The cryogenic air separation equipment achieves the separation of air components at low temperatures, and has high requirements for the water content of the gas entering the distillation tower for separation. Therefore, the configuration of high-performance trace water analyzers is crucial for the continuous, safe, and stable operation of the air separation equipment. At present, trace water analyzers widely used in the air separation industry can be divided into three types according to different measurement principles: electrolytic, capacitive, and cold mirror. Today, we will introduce the application and maintenance of one type of capacitive trace water analyzer in air separation equipment.
1. Measurement principle
The sensor part of the capacitive micro water analyzer (dew point meter) is equivalent to a capacitor. A thin layer of porous alumina is deposited on a conductive substrate, and then a thin layer of gold is coated on top of the alumina layer. The conductive substrate and the gold layer form the electrodes of the capacitor. The tested gas passes through a thin layer of gold, and its moisture is absorbed by the porous alumina. At this time, the impedance of the capacitor is proportional to the number of water molecules (i.e. water vapor partial pressure). By measuring the impedance or capacitance of the capacitor, the partial pressure of water and gas can be obtained, which can be converted to obtain the dew point value (i.e. trace water content). The thin film capacitive trace water analyzer uses polyurethane or acetate fiber polymer films deposited on two conductive electrodes. When the film absorbs or loses water, it will change the dielectric constant between the two electrodes. At present, there is also a technology that uses high-temperature resistant thermosetting polymers, which can enable continuous measurement of such sensors at temperatures above 100 ℃. The capacitive trace water analyzer can meet all the process detection requirements of air separation equipment, and has a relatively high cost-effectiveness, so it is widely used.
2. Advantages of Measurement
The advantages of capacitive trace water analyzer (dew point meter) are: ① It has a wide response range and can accurately measure the dew point between 110 ℃ and 20 ℃; ② It can be installed remotely or used on-site; ③ Relatively stable performance, fast response, small temperature coefficient, independent of flow rate changes; ④ Has high selectivity for moisture and can be used over a wide range of temperatures and pressures; ⑤ There are no consumables except for the power supply battery; ⑥ The daily maintenance workload is relatively small, and the service life is long, generally exceeding 10 years; ⑦ Equipped with automatic calibration function, it can calibrate automatically at any time. The disadvantage of capacitive trace water analyzer is that it uses indirect measurement method, which can easily cause instrument drift when operated at high temperatures, and the instrument needs to be calibrated after a certain period of operation.