Summary of the Use of Chlorine-Hydrogen Analyzer

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This post was last edited by sunjl1981 on 2013-1-6 23:17. Summary of the Use of Chlorine and Hydrogen Analyzers 1 Overview Chlorine and hydrogen are by-products that are inevitably generated during the electrolysis of saltwater to produce caustic soda. Hydrogen is a highly flammable gas, while chlorine is a toxic gas with strong oxidizing properties; when these two gases are mixed, an explosion can occur very easily. When the hydrogen content in chlorine is within the explosive range, an explosion can happen at any time under exposure to light or heat. In the chlor-alkali industry, explosion accidents caused by an increase in hydrogen content in chlorine pipelines are not uncommon. The main reason for the mixture of hydrogen and chlorine is the low level of brine in the anode chamber ； The hydrogen outlet of the electrolyzer is blocked, causing an increase in pressure in the cathode chamber ； The adsorption quality of the electrolyzer diaphragm is poor ； The asbestos fiber quality is poor ； The diaphragm was damaged during the installation of the electrolyzer, resulting in partial detachment of the diaphragm ； An excessive amount of salt water injected before power supply can damage the diaphragm, and such situations can all lead to an increase in the hydrogen content in chlorine. When this reaches the explosive limit, an explosion is likely to occur, resulting in serious consequences. The explosive range of hydrogen in chlorine (by volume percentage) is 5.5%–89%. Therefore, the \"Safety Regulations for Chlorine Gas (GB 11984-89)\」specify that the hydrogen content in the chlorine gas main pipe should be ≤0.4%. Therefore, to ensure the safe operation of the electrolysis process, it is necessary to strictly control the chlorine and hydrogen content in the chlorine main pipe. During the initial stage of operation, it is most likely that the hydrogen content in chlorine within the chlorine pipeline will reach the explosive range. Therefore, some companies invest significant human and financial resources in taking samples for analysis every 2 minutes or even more frequently, in order to closely monitor the hydrogen content in chlorine within such pipelines during this phase. For the analysis of hydrogen content in chlorine main pipes, the combustion method or explosion method is widely used at present; these detection techniques are relatively outdated, inaccurate, and pose a risk to operators due to chlorine leaks that occur during sampling. Another commonly used analysis method is chromatographic analysis; it is complex to operate, requires frequent maintenance, and is costly, with the chromatography column needing to be replaced every six months ； Moreover, the analysis results are far too delayed to enable real-time monitoring. Therefore, finding fast, accurate, and safe analysis methods to determine the hydrogen content in chlorine gas pipelines has always been one of the main challenges in the chlor-alkali industry. The chlorine-hydrogen analyzer offers accurate measurements, is simple, convenient, and safe to calibrate, requires virtually no maintenance costs, and has a long service life – making it fully suitable for analyzing the chlorine-hydrogen content in chlorine gas mainlines. For the above reasons, our company was the first in China to adopt Hitech’s KK650 chlorine and hydrogen analyzer for online monitoring of two key parameters – chlorine purity and chlorine hydrogen content – in the chlorine main pipeline of diaphragm caustic soda production plants. 2 Composition and Working Principle of the Chlorine-Hydrogen Analyzer 2.1 Composition of the Chlorine-Hydrogen Analyzer The chlorine-hydrogen analyzer consists of modules such as a sampling system, a sample gas treatment system, a sensor system, and a control and display system. 2.1 Working principle of the chlorine and hydrogen analyzer: The sample gas is collected by the sampling system and then fed into the sample gas treatment system, where particulate impurities, salts, and moisture are removed. It subsequently passes through the sensor system (sensor/reactor); this sensor system consists of two thermal conductivity detectors, with a reactor located between them. First, the thermal conductivity of the sample gas is measured, and then hydrogen is completely reacted in a reaction furnace. The gas after the reaction passes through the second detector. The difference in thermal conductivity measured by these two detectors is the hydrogen content in the sample gas. The data is displayed through the control/display table. 3 Applications of the chlorine and hydrogen analyzer: The chlorine and hydrogen analyzer arrived in mid-July 2008, was installed by the end of July 2008, and trial operation began on July 26. It has been in operation for over 2 months now, during which it has gone through the full startup process of the diaphragm caustic soda plant on 4 occasions, performing excellently throughout these startups, with analysis data that is sensitive, accurate, and reliable. 