Several issues in the application of air separation gas analysis instruments

In recent years, with the development of domestic air separation equipment towards large-scale, in order to meet the needs of large and medium-sized air separation production management and quality management, the number and types of gas composition analysis instruments introduced to match them have increased. These advanced gas analysis instruments have played a certain role in promoting air separation production management and improving the quality of gas products. However, due to some historical reasons, most of the personnel engaged in the application and management of analytical instruments are from the thermal instrumentation, automation engineering and instrument manufacturing professions and departments. They have not engaged in or have little contact with and studied the selection and application technology of gas analysis instruments. Therefore, some enterprises have improperly selected imported instruments and equipment, and the instrument functions cannot meet the production needs, causing economic waste. On the other hand, as a kind of high-tech product and a highly sensitive and high-precision scientific and technological tool, imported gas analysis instruments are often poorly applied due to insufficient understanding of their use requirements and low operating level of personnel, and cannot play their due role in air separation production and comprehensive quality management. The above problems are relatively common in the current domestic air separation industry. If this problem is not properly solved, it will be difficult to improve the management level of large and medium-sized air separation, and it will be difficult to effectively guarantee the safety of air separation equipment and gas quality (especially the quality of high-purity gas).

1. The application of gas analysis instruments is a professional technology.

Gas analysis instruments (specifically referring to instruments for trace gas analysis in this article) are tools used for gas composition analysis and testing. With the help of them, data on the types and contents of certain components can be obtained. However, gas analysis instruments are not simple tools. They are neither as simple in structure as flow meters and pressure gauges, nor as easy to operate and use as various thermal instruments. It is a type of tool with complex structure and high technical difficulty. The use of gas analysis instruments is a relatively complex and difficult-to-master specialized technology.

Generally speaking, the application of gas analysis instruments itself is a unique technical work, and it is a research-based work. However, this is not recognized and understood by outsiders. So far, the technical level of gas analysis instrument application in the domestic air separation industry is still at the primary stage compared with the petrochemical industry and the chemical fertilizer industry, and it is difficult to improve and develop rapidly. The main reason is this.

2. Analysis of Difficulties in the Application of Gas Analysis Instruments

Regarding the difficulties in the application of gas analysis instruments, we can get a general understanding of one or two from the following aspects.

2.1 Gas analysis is the realization of a series of chemical processes.

A gas analyzer or a gas analysis system is equivalent to a complete set of chemical process equipment. Therefore, the working process of the gas analysis instrument system is to realize a series of chemical processes. If you want to get accurate data through gas analysis, you must understand the situation and changes in each stage of this series of chemical processes, carefully study and master the laws, only in this way can you achieve the purpose of accurate measurement. It should be pointed out that not only does a gas analysis instrument have the same situation and conditions of a set of chemical process, but sometimes the sample gas pretreatment part (including sampling system) at the front stage of the instrument is also a set of chemical process. If you encounter more complex and special process technology conditions, then the chemical process reflected by the sample gas pretreatment system is still very complex, equivalent to the purification process of a small chemical plant. It can be seen that the process of gas analysis is to strictly control various factors that affect the measurement conditions on the premise of understanding and mastering the conditions of the entire chemical process system, so as to obtain accurate data required by process and management personnel.

2.2 It is difficult to control influencing factors and eliminate interference factors during the application process

. In the process of instrument application, there are many types of influencing factors and the changes are more complex. It is difficult to effectively control these influencing factors and eliminate the factors that interfere with the determination. For example, for the determination of trace oxygen, not only the system material and sealing must be strictly controlled, but also many factors such as the cleanliness of the system must be solved one by one, otherwise, the oxygen component analysis will not get accurate measurement results. For the determination of trace water content in gas, in addition to considering the various influencing factors mentioned above, the adsorption equilibrium of water in the sample gas in the pipeline must also be considered. The proper handling of this problem must rely on repeated experiments, understand its changes and laws, and master the operating techniques in order to obtain accurate results. Of course, the factors to be considered and controlled when using gas chromatograph to determine the ppm-ppb level impurity content in high-purity gas are more complicated.

