I. Overview
The position of online analytical instruments in chemical production is increasingly important. The use of online analyzers for quality control, process monitoring and optimization is one of the important signs of modern chemical production. The rational selection and installation of on-line analytical instruments is a guarantee for improving the quality of chemical products and achieving optimal control. It is also the key to the benefits of chemical production [1-4].
Among the ethylene crackers, the cracking furnace is the core of the energy consumption of the ethylene plant. The combustion control status of the cracking furnace directly determines the energy consumption level and operating efficiency of the plant, directly affecting the profitability of the plant.
At present, in the control of the cracking furnace, the helium oxide analyzer is generally used to measure the oxygen content of the flue gas in the cracking furnace. At the same time, the cracking furnace control scheme generally uses an online analyzer to analyze the CO content to monitor the cracking furnace flue. The CO content in the gas enables the monitoring of the combustion status of the cracking furnace, optimizing the operation status of the cracking furnace, making the combustion process of the cracking furnace, judging the fuel utilization rate, the thermal efficiency of the furnace, improving the fuel combustion efficiency, eliminating fuel waste and heat loss, and further Improve the efficiency of fuel use while achieving the goal of energy conservation and environmental protection.
Second, cracking furnace flue gas application features
The role of CO in cracking furnace analysis of ethylene plants is to continuously monitor the combustion conditions of the fuel in the cracking furnace and control the CO content in the flue gas to be lower than a certain parameter (general design value is 100 ppm).
The fuel of the cracking furnace is generally fuel oil and combustible exhaust gas generated by the device. Because the fuel is clean, the condition of the flue gas after combustion is ideal. The working temperature is generally between 120 and 170 degrees Celsius. The main component except nitrogen is the combustion of nitrogen fuel. The produced carbon dioxide and water, and a small amount of oxygen, etc., except for a small amount of particles, have no other impurities. It is an ideal and convenient measurement condition. Under normal circumstances, the design unit will select the infrared analyzer to extract the flue gas sample for sampling and analysis. measuring.
Our factory cracking furnace equipment currently uses a Siemens LDS6 laser analyzer to carry out in-situ analysis and measurement of the CO content in the flue gas of a single cracking furnace to achieve continuous monitoring of the combustion status of the cracking furnace. It has been working since its operation. normal.
Laser gas analyzers use in-situ (or in-line) measurement techniques to measure directly on the process pipeline being measured. Different from the traditional extraction gas analysis, the in-situ analysis does not require the sampling pipeline to extract and transport the sample gas, and there is no need to pretreat the sample. In this way, the basic configuration of the laser analyzer is much simpler than that of the extractor analyzer. The Siemens LDS6 laser gas analyzer system (shown in Figure 1) is divided into a central processing unit, a transmitting probe, a receiving probe, a signal cable and accessories.
According to the climatic conditions of Jilin, on the basis of the original system, the manufacturer provided another incubator for the heating and insulation of the probe in winter (see Figure 2 and Figure 3).
Figure 1: LDS6 Laser Gas Analyzer System Schematic
Since the laser gas analyzer does not require a sampling device, a sample transfer line, and a sample pretreatment system, the system is simple and the measurement is performed directly at the point to be measured, and sample collection, transportation, and pretreatment are not required, and the response time is shortened as much as possible.
Traditional extraction gas analyzer applications need to understand the composition of the medium to be measured, consider the possible mutual interference between the components, in order to determine and select the appropriate analyzer type. The laser wavelength emitted by the laser gas analyzer is modulated precisely to the absorption line of a specific gas. The emitted monochromatic laser light is selectively absorbed only by specific components having a specific absorption line in the scanning spectrum, and there is no measurement process. Interference. Relatively higher accuracy than traditional analysis methods. The absorption spectrum of a laser analyzer can also be referred to as a "single-line absorption spectrum."
The laser beam emitted by the laser analyzer launches a broad beam of laser light through the flue to be measured, and the signal is acquired by the receiving probe. The gas passed through by the entire beam participates in the analysis, and the “line†sampling can reflect the CO in the entire flue. Real content.
Figure 2: Field mounted LDS6 laser gas analyzer probe
Figure 3: LDS6 Laser Gas Analyzer Probe Incubator
Third, the analysis of the selection of analytical instruments
At present, the technology of continuous sample analysis for relatively clean samples such as cracking furnace flue gas is very mature and reliable, and its application is very extensive. However, its disadvantages are also apparent with respect to in situ analysis.
First of all, the real-time nature of continuous sampling analysis is affected by many factors, the system lags behind, and because the sample delivery pipe has been working under negative pressure conditions, there is a possibility that the sample pipe leakage sample receives external interference, and the sample lag is also continuous. One of the inherent characteristics of sampling analysis, and its lag time is affected by many factors such as sample pressure, temperature, flow rate, etc., and it is not stable, thus adversely affecting the accuracy and precise control of real-time acquisition of sample data.
Secondly, the typicality of continuous sampling and analysis is affected by many factors. Because the continuous sampling system only performs single point continuous sampling of the single position of the system, although the choice of sampling can be theoretically ideal, it is after all In spite of the ever-changing situation, the single-point sample and the actual media in the field will still have great differences influencing the accuracy and objectivity of the analysis results; moreover, the sample may also change due to changes in external conditions during sampling and transmission. This change will have a significant impact on the composition and content of the sample, thus affecting the accuracy of the measurement.
At the same time, long sample transfer lines can also cause malfunctions. Because the cracking furnace needs periodic defocusing during operation, a part of the coke can enter the sampling and conveying pipeline due to the suction of the sampling system and may be Stay in some places in the pipeline, and then block the pipeline and cause failures; and, because the flue gas contains a lot of gaseous water produced by combustion, when the water enters the flue sample pipeline, if the pipeline does not have good insulation The heat will condense into liquid water, or mix with the coke particles and other impurities in the pipeline to block the pipeline, or directly affect the instrument measurement, or even damage the instrument.
In addition, the installation and maintenance costs of the sample pretreatment system for continuous sampling and analysis are also a big investment. Take ethylene plant operating conditions as an example. If a continuous sampling infrared analysis system is used at this point, the system must be set up. The suction pump for sample suction and the preparation of pretreatment facilities such as sample lines with heat tracing and thermal insulation from the top flue sampling point to the analysis room will be a considerable investment. Plus, it basically offsets the original cost advantage of the infrared online analyzer; at the same time, because there are more moving parts and equipment, heat tracing system consumption and other equipment failures can not be avoided, it will raise the personnel maintenance and equipment follow-up investment higher Requirements, resulting in a large follow-up costs, and the use of laser analyzer system basically because there is no sampling system and moving parts required spare parts and consumption of materials is very small, personnel maintenance and repair content is very small, so its investment can be basically Completed, follow-up costs have greater advantages.
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