Trace moisture measurement in chlorine or hydrogen chloride gas
1. Moisture measurement and control
Chlorine gas turbine compressors are widely used in the chlor-alkali industry. In order to prevent damaging corrosion of the compressor by wet chlorine gas, it is very important to monitor the moisture content of the chlorine gas entering the compressor and interlock protection of the compressor.
To on-line monitor whether the pre-cooler is leaking, many compressor users also use MZD SMART trace moisture analyzer to monitor the chlorine gas at the outlet of the compressor to avoid the delays in monitoring chloride ion measurements in cooling water with conductivity and pH/ORP measurements, to avoid serious losses from chlorine compressor leak accidents. For the moisture content of chlorine is usually below 40ppm and the alarm value is 150ppm, 0~500ppm range is usually used.
There are two methods of online moisture measurement: direct measurement method and indirect measurement method. The direct measurement method includes electrolysis, laser method and fiber optic method, and the indirect measurement method includes dew point, cold mirror method and capacitance method.
1.1 Electrolysis method (phosphorus pentoxide P2O5)
The sensor pillar is plated with parallel spiral platinum layer as the electrode, and the hydrated phosphorus pentoxide film is coated between the platinum layer. Phosphorus pentoxide has a strong water absorption, when chlorine gas flows steadily through the sensor flow cell, where the water is absorbed to generate phosphoric acid, the reaction formula is as follows:
P2O5 + 3H2O → 2H3PO4
At the same time, between the two platinum layers to plus DC voltage, that has the electrolysis reaction, phosphoric acid is reductively decomposed into oxygen,chlorine, phosphorus pentoxide.The reaction formula is as follows:
4H3PO4→6H2+3O2+2P2O5
It is known from Faraday's electrolysis law: the electricity required to electrolyze 1 mole equivalent of substance is 96,500 coulombs, which is also the electricity required to electrolyze 9.01 grams of water, thus it can be seen that the electrolytic current between the platinum wire electrodes is a linear function of the moisture content in the quantitative gas.
1.2. Laser method
This method is based on the absorption of light of a specific wavelength by the detected molecule. After the light in a specific wavelength range passes through the sample gas, and then measure the intensity of the transmitted light (I) and incident light (I0) at the detector (Lambert-Beer law) to know the current gas concentration measurement chamber. The amount of absorption is proportional to the moisture content of the sample gas. Not suitable for contamination with impurity particles.
Sample gas needs to be very clean. TThe small range is 0~10ppm with high precision, which is suitable for the electronic industry, but not suitable for the occasions with impurity particle contamination.
To Alkali-Chlorine industry, chlorine and hydrogen chloride are highly corrosive, and in the chemical process, chlorine and hydrogen chloride gas pipeline is not as clean as the electronics industry, and the process equipment metal pipeline will also be corroded by chlorine and hydrogen chloride gas to produce impurities, so some laser analyzers need purge gas to prevent chlorine and hydrogen chloride corrosion to protect the laser lens detection unit to maintain clean. Obviously, if the moisture or cleanliness of the purge gas changes, it will inevitably cause errors in the final measurement results. For example, a Geman laser analyzer with a large range of 0~100,000 or 200,000ppm, accuracy is 2% or 10ppm (whichever is larger), repeatability is 0.25% or 50ppm (whichever is larger), the lowest detection limit or resolution is 1ppm or 5ppm; another example, a Japanese laser analyzer with a large range of 0~30,000ppm, linearity is 1% FS, it's 300ppm, repeatability is 2%. It is obvious that laser method is not suitable for chlorine or hydrogen chloride in chemical process.
1.3. Fiber optics method
The surface of the fiber optic humidity sensor is a laminated structure composed of silicon oxide and zirconium oxide with different reflection coefficients. Through advanced thermal curing technology, the pore size of the sensor surface is controlled at 0.3nm, and water molecules at 0.28nm can penetrate. The controller emits a beam of 790~820nm NIR light, which is transmitted to the sensor via fiber optic cable. The water molecules entering the sensor will change the reflection coefficient of the light, thus causing a change in wavelength, which is proportional to the moisture content of the medium. By measuring the wavelength of the received light, the dew point and moisture content of the medium can be obtained.
High accuracy, maintenance-free, very stable, can measure corrosive media containing H2S, HCl, etc., the transmission fiber is easy to break and needs protection, and not suitable for occasions with impurity particles pollution.
1.4. Dew-point method
When a certain volume of gas is cooled uniformly at a constant pressure, the partial pressure of the gas and the moisture in the gas remains constant until the moisture in the gas reaches saturation, and the temperature in this state is the temperature at which the number of water molecules in the gas reaches dynamic equilibrium to determine the dew point of the gas. The relative humidity is indirectly measured by the dew point based on temperature and pressure, and it is greatly influenced by pressure and temperature.
1.4.1 Cold mirror method
The formation of condensation on metal mirrors is directly observed by instrumentation or identified electronically. When condensation is formed at the dew point, the temperature and pressure at the mirror are measured with precision and the moisture content can then be calculated from the water vapor pressure and standard data tables. It is important to note at this point that there are two dew points: the dew point for moisture and the dew point for hydrocarbons.
High accuracy, no corrosion resistance, fast response, and the gas needs to be very clean. It is not suitable for applications with contamination by impurity particles or with corrosion.
1.4.2 Capacitance method
Capacitance sensor consists of a layer of aluminum trioxide (Al2O3) separated by an aluminum disk and a layer of gold, or film apacitance, when the gas passes through the sensor, the moisture content is measured according to the electric capacity.
Cheap, not corrosion resistant, very easy to drift, must be calibrated periodically, high post maintenance costs. Not suitable for applications with impurity particle contamination or corrosion.
2. Sampling system
The design of the sampling system is very important for the accuracy and reliability of the measurement of the moisture content in chlorine and hydrogen chloride gases. Sampling system components and materials require the following points:
1. Materials: PTFE, PVDF or PFA and other anti-corrosion materials.
2. Shorter sampling line, better.
3. Pressure reducer: pressure reducer is used when the pressure is too high.
4. Sampling pump: vacuum or pressure is too low when using the sampling pump.
5. Filter: in the case of sample gas is not very dirty, try not to use the filter.
3. Maintenance
Moisture analyzer accuracy can be affected by conditions such as impurities or sampling systems, requiring timely maintenance of the moisture analyzer.
4. Conclusion
The most of measurement problems are in the sampling system, especially the leakage of the sampling system, and the accuracy and anticorrosive reliability of the instrument depends on equipment selection, sampling system design, operator training, and regular maintenance of the instrument. In summary, electrolysis method is the most suitable method regardless of the selection and design, or long-term experience in use. Moreover, according to international standards, for trace moisture measurement in chlorine gas, electrolysis is the legal arbitration method.