any loss, the cathode is a carbon alloy, the anode is a platinum wire, and the world is only produced by the American company DELTA.F, the measuring principle is (1): on the anode plate (Platinum wire, sample gas outlet): 40H-→O2+2H2O+4e- on the cathode plate (carbon alloy, sample gas inlet): O2+2H2O+4e-→40H- The instrument only needs to periodically add purified water or deionized water during use, maintenance The amount is small, and the instrument can stabilize quickly after adding water. Suitable for analysis of N2, HE, Ar, H2, and hydrocarbons, etc. The minimum detection limit can reach 75PPt and the reliability is high.
2.2 Concentration cell method Concentration cell method, also known as zirconia battery method, it is the use of zirconia element as a key component of the detector, it is mainly composed of oxygen batteries, including zirconia tubes and painted on the bottom of the tube The molybdenum electrode and electrode lead, the electrode lead can lead the signal out; the heating furnace is used to heat the zirconia tube so that it is constant at the set temperature (780±10°C); the standard gas tube is used to connect the standard gas and calibrate the probe; Even used to measure the temperature in the oxygen battery, access to the transmitter temperature control system; wiring board with signal, thermocouple and heating three pairs of terminals, and other filters, mounting flanges and probe housing. As shown in Fig. 1, there are two platinum electrodes on the inner and outer surfaces of the bottom of the zirconia tube, namely a reference electrode and a measuring electrode, respectively with two platinum leads, forming a zirconia oxygen cell, ie, oxygen concentration cell, Its reaction principle at the platinum electrode is O2+4e→2O2-; 2O2-→O2+4e, so that a potential difference is formed between the two electrodes to form a dense cell (2). The influence of many factors such as aging of zirconia battery, dust accumulation, SO2 and SO3 corrosion on the battery, etc., during the operation process, the instrument parameters will gradually change, which will bring errors to the measurement, and the battery aging performance will increase the internal resistance. The bottom potential increases by two main parameters. When the internal resistance is greater than 800Ω or the background potential is increased to (―25 to ―30)mV, the oxygen amount shows a beating phenomenon and the response is slow.
In order to make the measurement accurate, it must be calibrated regularly with the standard gas. Oxygen in the nitrogen of our plant's products is analyzed using the ZR810 oxygen analyzer imported from SYSTECH UK. This instrument is a zirconia type and has two detectors. It can simultaneously analyze the oxygen and water content in the gas. The conversion can be displayed in the range of 0.01×10-4%-100%. When used, only the gas flow rate is 150ml/min, and the power is constant to 650°C, and the oxygen and water content in the sample gas can be analyzed. So this instrument is also very convenient to use. The main disadvantage of zirconia concentration batteries is that reducing impurities have an effect on the analysis of trace oxygen. Because in the case of 500-800 degrees Celsius, reducing substances can react with oxygen and consume oxygen to make the analysis result low, its main advantage is wide range, can cover constant to trace oxygen content analysis, easy to use, use long life.
2.3 GC Gas Chromatography The advantage of performing trace oxygen analysis is that multiple impurities can be detected at the same time because the separation of impurities in the air separation gas is relatively easy, so the configuration of the column system is simple. In the detection of multiple impurities containing traces of oxygen, the choice of chromatographic analysis is appropriate. Chromatographic detectors that can be selected include thermal conductivity detectors, electron capture detectors, cesium ionization detectors, argon ionization detectors, discharge ionization detectors, and atomic emission detectors (AEDs). The following is a brief introduction. Selecting a chromatographic detector mainly considers the properties of the sample being tested, such as the composition of the main component, the content of interfering impurities, and the suitability for the detector. For example, helium ionization detectors, discharge ionization detectors, and electromagnetic induction detectors perform trace oxygen in helium. Argon ionization detectors and electromagnetic induction detectors facilitate the analysis of trace oxygen in argon gas. Treatment of components. The second choice is based on the detection range of trace oxygen. The detection limit of the thermal conductivity detector is higher and it is not suitable for trace oxygen analysis at lower levels. The helium ionization detector, argon ionization detector, and discharge ionization detector PPb and trace oxygen analysis can be performed. The disadvantage of chromatographic analysis of trace oxygen is the inability to achieve true on-line analysis, that is, it is impossible to perform real-time monitoring of trace oxygen, requires intermittent detection, and the equipment system is complex, requiring carrier gas, auxiliary gas, etc.