3.1 Installation and commissioning: Thoroughly inspect all pipeline connections before introducing the sample gas. Chlorine is a highly toxic and highly corrosive gas; it is essential to ensure there are no leaks, in particular to verify that all connections are properly tightened, and to confirm that the fragile reaction reactor tubes are not damaged. After the instrument is initialized, the display will indicate that it is waiting for the reactor temperature to reach its operating temperature. Once it reaches the normal operating temperature (725°C), the analyzer will automatically enter normal detection mode. Keep the sample gas flow rate between 100 and 300 ml/min. After entering normal detection mode, carry out the calibration process in the following order: ① Perform zero-point calibration using air ； ②Calibrate the hydrogen range using air containing 2% hydrogen ； ③Calibrate the chlorine range using 100% Ar or a process gas with known composition. During the calibration process, it is essential to ensure that all other gases in the sample gas line are replaced by the standard gas being calibrated; otherwise, errors will occur. A more convenient way to verify the chlorine range is to use process sample gas, provided that the concentration of the components in the sample gas is known with certainty through some other reliable analytical method. 3.2 Problems that occurred during operation: After installation and commissioning were completed and the system was put into production, the overall situation was good, but some problems did arise. 3.2.1 Poor drainage of the filter: Four days after it was put into operation, issues were detected in the readings of the analyzer: the chlorine content reading was below 5%, while the hydrogen content reading was negative, which clearly did not correspond to the actual conditions. After analysis, it is likely caused by a blockage in the sample gas pipeline. Upon inspection of the analysis instrument, it was found that the chlorine-containing solution in the filter could not be discharged; the membrane filtration components of the entire filter were submerged in this chlorine solution, preventing the sample gas from passing through. The main reasons were the small inner diameter of the filter’s drainage pipe (φ4) and the small capacity of the water collection container (10 ml); most critically, there was no balance pipe, which prevented water from being discharged into the collection container. To address these issues, a balance pipe was added between the collection container and the filter, and a container with a larger capacity was used, thereby resolving the problem. The size of the collector’s volume can be determined based on the requirements of the collector’s drainage cycle. According to actual measurements, the filter discharges about 40 ml of water per day; if a collection container with a capacity of 2500 ml is used, it will be sufficient to meet the water collection needs for 30 days. If it is too small, the drainage cycle is short and the number of drainage operations increases; if it is too large, it increases the amount of sample gas in the sampling system, resulting in a longer delay in the analysis results. 3.2.2 Low hydrogen reading: After resolving the issue of poor drainage from the filter, readings showing a hydrogen content of nearly 0.00% appeared, which is significantly inaccurate compared to the actual situation ； The chlorine reading is normal. The reason for this is likely that the material of the collector added to address the drainage issue with the filter is transparent glass. Since hydrogen and chlorine are light-sensitive gases, they reacted under the influence of light as they passed through the transparent glass collector; as a result, the hydrogen concentration read as nearly 0.00%, while the chlorine reading remained normal. The solution to this problem is to use brown glass bottles as collectors, and at the same time cover the collectors with black cloth to ensure that the pipelines through which the sample gas passes are completely shielded from light. After the improvement, this issue was **solved**. 3.2.3 Dryer blockage: After approximately 15 days of operation, the sample gas flow rate dropped to near zero, and the readings on the gauge became abnormal; upon inspection, it was determined that the dryer was blocked. After removing the dryer, it was found that the drying membrane was deformed; the membrane, which was originally in the shape of a circular tube, had been flattened entirely, with some areas even folded. After cleaning with pure water and introducing nitrogen inside the dried membrane while immersing its outside in pure water for about 10 hours, the shape of the dried membrane returned to that of a circular tube. The reason for the deformation of the drying film is that the pressure of the nitrogen gas used for drying outside the film is too high, reaching 0.3 MPa; meanwhile, a vacuum is created inside the drying film, and deformation occurs as a result of the suction exerted by the external pressure. The fundamental solution to the problem is to reduce the pressure of the nitrogen gas used for drying outside the dry film. After analysis, the pressure of the nitrogen gas used for drying does not need to be very high; it is sufficient to ensure that its flow rate is at least 5 times that of the sample gas flow rate. In this way, we change the connection of the nitrogen gas used for drying from series with the air flow pump to parallel, allowing the nitrogen gas for drying and the nitrogen gas used by the air flow pump to be regulated separately. After the modification, with the sample gas flow rate remaining at 150 ml/min, the nitrogen pressure of the gas flow pump decreased from 0.3 MPa to 0.075 MPa. Keeping the nitrogen pressure entering the dryer below 0.01 MPa is sufficient to maintain a nitrogen flow rate of 3 L/min for drying. 