1. Background overview

In recent years, the emission of gas in the production process of sulfur acid is a project strictly controlled and required by the state. With the formulation of a new round of national environmental protection standards, higher requirements are put forward for the sulfur content in the production process of sulfur acid production, and the waste heat recovery tower is an important facility in sulfur acid production. However, under the abnormal condition of acid in the flue gas pipeline of the tower export, many safety hazards will arise, which will seriously lead to the corrosion of heat exchange equipment and the decrease of the total conversion rate, thus destroying the safe production environment. It also puts pressure on the environment [1]. Based on this, this paper analyzes the influence of acid fog produced by waste heat recovery tower in sulfur acid production process on the equipment, and discusses the reasons why the acid fog of the waste heat recovery tower exceeds the standard in sulfur acid production process. Finally, combining with the specific reasons, the corresponding optimization solution strategy is proposed to effectively reduce the acid fog of the waste heat recovery tower. Better realize the safe, stable, long, full and excellent healthy operation of the device.

2. Impact of excessive acid fog on the device

2.1 Corroding heat exchange tube and inhibiting secondary conversion rate

The flue gas in the shell of the heat exchanger mainly comes from the waste heat recovery tower. When the defogger inside the waste heat recovery tower has perforated the flower plate, the acid cup liquid seal fails, and the fiber bed has short circuit, etc., the defogging efficiency decreases, or the acid separator corrodes and reduces the absorption rate of the waste heat recovery tower, so that the SO3 concentration in the flue gas after absorption is higher, and more small droplets are formed. When the concentration and temperature of absorbed acid or the temperature of flue gas into the tower cause the amount of acid fog to exceed the processing capacity of the defogger, the acid fog carried by the flue gas will condense at the shell of the cold heat exchanger, causing corrosion to the tube plate, tube bundle, cylinder and other parts near the cold flue gas inlet side of the cold heat exchanger [2]. With the accumulation of time, the tube bundle will eventually be corroded and damaged, and a large amount of acid mud will increase the shell resistance [3]. For the heat exchanger of this structure, the leakage point is generally near the cold gas inlet side, and the SO3 gas in the pipe path leaks into the shell path and contacts with the cold gas from the absorption tower to form condensing acid, which accelerates the corrosion of the heat exchanger. The excessive acid fog will cause some SO3 flue gas to short-circuit into the fourth stage of the converter, affecting the conversion rate of the secondary conversion, resulting in an increase in the SO2 concentration at the outlet of the second suction tower, an increase in the cost of tail gas treatment, and a substandard device capacity.

3. Cause analysis

3.1 Inlet acid temperature, concentration and gas temperature

Acid temperature and concentration are the key factors affecting the absorption rate, but also affect the smoke particles. In an environment where the acid temperature is low, the flue gas in some areas of the tower condenses, producing condensed acid, and then forming susaturated acid vapor pressure, accompanied by obvious acid fog phenomenon, while the particle size of the generated smoke particles is generally small, and the foam catcher is difficult to intercept all aspects, thus showing a certain size of acid foam.

3.2 Demister health issues

As mentioned above, when the demister inside the waste heat recovery tower perforates the flower plate, the acid cup liquid seal fails, and the fiber bed short-circuit results in the decrease of the demister efficiency, it will affect the decrease of the demister efficiency, so that the SO3 concentration in the furnace gas after absorption is higher, and more small fog droplets are formed. Therefore, in the actual production process, it is necessary to timely detect whether the demister is in a healthy and stable operating state.

4. Technical advice

4.1 Control drying tower outlet moisture and acid distribution

The formation of sulfuric acid vapor in the conversion flue gas into the waste heat recovery tower, the most important reason is the water brought into the drying tower outlet, the more water brought into the sulfuric acid vapor pressure is higher, and the higher the steam pressure is not conducive to the uniform separation of fluids, easy to form acid fog. Therefore, it is recommended to measure whether the moisture content of dry air exceeds the standard at the exit of the drying tower; Or install a condensing acid emitter at the outlet of the drying tower and the outlet of the fan, and determine whether the drying efficiency of the drying tower and the acid separation state are uneven by the change of the amount of condensing acid.

4.2 Accurate control of process parameters

The main reason leading to the excess of acid mist at the exit of waste heat recovery tower is the concentration of circulating acid absorbed at the first and second levels and the temperature of the secondary absorbed acid. In order to better and accurately control the acid concentration, it is necessary to do the following aspects of work: First, the primary acid concentration of the waste heat recovery system and the acid concentration at the outlet of the circulating pump are accurately measured, and the PID automatic control is adopted for the water addition control of the primary acid concentration, rather than manual control, because the manual control can not respond to the control of the water addition of the device load changes in time. Secondly, for the control of the temperature of the secondary absorbed acid, when the load changes, the load from the first stage to the second stage changes with the change of the load. When the fixed flow rate or acid temperature is adopted, when the load becomes lower and the secondary absorbed acid reaches the position of the primary acid division tank, the acid temperature is > 104℃, which creates a good condition for the generation of acid fog. At the same time, the particle size of the generated smoke particles is generally small. The fog eliminator is difficult to intercept, thus showing excessive acid fog. Therefore, the acid amount and acid temperature should be adjusted with the load to ensure the temperature when the secondary absorbed acid reaches the first level. 104 ° C.

4.3 Acid Fog Status Monitoring

Due to the particularity of acid fog, at present, there is no special acid fog detection instrument for the sulfuric acid industry, and it is generally used to manually measure whether the acid fog exceeds the standard, and each measurement of acid fog takes a long time, so it can not meet the real-time production. When the acid fog exceeds the standard, the pipe from the tower outlet to the hot and cold heat exchanger will collide with the dew point temperature and acid fog particles to produce large particles of sulfuric acid, which will be deposited at the bottom of the pipe. Therefore, the amount of condensed acid produced in this section of pipeline can be monitored in real time to indirectly determine whether the acid fog exceeds the standard. The condensing acid emitter can be used to monitor the production rate of condensing acid in real time and analyze the data. When the condensing acid is abnormal, the alarm can be issued, and the cause of excessive acid fog can be checked in time to protect the hot and cold heat exchanger from corrosion and the conversion rate decline.

References:

[1] Zhou Biao. Analysis of dew point corrosion problem in sulfur acid making plant [J]. Sulfuric Acid Industry,2002(5):23-25. (in Chinese)

[2] Jiang Ning. Influence of furnace gas dew point on Design of waste Heat recovery System in sulfuric acid plant [J]. Sulfur and Phosphorus Design and Powder Engineering,2013(5):17-19. (in Chinese)

[3] shao-wu liu, qi yan, zhao built, etc., sulfuric acid production technology [M]. Nanjing: southeast university press, 1993:586.