Sulfuric acid is an important chemical raw material with wide application. The vast majority of sulfuric acid production in China is produced by the furnace gas containing S02 produced by pyrite roasting (a small amount is produced from the smelting flue gas containing S02). Sulfuric acid production process can be roughly divided into four main processes, namely roasting, purification, transformation, drying and absorption. In the purification process, there are pickling and water washing. There are two kinds of conversion processes: two turns, two mocks and one turn, one suction. The process pumps are mainly used in purification, drying and absorption of feed gas and storage and transportation of finished acid. There are two types of vacuum pump: horizontal vacuum pump and vertical vacuum pump. The main characteristic of vacuum pump is the strong corrosion resistance of dilute sulfuric acid and concentrated sulfuric acid. The concentration of sulfuric acid used can be divided into two types: dilute acid pump and concentrated acid pump.
Taking the process flow of purification and drying absorption of 80000 tons sulfuric acid plant with two conversion and two absorption pickling purification process as an example
1. Acid cleaning section
Acid cleaning process is to use dilute acid containing about 10% – 30% H2SO4 as the circulating absorption liquid to purify the furnace gas containing impurities. Compared with water washing purification, it has the advantages of realizing the closed cycle operation in the purification process, minimizing the discharge of harmful substances, reducing environmental pollution, and also producing part of dilute acid.
One of the key equipments to realize dilute acid circulation is dilute acid circulation pump. Dilute acid is recycled in the whole purification process. Acid concentration, acid temperature, impurities and solid particles contained in the acid are very important for the design and selection of circulating pump structure and materials. According to the different raw materials of gas production, the impurities in the raw gas are also different, resulting in different impurity composition and impurities in dilute acid. Dilute acid contains not only chloride ion, but also arsenic and its compounds or fluorine and its compounds, or both. In addition, dilute acid also contains a large number of washed solid particles. The concentration of dilute acid is generally no more than 30% (h2s04), and the acid temperature is 40-60 ℃. The dilute acid circulating pump used in sulfuric acid plant usually adopts the non-metallic horizontal pump which is both corrosion-resistant and abrasion resistant.
2. Drying absorption section
The drying and absorption section is to convert the purified feed gas containing S02 into S03 gas, which is then absorbed by concentrated flow acid to produce finished acid.
The drying and absorption of SO2 use high concentration sulfuric acid as the absorption medium. The concentrated acid circulates between the drying tower and the circulation tank by the circulation pump. The relative impurity content of concentrated acid is low, but the acid concentration and the acid temperature are high. The concentration of dried acid is generally above 93%, and the absorbed acid concentration is 98%. It is a strong oxidizing acid, and the acid temperature above 80 ℃ can reach 119 ℃. The acid temperature of strong oxidizing acid has a great influence on the corrosion rate.
As concentrated acid belongs to strong oxidizing corrosive medium, it requires high safety of equipment seal. When horizontal pump is used, the structure and material of shaft seal are difficult to solve, and maintenance is also difficult. At present, vertical submerged pump is mostly used, which is characterized by that the main body of vacuum pump is in the liquid phase, no leakage, no occupation of land, simple installation and maintenance, and the structure and material selection of shaft seal are relatively easy to solve. The hydraulic performance of vacuum pump is generally not required to be high. The material of key parts can be chrome cast iron, alloy, special stainless steel, etc. with strong acid corrosion resistance, but the price is higher than that of horizontal pump.
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