Low temperature waste heat recovery system of sulfur acid making plant (H2SO4) more than 99%, the characteristics of low corrosion of metal, the use of corrosion resistance special stainless steel series, its requirements are: in the production process of sulfuric acid, the concentration of sulfuric acid is strictly controlled in a small range. (H2SO4) (99.05~99.51%).

The low temperature waste heat recovery system balances the heat recovery at a specific H2SO4 temperature and concentration to maintain the operating life of the device. During the operation of the sulfuric acid plant, the flue gas temperature and SO3 concentration entering the low-temperature waste heat recovery system are basically not affected by the device load, so the temperature of the low-temperature waste heat recovery system is stable. But the system is also SO3 absorber, its outlet. (H2SO4) is 99.5%, in order to ensure the absorption efficiency of low temperature waste heat recovery system SO3, it is necessary to circulate sulfuric acid in the upper tower for water dilution, control the circulating sulfuric acid φ (H2SO4) is 99.05~99.2%. In this process, in order to ensure the safety of the low-temperature waste heat recovery tower, the acid concentration of the primary cycle must be controlled at &phi to ensure that the acid fog of the flue gas at the outlet of the primary SO3 is qualified. (H2SO4) ≥ 99% and φ (H2SO4) ≤ 99.2%.

Therefore, the accuracy and stability of the primary circulating acid concentration of the low-temperature waste heat recovery system is related to the safety and long-term stable operation of the low-temperature waste heat recovery tower system.

Importance of primary circulating acid concentration measurement

1. The sulfuric acid concentration adjustment of the low-temperature waste heat recovery system is realized by adding water, and the design of adding water is automatically controlled, and the process variable is also the sulfuric acid concentration of the first-stage cycle upper tower. Therefore, the acid concentration in the primary cycle is not only the safety instrument of the protection device in the low temperature waste heat recovery system, but also the measuring instrument. When the acid concentration of the first order cycle cannot be measured accurately, it will inevitably cause the acid concentration of the first order to be lower than &phi due to the change of production load. (H2SO4) 99.00 %, accelerated corrosion, resulting in serious corrosion of waste heat recovery tower walls, evaporators, heat exchangers, pumps and primary acid separators.

2, when the waste heat recovery system automatically adds water to lose the process control variables, it will increase the work intensity of the operator, if the load changes can not adjust the concentration of primary cycle sulfuric acid in time, it will lead to the concentration of primary cycle acid deviate from the allowable value of the process.

3, when the concentration of primary circulating acid cannot be accurately obtained, in order to reduce the corrosion of the equipment, the acid concentration is usually controlled to be higher, which will affect the acid fog exceeding the standard and corrode the back-end equipment of the system.

4, when the primary circulating acid concentration cannot be accurately obtained, in order to ensure the safe operation of the device, it is necessary to manually analyze the circulating acid concentration several times a day, increasing the workload of analysts and safety risks, and increasing the error of process control parameters.

Basic information

The customer is a large phosphating enterprise in Sichuan, there are two sets of 400Kt/a sulfur acid production devices, and the waste heat recovery system is used for primary absorption. At the outlet of the diluter, two sets of Rosemount conductivity sensors and 56E transmitters were selected to measure sulfuric acid concentration in combination. Since the primary circulating sulfuric acid measured is diluted sulfuric acid (temperature 180℃, pressure 0.1MPa), in order to achieve rapid and uniform mixing of water and sulfuric acid, the diluter adopts compressed air atomization to enhance the disturbance of desalted water. As a result, the conductivity sensor measures sulfuric acid containing air medium, which makes the measured value of acid concentration in primary cycle of the waste heat recovery system deviate greatly from the actual value from the operation to the transformation, and the measured data fluctuate greatly (see Figure 2). Therefore, the measured results can not be used to ensure the safety of the device interlock and automatic control of water.

