What are the working principles and common faults of the AMETEK zirconia oxygen analyzer?

1 Introduction

The semi-regenerative reforming process, proposed by UOP in 1949, was the first catalytic reforming method to utilize a platinum-coated catalyst. With the development of process and catalyst technology, treatment methods have been continuously improved. In 1971, UOP once again proposed CCR platinum reforming, innovating the contact reaction reforming process.

CCR platinum reforming adds a moving catalyst bed. This unique system allows catalyst to be added to and removed from the reactor while the unit is operating. The spent catalyst from the reactor flows to the regeneration tower, where the carbon covering the catalyst (coke) is burned off and the catalyst is reduced. The regenerated catalyst is returned to the top of the reactor.

2. The importance of zirconia oxygen analyzer

In the regenerator, the coke reacts with oxygen and burns out. However, excessive combustion will burn the catalyst, and insufficient combustion will cause the coke to burn out. This requires that the catalyst regeneration reaction must be controllable. This is accomplished by controlling the oxygen concentration. The catalyst is a key factor in the catalytic reforming process. Therefore, the measurement and control of oxygen concentration in the catalyst regeneration reaction is crucial. It is very important that the oxygen analyzer used in this process has stable and accurate performance and is easy to maintain and maintain.

3. Principle of zirconia oxygen measurement

Electrolyte solutions rely on ions to conduct electricity, and solid materials with ionic conductivity are called solid electrolytes. The solid electrolyte has an ionic crystal structure and relies on holes to move ions to conduct electricity, similar to the mechanism in which P-type semiconductors conduct electricity via holes.

Pure zirconium oxide (ZrO2) is non-conductive, and when doped with a certain proportion of low-priced metal oxides as stabilizers, such as calcium oxide (CaO), magnesium oxide (MgO), and yttrium oxide (Y2O3), it has high-temperature conductivity and becomes an oxide. Zirconium solid electrolyte.

The zirconia sensing element is a closed tube or disk made of ceramic zirconia balanced with yttria or calcium oxide. The inner and outer porous platinum coatings act as catalysts or electrodes. At high temperatures (generally above 1200℉/650℃), oxygen molecules in contact with the platinum electrode become ionized. As long as the oxygen partial pressure is the same on both sides of the element, there is no directed net flow of ions, that is, an electric current, on either side. But if the oxygen partial pressure on both sides of the element is different, a directional flow of ions will occur, thereby generating an electric potential and current. At this time, the magnitude of the potential generated at both ends of the electrode is a function of the ratio of the oxygen partial pressures on both sides. If the oxygen content in the gas in contact with one side of the electrode is known (the gas on this side is commonly called the reference gas), the oxygen content of the gas on the other end can be obtained through calculation. Under normal circumstances, because the oxygen content of air is always stable at 20.9%, it can be used as a good reference gas in the measurement of zirconia.

Notice:

1) The constant A is related to the operating temperature. The higher the temperature, the higher the A value, and the output millivolt value increases.

2) During calibration, 0 calibration gas cannot be used.

3) In general applications, pressure compensation needs to be added.

4. Simple structure and common faults of AMETEK zirconia

4.1 Simple structure

A chemical plant is using two zirconia oxygen analyzers, numbered AT-10 and AT-11 respectively, which belong to two sets of reforming catalyst regeneration devices. Although there is a difference of more than ten years in the commissioning time, because of this instrument It is already a relatively mature system. Its detection principle of zirconia and the core structure of the instrument gas circuit have almost remained unchanged. Apart from the upgrade of the electronic unit within the meter itself, the only major change is the method of injecting gas.

AT-10 uses the pressure difference between different locations in the system to ensure normal flow of sample gas, while AT-11 uses nitrogen as power to drive sample gas flow. (Analytical instrument sampling pretreatment) The sampling inlet of AT-10 is close to the process pipeline, the sampling pressure is 0.25Mpa, and the return outlet is set at a position where the process pressure is 0.24Mpa. The pressure difference of 0.01 is used to make the sample gas circulate in the pipeline. . The sample return pipeline is generally longer, up to about 10 meters. The sample circulation pipeline diagram is shown in the figure:

Sample circulation line

4.2 Common faults

Because the process conditions in the regenerator are extremely harsh, the gas in the regenerator is mixed with a large amount of chlorine sulfide, black impurities from coke, and oil and gas droplets. When the temperature drops to about 100°C, these black impurities condense and solidify, making it extremely difficult to remove. Therefore, full-time heavy heating needs to be added to the entire return pipeline to ensure the temperature of the sample gas and prevent impurities from condensing. In practical applications, common faults occur:

1. Poor heat tracing effect results in blockage of the return pipeline, decreased sample gas flow, and inaccurate detection leading to interlocking shutdown.

2. The process is not operating normally, and impurities such as oil and gas coking escape into the meter and block the probe, causing the probe's sensitivity to decrease or even become completely unresponsive.

3. Sudden changes in temperature, such as a sudden power outage when the process is shut down for cooling, and the temperature of the probe suddenly drops from a high temperature of 695°C to normal temperature. The thermal expansion rates between the probe body and the coating are different, which may cause the coating to fall off.

In October 2015, a low flow alarm occurred in the flow switch of AT-11. After on-site inspection, it was found that the thermal resistor value dropped, causing the indicated temperature to drop, triggering an interlock alarm. After a power outage, it was found that the thermal resistor resistance value was normal at ambient temperature. After reassembly, When the power is turned on, the resistance gradually decreases as the box heats up.

After inspection, it was found that there was an insulation problem at the connection between the resistor and the two wires. As the environment heated up, the wires heated up, causing the contact to form another loop in parallel with the original resistor body. After replacing the thermal resistor with good insulation, it returned to normal.

5. Summary

The AMETEK zirconia oxygen analyzer is an oxygen analyzer based on zirconia as the measurement principle. It is used to detect the oxygen content in the regenerator with a UOP-licensed continuous catalytic regeneration process. An AMETEK oxygen analyzer is used to analyze the oxygen content in the catalyst regenerator. Its importance is self-evident. Therefore, its maintenance is also very important. This article is just a little experience at work. Please correct me if there are any errors.