chlor-alkali industry

In chlor alkali production, it is crucial to control the oxygen content of the sealed gas in the chlorine compressor. Oxygen in contact with chlorine or hydrogen may cause explosions, so it is necessary to strictly control the oxygen content in the sealing gas (usually nitrogen). Traditional methods for measuring oxygen content include absorption and gas chromatography, which have disadvantages such as complex operation, high cost, or slow response speed.Plug-in analyzerDue to its high precision, low cost, and easy operation, it has become an ideal tool for measuring oxygen content in industrial sites.

　　1Plug-in analyzerWorking principle and structure

　　1.1 Working principle

The sensor used in the fuel cell method is a closed chemical cell composed of an inert metal electrode (cathode), a lead electrode (anode), and an electrolyte. Oxygen in the gas diffuses into the battery through a polymer film, where it is reduced at the cathode and oxidized at the anode. The current generated by the battery reaction, i.e. the output signal of the sensor, is directly proportional to the oxygen content in the gas. The electrons generated by the battery are extracted from the circuit and then compensated, corrected, and amplified to measure the oxygen content in the measured gas. The reaction equation is as follows:

Cathode O₂+2H₂O+4e→4OH^-;

Anode 2Pb+4OH^-→2 PbO+2H₂O+4e;

Complete chemical reaction O₂+2Pb→2 PbO。

　　1.2Plug-in analyzerstructure

Sensor: Core component, commonly used electrochemical or optical sensor, capable of quickly responding to trace oxygen changes.

Signal processing unit: Convert sensor signals into readable data.

Display screen: Real time display of oxygen mass fraction or volume fraction.

Power module: Supports battery power supply, suitable for on-site use.

Sampling system: including sampling probes and filters to ensure gas purity.

The gas path of the trace oxygen analyzer is shown in Figure 1.

　　Figure 1 Gas Path of Trace Oxygen Analyzer

　　2 Traditional methods for measuring oxygen content

　　2.1 Absorption method

The absorption method is suitable for separating gaseous nitrogen or liquid nitrogen produced from air, mainly used as protective gas, displacement gas, and low-temperature storage. It uses an Auger gas analyzer (see Figure 2) to determine the oxygen content using a caustic soda acid-base solution absorption method. A certain amount of sample gas is taken and reacted with the absorption liquid in a closed absorption bottle. The reduction in the volume of the sample gas due to oxygen absorption is called the oxygen content. This method has low cost, but is complex to operate, time-consuming, and has limited accuracy.

　　Figure 2 Logic of the first generation automatic liquid discharge program of the gas analyzer

　　2.2 Gas Chromatography Method

Gas chromatography is applicable to pure nitrogen, high-purity nitrogen, and ultra pure nitrogen in gaseous or liquid form, mainly used as protective gas, displacement gas, reaction gas, etc., obtained by air separation or purification. A gas chromatograph with zirconia detector and hydrogen flame ionization detector (Figure 3) is used to determine hydrogen, oxygen, carbon monoxide, methane, and carbon dioxide in nitrogen. Gas chromatography has high accuracy and sensitivity for the determination of trace oxygen, but the equipment is expensive, the operation is complex, and requires professional personnel to operate, making it suitable for laboratory environments.

　　Figure 3 Working principle of gas chromatograph

　　3Plug-in analyzerthe advantage of

　　3.1 High precision and fast response speed of oxygen sensor

Portable trace oxygen analyzer can quickly and accurately determine trace oxygen content, with short response time, suitable for real-time monitoring. Because the rate of oxygen molecules diffusing into the sensor is controlled by a polytetrafluoroethylene membrane, all diffusion processes are sensitive to temperature, pressure, and relative humidity. Temperature characteristics: The signal output of the sensor will vary with temperature changes. This change (2.5%/℃) is relatively stable. The temperature compensation circuit using thermistor offsets this effect with an accuracy of+5%/℃ or higher, and produces an output function that is independent of temperature. If calibration and sampling are performed at the same temperature, or measurements are taken immediately after calibration, there are no errors. A small temperature change (10-15 ℃) results in an error of less than+1%.