Cause analysis

Low temperature waste heat recovery primary cycle sulfuric acid process parameter control

Temperature: 180~190℃;

Pressure: 0.1~0.2MPa;

Flow: 800~1000m3/h;

Concentration: 99.05~99.15%;

Air content: sulfuric acid content 8~10%.

Sulfuric acid concentration analyzer sampling installation position: primary circulation sulfuric acid main pipeline intake tower horizontal pipe position;

Since the main pipeline into the tower is the acid separator, the pressure at the sampling position is basically the static pressure of the acid separator level. The selected 56E transmitter has a built-in 120 ° C φ (H2SO4) Sulfuric acid concentration curve corresponding to 96~100% conductivity, which can output sulfuric acid concentration or conductivity.

Then what causes the fluctuations in the measurement data?

See the following table for analysis reasons:

Specific reasons: Because the diluter adopts the technology of atomizing demineralized water with compressed air to enhance the disturbance, the circulating sulfuric acid from the diluter contains air medium. However, the current conductivity sensor can not accurately measure sulfuric acid concentration in air medium. When sulfuric acid containing air medium passes through the conductivity sensor, because the air is not conductive, the conductivity measured at this time is very low and unstable. Because the higher the concentration of sulfuric acid, the lower the conductivity, the measured value is larger than the actual and the fluctuation is large, so it can not be accurately measured.

Solution

To solve the problem of accurate measurement of sulfuric acid concentration in primary circulation, the separation of the air medium is the key. Therefore, it is necessary to design a set of equipment that can stably and efficiently separate the air contained in sulfuric acid according to the characteristics of primary cycle sulfuric acid.

Lilly Technology developed HAS110 high temperature concentrated sulfuric acid analyzer to solve the problem of measuring the concentration of primary circulating sulfuric acid at present. The air medium contained in the first stage circulating sulfuric acid is completely separated, and the analytical flow stabilizer is designed to reduce the interference of the unsteady flow of the analytical medium to the measurement results. (See Figure 3)

In order to save costs for production enterprises, Eli Zeliza Technology makes full use of customers' original normal measuring instruments: conductivity sensors and 56E transmitters, and applies them to HSA110A high temperature concentrated sulfuric acid analyzer, not only to help users reduce procurement costs, but also to maximize the value of existing equipment.

Lilly Technology developed the XA96-100 sulfuric acid concentration line function meter according to the sulfuric acid concentration curve corresponding to the conductivity of high temperature concentrated sulfuric acid, which has been running stably for three months at the production site, data (see Figure 4). The modified first-stage circulating acid concentrator has realized safety interlock control to protect the device from corrosion, provide accurate and stable process variables for automatic water addition, and realize automatic water addition control.

High temperature concentrated sulfuric acid analyzer Technical characteristics:

Adopt stable air separation technology, equipped with conductivity sensor, line function meter, composed of high-performance low-temperature waste heat recovery system special analysis instrument. It is suitable for measuring acid concentration in primary circulation, acid concentration at circulation pump outlet, acid concentration at heater outlet and acid concentration at preheater outlet. The problem caused by the failure of the current high temperature concentrated sulfuric acid analyzer is solved. Fast and accurate process measurement for automatic control of diluter water addition is realized, and low temperature waste heat recovery system is protected from low concentration sulfuric acid corrosion.

Technical parameter list

Application effect of high temperature concentrated sulfuric acid analyzer

The low temperature waste heat recovery system of the sulfur acid production plant replaced the primary circulating sulfuric acid high-temperature concentrated sulfuric acid analyzer in December 2022. After putting into use, the concentration of sulfuric acid in the first stage circulation is measured accurately, the data is stable, the measured value is reacted quickly, there is no sulfuric acid leakage at the sensor and no maintenance work.

The production practice has proved that the design and material selection of the high temperature concentrated sulfuric acid analyzer are reasonable, which can meet the requirements of the low-temperature waste heat recovery system, and completely solve the current problem of the measurement of primary circulating sulfuric acid. It is a core measuring instrument to ensure the safety, long period and stable operation of low temperature waste heat recovery system, and is worth promoting and applying.

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