2.3 The influencing factors of trace gas component analysis are more complex.

The microscopic changes that occur when gas components flow in pipelines and equipment are complex and changeable. Many influencing factors that can be ignored in the analysis of constant gas components cannot be ignored in the analysis of trace gas components. Instead, they must be taken seriously. At this time, these factors have become the main contradictions affecting the correct results of trace gas component analysis. They must be eliminated and resolved one by one to ensure the smooth operation of the trace gas analysis instrument. These influencing factors mainly include the following aspects: ① repeated mixing of gases in the sampling pipeline; ② physical and chemical effects between the tube wall and the gas components; ③ pipeline material; ④ pipeline connection method; ⑤ pipeline cleanliness.

2.4 Instrument and method verification is one of the keys to obtaining accurate data.

As a measurement and detection tool, the data given by the instrument under normal operation are mostly relative values. Whether the measurement data is accurate and the degree of accuracy (precision) cannot be provided or verified by the instrument itself. It must be completed by peripheral technical work, which is the verification of analytical data.

(1) Verification of instrument linearity. First of all, in order to ensure the normal operation of the instrument, the analytical instrument, as a kind of measurement instrument, must be tested by the authoritative measurement department in accordance with the national regulations every year before it can be used. At the same time, it is also necessary to use a series of standard gases every year to check whether the linear relationship of the instrument in the entire linear range remains normal. Otherwise, blindly trusting the integrity of analytical instruments (even imported instruments) will definitely lead to errors in production management and quality management due to erroneous data.

(2) Error analysis. In the application process of analytical instruments, error analysis must be conducted for each measurement result to determine the authenticity, reliability and credibility of the data analysis. A qualified analyst will not and should not casually report or publish the results of each analysis and measurement. Generally, after the measurement results are obtained, after error analysis, when it is determined that the sum of the errors of the analytical data is less than the specified allowable error, this one (or a group of) data will be considered as the correct measurement result and reported or published. Otherwise, inaccurate data will bring serious adverse consequences to production managers.

(3) Instrument calibration commonly used in quantitative analysis. As a quantitative analysis instrument, gas analyzers must be calibrated (or calibrated) with standard gas before quantitative analysis. Standard gas is generally purchased from national metrology departments or legal factories. In special cases, it can also be configured by yourself (but you must have the qualifications and ability to configure standard gas and related equipment). The shelf life of standard gas is one year. When using standard gas to calibrate analytical instruments, you must also have a deep understanding of normal procedures and usage rules. If you purchase and use standard gases that do not meet the requirements, it will lead to great deviations in the analysis data. If you do not fully understand the requirements for the use of standard gases, it will also cause trouble for air separation production due to the inability to obtain accurate data results.

2.5 Analytical engineers must continuously improve and enhance analytical detection technology.

A qualified analytical engineer needs to continuously learn and research new technologies for analytical instruments and new technologies for instrumental analysis, and promptly apply them to his or her job in order to achieve the purpose of continuously improving and enhancing analytical detection technology. An analytical engineer must not only be able to make the best possible use of existing equipment, but also should, based on a deep understanding of the principles, structures and performance of the instruments currently used, absorb advanced foreign and domestic analytical technologies at any time, continuously innovate technology, and further improve and enhance the detection level of existing instruments, rather than just being satisfied with simple operations.

3 Conclusion

In short, trace gas analysis is a specialized technology and a research-based work, which determines the application effect and level of gas analysis instruments. Trace gas analysis technology is also a practical science. It is necessary to go through a lot of experimental practice to explore the regularity of the chemical, master it well, and satisfactorily solve various specific trace gas analysis topics. This is also the experience summary of the first generation of gas analyzers in my country when they began to develop high-purity gases in the 1960s and 1970s. This conclusion drawn from their decades of gas analysis practice should attract the attention of future generations. We hope that gas analysis technicians in different industries can achieve faster and greater development under the new situation of rapid development of air separation.