　　Figure 4 Temperature characteristic curve of sensor

Pressure and flow characteristics: The oxygen sensor of the plug-in analyzer measures the partial pressure of oxygen, and its output is affected by changes in sample gas pressure and atmospheric pressure. The output signal is linearly proportional to the partial pressure of oxygen in the mixed gas, and any change in atmospheric pressure will result in an equivalent change in the sensor output current. Therefore, the reading of the analyzer is only correct when there is no change in the total pressure of the analyzed gas or when it is compensated by the atmospheric pressure signal. If the sample pressure is controlled to be constant, the analyzer should calibrate the sample gas under conditions similar to the measured pressure and flow rate. If a separate sensor is used to measure pressure, the oxygen concentration can be calculated based on the absolute pressure of the sampled gas. The output of the sensor is not directly affected by the flow rate of the sample gas. 0.0283~0.1415m³/The flow rate of h will not cause significant changes in oxygen readings. However, the traffic exceeds 0.1415m³/H may generate back pressure and incorrect oxygen readings. For optimal performance, it is recommended to have a gas flow rate of 0.0566m³/H or 1L/min.

Response time: The response time of the sensor depends on many factors such as the concentration of the measured sample gas. When the oxygen sensor reaches equilibrium in the trace range, its response time to changes in oxygen concentration is very fast. If exposed to oxygen with a volume fraction exceeding 1%, it may cause the liquid electrolyte of the sensor to temporarily saturate, requiring additional time to reach equilibrium and obtain rapid response again.

The response time available for reference is as follows:

When decreasing from 1% to 0.1%, less than 180 seconds;

When it drops from 0.1% to 0.00l%, it takes less than 10 minutes;

From 10 × 10^-6Decrease to less than 10^-6Less than 10 hours.

Humidity effect: The output of the sensor is proportional to the relative humidity of oxygen. Oxygen sensors are suitable for non condensing situations where the relative humidity of the sample gas is between 0 and 99%. The non condensing humidity has no effect on the performance of the sensor, except for the dilution gas. However, it should be noted that the way water vapor generates pressure is the same as the oxygen in the sample airflow. In a gas sample with a constant absolute pressure, the higher the humidity, the lower the partial pressure of the oxygen component. When measuring oxygen in sample gases, if calculated by volume fraction or mass fraction, the influence of relative humidity must also be considered. If the relative humidity generally remains constant from the calibration date, the offset caused by relative humidity will be minimal. If the user calibrates the sensor in dry gas and humidifies the sample gas, the sensor output signal will be slightly lower than the previous output signal. This is caused by the dilution of oxygen in the sample gas by water vapor. This characteristic is crucial in systems where both wet and dry air flows coexist. In addition, high humidity airflow may condense on the sensor and ultimately affect its performance.

Sensor influencing factors: The chemical composition of the sensor is rarely affected by other gases, including refrigerants and hydrocarbon compounds. However, based on the chemical properties of the electrolyte, the sensor is still extremely sensitive to some high concentrations of corrosive gases. Table 1 lists the effects of various gases on sensors.

　　Table 1 Effects of Various Gases on Sensors

The interference level is compared to a non polluting gas mixture, containing 0-2000 × 10^-6The rate of change of sensor output in a gas mixture of oxygen.

If Cl₂The interference level is considered to be in a 1:2 ratio, so it contains Cl₂Add 100 x 10^-6The output signal of the sensor in the sample gas of oxygen is equivalent to 300 × 10^-6The normal oxygen signal is 100 × 10^-6Additionally, 200 × 10^-6For Cl₂The interference caused.