3. Note on the use of oxygen analyzer When conducting trace oxygen analysis, because the oxygen content in the air is as high as 21% 02, if it is handled improperly, it will easily cause pollution and interference to the sample, and the analysis result data will be abnormal. The main reason is the improper operation of the instrument. The following is only a few comments for reference.
(1) Leakage. Oxygen analyzers must be strictly leak-checked before being used for the first time. The instrument can only obtain the data result if it is strictly leakproof. Any imperfections at any connection points, solder joints, valves, etc., will cause oxygen in the air to infiltrate into the pipeline and the instrument, resulting in a high oxygen content.
(2) Pollution. When reusing the instrument, it is first important to note that no air is leaking when connecting the sampling line. In addition, the air must be blown out and the oxygen should be passed through the sensor to extend the life of the sensor. In the purification process of the pipeline system, a certain method is needed to shorten the purification time. Generally, high-pressure gas discharge and small-flow blow-off are alternately used to quickly clean the pipeline.
(3) The choice of pipe material. The material and surface roughness of the pipe will also affect the oxygen content of the sample gas. In general, plastic pipes, rubber pipes, etc. should not be used as connecting pipes. Copper tubes or stainless steel tubes are usually used. For ultra-micro analysis (<0.1×10-4%), polished stainless steel tubes must be used.
(4) Simplification and cleanliness of the airway system. Micro-analysis requires that the contamination of the sample gas must be effectively eliminated in the dead ends of various fittings, valves, and gauges on the gas line. Therefore, gas path systems should be simplified as much as possible, and connectors with small dead angles should be used. Also, avoid the use of water seals, oil seals, and wax seals to prevent the dissolved oxygen from escaping and causing pollution. It is even more necessary to avoid the addition of purifying equipment that can easily cause pollution when the sample gas is drawn to the instrument inlet. Only in this way can the system be clean and the data obtained be accurate.
(5) Background gas interference. For fuel cell analyzers, different main background gases (related to the molar mass of the background gas) will affect the analysis results. And a small amount of hydrogen will also have a greater impact on the analysis results. For sample gas containing traces of acid gas, only a special type of Hertz Battery Oxygen Analyzer (produced by American â–³F Corporation) can eliminate interference, prevent sensor poisoning, and provide correct analytical data. No other fuel cell oxygen analyzer can be used for acid gas analysis.
(6) Interference with impurities. Interference impurities mainly refers to the presence of a small amount of unknown impurities in addition to the background gas and the components to be tested, such as the fuel cell type micro-oxygen analyzer is sensitive to trace acid, basic gas impurities, if the gas contains traces of acid gas components, such as C02, H2S, HCl , HCN, etc., then the fuel pool sensor is susceptible to poisoning failure. The trace amount of reducing gas affects the analysis results of the zirconia type micro oxygen analyzer. Recently, it has been found that trace amounts of N2O have an influence on the zirconia type micro-oxygen analyzer. Therefore, the presence of trace impurities must be fully taken into account during the inspection. The analyzer can get accurate results.
4. Concluding remarks Since there are many kinds of trace oxygen analyzers and they are widely used, in production, we must consider the sample, interference, and application conditions in order to meet the needs of practical applications. We must select the appropriate analytical instruments to obtain accurate analysis results. Better Gas production service.
