1. The origin of condensed acid in the system

In the normal production process of the plant, the flue gas contains sulfur trioxide, water vapor and sulfuric acid vapor. Among them, sulfur trioxide is converted from sulfur dioxide, and its content is usually between 0.25% and 0.5% in the sulfur burning furnace, and the specific content will vary due to the temperature of the sulfur burning furnace and the organic content of the liquid sulfur. In the converter, the conversion rate of each stage will vary slightly depending on the type of catalyst selected. For the 3+1 two-conversion two-absorption process, the conversion rate can reach more than 95%. Therefore, the concentration of sulfur trioxide in the pipeline and equipment between the three exits and the one suction tower will reach a peak.

In normal operation, water vapor is mainly brought in through the air. Although the air is treated by a drying tower, the drying efficiency cannot reach 100%. The moisture content of the drying tower outlet is usually controlled at 0.1g/Nm³ The following. After these residual moisture enters the system, part of it exists in the form of vapor, and the other part reacts with sulfur trioxide to form sulfuric acid vapor and exists in the form of sulfuric acid vapor.

The outlet of the drying tower is installed with a screen demister, but the demister efficiency can not reach 100%. Generally, the acid fog content at the exit of the drying tower is controlled at 30mg/Nm³ The following. Some of this acid fog accumulates in the pipe, exits the system through the acid discharge pipe and is collected, and the other part exists in the form of sulfuric acid steam after heating in the sulfur burning furnace.

The flue gas mixture is equivalent to the composition of a concentration of sulfuric acid saturated vapor at a specific temperature, and the temperature is the dew point of the concentration of sulfuric acid vapor. When the temperature of the mixed flue gas is lower than the dew point temperature, the sulfuric acid vapor will condense into liquid sulfuric acid, which is what we call condensed acid.

2. Factors affecting dew point temperature

1, moisture: moisture content increases, will increase the dew point temperature

2, the concentration of sulfur trioxide: the higher the concentration of sulfur trioxide, the easier it is to form sulfuric acid with water vapor in the air, resulting in a rise in dew point temperature.

3, smoke pressure: the higher the smoke pressure, the higher the dew point temperature of sulfuric acid.

4, impurities in the flue gas: If the impurities in the flue gas can react with sulfuric acid, change the concentration of sulfuric acid, so that the specific sulfuric acid concentration changes, the dew point temperature also changes.

3. Analysis of the causes of condensing acid during operation

1. During the normal operation of the sulfuric acid plant, the flue gas in the equipment and pipe between the outlet of the third stage and the suction tower will reach a peak sulfur trioxide concentration. These flue gases need to be heat treated before entering a suction tower, and after heat transfer through the economizer, the flue gas temperature will drop to a very low value. If this temperature is lower than the dew point temperature, it will result in the formation of condensing acid. Therefore, in the normal operation of the device, the outlet of economizer 2 is prone to condensate acid.

2, more than 99.9% of sulfur trioxide in the flue gas at the outlet of the suction tower is absorbed, and the concentration of sulfur trioxide is significantly reduced after the flue gas passes through the suction tower, resulting in a decrease in the dew point temperature under this condition. However, the flue gas temperature at the outlet of the first suction tower is also greatly reduced by the limit of the demister and the acid temperature of the first suction tower. In a suction tower, the contact between flue gas and sulfuric acid will produce acid fog, most of which are intercepted by the demister (the concentration of acid fog at the outlet of a suction tower is usually controlled at 20mg/Nm³ Below, some devices may have higher control standards). The unintercepted acid fog enters the flue gas pipe, where some of it is deposited to form condensing acid, while the rest enters the subsequent section and is converted into sulfuric acid vapor. Therefore, condensed acid will be produced at the outlet of the suction tower, and condensed acid may also appear on the side of the tube of the hot and cold heat exchanger.