3.2.4 Poor exhaust gas emission: During operation, the treatment of the analyzed exhaust gases (mainly chlorine and hydrogen chloride) is a key aspect. At the beginning of commissioning, liquid caustic in barrels was used to absorb the exhaust gases; although this caused large fluctuations in the flow rate of the sample gas, it still allowed the instrument to function properly. The disadvantage of using liquid caustic in barrels to absorb waste gas is that the absorption is incomplete, the liquid caustic needs to be replaced frequently, and there is a risk of it spilling out of the barrel at the site; it is not a very good solution. The best method is to send the exhaust gas to the accident chlorine treatment process for processing. When we connected the exhaust gas using the φ10 pipe that came with the instrument to the main chlorine pipeline in case of an accident, no sample gas flow was achieved no matter how we adjusted the air flow pump. After analysis, it is the small φ10 pipe that comes with the instrument, which causes a large pressure drop after the air flow pump; as a result, the air flow pump cannot function properly. The solution is to enlarge the pipe downstream of the air flow pump. This problem was successfully resolved after enlarging the pipe downstream of the air pump to φ32. 4. Analysis of data comparison: The chlorine and hydrogen analyzer began trial operation on July 26, and it was used during 4 startup tests of the diaphragm caustic soda plant. The instrument performed excellently during these startups, providing sensitive, accurate, and reliable analysis data. Taking the startup data of the diaphragm caustic soda plant on September 23, 2008, as an example, a comparison is made between the chlorine purity and the readings from the chlorine hydrogen analyzer versus the results of manual analysis. Figure 4 shows the comparison between the machine-read data and manually analyzed data for chlorine purity, while Figure 5 shows the comparison between the machine-read data and manually analyzed data for chlorine hydrogen content. Figure 4 shows the comparison curve between the machine-read data and the manually analyzed data regarding chlorine purity. Figure 5 presents the comparison curve between the machine-read data and the manually analyzed data for chlorine hydrogen content. As can be seen from Figure 4, the trend of the comparison curves for chlorine purity data is exactly the same for both machine-read and manually analyzed values, with good agreement between them. This indicates that the chlorine hydrogen analyzer provides results that are essentially consistent with those obtained through manual analysis, thus meeting the requirements of production. As can be seen from Figure 5, regarding the chlorine-hydrogen data, the manually analyzed data show greater fluctuations, while the machine-read data remain stable. The reason for this phenomenon is that manual data analysis suffers from significant jumps due to the volume scale of the analytical instruments (in practical manual analysis of chlorine and hydrogen, this is the main cause of large errors at low concentrations). Although the data from the two methods do not match perfectly, the trend lines of the machine-read data and the manually analyzed data are generally similar. It can therefore be concluded that the chlorine hydrogen analyzer provides results that are consistent with those obtained through manual analysis when measuring chlorine gas, and it is suitable for meeting production requirements. 5 Conclusion The use of chlorine-hydrogen analyzers in our company has been successful. As this type of instrument was used for the first time in China for online measurement of chlorine purity and chlorine hydrogen content in chlorine pipelines, some minor issues arose during operation. By making improvements to components such as the water collection container and nitrogen supply lines, problems such as poor drainage, low hydrogen readings, and clogged desiccators were resolved, thus meeting all the requirements for safe production. The improved instrument operates more stably and reliably. The chlorine hydrogen analyzer enables online analysis of chlorine purity and chlorine hydrogen content; it features automatic sampling, eliminating the need for manual sampling and thus reducing the risk of leaks, making it very safe for operators ； The sensor is not affected by fluctuations in sample gas pressure and ambient pressure, ensuring stable operation ； It also features fast response, displaying readings directly; it is connected to the DCS system, and automatic audio-visual alarms can be activated after setting alarm parameters. No special requirements are needed for the operators or those who read the measurements. In terms of maintenance, the instrument only requires calibration 1–2 times a year; the calibration method is simple and easy to operate. The machine-readable data was compared with those obtained through manual chemical analysis, and the results were generally consistent, showing similar trends. It was also tested during 4 startup processes of diaphragm caustic soda production in our company. In summary, the use of the KK650 chlorine and hydrogen analyzer on the chlorine main pipeline enables convenient and safe online monitoring of chlorine purity and chlorine hydrogen content; it allows for the acquisition of data on these parameters in real time, thus fulfilling the dual purposes of monitoring chlorine purity and chlorine hydrogen content in the pipeline, as well as ensuring both quality control and safety oversight of the chlorine. It is of great significance for guiding the safe production of caustic soda electrolysis systems. # + + .