2. Classification, principles and characteristics Trace oxygen analysis methods are mainly colorimetric method, chemical cell method, yellow phosphorescence method, concentration cell method and gas chromatography. Among them, the colorimetric method is an earlier analysis method. It is a method prescribed by the national standard. Colorimetric analysis using copper-ammonia solution is difficult because the operation is complicated and the automatic online analysis cannot be implemented. It is rarely used nowadays. , but it is still an arbitration method. The yellow phosphorus luminescence method utilizes the oxygen and yellow phosphorus oxidative combustion for analysis. It has the characteristics of rapid analysis and continuous analysis. However, the yellow phosphorus used in this method is a dangerous chemical. The resulting product is corrosive and the detection limit is low. So it is rarely used now. This section mainly introduces the chemical battery method, the concentration battery method, and the gas chromatography method.
2.1.Chemical Battery Method The trace oxygen analyzer of the chemical battery method refers to the principle of using the redox battery to perform trace oxygen analysis. Its sensor (detector) is a chemical primary battery, mainly composed of a cathode, an anode, and an electrolyte. The above components are sealed in an inert shell. Oxygen in the gas to be measured enters the O2 near the cathode of the battery to obtain electrons. The anode is made of lead metal. The lost electrons themselves are oxidized. The electrons generated by the battery are extracted by the circuit and then compensated for amplification. The oxygen content in the measured gas can be measured. The reaction formula is as follows: O2+2H2O+4e-→40H-cathode Pb2OH-→pbo H2O2e The total anode reaction formula 2pb+O2→2pbO can be divided into primary cell method, fuel cell method, and Hertz cell method depending on the implementation, which are described below.
2.1.1. Primary cell method The primary oxygen cell micro oxygen analyzer is an early product. It is characterized by a simple structure, the detection cell is a development type, the need to add electrolyte, and can be cleaned and replaced electrodes, the use of lower cost. However, the use of lye to use, but also often add distilled water, replace the silica gel desiccant, air tightness is not easy to guarantee, maintenance is more troublesome. The nitrogen oxygen content analysis of an air separation product in our factory was originally performed using the DH-01 oxygen analyzer produced by Sichuan Analytical Instrument Factory, and also used the DH-3 type micro-oxygen analyzer from Nanjing Analytical Instrument Factory. Similar to the DH-01, the foreign product is the Elcoflux-C5 micro Oxygen Analyzer from German H&B Company. The sensitivity and stability are also good, as long as they are mastered or can be used. The electrolyte of this kind of instrument has two kinds of acidity and alkalinity, the lead generally adopts the lead electrode, the cathode adopts the inert silver or gold electrode, when the sensitivity is reduced, can deal with changing the electrode.
2.1.2 Fuel cell method This kind of instrument is an integrated, miniaturized primary battery instrument. Its sensor (detector) is a micro-fuel cell, which is mainly composed of a cathode, an anode, and electrolyte. The above components are sealed in inert. In the shell, the oxygen in the measured gas enters the cell through the (semi-permeable membrane) diffusion membrane on the cell side, but the internal electrolyte cannot seep out. The cathode is made of ruthenium metal and does not participate in the redox reaction itself. O2 near the cathode receives electrons, and the anode is made of lead metal. The lost electrons themselves are oxidized. The electrons generated by the battery are detected by the circuit, which means that the oxygen content in the measured gas is measured. Its key technology is diffuse diffusion membrane, which can quantitatively control oxygen infiltration and prevent water from seeping out. At present, there is no reliable product in China. The main disadvantage of this type of instrument is that it requires periodic replacement of the sensor, and the sensor cannot be stored for long periods of time and is sensitive to the temperature and pressure of the sample. The common instruments in China mainly include EC90 and EC910 series from British SYSTECH, 3000 series from American TELEDYNE, and GPR series from American AII. The technologies are relatively mature. When using such instruments, attention should be paid to background gas and interference impurities. Choose to get accurate results.