3, the flue gas in economizer 1 is the flue gas after secondary conversion, and its sulfur trioxide concentration is significantly reduced. Due to the change in the composition of the flue gas, the dew point temperature also decreases, so it can be safer to control the flue gas temperature at the exit of economizer 1 at a lower level, and it is not easy to form condensing acid.

4, heat heat exchanger and high temperature superheater due to their high operating temperature, much higher than the dew point temperature, so under normal circumstances will not produce condensing acid.

4. Analysis of the causes of condensing acid in sulfuric acid plant

1, drying tower exit

The main reason is that the defroming effect of the mesh defromer is poor, resulting in acid fog entering the pipeline and accumulation to form condensation. The defrothing effect is not only affected by the performance of the defrothing equipment itself, but also closely related to the inlet air velocity of the drying tower. Too much or too little air speed will affect the fog removal effect. In addition, the change of acid separation process and acid temperature may lead to a large number of acid droplets, which exceeds the processing capacity of the wire mesh defroster, resulting in an increase in acid droplets.

2, economizer 2 outlet

Outlet flue gas temperature is low, lower than the dew point temperature will generate condensing acid; The feed water temperature of the economizer is low, and the local temperature of the water pipe in contact with the flue gas is lower than the dew point temperature to generate condensing acid. The high moisture content of dry air leads to the increase of dew point temperature and the formation of condensing acid. The gas concentration of the converter is high or the conversion rate is high, resulting in the increase of the concentration of sulfur trioxide, the increase of the dew point temperature, and the formation of condensing acid; Liquid sulfur contains impurities that can react with sulfuric acid, which will also change the dew point temperature to produce condensing acid. Water vapor leaks into the system, resulting in an increase in dew point temperature and the formation of condensing acid, including leakage of the sulfonator jacket, boiler leakage, and high temperature superheater leakage.

3, a suction tower outlet pipe or heat exchanger shell

The main reason is that the de-fogging effect of a tower demister is not good, resulting in acid mist entering the pipeline and accumulating to form condensing acid. The effect of the demister is not only affected by the performance of the equipment itself, but also related to the installation flange of the demister and the amount of acid in the acid seal cup. In addition, the uneven acid distribution or acid temperature change in a suction tower may lead to a large number of acid droplets, beyond the processing capacity of the mist remover, thereby increasing the amount of acid foam generation. On the other hand, the internal leakage of hot and cold heat exchangers is also an important factor. When a large number of high-concentration sulfur trioxide leaks from the pipe to the low-temperature side of the shell, it will cause the low-temperature sulfur trioxide concentration to rise, and the dew point temperature to rise, and then generate condensing acid.

4, heat heat exchanger and high temperature superheater, economizer 1

Mainly because water vapor leaks into the system, and the dew point temperature rises to form condensing acid. Economizer 1 May also generate condensing acid because the smoke temperature is too low, below the dew point temperature.

5. The treatment and harm of condensed acid

Condensed acid must be discharged from the equipment in time and properly contained, or directly into the sulfuric acid underground tank, or neutralized by lime. If the condensing acid is not discharged in time, it will corrode the pipe of the equipment and cause irreversible damage to the equipment. Secondly, the presence of sulfuric acid in the equipment pipeline will cause the concentration of sulfuric acid to change, which will cause the change of the dew point temperature of the gas phase component, and then produce a steady stream of condensing acid, and the phenomenon that condensing acid can not be drained. Especially during the first driving of the new device, the drying efficiency of the drying tower is difficult to guarantee, and a large amount of moisture may enter the system. In addition, the temperature of the deoxygenated water entering the economizer is also relatively low, resulting in the flue gas temperature at the outlet of the economizer unable to rise quickly above the dew point temperature, resulting in a large amount of condensed acid. If the condensed acid is not discharged in time, a vicious cycle will be formed, resulting in an increasing amount of condensed acid. Therefore, when discharging condensed acid, it must be ensured that it is exhausted, otherwise condensed acid will continue to increase.