1. Causes of acid mist formation

During the sulfuric acid production process, the return acid in the drying circulation tank and the primary and secondary absorption circulation tanks will dissolve saturated SO₂. Since the circulating tank is under normal pressure, the dissolved SO₂ will be desorbed out of the tank. To adjust the concentration of sulfuric acid, water is usually added to the circulating tank for dilution. However, as water reacts vigorously with acid and releases a large amount of heat, some of the added water will be evaporated into water vapor. Therefore, during the production process, a mixture composed of desorbed SO₂, water vapor and acid mist will form above the liquid level in the acid circulation tank. To prevent the circulation tank from generating negative pressure due to the drop in liquid level, each circulation tank must be connected to the atmosphere. This leads to the discharge of gases mainly containing SO₂, SO₃ and sulfuric acid mist into the environment, causing pollution and having adverse effects on human health.

2. Hazards of Acid mist

According to the "Design Standard for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industry" (GB/T50493-2019), since 2020, China has formulated strict detection standards for toxic and acidic gases emitted by the petroleum industry. As an important component of the petrochemical industry, the detection of flammable and toxic gases in the sulfuric acid industry also needs to comply with the relevant requirements of this standard. These requirements impose strict limits on the emissions of acidic gases and toxic pollutants in the sulfuric acid industry, including emission limits for pollutants such as sulfur dioxide (SO₂) and sulfuric acid mist, as well as regulations on unorganized emissions of air pollutants at the boundaries of enterprises, in order to reduce environmental pollution. Not only new enterprises need to meet this standard, but existing enterprises also need to gradually achieve and maintain this emission standard through technological transformation and upgrading. In addition, the environmental protection department will regularly monitor the emission of acidic and toxic gases from enterprises to ensure that they meet the emission standards. However, the circulation tank has become the largest unorganized emission point of acid mist in the sulfuric acid plant. Many sulfuric acid enterprises have attempted to use electric fans and main fans for suction, but the results have not been satisfactory. This has become a persistent problem of acid mist emission in the sulfuric acid plant. The International Agency for Research on Cancer (IARC), an affiliate of the World Health Organization, has classified sulfuric acid mist as a Group G1 carcinogen, and Group G1 carcinogens have been directly identified as extremely hazardous media. After measurement, the emissions of SO₂ and acid mist from each circulation tank to the environment due to desorption are approximately 30 cubic meters per hour. The total amount of acidic gases emitted into the environment by each set of equipment throughout the year is approximately 480,000 cubic meters. This has caused serious pollution to the environment and corrosion to the equipment.

3. Current Situation

At present, in order to recover the acidic gas produced in the drying circulation tank and the secondary suction tower circulation tank of the sulfuric acid plant, pipelines are used to connect the circulation tank to the inlet of the main fan. The negative pressure at the inlet of the main fan is utilized for recovery, allowing the gas to re-enter the acid production system to produce sulfuric acid. However, as the circulating acid pump tanks of the drying tower and the secondary suction tower are lined with acid-resistant bricks, they cannot withstand vibration loads. Therefore, the pumps cannot be directly installed on the pump tank body and a separate pump frame needs to be set up. Due to the existence of a gap between the pump frame and the pump trough that could not be eliminated, after a period of use, it was found that the connecting pipeline could not achieve complete sealing between the drying pump base and the secondary suction pump base. As a result, water-containing air entered the pump trough through the gap and mixed with the acidic liquid to form dilute acid. The acid sludge formed after the dilute acid corrodes the pipeline will block the passage, preventing the acidic gas from being drawn to the inlet of the main fan. Ultimately, acidic gases are directly discharged into the atmosphere, seriously polluting the environment. To solve this problem, some sulfuric acid plants have improved the original waste gas recovery process and adopted fans for suction.

2. Due to the gap between the pump trough and the pump frame, to prevent the leakage of exhaust gas into the atmosphere, it is necessary to maintain a slight negative pressure inside the pump trough. The existing sulfuric acid plant is equipped with a tail gas desulfurization tower. The fan impeller and bearings of the tower are made of 316L stainless steel, while the fan casing is made of plastic. However, after the modification, it was found that in the acidic gas in the pump trough, moisture and acid mist formed dilute acid, which caused severe corrosion to the fan impeller and made it impossible for the impeller to maintain dynamic balance. In addition, the condensate acid condensed inside the fan intensifies the fan's vibration. The seal at the connection between the fan casing and the inlet back cover often experiences acid leakage and a small amount of exhaust gas. The bolts at the connection are also frequently corroded. The sealing packing between the shaft and the fan casing also frequently leaks dilute acid and a small amount of exhaust gas. The area where the shaft comes into contact with the exhaust gas is severely corroded, and usually the impeller and connecting bolts need to be replaced once a month. The newly replaced impeller needs to be thoroughly rinsed with clean water and re-tested for dynamic balance in preparation for the next replacement. During the operation of the fan, it is also necessary to regularly discharge the dilute acid inside the fan casing. This not only increases the inconvenience of operation but also leads to frequent maintenance and high maintenance costs. Meanwhile, during the maintenance process, the risk of exhaust gas leakage increases, and the condensing acid inside the fan also makes the maintenance environment worse. Therefore, solving the problem of acid gas recovery in the circulation tank has become a major challenge in the industry.

4. Solution: Acid mist suction device

The acid mist aspirator is a maintenance-free device without moving parts, specially designed for the characteristics of acid mist in the circulation tank. This equipment uses the compressed air generated by the main fan as power, eliminating the need for additional power devices and thus extending the service life of the equipment. It can automatically adjust the suction capacity according to the load of the device to ensure that the circulating tank is always in a slightly negative pressure state (-300Pa). Through the regulating valve, users can set the required negative pressure value to prevent the pump base of the circulating tank from being corroded by dilute acid. In addition, the acid mist suction device is equipped with a highly efficient demister, which can effectively remove 90% of the acid mist components in the acid mist, converting these acid mists into sulfuric acid and refluxing it back into the circulation tank. It is estimated that about 900 tons of sulfuric acid can be recovered throughout the year. This not only reduces environmental pollution, but also creates economic benefits of approximately 540,000 yuan. The acid mist suction device is made of engineering materials resistant to dilute acid corrosion to ensure long-term stable operation.

5. Product Performance

Name: Acid Mist Suction Device

Installation location: Drying circulation tank, secondary suction circulation tank, primary suction circulation tank

Power usage: Air at the outlet of the main fan

Suction capacity: 50-200m ³ /h

Suction negative pressure: maximum -6000Pa The adjustable range of suction pressure is 0 to -2000Pa

Air consumption: 50-200 m ³/h.

Equipment material: 316 stainless steel and corrosion-resistant engineering plastic.

Demisting efficiency: >90%

Product characteristics

It has no moving parts and uses the air from the outlet of the main fan as the power source.

2. Corrosion-resistant materials are adopted to effectively prevent dilute acid corrosion, thereby extending the service life.

3. The suction negative pressure of the circulating tank can be adjusted, which can prevent wet air from entering the circulating tank and corroding the pump base to the greatest extent.

4. It can efficiently recover the acidic gas generated in the circulation tank into the tail gas system, achieving zero emission of acidic gas on the site of the device.

5. It operates automatically, is easy to operate, maintenance-free, and simple to install. Once installed, you will benefit continuously.

A set of acid mist suction devices can restore a clean environment to your sulfuric acid plant site, solving long-standing problems. Contact us for a free technical solution.