2.1.3 Hertz Battery The Fahrenheit battery type micro oxygen analyzer is a special case of the chemical battery method. The electrode is not consumed during use. It is similar to an electrolytic cell. The instrument provides 1.300V DC voltage to provide electron transfer energy. The gas inlet enters, the sample gas outlet discharges, the electrode, the electrolyte and the sample do not have any loss, the cathode is a carbon alloy, the anode is a platinum wire, and the world is only produced by the American company DELTA.F, the measuring principle is (1): on the anode plate (Platinum wire, sample gas outlet): 40H-→O2+2H2O+4e- on the cathode plate (carbon alloy, sample gas inlet): O2+2H2O+4e-→40H- The instrument only needs to periodically add purified water or deionized water during use, maintenance The amount is small, and the instrument can stabilize quickly after adding water. Suitable for analysis of N2, HE, Ar, H2, and hydrocarbons, etc. The minimum detection limit can reach 75PPt and the reliability is high.
2.2 Concentration cell method Concentration cell method, also known as zirconia battery method, it is the use of zirconia element as a key component of the detector, it is mainly composed of oxygen batteries, including zirconia tubes and painted on the bottom of the tube The molybdenum electrode and electrode lead, the electrode lead can lead the signal out; the heating furnace is used to heat the zirconia tube so that it is constant at the set temperature (780±10°C); the standard gas tube is used to connect the standard gas and calibrate the probe; Even used to measure the temperature in the oxygen battery, access to the transmitter temperature control system; wiring board with signal, thermocouple and heating three pairs of terminals, and other filters, mounting flanges and probe housing. As shown in Fig. 1, there are two platinum electrodes on the inner and outer surfaces of the bottom of the zirconia tube, namely a reference electrode and a measuring electrode, respectively with two platinum leads, forming a zirconia oxygen cell, ie, oxygen concentration cell, Its reaction principle at the platinum electrode is O2+4e→2O2-; 2O2-→O2+4e, so that a potential difference is formed between the two electrodes to form a dense cell (2). The influence of many factors such as aging of zirconia battery, dust accumulation, SO2 and SO3 corrosion on the battery, etc., during the operation process, the instrument parameters will gradually change, which will bring errors to the measurement, and the battery aging performance will increase the internal resistance. The bottom potential increases by two main parameters. When the internal resistance is greater than 800Ω or the background potential is increased to (―25 to ―30)mV, the oxygen amount shows a beating phenomenon and the response is slow.
In order to make the measurement accurate, it must be calibrated regularly with the standard gas. Oxygen in the nitrogen of our plant's products is analyzed using the ZR810 oxygen analyzer imported from SYSTECH UK. This instrument is a zirconia type and has two detectors. It can simultaneously analyze the oxygen and water content in the gas. The conversion can be displayed in the range of 0.01×10-4%-100%. When used, only the gas flow rate is 150ml/min, and the power is constant to 650°C, and the oxygen and water content in the sample gas can be analyzed. So this instrument is also very convenient to use. The main disadvantage of zirconia concentration batteries is that reducing impurities have an effect on the analysis of trace oxygen. Because in the case of 500-800 degrees Celsius, reducing substances can react with oxygen and consume oxygen to make the analysis result low, its main advantage is wide range, can cover constant to trace oxygen content analysis, easy to use, use long life.
2.3 GC Gas Chromatography The advantage of performing trace oxygen analysis is that multiple impurities can be detected at the same time because the separation of impurities in the air separation gas is relatively easy, so the configuration of the column system is simple. In the detection of multiple impurities containing traces of oxygen, the choice of chromatographic analysis is appropriate. Chromatographic detectors that can be selected include thermal conductivity detectors, electron capture detectors, cesium ionization detectors, argon ionization detectors, discharge ionization detectors, and atomic emission detectors (AEDs). The following is a brief introduction. Selecting a chromatographic detector mainly considers the properties of the sample being tested, such as the composition of the main component, the content of interfering impurities, and the suitability for the detector. For example, helium ionization detectors, discharge ionization detectors, and electromagnetic induction detectors perform trace oxygen in helium. Argon ionization detectors and electromagnetic induction detectors facilitate the analysis of trace oxygen in argon gas. Treatment of components. The second choice is based on the detection range of trace oxygen. The detection limit of the thermal conductivity detector is higher and it is not suitable for trace oxygen analysis at lower levels. The helium ionization detector, argon ionization detector, and discharge ionization detector PPb and trace oxygen analysis can be performed. The disadvantage of chromatographic analysis of trace oxygen is the inability to achieve true on-line analysis, that is, it is impossible to perform real-time monitoring of trace oxygen, requires intermittent detection, and the equipment system is complex, requiring carrier gas, auxiliary gas, etc.