　　3.2 Low cost

Compared to gas chromatography, portable trace oxygen analyzers have significantly reduced procurement costs, low maintenance costs, no complex mechanical structures, long sensor life, and do not require frequent replacement of consumables (such as desiccants, electrolytes, etc.), making them suitable for large-scale applications. When oxygen sensors are exposed to trace amounts of oxygen, they typically provide good performance within a 2-year lifespan. Regular exposure to sample gases containing a certain humidity can help restore the water concentration in the electrolyte to a normal state, thereby extending the lifespan of the sensor. The portable trace oxygen analyzer adopts a low-power design, supports battery power supply, and is suitable for outdoor or industrial sites without stable power supply. The long-term electricity cost tends to be zero, and the energy consumption is low. Portable trace oxygen analyzer saves labor costs, reduces laboratory sample delivery process for rapid detection, avoids the need for dedicated personnel on duty, and is particularly suitable for multi-point inspections (such as electrolytic cells, air compression stations, etc.).

　　3.3 Easy to operate

Plug in analyzer is ready to use, without the need for preheating or complex initialization. It can directly enter measurement mode after startup, improving emergency detection efficiency (such as sudden situations in chemical production). Equipped with a touch screen or one click buttons, the menu logic is clear, and it supports a multilingual interface, reducing the operating threshold for non-technical personnel. Built in self checking and calibration programs reduce manual intervention and avoid the tedious steps of manual calibration. Most brands and models of portable trace oxygen analyzers can output results within 1-3 minutes, making them particularly suitable for real-time monitoring of production lines to avoid downtime and waiting.

　　3.4 Multifunctionality

In addition to measuring deoxygenation content, some plug-in analyzers can also measure other gas parameters to meet various industrial needs, with a wide range coverage and a detection range from 10^-6From trace oxygen to high concentration oxygen content in percentage levels, it meets the needs of various scenarios and has good data scalability. It has built-in storage and Bluetooth/WiFi transmission functions, supports generating trend analysis reports, or interfaces with LIMS systems (laboratory information management systems). Most portable trace oxygen analyzers have strong environmental adaptability and are designed to withstand high temperatures and corrosion. They can work stably in harsh environments such as petrochemicals and pharmaceuticals, and some models are compatible with explosion-proof requirements.

　　4 Application Examples

Use an insertion analyzer to monitor the oxygen content in the nitrogen gas of the chlorine pressure seal.

　　4.1 Calibration

Firstly, confirm that the four-way valve is in BYPASS state and the flow regulating needle valve is closed. Press and hold the power button for about 3 seconds to turn on the machine, and connect the oxygen content to 5 × 10^-6After purging the analyzer pipeline with the standard gas for 3-5 minutes, open the four-way valve to the SAMPLE state, wait for the data to stabilize, enter the calibration interface, select low point calibration, input the standard gas concentration value into the target value, press √ to confirm, and complete the calibration. Exit the calibration interface, adjust the four-way valve to BYPASS state, and close the flow control needle valve.

　　4.2 Sample determination

Firstly, confirm that the four-way valve is in BYPASS state and the flow regulating needle valve is closed. Press and hold the power button for about 3 seconds to turn on the device. Connect the pre processed sampling gas (recommended sampling gas temperature is 5-45 ℃, gas pressure is 34.48-68.95kPa) to the equipment inlet and adjust the flow rate to 0.0566m³/h, After purging the analyzer pipeline for about 3 minutes, open the four-way valve to the SAMPLE state, wait for the data to stabilize, record the measurement value, or save the measurement value. After the test is complete, adjust the four-way valve to BYPASS state, close the flow control needle valve, press and hold the power button for about 3 seconds to shut down, and disconnect the sampling pipeline. The parallel measurement results are shown in Table 2.

　　Table 2 Parallel Measurement Results

The parallel measurement results in Table 2 indicate that the instrument can measure oxygen content in real time and accurately, ensuring that the oxygen content in the sealed gas of the chlorine pressure chamber is below 10 × 10 ^ 2^-6Meet the requirements of safety production. Compared with traditional methods, portable trace oxygen analyzers significantly improve detection efficiency and reduce costs.

　　5 Conclusion

The insertion analyzer has shown advantages of high accuracy, low cost, and easy operation in the determination of oxygen content in chlorine compressor seals, making it suitable for industrial field applications. Compared to traditional absorption and gas chromatography methods, portable trace oxygen analyzers are more competitive in terms of efficiency and economy, making them an ideal choice for oxygen content determination in the chlor alkali industry.