3. Note on the use of oxygen analyzer When conducting trace oxygen analysis, because the oxygen content in the air is as high as 21% 02, if it is handled improperly, it will easily cause pollution and interference to the sample, and the analysis result data will be abnormal. The main reason is the improper operation of the instrument. The following is only a few comments for reference.
(1) Leakage. Oxygen analyzers must be strictly leak-checked before being used for the first time. The instrument can only obtain the data result if it is strictly leakproof. Any imperfections at any connection points, solder joints, valves, etc., will cause oxygen in the air to infiltrate into the pipeline and the instrument, resulting in a high oxygen content.
(2) Pollution. When reusing the instrument, it is first important to note that no air is leaking when connecting the sampling line. In addition, the air must be blown out and the oxygen should be passed through the sensor to extend the life of the sensor. In the purification process of the pipeline system, a certain method is needed to shorten the purification time. Generally, high-pressure gas discharge and small-flow blow-off are alternately used to quickly clean the pipeline.
(3) The choice of pipe material. The material and surface roughness of the pipe will also affect the oxygen content of the sample gas. In general, plastic pipes, rubber pipes, etc. should not be used as connecting pipes. Copper tubes or stainless steel tubes are usually used. For ultra-micro analysis (<0.1×10-4%), polished stainless steel tubes must be used.
(4) Simplification and cleanliness of the airway system. Micro-analysis requires that the contamination of the sample gas must be effectively eliminated in the dead ends of various fittings, valves, and gauges on the gas line. Therefore, gas path systems should be simplified as much as possible, and connectors with small dead angles should be used. Also, avoid the use of water seals, oil seals, and wax seals to prevent the dissolved oxygen from escaping and causing pollution. It is even more necessary to avoid the addition of purifying equipment that can easily cause pollution when the sample gas is drawn to the instrument inlet. Only in this way can the system be clean and the data obtained be accurate.
(5) Background gas interference. For fuel cell analyzers, different main background gases (related to the molar mass of the background gas) will affect the analysis results. And a small amount of hydrogen will also have a greater impact on the analysis results. For sample gas containing traces of acid gas, only a special type of Hertz Battery Oxygen Analyzer (produced by American â–³F Corporation) can eliminate interference, prevent sensor poisoning, and provide correct analytical data. No other fuel cell oxygen analyzer can be used for acid gas analysis.
(6) Interference with impurities. Interference impurities mainly refers to the presence of a small amount of unknown impurities in addition to the background gas and the components to be tested, such as the fuel cell type micro-oxygen analyzer is sensitive to trace acid, basic gas impurities, if the gas contains traces of acid gas components, such as C02, H2S, HCl , HCN, etc., then the fuel pool sensor is susceptible to poisoning failure. The trace amount of reducing gas affects the analysis results of the zirconia type micro oxygen analyzer. Recently, it has been found that trace amounts of N2O have an influence on the zirconia type micro-oxygen analyzer. Therefore, the presence of trace impurities must be fully taken into account during the inspection. The analyzer can get accurate results.
4. Concluding remarks Since there are many kinds of trace oxygen analyzers and they are widely used, in production, we must consider the sample, interference, and application conditions in order to meet the needs of practical applications. We must select the appropriate analytical instruments to obtain accurate analysis results. Better Gas production service.
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